- Email: [email protected]
Thermal history of Early Jurassic eclogite facies metamorphism in the Nagaland Ophiolite Complex, NE India: New insights into pre-Cretaceous subduction channel tectonics within the Neo-Tethys
Accepted Manuscript Thermal history of Early Jurassic eclogite facies metamorphism in the Nagaland Ophiolite Complex, NE India: New insights into pre-Cretaceous subduction channel tectonics within the NeoTethys
M. Rajkakati, S.K. Bhowmik, A. Ao, T.R. Ireland, J. Avila, G.L. Clarke, A. Bhandari, J.C. Aitchison PII: DOI: Article Number: Reference:
S0024-4937(19)30323-8 https://doi.org/10.1016/j.lithos.2019.105166 105166 LITHOS 105166
To appear in:
LITHOS
Received date: Revised date: Accepted date:
31 January 2019 4 August 2019 5 August 2019
Please cite this article as: M. Rajkakati, S.K. Bhowmik, A. Ao, et al., Thermal history of Early Jurassic eclogite facies metamorphism in the Nagaland Ophiolite Complex, NE India: New insights into pre-Cretaceous subduction channel tectonics within the NeoTethys, LITHOS, https://doi.org/10.1016/j.lithos.2019.105166
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Thermal history of Early Jurassic eclogite facies metamorphism in the Nagaland Ophiolite Complex, NE India: New insights into pre-Cretaceous subduction channel tectonics within the NeoTethys
SC RI PT
M. Rajkakati1, S.K.Bhowmik1,*, A. Ao2, T. R. Ireland3, J. Avila3, G.L. Clarke4, A. Bhandari5, J.C. Aitchison6 1
Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur 721 302,
NU
India
Wadia Institute of Himalayan Geology, Dehradun, Uttarakhand 248001, India
3
Research School of Earth Sciences, Australian National University, Canberra ACT 0200,
MA
2
Australia
School of Geosciences, University of Sydney, Sydney NSW 2006, Australia
5
Geological Survey of India, Faridabad, Haryana 121001, India
6
School of Earth and Environmental Sciences, The University of Queensland, St Lucia,
PT
ED
4
AC
ABSTRACT
CE
Queensland 4072, Australia
Eclogite from two locations in a fossil accretionary complex now exposed in Nagaland, NE India, at the northern end of the Indo-Myanmar ranges, provides the oldest evidence for Neo-Tethyan subduction along the Yarlung-Tsangpo suture zone. Metamorphic texture, mineral composition, peak metamorphic P-T estimates, reconstructed metamorphic P-T paths, and U-Pb zircon ages from three eclogite samples collected near Mokie and Thewati villages collectively reveal snapshots of a c. 205–172 Ma subduction burialexhumation cycle in the eastern Neo-Tethys. The Thewati eclogite records a clockwise
ACCEPTED MANUSCRIPT metamorphic P-T path involving: (1) prograde metamorphism traversing the epidote blueschist facies and culminated in peak eclogite facies conditions at 25-28 kbar and ~650 o
C; (2) an early retrograde stage involving decompression with cooling in the eclogite facies
at ~18.3 kbar and 630 oC; followed by (3) cooling through the epidote blueschist, transitional lawsonite blueschist and greenschist facies to ~6 kbar and 300 oC. Circa 189-185 Ma peak
SC RI PT
metamorphism is inferred for the Mokie and Thewati eclogites, and was accompanied by an apparently low thermal gradient (~7-8 oC/km). Such conditions and the clockwise P-T path are associated with a tectonic scenario involving cold mature subduction within the NeoTethys in Nagaland. This
is thus the first comprehensive dataset of an Early Jurassic
NU
subduction channel (with respect to present geographic co-ordinates) for the eastern margin of the Neo-Tethyan suture. The Nagaland samples offer a critical record of a pre-Cretaceous
MA
subduction system within the Neo-Tethys, which incorporated an Early-Middle Jurassic Andean-type convergent margin extending along the southern Eurasian margin from Pakistan
ED
to the Indo-Myanmar Ranges.
PT
Key words: Eclogite; U-Pb zircon geochronology; Metamorphism; CW P-T Path;
AC
1. Introduction
CE
Subduction; Neo-Tethys; Yarlung-Tsangpo Suture Zone
Much of our understanding of Mesozoic plate tectonics, including the sequential amalgamation of Gondwanan continental fragments that assembled the Eurasian continent, derives from metamorphic and isotopic studies of three Tethyan suture zones that dissect Tibet. These are: (1) the Longmu Co-Shuanghu (LCS); (2) Bangong-Nujiang (BNS); and the (3) Yarlung-Tsangpo (YTS) sutures (Fig.1). They include blueschist (Anczkiewicz et al., 2000; Groppo et al., 2016), eclogite (Zhang, Y-X et al., 2018) and amphibolite facies rocks
ACCEPTED MANUSCRIPT (Guilmette et al., 2012; Wang et al 2008). These rocks provide information critical to understanding tectonic processes linked to the closure of the Palaeo- (Devonian-Triassic), Meso- (Early Permian to Late Cretaceous) and Neo-Tethyan (Late Triassic-Paleogene) oceans (see Zhu et al., 2013 for a review). The Yarlung-Tsangpo suture zone (YTSZ) is the youngest and southernmost of the
SC RI PT
three sutures that dissect Tibet. It is potentially the most cryptic in terms of understanding the timing, duration, and periodicity of the Neo-Tethyan subduction (see Hébert et al., 2012 for a review). Tectonic models incorporating the northward subduction of the Neo-Tethyan oceanic lithosphere beneath southern Tibet (Lhasa continental block) are well established,
NU
covering much of the Cretaceous and extending to the Early Cenozoic collision of India and Asia (e.g. Yin and Harrison, 2000). However, the pre-Cretaceous geodynamic history of the
MA
Neo-Tethyan lithosphere remains uncertain and subject to on-going discussion. Models include the opening of the Neo-Tethyan ocean as a Late Triassic back-arc basin due to the
ED
separation of the Lhasa block from northern Australia (Zhu et al., 2013), and the initiation of its subduction at c.180 Ma (Hébert et al., 2012). The existence of Late Triassic to Early
PT
Jurassic Neo-Tethyan subduction beneath the Lhasa Terrane has also been inferred on the
CE
basis of the age and geochemistry of arc components of the Gangdese magmatic belt (Yu et al., 2018). However, in the absence of direct evidence of related HP/LT metamorphism, the
AC
subduction channel dynamics of such early events within the Neo-Tethyan sutures are not well understood.
The Nagaland Ophiolite Complex (NOC), at the northern extent of the Indo-Myanmar Ranges (IMR) (Fig. 2a), is considered a southeastern extension of the Yarlung-Tsangpo suture. It exposes a rare Neo-Tethyan metamorphic sequence including pumpellyite-diopside, greenschist, blueschist and eclogite facies rocks (Chatterjee and Ghose, 2010; Ao and Bhowmik, 2014; Bhowmik and Ao, 2016) (Fig. 2b). Preliminary chronological constraints
ACCEPTED MANUSCRIPT indicate Middle to Upper Jurassic protolith ages for elements of the metamorphosed oceanic crust (Sarkar et al., 1996; Baxter et al., 2011; Aitchison et al., 2019). The timing and thermal history of eclogite facies metamorphism and its relationship with other HP metamorphites in the terrane are not known (see also Ao and Bhowmik, 2014; Bhowmik and Ao, 2016). Numerous studies indicate that eclogite facies rocks preserve critical evidence for the timing
SC RI PT
and thermal history of oceanic subduction (Agard et al., 2009 and references cited therein). In particular, low to medium temperature oceanic eclogites commonly preserve excellent textural and mineral compositional (e.g. compositional zoning in garnet, omphacite, epidote, amphibole and phengite) features amenable to the near complete reconstructions of
NU
metamorphic P-T paths, including well-constrained prograde histories (e.g. Franciscan highgrade eclogite block, California, Page et al., 2007; lawsonite eclogite pods, Sivrihisar Massif,
MA
Turkey, Davis and Whitney, 2008; lawsonite eclogite, Alpine Corsica, Brovarone et al., 2011; Zermatt-Saas, Western Alps, Reinecke, 1998, Angiboust et al., 2009; Monviso, Alps,
ED
Groppo & Castelli, 2010; Angiboust et al., 2012; Schistes-Lustres, Alps, Agard et al., 2001; eclogites of Pouebo terrane, Pam Peninsula, NE New Caledonia, Clarke et al., 1997;
PT
Dominican Republic, Krebs et al., 2011; Rio San Juan mélange Complex, Krebs et al., 2011).
CE
Moreover, recent analytical and theoretical advances that combine in situ U-Pb dating and trace element compositional analysis of zircon offer unprecedented insights into the
AC
timescales involved in forming high-pressure/ultra-high-pressure (HP/UHP) metamorphic terranes (Rubatto and Hermann, 2007; Rubatto, 2017). The Neo-Tethyan tectonic models for Southeast Asia, in general, are strongly influenced by data from the Tibetan belts with a lack of detail from the IMR and adjoining ophiolite belts in eastern and north-eastern Myanmar. Seen in this context, the Nagaland Ophiolite Complex presents a unique opportunity to: (a) test existing predictions for the existence of a pre-Cretaceous subduction system within the Neo-Tethys (e.g. Zedong terrane
ACCEPTED MANUSCRIPT - McDermid et al. 2002; Aitchison et al. 2007) and (b) determine the thermal and dynamic history of such a subduction zone using well-constrained metamorphic P-T paths from the eclogites. In this investigation, we establish the thermal history of an Early Jurassic NeoTethyan subduction system, considered to be the oldest within the Yarlung-Tsangpo suture zone, using textural and mineral compositional data, metamorphic reaction history, peak
SC RI PT
metamorphic P-T estimates, reconstructed metamorphic P-T path and SHRIMP U-Pb zircon age data from two new eclogites locations in the NOC. Given the importance of these findings in the tectonic frame work of Southeast Asia, we also review the published literature from the ophiolite belts of IMR and eastern and northeastern Myanmar. These new results of
NU
Neo-Tethyan tectonics from Nagaland and adjoining Myanmar are correlated with those from
MA
the YTSZ in the Tibetan sector.
2. Geological setting of the Nagaland eclogites
ED
Eclogite samples investigated in this work come from the Nagaland Ophiolite Complex at the northern end of the Indo Myanmar Ranges. The IMR are located at the junction between two
PT
orogenic systems: the Alpine-Himalayan continental collision belt in the north, and the active
CE
subduction zone of the Sunda–Andaman arc to the south (Fig. 2a). These orogenic systems resulted from the interaction of three tectonic plates: the Indian Plate in the southwest, the
AC
West Myanmar Block (alternatively Myanmar Microplate) in the east, and the Asian Plate to the north and northeast (Fig. 2a) (Searle et al., 2007). The IMR, along the western margin of the West Myanmar Block, is considered an accretionary wedge that developed in response to eastward subduction of the Neo-Tethys (attached to the Indian plate) beneath the West Myanmar Block (Searle et al. 2007; Acharyya, 2015; Aitchison et al., 2019). Ophiolitic rocks, with or without HP metamorphic rocks, in the IMR and the West Myanmar Block occur in three sub-parallel, N-trending ophiolite belts, called the Western
ACCEPTED MANUSCRIPT (WOB), Central (COB) and Eastern Ophiolite Belts (EOB) (terminology after Htay et al. 2017) (Fig. 2a). The WOB is the longest of the three belts and separates the eastern margin of the IMR from the West Myanmar Block. It can be traced for ~3000 km from Mayodia (Arunachal Pradesh) in the north near the eastern Himalayan syntaxis through the Naga and Manipur Hills, Kalaymyo, Chin Hills, Mt. Victoria-Kawlun belt in the middle section to the
SC RI PT
Andaman-Nicobar Islands in the south (Fig. 2a). Ophiolitic sequences in this belt generally occur as small, rootless and tectonised bodies with shallow-dipping foliations in low-grade metamorphic rocks, which are overlain by Eocene–Oligocene sedimentary rocks of flysch affinity (Acharyya, 2007). A Late Jurassic age for ultramafic rocks of the WOB was posited
NU
from a K–Ar age for hornblende-bearing pegmatite (158 ± 20 Ma) that intruded serpentinites in the Mt Victoria area (Mitchell, 1986). However, recent U-Pb dating of zircon indicates
MA
Early Cretaceous protoliths for ophiolitic components in this belt (e.g. c.116-119 Ma Manipur ophiolite, Singh et al, 2017, Aitchison et al., 2019; 127 Ma Kalaymyo ophiolite, Liu et al.,
ED
2016a). The formation of amphibolite-facies rocks in Kalaymyo has been interpreted as a metamorphic sole to the ophiolite and is dated at 115-119 Ma (Zhang et al., 2017). In
PT
summary, the ophiolites of the WOB are thought to mark a suture zone, where the Neo-
CE
Tethyan Ocean once existed (Searle et al., 2007; Liu et al., 2016a). The Central Ophiolite Belt extends from the Putao area of Kachin State in the north
AC
through the Jade Mines Belt (JMB) to the Sagaing-Minwun ranges in the south (Fig. 2a). It is composed of a dismembered, incompletely preserved ophiolitic sequence of serpentinite and peridotite (Htay et al., 2017). The Jade Mines Belt (JMB) includes jadeitite, eclogite, amphibolite, blueschist, chromitite, and serpentinized peridotite (Goffé et al., 2002) that reflect the effects of high-P/low-T metamorphism. The P–T conditions have been variously estimated at 10–15 kbar and 300–500 °C (Mével & Kiénast, 1986), >14 kbar and 400-450 °C (Goffé et al., 2002), >10 kbar and 250–370 °C (Shi et al., 2003) and ~15 kbar and ~380 °C
ACCEPTED MANUSCRIPT (Oberhänsli et al., 2007). U–Pb Jurassic ages of zircon taken from jadeitite (163 – 160 Ma) have been interpreted as protolith ages for the ophiolite in the JMB (Shi et al., 2008; Yui et al., 2013). The timing of the HP metamorphic event is not well defined, with age estimates ranging from the Late Jurassic (U–Pb zircon age between 158 and 147 Ma, Shi et al., 2008; Qiu et al., 2009) to the Late Cretaceous (U–Pb zircon age of 77 ± 3 Ma, Yui et al., 2013 and 40
Ar/39Ar age of 80 Ma for phengite in eclogite, blueschist, jadeitite, and amphibolite,
SC RI PT
an
Goffé et al., 2002). Zhang, J. et al. (2018) recently reinterpreted all published zircon age data from the JMB and concluded that the main HP metamorphism and accretionary orogenesis in this segment of the COB was of Early to Middle Jurassic age.
NU
The Eastern Ophiolite Belt (EOB) is represented by the Tagaung-Myitkyina belt (Fig. 2a) comprising massive peridotite of harzburgite to lherzolite composition, dacite and
MA
andesite, hornblende gabbro, diorite, granodiorite and veins of gabbro, rodingite and plagiogranite in peridotite (Liu et al., 2016b; Zhang, J. et al., 2018). The volcanic and
ED
gabbroic to dioritic rocks have supra-subduction-zone-type chemical signatures (Yang et al., 2012; Xu et al., 2017). Recent U-Pb dating of zircon from a variety of lithologies in the
PT
Tagaung-Myitkyina belt has yielded a Middle Jurassic protolith age (ca. 177-166 Ma, Yang
CE
et al., 2012, Liu et al., 2016a; Gardiner et al., 2018). The EOB has been interpreted as: (a) the southern continuation of the Meso-Tethyan suture in central Tibet (Liu et al., 2016a); (b) a
AC
klippen of the Medial Myanmar Suture Zone that developed along the eastern margin of the Mogok Metamorphic and Slate Belt (Mitchell et al., 2015); and (c) a Jurassic continental margin arc (Zhang, J. et al., 2018). The 200 km long and 5-15 km wide, NNE-trending Nagaland Ophiolite Complex can be distinguished from other ophiolite bodies in the WOB by: (1) the presence of a variety of HP/LT metamorphic rocks in ophiolitic mélange; and (2) its association with a disrupted oceanic plate sequence of chert, pillowed basalt, plagiogranite, mafic, and ultramafic
ACCEPTED MANUSCRIPT cumulate and serpentinite (Fig. 2b) (Acharyya, 1986). Some of its serpentinite has mantle peridotite lineage (Brunnschweiler, 1966). Ophiolitic mélange structurally overlies the Disang Formation (Fig. 2b), which has Indian continent-derived clastic sedimentary rock of Eocene age (Imchen et al., 2014). The ophiolite mélange is in turn structurally overlain by an Early Paleozoic metasedimentary sequence (Aitchison et al., 2019) forming the Naga
SC RI PT
Metamorphics (Brunnschweiler, 1966, Fig. 2b). The NOC includes accretionary metasedimentary rocks, dominantly of pelagic chert-limestone association, with attenuated metabasalt, metagabbro, serpentinite, and pillow basalt with igneous texture and mineralogy (Acharyya, 1986; Ao and Bhowmik, 2014; Bhowmik and Ao, 2016). Metamorphic
NU
assemblages in the NOC reflect greenschist, pumpellyite-diopside, and blueschist to eclogite facies conditions (Fig. 2b) (Ao and Bhowmik, 2014; Bhowmik and Ao, 2016; Chatterjee and
MA
Ghose, 2010). Ao and Bhowmik (2014) previously established evidence for a disrupted field array in the Mokie-Satuza sequence in the western part of the mapped area, with
ED
metamorphic grade increasing eastward from greenschist through pumpellyite-diopside to lawsonite blueschist facies (Fig. 2b).
PT
Preliminary whole rock K-Ar geochronology of metabasalt (148 ± 4 Ma, Sarkar et al.,
CE
1996) and radiolarian microfossils from elements of the NOC (latest Bathonian to early Callovian, Aitchison et al., 2019) indicate Middle-Upper Jurassic protoliths that are
AC
interpreted as having been scraped off the subducting oceanic lithosphere. Chatterjee and Ghose (2010) estimated peak metamorphic conditions for eclogite in the NOC at 17–20 kbar and 580–610°C. Bhowmik and Ao (2016) recently reported a hornblende eclogite facies metamorphic sole (peak metamorphism at 13.8 kbar, 625 oC) from a nearby area. In this work, we describe two new eclogite locations: (1) near Mokie village (sample N22F, Mokie eclogite from now on), and (2) ~2 km east of Old Thewati village (samples N23B, N40B, hereafter Thewati eclogite) (Fig. 2). In both locations, eclogite occurs as
ACCEPTED MANUSCRIPT small, meter-scale lensoidal bodies within a lawsonite blueschist facies sequence of basalt, chert and limestone (Fig. 3a-b). Thin discontinuous bands rich in tremolite and/or white mica separate the eclogite bodies from the host metabasaltic rocks (Fig. 3a-b).
2.1 Sample description
SC RI PT
Sample N23B is a coarse-grained eclogite, collected from the core region of the Thewati eclogite body (Fig. 3b). It consists of porphyroblastic garnet (5-7 mm grain diameter) (~18 modal%) and omphacite (2-5 mm grain diameter) (~19%) enveloped by a matrix of phengite (~31%), barroisite (Fig. 3c) and glaucophane (collectively ~22%), and chlorite (~8%), with
NU
minor epidote (~3%), rutile, sphene and accessory zircon (Fig. 3a, 3b, 3d). Sample N40B is a composite, omphacite-rich (~37-57%) eclogite (Fig. 3e) collected from the outer margin of
MA
the Thewati eclogite body (Fig. 3b). Pale green layers dominated by foliated omphacite (Fig. 3f) alternate with darker barroisite-rich layers. Porphyroblastic garnet (1-5 mm in diameter),
ED
enveloped by finely foliated omphacite (Fig. 3e, f) occurs throughout the sample (Fig. 3e). Thin discontinuous, layer-parallel segregations of epidote are also present in the rock. Sample
PT
N22F is a fine-grained, schistose rock collected from the outer margin of the Mokie eclogite
CE
body. It consists of garnet and Na-Ca pyroxene of omphacite to aegirine-augite composition, enveloped by Na-Ca amphibole of barroisitic to magnesio-kataphoritic composition, sphene,
AC
minor glaucophane, phengite, epidote and rutile and accessory zircon (Fig. 3g).The foliation is cut by phengite-rich veins. A total of five thin sections (two from N23B, here named N23B-1 and N23B-3 and three from N40B, named N40B-2, N40B-4 and N40B-6) was utilised to recover the metamorphic history of the Thewati eclogite. Sample N23B-1 was used for the geochronology. The Mokie eclogite sample N22F was used for both geochronological and metamorphic studies.
ACCEPTED MANUSCRIPT 3.Methods 3.1. Electron probe micro-analytical technique Major-element mineral chemistry was determined using a CAMECA SX-100 electron microprobe fitted with four spectrometers at the Department of Geology and Geophysics, Indian Institute of Technology (IIT), Kharagpur. Analyses were performed with an
SC RI PT
accelerating voltage of 15 kV, a beam current of 15 nA and a beam size of ~1µm (~5 μm for phengite). Natural mineral standards were used for calibration, including orthoclase (for K and Si), jadeite (Na), wollastonite (Ca), periclase (Mg), hematite (Fe), corundum (Al), rhodonite (Mn) and chromite (Cr). Analytical variation within standards was kept at or below
NU
∼1.5wt% for primary standards. Representative mineral chemical analyses (presented in
3.2. Metamorphic Reconstruction
MA
Tables 1-3) were recalculated using the AX program (THERMOCALC software, v. 3.26).
ED
3.2.1. Methods of determination of metamorphic pressures (P) and temperatures (T) We adopt a conventional thermobarometric approach to recover the P–T conditions of
PT
eclogite facies metamorphism at the two locations. This utilizes simultaneous solutions of
CE
garnet-clinopyroxene Fe2+-Mg exchange thermometric (reaction R1, 2 pyrope + 3 ferrosilite = 2 almandine + 3 enstatite, after Ravna, 2000) and garnet-clinopyroxene-phengite
AC
barometric (reaction R2, 6 diopside + 3 muscovite = 2 grossular + 1 pyrope + 3 celadonite, after Ravna and Terry, 2004) equations. This methodology has been widely applied to eclogite localities from around the world. P–T estimates made using these calibrations (from now on R00_RT04 and Method 1) are also compared with the results of computations of the same equilibria using the internally consistent thermodynamic dataset of Holland and Powell (1998) and THERMOCALC software (version 3.31, thermodynamic database tc-ds55, 22 November 2003) (Method 2). Mineral end-member activities and their uncertainties were
ACCEPTED MANUSCRIPT calculated using activity-composition (a-X) datafile coding for pseudosection calculations, following the approach outlined by Powell and Holland (2008). Solid solution models used for this purpose are clinopyroxene (Green et al., 2007), garnet (White et al., 2007) and phengite (Coggon and Holland, 2002). For the spessartine-rich garnet composition (Sps17) in N22F, the garnet activity model of Ganguly et al. (1996) was adopted for the Method 2. The
SC RI PT
model incorporates Mg-Mn interaction relevant to the peak mineral assemblage in the sample and for the computations of garnet end-member activities. Finally, we have also estimated peak conditions using the THERMOCALC average P-T method (Method 3) (see Bhowmik and Ao, 2016 for additional information on the methodology). The average P-T computations
NU
were carried out using the same garnet-clinopyroxene-phengite assemblage and mineral chemistry. The same THERMOCALC software, thermodynamic database and mineral end
MA
member activity models as adopted in the Method 2 has been applied here. For all the three methodologies, we have estimated ferric content in clinopyroxene using charge-balance
ED
criterion and in garnet following the methodology of Ravna (2000). The P-T results are
PT
presented in Table 4.
CE
3.2.2 P-T Pseudosection modelling
Estimates of peak metamorphic conditions and the likely reaction history of the Nagaland
AC
eclogite were also evaluated using calculated metamorphic phase diagrams. Sample N40B preserves excellent reaction textures (Fig. 4) and mineral compositional features (Fig. 5-6, Tables 1-3) consistent with evidence for prograde, peak and retrograde conditions being preserved in selected domains of the rock (Section 4). For this reason, we calculated P-T pseudosections using effective bulk rock compositions for two domains in the N40B: domain N40B-4 represents the garnet-pyroxene-rich peak assemblage; and domain N40B-6 is rich in retrograde minerals such as barroisite and epidote and represents the retrograde stage.
ACCEPTED MANUSCRIPT Effective bulk rock compositions were calculated using XMapTools software, following the methodology of Lanari (2014). For each domain, a mineral mask file was prepared representing the modes of constituent minerals (Fig. 7a-b, Table 5). Mineral modes were then converted to their mass fractions and then combined with the calculated average oxide weight percentage of all major elements of minerals present in the chosen domain to obtain effective
SC RI PT
rock compositions (Table 5).
The chemical system Na2O - CaO - K2O - FeO - MgO - Al2O3 - SiO2 - H2O- TiO2 - O2 (NCKFMASHTO) was used for the computation of the pseudosections, together with the same version of the THEMOCALC software and thermodynamic dataset used in the
NU
thermobarometry. The component MnO was omitted because all minerals other than garnet in the mafic eclogites have very low manganese content, and there are currently no manganese
MA
solid-solution models for several of the rock-forming minerals. The inclusion of manganese in the model system has the main effect of subtly expanding garnet-bearing equilibria to
ED
lower-grade conditions. However, as the spessartine content of garnet grain cores can be comparatively high (Sps11) the proportion of MnO was combined with the FeO content in the
PT
rock. The following mixing models were applied: clinopyroxene (Green et al., 2007),
CE
amphibole (Diener et al., 2007), garnet (White et al., 2007), epidote (Holland and Powell, 1998), plagioclase (Holland and Powell, 2003) and phengite (Coggon and Holland, 2002).
AC
Quartz, sphene, rutile and lawsonite are treated as pure phases. Fe3+ contents in the effective rock compositions have estimated on the basis of average Fe3+ contents in the constituent minerals and their proportions. Previous studies have demonstrated a strong influence of the redox state of the rock in the P-T stability of epidote-, pyroxene- and amphibole-bearing mineral assemblages in eclogite facies metamorphic rocks (e.g. Rebay et al., 2010). Although, we are not in a position to provide an accurate estimate of the O2 content in the modelled rock compositions, the approach adopted here provides a reasonable approximation
ACCEPTED MANUSCRIPT of phase stabilities in HP metabasites as in this study. The metamorphic phase relationships for water-saturated conditions are presented in Figs. 8-9.
3.3. U-Pb zircon geochronology Zircon grains from the two Nagaland eclogite samples (N22F and N23B-1) were analysed for
SC RI PT
U-Th-Pb content using the Sensitive High Resolution Ion Microprobe (SHRIMP-II) instrument at the Research School of Earth Sciences (RSES), The Australian National University (ANU). Zircon grains in the samples were initially identified in polished thin sections using a JEOL JSM 6490 scanning electron microscope (SEM) with EDAX
NU
attachment at the EPMA National facility in IIT Kharagpur. Although small (~10-20 μm), zircon is common throughout the two samples in a variety of textural settings. Back-
MA
scattered-electron (BSE) imaging revealed diverse zircon morphology, internal structure, and mineral inclusions. Because of their small grain size, parts of the thin sections with the most
ED
zircon were cut for in-situ analysis using an ultrasonic disk cutter. These thin section pieces were then mounted on double-sided tape together with Temora 2 reference zircon prior to
PT
casting in 25mm diameter epoxy mounts. The Temora 2 grains were grounded to expose their
CE
mid-sections and polished with 1µm diamond paste prior to mounting together with the thin section pieces. Mounts were then mapped with an automated Leica D6000 microscope in
AC
reflected and transmitted light in order to locate the previously identified zircon grains based on textural positioning in the original thin section. Subsequently, Cathodoluminescence (CL) imaging of zircon grains in the mounts was completed using a JEOL-JSM-6610A SEM at RSES-ANU to characterise their internal structure. For U-Pb isotopic analysis on SHRIMP-II, the grain mounts were coated with ca. of 10 nm of Au. After coating, the epoxy mounts were loaded into a steel holder for insertion into SHRIMP. A reflected light map of each mount was uploaded onto the SHRIMP-II
ACCEPTED MANUSCRIPT computer. Every individual spot location was checked for correct positioning. SHRIMP measurements were performed with a O2- primary beam of ca. 1.5nA, with an accelerating voltage of 10 kV and focused to sputter an area of ~ 10 μm x 15 μm (Köhler illumination). Secondary ions were extracted using an accelerating voltage of 10 kV and measured by single collector analysis on an ETP electron multiplier. Masses were selected by magnetic
SC RI PT
peak switching. SHRIMP-II is operated at a mass resolving power of
~ 6500 (at 10%
peak height). The acquisition time for each spot was ca. 16 min, which consists of six cycles through the following peaks: 248
ThO+, and
254
196
Zr2O+,
204
UO+. The peak position of
Pb+, background, 204
206
Pb+,
207
Pb+,
Pb is fixed relative to
196
208
Pb+,
238
U+,
Zr2O, and the peak
NU
positions of 207Pb and 208Pb are fixed relative to 206Pb.
Uranium concentrations are determined by normalizing to SL13 zircon (238 ppm U;
MA
Roddick and van Breemen, 1994) which were measured at the beginning of every session. The analysis of every five unknown spots was both preceded and followed by the analysis of
ED
Temora2 standard and used as a U/Pb normalizing reference (416.8 ± 1.3 Ma; Black et al., 2003). All analyses were corrected for common Pb using the measured
204
Pb content. Data
PT
were reduced using POXI, an in-house ANU program that follows the data reduction
CE
methods of SQUID 1. The program Isoplot/Ex version 3.0 (Ludwig, 2003) was used to generate probability density and concordia plots and explore possible mixed age populations
AC
through the "mixture modelling" approach of Sambridge and Compston (1994). Single analyses in the data table (Table 6) and concordia plots are presented with 1σ errors.
4. Petrography and mineral chemistry 4.1 Petrography 4.1.1. Thewati eclogite
ACCEPTED MANUSCRIPT In the Thewati eclogite sampleN23B, garnet (Fig. 4a-b), omphacite (Fig. 4c) and epidote (Fig. 4a) occur as large to medium-sized porphyroblasts, enveloped by foliated phengite (Fig. 3d), barroisite (Fig. 4d), epidote and sphene. Rutile, quartz and omphacite occur as inclusions in garnet (Fig. 4a) and omphacite (Fig. 4c). Evidence for an early prograde stage is indicated by foliated, fine-grained glaucophane (glaucophane1), epidote (epidote1), sphene (sphene1),
SC RI PT
phengite and quartz inclusions concentrated in garnet grain cores (Fig. 4a-b), and also rarely in omphacite (Fig. 4c). When present, inclusions in garnet grain rims are comparatively coarse-grained and dominated by omphacite and rutile, and only very rare epidote (Fig. 4a). The peak metamorphic assemblage for this sample is inferred to have involved garnet,
NU
omphacite, epidote, rutile, phengite and quartz.
The composite Thewati eclogite (sample N40B) contains garnet similar to that in
MA
sample N23B (Fig. 4f), and a second variety of garnet that is characteristically smaller (1-2 mm grain diameter) and has an idioblastic habit (Fig. 4e). These garnet types are named
ED
garnet-A and garnet-B, respectively. The cores of garnet-B grains have inclusions of epidote (epidote1) and omphacite (Fig. 4e), but in smaller proportions than occur in garnet-A (Fig.
PT
4f). The outer rims of garnet-B grains are inclusion free. Matrix pyroxene, amphibole and
CE
epidote envelop both garnet-A and garnet-B grains. The post-peak history of the Thewati eclogite involved a series of retrograde stages,
AC
that successively formed: (a) structurally-late epidote (epidote2) with inclusions of garnet and omphacite as aggregates and discontinuous layer-parallel segregations (Fig. 4g, h); (b) the syn- (Fig. 4i), to post-tectonic (with respect to the pervasive matrix foliation) growth of barroisite that partially pseudomorphs garnet, epidote2, and omphacite (Fig. 4d, j–k); (c) a late generation of idioblastic glaucophane (glaucophane2) and sphene (sphene2) that form rims on barroisite (Fig. 4d, l) and rutile, respectively; (d) post-tectonic rims of chlorite and phengite on garnet (Fig. 4m) and (e) cross-cutting veins of lawsonite, albite, winchite-
ACCEPTED MANUSCRIPT ferrowinchite, aegirine-augite, and sphene2 (Fig. 4n-q). The minerals constituting individual veins varies along their length and they can be classified into three types: (i) albite + winchite + aegirine-augite + sphene veins cutting garnet-A grain cores of (Fig. 4n-o); (ii) lawsonite + aegirine-augite veins cutting grains of garnet-B (Fig. 4p) and (iii) lawsonite + albite + ferro-
SC RI PT
winchite veins cutting barroisite porphyroblasts (Fig. 4q).
4.1.2. Mokie eclogite
The Mokie eclogite records prograde, peak and retrograde stages (Fig.2g, 3r) similar to the Thewati eclogite. The rock lacks the texturally late epidote, and late lawsonite, albite, sphene,
prograde history of the Mokie
NU
(ferro-)winchite, aegirine-augite-bearing veins observed in the Thewati eclogite. The sample is preserved by inclusions of glaucophane1 in
MA
omphacite (Fig.4s) and sphene1 in rutile (Fig. 4t). The peak mineral assemblage is inferred to have involved garnet, omphacite/aegirine-augite, rutile, phengite and epidote (Fig.4r). In
ED
contrast to the porphyroblastic garnet in the Thewati eclogite, garnet in the Mokie eclogite occurs as small idioblastic grains, and occasionally as clusters within pyroxene and the rock
PT
matrix (Fig. 4r-s). An intergrowth of barroisite and sphene partially rims pyroxene and
CE
encloses garnet, rutile and epidote (Fig. 4s-u). Barroisite in such intergrowths is in turn
AC
partially pseudomorphed by a thin rim of glaucophane2 (Fig. 4v) and chlorite (Fig. 4u).
4.2. Mineral chemistry Garnet in the Thewati eclogite is dominantly a solid solution among spessartine (Sps 11 to Sps00-01), pyrope (Prp06 to Prp17), almandine (Alm54 to Alm64) and grossular (Grs19 to Grs28) with minor andradite (Adr02 to Adr05) contents (Table 1). Mn-, Mg-, Fe- and Ca-X-ray element images of three representative large garnet grains in samples N23B and N40B (cf. garnet-A in N40B-6) show a uniform feature of Mn-rich core and Mn-poor rim and a
ACCEPTED MANUSCRIPT continuous rim-ward enrichment in Mg contents (Fig. 5a-b, e-f, i-j). The Fe and Ca images show a feature of complementary increase and fall in their concentrations up to the inner rim followed by fall and rise towards the outer rim respectively (Fig. 5c-d, g-h, k-l). The compositional zonation of these elements, inferred from the X-ray element images is also confirmed in detailed compositional profiles (Fig. 5q-ae). The rim-ward depletions in
SC RI PT
spessartine (11-06→00-01 mol%) and enrichments in pyrope (06-08→15-17) and XMg (1113→20-23) contents in garnet (Table 1) are attributed to growth during prograde metamorphism. Characteristic features of elemental zoning in these garnet grains include discontinuities in almandine [54-58 (core)→60-64 (inner rim)→55-61 (outer rim)] and
NU
combined grossular + andradite [26-30 (core)→22-24 (inner rim)→24-27 (outer rim)] profiles (Fig. 5s-t, x-y, ac-ad). On the basis of these compositional features, we consider
MA
garnets in N23B to be comparable with garnet-A in N40B. Type-B (smaller) garnet grains in N40B also show features of growth zoning in the
ED
X-ray element images (Fig. 5m-p) and compositional profiles (Fig. 5af-aj). However, their
PT
grain cores are deficient in spessartine (Sps02 vs. Sps11-06) but enriched in pyrope (Prp13 vs. Prp06-08) and XMg (17 vs. 11-13) contents compared to that in the garnet-A cores (Table 1).
CE
These distinctions are attributed to the growth of garnet-B in sample N40B from a
AC
fractionated bulk rock composition (e.g. in Mn) following the early growth of (the cores of) large garnet grains, and at elevated temperature conditions. However, the spessartine-poor and pyrope-rich compositions in the garnet outer rims are the same in all the textural varieties of garnet. Garnet-B, is also distinct from the other garnet grains by its developments of a narrow (~50μm wide) rims marked by sharp fall in pyrope and XMg and complementary rise in almandine contents (Fig. 5ah-aj) at its outer edge in contact with the matrix omphacite (Fig.4e). These are attributed to localized diffusive re-equilibration.
ACCEPTED MANUSCRIPT Garnet in the Mokie eclogite is characteristically Mn-rich (Sps14-26) and growthzoned [Sps26 Prp07 Alm45 Grs19 (C)→Sps14-17 Prp10-12 Alm52-57 Grs13-23 (R)] as in the Thewati eclogite (Table 1). The XPrp and XMg contents in its outer rim are, however, lower than that in the Thewati eclogite. Pyroxenes in the Nagaland eclogites show a variation in composition from aegirine-
SC RI PT
augite to omphacite (Fig. 6a-b, Table 2), which can be related to their textural setting. In the Thewati eclogites, pyroxene inclusions (Type 1 pyroxene) in the cores of large garnet grains straddle the boundary of the aegirine-augite and omphacite fields (Fig. 6a). Pyroxene in the garnets rims is omphacite (Fig. 6a). Pyroxene inclusions in the smaller garnet-B grains in
NU
N40B are omphacite (Fig. 6a). Pyroxene grain in the matrix (Type 2A pyroxene) or its relict within the texturally late barroisite grains (Type 2B pyroxene) is omphacite (Fig. 6b). The
MA
pyroxene porphyroblasts in the Mokie eclogite are omphacite to aegirine-augite (Fig. 6b). The vein-type pyroxenes are characteristically more oxidised and plot in the field of aegirine-
ED
augite in the ternary Q (Quad)-Jd (Jadeite)-Aeg (Aegirine) diagram (Fig. 6c). Na-Ca amphibole in the Thewati eclogite is barroisite. The Mokie eclogite has
PT
amphibole ranging in composition from barroisite to magnesio-kataphorite (Fig. 6d-e, Table
CE
3). The Na-Ca amphiboles in the veins are ferro-winchite (Fig. 6d). The texturally early and later sodic amphiboles in the Nagaland eclogites are glaucophane (Fig. 6f). In the binary NaB
AC
vs. (Al+Fe3+)VI plot, both the sodic and sodic-calcic amphibole groups show compositional spreads along the glaucophane substitution vector (Fig. 6g). The sodic-calcic amphibole group additionally shows minor tschermak substitution (Fig. 6g). Ferro-winchite in the veins is comparatively enriched in NaB content and lies very close to the boundary between sodiccalcic and sodic amphibole groups (Fig. 6g).
ACCEPTED MANUSCRIPT Phengite in the Thewati eclogite sample N23B is more siliceous [Si (c.p.f.u.) = 3.533.54] than that in sample N40B (Si = 3.46). Phengite of even higher Si-content is found in the Mokie eclogite sample N22F (Si = 3.63). In the Thewati eclogite, epidote1 inclusions within garnet are more pistacite-rich
the Mokie eclogite is pistacite-poor (XPs = 0.17).
SC RI PT
(XPs=0.25) than the foliation-defining epidote2 (XPs= 0.16-0.20). Texturally early epidote in
Chlorite coronae around garnet in the Thewati eclogite is clinochlore (XMg=0.68). Sphene1 and sphene2 in the Thewati and Mokie eclogites contain small amount of alumina (Al2O3 = 1.04-1.43 wt %). Albite, lawsonite and rutile are near pure phases.
NU
Summarising, the textural and mineral compositional features of the Nagaland eclogites reveal a multistage mineral assemblage evolution comprising: (1) early epidote
MA
blueschist facies conditions that accompanied prograde garnet growth (Fig. 5) together with Na-Ca pyroxene (Fig. 6a), glaucophane1, epidote1, sphene1, phengite and quartz; (2) peak
ED
conditions represented by garnet outer rims of spessartine-poor and pyrope-rich compositions, jadeite-rich omphacite in the matrix and inclusions in the garnet outer rim,
PT
rutile and phengite with or without epidote and quartz; and (3) a protracted post-peak
CE
evolution marked by sequential appearances of epidote2 (Fig.4g-h), barroisite + sphene2 ± epidote2 (Fig. 4i-k, s-u) and glaucophane2 + chlorite (Fig.4k-l, m, t-v). The Thewati eclogite
AC
additionally records the development of lawsonite with or without albite, ferro-winchite and aegirine-augite in cross-cutting veins (Fig. 4n-q).
4.3. Peak P-T conditions of metamorphism Mineral compositions from the pyrope-rich, growth-zoned garnet rims were paired with jadeite-rich omphacite from either the cores of larger matrix grains or inclusions in garnet rims together with the high-Si matrix phengite to estimate peak metamorphic
ACCEPTED MANUSCRIPT conditions. According to equations R1 and R2, these chemical parameters most likely reflect the maximum pressure conditions experienced by eclogite facies basites. For the Thewati eclogites, the applications of Method 1 yielded peak P-T estimates of 28.4 ± 1.6 kbar (weighted mean value with errors at 2 sigma) and 630 ± 50°C for the sample N23B, which are similar to those obtained by the Method 2 (28.1 ± 2.8kbar, 645 ± 80 °C) (Table 4). For
SC RI PT
the sample N40B, the estimated pressures (P = 26.1 ± 2.0 kbar, Method 1 and 25.5 ± 3.3 kbar, Method 2) are lower by ~2.3 to 2.6 kbar at similar TMax (T= 635±60°C, Method 1 and T= 650±100 °C, Method 2). We attribute the higher P-estimates in N23B to a more siliceous composition of phengite in the sample [Si = 3.54 (c.p.f.u.) vs. 3.46]. Compared to the
NU
Methods 1 and 2, the Method 3 yielded temperatures, which exceed by ~25 oC in sample N23B and ~ 100 oC for N40B. In particular, the peak estimate of ~755 oC in N40B is much
MA
higher than the more robust garnet-clinopyroxene Fe2+-Mg exchange thermometry results. For this reason, we discount the results of the Method 3 from further discussions on the P-T
ED
constraints of peak metamorphism in the Nagaland eclogites. Although the results from the first two methods are very close, we favour the computations by the Method 2. This is
PT
because these P-T values can be directly compared with the results of the phase equilibria
CE
modelling as the two methodologies adopt the same thermodynamic dataset and mineral solid solution models. Accordingly, we consider P ~ 25.5 to 28.1 kbar, T ~650 oC as the peak P-T
AC
estimates of the Thewati eclogites. Using the same reasoning, the peak P-T estimate of the Mokie eclogite has been constrained at P ~ 23.8 ± 5.1 kbar, T~ 555 ± 90 oC (Table 4).
4.4. Calculations of metamorphic phase diagrams and deduction of metamorphic P-T path Phase diagrams calculated for the Thewati eclogite are presented in Figs. 8 & 9. For the pyroxene-garnet-rich domains of eclogite sample N40B (cf. N40B-4), the phase diagram predicts garnetiferous assemblages at T>475-550 oC, and the loss of chlorite at T>550-575 oC
ACCEPTED MANUSCRIPT and amphibole at T>500-725 oC, depending on metamorphic pressures (Fig. 8a). The positive dP/dT slope of the lawsonite-out curve and the stability of the lawsonite-bearing assemblages at T<525-650 oC is nearly identical to the experimentally determined lawsonite-out curve (e.g. Liou, 1971; Schmidt, 1995). Epidote-bearing assemblages define an inverted V-shaped field at P<25 kbar and at T conditions between 525 and 675 oC. The early prograde loss of
SC RI PT
lawsonite before epidote during heating at P<25 kbar is consistent with phase diagram predictions made for metabasic bulk rock composition elsewhere (Wei and Clarke, 2011). The phase diagram also predicts peak eclogite facies conditions of T>625-675oC, which are consistent with the estimates for the sample N40B (P~ 25.5 kbar, T~650 oC) (Fig. 8a).
NU
We now evaluate the progressive changes in garnet composition (Fig. 8b) and garnet (Fig.8c), pyroxene (Fig. 8d) and amphibole (Fig. 8e) modes along three model P-T paths:
MA
Path1, a prograde path across the epidote-blueschist facies; Path 2, a counter-clockwise (CCW) P-T trajectory involving cooling and decompression; and Path 3, a clockwise (CW)
ED
P-T trajectory involving decompression followed by cooling. Path P1 predicts (a) the sequential intersections of the garnet-in, chlorite-out, amphibole-out and epidote-out curves;
PT
(b) prograde garnet growth with garnet composition becoming progressively magnesian
CE
(decreasing x(g) isopleths, see Table 4 for the terminology) and Ca-poor (decreasing z(g) isopleths) and (c) increasing omphacite and decreasing amphibole contents in the rock. This
AC
is consistent with the observed textural and garnet compositional features. Assuming the rock was not appreciably fractionated prior to the growth of garnet, the P–T condition at which garnet is predicted to first formed can be estimated using garnet core compositions and the isopleth thermobarometry approach (after Vance and Mahar, 1998). For this purpose, we have used the core composition of the largest garnet-A grain in sample N40B, which has the highest spessartine, lowest pyrope and highest grossular + andradite content. The x(g) =0.85 and z(g) =0.30) garnet isopleths intersect at ~18.8kbar, 555oC (Fig. 8f). This P-T estimate
ACCEPTED MANUSCRIPT falls in the epidote blueschist facies, in good agreement with the reconstructed mineral reaction history, and can be considered a reasonable approximation of the conditions accompanying early garnet growth for the Thewati eclogite. An expanded MnO-bearing system would likely displace this result to subtly lower grade (see, for example, Marmo et al., 2002). Both of the retrograde paths P2 and P3 (Fig. 8c) can be expected to induce the
SC RI PT
decomposition of garnet and clinopyroxene, and growth of epidote and amphibole. Path 2 posits the growth of lawsonite before amphibole (Fig. 8f), inconsistent with the deduced mineral reaction history. In contrast, Path 3 posits the sequential growth of epidote, then epidote with amphibole, then chlorite (Fig. 8f), in good agreement with the deduced sequence
NU
of retrograde minerals.
The phase diagram for the amphibole-rich compositions of sample N40B resembles
MA
that for the pyroxene-rich compositions in having similar P-T stability fields for lawsoniteand chlorite-bearing assemblages (Fig. 9a. However, the P-T range of epidote-bearing
ED
mineral assemblages is significantly enlarged (Fig. 9a), due to the higher calcic and alumina content of the modelled composition (Table 5). The phase diagram is contoured for garnet
PT
composition and garnet, pyroxene and amphibole modes (Fig. 9b-e). Whereas the phase
CE
diagram broadly predicts prograde and retrograde growth sequences similar to that of the garnet-pyroxene-rich composition (compare Figs 8f and 9f), the amphibole-rich bulk
AC
composition better matches discontinuities in grossular zoning during the prograde garnet growth. The prediction of a progressive decrease in z(g) content from core to the inner rim followed by an increase towards the garnet outer rim (along P1, Fig. 9b) resembles the measured garnet composition (Fig. 5t, y, ad). The phase diagram is also contoured with isopleths of NaB content in amphibole for values 0.5 and 1.5 [where the numbers refer to cations per formula unit (c.p.f.u.) in the B structural site of amphibole] (Fig. 9e). When considered together with the positions of
ACCEPTED MANUSCRIPT chlorite-in and sphene-in curves, the phase diagram with the modelled amphibole compositions predict the observed growth of glaucophane after barroisite, chlorite after garnet and sphene after rutile (Fig. 4d, l-m) along the retrograde path P2 (Fig. 9f). This gives confidence in using the phase diagram together with detail from chemically-zoned amphibole grains to assess the validity of the retrograde paths P2 and P3 (Fig. 9f).
SC RI PT
We now combine the reconstructed metamorphic evolutionary pathways, key details of the phase equilibria modelling, estimated peak P-T conditions of metamorphism and P-T positions of the experimentally determined lawsonite-out curve and calculated mineral equilibria, corresponding to greenschist-blueschist-facies transition and transformation of
NU
albite to jadeite + quartz to establish the metamorphic P-T path of evolution from the Thewati eclogite sample (Fig. 10). We additionally calculate and plot four activity-corrected end
MA
member mineral reactions (reactions R3 to R6) to supplement phase equilibria predictions of mineral stabilities, particularly involving retrograde minerals that formed in localised
ED
domains. These are:10 (jadeite)Omphacite + (4 grossular + 5 pyrope)Garnet + 11 quartz + 8 H2O = 5 (glaucophane)Barroisite+ 6 (clinozoisite)Epidote (R3).
PT
5 (glaucophane)Barroisite+12 (clinozoisite)Epidote + 14 quartz + 28 H2O = 3 (tremolite)Fe-winchite +
CE
10 albite + 18 lawsonite (R4).
3 daphnite + 5 pyrope = 3 clinochlore + 5 almandine (R5).
AC
Ab = Jd17 + Qtz (R6).
These reactions reveal the thermal and baric conditions for the stability of retrograde barroisite + epidote, chlorite and lawsonite, albite, ferro-winchite and aegirine-augite in late veins. In calculating R3-R5, we have applied the same thermodynamic dataset and activitycomposition models of mineral end members as adopted during the phase equilibria modelling. This makes the P-T positions of these equilibria to be internally consistent and comparable with the results of phase equilibria modelling.
ACCEPTED MANUSCRIPT The disposition of the sphene-out, garnet-out, chlorite-out, amphibole-out and epidote-out curves in sequence and the P-T conditions of formation of the prograde garnet core at 18.8 kbar, 540 oC, when considered together provide a tight constraint for the prograde P-T path (cf. MPR metamorphic stage, where PR represents prograde) that culminated in peak eclogite facies metamorphism at 25.5 kbar, 650 oC (MP metamorphism as
SC RI PT
P represents peak, Fig. 10). The intersections of epidote-out and amphibole-out curves in an opposite sense and reaction R3, leading to the successive appearances of retrograde epidote2 and barroisite2 + epidote2 constrain the first stage of retrogression (cf. MR1 metamorphic stage, where R refers to retrograde) as one of decompression with minor cooling to ~18 kbar,
NU
~620 oC (at amphibole eclogite facies condition) (Fig. 10). The subsequent crossing of the NaB =1.5 isopleth in amphibole and intersections of the chlorite-out and sphene-out curves in
MA
the reverse sense and R5 indicates further decompression and cooling in the epidote blueschist facies at ~10kbar, 510 oC (cf. MR2 metamorphic stage) (Fig. 10). The dispositions
ED
of the reactions, R4 and R6, reflecting the stability of aegirine-augite + lawsonite + albite + ferro-winchite assemblage indicate a terminal phase of retrogression to one of protracted
PT
cooling (∆T ~200 oC) at low-pressures (P~6 kbar) (cf. M3R metamorphic stage, Fig. 10). The
CE
development of a narrow diffusion zoning in garnet-B (Fig.5aj) in contact with omphacite provides additional support for post-peak cooling. Summarising, our study reveals that this
AC
progressive change in P-T vectors from loading accompanying heating (MPR) culminating in peak metamorphism (MP) and three successive stages of decompression and cooling (MR1 to MR3) defines a clockwise P-T path of evolution of the Thewati eclogites (Fig. 10). Although we have not quantified the metamorphic P-T path of evolution of the Mokie eclogites, the broadly analogous metamorphic reaction pathways in the Mokie eclogite with the Thewati eclogite raise the possibility that the former also records a broadly similar subduction burialexhumation history in the Nagaland subduction channel.
ACCEPTED MANUSCRIPT
5. Results of SHRIMP U-Pb zircon geochronology
Analytical data for zircon are presented in Table 6. Zircon textural and cathodoluminescence (CL) features are summarized in Fig. 11 and chemical composition and Th/U vs. age
SC RI PT
distribution shown in Fig. 12. A total of 34 spot analyses from 33 zircon grains were completed on the two eclogite samples (17 grains in N22F and 16 grains in N23B-1). In both samples, zircon grains are common but small (grain diameter range ~ 8-20 μm). They occur in various textural settings: as inclusions in garnet (Fig. 11a), Na-Ca pyroxene (Fig. 11b) and
NU
rutile, and also in the matrix foliation-defining barroisite, phengite and sphene. Zircon in N22F generally occurs as elongate grains with pyramidal terminations and has grain cores
MA
dominated by ovoid inclusions (Fig. 11c). Zircon in N23B-1 mostly occurs as equidimensional grains with well-developed crystal faces that are free of mineral inclusions (Fig.
ED
11d). Where inclusions are relatively coarse they are determined to be garnet (Fig. 11e), omphacite (Fig. 11f), and rutile.
PT
Their small size partially inhibits the high resolution CL images of zircon in the two
CE
Nagaland eclogites. However, they have a variety of CL structures (Fig. 11g-x) that can be used to identify three types of CL domains that reflect stages of grain growth,
AC
recrystallisation and alteration (Table 7). Type 1 zircon is marked by darkly luminescent, faintly banded (Fig. 11g, i, n-p) elongate domains that characteristically have euhedral shapes and pyramidal terminations (Fig. 11j-l, r-t). These features are consistent with zircon growth in the presence of a fluid or melt (e.g. Rubatto, 2017). Type 2 zircon is marked by (a) structure-less grain domains with morphology similar to type-1 zircon (Fig. 11h, j-m, r, u), commonly with patchy, irregular zones of bright luminescence (Fig. 11q, s-t); (b) textural modification of zircon with type-1 grain form involving grain rounding and/or the truncation
ACCEPTED MANUSCRIPT of pyramidal faces (Fig. 11j, l, p, r-s, u-v); and (c) development of bright luminescent narrow rims to type-1 zircon (Fig. 11h, j-m, p-v), commonly with equant and euhedral external shape oblique to the enclosed type-1 zircon core (Fig. 11j-k). These features are attributed to the recrystallisation and/or replacement of type 1 zircon. Type 3 zircon only occurs in N22F and is marked by irregular bright-luminescent domains that partially replace type-1 zircon cores
SC RI PT
(e.g. domain 1 in grain Z-9) (Fig. 11v). The zones of replacement have serrated to curvilinear inner boundaries (e.g. in grains Z-9, 33) (Fig. 11v-w). In some zircon grains, the partial replacement of the older core is extensive, and gives rise to ghost and fuzzy zoning (e.g. grain Z-28) (Fig. 11x). Features of this third stage are consistent with grain modification during the
NU
localised fluid-mediated alteration of type 1 zircon.
Types 1 and 2 zircon grains (both from primary and recrystallised varieties) in N23B-
MA
1 have limited compositional variation with low Th (0.05-1.62 ppm) and U (29-200 ppm) contents (Fig. 12a). Equivalent zircon domains in N22F are characteristically uraniferous (U:
ED
1110-2478 ppm) and more thorian (Th: 8-66 ppm) (Fig. 12a). A Type 2 zircon rim in N23B-1 is depleted in U relative to the recrystallised grain core (e.g. 13 ppm in rim vs. 29 ppm in
PT
core, grain Z-66, N23B-1) (Fig. 12a). Type 3 zircon is chemically distinct by being more
CE
thorian (Th: 94-271 ppm) and has high Th/U ratios (0.09 to 0.20) (Table 6, Fig.12a-b). Inter-sample compositional variability of Th-U in the Nagaland zircon grains is a
AC
feature common in many HP metamorphic rocks. It is generally attributed to zircon growth in the absence of a dominant buffering fluid (Rubatto and Hermann, 2007). Type 1 zircon in the Nagaland samples also has low Th/U ratios (≤0.02-0.03) (Table 7,Fig. 12b), that are common in metamorphic zircon from blueschist and eclogite facies rocks (Rubatto and Gebauer, 2000; Rubatto and Hermann, 2007). Given the small size of Nagaland zircon grains, it is not possible to analyse most type2 zircon rims in the two samples. One analysis was made of a comparatively thick rim in
ACCEPTED MANUSCRIPT N23B-1 (see below). The SHRIMP analysis sessions thus targeted zircon core domains, including the type1 zircon, and additionally the type3 zircon in N22F (See Fig. 11 for the analytical spots in representative zircon grains). Among the 17 analysed spots in N22F, six spots were excluded from the calculation of concordia and mean ages based on the following considerations: (a) these analyses reveal discordance in the concordia plot (Fig. 13a); (b) four
SC RI PT
of the analyses are from the Th-rich altered domain (type 3 zircon); and (c) all have yielded older dates (between 337 and 234 Ma). Following previous studies that have indicated the isotopic disturbance of altered zircon grains in HP metamorphic terranes, we are of the opinion that zircon ages from such texturally late, fluid-mediated alteration in N22F are
NU
likely geologically spurious and lack geological significance (e.g. Rubatto and Hermann 2003).
MA
The remaining 11 analyses show a spread in zircon ages from 216 to 171 Ma (Table 6) and reveal three groups at 206 ± 4 Ma (error 2s), 189 ± 3 Ma, and a minor one at 172 ± 4
ED
Ma (Fig. 13b). Concordia ages of the two dominant age groups are 205± 6 Ma [n=4, Mean Square Weighted Deviates (MSWD) =0.49, probability of concordance =0.48] (Fig. 13c) and
PT
189 ±1Ma (n=5,MSWD=0.05, probability=0.82) (Fig. 13d).
CE
In N23B-1, 18 analyses show a spread in zircon dates from ~217 Ma to ~152 Ma, with two broad age groups at 204 ± 10 Ma and 187 ± 7 Ma (Fig. 13e) similar to that observed
AC
in N22F. Two analyses from core-rim domains of one zircon grain (grain Z-66, Fig. 11h) give dates at 190 ± 18 Ma for the grain core (type 1 zircon) and 152 ± 42 Ma for the grain rim (type 2 zircon). Although this is consistent with zircon recrystallisation being younger than the growth of the core, the large age error due to the presence of high common Pb does not allow this age to be useful for estimating the timing of the recrystallisation event. For concordia age calculations, we have excluded 9 analyses because of negative or indeterminate radiogenic
207
Pb/235U ratios. The remaining 9 analyses have yielded two
ACCEPTED MANUSCRIPT concordia ages at 205± 4 Ma (n=5, MSWD = 0.10, probability = 0.75) (Fig. 13f) and 185 ± 4Ma (n=4, MSWD = 0.08, probability=0.78) (Fig. 13g) that are near identical to that obtained from N22F.
6. Discussion
SC RI PT
6.1. Early Jurassic eclogite facies metamorphism in the Nagaland Ophiolite Complex Nearly identical zircon age groupings of ~205 Ma, ~185-189 Ma and (minor) ~172 Ma have been identified in two separated eclogite locations in Nagaland. We recognise two stages of zircon growth and/or recrystallisation, in small zircon grains that challenge the analytical
NU
method. Despite minor overlap, the data largely define two end member textural types (1 and 2, below). In the context of petrological observations, the concordia ages of the first two age
MA
groups are most probably geologically meaningful. The oldest zircon age group was recovered from type 1 zircon, and reflects the earliest stage of zircon growth (e.g. in N23B).
ED
There are three possible interpretations of the c. 205 Ma date: (a) the growth of zircon in the eclogitic protolith; (b) a partially reset growth age due to the effects of post-growth heating
PT
during later prograde conditions; and (c) zircon growth during prograde metamorphism. In
CE
the absence of zircon trace element data it is difficult to distinguish between these possibilities, but the characteristic low Th/U ratios (<0.01) of these zircons are most
AC
consistent with the c. 205 Ma date to being a minimum age for the eclogitic protolith [option (b) above]. The c.185-189 Ma zircon ages were recovered mostly from stage 2 recrystallised grain cores, which include eclogite facies mineral inclusions. Stage 2 zircon is thus interpreted to have formed during or shortly after peak conditions in the eclogitic facies, at c. 189-185 Ma. The limited dataset of the c. 172 Ma concordant zircon ages is interpreted as recording the timing of retrograde cooling and exhumation. However, more detailed work is needed to firmly establish its significance. An important outcome of these data is the
ACCEPTED MANUSCRIPT identification of a Triassic age (≥ c. 205 Ma) for this segment of the Neo-Tethyan oceanic crust at Nagaland.
6.2. Thermal history and accretionary processes in an Early Mesozoic Neo-Tethyan subduction channel
SC RI PT
Eclogite from two locations in the Nagaland Ophiolite Complex preserve a record of broadly coeval eclogite facies conditions in the Early Jurassic: ~28.1 to 25.5 kbar, ~ 650 oC for the Thewati body and 23.8 kbar, 555 oC for the Mokie body. Whereas the Mokie eclogite records low-temperature (LT) eclogite facies conditions (terminology and P-T field
NU
of global LT eclogites are after Fig. 3a in Agard et al., 2016), the Thewati eclogites underwent burial to deeper levels (~80-90 km depths) that were transitional to ultra-high
MA
pressure metamorphic conditions (Fig. 14a). In a previous study, Chatterjee and Ghose (2010) reported eclogite from a location near the Satuza area, and inferred a CW P-T path
ED
with peak conditions of 17–20 kbar and 580–610 °C (Path 3, Fig.14a). Whereas the prograde and retrograde segments of the Satuza P-T path resemble that of the Thewati
PT
eclogite (Path 1, Fig. 14a), the Satuza locality records shallower levels and higher thermal
CE
gradients (>10 oC/km) (Fig. 14a). Our findings, however, suggest that the Thewati eclogite falls under the select category of oceanic eclogites that are recovered from the maximum
AC
depth in the subduction zone (PMax ~28 kbar) and with a low apparent peak thermal gradient of ≤6–9 °C/km (Fig. 15). In contrast to the Nagaland samples, jadeite-glaucophane-bearing and garnetphengite-bearing schists from the adjoining Central Ophiolite Belt of Myanmar preserve a much larger spread in peak conditions, generally in blueschist to low-T eclogite facies (reviewed in Nyunt et al., 2017; Fig. 14a). A well-constrained metamorphic P-T path for the evolution of the terrane is lacking. Earlier studies of the Himalayan blueschists (see O'Brien,
ACCEPTED MANUSCRIPT 2018 for the latest review on the Himalayan HP/LT metamorphism) inferred low-T conditions and CW P-T paths (Paths 4 & 6, Fig. 14a) (Guiraud, 1982; Jan, 1985; Honegger et al., 1989; Anczkiewicz et al., 2000; Oberhänsli, 2013). On the basis of a detailed metamorphic investigation of Sapi-Shergol garnetiferous lawsonite blueschists, Groppo et al. (2016) recently established a significantly deeper and cold subduction system of possible
SC RI PT
Cretaceous age (cf. Anczkiewicz et al., 2000) within the Neo-Tethys from the NW Himalayas (peak P-T at 19 kbar, 470 oC) and a clockwise hairpin P-T loop along low thermal gradients (< 8-9 °C/km; Path 5, Fig. 14a). While the geometry of the P-T path and lower thermal gradients of the Shangla lawsonite blueschist resemble with that of the
NU
Nagaland blueschist (Path 2, Fig. 14a), our new geochronological constraints indicate the subduction zone metamorphism in the eastern Neo-Tethys initiated much earlier in the Early
MA
Jurassic.
The lower apparent thermal gradient at metamorphic peak in the Nagaland eclogite
ED
samples (7-8 oC/km) (Fig. 14a) is comparable to the gradient recovered from the associated lawsonite blueschist in Mokie (~9 oC/km, Ao and Bhowmik, 2014) and other HP
PT
metamorphic belts in the Zagros suture zone, Southern Iran (Angiboust et al., 2016), Sapi-
CE
Shergol lawsonite blueschist, Western Himalaya in the YTSZ (Groppo et al., 2016), Dominican Republic and Rio San Juan Mélange Complex (Krebs et al., 2011), the Western
AC
Alps (Angiboust et al., 2009; Agard et al., 2009, 2016 and reference cited therein), the Bantimala Complex, South Sulawesi, Indonesia (Setiawan et al., 2016) and also predicted by thermo-mechanical models (Gerya et al., 2002) (Fig. 15). The inferred Nagaland P-T path is similar to that inferred for the Sulawesi eclogite (Path 6, Fig. 15). For the latter, subduction of an old and cold oceanic crust was suggested (Setiawan et al., 2016). For the present study, we relate this characteristic lower thermal gradient during peak eclogite and blueschist facies metamorphism, the (hairpin) clockwise P-T paths of blueschists and
ACCEPTED MANUSCRIPT eclogites (Fig. 14a) and the geological setting with the development of a cold mature stage of an intra-oceanic subduction system within the Neo-Tethys in Nagaland (see also Ao and Bhowmik, 2014; Bhowmik and Ao, 2016). In many subduction-related HP-LT terranes, different localities of adjacent tectonometamorphic units record a systematic increase of peak-T and associated P, producing a
SC RI PT
positive dP/dT slope of the P-T array (e.g. Fig. 13b of Groppo et al., 2016). In the NOC, our studies (e.g. Ao and Bhowmik, 2014 and this work) demonstrate a near-complete preservation of a progressive metamorphic sequence from greenschist through pumpellyitediopside, blueschist to eclogite facies from the Mokie-Satuza area (Figs. 2c, 16). The peak P-
NU
T estimates of these rocks reveal a metamorphic field gradient (Fig. 14b) that is a characteristic feature of many HP/LT metamorphic terranes worldwide (e.g. Oman: Yamato
MA
et al., 2007; Turkey: Plunder et al., 2015; New Caledonia: Vitale Brovarone and Agard, 2013; Corsica: Vitale Brovarone et al., 2014; Schistes Lustres of the Western Alps: Plunder et al.,
ED
2012), also predicted by thermo-mechanical models (Fig. 14b). We attribute this metamorphic zonation in the NOC to repeated accretion of the top layer of the oceanic crust
PT
subducted at different depths and subsequently off-scraped at the base of the accretionary
CE
prism (e.g. Agard et al., 2009, 2018 and references therein). In the NOC, these HP metamorphic rocks are tectonically imbricated with serpentinites and mafic and ultramafic
AC
cumulate rocks, giving rise to an apparent chaotic internal arrangement in the ophiolitic mélange, though we note the coherence of individual members of the HP metamorphic sequence, in particular from the Mokie-Satuza area. The Nagaland P-T path reveals a two stage exhumation of the Nagaland eclogites: (a) initial exhumation (from 25.5 kbar, 650 oC to 10 kbar, 510 oC, and combining MR1 and MR2 metamorphic stages) took place along a steep dP/dT gradient (Fig. 14a). Such an exhumation path resembles that deduced for cold oceanic eclogites from different metamorphic belts
ACCEPTED MANUSCRIPT worldwide (e.g. Fig. 15); (b) later exhumation of the eclogites took place along a gentler dP/dT gradient (cf. MR3 metamorphic stage, Fig. 14a), and is distinctive for the Nagaland rocks. Three tectonic scenarios have been invoked to account for the exhumation of deeply buried eclogite along a steep dP/dT gradient (see Agard et al., 2009 for a general review). (1) In the first scenario that involves high-grade blocks (amphibolites/eclogites along CCW P-T
SC RI PT
path), the exhumation takes place within the first 5–15 Myrs of subduction infancy, in rapidly cooling subduction zones (e.g. Agard et al., 2009). In this scenario, exhumation is triggered by rheological weakening of the subducted oceanic crust due to partial melting in warm subduction zones (Sorensen and Barton, 1987). It has been proposed that this leads to the
NU
boudinage and detachment of the high-grade blocks in a weaker, sedimentary mud-matrix mélange (Cloos and Shreve, 1988). (2) This scenario involves buoyancy-driven exhumation
MA
along the subduction channel as a consequence of serpentinisation of the mantle wedge, controlled by fluid released during dehydration of hydrous phases in the subducting oceanic
ED
crust (Cloos, 1982, 1985; Gerya et al., 2002; Hermann et al., 2000; Guillot et al., 2009). In this setting, oceanic blueschist and eclogite are thought to occur as thin, disrupted bodies in
PT
association with serpentinite and/or a mechanically weak mud matrix (e.g. Gerya et al.,
CE
2002). The scenario predicts the exhumation of eclogite as part of material transport in a large-scale convective circulation system in the subduction channel that is evolving to a cold
AC
and mature stage (Cloos, 1982; Peacock & Wang, 1999; Gerya et al., 2002). The exhumation is inferred to be facilitated by the buyoancy and low viscosity of the serpentinites in the channel (Guillot et al., 2009). (3) A third scenario that involves the exhumation of eclogite as a consequence of the positively buoyant continental crust entering the subduction channel towards the end of the oceanic subduction, allowing the extrusion of large volumes of deeply buried continental crust (e.g. Norway, Andersen et al., 1991; Oman, Chemenda et al., 1996; Dabie Shan, Hacker et al., 1996; Zermatt-Saas eclogite, Western Alps, Angiboust et al.,
ACCEPTED MANUSCRIPT 2009; New Caledonia, Fitzherbert et al., 2004). In this scenario, oceanic eclogite is exhumed as part of a forced return flow of buyoant continental material with or without slab break-off (Von Blanckenburg and Huw Davies, 1995). For the Nagaland eclogites, features such as the (a) characteristic LT eclogite facies metamorphism along a CW P-T path, (b) occurrence of eclogite as rare, small lensoidal bodies within blueschist, (c) presence of thin, fluid-assisted
SC RI PT
metasomatic bands rich in amphibole + white mica as rims around the eclogitic cores and (d) absence of deeply buried continental crustal fragment in the ophiolitic mélange are collectively most consistent with exhumation via the second scenario.
The stage 2 exhumation that involved protracted cooling at shallower levels (Fig. 14a)
NU
can be linked with thrust stacking of partially exhumed eclogites with the cooler, prograde blueschists at shallower crustal levels. Despite differences in peak P-T conditions of
MA
metamorphism in the eclogites and blueschists, the P-T conditions of terminal retrogression in these rocks are broadly similar (P~5-6 kbar, T~300 oC, Fig. 14a). This suggest that the
ED
eclogites and blueschists were finally tectonically juxtaposed along their exhumation paths. On the basis of this deduction, we propose that the final mixing of rocks of contrasting
PT
lithologies, originally placed at different depths in the subduction channel, and producing a
CE
tectonic collage took place at a shallower crustal level during the development of the Nagaland accretionary complex. Future work should be directed to provide structural details
AC
in support of the suggested model above.
6.3. Regional correlations and Early Mesozoic tectonic framework of SE Asia The Nagaland eclogite samples dated in this work are older than the Early Cretaceous NeoTethyan ophiolites of the WOB (e.g. Manipur and Kalaymyo ophiolite belts) and its amphibolite-facies metamorphic sole (119-115 Ma, Liu et al., 2016a; Zhang et al., 2017). The Nagaland Ophiolite also predates the Mid-Jurassic aged Myitkyina ophiolite of the EOB by
ACCEPTED MANUSCRIPT more than ~30 Myr. It is also older than Cretaceous blueschists (e.g. 100-80 Ma Sapi-Shergol and Shangla blueschist, NW Himalayas, Anczkiewicz et al., 2000) and amphibolites (e.g. Saga amphibolite metamorphic sole between 135 and 127 Ma, Guilmette et al., 2012) (locations in Fig. 1 of the YTSZ). Nevertheless, it is contemporary with Late Triassic-Early Jurassic arc magmatism in the Gangdese magmatic belt (e.g. Yu et al., 2018). We propose
SC RI PT
that this cryptic record for the earliest phase of Neo-Tethyan subduction can be traced westwards from Nagaland-Myanmar, along the southern Eurasian margin to Pakistan as part of an Early to Middle-Jurassic Andean-type convergent margin (Searle et al., 1999; Yu et al., 2018).
NU
Several recent studies present interpretations for the Early Mesozoic tectonic framework of northern Myanmar and southeastern Tibet (see Searle et al., 2017; Gardiner et
MA
al., 2018; Zhang, J. et al., 2018 for general reviews). However, there remains considerable uncertainty regarding the: (a) polarity (easterly, Zhang, J. et al., 2018 or westerly, Mitchell,
ED
1993) and type of the subduction (single long-lived, e.g. Zhang, J. et al., 2018; or multiple, Mitchell, 1993; Acharyya, 2007, 2015; Mitchell et al., 2015; Searle et al., 2017; Gardiner et
PT
al., 2018); (b) status of the Western Myanmar block [e.g. a western extension of the Shan
CE
Plateau, an extension of the Lhasa Block, through Tengchong and Baoshan in Yunnan or as a part of the west Sumatra block (e.g. Zhang, J. et al., 2018)] and (c) any continuity of the
AC
three major Mesozoic Tibetan sutures into Myanmar. Liu et al. (2016a) interpret the EOB as a Jurassic suture of the Meso-Tethys (equivalent to the BN suture in central Tibet). This extended suture would pass between the Sibumasu block to the east and the West Myanmar block to the west (Fig. 17a). Gardiner et al. (2018) recently proposed a southerly termination of the Meso-Tethyan BN suture in NE Myanmar, extending the suture between the Tengchong block to the west and the coalesced Sibumasu–Baoshan Blocks to the east (Fig. 17b). In this model, an east-subducting Meso-Tethyan sea floor developed an Andean-type
ACCEPTED MANUSCRIPT active continental margin setting above the Sibumasu block between the Late Triassic and Early Jurassic. The closure of this Meso-Tethyan subduction through the amalgamation of the two continental blocks is inferred to have occurred before the Late Cretaceous. Alternatively, Zhang, J. et al. (2018) propose a long-lived east-dipping subduction within the Neo-Tethyan Ocean that progressively migrated westwards from the Early Jurassic – Middle Eocene and
SC RI PT
the present day. In this model, the subduction of the Neo-Tethyan ocean beneath the Sibumasu continental block during Early to Middle Jurassic time resulted in an accretionary complex of HP metamorphic rocks (now represented by the Jade Mines Belt and Kumon range), and a broadly coeval continental margin Jurassic magmatism in the Myitkyina
NU
ophiolite and Mogok metamorphic belt (Fig. 17c). However, this model predicts peak and retrograde metamorphism in the NOC, which is situated west of the Jade Mines Belt, to be
MA
the product of a second accretionary event in the Late Cretaceous. The reconstruction of an Early Jurassic accretionary complex in the NOC (this study)
ED
raises the possibility that it was continuous with the accretionary complex of the Jade Mines Belt further northeast (with respect to current geographic co-ordinates; Fig. 17c). The two
PT
ophiolites have been disrupted by younger strike-slip faulting along a structure parallel to the
CE
N-trending Sagaing fault. This proposition is supported by the analogous spatial association of continental slivers of Gondwanan affinity to the east of the HP accretionary complexes in
AC
both the locations: continental Naga Metamorphics of Lower Palaeozoic age (Aitchison et al., 2019) in the NOC and Mogok Metamorphic and Slate Belt as part of the Sibumasu Continental block in northern Myanmar (cf. Metcalfe, 1995; Zhang, J. et al., 2018) (Fig. 17c). Future studies should be directed to understand the Neo-Tethyan tectonics that was responsible for the present association of Jurassic and Cretaceous accretionary complexes in the WOB.
ACCEPTED MANUSCRIPT 7. Conclusions 1. Oceanic eclogite from two new locations near Mokie and Thewati in the Nagaland Ophiolite Complex, NE India, occurs as meter-scale lensoidal bodies within a lawsoniteblueschist facies metamorphosed oceanic sequence of basalt-chert-limestone. 2. SHRIMP U-Pb zircon dating of the eclogites yielded age groupings at ~205 Ma,
SC RI PT
~185-189 Ma and ~172 Ma (minor). The c. 205 Ma date is interpreted as a minimum age for the oceanic protolith, the c.185-189 Ma age is thought to reflect zircon growth at eclogite facies conditions and the c.172 Ma age possibly represents the onset of retrograde cooling and exhumation.
NU
3. Maximum P-T conditions recorded by the eclogite samples (~28.1 to 25.5 kbar and ~ 650 oC for the Thewati eclogite and ~23.8 kbar and ~555 oC for the Mokie eclogite)
MA
involved a low apparent thermal gradient (~7-8 oC/km).
4. The Thewati eclogite was buried to depths of ~80-90 km, transitional to UHP
ED
conditions, thus falling in the select category of oceanic eclogites worldwide that are recovered from the maximum depths in the subduction zone.
PT
5. The Thewati eclogite records a near complete clockwise metamorphic P-T path of
CE
evolution that involved an epidote blueschist facies prograde stage, peak metamorphism in the epidote eclogite facies and multistage retrogression through amphibole-eclogite facies at
AC
~18.3 kbar, 630 oC, epidote blueschist facies to the transitional lawsonite blueschist and greenschist facies metamorphic conditions at ~6 kbar, 300 oC. 6. The low thermal gradients (~7-9 oC/km) that accompanied peak conditions, together with the clockwise P-T paths of the blueschists and eclogites are consistent with a Lower Jurassic-aged, cold intra-oceanic subduction system within the Neo-Tethys in Nagaland.
ACCEPTED MANUSCRIPT 7. The Nagaland eclogites were exhumed in two stages: (a) the initial exhumation (from 25.5 kbar, 650 oC to 10 kbar, 510 oC, and along a steep dP/dT gradient) was likely to have taken place in the subduction channel as a consequence of buoyancy-driven material transport in a rheologically weak and fluidised mantle; (b) in the latter phase, the exhumation of the eclogites along a gentler dP/dT gradient was accompanied by extended cooling at
SC RI PT
shallower levels. This exhumation stage is linked with thrust stacking of the partially exhumed eclogites with the cooler, prograde blueschists at shallower crustal levels. 8. The current disposition of the Nagaland eclogites in lower grade blueschist likely reflects stage 2 of their exhumation in the subduction channel, when the Nagaland
NU
accretionary complex was assembled.
10. An Early Jurassic accretionary complex in the NOC was likely continuous with
current geographic co-ordinates).
MA
the accretionary complex of the Jade Mines Belt of northern Myanmar (with respect to
ED
11. The Nagaland eclogite samples present the oldest Neo-Tethyan remnant (cf. Late Triassic age) and evidence of the earliest subduction (cf. Early Jurassic) within the Yarlung-
PT
Tsangpo Suture Zone. The cryptic record of this pre-Cretaceous subduction system within the
CE
Neo-Tethys that incorporated an Early-Middle Jurassic Andean-type convergent margin can
Pakistan.
AC
be traced westwards from the Indo-Myanmar Ranges along the southern Eurasian margin to
Acknowledgements The paper constitutes a part of the doctoral thesis work of MR for which she acknowledges funding from the Indian Institute of Technology Kharagpur (IIT KGP) for an Institute doctoral fellowship and to carry out geological fieldwork. S.K.B. acknowledges financial support from the Indian Space Research Organization (Grant No. IIT/KCSTC/Chairman/New
ACCEPTED MANUSCRIPT Approval/15-16/09) and Australia-India Strategic Research Fund (DST/INT/AUS/P-55/2013 and AISRF07021) for fieldwork and analyses. A.A. thanks the Director, Wadia Institute of Himalayan Geology. We would also like to acknowledge the generous support of the Nagaland State Mineral Development Corporation Ltd. and their support staff at the Weziho Field Hostel without which, it wouldn't have been possible for us to undertake geological
SC RI PT
fieldwork in this remote and inaccessible part of the Nagaland state bordering Myanmar. The manuscript has benefitted from earlier reviews by S.A. Wilde and T. Johnson. We thank two anonymous journal reviewers for detailed reviews, and Marco Scambelluri for efficient
NU
editorial handling.
References
MA
Acharyya, S.K., 1986. Tectono-stratigraphic history of Naga Hills Ophiolites. In: Geology of the Nagaland Ophiolite (ed. Ghosh, D.B.), Geological Survey of India Memoirs
ED
119,94–103.
Acharyya, S.K., 2007. Collisional emplacement history of the Naga-Andaman ophiolites and
PT
the position of the eastern Indian suture. Journal of Asian Earth Sciences 29, 229-242.
CE
Acharyya, S.K., 2015. Indo-Burma Range: a belt of accreted microcontinents, ophiolites and Mesozoic–Paleogene flyschoid sediments. International Journal of Earth Sciences 104,
AC
1235-1251.
Agard, P., Jolivet, L., Goffé, B., 2001. Tectonometamorphic evolution of the Schistes Lustrés complex: implications for the exhumation of HP and UHP rocks in the Western Alps. Bulletin de la Société Géologique de France 172 (5), 617–636. Agard, P., Yamato, P., Jolivet, L., Burov, J.E., 2009. Exhumation of oceanic blueschists and eclogites in subduction zones: Timing and mechanisms. Earth Science Reviews 92,5379.
ACCEPTED MANUSCRIPT Agard, P., Yamato, P., Soret, M., Prigent, C., Guillot, S., Plunder, A., Dubacq, B., Chauvet,A., Monié, P., 2016. Plate interface rheological switches during subduction infancy: control on slab penetration and metamorphic sole formation. Earth and Planetary Science Letters 451, 208–220. Agard, P., Plunder, A., Angiboust, S., Bonnet, G., Ruh, J., 2018. The subduction plate
SC RI PT
interface: Rock record and mechanical coupling (from long to short time scales). Lithos 320-321, 537-566. https://doi.org/10.1016/j.lithos.2018.09.029.
Aitchison, J.C., McDermid, I.R.C., Ali, J.R., Davis, A.M., Zyabrev, S.V., 2007. Shoshonites in southern Tibet record Late Jurassic rifting of a Tethyan intra-oceanic island arc.
NU
Journal of Geology, 115: 197-213.
Aitchison, J.C., Ao, A., Bhowmik, S., Clarke, G.L., Ireland, T.R., Kachovich, S., Lokho,
MA
K.,Stojanovic, D., Roeder, T., Truscott, N., Zhen, Y., Zhou, R., 2019. Tectonic evolution of the western margin of the Myanmar microplate based on new fossil and
ED
radiometric age constraints. Tectonics 38, 1718–1741. https://doi.org/10.1029/2018TC005049.
PT
Anczkiewicz, R., Burg, J.P., Villa, I.M., Meier, M., 2000. Late Cretaceous blueschist
CE
metamorphism in the Indus suture zone, Shangla region, Pakistan Himalaya. Tectonophysics 324, 111-134.
AC
Andersen, T. B., Jamtveit, B., Dewey, J. F., Swensson, E., 1991. Subduction and eduction of continental crust: major mechanisms during continent‐continent collision and orogenic extensional collapse, a model based on the south Norwegian Caledonides. Terra Nova 3(3), 303-310. Angiboust, S., Agard, P., Jolivet, L., Beyssac, O., 2009. The Zermatt‐Saas ophiolite: the largest (60km wide) and deepest (c. 70–80 km) continuous slice of oceanic lithosphere detached from a subduction zone? Terra Nova 21(3), 171-180.
ACCEPTED MANUSCRIPT Angiboust, S., Langdon, R., Agard, P., Waters, D., Chopin, C., 2012. Eclogitization of the Monviso ophiolite (W. Alps) and implications on subduction dynamics. Journal of Metamorphic Geology 30, 37-61. Angiboust, S., Agard, P., Glodny, J., Omrani, J., Oncken, O., 2016. Zagros blueschists: Episodic underplating and long-lived cooling of a subduction zone. Earth and Planetary
SC RI PT
Science Letters 443, 48-58. https://doi.org/10.1016/j.epsl.2016.03.017. Anon., 1986. Geology of Nagaland ophiolite. Geological Survey of India Memoir, 119, 113pp.
Ao, A., Bhowmik, S.K., 2014. Cold subduction of the Neotethys: The metamorphic record
NU
from finely banded lawsonite and epidote blueschists and associated metabasalts of the Nagaland Ophiolite Complex, India. Journal of Metamorphic Geology 32(8), 829-860.
MA
Baxter, A.T., Aitchison, J.C., Zyabrev, S.V., Ali, J.R., 2011. Upper Jurassic radiolarians from the Naga ophiolite, Nagaland, northeast India. Gondwana Research 20, 638-644.
ED
Bhowmik, S.K., Ao, A., 2016. Subduction initiation in the Neo‐Tethys: Constraints from counterclockwise P–T paths in amphibolite rocks of the Nagaland Ophiolite Complex,
PT
India. Journal of Metamorphic Geology 34(1), 17-44.
CE
Black, L.P., Kamo, S.L., Allen, C.M., Aleinikoff, J.N., 2003. TEMORA 1: a new zircon standard for Phanerozoic U-Pb geochronology. Chemical Geology 200, 155–17.
AC
Bohlen, S.R., Boettcher, A.L., 1982. The quartz–coesite transformation: a pressure determination and the effects of other components. Journal of Geophysical Research104,208–224. Brovarone, A.V., Agard, P., 2013. True metamorphic isograds or tectonically sliced metamorphic sequence? New high-spatial resolution petrological data for the New Caledonia case study. Contributions to Mineralogy and Petrology 166(2), 451-469.
ACCEPTED MANUSCRIPT Brovarone, A. V., Groppo, C., Hetenyi, G., Compagnoni, R., Malavieille, J., 2011. Coexistence of lawsonite‐bearing eclogite and blueschist: phase equilibria modelling of Alpine Corsica metabasalts and petrological evolution of subducting slabs. Journal of Metamorphic Geology 29(5), 583-600. Brovarone, A. V., Picatto, M., Beyssac, O., Lagabrielle, Y., Castelli, D. 2014. The
SC RI PT
blueschist–eclogite transition in the Alpine chain: P–T paths and the role of slowspreading extensional structures in the evolution of HP–LT mountain belts.Tectonophysics 615, 96-121.
Brunnschweiler, R.O., 1966. On the geology of Indo-Burman Ranges. Geological Society of
NU
Australia 13, 137–195.
Chatterjee, N., Ghose, N.C., 2010. Metamorphic evolution of the Naga Hills eclogite and
MA
blueschist, Northeast India: Implications for early subduction of the Indian plate under the Burma microplate. Journal of Metamorphic Geology 28(2), 209-225.
ED
Chemenda, A. I., Mattauer, M., Bokun, A. N., 1996. Continental subduction and a mechanism for exhumation of high-pressure metamorphic rocks: new modelling and
PT
field data from Oman. Earth and Planetary Science Letters 143(1-4), 173-182.
CE
Clarke, G. L., Aitchison, J. C., Cluzel, D., 1997. Eclogites and blueschists of the Pam Peninsula, NE New Caledonia: a reappraisal. Journal of Petrology 38(7), 843-876.
AC
Cloos, M., 1982. Flow melanges: numerical modelling and geologic constraints on their origin in the Franciscan subduction complex, California. Bulletin of Geological Society of America 93, 330–345. Cloos, M., 1985. Thermal evolution of convergent plate margins: Thermal modeling and reevaluation of isotopic Ar‐ages for blueschists in the Franciscan Complex of California. Tectonics 4(5), 421-433.
ACCEPTED MANUSCRIPT Cloos, M., Shreve, R., 1988. Subduction channel mode of prism accretion, mélange formation, sediment subduction, and subduction erosion at convergent plate margins: 2. Implications and discussions. Pure and Applied Geophysics 128, 501–541. Coggon, R., Holland, T.J.B., 2002. Mixing properties of phengitic micas and revised garnet‐phengite thermobarometers. Journal of Metamorphic Geology 20(7), 683-696.
SC RI PT
Davis, P.B. Whitney, D.L., 2008. Petrogenesis and structural petrology of high-pressure metabasalt pods, Sivrihisar, Turkey. Contributions to Mineralogy and Petrology 156, 217– 241.
Diener, J.F.A., Powell, R., White, R.W. Holland, T.J.B., 2007. A new thermodynamic model
NU
for clino- and orthoamphiboles in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2OO. Journal of Metamorphic Geology 25, 631–656.
MA
Fitzherbert, J. A., Clarke, G. L., Marmo, B.,Powell, R., 2004. The origin and P–T evolution of peridotites and serpentinites of NE New Caledonia: prograde interaction between
ED
continental margin and the mantle wedge. Journal of Metamorphic Geology 22(4), 327344. https://doi.org/10.1111/j.1525-1314.2004.00517.x.
PT
Ganguly, J., Cheng, W., Tirone, M., 1996. Thermodynamics of aluminosilicate garnet solid
CE
solution: new experimental data, an optimized model, and thermometric applications. Contributions to Mineralogy and Petrology 126, 137–151.
AC
Gardiner, N.J., Searle, M.P., Morley, C.K., Robb, L.J., Whitehouse, M.J., Roberts, N.M., Kirkland, C.L., Spencer, C.J., 2018. The crustal architecture of Myanmar imaged through zircon U-Pb, Lu-Hf and O isotopes: Tectonic and metallogenic implications. Gondwana Research 62, 27-60. doi:10.1016/j.gr.2018.02.008. Gerya, T.V., Stoeckhert, B., Perchuk, A.L., 2002. Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics 21, 61-69, 1056. doi:10.1029/2002TC001406.
ACCEPTED MANUSCRIPT Goffé, B., Rangin, C., Maluski, H., 2000. Jade and associated rocks from Jade Mines area, northern Myanmar, as record of a polyphased high-pressure metamorphism. EOS Transactions AGU 81, Fall Meeting Supplement, F1365. Goffé, B., Rangin, C., Maluski, H., 2002. Jade and associated rocks from the Jade Mines area, northern Myanmar as record of a polyphased high-pressure metamorphism.
SC RI PT
Himalaya Karakoram-Tibet Workshop Meeting (Abstracts). Journal of Asian Earth Sciences 20, 16–17.
Green, E.C.R., Holland, T.J.B., Powell, R., 2007. An order disorder model for omphacitic pyroxenes in the system jadeite- diopside-hedenbergite-acmite, with applications to
NU
eclogite rocks. American Mineralogist 92, 1181–1189.
Groppo, C., Castelli, D., 2010. Prograde P-T evolution of a lawsonite eclogite from the
MA
Monviso metaophiolite (Western Alps): dehydration and redox reactions during subduction of oceanic FeTi-oxide gabbro. Journal of Petrology 51, 2489–2514.
ED
Groppo, C., Rolfo, F., Sachan, H.K., Rai, S.K., 2016. Petrology of blueschist from the Western Himalaya (Ladakh, NW India): Exploring the complex behavior of a
PT
lawsonite-bearing system in a paleo-accretionary setting. Lithos 252, 41-56.
CE
Guillot, S., Hattori, K., Agard, P., Schwartz, S., Vidal, O., 2009. Exhumation processes in oceanic and continental subduction contexts: a review. In: Lallemand, S., Funcinello, F.
AC
(Eds.), Subduction Zone Geodynamics: pp. 175–205. http://dx.doi.org/10.1007/978-3540-87974-9.
Guilmette, C., Hébert, R., Dostal, J., Indares, A., Ullrich, T., Bédard, É., Wang, C., 2012. Discovery of a dismembered metamorphic sole in the Saga ophiolitic mélange, South Tibet: Assessing an Early Cretaceous disruption of the Neo-Tethyan supra-subduction zone and consequences on basin closing. Gondwana Research 22(2), 398-414.
ACCEPTED MANUSCRIPT Guiraud, M., 1982. Géothermométrie du faciés schistes vert à glaucophane. Modélisation et applications (Afghanistan, Pakistan, Corse, Bohème). Ph.D. thesis, University of Montpellier, France. Hacker, B.R., Wang, X., Eide, E.A., Ratsbacher, L., 1996. The Qinling‐Dabie ultrahigh‐pressure collisional orogen. In: Tectonic Evolution of Asia (Yin, A.,
SC RI PT
Harrison, T.M. eds), Cambridge University Press, Cambridge, pp. 345-370. Hébert, R., Bezard, R., Guilmette, C., Dostal, J., Wang, C.S., Liu, Z.F., 2012. The Indus– Yarlung Zangbo ophiolites from Nanga Parbat to Namche Barwa syntaxes, southern Tibet: First synthesis of petrology, geochemistry, and geochronology with incidences
NU
on geodynamic reconstructions of Neo-Tethys. Gondwana Research 22, 377-397. Hermann, J., Müntener, O., Scambelluri, M., 2000. The importance of serpentinite mylonites
MA
for subduction and exhumation of oceanic crust. Tectonophysics 327(3-4), 225-238. Holland, T.J.B., 1980. The reaction albite = jadeite+ quartz determined experimentally in the
ED
range 600–1200 oC. American Mineralogist 65(1-2), 129-134. Holland, T.J.B., Powell, R., 1998. An internally consistent thermodynamic dataset for phases
PT
of petrological interest. Journal of Metamorphic Geology 16, 309–343.
CE
Holland, T.J.B., Powell, R., 2003. Activity–composition relations for phases in petrological calculations: an asymmetric multicomponent formulation. Contributions to Mineralogy
AC
and Petrology 145, 492–501. Honegger, K., Le Fort, P., Mascle, G., Zimmerman, J.L., 1989. The blueschists along the Indus Suture Zone in Ladakh, N.W. Himalaya. Journal of Metamorphic Geology 7, 57– 72. Htay, H., Zaw, K., Oo, T.T., 2017. The mafic–ultramafic (ophiolitic) rocks of Myanmar. In: Barber, A.J., Khin, Z., & Crow, M.J. (eds) Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs 48, 117–141.
ACCEPTED MANUSCRIPT Imchen, W., Thong, G.T., Pongen, T., 2014. Provenance, tectonic setting and age of the sediments of the Upper Disang Formation in the Phek District, Nagaland. Journal of Asian Earth Sciences 88, 11–27. Jan, M.Q., 1985. High-P rocks along the suture zone around Indo-Pakistan plate and phase chemistry of blueschists from eastern Ladakh. Geological Bulletin University of
SC RI PT
Peshawar 18, 1–40.
Krebs, M., Schertl, H.P., Maresch, W.V., Draper, G., 2011. Mass flow in serpentinite-hosted subduction channels: P-T-t path patterns of metamorphic blocks in the Rio San Juan melange (Dominican Republic). Journal of Asian Earth Sciences 42, 569–595.
NU
Lanari, P., Vidal, O., De Andrade, V., Dubacq, B., Lewin, E., Grosch, E.G., Schwartz, S.,2014. XMapTools: A MATLAB©-based program for electron microprobe X-ray
MA
image processing and geothermobarometry. Computers & Geosciences 62, 227-240. Leake, B.E., Wooley, A.R., Arps, C.E., Birch, W.D., Gilbert, M.C., Grice, J.D., Hawthorne,
ED
F.C., Kato, A., Kisch, H.J., Krivovichec, V.G. and Linthout, K., 1997. Nomenclature of amphiboles: report of the subcommittee on amphiboles of the International
PT
Mineralogical Association, commission on new minerals and mineral names. American
CE
Mineralogist 82, 1019–1037.
Liou, J.G., 1971. P-T Stabilities of laumontite, wairakite, lawsonite, and related minerals in
AC
the system CaAl2Si2O8-SiO2-H2O. Journal of Petrology 12(2), 379-411. Liu, C.Z., Chung, S.L., Wu, F.Y., Zhang, C., Xu, Y., Wang, J.G., Chen, Y., Guo, S., 2016a. Tethyan suturing in Southeast Asia: zircon U-Pb and Hf-O isotopic constraints from Myanmar ophiolites. Geology 44, 311–314. Liu, C.Z., Zhang, C., Xu, Y., Wang, J.G., Chen, Y., Guo, S., Wu, F.Y., Sein, K., 2016b. Petrology and geochemistry of mantle peridotites from the Kalaymyo and Myitkyina ophiolites (Myanmar): implications for tectonic settings. Lithos 264, 495–508.
ACCEPTED MANUSCRIPT Ludwig, K.R., 2003. User’s manual for a geochronological toolkit for Microsoft Excel (Isoplot/Ex version 3.0). Berkeley Geochronology Center, Special Publication 4, 1-71. Mahéo, G., Fayoux, X., Guillot, S., Garzanti, E., Capiez, P., Mascle, G., 2006. Relicts of an intra-oceanic arc in the Sapi-Shergol mélange zone (Ladakh, NW Himalaya, India): implications for the closure of the Neo-Tethys Ocean. Journal of Asian Earth Sciences
SC RI PT
26, 695–707.
Maruyama, S., Cho, M., Liou, J. G., 1986. Experimental investigations of blueschistgreenschist transition equilibria: Pressure dependence of Al2O3 contents in sodic amphiboles- A new geobarometer. Blueschists and Eclogites 164, 1-16.
NU
McDermid, I.R.C., Aitchison, J.C., Davis, A.M., Harrison, T.M., Grove, M., 2002. The Zedong terrane: A Late Jurassic intra-oceanic magmatic arc within the Yarlung-Zangbo
MA
suture zone, southeastern Tibet. Chemical Geology 187, 267-277. Metcalfe, I., 1995. Gondwana dispersion and Asian accretion. Journal of Geology 5-6, 223–
ED
266.
Mével, C., Kiénast, J.R., 1986. Jadeite-kosmochlor solid solution and chromian sodic
PT
amphiboles in jadeitites and associated rocks from Tawmaw (Burma). Bulletin de
CE
Mineralogie 109, 617–633.
Mitchell, A.H.G., 1986. Ophiolite and associated rocks in four settings: relationships to
AC
subduction and collision. Tectonophysics 125, 269-285. Mitchell, A.H.G., 1993. Cretaceous-Cenozoic Tectonic events in the western Myanmar (Burma)-Assam region. Journal of Geological Society, London 150, 1089–1102. Mitchell, A.H.G., Htay, M.T., Htun, K.M., 2015. The medial Myanmar suture zone and the western Myanmar Mogok foreland. Journal of the Myanmar Geosciences Society 6,73– 88.
ACCEPTED MANUSCRIPT Nyunt, T.T., Massonne, H.J., Sun, T.T., 2017. Jadeitite and other high-pressure metamorphic rocks from the Jade Mines Belt, Tawmaw area, Kachin State, northern Myanmar. Geological Society, London, Memoirs 48(1), 295-315. Oberhhänsli, R., 2013. High-pressure – low-temperature evolution in the Indus-Tsangpo Suture along the Kohistan Arc (Kaghan Valley, NE Pakistan). Episodes 36, 87–93.
SC RI PT
Oberhänsli, R., Bousquet, R., Moinzadeh, H., Moazzen, M., Arvin, M., 2007. The field of stability of blue jadeite: a new occurrence of jadeitite from Sorkhan, Iran, as a case study. Canadian Mineralogist 45, 1501–1509.
O'Brien, P.J., 2018. Eclogites and other high-pressure rocks in the Himalaya: a review.
NU
Geological Society, London, Special Publications 483,https://doi.org/10.1144/SP483.13 Okamoto, K., Maruyama, S., 1999. The high-pressure synthesis of lawsonite in the
MA
MORB+H2O system. American Mineralogist 84(3), 362-373. Page, F.Z., Armstrong, L.S., Essene, E.J., Mukasa, S.B., 2007. Prograde and retrograde
ED
history of the Junction School eclogite, California, and an evaluation of garnet-
153, 533–555.
PT
phengite-clinopyroxene thermobarometry. Contributions to Mineralogy and Petrology
CE
Peacock, S.M., Wang, K., 1999. Seismic consequences of warm versus cool subduction metamorphism: examples from southwest and northeast Japan. Science 286, 937–939.
AC
Plunder, A., Agard, P., Dubacq, B., Chopin, C., Bellanger, M., 2012. How continuous and precise is the record of P–T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustrés, W. Alps). Journal of Metamorphic Geology 30(3), 323-346. Plunder, A., Agard, P., Chopin, C., Pourteau, A., Okay, A. I., 2015. Accretion, underplating and exhumation along a subduction interface: From subduction initiation to continental subduction (Tavşanlı zone, W. Turkey). Lithos 226, 233-254.
ACCEPTED MANUSCRIPT Powell, R., Holland, T., 2008. On thermobarometry. Journal of Metamorphic Geology 26, 155–179. Qiu, Z.L., Wu, F.Y., Yang, S.F., Zhu, M., Sun, J.F., Yang, P., 2009. Age and genesis of the Myanmar jadeite: constraints from U-Pb ages and Hf isotopes of zircon inclusions. Chinese Science Bulletin 54, 658–668.
SC RI PT
Ravna, K., 2000. The garnet–clinopyroxene Fe2+–Mg geothermometer: an updated calibration. Journal of Metamorphic Geology 18, 211-219.
Ravna, E.J., Terry, M.P., 2004. Geothermobarometry of UHP and HP eclogites and schists– an evaluation of equilibria among garnet–clinopyroxene–kyanite–phengite–
NU
coesite/quartz. Journal of Metamorphic Geology 22, 57-592.
Rebay, G., Powell, R., Diener, J.F.A., 2010. Calculated phase equilibria for a MORB
MA
composition in a P–T range, 450–650°C and 18–28 kbar: the stability of eclogite. Journal of Metamorphic Geology 28, 635–645.
ED
Reinecke, T., 1998. Prograde high- to ultrahigh-pressure metamorphism and exhumation of oceanic sediments at Lago di Cignana, Zermatt-Saas Zone, western Alps. Lithos 42,
PT
147–189.
CE
Roddick, J.C., van Breemen, O., 1994. U-Pb zircon dating: A comparison of ion microprobe and single grain conventional analyses. In Radiogenic Age and Isotopic Studies,
AC
Report8. Geological Survey of Canada Current Res 1994-F, p 1-9. Rubatto, D., 2017. Zircon: the metamorphic mineral. Reviews in Mineralogy and Geochemistry 83, 261-295. Rubatto, D., Gebauer, D., 2000. Use of cathodoluminescence for U-Pb zircon dating by ion microprobe: some examples from the Western Alps. In Cathodoluminescence in geosciences. Springer, Berlin, Heidelberg. pp. 373-400.
ACCEPTED MANUSCRIPT Rubatto, D., Hermann, J., 2003. Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones. Geochimica et Cosmochimica Acta 67, 2173-2187. Rubatto, D., Hermann, J., 2007. Zircon behaviour in deeply subducted rocks. Elements 3, 3135.
SC RI PT
Sambridge, M.S., Compston, W., 1994. Mixture modeling of multi-component data sets with application to ion-probe zircon ages. Earth and Planetary Science Letters 128, 373–390. Sarkar, A., Datta, A.K., Poddar, B.C., Bhattacharyya, B.K., Kollapuri, V.K., Sanwal, R.,1996. Geochronological studies of Mesozoic igneous rocks from eastern India.
NU
Journal of Southeast Asian Earth Sciences 13, 77–81.
Schmidt, M.W., 1995. Lawsonite: upper pressure stability and formation of higher density
MA
hydrous phases. American Mineralogist 80 (11-12), 1286-1292. Searle, M.P., Kahn, M.A., Fraser, J.E., Gough, S.J., 1999. The tectonic evolution of the
ED
Kohistan-Karakoram collision belt along the Karakoram Highway transect, north Pakistan. Tectonics 18, 929–949.
PT
Searle, M.P., Noble, S.R., Cottle, J.M. et al., 2007. Tectonic evolution of the Mogok
CE
Metamorphic belt, Burma (Myanmar) constrained by U-Th-Pb dating of metamorphic and magmatic rocks. Tectonics 26, TC3014. https://doi:10.1029/2006TC002083.
AC
Searle, M.P., Morley, C.K., Waters, D.J., Gardiner, N.J., Htun, U. K., Nu, T.T., Robb, L.J., 2017.Tectonic and metamorphic evolution of the Mogok Metamorphic and Jade Mines belts and ophiolitic terranes of Burma (Myanmar). In: Barber, A.J., Khin, Z., & Crow, M.J. (eds) Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs 48, 261–293, https://doi.org/10.1144/M48.12.
ACCEPTED MANUSCRIPT Setiawan, N.I., Osanai, Y., Nakano, N., Adachi, T., Yonemura, K., Yoshimoto, A., 2016. Prograde and retrograde evolution of eclogites from the Bantimala Complex in South Sulawesi, Indonesia. Journal of Mineralogical and Petrological Sciences 111, 211–225. Shi, G.H., Cui, W.Y., Tropper, P., Wang, C.Q., Shu, G.M., Yu, H.X., 2003. The petrology of a complex sodic and sodic-calcic amphibole association and its implications for the
SC RI PT
metasomatic processes in the jadeitite area in northwestern Myanmar, formerly Burma. Contributions to Mineralogy and Petrology 145, 355–376.
Shi, G.H., Cui, W.Y., Cao, S.M., Jiang, N., Jian, P., Liu, D.Y., Miao, L.C., Chu, B.B., 2008. Ion microprobe zircon U–Pb age and geochemistry of the Myanmar jadeitite. Journal of
NU
the Geological Society, London 165, 221–234.
Singh, A.K., Chung, S.-L., Bikramaditya, R.K., Lee, H.Y., 2017. New U–Pb zircon ages of
MA
plagiogranites from the Nagaland-Manipur Ophiolites, Indo-Myanmar Orogenic Belt, NE India. Journal of the Geological Society, London 74, 170-179.
ED
Sorensen, S. S., Barton, M. D., 1987. Metasomatism and partial melting in a subduction complex: Catalina Schist, southern California. Geology 15(2), 115-118.
PT
Thet Tin Nyunt., 2009. Petrological and geochemical contribution to the origin of jadeitite
CE
and associated rocks of the Tawmaw area, Kachin State, Myanmar. Doctoral dissertation, Institut für Mineralogie und Kristallchemie der Universität Stuttgart.
AC
Vance, D., Mahar, E., 1998. Pressure-temperature paths from P-T pseudosections and zoned garnets: potential, limitations and examples from the Zanskar Himalaya, NW India. Contributions to Mineralogy and Petrology 132(3), 225-245. Von Blanckenburg, F., Davies, J. H., 1995. Slab break off: a model for syncollisional magmatism and tectonics in the Alps. Tectonics 14(1), 120-131.
ACCEPTED MANUSCRIPT Wang, W.L., Aitchison, J.C., Lo, C.H., Zeng, Q.G., 2008. Geochemistry and geochronology of the amphibolite blocks in ophiolitic mélanges along Bangong-Nujiang suture, central Tibet. Journal of Asian Earth Sciences 33, 122-138. Wei, C.J., Clarke, G.L., 2011. Calculated phase equilibria for MORB compositions: a reappraisal of the metamorphic evolution of lawsonite eclogite. Journal of
SC RI PT
Metamorphic Geology 29 (9), 939-952.
White, R.W., Powell, R., Holland, T.J.B., 2007. Progress relating to calculation of partial melting equilibria for metapelites. Journal of Metamorphic Geology 25, 511–527. Whitney, D.L., Evans, B.W., 2010. Abbreviations for names of rock-forming minerals.
NU
American Mineralogist 95, 185-187.
Xu, Y., Liu, C.Z., Guo, S., Wang, J., Sein, K., 2017. Petrogenesis and tectonic implications
MA
of gabbro and plagiogranite intrusions in mantle peridotites of the Myitkyina ophiolite, Myanmar. Lithos 284-285, 180–193.
ED
Yang, J.S., Xu, Z.Q., Duan, X.D., Li, J., Xiong, F.H., Liu, Z., Cai, Z.H., Li, H.Q., 2012. Discovery of a Jurassic SSZ ophiolite in the Myitkyina region of Myanmar. Acta
PT
Petrologica Sinica 28, 1710–1730 (in Chinese with English abstract).
CE
Yamato, P., Agard, P., Burov, E., Le Pourhiet, L., Jolivet, L., Tiberi, C., 2007. Burial and exhumation in a subduction wedge: mutual constraints from thermo-mechanical
AC
modelling and natural P-T-t data (Schistes Lustres, W. Alps). Journal of Geophysical Research 112, B07410. doi:10.1029/2006JB004441. Yin, A., Harrison, T.M., 2000. Geologic evolution of the Himalayan Tibetan orogen. Annual Review of Earth and Planetary Sciences 28, 211–80. Yu, Y-P., Xie, C-M., Fan, J-J., Wang, M., Dong, Y-C, Wang, B., Hao, Y-J., 2018. Zircon U– Pb geochronology and geochemistry of Early Jurassic granodiorites in Sumdo area,
ACCEPTED MANUSCRIPT Tibet: Constraints on petrogenesis and the evolution of the Neo-Tethyan Ocean. Lithos 320-321, 134-143. Yui, T.F., Fukoyama, M., Iizuka, Y. et al., 2013. Is Myanmar jadeitite of Jurassic age? A result from incompletely recrystallized inherited zircon. Lithos 160–161, 268–282. Zhang, J., Xiao, W., Windley, B.F., Cai, F., Sein, K., Naing, S., 2017. Early Cretaceous
SC RI PT
wedge extrusion in the Indo-Burma Range accretionary complex: Implications for the Mesozoic subduction of Neotethys in SE Asia. International Journal of Earth Sciences 106, 1391–1408.
Zhang, J., Xiao, W., Windley, B., Wakabayashi, J., Cai, F., Sei, K., Wu, H., Naing, S., 2018.
NU
Multiple alternating forearc- and backarc-ward migration of magmatism in the IndoMyanmar Orogenic Belt since the Jurassic: Documentation of the orogenic architecture
MA
of eastern Neotethys in SE Asia. Earth Science Reviews 185, 704-731. Zhang, Y.X., Jin, X., Zhang, K.J., Sun, W.D., Liu, J.M., Zhou, X.Y., Yan, L.L., 2018. Newly
ED
discovered Late Triassic Baqing eclogite in central Tibet indicates an anticlockwise West–East Qiangtang collision. Scientific Reports. 8(1), 966.
PT
https://doi:10.1038/s41598-018-19342-w.
CE
Zhu, D-C., Zhao, Z-D., Niu, Y., Dilek, Y., Hou, Z-Q., Mo, X-X., 2013. The origin and pre-
AC
Cenozoic evolution of the Tibetan Plateau. Gondwana Research 23, 1429-1454.
Figure Captions
Fig. 1: A tectonic map of eastern Tibet, SE China, Myanmar and NE India showing the occurrences of different Tethyan sutures (LCS, Longmu Co-Shuanghu suture; BNS, Bangong-Nujiang suture; YTS, Yarlung-Tsangpo suture (after Liu et al., 2016a) and locations of oceanic blueschists and amphibolites (1-4) along the YTS zone. N/M: Nagaland/Myktyina ophiolites. Other abbreviations used: IMR, Indo-Myanmar Ranges;
ACCEPTED MANUSCRIPT WMB, West Myanmar Block; MCT, Main Central thrust; STDS, South Tibetan Detachment System. Fig. 2: (a) Locations of the Indo-Myanmar Ranges (IMR) and western, central and eastern ophiolite belts (WOB, COB and EOB) in the tectonic framework of Indian, Asian and Burmese plates (modified after Searle et al., 2007). The outlined box in the northern margin
SC RI PT
of the IMR marks the location of the present study in the Nagaland Ophiolite Complex (NOC), shown in detail in Fig. 2b. Geochronological data sources for magmatic and metamorphic events are superscripted and subscripted as follows: Superscript:1. Shi et al.(2008); 2. Yui et al.(2013); 3. Goffé et al. (2002); 4. Singh et al. (2017); 5. Liu et al.
NU
(2016a); 6. Yang et al. (2012); 7. Zhang et al. (2017). Subscript: a, Jadeitite; b, Eclogite; c, Plagiogranite; d, Diorite and Gabbro; e, Rodingites; f, Amphibolite; g, Andesitic basalt,
MA
Leucogabbro and Plagiogranite; h, Trondhjemite. (b) Geological map of the NOC (modified after Anon., 1986; Ao and Bhowmik, 2014). Also shown are the distributions of different
ED
metamorphic facies and locations of eclogite samples [(Mokie eclogite, N22F and Thewati eclogites, N23B and N40B, this study and of previous study, abbreviated as CG10 (after
PT
Chatterjee & Ghose, 2010), along with their peak P-T estimates], blueschist occurrences (BS-
CE
A86, after Anon., 1986) and dated basalt sample (S-96, after Sarkar et al., 1996).
Eclogite.
AC
Abbreviations used: GS, Greenschist; PD, Pumpellyite-diopside; BS, Blueschist, EC,
Fig. 3: (a-b) Outcrop-scale maps showing the occurrences of Mokie (Sample N22F) (a) and Thewati (Samples N23B and N40B) (b) eclogites in a blueschist facies ensemble of metabasalt, marble and metachert. Photographs of hand specimens (c, e, g), photomicrograph (f) and false-colour thin section map (d) of Thewati(c-f) and Mokie (g) eclogite samples showing the mineralogy, texture and micro-structure of these rocks. See text for
ACCEPTED MANUSCRIPT more details. Mineral abbreviations in this and other figures and tables are after Whitney & Evans (2010). Fig. 4: Back-scattered electron images (BSEI, a-b, e, h, n-q, s-u) and photo-micrographs (c-d, f-g, i-m, r) of Thewati (a-q) and Mokie eclogites showing textural features of prograde, peak and multi-stage retrograde metamorphism. (a-c): Porphyroblastic Grt (a-b) and omphacitic
SC RI PT
Cpx (c) contain prograde inclusions of Ep1, Gln1 and Ph. (d) Relics of Omp and Rt occur within Brs porphyroblast. Brs is rimmed by a thin corona of Gln2. (e-f) The Thewati eclogite sample N40B shows a second textural variety of Grt (cf. Grt-B) (e-f), which is smaller in size compared to the larger Grt porphyroblasts (cf. Grt-A) (f). Grt-A shows inclusion-rich core
NU
and inclusion-poor rim. (g-h) The occurrence of Ep2 as very coarse aggregates in layer parallel segregations (g) or in the matrix foliation domain (h). Ep2 contains relics of Grt and
MA
Omp (g-h). (i-l) Syn- (i) to post-tectonic (j-l) growth of Brs. Brs includes Rt, Ep2, Grt and Ttn. Thin idioblastic corona of Gln2 occurs around Brs porphyroblast. (m) Composite corona
ED
of Chl + Ph occurs around Grt porphyroblast. (n-q) The developments of late Lws-bearing assemblages with or without Ab, (F)Wnc, Agt and Spn as veins in Grt (n-p) and Brs (q)
PT
porphyroblasts. (r-v) Sample N22F showing peak minerals of Omp porphyroblast and small
CE
Rt and Grt (r-s). Grt crystals, with an idioblastic habit and often in clusters occur in a matrix of retrograde Brs, Gln and Spn. Prograde inclusion of Gln1 occurs within the Omp
AC
porphyroblast. An intergrowth of Brs + Spn occurs around Omp and includes relics of Grt, Rt and Ep (s-u). Brs is later rimmed by Chl and Gln2 (t-v). Fig. 5: Mn-, Mg-, Fe-, and Ca- X-ray element images of representative porphyroblastic Grt crystals in Thewati eclogite samples N23B (a-h) and N40B (Grt-A in Fig. i-l and Grt-B in Fig. m-p) and corresponding compositional profiles along selected traverses (q-aj). The compositional features are attributed to growth zoning in Grt.
ACCEPTED MANUSCRIPT Fig. 6: (a-c) Ternary quadrilateral pyroxene-jadeite-aegirine diagram showing the compositional features of three textural varieties of Na-Ca Px crystals in the investigated samples: (i) Type 1 (T1) pyroxene as inclusions within Grt (a); (ii) Type 2 pyroxenes in the matrix (T2A type) or as relics in barroisite (T2B type) (b); (iii) T3 pyroxenes in veins (c). (df) Compositional varieties of Na-Ca (d-e) and Na amphiboles (f). (g) Binary NaB vs.
SC RI PT
(Al+Fe3+)VI diagram showing coupled substitution vectors in Na-Ca (dark-grey field) and Na(grey field) amphiboles.
Fig. 7: Colour coded mineral maps of two selected compositional layers in the banded Thewati eclogite sample N40B: Pyroxene-garnet-rich layer (a) and Barroisite-epidote-rich
NU
layer (b) using the software XMapTools (after Lanari et al., 2014). These two compositional layers are chosen for the computations of effective bulk rock compositions for phase
MA
equilibria modelling.
Fig. 8:(a) NCKFMASHTO P-T pseudosection for the Px-Grt-rich bulk rock compositional
ED
domain in sample N40B, showing stability of equilibrium mineral assemblage fields and key index minerals. The P-T estimate (with 2σ error) plotted in the diagram refers to the peak Ep-
PT
absent, eclogite facies metamorphism for the Thewati eclogite. (b-e) The mineral assemblage
CE
fields are contoured with isopleths of Grt compositions [x(g) and z(g) parameters, see Table 4 for the nomenclature] and modes of Grt [abbreviated as (Grt(m)], Omp [O(m)] and Amp
AC
[Amp(m)]. The reconstructed metamorphic reaction pathways in the Thewati eclogites are evaluated with compositional (e.g. Grt composition) and modal variations of these minerals along three representative metamorphic P-T paths of evolution (P1, P2 and P3). (f) A summary P-T diagram showing the stability fields of key index minerals, the P-T condition of formation
of
the
prograde
Grt
core
and
the
possible
sequences
of
mineral
appearances/disappearances along the three model P-T paths (see text for details). The numbers in Fig. 9a refer to different mineral assemblage fields, listed as: (1) Cpx Ph Ep Grt
ACCEPTED MANUSCRIPT Lws Amp Qz Rt, (2) Cpx Ph Ep Grt Lws Amp Rt, (3) Cpx Chl Ph Ep Grt Lws Amp Rt, (4) Cpx Ph Ep Grt Amp Rt, (5) Cpx Chl Ph Ep Amp Rt, (6) Cpx Chl Ph Amp Lws Spn Rt, (7) Cpx Ph Ep Grt Lws Qz Rt, (8) Cpx Ph Grt Amp Lws Qz Rt, (9) Cpx Ph Grt Amp Lws Rt, (10) Cpx Chl Ph Amp Lws Spn, (11) Cpx Ph Grt Lws Rt, (12) Cpx Pl Ph Grt Qz Rt, (13) Cpx Chl Ph Ep Lws Amp Rt, (14) Cpx Ph Grt Lws Coe Rt, (15) Cpx Ph Grt Amp Qz Rt, (16) Cpx
SC RI PT
Chl Ph Grt Lws Rt.
Fig. 9: (a) NCKFMASHTO P-T pseudosection for the Amp-Ep-rich bulk rock compositional domain in sample N40B, showing the stability of equilibrium mineral assemblage fields and key index minerals. (b-e) The mineral assemblage fields are contoured with compositional
NU
isopleths of Grt and Amp (NaB values at 0.5 and 1.5, where the values represent cations per formula unit) and modes of Grt, Omp and Amp. The prograde P-T path P1 predicts a Grt
MA
compositional zoning with progressive depletion in Grs + Adr content from core to inner rim of Grt followed by its enrichment toward the Grt outer rim, consistent with the measured Grt
ED
composition. The Amp compositional isopleths reveal the P-T windows at which Na-Ca (cf. Brs) and Na- (cf. Gln) amphiboles are likely to become stable along the retrograde path, P3.
PT
(f) The summary P-T diagram showing the stability fields of key index minerals and the
CE
possible sequences of mineral appearances along the two model retrograde P-T paths (see text for details). The numbers in Fig. 9a refer to different mineral assemblage fields, listed as:
AC
(1) Cpx Chl Ph Grt Lws Ep Spn, (2) Cpx Chl Ph Amp Lws Ep Spn, (3) Cpx Chl Ph Grt Amp Lws Ep Rt, (4) Cpx Chl Ph Grt Lws Ep Rt, (5) Cpx Ph Amp Pl Ep Rt, (6) Cpx Ph Amp Grt Pl Ep Rt, (7) Cpx Ph Grt Lws Qz Rt, (8) Cpx Ph Grt Lws Coe Rt, (9) Cpx Ph Grt Ep Lws Qz Rt, (10) Cpx Ph Grt Ep Lws Rt, (11) Cpx Ph Grt Amp Ep Rt, (12) Cpx Ph Amp Ep Rt, (13) Cpx Ph Amp Ep Qz Rt, (14) Cpx Ph Amp Pl Ep GrtQz Rt, (15) Cpx Chl Ph Amp Ep Lws Rt, (16) Cpx Chl Ph Ep Lws Rt, (17) Cpx Chl Ph Lws Ep Spn, (18) Cpx Ph Grt Ep Coe Rt, (19) Cpx Ph Grt Ep Lws Coe Rt.
ACCEPTED MANUSCRIPT Fig. 10: Reconstruction of the metamorphic P-T path of evolution of the Thewati eclogite using the following approaches: (a) plotting the P-T locus of the first appearance (shown by the symbol +) or disappearance (symbol -) of key minerals calculated through phase equilibria modelling, (b) calculating the P-T positions of activity corrected mineral reactions R3 to R6 (see text for details for the calculation procedures), (c) placing the mineral
SC RI PT
equilibria, R7 (after Maruyama et al., 1986), R8 (after Bohlen and Boettcher, 1982), R9 (after Holland, 1980) and experimentally determined Lws-out curve after Liou (1971) and Schmidt (1995) (Curve R10) in the P-T space and (d) the estimated P-T conditions of formation of prograde Grt core following the isopleth thermobarometry method and peak metamorphism
NU
through conventional thermobarometry. The deduced metamorphic evolutionary pathways when evaluated with (a) to (d) indicate a CW P-T path of evolution involving a prograde
MA
segment (MPR stage) in the epidote blueschist facies, and culminating in peak metamorphism in the eclogite facies (cf. MP stage) and three successive stages of retrograde evolution in
ED
amphibole eclogite facies (cf. MR1 stage), epidote blueschist facies (cf. MR2 stage) and transitional between greenschist and lawsonite blueschist facies (cf. M R3 stage). Mineral
PT
reactions: R3, 10(Jd)Omp+ (4Gr+5Prp)Grt+ 11Qtz + 8H2O → 5(Gl)Brs+6(Cz)Ep; R4, 5(Gl)Brs+
CE
12(Cz)Ep+ 14Qtz + 28H2O → 3(Tr)Fe-Wnc+ 10 Ab + 18 Lws; R5, 3(Dph)Chl + 5(Prp)Grt= 3(Clc)Chl + 5(Alm)Grt; R6, Ab = Jd17 + Qtz; R7, 25Gln + 6Cz + 7 Qtz + 14H2O = 6Tr + 9Chl
AC
+ 50Ab; R8; Qtz = Coe; R9; LAb = Jd + Qtz. Fig. 11: (a-b) BSE images showing the textural locations of zircon in N22F. (c-d) BSE images showing the distribution patterns of mineral inclusions and shape of host zircon crystal in N22F (c) and N23B (d). (e-f) Cathodoluminescence (CL) (e) and BSE (f) images of zircon showing the presence of Grt and Omp inclusions. (g-x) CL structures of representative analysed zircon grains in samples N23B (g-n) and N22F (o-x). 1-2-3 represent different CL domains (see text for details).
ACCEPTED MANUSCRIPT Fig. 12: Binary plots of Th vs. U compositions (in ppm) (a) and Th/U ratio vs. age (in Ma) (b) of analysed zircon grains in the two eclogite samples. Fig. 13: Concordia plots (a, c-d, f-g) and probability distribution diagrams (b, e) of 206Pb/238U ages (in Ma) in zircon grains from samples N22F (a-d) and N23B (e-g). Compositional ellipses in the concordia diagrams are linked with individual zircon grains (abbreviated as Z-
SC RI PT
10 etc.).
Fig. 14: Peak P-T conditions of metamorphism and metamorphic P–T path of the Nagaland eclogites (e.g. the Thewati eclogite path 1, this study) are compared with those from the Mokie blueschist (path 2) and Satuza eclogite (Path 3), all from the NOC, Himalayan
NU
blueschists (paths 4-6) and jadeitite and associated HP metamorphic rocks in the Central Ophiolite Belt (COB) of Myanmar. The P–T results/paths are plotted in the metamorphic
MA
facies diagram (after Okamoto and Maruyama, 1999). The apparent peak thermal gradients of the Nagaland eclogites are compared with model geotherms. The P-T field of global LT
ED
eclogites, and outlined by a red circle is adopted from Fig. 3a of Agard et al. (2016). See text for details. Abbreviations used: AM, Amphibolites facies; Ep-EC, Epidote eclogite facies;
PT
Amp-EC, Amphibole eclogite facies; BS, Blueschist facies; Dry-EC, Dry eclogite facies; EA,
CE
Epidote amphibolites facies; GS, Greenschist facies. References for metamorphic P-T paths and P-T conditions of metamorphism: 1-6: Metamorphic P-T paths of Neo-Tethyan HP/LT
AC
rocks; 1, Thewati EC (N40B), NOC (this study); 2, Mokie BS, NOC (after Ao & Bhowmik, 2014); 3, Satuza EC, NOC (Chatterjee & Ghose, 2010); 4,Sapi-Shergol-Zildat BS (Mahéo et al., 2006); 5, Sapi-Shergol LBS (Groppo et al., 2016); 6, Shangla BS (Guiraud, 1982, Jan, 1985); 7-11: P-T conditions of metamorphism of jadeitite and associated HP metamorphic rocks in the COB; 7, Mével et al., 1986; 8, Goffé et al., 2000; 9, Shi et al., 2003; 10, Oberhänsli et al., 2007; 11a/11b, Gln schist/Grt-Mica schist (after Thet Tin Nyunt, 2009). (b) P-T diagram comparing the peak P-T conditions of pumpellyite-diopside (PD), BS and
ACCEPTED MANUSCRIPT eclogite facies metamorphic rocks from the Mokie-Satuza (red circles) and Thewati (green circles) areas in the NOC. For a comparison, the P-T gradient of the subducted oceanic crust in a thermo-mechanical model-simulated intra-oceanic subduction system (after Gerya et al., 2002) is also plotted. The Nagaland HP/LT metamorphic rocks lie on the P-T gradient predicted by the thermo-mechanical model.
SC RI PT
Fig. 15: P–T path of the Nagaland eclogite (Path 1, NOC, this study) is compared with that from oceanic eclogites in metamorphic belts of the world. The grey P-T band refers to the average low T/P gradients for mature subduction (after Agard et al., 2018). The curve G02 (after Gerya et al., 2002) is the numerical simulation of prograde paths computed for
NU
subduction interface rocks. References for other paths: 2, Zermatt-Saas, Western Alps (Reinecke, 1998; Angiboust et al., 2009); 3, Dominican Republic (Krebs et al., 2011); 4,
MA
Monviso, Alps (Groppo and Castelli, 2010, Angiboust et al., 2012); 5: Rio San Juan mélange Complex (Krebs et al., 2011); 6: Sulawesi, Indonesia (Setiawan et al., 2016).
ED
Fig. 16: A simplified geological map of the NOC showing a progressive metamorphic sequence from GS through PD, BS to EC facies ensemble of basalt-limestone and chert. Also
PT
shown are the locations of Middle-Upper Jurassic-aged basalt and radiolarian chert samples
CE
of previous studies and PD, BS and eclogite facies samples with peak P-T estimates (Ao and Bhowmik, 2014 and this study). Ages of protolith formation and peak eclogite facies
AC
metamorphism (abbreviated as P and M respectively) of the eclogite samples (this study) are also indicated. To the east, the ophiolite terrane is bounded by Early Paleozoic Naga Metamorphics, a Lhasa/Tengchong-equivalent continental slice. Fig. 17: Cartoon diagrams showing the different tectonic models that correlate the Western, Central and Eastern Ophiolite belts in NE India and adjoining Myanmar and SE China with the Tibetan sutures (modified after Zhang, J.et al., 2018). We suggest that the HP metamorphic rocks of the Nagaland Ophiolite Complex and Jade Mines Belt together
ACCEPTED MANUSCRIPT constitute once continuous Early-Middle Jurassic accretionary complex within the NeoTethys.
Table 1: Representative mineral chemical analyses of garnet in eclogites from the NOC. Table 2: Representative mineral chemical analyses of pyroxene in eclogites from the NOC.
sphene, lawsonite, albite and rutile from the NOC.
SC RI PT
Table 3: Representative mineral chemical analysis of phengite, epidote, amphibole, chlorite,
Table 4: Results of thermobarometry of peak eclogite facies metamorphism in the NOC. Table 5: Calculations of effective bulk rock compositions of two representative chemical
NU
domains in sample N40B.
Table 6: SHRIMP U-Pb analyses of representative zircon grains in Mokie and Thewati
MA
eclogites from the NOC.
AC
CE
PT
ED
Table 7: A summary of characteristics of zircon from the Nagaland eclogite samples.
ACCEPTED MANUSCRIPT Table 1. Representative mineral chemical analysis of garnet in eclogite from the Nagaland Ophiolite complex, India N23B-1 133_I1 Grt C 37.14 0.17 21.03 0.01 27.08 2.90 2.21 9.56 0.09 b.d.l. 100.18 12 2.95 1.95 0.01 — 0.04 1.74 0.19 0.26 0.81 0.01 — 7.96 0.06 0.09 0.58 0.25 0.02 0.13
N23B-1 40_23_I1 Grt IR 37.13 0.06 21.13 0.13 30.11 0.49 3.34 7.99 0.04 0.02 100.42 12 2.93 1.94 — 0.01 0.05 1.91 0.03 0.39 0.67 0.01 — 7.93 0.01 0.13 0.64 0.20 0.02 0.17
N23B-1 57_1F2 Grt OR 37.78 0.12 21.13 b.d.l. 28.39 0.19 3.81 8.43 0.02 0.01 99.88 12 2.98 1.96 0.01 — 0.04 1.83 0.01 0.45 0.71 — — 7.98 b.d.l. 0.15 0.61 0.22 0.02 0.20
N23B-1 10_1F1 Grt OR 38.57 0.05 21.52 0.08 28.24 0.37 4.17 8.96 0.03 b.d.l. 101.98 12 2.97 1.95 — — 0.05 1.76 0.02 0.48 0.74 — — 7.96 0.01 0.16 0.59 0.22 0.02 0.21
N23B-3 1_1_I2 Grt C 37.76 0.62 20.71 0.04 25.60 4.40 2.09 9.86 0.06 b.d.l. 101.14 12 2.97 1.92 0.04 — 0.05 1.63 0.29 0.24 0.83 0.01 — 7.98 0.10 0.08 0.54 0.25 0.02 0.13
N23B-3 1_34_I2 Grt IR 37.92 0.02 21.35 b.d.l. 28.75 0.46 4.06 8.13 b.d.l. 0.02 100.71 12 2.97 1.95 — — 0.04 1.82 0.03 0.47 0.68 — — 7.96 0.01 0.16 0.61 0.20 0.02 0.20
SC RI PT
N23B-1 8_1F2 Grt C 37.32 0.11 20.82 0.05 25.64 3.55 1.70 10.67 0.01 b.d.l. 99.84 12 2.97 1.95 0.01 — 0.05 1.65 0.24 0.20 0.91 — — 7.97 0.08 0.07 0.55 0.28 0.02 0.11
NU
N23B-1 40_2_I1 Grt C 37.85 0.16 21.35 b.d.l. 27.87 3.14 2.17 9.29 0.03 b.d.l. 101.85 12 2.96 1.95 0.01 — 0.04 1.77 0.21 0.25 0.77 — — 7.96 0.07 0.08 0.59 0.24 0.02 0.12
MA
Sample Anal no. Mineral Site SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XSps XPrp XAlm XGrs XAdr XMg
N23B-3 1_39_I2 Grt OR 38.15 0.06 21.44 b.d.l. 27.73 0.40 3.97 9.57 0.08 b.d.l. 101.41 12 2.95 1.94 — — 0.05 1.73 0.03 0.45 0.79 0.01 — 7.96 0.01 0.15 0.58 0.24 0.03 0.21
N40B-4 5 _I4 GrtA C 37.85 0.25 20.81 b.d.l. 26.02 3.62 2.56 9.27 0.04 b.d.l. 100.42 12 2.99 1.94 0.01 — 0.05 1.67 0.24 0.30 0.78 0.01 — 8.00 0.08 0.10 0.56 0.24 0.02 0.15
N40B-4 30_I4 GrtA C 38.06 0.25 20.81 0.02 25.66 4.06 2.48 9.47 0.05 b.d.l. 100.86 12 3.00 1.93 0.02 — 0.05 1.64 0.27 0.29 0.80 0.01 — 8.00 0.09 0.10 0.55 0.24 0.03 0.15
N40B 35_I4 GrtA OR 38.03 0.11 21.17 b.d.l 27.49 0.36 4.20 9.17 0.04 b.d.l 100.5 12 2.97 1.94 0.01 — 0.06 1.73 0.02 0.49 0.76 0.01 — 7.97 0.01 0.16 0.58 0.23 0.03 0.21
AC
CE
PT
ED
Calculation of Fe3+ in Grt after Ravna & Terry (2004) and in pyroxenes, phengite, amphiboles, epidote and chlorite are bas abbreviations are after Whitney and Evans (2010); other abbreviations used: C/IR/OR/R: Core/Inner Rim/Outer Rim/Rim; I/M: In Porphyroblast/Vein; Fol/Cor: Foliation domain/Corona; Garnet and Pyroxene end members are in mole fraction; X Mg= Mg/( b.d.l: below detection limit.
Table 1. Continued Sample Anal no. Mineral Site SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen
N40B-4 P114_I3 GrtB Edge 38.81 0.04 21.81 b.d.l. 27.11 0.32 3.87 9.16 b.d.l. b.d.l. 101.11 12
N40B-4 23_I3 GrtB OR 38.30 0.04 20.95 0.08 28.52 0.31 4.32 8.41 0.05 b.d.l. 100.99 12
N40B-4 66_I3 GrtB OR 37.89 0.07 20.82 b.d.l. 27.47 0.34 4.20 8.72 b.d.l. b.d.l. 99.53 12
N40B-4 P96_I3 GrtB OR 38.43 0.01 21.23 b.d.l. 27.00 0.43 4.30 8.64 0.03 b.d.l. 100.06 12
N40B-4 15_I3 GrtB OR 38.27 0.04 21.13 b.d.l. 26.42 0.38 4.16 9.02 b.d.l. 0.02 99.44 12
N40B-6 P3_I3 GrtB C 37.08 0.20 20.19 b.d.l. 26.00 4.72 2.10 9.09 0.09 b.d.l. 99.46 12
N40B-6 P56_I3 GrtB IR 37.19 0.17 20.00 b.d.l. 28.28 0.43 3.88 8.34 0.02 b.d.l. 98.31 12
N40B-6 58_I3 GrtB IR 37.73 0.04 20.82 b.d.l. 26.48 0.49 4.25 9.16 b.d.l. b.d.l. 98.96 12
N40B-6 P61_I3 GrtB OR 37.69 0.07 20.38 b.d.l. 26.35 0.43 4.22 9.50 0.01 b.d.l. 98.63 12
N22F 69_F2 Grt C 37.47 0.22 20.16 b.d.l. 21.05 11.27 1.81 7.77 0.10 0.01 99.86 12
N22F 53_F2 Grt OR 37.63 0.06 20.88 0.02 23.29 6.23 2.54 8.11 0.10 b.d.l. 98.85 12
N22F 3_I1 Grt I(R)^Ttn 37.25 0.35 20.77 b.d.l. 26.21 7.78 3.02 5.34 0.06 b.d.l. 100.78 12
ACCEPTED MANUSCRIPT 2.98 1.92 — — 0.07 1.78 0.02 0.50 0.70 0.01 — 7.98 0.01 0.17 0.59 0.20 0.04 0.22
2.99 1.93 — — 0.06 1.75 0.02 0.49 0.74 — — 7.99 0.01 0.16 0.58 0.21 0.03 0.22
3.01 1.97 — — 0.03 1.74 0.03 0.50 0.73 — — 8.00 0.01 0.17 0.58 0.23 0.02 0.22
3.02 1.97 — — 0.03 1.72 0.03 0.49 0.76 — — 8.00 0.01 0.16 0.57 0.24 0.01 0.22
2.97 1.90 0.01 — 0.09 1.65 0.32 0.25 0.78 0.01 — 7.99 0.11 0.08 0.55 0.22 0.04 0.13
2.99 1.89 0.01 — 0.10 1.79 0.03 0.46 0.72 — — 7.99 0.01 0.15 0.60 0.19 0.05 0.21
2.99 1.94 — — 0.06 1.69 0.03 0.50 0.78 — — 7.99 0.01 0.17 0.56 0.23 0.03 0.23
SC RI PT
3.01 2.00 — — — 1.76 0.02 0.45 0.76 — — 8.00 0.01 0.15 0.59 0.25 0.00 0.20
NU
Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XSps XPrp XAlm XGrs XAdr XMg
3.00 1.91 — — 0.09 1.66 0.03 0.50 0.81 — — 7.99 0.01 0.17 0.55 0.23 0.04 0.23
3.00 1.91 0.01 — 0.07 1.35 0.77 0.22 0.67 0.02 — 8.00 0.26 0.07 0.19 0.45 0.04 0.14
3.02 1.99 — — 0.01 1.57 0.43 0.31 0.70 0.02 — 8.00 0.14 0.10 0.23 0.52 0.00 0.16
2.96 1.93 0.02 — 0.05 1.68 0.52 0.35 0.45 0.01 — 7.96 0.17 0.12 0.13 0.56 0.03 0.17
Table 2. Representative mineral chemical analysis of pyroxenes in eclogite from the Nagaland Ophiolite Complex, India. N23B-1 9_F1 Omp I^Grt (R) 55.80 0.11 8.01 0.01 8.57 b.d.l. 7.93 13.68 6.45 0.01 100.57 6 2.00 0.34 — — 0.10 0.16 — 0.42 0.53 0.45 — 4.00 0.55 0.35 0.10 0.73
MA
N23B-1 10_1F2 Agt I^Grt (C) 53.62 0.04 4.50 0.03 11.01 0.17 9.10 16.20 4.82 0.01 99.49 6 1.98 0.20 — — 0.20 0.14 0.01 0.50 0.64 0.34 — 4.00 0.65 0.16 0.19 0.78
ED
N23B-1 7_1F2 Omp I^Grt (C) 53.67 0.08 4.26 0.01 10.17 0.26 9.67 16.71 4.17 b.d.l. 99.00 6 1.99 0.19 — — 0.13 0.19 0.01 0.53 0.66 0.30 — 4.00 0.70 0.17 0.13 0.74
PT
N23B-1 15_F1 Omp I^Grt (C) 54.89 0.04 5.32 b.d.l. 10.85 0.13 8.73 16.15 5.05 0.02 101.16 6 1.99 0.23 — — 0.15 0.18 — 0.47 0.63 0.35 — 4.00 0.64 0.21 0.15 0.73
CE
Site SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XQuad XJd XAeg XMg
N23B-1 14_F1 Omp I^Grt (C) 52.57 0.01 5.53 0.01 12.95 0.19 9.30 14.47 4.38 0.08 99.48 6 1.95 0.24 — — 0.18 0.22 0.01 0.51 0.57 0.31 — 4.00 0.67 0.16 0.16 0.70
AC
Sample Anal no. Mineral
N23B-1 53_1F2 Omp I^Grt (R) 55.59 0.16 8.58 0.03 8.42 0.03 7.26 12.76 6.83 b.d.l. 99.66 6 2.01 0.37 — — 0.09 0.17 — 0.39 0.49 0.48 — 3.99 0.52 0.39 0.09 0.70
N23B-1 13_4F1 Omp
N23B-1 16_4F1 Omp
P(C)
P(R)
55.81 0.08 6.68 0.04 10.47 0.31 7.39 11.82 7.39 0.01 99.99 6 2.02 0.28 — — 0.20 0.12 0.01 0.40 0.46 0.52 — 4.00 0.48 0.30 0.21 0.77
55.51 0.15 5.78 b.d.l. 11.31 0.23 8.03 12.06 6.94 0.01 100.03 6 2.01 0.25 — — 0.21 0.13 0.01 0.43 0.47 0.49 — 4.00 0.51 0.26 0.23 0.99
N23B-1 59 _IF2 Omp Relics^ Brs(P) 54.45 b.d.l. 7.97 0.04 7.75 b.d.l. 8.45 13.93 6.16 0.02 98.76 6 1.98 0.34 — — 0.13 0.11 — 0.46 0.54 0.44 — 4.00 0.56 0.32 0.12 0.81
N40B-4 4_I4 Omp I^GrtA (C) 54.90 0.06 6.04 0.06 9.61 0.18 8.66 15.09 5.62 b.d.l. 100.21 6 1.99 0.26 — — 0.15 0.14 0.01 0.47 0.59 0.40 — 4.00 0.60 0.25 0.15 0.77
N40B-4 29_I4 Omp I^GrtA (C) 54.69 0.05 6.46 0.01 10.10 0.15 8.45 14.97 5.85 0.01 100.74 6 1.97 0.28 — — 0.19 0.12 0.01 0.45 0.58 0.41 — 4.00 0.58 0.24 0.18 0.80
N40B40_I4 Omp I^GrtA (R) 54.19 0.08 8.37 0.05 9.57 0.13 7.36 12.99 6.77 0.01 99.50 6 1.97 0.36 — — 0.18 0.11 — 0.40 0.51 0.48 — 3.99 0.52 0.31 0.17 0.79
ACCEPTED MANUSCRIPT
Table 2. Continued N40B-4 42_I3 Omp
Fol(C)
Fol(R)
54.94 0.01 8.16 b.d.l. 6.81 b.d.l. 8.64 14.45 5.94 0.03 98.98 6 2.00 0.35 — — 0.08 0.13 — 0.47 0.56 0.42 — 4.00 0.58 0.34 0.08 0.79
55.23 0.10 7.93 0.06 7.17 0.08 8.43 14.37 6.01 b.d.l. 99.37 6 2.00 0.34 — — 0.07 0.14 — 0.46 0.56 0.42 — 3.99 0.58 0.35 0.08 0.76
N40B-4 84_I3 Agt V(+Lws)^ GrtB 53.23 0.06 3.79 b.d.l. 19.92 0.33 3.81 6.77 9.47 b.d.l. 97.38 6 2.01 0.17 — — 0.50 0.13 0.01 0.21 0.27 0.69 — 4.00 0.31 0.17 0.52 0.63
N40B-4 23_I4 Agt V(+Ab)^ GrtA 53.74 0.22 4.27 0.02 21.82 0.33 2.23 4.84 10.97 0.01 98.44 6 2.00 0.19 0.01 — 0.59 0.10 0.01 0.12 0.19 0.79 — 4.00 0.21 0.19 0.60 0.57
N40B-4 74_I3 Agt V^MOmp 53.92 0.03 5.32 0.01 16.25 0.25 4.63 7.91 9.21 0.03 97.56 6 2.01 0.23 — — 0.41 0.10 0.01 0.26 0.32 0.67 — 4.00 0.33 0.24 0.42 0.73
SC RI PT
N40B-4 41_I3 Omp
NU
N40B-4 14_I3 Omp I^GrtB (OR) 54.67 b.d.l. 7.77 0.07 8.70 0.06 7.73 13.77 6.25 b.d.l. 99.01 6 2.00 0.33 — — 0.12 0.15 — 0.42 0.54 0.44 — 4.00 0.56 0.33 0.12 0.74
MA
Site
N40B-6 11_I3 Omp I^GrtB (OC) 54.78 0.10 7.51 b.d.l. 8.57 0.20 8.08 14.38 6.08 0.02 99.72 6 1.99 0.32 — — 0.13 0.13 0.01 0.44 0.56 0.43 — 4.00 0.57 0.30 0.13 0.77
N40B-6 32_I3 Omp I^GrtB (IR) 54.34 0.08 8.40 b.d.l. 8.99 0.19 7.12 13.18 6.71 0.01 99.02 6 1.98 0.36 — — 0.15 0.13 0.01 0.39 0.52 0.47 — 4.00 0.52 0.33 0.14 0.75
N40B-6 39_I3 Omp I^GrtB (IR) 54.82 0.05 9.25 b.d.l. 8.58 0.10 7.27 12.56 6.92 b.d.l. 99.54 6 1.98 0.39 — — 0.13 0.13 — 0.39 0.49 0.48 — 4.00 0.51 0.37 0.12 0.75
N40B-6 48_I3 Omp Fol 54.55 0.01 7.63 0.01 9.31 0.02 7.75 13.37 6.55 0.03 99.23 6 1.98 0.33 — — 0.17 0.12 — 0.42 0.52 0.46 — 4.00 0.53 0.30 0.16 0.78
CE
PT
SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XQuad XJd XAeg XMg
N40B-4 65_I3 Omp I^GrtB (OR) 55.23 0.06 9.21 0.02 7.90 0.07 7.46 12.58 6.93 b.d.l. 99.46 6 1.99 0.39 — — 0.10 0.14 — 0.40 0.49 0.49 — 4.00 0.51 0.38 0.10 0.75
ED
Sample Anal no. Mineral
AC
Table 3. Representative mineral chemical analysis of phengite, epidote, amphiboles, chlorite, sphene, lawsonite, albite and Nagaland Ophiolite Complex, India. Sample Anal no. Mineral
N23B-1 103_I1 Ph
N23B-1 56_2_I2 Ph
N40B-4 26 _I4 Ph
N40B-4 67_I3 Ph
N22F 44_I1 Ph
N22F 1_I4 Ep
N23B-1 2_4F1 Ep1
N40B-4 3_I4 Ep1
N40B-4 29_I3 Ep2
N40B-6 49_I3 Ep2
N23B-1 42_2_I2 Brs
N40B-4 93_I3 Brs
Site
I^Grt(R)
M
I^GrtA(C)
M(C)
M
I^Omp
I^Grt(C)
I^Grt(C)
Fol
I^Brs(P)
Fol
P
SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen
53.57 0.10 23.25 0.18 5.06 0.05 4.13 0.04 0.17 10.93 97.48 11
52.12 0.25 23.25 0.02 3.74 b.d.l. 4.22 0.01 0.14 11.12 94.87 11
51.85 0.07 22.11 b.d.l. 5.32 0.15 3.49 0.08 0.12 11.09 94.28 11
52.66 0.43 26.03 b.d.l. 3.87 b.d.l. 3.18 b.d.l. 0.67 10.49 97.33 11
55.59 0.02 21.86 b.d.l. 2.55 0.09 5.87 b.d.l. 0.12 11.60 97.70 11
38.13 0.17 26.62 0.13 8.35 0.21 0.11 23.23 0.01 0.00 96.95 12.50
37.80 0.07 23.86 0.01 11.51 0.24 0.04 22.78 b.d.l. b.d.l. 96.31 12.5
38.27 0.04 24.03 0.02 11.50 0.31 0.03 23.37 0.05 b.d.l. 97.62 12.5
38.06 0.16 24.46 b.d.l. 9.51 0.11 0.04 23.77 0.02 b.d.l. 96.13 12.5
38.26 0.06 25.13 b.d.l. 8.74 0.18 0.01 24.16 b.d.l. b.d.l. 96.54 12.5
51.70 0.17 7.43 b.d.l. 10.72 0.16 14.99 9.42 2.85 0.29 97.70 23
50.97 0.12 6.80 b.d.l. 12.89 0.15 13.36 8.18 3.22 0.20 95.88 23
ACCEPTED MANUSCRIPT 3.54 1.81 0.01 0.01 0.05 0.23 — 0.41 — 0.02 0.92 6.99 0.64 –
3.53 1.86 0.01 — — 0.21 — 0.43 — 0.02 0.96 7.02 0.67 –
3.57 1.79 — — — 0.31 0.01 0.36 0.01 0.02 0.97 7.03 0.46 –
3.46 2.02 0.02 — — 0.21 — 0.31 — 0.09 0.88 6.99 0.59 –
3.63 1.68 — — — 0.14 0.01 0.57 — 0.02 0.97 7.02 0.80 –
2.99 2.46 0.01 0.01 0.52 0.03 0.01 0.01 1.95 0.00 0.00 8.00 – 0.17
3.01 2.24 b.d.l. b.d.l. 0.76 0.01 0.02 b.d.l. 1.94 b.d.l. b.d.l. 7.99 – 0.25
3.01 2.23 — — 0.75 0.01 0.02 — 1.97 0.01 — 8.00 – 0.25
3.05 2.31 0.01 — 0.44 0.20 0.01 0.01 2.04 — — 8.07 – 0.16
3.03 2.34 — — 0.57 0.01 0.01 — 2.05 — — 8.01 – 0.20
7.34 1.24 0.02 — 0.22 1.06 0.02 3.17 1.43 0.78 0.05 15.33 0.75 –
7.42 1.17 0.01 — 0.30 1.27 0.02 2.90 1.28 0.91 0.04 15.31 0.70 –
NU
SC RI PT
Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XMg XPs
I^Grt(C)
COR
55.81 b.d.l. 7.21 0.13 15.88 0.21 9.33 1.90 6.30 0.01 96.79 23 7.95 1.21 — 0.01 0.39 1.51 0.03 1.98 0.29 1.74 — 15.11 0.57 –
57.52 0.01 6.22 b.d.l. 14.52 0.01 11.23 0.96 6.81 b.d.l. 97.27 23 8.00 1.02 — — 0.81 0.88 — 2.33 0.14 1.84 — 15.01 0.73 –
ED
N23B-1 50_I2 Gln2
PT
SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XMg XPs
N23B-1 12_I1 Gln1
CE
Site
N40B-4 101_I3 Fwn V(+Lws) ^Brs 52.88 0.03 1.41 b.d.l. 25.59 0.57 7.21 2.88 5.19 0.04 95.79 23 7.95 0.25 — — 1.17 2.05 0.07 1.62 0.46 1.51 0.01 15.10 0.44 –
AC
Sample Anal no. Mineral
N40B-2 72_I3 Gln2
N22F 101_I2 Gln1
N22F 63_I2 Gln2
N23B-1 38_I2 Chl
70_I1 Chl
N40B-4 6_I4 Spn1
N23B-1 85_I1 Spn2
COR
I^Agt
Cor
Cor
Cor
I^Grt(C)
Cor
56.63 0.05 7.67 0.06 12.79 0.21 11.17 1.85 6.08 0.02 96.53 23 7.97 1.27 0.01 0.01 0.31 1.20 0.03 2.34 0.28 1.66 — 15.06 0.66 –
57.58 0.02 9.79 0.02 11.09 0.24 11.52 1.09 7.13 0.02 98.49 23 7.87 1.58 — — 0.30 0.97 0.03 2.35 0.16 1.89 — 15.13 0.71 –
57.95 b.d.l. 8.80 b.d.l. 10.71 0.16 12.05 1.28 6.97 0.05 97.97 23 7.95 1.42 — — 0.28 0.95 0.02 2.46 0.19 1.85 0.01 15.13 0.72 –
28.37 0.02 18.48 b.d.l. 18.33 0.61 21.65 0.04 0.05 0.01 87.56 14 2.88 2.21 — — 0.03 1.53 0.05 3.28 0.01 0.01 — 10.00 0.68 –
29.06 0.05 18.97 0.00 14.75 0.33 24.86 0.05 0.00 0.00 88.09 14.00 2.87 2.21 — — 0.04 1.18 0.03 3.66 0.01 0.00 0.00 10.00 0.76 –
30.10 38.43 1.07 0.04 1.12 0.14 b.d.l. 28.35 0.03 0.02 99.30 5 0.99 0.04 0.95 — — 0.03 — — 1.00 — — 3.02 – –
30.70 38.25 1.43 b.d.l. 0.54 0.11 0.01 29.45 0.06 0.01 100.57 5 1.00 0.05 0.94 — — 0.01 — — 1.03 — — 3.03 – –
MA
Table 3. Continued
N22F
ACCEPTED MANUSCRIPT
ts of thermobarometry of peak eclogite facies metamorphism in the NOC. Grt
Cpx
Simultaneous solution of Grt-Cpx thermometry and Grt-Cpx-Phe barometry
Phe
Thermocalc average Cpx-Ph ass
t)
z(grt)
f(grt)
m(grt)
x(cpx)
j(cpx)
f(cpx)
Si(mu)
x(mu)
y(mu)
na(mu)
PRT , TRAV
PTC , TTC
PTC(Av.)(SdP),TTC(Av.)(S
0
0.24, ¹0.24
0.02
0.00
0.30
0.48
0.19
3.54
0.36
0.39
0.02
29.0±2, 650±60
28.7±3.7, 665±110
29.8±3.7, 685±70
29.1±3.7, 665±110
29.2±3.5, 650±65
0.24, ¹0.24
0.02
0.00
0.30
0.48
0.19
3.53
0.33
0.39
0.02
0
0.24, ¹0.24
0.02
0.00
0.27
0.45
0.23
3.54
0.36
0.39
0.02
27.3±2, 600±60
27.1±3.5, 615±90
29.3±3.6, 675±65
0
0.24, ¹0.24
0.02
0.00
0.27
0.45
0.23
3.53
0.33
0.39
0.02
27.6±2, 605±60
27.5±3.5, 620±90
28.7±3.5, 645±60
0.39
0.02
28.5±2, 640±60
28.1±3.4, 655±100
30.5±3.7, 710±70
0.33 0.39 Weighted average
0.02
28.8±2, 640±60
28.5±3.4, 660±100 28.1±2.8, 645±80
29.7±3.5, 675±65
28.4±1.6, 630±50 0.09
26.5±2, 675±60
26.0±3.3, 695±110
28.6±3.5, 770±75
0.09
25.6±2, 605±60
25.0±3.6, 625±100
30.2±4.7, 735±85
0.09
26.6±2, 645±60
26.0±3.1, 660±100
28.9±3.3, 765±75
9
0.25, ¹0.25
0.02
0.01
0.27
0.45
0.23
3.54
9
0.25, ¹0.25
0.02
0.01
0.27
0.45
0.23
3.53
0.36
8
0.24, ¹0.25
0.02
0.01
0.29
0.45
0.22
3.46
0.41
0.49
8
0.23, ¹0.24
0.04
0.01
0.26
0.51
0.25
3.46
0.41
0.49
8
0.26, ¹0.26
0.01
0.01
0.26
0.44
0.26
3.46
0.41
0.49
8
0.25, ¹0.25
0.03
0.01
0.25
0.49
0.21
3.46
0.41
0.49
Weighted average
7
0.26, ¹0.26
0.03
0.01
0.25
0.48
0.25
7
0.26, ¹0.26
0.03
0.01
0.25
0.47
0.30 Weighted average
0.15, ¹0.18
0.03
0.17
0.23
0.50
0.38
3.63
0.20
0.31
3
0.15, ¹0.18
0.03
0.17
0.24
0.46
0.41
3.63
0.20
0.31
4
0.16, ¹0.19
0.02
0.16
0.24
0.46
0.41
3.63
0.20
0.09
0.31
Weighted average
25.6±2, 605±60
24.8±3.4, 620±100
29.7±4.6, 739±90
25.5±3.3, 650±100
29.2±3.8, 755±80
²635±60
²645±95 ²640±90 ²640±130
0.02
²630±85 24.2±2, 550±60
24.1±4.8, 550±80
0.02
23.6±2, 565±60
23.4±4.4, 570±80
0.02
29.5±2.9, 670±50
26.1±2.0, 635±60 ²625±60
NU
3
SC RI PT
0
29.4±2, 650±60
24.0±2, 540±60 23.9±2.3, 550±70
24.0±4.3, 545±75 23.8±5.1, 555±90 2+
2+
#
24.1±4.2, #475±80
#
22.9±4.1, #470±80
#
23.1±3.6, #455±75 23.3±4.5, 465±90
AC
CE
PT
ED
MA
used: Chemical compositions of garnet (grt), clinopyroxene (cpx) and phengite (mu), x(mineral)=Fe /(Fe +Mg); z(grt)=Ca/(Ca+M m(grt)=Mn/(Ca+Mg+Fe2++Mn); j(cpx)=Na(total); f(cpx)=Fe3+/(Fe3++Al(VI)); y(mu)=XAlM2A; na (mu)=XNaA ; TRAV/PRT , temperatur ed using Grt-Cpx thermometry of Ravna (2000) & Grt-Cpx-Ph barometry of Ravna and Terry (2004); TTC/PTC: temperature (⁰C)/ g Grt-Cpx thermometric and Grt-Cpx-Ph barometric reactions calculated by using Thermocalc software (v.3.31); T TC(Av)/PTC(Av): Therm C)/pressure (kbar) calculated using Thermocalc software (v.3.31) (see text for details); #Average P-T computations with Grt activity mo , z(grt)=Ca/(Ca+Mg+Fe2+); 2,temperature calculated at a PRef=26 kbar.
ACCEPTED MANUSCRIPT
Table 5: Calculations of effective bulk rock compositions of two representative chemical domains in sample N40B NOC. XMapTools Data
Mole %
52.44±1.57 0.11±0.07 5.97±1.96 — 15.09±3.01 0.22±0.15 12.31±1.39 6.81±2.09 3.88±0.83 0.18±0.08 —
50.31±0.88 0.24±0.08 12.96±1.66 0.03±0.03 12.77±2.52 0.19±0.17 6.29±0.93 10.76±1.26 4.04±0.58 1.38±0.05 —
52.56 0.19 7.98 0.00 11.27 0.17 9.79 12.04 4.09 0.92 0.99
97.00±4.73
98.97±3.70
100.00
Calculated Bulk
Mole%
48.50±0.45 0.37±0.22 13.57±0.67 0.02±0.04 11.97±0.84 0.36±0.26 6.40±0.39 13.77±0.47 3.34±0.21 0.71±0.07 — 99.01±1.56
50.48 0.29 8.34 0.00 10.52 0.32 9.93 15.36 3.37 0.47 0.92 100.00
Grt
Ep
Ttn
Ph
Omp
Brs
17.04 4.00 0.34 15.40 57.11 6.11 100.00
SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O O2
38.07±0.46 0.07±0.05 20.82±0.34 0.02±0.03 27.49±0.88 0.53±0.17 3.88±0.32 8.80±0.46 0.04±0.03 0.00±0.01 —
38.40±0.30 0.09±0.03 24.77±0.64 0.04±0.04 10.11±1.03 0.06±0.06 0.10±0.11 23.40±0.50 0.03±0.02 0.00±0.00 —
30.96±0.09 36.92±1.64 2.09±0.97 — 0.86±0.47 0.04±0.06 — 29.09±0.11 0.02±0.03 0.02±0.01 —
52.84±1.25 0.34±0.12 25.12±1.80 0.01±0.02 3.96±0.21 0.03±0.03 3.56±0.51 0.01±0.02 0.45±0.22 10.68±0.29 —
54.78±0.93 0.07±0.07 7.40±2.13 0.03±0.04 9.65±3.68 0.12±0.22 7.65±1.25 13.20±1.80 6.56±0.87 0.01±0.01 —
Total
99.73±1.20
97.00±1.35
100.00±1.97
96.99±2.30
99.46±4.96
N40B-6
SC RI PT
Mineral
N40B-6
Sample
Average mineral composition(EPMA)
Mineral
Grt
Ep
Ttn
13.48 20.94 0.62 7.81 37.39 19.76 100.00
SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O O2 Total
37.80±0.45 0.24±0.39 20.29±0.38 0.02±0.03 27.01±1.17 1.97±1.42 3.19±0.66 9.38±0.52 0.05±0.06 0.05±0.19 — 100±2.15
38.60±0.32 0.23±0.64 24.70±0.65 0.04±0.08 10.34±0.92 0.10±0.08 0.08±0.08 24.38±0.28 0.03±0.05 0.01±0.01 — 98.50±1.37
29.85±0.15 39.00±0.33 1.26±0.10 0.04±0.05 0.72±0.29 0.01±0.01 — 29.05±0.16 0.05±0.03 0.02±0.01 — 100.00±0.51
PT CE
Ph
Omp
Brs
51.92±1.93 0.35±0.25 23.72±2.73 0.01±0.02 4.27±0.16 0.06±0.04 3.89±0.80 0.07±0.10 0.43±0.27 11.28±0.50 — 96.00±3.50
54.96±0.34 0.07±0.04 7.93±0.67 0.02±0.03 8.48±0.99 0.08±0.07 8.08±0.45 14.13±0.65 6.26±0.35 0.01±0.01 — 100.00±1.52
51.24±0.38 0.15±0.04 6.67±0.21 0.03±0.05 13.53±0.30 0.11±0.08 13.41±0.18 8.35±0.29 3.27±0.14 0.24±0.03 — 97.00±0.65
NU
Mode
AC
Grt Ep Spn Ph Omp Brs Total
Calculated Bulk
Mode
XMapTools Data Phases
Average mineral composition(EPMA)
MA
Grt Ep Spn Ph Omp Brs Total
N40B-4
Sample
ED
Phases
N40B-4
ACCEPTED MANUSCRIPT Table 6: SHRIMP U-Pb analyses of representative zircon grains in Mokie and Thewati eclogites from the Nagaland Ophiolite Complex ²⁰⁴Pb /²⁰⁶Pb
% comm 206 Pb
4corr ²⁰⁷Pb* /²⁰⁶Pb*
% err
4corr ²⁰⁷Pb* /²³⁵U
% err
4corr ²⁰⁶Pb* /²³⁸U
% err
204corr ²⁰⁶Pb* /²³⁸U Age(Ma)
31.23
4.6E-4
0.84
.0477
5.5
0.177
5.7
.0269
1.4
171
0.15
35.05
7.0E-4
1.28
.0701
5.8
0.519
6.0
.0536
1.5
337
66
0.03
60.67
5.0E-4
0.92
.0476
4.9
0.212
5.1
.0322
1.4
204
1267
30
0.02
40.33
2.1E-4
0.38
.0600
7.5
0.305
7.6
.0369
1.4
234
Zrn-13(O)
1110
36
0.03
26.37
9.0E-4
1.64
.0446
9.3
0.167
9.4
.0272
1.6
173
Zrn-22(N1)
1705
13
0.01
46.00
2.3E-4
0.42
.0491
2.9
0.212
3.3
.0313
1.6
199
Zrn-28(N1)
1380
271
0.20
52.09
3.1E-4
0.57
.0528
5.2
0.318
5.5
.0437
1.6
276
Zrn-33(N1)
1091
94
0.09
41.31
1.4E-3
2.58
.0566
13.3
0.335
13.4
.0429
1.8
271
Zrn-35(N1)
1511
88
0.06
44.94
8.2E-4
1.51
.0496
7.9
0.233
8.1
.0341
1.7
216
Zrn-36(N1)
2387
15
0.01
63.24
1.0E-3
1.90
.0495
7.9
0.207
8.1
.0302
1.6
192
Zrn-40(N1)
1197
10
0.01
33.80
6.8E-4
1.24
.0565
6.4
0.253
6.6
.0325
1.7
206
Zrn-45(N1)
1939
24
0.01
50.06
3.5E-4
0.65
.0480
3.4
0.198
3.8
.0299
1.7
190
Zrn-46(N1)
2478
8
0.003
63.27
1.1E-4
0.19
.0501
1.8
0.205
2.4
.0297
1.6
188
Zrn-47(N1)
1469
40
0.03
38.31
1.9E-3
3.57
.0591
11.6
0.238
11.8
.0293
1.8
186
Zrn-49(N1)
1561
29
0.02
57.64
1.3E-4
0.24
.0555
3.4
0.328
3.8
.0429
1.7
271
Zrn-50(N1)
2051
36
0.02
52.05
1.8E-4
0.34
.0497
2.8
0.202
3.2
.0294
1.6
187
Zrn-55(N1) Sample N23B
1765
163
0.09
61.25
3.5E-3
6.49
.0553
16.6
0.288
16.7
.0378
1.7
239
Zrn-10
106
0.88
0.01
3.03
4.8E-3
8.77
.0339
91.2
0.142
91.4
.0305
5.6
194
Zrn-16
97
0.85
0.01
2.48
1.2E-3
2.11
.0488
21.1
0.196
21.3
.0291
2.9
185
Zrn-21
45
0.16
0.004
1.19
7.7E-3
14.05
.0694
84.9
0.253
85.3
.0265
7.8
168
Zrn-22
200
0.57
0.003
6.22
1.2E-2
22.72
‒
‒
‒
‒
.0280
5.3
178
Zrn-23
92
0.24
0.003
2.98
1.4E-2
25.27
‒
‒
‒
‒
.0281
7.0
178
Zrn-34
136
0.39
0.003
3.78
6.7E-4
1.22
.0479
19.0
0.212
19.3
.0320
3.5
203
Zrn-46
61
0.002
1.97
6.5E-3
11.20
.0756
65.2
0.347
65.6
.0333
7.0
211
²⁰⁴Pb /²⁰⁶Pb
% comm 206
4corr ²⁰⁷Pb* /²⁰⁶Pb*
% err
4corr ²⁰⁷Pb* /²³⁵U
% err
4corr ²⁰⁶Pb* /²³⁸U
% err
204corr ²⁰⁶Pb* /²³⁸U Age(Ma)
U (ppm)
Th (ppm)
Th/U
²⁰⁶Pb* (ppm)
Zrn-8(O)
1340
27
0.02
Zrn-9(O)
751
113
Zrn-10(O)
2172
Zrn-11(O)
Spot No.
NU
MA
ED
PT
CE
AC
Table 6: contd.
0.14
SC RI PT
Sample N22F
U (ppm)
Th (ppm)
Th/U
²⁰⁶Pb* (ppm)
57
0.18
0.003
1.84
6.9E-3
12.72
.0235
79.5
0.106
79.6
.0328
4.1
208
Zrn-53
126
0.23
0.002
3.43
1.0E-3
1.87
.0408
29.2
0.175
29.4
.0311
3.7
198
Zrn-54
55
0.12
0.002
1.52
3.6E-3
6.60
‒
‒
‒
‒
.0298
5.2
189
Zrn-62
59
0.11
0.002
1.70
6.4E-3
11.80
‒
‒
‒
‒
.0297
5.5
189
Zrn-64
42
0.15
0.004
1.32
1.5E-2
27.27
‒
‒
‒
‒
.0266
10.8
169
Zrn-66
29
0.04
0.001
0.88
7.9E-3
14.41
‒
‒
‒
‒
.0299
9.7
190
Zrn-66.2
13
0.06
0.004
0.47
2.2E-2
41.12
‒
‒
‒
‒
.0239
27.7
152
Zrn-69
55
0.24
0.004
1.70
7.8E-3
14.28
‒
‒
‒
‒
.0307
6.8
195
Zrn-71
83
0.27
0.003
2.52
1.4E-3
2.66
.0828
19.4
0.391
19.8
.0343
4.0
217
Zrn-76
93
1.62
0.02
2.48
2.4E-3
4.37
.0411
62.6
0.168
62.7
.0297
4.8
188
Spot No.
Zrn-49
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
MA
NU
SC RI PT
Notes: Pb* indicate the radiogenic Pb corrected using measured ²⁰⁴Pb & ²⁰⁷Pb . The “-” means undetected or imponderable.
ACCEPTED MANUSCRIPT Table 7: A summary of characteristics of zircon from two Nagaland eclogite samples Stage-1
Stage-2
Stage-3
Dark luminescent, faintly banded elongated domain in Zrn cores (cf. Type 1 Zrn); Euhedral external shape, often with pyramidal termination
Only in N22F; Irregular bright luminiscent domain that replaces stage 2 Zrn cores (cf. Type 3 Zrn)
Composition
Low Th (0.12-0.27 ppm) and U (55-126 ppm) concentrations in N23B; More uraniferous (15112387 ppm) and thoriferous (15-88 ppm) in N22F; Low Th/U ratios in both sample groups : <0.01 (N23B); 0.01-0.06 (N22F) ~205 Ma
Commonly structure-less dark luminescent core, locally with patchy, irregular zones of bright luminescence (cf. Type 2 Zrn); Change of regular shape of Type-1 Zrn; Bright luminescent narrow rim around Zrn cores; Contains inclusions of Grt, Omp and Rt Low Th (0.05-1.62 ppm) and U (29-200 ppm) concentrations in N23B; uraniferous (1110-2478 ppm) and thoriferous (8-66 ppm) in N-22F; Low Th/U ratios in both sample groups : ≤0.02 (N-23B); ≤0.03 (N22F) ~189-185 Ma
Older 206Pb/238U dates between 337 and 239 Ma; Discordant in concordia plot Localised fluid-mediated alteration of recrystallised Zrn core
Statistically Th-rich (94271) and U-poor (751-1765 ppm) than other textural domains in N22F; High Th/U ratios (0.09-0.20)
MA
Recrystallization/replacement of growth-zoned Zrn due to in situ dissolution–reprecipitation in sub-solidus metamorphic conditions Metamorphic/ Minimum age of protolith Peak eclogite facies Timing unknown; Older Magmatic formation metamorphism at ~189-185 discordant dates context Ma geologically meaningless Mineral abbreviations after Whitney & Evans (2010); Other abbreviations used: CL, Cathodoluminescence; Soln, Solution.
CE
PT
ED
Primary growth due to precipitation from fluid/melt
AC
Origin
NU
Age
SC RI PT
Zrn evolution stages CL Zoning and texture
ACCEPTED MANUSCRIPT Highlights Nagaland eclogites record the oldest evidence for Neo-Tethyan subduction in the YTSZ. Minimum age of Nagaland eclogitic protolith is Late Triassic (≥ c. 205 Ma). Peak eclogite facies metamorphism occurred between 189 and 185 Ma.
SC RI PT
Metamorphic peak P-T conditions range from 25.5-28.1 kbar, ~650 oC to 23.8 kbar, ~555oC.
Eclogites record low thermal gradient (~7-8 oC/km) at metamorphic peak and a CW
AC
CE
PT
ED
MA
NU
P-T path of evolution.
M. Rajkakati, S.K. Bhowmik, A. Ao, T.R. Ireland, J. Avila, G.L. Clarke, A. Bhandari, J.C. Aitchison PII: DOI: Article Number: Reference:
S0024-4937(19)30323-8 https://doi.org/10.1016/j.lithos.2019.105166 105166 LITHOS 105166
To appear in:
LITHOS
Received date: Revised date: Accepted date:
31 January 2019 4 August 2019 5 August 2019
Please cite this article as: M. Rajkakati, S.K. Bhowmik, A. Ao, et al., Thermal history of Early Jurassic eclogite facies metamorphism in the Nagaland Ophiolite Complex, NE India: New insights into pre-Cretaceous subduction channel tectonics within the NeoTethys, LITHOS, https://doi.org/10.1016/j.lithos.2019.105166
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Thermal history of Early Jurassic eclogite facies metamorphism in the Nagaland Ophiolite Complex, NE India: New insights into pre-Cretaceous subduction channel tectonics within the NeoTethys
SC RI PT
M. Rajkakati1, S.K.Bhowmik1,*, A. Ao2, T. R. Ireland3, J. Avila3, G.L. Clarke4, A. Bhandari5, J.C. Aitchison6 1
Department of Geology & Geophysics, Indian Institute of Technology, Kharagpur 721 302,
NU
India
Wadia Institute of Himalayan Geology, Dehradun, Uttarakhand 248001, India
3
Research School of Earth Sciences, Australian National University, Canberra ACT 0200,
MA
2
Australia
School of Geosciences, University of Sydney, Sydney NSW 2006, Australia
5
Geological Survey of India, Faridabad, Haryana 121001, India
6
School of Earth and Environmental Sciences, The University of Queensland, St Lucia,
PT
ED
4
AC
ABSTRACT
CE
Queensland 4072, Australia
Eclogite from two locations in a fossil accretionary complex now exposed in Nagaland, NE India, at the northern end of the Indo-Myanmar ranges, provides the oldest evidence for Neo-Tethyan subduction along the Yarlung-Tsangpo suture zone. Metamorphic texture, mineral composition, peak metamorphic P-T estimates, reconstructed metamorphic P-T paths, and U-Pb zircon ages from three eclogite samples collected near Mokie and Thewati villages collectively reveal snapshots of a c. 205–172 Ma subduction burialexhumation cycle in the eastern Neo-Tethys. The Thewati eclogite records a clockwise
ACCEPTED MANUSCRIPT metamorphic P-T path involving: (1) prograde metamorphism traversing the epidote blueschist facies and culminated in peak eclogite facies conditions at 25-28 kbar and ~650 o
C; (2) an early retrograde stage involving decompression with cooling in the eclogite facies
at ~18.3 kbar and 630 oC; followed by (3) cooling through the epidote blueschist, transitional lawsonite blueschist and greenschist facies to ~6 kbar and 300 oC. Circa 189-185 Ma peak
SC RI PT
metamorphism is inferred for the Mokie and Thewati eclogites, and was accompanied by an apparently low thermal gradient (~7-8 oC/km). Such conditions and the clockwise P-T path are associated with a tectonic scenario involving cold mature subduction within the NeoTethys in Nagaland. This
is thus the first comprehensive dataset of an Early Jurassic
NU
subduction channel (with respect to present geographic co-ordinates) for the eastern margin of the Neo-Tethyan suture. The Nagaland samples offer a critical record of a pre-Cretaceous
MA
subduction system within the Neo-Tethys, which incorporated an Early-Middle Jurassic Andean-type convergent margin extending along the southern Eurasian margin from Pakistan
ED
to the Indo-Myanmar Ranges.
PT
Key words: Eclogite; U-Pb zircon geochronology; Metamorphism; CW P-T Path;
AC
1. Introduction
CE
Subduction; Neo-Tethys; Yarlung-Tsangpo Suture Zone
Much of our understanding of Mesozoic plate tectonics, including the sequential amalgamation of Gondwanan continental fragments that assembled the Eurasian continent, derives from metamorphic and isotopic studies of three Tethyan suture zones that dissect Tibet. These are: (1) the Longmu Co-Shuanghu (LCS); (2) Bangong-Nujiang (BNS); and the (3) Yarlung-Tsangpo (YTS) sutures (Fig.1). They include blueschist (Anczkiewicz et al., 2000; Groppo et al., 2016), eclogite (Zhang, Y-X et al., 2018) and amphibolite facies rocks
ACCEPTED MANUSCRIPT (Guilmette et al., 2012; Wang et al 2008). These rocks provide information critical to understanding tectonic processes linked to the closure of the Palaeo- (Devonian-Triassic), Meso- (Early Permian to Late Cretaceous) and Neo-Tethyan (Late Triassic-Paleogene) oceans (see Zhu et al., 2013 for a review). The Yarlung-Tsangpo suture zone (YTSZ) is the youngest and southernmost of the
SC RI PT
three sutures that dissect Tibet. It is potentially the most cryptic in terms of understanding the timing, duration, and periodicity of the Neo-Tethyan subduction (see Hébert et al., 2012 for a review). Tectonic models incorporating the northward subduction of the Neo-Tethyan oceanic lithosphere beneath southern Tibet (Lhasa continental block) are well established,
NU
covering much of the Cretaceous and extending to the Early Cenozoic collision of India and Asia (e.g. Yin and Harrison, 2000). However, the pre-Cretaceous geodynamic history of the
MA
Neo-Tethyan lithosphere remains uncertain and subject to on-going discussion. Models include the opening of the Neo-Tethyan ocean as a Late Triassic back-arc basin due to the
ED
separation of the Lhasa block from northern Australia (Zhu et al., 2013), and the initiation of its subduction at c.180 Ma (Hébert et al., 2012). The existence of Late Triassic to Early
PT
Jurassic Neo-Tethyan subduction beneath the Lhasa Terrane has also been inferred on the
CE
basis of the age and geochemistry of arc components of the Gangdese magmatic belt (Yu et al., 2018). However, in the absence of direct evidence of related HP/LT metamorphism, the
AC
subduction channel dynamics of such early events within the Neo-Tethyan sutures are not well understood.
The Nagaland Ophiolite Complex (NOC), at the northern extent of the Indo-Myanmar Ranges (IMR) (Fig. 2a), is considered a southeastern extension of the Yarlung-Tsangpo suture. It exposes a rare Neo-Tethyan metamorphic sequence including pumpellyite-diopside, greenschist, blueschist and eclogite facies rocks (Chatterjee and Ghose, 2010; Ao and Bhowmik, 2014; Bhowmik and Ao, 2016) (Fig. 2b). Preliminary chronological constraints
ACCEPTED MANUSCRIPT indicate Middle to Upper Jurassic protolith ages for elements of the metamorphosed oceanic crust (Sarkar et al., 1996; Baxter et al., 2011; Aitchison et al., 2019). The timing and thermal history of eclogite facies metamorphism and its relationship with other HP metamorphites in the terrane are not known (see also Ao and Bhowmik, 2014; Bhowmik and Ao, 2016). Numerous studies indicate that eclogite facies rocks preserve critical evidence for the timing
SC RI PT
and thermal history of oceanic subduction (Agard et al., 2009 and references cited therein). In particular, low to medium temperature oceanic eclogites commonly preserve excellent textural and mineral compositional (e.g. compositional zoning in garnet, omphacite, epidote, amphibole and phengite) features amenable to the near complete reconstructions of
NU
metamorphic P-T paths, including well-constrained prograde histories (e.g. Franciscan highgrade eclogite block, California, Page et al., 2007; lawsonite eclogite pods, Sivrihisar Massif,
MA
Turkey, Davis and Whitney, 2008; lawsonite eclogite, Alpine Corsica, Brovarone et al., 2011; Zermatt-Saas, Western Alps, Reinecke, 1998, Angiboust et al., 2009; Monviso, Alps,
ED
Groppo & Castelli, 2010; Angiboust et al., 2012; Schistes-Lustres, Alps, Agard et al., 2001; eclogites of Pouebo terrane, Pam Peninsula, NE New Caledonia, Clarke et al., 1997;
PT
Dominican Republic, Krebs et al., 2011; Rio San Juan mélange Complex, Krebs et al., 2011).
CE
Moreover, recent analytical and theoretical advances that combine in situ U-Pb dating and trace element compositional analysis of zircon offer unprecedented insights into the
AC
timescales involved in forming high-pressure/ultra-high-pressure (HP/UHP) metamorphic terranes (Rubatto and Hermann, 2007; Rubatto, 2017). The Neo-Tethyan tectonic models for Southeast Asia, in general, are strongly influenced by data from the Tibetan belts with a lack of detail from the IMR and adjoining ophiolite belts in eastern and north-eastern Myanmar. Seen in this context, the Nagaland Ophiolite Complex presents a unique opportunity to: (a) test existing predictions for the existence of a pre-Cretaceous subduction system within the Neo-Tethys (e.g. Zedong terrane
ACCEPTED MANUSCRIPT - McDermid et al. 2002; Aitchison et al. 2007) and (b) determine the thermal and dynamic history of such a subduction zone using well-constrained metamorphic P-T paths from the eclogites. In this investigation, we establish the thermal history of an Early Jurassic NeoTethyan subduction system, considered to be the oldest within the Yarlung-Tsangpo suture zone, using textural and mineral compositional data, metamorphic reaction history, peak
SC RI PT
metamorphic P-T estimates, reconstructed metamorphic P-T path and SHRIMP U-Pb zircon age data from two new eclogites locations in the NOC. Given the importance of these findings in the tectonic frame work of Southeast Asia, we also review the published literature from the ophiolite belts of IMR and eastern and northeastern Myanmar. These new results of
NU
Neo-Tethyan tectonics from Nagaland and adjoining Myanmar are correlated with those from
MA
the YTSZ in the Tibetan sector.
2. Geological setting of the Nagaland eclogites
ED
Eclogite samples investigated in this work come from the Nagaland Ophiolite Complex at the northern end of the Indo Myanmar Ranges. The IMR are located at the junction between two
PT
orogenic systems: the Alpine-Himalayan continental collision belt in the north, and the active
CE
subduction zone of the Sunda–Andaman arc to the south (Fig. 2a). These orogenic systems resulted from the interaction of three tectonic plates: the Indian Plate in the southwest, the
AC
West Myanmar Block (alternatively Myanmar Microplate) in the east, and the Asian Plate to the north and northeast (Fig. 2a) (Searle et al., 2007). The IMR, along the western margin of the West Myanmar Block, is considered an accretionary wedge that developed in response to eastward subduction of the Neo-Tethys (attached to the Indian plate) beneath the West Myanmar Block (Searle et al. 2007; Acharyya, 2015; Aitchison et al., 2019). Ophiolitic rocks, with or without HP metamorphic rocks, in the IMR and the West Myanmar Block occur in three sub-parallel, N-trending ophiolite belts, called the Western
ACCEPTED MANUSCRIPT (WOB), Central (COB) and Eastern Ophiolite Belts (EOB) (terminology after Htay et al. 2017) (Fig. 2a). The WOB is the longest of the three belts and separates the eastern margin of the IMR from the West Myanmar Block. It can be traced for ~3000 km from Mayodia (Arunachal Pradesh) in the north near the eastern Himalayan syntaxis through the Naga and Manipur Hills, Kalaymyo, Chin Hills, Mt. Victoria-Kawlun belt in the middle section to the
SC RI PT
Andaman-Nicobar Islands in the south (Fig. 2a). Ophiolitic sequences in this belt generally occur as small, rootless and tectonised bodies with shallow-dipping foliations in low-grade metamorphic rocks, which are overlain by Eocene–Oligocene sedimentary rocks of flysch affinity (Acharyya, 2007). A Late Jurassic age for ultramafic rocks of the WOB was posited
NU
from a K–Ar age for hornblende-bearing pegmatite (158 ± 20 Ma) that intruded serpentinites in the Mt Victoria area (Mitchell, 1986). However, recent U-Pb dating of zircon indicates
MA
Early Cretaceous protoliths for ophiolitic components in this belt (e.g. c.116-119 Ma Manipur ophiolite, Singh et al, 2017, Aitchison et al., 2019; 127 Ma Kalaymyo ophiolite, Liu et al.,
ED
2016a). The formation of amphibolite-facies rocks in Kalaymyo has been interpreted as a metamorphic sole to the ophiolite and is dated at 115-119 Ma (Zhang et al., 2017). In
PT
summary, the ophiolites of the WOB are thought to mark a suture zone, where the Neo-
CE
Tethyan Ocean once existed (Searle et al., 2007; Liu et al., 2016a). The Central Ophiolite Belt extends from the Putao area of Kachin State in the north
AC
through the Jade Mines Belt (JMB) to the Sagaing-Minwun ranges in the south (Fig. 2a). It is composed of a dismembered, incompletely preserved ophiolitic sequence of serpentinite and peridotite (Htay et al., 2017). The Jade Mines Belt (JMB) includes jadeitite, eclogite, amphibolite, blueschist, chromitite, and serpentinized peridotite (Goffé et al., 2002) that reflect the effects of high-P/low-T metamorphism. The P–T conditions have been variously estimated at 10–15 kbar and 300–500 °C (Mével & Kiénast, 1986), >14 kbar and 400-450 °C (Goffé et al., 2002), >10 kbar and 250–370 °C (Shi et al., 2003) and ~15 kbar and ~380 °C
ACCEPTED MANUSCRIPT (Oberhänsli et al., 2007). U–Pb Jurassic ages of zircon taken from jadeitite (163 – 160 Ma) have been interpreted as protolith ages for the ophiolite in the JMB (Shi et al., 2008; Yui et al., 2013). The timing of the HP metamorphic event is not well defined, with age estimates ranging from the Late Jurassic (U–Pb zircon age between 158 and 147 Ma, Shi et al., 2008; Qiu et al., 2009) to the Late Cretaceous (U–Pb zircon age of 77 ± 3 Ma, Yui et al., 2013 and 40
Ar/39Ar age of 80 Ma for phengite in eclogite, blueschist, jadeitite, and amphibolite,
SC RI PT
an
Goffé et al., 2002). Zhang, J. et al. (2018) recently reinterpreted all published zircon age data from the JMB and concluded that the main HP metamorphism and accretionary orogenesis in this segment of the COB was of Early to Middle Jurassic age.
NU
The Eastern Ophiolite Belt (EOB) is represented by the Tagaung-Myitkyina belt (Fig. 2a) comprising massive peridotite of harzburgite to lherzolite composition, dacite and
MA
andesite, hornblende gabbro, diorite, granodiorite and veins of gabbro, rodingite and plagiogranite in peridotite (Liu et al., 2016b; Zhang, J. et al., 2018). The volcanic and
ED
gabbroic to dioritic rocks have supra-subduction-zone-type chemical signatures (Yang et al., 2012; Xu et al., 2017). Recent U-Pb dating of zircon from a variety of lithologies in the
PT
Tagaung-Myitkyina belt has yielded a Middle Jurassic protolith age (ca. 177-166 Ma, Yang
CE
et al., 2012, Liu et al., 2016a; Gardiner et al., 2018). The EOB has been interpreted as: (a) the southern continuation of the Meso-Tethyan suture in central Tibet (Liu et al., 2016a); (b) a
AC
klippen of the Medial Myanmar Suture Zone that developed along the eastern margin of the Mogok Metamorphic and Slate Belt (Mitchell et al., 2015); and (c) a Jurassic continental margin arc (Zhang, J. et al., 2018). The 200 km long and 5-15 km wide, NNE-trending Nagaland Ophiolite Complex can be distinguished from other ophiolite bodies in the WOB by: (1) the presence of a variety of HP/LT metamorphic rocks in ophiolitic mélange; and (2) its association with a disrupted oceanic plate sequence of chert, pillowed basalt, plagiogranite, mafic, and ultramafic
ACCEPTED MANUSCRIPT cumulate and serpentinite (Fig. 2b) (Acharyya, 1986). Some of its serpentinite has mantle peridotite lineage (Brunnschweiler, 1966). Ophiolitic mélange structurally overlies the Disang Formation (Fig. 2b), which has Indian continent-derived clastic sedimentary rock of Eocene age (Imchen et al., 2014). The ophiolite mélange is in turn structurally overlain by an Early Paleozoic metasedimentary sequence (Aitchison et al., 2019) forming the Naga
SC RI PT
Metamorphics (Brunnschweiler, 1966, Fig. 2b). The NOC includes accretionary metasedimentary rocks, dominantly of pelagic chert-limestone association, with attenuated metabasalt, metagabbro, serpentinite, and pillow basalt with igneous texture and mineralogy (Acharyya, 1986; Ao and Bhowmik, 2014; Bhowmik and Ao, 2016). Metamorphic
NU
assemblages in the NOC reflect greenschist, pumpellyite-diopside, and blueschist to eclogite facies conditions (Fig. 2b) (Ao and Bhowmik, 2014; Bhowmik and Ao, 2016; Chatterjee and
MA
Ghose, 2010). Ao and Bhowmik (2014) previously established evidence for a disrupted field array in the Mokie-Satuza sequence in the western part of the mapped area, with
ED
metamorphic grade increasing eastward from greenschist through pumpellyite-diopside to lawsonite blueschist facies (Fig. 2b).
PT
Preliminary whole rock K-Ar geochronology of metabasalt (148 ± 4 Ma, Sarkar et al.,
CE
1996) and radiolarian microfossils from elements of the NOC (latest Bathonian to early Callovian, Aitchison et al., 2019) indicate Middle-Upper Jurassic protoliths that are
AC
interpreted as having been scraped off the subducting oceanic lithosphere. Chatterjee and Ghose (2010) estimated peak metamorphic conditions for eclogite in the NOC at 17–20 kbar and 580–610°C. Bhowmik and Ao (2016) recently reported a hornblende eclogite facies metamorphic sole (peak metamorphism at 13.8 kbar, 625 oC) from a nearby area. In this work, we describe two new eclogite locations: (1) near Mokie village (sample N22F, Mokie eclogite from now on), and (2) ~2 km east of Old Thewati village (samples N23B, N40B, hereafter Thewati eclogite) (Fig. 2). In both locations, eclogite occurs as
ACCEPTED MANUSCRIPT small, meter-scale lensoidal bodies within a lawsonite blueschist facies sequence of basalt, chert and limestone (Fig. 3a-b). Thin discontinuous bands rich in tremolite and/or white mica separate the eclogite bodies from the host metabasaltic rocks (Fig. 3a-b).
2.1 Sample description
SC RI PT
Sample N23B is a coarse-grained eclogite, collected from the core region of the Thewati eclogite body (Fig. 3b). It consists of porphyroblastic garnet (5-7 mm grain diameter) (~18 modal%) and omphacite (2-5 mm grain diameter) (~19%) enveloped by a matrix of phengite (~31%), barroisite (Fig. 3c) and glaucophane (collectively ~22%), and chlorite (~8%), with
NU
minor epidote (~3%), rutile, sphene and accessory zircon (Fig. 3a, 3b, 3d). Sample N40B is a composite, omphacite-rich (~37-57%) eclogite (Fig. 3e) collected from the outer margin of
MA
the Thewati eclogite body (Fig. 3b). Pale green layers dominated by foliated omphacite (Fig. 3f) alternate with darker barroisite-rich layers. Porphyroblastic garnet (1-5 mm in diameter),
ED
enveloped by finely foliated omphacite (Fig. 3e, f) occurs throughout the sample (Fig. 3e). Thin discontinuous, layer-parallel segregations of epidote are also present in the rock. Sample
PT
N22F is a fine-grained, schistose rock collected from the outer margin of the Mokie eclogite
CE
body. It consists of garnet and Na-Ca pyroxene of omphacite to aegirine-augite composition, enveloped by Na-Ca amphibole of barroisitic to magnesio-kataphoritic composition, sphene,
AC
minor glaucophane, phengite, epidote and rutile and accessory zircon (Fig. 3g).The foliation is cut by phengite-rich veins. A total of five thin sections (two from N23B, here named N23B-1 and N23B-3 and three from N40B, named N40B-2, N40B-4 and N40B-6) was utilised to recover the metamorphic history of the Thewati eclogite. Sample N23B-1 was used for the geochronology. The Mokie eclogite sample N22F was used for both geochronological and metamorphic studies.
ACCEPTED MANUSCRIPT 3.Methods 3.1. Electron probe micro-analytical technique Major-element mineral chemistry was determined using a CAMECA SX-100 electron microprobe fitted with four spectrometers at the Department of Geology and Geophysics, Indian Institute of Technology (IIT), Kharagpur. Analyses were performed with an
SC RI PT
accelerating voltage of 15 kV, a beam current of 15 nA and a beam size of ~1µm (~5 μm for phengite). Natural mineral standards were used for calibration, including orthoclase (for K and Si), jadeite (Na), wollastonite (Ca), periclase (Mg), hematite (Fe), corundum (Al), rhodonite (Mn) and chromite (Cr). Analytical variation within standards was kept at or below
NU
∼1.5wt% for primary standards. Representative mineral chemical analyses (presented in
3.2. Metamorphic Reconstruction
MA
Tables 1-3) were recalculated using the AX program (THERMOCALC software, v. 3.26).
ED
3.2.1. Methods of determination of metamorphic pressures (P) and temperatures (T) We adopt a conventional thermobarometric approach to recover the P–T conditions of
PT
eclogite facies metamorphism at the two locations. This utilizes simultaneous solutions of
CE
garnet-clinopyroxene Fe2+-Mg exchange thermometric (reaction R1, 2 pyrope + 3 ferrosilite = 2 almandine + 3 enstatite, after Ravna, 2000) and garnet-clinopyroxene-phengite
AC
barometric (reaction R2, 6 diopside + 3 muscovite = 2 grossular + 1 pyrope + 3 celadonite, after Ravna and Terry, 2004) equations. This methodology has been widely applied to eclogite localities from around the world. P–T estimates made using these calibrations (from now on R00_RT04 and Method 1) are also compared with the results of computations of the same equilibria using the internally consistent thermodynamic dataset of Holland and Powell (1998) and THERMOCALC software (version 3.31, thermodynamic database tc-ds55, 22 November 2003) (Method 2). Mineral end-member activities and their uncertainties were
ACCEPTED MANUSCRIPT calculated using activity-composition (a-X) datafile coding for pseudosection calculations, following the approach outlined by Powell and Holland (2008). Solid solution models used for this purpose are clinopyroxene (Green et al., 2007), garnet (White et al., 2007) and phengite (Coggon and Holland, 2002). For the spessartine-rich garnet composition (Sps17) in N22F, the garnet activity model of Ganguly et al. (1996) was adopted for the Method 2. The
SC RI PT
model incorporates Mg-Mn interaction relevant to the peak mineral assemblage in the sample and for the computations of garnet end-member activities. Finally, we have also estimated peak conditions using the THERMOCALC average P-T method (Method 3) (see Bhowmik and Ao, 2016 for additional information on the methodology). The average P-T computations
NU
were carried out using the same garnet-clinopyroxene-phengite assemblage and mineral chemistry. The same THERMOCALC software, thermodynamic database and mineral end
MA
member activity models as adopted in the Method 2 has been applied here. For all the three methodologies, we have estimated ferric content in clinopyroxene using charge-balance
ED
criterion and in garnet following the methodology of Ravna (2000). The P-T results are
PT
presented in Table 4.
CE
3.2.2 P-T Pseudosection modelling
Estimates of peak metamorphic conditions and the likely reaction history of the Nagaland
AC
eclogite were also evaluated using calculated metamorphic phase diagrams. Sample N40B preserves excellent reaction textures (Fig. 4) and mineral compositional features (Fig. 5-6, Tables 1-3) consistent with evidence for prograde, peak and retrograde conditions being preserved in selected domains of the rock (Section 4). For this reason, we calculated P-T pseudosections using effective bulk rock compositions for two domains in the N40B: domain N40B-4 represents the garnet-pyroxene-rich peak assemblage; and domain N40B-6 is rich in retrograde minerals such as barroisite and epidote and represents the retrograde stage.
ACCEPTED MANUSCRIPT Effective bulk rock compositions were calculated using XMapTools software, following the methodology of Lanari (2014). For each domain, a mineral mask file was prepared representing the modes of constituent minerals (Fig. 7a-b, Table 5). Mineral modes were then converted to their mass fractions and then combined with the calculated average oxide weight percentage of all major elements of minerals present in the chosen domain to obtain effective
SC RI PT
rock compositions (Table 5).
The chemical system Na2O - CaO - K2O - FeO - MgO - Al2O3 - SiO2 - H2O- TiO2 - O2 (NCKFMASHTO) was used for the computation of the pseudosections, together with the same version of the THEMOCALC software and thermodynamic dataset used in the
NU
thermobarometry. The component MnO was omitted because all minerals other than garnet in the mafic eclogites have very low manganese content, and there are currently no manganese
MA
solid-solution models for several of the rock-forming minerals. The inclusion of manganese in the model system has the main effect of subtly expanding garnet-bearing equilibria to
ED
lower-grade conditions. However, as the spessartine content of garnet grain cores can be comparatively high (Sps11) the proportion of MnO was combined with the FeO content in the
PT
rock. The following mixing models were applied: clinopyroxene (Green et al., 2007),
CE
amphibole (Diener et al., 2007), garnet (White et al., 2007), epidote (Holland and Powell, 1998), plagioclase (Holland and Powell, 2003) and phengite (Coggon and Holland, 2002).
AC
Quartz, sphene, rutile and lawsonite are treated as pure phases. Fe3+ contents in the effective rock compositions have estimated on the basis of average Fe3+ contents in the constituent minerals and their proportions. Previous studies have demonstrated a strong influence of the redox state of the rock in the P-T stability of epidote-, pyroxene- and amphibole-bearing mineral assemblages in eclogite facies metamorphic rocks (e.g. Rebay et al., 2010). Although, we are not in a position to provide an accurate estimate of the O2 content in the modelled rock compositions, the approach adopted here provides a reasonable approximation
ACCEPTED MANUSCRIPT of phase stabilities in HP metabasites as in this study. The metamorphic phase relationships for water-saturated conditions are presented in Figs. 8-9.
3.3. U-Pb zircon geochronology Zircon grains from the two Nagaland eclogite samples (N22F and N23B-1) were analysed for
SC RI PT
U-Th-Pb content using the Sensitive High Resolution Ion Microprobe (SHRIMP-II) instrument at the Research School of Earth Sciences (RSES), The Australian National University (ANU). Zircon grains in the samples were initially identified in polished thin sections using a JEOL JSM 6490 scanning electron microscope (SEM) with EDAX
NU
attachment at the EPMA National facility in IIT Kharagpur. Although small (~10-20 μm), zircon is common throughout the two samples in a variety of textural settings. Back-
MA
scattered-electron (BSE) imaging revealed diverse zircon morphology, internal structure, and mineral inclusions. Because of their small grain size, parts of the thin sections with the most
ED
zircon were cut for in-situ analysis using an ultrasonic disk cutter. These thin section pieces were then mounted on double-sided tape together with Temora 2 reference zircon prior to
PT
casting in 25mm diameter epoxy mounts. The Temora 2 grains were grounded to expose their
CE
mid-sections and polished with 1µm diamond paste prior to mounting together with the thin section pieces. Mounts were then mapped with an automated Leica D6000 microscope in
AC
reflected and transmitted light in order to locate the previously identified zircon grains based on textural positioning in the original thin section. Subsequently, Cathodoluminescence (CL) imaging of zircon grains in the mounts was completed using a JEOL-JSM-6610A SEM at RSES-ANU to characterise their internal structure. For U-Pb isotopic analysis on SHRIMP-II, the grain mounts were coated with ca. of 10 nm of Au. After coating, the epoxy mounts were loaded into a steel holder for insertion into SHRIMP. A reflected light map of each mount was uploaded onto the SHRIMP-II
ACCEPTED MANUSCRIPT computer. Every individual spot location was checked for correct positioning. SHRIMP measurements were performed with a O2- primary beam of ca. 1.5nA, with an accelerating voltage of 10 kV and focused to sputter an area of ~ 10 μm x 15 μm (Köhler illumination). Secondary ions were extracted using an accelerating voltage of 10 kV and measured by single collector analysis on an ETP electron multiplier. Masses were selected by magnetic
SC RI PT
peak switching. SHRIMP-II is operated at a mass resolving power of
~ 6500 (at 10%
peak height). The acquisition time for each spot was ca. 16 min, which consists of six cycles through the following peaks: 248
ThO+, and
254
196
Zr2O+,
204
UO+. The peak position of
Pb+, background, 204
206
Pb+,
207
Pb+,
Pb is fixed relative to
196
208
Pb+,
238
U+,
Zr2O, and the peak
NU
positions of 207Pb and 208Pb are fixed relative to 206Pb.
Uranium concentrations are determined by normalizing to SL13 zircon (238 ppm U;
MA
Roddick and van Breemen, 1994) which were measured at the beginning of every session. The analysis of every five unknown spots was both preceded and followed by the analysis of
ED
Temora2 standard and used as a U/Pb normalizing reference (416.8 ± 1.3 Ma; Black et al., 2003). All analyses were corrected for common Pb using the measured
204
Pb content. Data
PT
were reduced using POXI, an in-house ANU program that follows the data reduction
CE
methods of SQUID 1. The program Isoplot/Ex version 3.0 (Ludwig, 2003) was used to generate probability density and concordia plots and explore possible mixed age populations
AC
through the "mixture modelling" approach of Sambridge and Compston (1994). Single analyses in the data table (Table 6) and concordia plots are presented with 1σ errors.
4. Petrography and mineral chemistry 4.1 Petrography 4.1.1. Thewati eclogite
ACCEPTED MANUSCRIPT In the Thewati eclogite sampleN23B, garnet (Fig. 4a-b), omphacite (Fig. 4c) and epidote (Fig. 4a) occur as large to medium-sized porphyroblasts, enveloped by foliated phengite (Fig. 3d), barroisite (Fig. 4d), epidote and sphene. Rutile, quartz and omphacite occur as inclusions in garnet (Fig. 4a) and omphacite (Fig. 4c). Evidence for an early prograde stage is indicated by foliated, fine-grained glaucophane (glaucophane1), epidote (epidote1), sphene (sphene1),
SC RI PT
phengite and quartz inclusions concentrated in garnet grain cores (Fig. 4a-b), and also rarely in omphacite (Fig. 4c). When present, inclusions in garnet grain rims are comparatively coarse-grained and dominated by omphacite and rutile, and only very rare epidote (Fig. 4a). The peak metamorphic assemblage for this sample is inferred to have involved garnet,
NU
omphacite, epidote, rutile, phengite and quartz.
The composite Thewati eclogite (sample N40B) contains garnet similar to that in
MA
sample N23B (Fig. 4f), and a second variety of garnet that is characteristically smaller (1-2 mm grain diameter) and has an idioblastic habit (Fig. 4e). These garnet types are named
ED
garnet-A and garnet-B, respectively. The cores of garnet-B grains have inclusions of epidote (epidote1) and omphacite (Fig. 4e), but in smaller proportions than occur in garnet-A (Fig.
PT
4f). The outer rims of garnet-B grains are inclusion free. Matrix pyroxene, amphibole and
CE
epidote envelop both garnet-A and garnet-B grains. The post-peak history of the Thewati eclogite involved a series of retrograde stages,
AC
that successively formed: (a) structurally-late epidote (epidote2) with inclusions of garnet and omphacite as aggregates and discontinuous layer-parallel segregations (Fig. 4g, h); (b) the syn- (Fig. 4i), to post-tectonic (with respect to the pervasive matrix foliation) growth of barroisite that partially pseudomorphs garnet, epidote2, and omphacite (Fig. 4d, j–k); (c) a late generation of idioblastic glaucophane (glaucophane2) and sphene (sphene2) that form rims on barroisite (Fig. 4d, l) and rutile, respectively; (d) post-tectonic rims of chlorite and phengite on garnet (Fig. 4m) and (e) cross-cutting veins of lawsonite, albite, winchite-
ACCEPTED MANUSCRIPT ferrowinchite, aegirine-augite, and sphene2 (Fig. 4n-q). The minerals constituting individual veins varies along their length and they can be classified into three types: (i) albite + winchite + aegirine-augite + sphene veins cutting garnet-A grain cores of (Fig. 4n-o); (ii) lawsonite + aegirine-augite veins cutting grains of garnet-B (Fig. 4p) and (iii) lawsonite + albite + ferro-
SC RI PT
winchite veins cutting barroisite porphyroblasts (Fig. 4q).
4.1.2. Mokie eclogite
The Mokie eclogite records prograde, peak and retrograde stages (Fig.2g, 3r) similar to the Thewati eclogite. The rock lacks the texturally late epidote, and late lawsonite, albite, sphene,
prograde history of the Mokie
NU
(ferro-)winchite, aegirine-augite-bearing veins observed in the Thewati eclogite. The sample is preserved by inclusions of glaucophane1 in
MA
omphacite (Fig.4s) and sphene1 in rutile (Fig. 4t). The peak mineral assemblage is inferred to have involved garnet, omphacite/aegirine-augite, rutile, phengite and epidote (Fig.4r). In
ED
contrast to the porphyroblastic garnet in the Thewati eclogite, garnet in the Mokie eclogite occurs as small idioblastic grains, and occasionally as clusters within pyroxene and the rock
PT
matrix (Fig. 4r-s). An intergrowth of barroisite and sphene partially rims pyroxene and
CE
encloses garnet, rutile and epidote (Fig. 4s-u). Barroisite in such intergrowths is in turn
AC
partially pseudomorphed by a thin rim of glaucophane2 (Fig. 4v) and chlorite (Fig. 4u).
4.2. Mineral chemistry Garnet in the Thewati eclogite is dominantly a solid solution among spessartine (Sps 11 to Sps00-01), pyrope (Prp06 to Prp17), almandine (Alm54 to Alm64) and grossular (Grs19 to Grs28) with minor andradite (Adr02 to Adr05) contents (Table 1). Mn-, Mg-, Fe- and Ca-X-ray element images of three representative large garnet grains in samples N23B and N40B (cf. garnet-A in N40B-6) show a uniform feature of Mn-rich core and Mn-poor rim and a
ACCEPTED MANUSCRIPT continuous rim-ward enrichment in Mg contents (Fig. 5a-b, e-f, i-j). The Fe and Ca images show a feature of complementary increase and fall in their concentrations up to the inner rim followed by fall and rise towards the outer rim respectively (Fig. 5c-d, g-h, k-l). The compositional zonation of these elements, inferred from the X-ray element images is also confirmed in detailed compositional profiles (Fig. 5q-ae). The rim-ward depletions in
SC RI PT
spessartine (11-06→00-01 mol%) and enrichments in pyrope (06-08→15-17) and XMg (1113→20-23) contents in garnet (Table 1) are attributed to growth during prograde metamorphism. Characteristic features of elemental zoning in these garnet grains include discontinuities in almandine [54-58 (core)→60-64 (inner rim)→55-61 (outer rim)] and
NU
combined grossular + andradite [26-30 (core)→22-24 (inner rim)→24-27 (outer rim)] profiles (Fig. 5s-t, x-y, ac-ad). On the basis of these compositional features, we consider
MA
garnets in N23B to be comparable with garnet-A in N40B. Type-B (smaller) garnet grains in N40B also show features of growth zoning in the
ED
X-ray element images (Fig. 5m-p) and compositional profiles (Fig. 5af-aj). However, their
PT
grain cores are deficient in spessartine (Sps02 vs. Sps11-06) but enriched in pyrope (Prp13 vs. Prp06-08) and XMg (17 vs. 11-13) contents compared to that in the garnet-A cores (Table 1).
CE
These distinctions are attributed to the growth of garnet-B in sample N40B from a
AC
fractionated bulk rock composition (e.g. in Mn) following the early growth of (the cores of) large garnet grains, and at elevated temperature conditions. However, the spessartine-poor and pyrope-rich compositions in the garnet outer rims are the same in all the textural varieties of garnet. Garnet-B, is also distinct from the other garnet grains by its developments of a narrow (~50μm wide) rims marked by sharp fall in pyrope and XMg and complementary rise in almandine contents (Fig. 5ah-aj) at its outer edge in contact with the matrix omphacite (Fig.4e). These are attributed to localized diffusive re-equilibration.
ACCEPTED MANUSCRIPT Garnet in the Mokie eclogite is characteristically Mn-rich (Sps14-26) and growthzoned [Sps26 Prp07 Alm45 Grs19 (C)→Sps14-17 Prp10-12 Alm52-57 Grs13-23 (R)] as in the Thewati eclogite (Table 1). The XPrp and XMg contents in its outer rim are, however, lower than that in the Thewati eclogite. Pyroxenes in the Nagaland eclogites show a variation in composition from aegirine-
SC RI PT
augite to omphacite (Fig. 6a-b, Table 2), which can be related to their textural setting. In the Thewati eclogites, pyroxene inclusions (Type 1 pyroxene) in the cores of large garnet grains straddle the boundary of the aegirine-augite and omphacite fields (Fig. 6a). Pyroxene in the garnets rims is omphacite (Fig. 6a). Pyroxene inclusions in the smaller garnet-B grains in
NU
N40B are omphacite (Fig. 6a). Pyroxene grain in the matrix (Type 2A pyroxene) or its relict within the texturally late barroisite grains (Type 2B pyroxene) is omphacite (Fig. 6b). The
MA
pyroxene porphyroblasts in the Mokie eclogite are omphacite to aegirine-augite (Fig. 6b). The vein-type pyroxenes are characteristically more oxidised and plot in the field of aegirine-
ED
augite in the ternary Q (Quad)-Jd (Jadeite)-Aeg (Aegirine) diagram (Fig. 6c). Na-Ca amphibole in the Thewati eclogite is barroisite. The Mokie eclogite has
PT
amphibole ranging in composition from barroisite to magnesio-kataphorite (Fig. 6d-e, Table
CE
3). The Na-Ca amphiboles in the veins are ferro-winchite (Fig. 6d). The texturally early and later sodic amphiboles in the Nagaland eclogites are glaucophane (Fig. 6f). In the binary NaB
AC
vs. (Al+Fe3+)VI plot, both the sodic and sodic-calcic amphibole groups show compositional spreads along the glaucophane substitution vector (Fig. 6g). The sodic-calcic amphibole group additionally shows minor tschermak substitution (Fig. 6g). Ferro-winchite in the veins is comparatively enriched in NaB content and lies very close to the boundary between sodiccalcic and sodic amphibole groups (Fig. 6g).
ACCEPTED MANUSCRIPT Phengite in the Thewati eclogite sample N23B is more siliceous [Si (c.p.f.u.) = 3.533.54] than that in sample N40B (Si = 3.46). Phengite of even higher Si-content is found in the Mokie eclogite sample N22F (Si = 3.63). In the Thewati eclogite, epidote1 inclusions within garnet are more pistacite-rich
the Mokie eclogite is pistacite-poor (XPs = 0.17).
SC RI PT
(XPs=0.25) than the foliation-defining epidote2 (XPs= 0.16-0.20). Texturally early epidote in
Chlorite coronae around garnet in the Thewati eclogite is clinochlore (XMg=0.68). Sphene1 and sphene2 in the Thewati and Mokie eclogites contain small amount of alumina (Al2O3 = 1.04-1.43 wt %). Albite, lawsonite and rutile are near pure phases.
NU
Summarising, the textural and mineral compositional features of the Nagaland eclogites reveal a multistage mineral assemblage evolution comprising: (1) early epidote
MA
blueschist facies conditions that accompanied prograde garnet growth (Fig. 5) together with Na-Ca pyroxene (Fig. 6a), glaucophane1, epidote1, sphene1, phengite and quartz; (2) peak
ED
conditions represented by garnet outer rims of spessartine-poor and pyrope-rich compositions, jadeite-rich omphacite in the matrix and inclusions in the garnet outer rim,
PT
rutile and phengite with or without epidote and quartz; and (3) a protracted post-peak
CE
evolution marked by sequential appearances of epidote2 (Fig.4g-h), barroisite + sphene2 ± epidote2 (Fig. 4i-k, s-u) and glaucophane2 + chlorite (Fig.4k-l, m, t-v). The Thewati eclogite
AC
additionally records the development of lawsonite with or without albite, ferro-winchite and aegirine-augite in cross-cutting veins (Fig. 4n-q).
4.3. Peak P-T conditions of metamorphism Mineral compositions from the pyrope-rich, growth-zoned garnet rims were paired with jadeite-rich omphacite from either the cores of larger matrix grains or inclusions in garnet rims together with the high-Si matrix phengite to estimate peak metamorphic
ACCEPTED MANUSCRIPT conditions. According to equations R1 and R2, these chemical parameters most likely reflect the maximum pressure conditions experienced by eclogite facies basites. For the Thewati eclogites, the applications of Method 1 yielded peak P-T estimates of 28.4 ± 1.6 kbar (weighted mean value with errors at 2 sigma) and 630 ± 50°C for the sample N23B, which are similar to those obtained by the Method 2 (28.1 ± 2.8kbar, 645 ± 80 °C) (Table 4). For
SC RI PT
the sample N40B, the estimated pressures (P = 26.1 ± 2.0 kbar, Method 1 and 25.5 ± 3.3 kbar, Method 2) are lower by ~2.3 to 2.6 kbar at similar TMax (T= 635±60°C, Method 1 and T= 650±100 °C, Method 2). We attribute the higher P-estimates in N23B to a more siliceous composition of phengite in the sample [Si = 3.54 (c.p.f.u.) vs. 3.46]. Compared to the
NU
Methods 1 and 2, the Method 3 yielded temperatures, which exceed by ~25 oC in sample N23B and ~ 100 oC for N40B. In particular, the peak estimate of ~755 oC in N40B is much
MA
higher than the more robust garnet-clinopyroxene Fe2+-Mg exchange thermometry results. For this reason, we discount the results of the Method 3 from further discussions on the P-T
ED
constraints of peak metamorphism in the Nagaland eclogites. Although the results from the first two methods are very close, we favour the computations by the Method 2. This is
PT
because these P-T values can be directly compared with the results of the phase equilibria
CE
modelling as the two methodologies adopt the same thermodynamic dataset and mineral solid solution models. Accordingly, we consider P ~ 25.5 to 28.1 kbar, T ~650 oC as the peak P-T
AC
estimates of the Thewati eclogites. Using the same reasoning, the peak P-T estimate of the Mokie eclogite has been constrained at P ~ 23.8 ± 5.1 kbar, T~ 555 ± 90 oC (Table 4).
4.4. Calculations of metamorphic phase diagrams and deduction of metamorphic P-T path Phase diagrams calculated for the Thewati eclogite are presented in Figs. 8 & 9. For the pyroxene-garnet-rich domains of eclogite sample N40B (cf. N40B-4), the phase diagram predicts garnetiferous assemblages at T>475-550 oC, and the loss of chlorite at T>550-575 oC
ACCEPTED MANUSCRIPT and amphibole at T>500-725 oC, depending on metamorphic pressures (Fig. 8a). The positive dP/dT slope of the lawsonite-out curve and the stability of the lawsonite-bearing assemblages at T<525-650 oC is nearly identical to the experimentally determined lawsonite-out curve (e.g. Liou, 1971; Schmidt, 1995). Epidote-bearing assemblages define an inverted V-shaped field at P<25 kbar and at T conditions between 525 and 675 oC. The early prograde loss of
SC RI PT
lawsonite before epidote during heating at P<25 kbar is consistent with phase diagram predictions made for metabasic bulk rock composition elsewhere (Wei and Clarke, 2011). The phase diagram also predicts peak eclogite facies conditions of T>625-675oC, which are consistent with the estimates for the sample N40B (P~ 25.5 kbar, T~650 oC) (Fig. 8a).
NU
We now evaluate the progressive changes in garnet composition (Fig. 8b) and garnet (Fig.8c), pyroxene (Fig. 8d) and amphibole (Fig. 8e) modes along three model P-T paths:
MA
Path1, a prograde path across the epidote-blueschist facies; Path 2, a counter-clockwise (CCW) P-T trajectory involving cooling and decompression; and Path 3, a clockwise (CW)
ED
P-T trajectory involving decompression followed by cooling. Path P1 predicts (a) the sequential intersections of the garnet-in, chlorite-out, amphibole-out and epidote-out curves;
PT
(b) prograde garnet growth with garnet composition becoming progressively magnesian
CE
(decreasing x(g) isopleths, see Table 4 for the terminology) and Ca-poor (decreasing z(g) isopleths) and (c) increasing omphacite and decreasing amphibole contents in the rock. This
AC
is consistent with the observed textural and garnet compositional features. Assuming the rock was not appreciably fractionated prior to the growth of garnet, the P–T condition at which garnet is predicted to first formed can be estimated using garnet core compositions and the isopleth thermobarometry approach (after Vance and Mahar, 1998). For this purpose, we have used the core composition of the largest garnet-A grain in sample N40B, which has the highest spessartine, lowest pyrope and highest grossular + andradite content. The x(g) =0.85 and z(g) =0.30) garnet isopleths intersect at ~18.8kbar, 555oC (Fig. 8f). This P-T estimate
ACCEPTED MANUSCRIPT falls in the epidote blueschist facies, in good agreement with the reconstructed mineral reaction history, and can be considered a reasonable approximation of the conditions accompanying early garnet growth for the Thewati eclogite. An expanded MnO-bearing system would likely displace this result to subtly lower grade (see, for example, Marmo et al., 2002). Both of the retrograde paths P2 and P3 (Fig. 8c) can be expected to induce the
SC RI PT
decomposition of garnet and clinopyroxene, and growth of epidote and amphibole. Path 2 posits the growth of lawsonite before amphibole (Fig. 8f), inconsistent with the deduced mineral reaction history. In contrast, Path 3 posits the sequential growth of epidote, then epidote with amphibole, then chlorite (Fig. 8f), in good agreement with the deduced sequence
NU
of retrograde minerals.
The phase diagram for the amphibole-rich compositions of sample N40B resembles
MA
that for the pyroxene-rich compositions in having similar P-T stability fields for lawsoniteand chlorite-bearing assemblages (Fig. 9a. However, the P-T range of epidote-bearing
ED
mineral assemblages is significantly enlarged (Fig. 9a), due to the higher calcic and alumina content of the modelled composition (Table 5). The phase diagram is contoured for garnet
PT
composition and garnet, pyroxene and amphibole modes (Fig. 9b-e). Whereas the phase
CE
diagram broadly predicts prograde and retrograde growth sequences similar to that of the garnet-pyroxene-rich composition (compare Figs 8f and 9f), the amphibole-rich bulk
AC
composition better matches discontinuities in grossular zoning during the prograde garnet growth. The prediction of a progressive decrease in z(g) content from core to the inner rim followed by an increase towards the garnet outer rim (along P1, Fig. 9b) resembles the measured garnet composition (Fig. 5t, y, ad). The phase diagram is also contoured with isopleths of NaB content in amphibole for values 0.5 and 1.5 [where the numbers refer to cations per formula unit (c.p.f.u.) in the B structural site of amphibole] (Fig. 9e). When considered together with the positions of
ACCEPTED MANUSCRIPT chlorite-in and sphene-in curves, the phase diagram with the modelled amphibole compositions predict the observed growth of glaucophane after barroisite, chlorite after garnet and sphene after rutile (Fig. 4d, l-m) along the retrograde path P2 (Fig. 9f). This gives confidence in using the phase diagram together with detail from chemically-zoned amphibole grains to assess the validity of the retrograde paths P2 and P3 (Fig. 9f).
SC RI PT
We now combine the reconstructed metamorphic evolutionary pathways, key details of the phase equilibria modelling, estimated peak P-T conditions of metamorphism and P-T positions of the experimentally determined lawsonite-out curve and calculated mineral equilibria, corresponding to greenschist-blueschist-facies transition and transformation of
NU
albite to jadeite + quartz to establish the metamorphic P-T path of evolution from the Thewati eclogite sample (Fig. 10). We additionally calculate and plot four activity-corrected end
MA
member mineral reactions (reactions R3 to R6) to supplement phase equilibria predictions of mineral stabilities, particularly involving retrograde minerals that formed in localised
ED
domains. These are:10 (jadeite)Omphacite + (4 grossular + 5 pyrope)Garnet + 11 quartz + 8 H2O = 5 (glaucophane)Barroisite+ 6 (clinozoisite)Epidote (R3).
PT
5 (glaucophane)Barroisite+12 (clinozoisite)Epidote + 14 quartz + 28 H2O = 3 (tremolite)Fe-winchite +
CE
10 albite + 18 lawsonite (R4).
3 daphnite + 5 pyrope = 3 clinochlore + 5 almandine (R5).
AC
Ab = Jd17 + Qtz (R6).
These reactions reveal the thermal and baric conditions for the stability of retrograde barroisite + epidote, chlorite and lawsonite, albite, ferro-winchite and aegirine-augite in late veins. In calculating R3-R5, we have applied the same thermodynamic dataset and activitycomposition models of mineral end members as adopted during the phase equilibria modelling. This makes the P-T positions of these equilibria to be internally consistent and comparable with the results of phase equilibria modelling.
ACCEPTED MANUSCRIPT The disposition of the sphene-out, garnet-out, chlorite-out, amphibole-out and epidote-out curves in sequence and the P-T conditions of formation of the prograde garnet core at 18.8 kbar, 540 oC, when considered together provide a tight constraint for the prograde P-T path (cf. MPR metamorphic stage, where PR represents prograde) that culminated in peak eclogite facies metamorphism at 25.5 kbar, 650 oC (MP metamorphism as
SC RI PT
P represents peak, Fig. 10). The intersections of epidote-out and amphibole-out curves in an opposite sense and reaction R3, leading to the successive appearances of retrograde epidote2 and barroisite2 + epidote2 constrain the first stage of retrogression (cf. MR1 metamorphic stage, where R refers to retrograde) as one of decompression with minor cooling to ~18 kbar,
NU
~620 oC (at amphibole eclogite facies condition) (Fig. 10). The subsequent crossing of the NaB =1.5 isopleth in amphibole and intersections of the chlorite-out and sphene-out curves in
MA
the reverse sense and R5 indicates further decompression and cooling in the epidote blueschist facies at ~10kbar, 510 oC (cf. MR2 metamorphic stage) (Fig. 10). The dispositions
ED
of the reactions, R4 and R6, reflecting the stability of aegirine-augite + lawsonite + albite + ferro-winchite assemblage indicate a terminal phase of retrogression to one of protracted
PT
cooling (∆T ~200 oC) at low-pressures (P~6 kbar) (cf. M3R metamorphic stage, Fig. 10). The
CE
development of a narrow diffusion zoning in garnet-B (Fig.5aj) in contact with omphacite provides additional support for post-peak cooling. Summarising, our study reveals that this
AC
progressive change in P-T vectors from loading accompanying heating (MPR) culminating in peak metamorphism (MP) and three successive stages of decompression and cooling (MR1 to MR3) defines a clockwise P-T path of evolution of the Thewati eclogites (Fig. 10). Although we have not quantified the metamorphic P-T path of evolution of the Mokie eclogites, the broadly analogous metamorphic reaction pathways in the Mokie eclogite with the Thewati eclogite raise the possibility that the former also records a broadly similar subduction burialexhumation history in the Nagaland subduction channel.
ACCEPTED MANUSCRIPT
5. Results of SHRIMP U-Pb zircon geochronology
Analytical data for zircon are presented in Table 6. Zircon textural and cathodoluminescence (CL) features are summarized in Fig. 11 and chemical composition and Th/U vs. age
SC RI PT
distribution shown in Fig. 12. A total of 34 spot analyses from 33 zircon grains were completed on the two eclogite samples (17 grains in N22F and 16 grains in N23B-1). In both samples, zircon grains are common but small (grain diameter range ~ 8-20 μm). They occur in various textural settings: as inclusions in garnet (Fig. 11a), Na-Ca pyroxene (Fig. 11b) and
NU
rutile, and also in the matrix foliation-defining barroisite, phengite and sphene. Zircon in N22F generally occurs as elongate grains with pyramidal terminations and has grain cores
MA
dominated by ovoid inclusions (Fig. 11c). Zircon in N23B-1 mostly occurs as equidimensional grains with well-developed crystal faces that are free of mineral inclusions (Fig.
ED
11d). Where inclusions are relatively coarse they are determined to be garnet (Fig. 11e), omphacite (Fig. 11f), and rutile.
PT
Their small size partially inhibits the high resolution CL images of zircon in the two
CE
Nagaland eclogites. However, they have a variety of CL structures (Fig. 11g-x) that can be used to identify three types of CL domains that reflect stages of grain growth,
AC
recrystallisation and alteration (Table 7). Type 1 zircon is marked by darkly luminescent, faintly banded (Fig. 11g, i, n-p) elongate domains that characteristically have euhedral shapes and pyramidal terminations (Fig. 11j-l, r-t). These features are consistent with zircon growth in the presence of a fluid or melt (e.g. Rubatto, 2017). Type 2 zircon is marked by (a) structure-less grain domains with morphology similar to type-1 zircon (Fig. 11h, j-m, r, u), commonly with patchy, irregular zones of bright luminescence (Fig. 11q, s-t); (b) textural modification of zircon with type-1 grain form involving grain rounding and/or the truncation
ACCEPTED MANUSCRIPT of pyramidal faces (Fig. 11j, l, p, r-s, u-v); and (c) development of bright luminescent narrow rims to type-1 zircon (Fig. 11h, j-m, p-v), commonly with equant and euhedral external shape oblique to the enclosed type-1 zircon core (Fig. 11j-k). These features are attributed to the recrystallisation and/or replacement of type 1 zircon. Type 3 zircon only occurs in N22F and is marked by irregular bright-luminescent domains that partially replace type-1 zircon cores
SC RI PT
(e.g. domain 1 in grain Z-9) (Fig. 11v). The zones of replacement have serrated to curvilinear inner boundaries (e.g. in grains Z-9, 33) (Fig. 11v-w). In some zircon grains, the partial replacement of the older core is extensive, and gives rise to ghost and fuzzy zoning (e.g. grain Z-28) (Fig. 11x). Features of this third stage are consistent with grain modification during the
NU
localised fluid-mediated alteration of type 1 zircon.
Types 1 and 2 zircon grains (both from primary and recrystallised varieties) in N23B-
MA
1 have limited compositional variation with low Th (0.05-1.62 ppm) and U (29-200 ppm) contents (Fig. 12a). Equivalent zircon domains in N22F are characteristically uraniferous (U:
ED
1110-2478 ppm) and more thorian (Th: 8-66 ppm) (Fig. 12a). A Type 2 zircon rim in N23B-1 is depleted in U relative to the recrystallised grain core (e.g. 13 ppm in rim vs. 29 ppm in
PT
core, grain Z-66, N23B-1) (Fig. 12a). Type 3 zircon is chemically distinct by being more
CE
thorian (Th: 94-271 ppm) and has high Th/U ratios (0.09 to 0.20) (Table 6, Fig.12a-b). Inter-sample compositional variability of Th-U in the Nagaland zircon grains is a
AC
feature common in many HP metamorphic rocks. It is generally attributed to zircon growth in the absence of a dominant buffering fluid (Rubatto and Hermann, 2007). Type 1 zircon in the Nagaland samples also has low Th/U ratios (≤0.02-0.03) (Table 7,Fig. 12b), that are common in metamorphic zircon from blueschist and eclogite facies rocks (Rubatto and Gebauer, 2000; Rubatto and Hermann, 2007). Given the small size of Nagaland zircon grains, it is not possible to analyse most type2 zircon rims in the two samples. One analysis was made of a comparatively thick rim in
ACCEPTED MANUSCRIPT N23B-1 (see below). The SHRIMP analysis sessions thus targeted zircon core domains, including the type1 zircon, and additionally the type3 zircon in N22F (See Fig. 11 for the analytical spots in representative zircon grains). Among the 17 analysed spots in N22F, six spots were excluded from the calculation of concordia and mean ages based on the following considerations: (a) these analyses reveal discordance in the concordia plot (Fig. 13a); (b) four
SC RI PT
of the analyses are from the Th-rich altered domain (type 3 zircon); and (c) all have yielded older dates (between 337 and 234 Ma). Following previous studies that have indicated the isotopic disturbance of altered zircon grains in HP metamorphic terranes, we are of the opinion that zircon ages from such texturally late, fluid-mediated alteration in N22F are
NU
likely geologically spurious and lack geological significance (e.g. Rubatto and Hermann 2003).
MA
The remaining 11 analyses show a spread in zircon ages from 216 to 171 Ma (Table 6) and reveal three groups at 206 ± 4 Ma (error 2s), 189 ± 3 Ma, and a minor one at 172 ± 4
ED
Ma (Fig. 13b). Concordia ages of the two dominant age groups are 205± 6 Ma [n=4, Mean Square Weighted Deviates (MSWD) =0.49, probability of concordance =0.48] (Fig. 13c) and
PT
189 ±1Ma (n=5,MSWD=0.05, probability=0.82) (Fig. 13d).
CE
In N23B-1, 18 analyses show a spread in zircon dates from ~217 Ma to ~152 Ma, with two broad age groups at 204 ± 10 Ma and 187 ± 7 Ma (Fig. 13e) similar to that observed
AC
in N22F. Two analyses from core-rim domains of one zircon grain (grain Z-66, Fig. 11h) give dates at 190 ± 18 Ma for the grain core (type 1 zircon) and 152 ± 42 Ma for the grain rim (type 2 zircon). Although this is consistent with zircon recrystallisation being younger than the growth of the core, the large age error due to the presence of high common Pb does not allow this age to be useful for estimating the timing of the recrystallisation event. For concordia age calculations, we have excluded 9 analyses because of negative or indeterminate radiogenic
207
Pb/235U ratios. The remaining 9 analyses have yielded two
ACCEPTED MANUSCRIPT concordia ages at 205± 4 Ma (n=5, MSWD = 0.10, probability = 0.75) (Fig. 13f) and 185 ± 4Ma (n=4, MSWD = 0.08, probability=0.78) (Fig. 13g) that are near identical to that obtained from N22F.
6. Discussion
SC RI PT
6.1. Early Jurassic eclogite facies metamorphism in the Nagaland Ophiolite Complex Nearly identical zircon age groupings of ~205 Ma, ~185-189 Ma and (minor) ~172 Ma have been identified in two separated eclogite locations in Nagaland. We recognise two stages of zircon growth and/or recrystallisation, in small zircon grains that challenge the analytical
NU
method. Despite minor overlap, the data largely define two end member textural types (1 and 2, below). In the context of petrological observations, the concordia ages of the first two age
MA
groups are most probably geologically meaningful. The oldest zircon age group was recovered from type 1 zircon, and reflects the earliest stage of zircon growth (e.g. in N23B).
ED
There are three possible interpretations of the c. 205 Ma date: (a) the growth of zircon in the eclogitic protolith; (b) a partially reset growth age due to the effects of post-growth heating
PT
during later prograde conditions; and (c) zircon growth during prograde metamorphism. In
CE
the absence of zircon trace element data it is difficult to distinguish between these possibilities, but the characteristic low Th/U ratios (<0.01) of these zircons are most
AC
consistent with the c. 205 Ma date to being a minimum age for the eclogitic protolith [option (b) above]. The c.185-189 Ma zircon ages were recovered mostly from stage 2 recrystallised grain cores, which include eclogite facies mineral inclusions. Stage 2 zircon is thus interpreted to have formed during or shortly after peak conditions in the eclogitic facies, at c. 189-185 Ma. The limited dataset of the c. 172 Ma concordant zircon ages is interpreted as recording the timing of retrograde cooling and exhumation. However, more detailed work is needed to firmly establish its significance. An important outcome of these data is the
ACCEPTED MANUSCRIPT identification of a Triassic age (≥ c. 205 Ma) for this segment of the Neo-Tethyan oceanic crust at Nagaland.
6.2. Thermal history and accretionary processes in an Early Mesozoic Neo-Tethyan subduction channel
SC RI PT
Eclogite from two locations in the Nagaland Ophiolite Complex preserve a record of broadly coeval eclogite facies conditions in the Early Jurassic: ~28.1 to 25.5 kbar, ~ 650 oC for the Thewati body and 23.8 kbar, 555 oC for the Mokie body. Whereas the Mokie eclogite records low-temperature (LT) eclogite facies conditions (terminology and P-T field
NU
of global LT eclogites are after Fig. 3a in Agard et al., 2016), the Thewati eclogites underwent burial to deeper levels (~80-90 km depths) that were transitional to ultra-high
MA
pressure metamorphic conditions (Fig. 14a). In a previous study, Chatterjee and Ghose (2010) reported eclogite from a location near the Satuza area, and inferred a CW P-T path
ED
with peak conditions of 17–20 kbar and 580–610 °C (Path 3, Fig.14a). Whereas the prograde and retrograde segments of the Satuza P-T path resemble that of the Thewati
PT
eclogite (Path 1, Fig. 14a), the Satuza locality records shallower levels and higher thermal
CE
gradients (>10 oC/km) (Fig. 14a). Our findings, however, suggest that the Thewati eclogite falls under the select category of oceanic eclogites that are recovered from the maximum
AC
depth in the subduction zone (PMax ~28 kbar) and with a low apparent peak thermal gradient of ≤6–9 °C/km (Fig. 15). In contrast to the Nagaland samples, jadeite-glaucophane-bearing and garnetphengite-bearing schists from the adjoining Central Ophiolite Belt of Myanmar preserve a much larger spread in peak conditions, generally in blueschist to low-T eclogite facies (reviewed in Nyunt et al., 2017; Fig. 14a). A well-constrained metamorphic P-T path for the evolution of the terrane is lacking. Earlier studies of the Himalayan blueschists (see O'Brien,
ACCEPTED MANUSCRIPT 2018 for the latest review on the Himalayan HP/LT metamorphism) inferred low-T conditions and CW P-T paths (Paths 4 & 6, Fig. 14a) (Guiraud, 1982; Jan, 1985; Honegger et al., 1989; Anczkiewicz et al., 2000; Oberhänsli, 2013). On the basis of a detailed metamorphic investigation of Sapi-Shergol garnetiferous lawsonite blueschists, Groppo et al. (2016) recently established a significantly deeper and cold subduction system of possible
SC RI PT
Cretaceous age (cf. Anczkiewicz et al., 2000) within the Neo-Tethys from the NW Himalayas (peak P-T at 19 kbar, 470 oC) and a clockwise hairpin P-T loop along low thermal gradients (< 8-9 °C/km; Path 5, Fig. 14a). While the geometry of the P-T path and lower thermal gradients of the Shangla lawsonite blueschist resemble with that of the
NU
Nagaland blueschist (Path 2, Fig. 14a), our new geochronological constraints indicate the subduction zone metamorphism in the eastern Neo-Tethys initiated much earlier in the Early
MA
Jurassic.
The lower apparent thermal gradient at metamorphic peak in the Nagaland eclogite
ED
samples (7-8 oC/km) (Fig. 14a) is comparable to the gradient recovered from the associated lawsonite blueschist in Mokie (~9 oC/km, Ao and Bhowmik, 2014) and other HP
PT
metamorphic belts in the Zagros suture zone, Southern Iran (Angiboust et al., 2016), Sapi-
CE
Shergol lawsonite blueschist, Western Himalaya in the YTSZ (Groppo et al., 2016), Dominican Republic and Rio San Juan Mélange Complex (Krebs et al., 2011), the Western
AC
Alps (Angiboust et al., 2009; Agard et al., 2009, 2016 and reference cited therein), the Bantimala Complex, South Sulawesi, Indonesia (Setiawan et al., 2016) and also predicted by thermo-mechanical models (Gerya et al., 2002) (Fig. 15). The inferred Nagaland P-T path is similar to that inferred for the Sulawesi eclogite (Path 6, Fig. 15). For the latter, subduction of an old and cold oceanic crust was suggested (Setiawan et al., 2016). For the present study, we relate this characteristic lower thermal gradient during peak eclogite and blueschist facies metamorphism, the (hairpin) clockwise P-T paths of blueschists and
ACCEPTED MANUSCRIPT eclogites (Fig. 14a) and the geological setting with the development of a cold mature stage of an intra-oceanic subduction system within the Neo-Tethys in Nagaland (see also Ao and Bhowmik, 2014; Bhowmik and Ao, 2016). In many subduction-related HP-LT terranes, different localities of adjacent tectonometamorphic units record a systematic increase of peak-T and associated P, producing a
SC RI PT
positive dP/dT slope of the P-T array (e.g. Fig. 13b of Groppo et al., 2016). In the NOC, our studies (e.g. Ao and Bhowmik, 2014 and this work) demonstrate a near-complete preservation of a progressive metamorphic sequence from greenschist through pumpellyitediopside, blueschist to eclogite facies from the Mokie-Satuza area (Figs. 2c, 16). The peak P-
NU
T estimates of these rocks reveal a metamorphic field gradient (Fig. 14b) that is a characteristic feature of many HP/LT metamorphic terranes worldwide (e.g. Oman: Yamato
MA
et al., 2007; Turkey: Plunder et al., 2015; New Caledonia: Vitale Brovarone and Agard, 2013; Corsica: Vitale Brovarone et al., 2014; Schistes Lustres of the Western Alps: Plunder et al.,
ED
2012), also predicted by thermo-mechanical models (Fig. 14b). We attribute this metamorphic zonation in the NOC to repeated accretion of the top layer of the oceanic crust
PT
subducted at different depths and subsequently off-scraped at the base of the accretionary
CE
prism (e.g. Agard et al., 2009, 2018 and references therein). In the NOC, these HP metamorphic rocks are tectonically imbricated with serpentinites and mafic and ultramafic
AC
cumulate rocks, giving rise to an apparent chaotic internal arrangement in the ophiolitic mélange, though we note the coherence of individual members of the HP metamorphic sequence, in particular from the Mokie-Satuza area. The Nagaland P-T path reveals a two stage exhumation of the Nagaland eclogites: (a) initial exhumation (from 25.5 kbar, 650 oC to 10 kbar, 510 oC, and combining MR1 and MR2 metamorphic stages) took place along a steep dP/dT gradient (Fig. 14a). Such an exhumation path resembles that deduced for cold oceanic eclogites from different metamorphic belts
ACCEPTED MANUSCRIPT worldwide (e.g. Fig. 15); (b) later exhumation of the eclogites took place along a gentler dP/dT gradient (cf. MR3 metamorphic stage, Fig. 14a), and is distinctive for the Nagaland rocks. Three tectonic scenarios have been invoked to account for the exhumation of deeply buried eclogite along a steep dP/dT gradient (see Agard et al., 2009 for a general review). (1) In the first scenario that involves high-grade blocks (amphibolites/eclogites along CCW P-T
SC RI PT
path), the exhumation takes place within the first 5–15 Myrs of subduction infancy, in rapidly cooling subduction zones (e.g. Agard et al., 2009). In this scenario, exhumation is triggered by rheological weakening of the subducted oceanic crust due to partial melting in warm subduction zones (Sorensen and Barton, 1987). It has been proposed that this leads to the
NU
boudinage and detachment of the high-grade blocks in a weaker, sedimentary mud-matrix mélange (Cloos and Shreve, 1988). (2) This scenario involves buoyancy-driven exhumation
MA
along the subduction channel as a consequence of serpentinisation of the mantle wedge, controlled by fluid released during dehydration of hydrous phases in the subducting oceanic
ED
crust (Cloos, 1982, 1985; Gerya et al., 2002; Hermann et al., 2000; Guillot et al., 2009). In this setting, oceanic blueschist and eclogite are thought to occur as thin, disrupted bodies in
PT
association with serpentinite and/or a mechanically weak mud matrix (e.g. Gerya et al.,
CE
2002). The scenario predicts the exhumation of eclogite as part of material transport in a large-scale convective circulation system in the subduction channel that is evolving to a cold
AC
and mature stage (Cloos, 1982; Peacock & Wang, 1999; Gerya et al., 2002). The exhumation is inferred to be facilitated by the buyoancy and low viscosity of the serpentinites in the channel (Guillot et al., 2009). (3) A third scenario that involves the exhumation of eclogite as a consequence of the positively buoyant continental crust entering the subduction channel towards the end of the oceanic subduction, allowing the extrusion of large volumes of deeply buried continental crust (e.g. Norway, Andersen et al., 1991; Oman, Chemenda et al., 1996; Dabie Shan, Hacker et al., 1996; Zermatt-Saas eclogite, Western Alps, Angiboust et al.,
ACCEPTED MANUSCRIPT 2009; New Caledonia, Fitzherbert et al., 2004). In this scenario, oceanic eclogite is exhumed as part of a forced return flow of buyoant continental material with or without slab break-off (Von Blanckenburg and Huw Davies, 1995). For the Nagaland eclogites, features such as the (a) characteristic LT eclogite facies metamorphism along a CW P-T path, (b) occurrence of eclogite as rare, small lensoidal bodies within blueschist, (c) presence of thin, fluid-assisted
SC RI PT
metasomatic bands rich in amphibole + white mica as rims around the eclogitic cores and (d) absence of deeply buried continental crustal fragment in the ophiolitic mélange are collectively most consistent with exhumation via the second scenario.
The stage 2 exhumation that involved protracted cooling at shallower levels (Fig. 14a)
NU
can be linked with thrust stacking of partially exhumed eclogites with the cooler, prograde blueschists at shallower crustal levels. Despite differences in peak P-T conditions of
MA
metamorphism in the eclogites and blueschists, the P-T conditions of terminal retrogression in these rocks are broadly similar (P~5-6 kbar, T~300 oC, Fig. 14a). This suggest that the
ED
eclogites and blueschists were finally tectonically juxtaposed along their exhumation paths. On the basis of this deduction, we propose that the final mixing of rocks of contrasting
PT
lithologies, originally placed at different depths in the subduction channel, and producing a
CE
tectonic collage took place at a shallower crustal level during the development of the Nagaland accretionary complex. Future work should be directed to provide structural details
AC
in support of the suggested model above.
6.3. Regional correlations and Early Mesozoic tectonic framework of SE Asia The Nagaland eclogite samples dated in this work are older than the Early Cretaceous NeoTethyan ophiolites of the WOB (e.g. Manipur and Kalaymyo ophiolite belts) and its amphibolite-facies metamorphic sole (119-115 Ma, Liu et al., 2016a; Zhang et al., 2017). The Nagaland Ophiolite also predates the Mid-Jurassic aged Myitkyina ophiolite of the EOB by
ACCEPTED MANUSCRIPT more than ~30 Myr. It is also older than Cretaceous blueschists (e.g. 100-80 Ma Sapi-Shergol and Shangla blueschist, NW Himalayas, Anczkiewicz et al., 2000) and amphibolites (e.g. Saga amphibolite metamorphic sole between 135 and 127 Ma, Guilmette et al., 2012) (locations in Fig. 1 of the YTSZ). Nevertheless, it is contemporary with Late Triassic-Early Jurassic arc magmatism in the Gangdese magmatic belt (e.g. Yu et al., 2018). We propose
SC RI PT
that this cryptic record for the earliest phase of Neo-Tethyan subduction can be traced westwards from Nagaland-Myanmar, along the southern Eurasian margin to Pakistan as part of an Early to Middle-Jurassic Andean-type convergent margin (Searle et al., 1999; Yu et al., 2018).
NU
Several recent studies present interpretations for the Early Mesozoic tectonic framework of northern Myanmar and southeastern Tibet (see Searle et al., 2017; Gardiner et
MA
al., 2018; Zhang, J. et al., 2018 for general reviews). However, there remains considerable uncertainty regarding the: (a) polarity (easterly, Zhang, J. et al., 2018 or westerly, Mitchell,
ED
1993) and type of the subduction (single long-lived, e.g. Zhang, J. et al., 2018; or multiple, Mitchell, 1993; Acharyya, 2007, 2015; Mitchell et al., 2015; Searle et al., 2017; Gardiner et
PT
al., 2018); (b) status of the Western Myanmar block [e.g. a western extension of the Shan
CE
Plateau, an extension of the Lhasa Block, through Tengchong and Baoshan in Yunnan or as a part of the west Sumatra block (e.g. Zhang, J. et al., 2018)] and (c) any continuity of the
AC
three major Mesozoic Tibetan sutures into Myanmar. Liu et al. (2016a) interpret the EOB as a Jurassic suture of the Meso-Tethys (equivalent to the BN suture in central Tibet). This extended suture would pass between the Sibumasu block to the east and the West Myanmar block to the west (Fig. 17a). Gardiner et al. (2018) recently proposed a southerly termination of the Meso-Tethyan BN suture in NE Myanmar, extending the suture between the Tengchong block to the west and the coalesced Sibumasu–Baoshan Blocks to the east (Fig. 17b). In this model, an east-subducting Meso-Tethyan sea floor developed an Andean-type
ACCEPTED MANUSCRIPT active continental margin setting above the Sibumasu block between the Late Triassic and Early Jurassic. The closure of this Meso-Tethyan subduction through the amalgamation of the two continental blocks is inferred to have occurred before the Late Cretaceous. Alternatively, Zhang, J. et al. (2018) propose a long-lived east-dipping subduction within the Neo-Tethyan Ocean that progressively migrated westwards from the Early Jurassic – Middle Eocene and
SC RI PT
the present day. In this model, the subduction of the Neo-Tethyan ocean beneath the Sibumasu continental block during Early to Middle Jurassic time resulted in an accretionary complex of HP metamorphic rocks (now represented by the Jade Mines Belt and Kumon range), and a broadly coeval continental margin Jurassic magmatism in the Myitkyina
NU
ophiolite and Mogok metamorphic belt (Fig. 17c). However, this model predicts peak and retrograde metamorphism in the NOC, which is situated west of the Jade Mines Belt, to be
MA
the product of a second accretionary event in the Late Cretaceous. The reconstruction of an Early Jurassic accretionary complex in the NOC (this study)
ED
raises the possibility that it was continuous with the accretionary complex of the Jade Mines Belt further northeast (with respect to current geographic co-ordinates; Fig. 17c). The two
PT
ophiolites have been disrupted by younger strike-slip faulting along a structure parallel to the
CE
N-trending Sagaing fault. This proposition is supported by the analogous spatial association of continental slivers of Gondwanan affinity to the east of the HP accretionary complexes in
AC
both the locations: continental Naga Metamorphics of Lower Palaeozoic age (Aitchison et al., 2019) in the NOC and Mogok Metamorphic and Slate Belt as part of the Sibumasu Continental block in northern Myanmar (cf. Metcalfe, 1995; Zhang, J. et al., 2018) (Fig. 17c). Future studies should be directed to understand the Neo-Tethyan tectonics that was responsible for the present association of Jurassic and Cretaceous accretionary complexes in the WOB.
ACCEPTED MANUSCRIPT 7. Conclusions 1. Oceanic eclogite from two new locations near Mokie and Thewati in the Nagaland Ophiolite Complex, NE India, occurs as meter-scale lensoidal bodies within a lawsoniteblueschist facies metamorphosed oceanic sequence of basalt-chert-limestone. 2. SHRIMP U-Pb zircon dating of the eclogites yielded age groupings at ~205 Ma,
SC RI PT
~185-189 Ma and ~172 Ma (minor). The c. 205 Ma date is interpreted as a minimum age for the oceanic protolith, the c.185-189 Ma age is thought to reflect zircon growth at eclogite facies conditions and the c.172 Ma age possibly represents the onset of retrograde cooling and exhumation.
NU
3. Maximum P-T conditions recorded by the eclogite samples (~28.1 to 25.5 kbar and ~ 650 oC for the Thewati eclogite and ~23.8 kbar and ~555 oC for the Mokie eclogite)
MA
involved a low apparent thermal gradient (~7-8 oC/km).
4. The Thewati eclogite was buried to depths of ~80-90 km, transitional to UHP
ED
conditions, thus falling in the select category of oceanic eclogites worldwide that are recovered from the maximum depths in the subduction zone.
PT
5. The Thewati eclogite records a near complete clockwise metamorphic P-T path of
CE
evolution that involved an epidote blueschist facies prograde stage, peak metamorphism in the epidote eclogite facies and multistage retrogression through amphibole-eclogite facies at
AC
~18.3 kbar, 630 oC, epidote blueschist facies to the transitional lawsonite blueschist and greenschist facies metamorphic conditions at ~6 kbar, 300 oC. 6. The low thermal gradients (~7-9 oC/km) that accompanied peak conditions, together with the clockwise P-T paths of the blueschists and eclogites are consistent with a Lower Jurassic-aged, cold intra-oceanic subduction system within the Neo-Tethys in Nagaland.
ACCEPTED MANUSCRIPT 7. The Nagaland eclogites were exhumed in two stages: (a) the initial exhumation (from 25.5 kbar, 650 oC to 10 kbar, 510 oC, and along a steep dP/dT gradient) was likely to have taken place in the subduction channel as a consequence of buoyancy-driven material transport in a rheologically weak and fluidised mantle; (b) in the latter phase, the exhumation of the eclogites along a gentler dP/dT gradient was accompanied by extended cooling at
SC RI PT
shallower levels. This exhumation stage is linked with thrust stacking of the partially exhumed eclogites with the cooler, prograde blueschists at shallower crustal levels. 8. The current disposition of the Nagaland eclogites in lower grade blueschist likely reflects stage 2 of their exhumation in the subduction channel, when the Nagaland
NU
accretionary complex was assembled.
10. An Early Jurassic accretionary complex in the NOC was likely continuous with
current geographic co-ordinates).
MA
the accretionary complex of the Jade Mines Belt of northern Myanmar (with respect to
ED
11. The Nagaland eclogite samples present the oldest Neo-Tethyan remnant (cf. Late Triassic age) and evidence of the earliest subduction (cf. Early Jurassic) within the Yarlung-
PT
Tsangpo Suture Zone. The cryptic record of this pre-Cretaceous subduction system within the
CE
Neo-Tethys that incorporated an Early-Middle Jurassic Andean-type convergent margin can
Pakistan.
AC
be traced westwards from the Indo-Myanmar Ranges along the southern Eurasian margin to
Acknowledgements The paper constitutes a part of the doctoral thesis work of MR for which she acknowledges funding from the Indian Institute of Technology Kharagpur (IIT KGP) for an Institute doctoral fellowship and to carry out geological fieldwork. S.K.B. acknowledges financial support from the Indian Space Research Organization (Grant No. IIT/KCSTC/Chairman/New
ACCEPTED MANUSCRIPT Approval/15-16/09) and Australia-India Strategic Research Fund (DST/INT/AUS/P-55/2013 and AISRF07021) for fieldwork and analyses. A.A. thanks the Director, Wadia Institute of Himalayan Geology. We would also like to acknowledge the generous support of the Nagaland State Mineral Development Corporation Ltd. and their support staff at the Weziho Field Hostel without which, it wouldn't have been possible for us to undertake geological
SC RI PT
fieldwork in this remote and inaccessible part of the Nagaland state bordering Myanmar. The manuscript has benefitted from earlier reviews by S.A. Wilde and T. Johnson. We thank two anonymous journal reviewers for detailed reviews, and Marco Scambelluri for efficient
NU
editorial handling.
References
MA
Acharyya, S.K., 1986. Tectono-stratigraphic history of Naga Hills Ophiolites. In: Geology of the Nagaland Ophiolite (ed. Ghosh, D.B.), Geological Survey of India Memoirs
ED
119,94–103.
Acharyya, S.K., 2007. Collisional emplacement history of the Naga-Andaman ophiolites and
PT
the position of the eastern Indian suture. Journal of Asian Earth Sciences 29, 229-242.
CE
Acharyya, S.K., 2015. Indo-Burma Range: a belt of accreted microcontinents, ophiolites and Mesozoic–Paleogene flyschoid sediments. International Journal of Earth Sciences 104,
AC
1235-1251.
Agard, P., Jolivet, L., Goffé, B., 2001. Tectonometamorphic evolution of the Schistes Lustrés complex: implications for the exhumation of HP and UHP rocks in the Western Alps. Bulletin de la Société Géologique de France 172 (5), 617–636. Agard, P., Yamato, P., Jolivet, L., Burov, J.E., 2009. Exhumation of oceanic blueschists and eclogites in subduction zones: Timing and mechanisms. Earth Science Reviews 92,5379.
ACCEPTED MANUSCRIPT Agard, P., Yamato, P., Soret, M., Prigent, C., Guillot, S., Plunder, A., Dubacq, B., Chauvet,A., Monié, P., 2016. Plate interface rheological switches during subduction infancy: control on slab penetration and metamorphic sole formation. Earth and Planetary Science Letters 451, 208–220. Agard, P., Plunder, A., Angiboust, S., Bonnet, G., Ruh, J., 2018. The subduction plate
SC RI PT
interface: Rock record and mechanical coupling (from long to short time scales). Lithos 320-321, 537-566. https://doi.org/10.1016/j.lithos.2018.09.029.
Aitchison, J.C., McDermid, I.R.C., Ali, J.R., Davis, A.M., Zyabrev, S.V., 2007. Shoshonites in southern Tibet record Late Jurassic rifting of a Tethyan intra-oceanic island arc.
NU
Journal of Geology, 115: 197-213.
Aitchison, J.C., Ao, A., Bhowmik, S., Clarke, G.L., Ireland, T.R., Kachovich, S., Lokho,
MA
K.,Stojanovic, D., Roeder, T., Truscott, N., Zhen, Y., Zhou, R., 2019. Tectonic evolution of the western margin of the Myanmar microplate based on new fossil and
ED
radiometric age constraints. Tectonics 38, 1718–1741. https://doi.org/10.1029/2018TC005049.
PT
Anczkiewicz, R., Burg, J.P., Villa, I.M., Meier, M., 2000. Late Cretaceous blueschist
CE
metamorphism in the Indus suture zone, Shangla region, Pakistan Himalaya. Tectonophysics 324, 111-134.
AC
Andersen, T. B., Jamtveit, B., Dewey, J. F., Swensson, E., 1991. Subduction and eduction of continental crust: major mechanisms during continent‐continent collision and orogenic extensional collapse, a model based on the south Norwegian Caledonides. Terra Nova 3(3), 303-310. Angiboust, S., Agard, P., Jolivet, L., Beyssac, O., 2009. The Zermatt‐Saas ophiolite: the largest (60km wide) and deepest (c. 70–80 km) continuous slice of oceanic lithosphere detached from a subduction zone? Terra Nova 21(3), 171-180.
ACCEPTED MANUSCRIPT Angiboust, S., Langdon, R., Agard, P., Waters, D., Chopin, C., 2012. Eclogitization of the Monviso ophiolite (W. Alps) and implications on subduction dynamics. Journal of Metamorphic Geology 30, 37-61. Angiboust, S., Agard, P., Glodny, J., Omrani, J., Oncken, O., 2016. Zagros blueschists: Episodic underplating and long-lived cooling of a subduction zone. Earth and Planetary
SC RI PT
Science Letters 443, 48-58. https://doi.org/10.1016/j.epsl.2016.03.017. Anon., 1986. Geology of Nagaland ophiolite. Geological Survey of India Memoir, 119, 113pp.
Ao, A., Bhowmik, S.K., 2014. Cold subduction of the Neotethys: The metamorphic record
NU
from finely banded lawsonite and epidote blueschists and associated metabasalts of the Nagaland Ophiolite Complex, India. Journal of Metamorphic Geology 32(8), 829-860.
MA
Baxter, A.T., Aitchison, J.C., Zyabrev, S.V., Ali, J.R., 2011. Upper Jurassic radiolarians from the Naga ophiolite, Nagaland, northeast India. Gondwana Research 20, 638-644.
ED
Bhowmik, S.K., Ao, A., 2016. Subduction initiation in the Neo‐Tethys: Constraints from counterclockwise P–T paths in amphibolite rocks of the Nagaland Ophiolite Complex,
PT
India. Journal of Metamorphic Geology 34(1), 17-44.
CE
Black, L.P., Kamo, S.L., Allen, C.M., Aleinikoff, J.N., 2003. TEMORA 1: a new zircon standard for Phanerozoic U-Pb geochronology. Chemical Geology 200, 155–17.
AC
Bohlen, S.R., Boettcher, A.L., 1982. The quartz–coesite transformation: a pressure determination and the effects of other components. Journal of Geophysical Research104,208–224. Brovarone, A.V., Agard, P., 2013. True metamorphic isograds or tectonically sliced metamorphic sequence? New high-spatial resolution petrological data for the New Caledonia case study. Contributions to Mineralogy and Petrology 166(2), 451-469.
ACCEPTED MANUSCRIPT Brovarone, A. V., Groppo, C., Hetenyi, G., Compagnoni, R., Malavieille, J., 2011. Coexistence of lawsonite‐bearing eclogite and blueschist: phase equilibria modelling of Alpine Corsica metabasalts and petrological evolution of subducting slabs. Journal of Metamorphic Geology 29(5), 583-600. Brovarone, A. V., Picatto, M., Beyssac, O., Lagabrielle, Y., Castelli, D. 2014. The
SC RI PT
blueschist–eclogite transition in the Alpine chain: P–T paths and the role of slowspreading extensional structures in the evolution of HP–LT mountain belts.Tectonophysics 615, 96-121.
Brunnschweiler, R.O., 1966. On the geology of Indo-Burman Ranges. Geological Society of
NU
Australia 13, 137–195.
Chatterjee, N., Ghose, N.C., 2010. Metamorphic evolution of the Naga Hills eclogite and
MA
blueschist, Northeast India: Implications for early subduction of the Indian plate under the Burma microplate. Journal of Metamorphic Geology 28(2), 209-225.
ED
Chemenda, A. I., Mattauer, M., Bokun, A. N., 1996. Continental subduction and a mechanism for exhumation of high-pressure metamorphic rocks: new modelling and
PT
field data from Oman. Earth and Planetary Science Letters 143(1-4), 173-182.
CE
Clarke, G. L., Aitchison, J. C., Cluzel, D., 1997. Eclogites and blueschists of the Pam Peninsula, NE New Caledonia: a reappraisal. Journal of Petrology 38(7), 843-876.
AC
Cloos, M., 1982. Flow melanges: numerical modelling and geologic constraints on their origin in the Franciscan subduction complex, California. Bulletin of Geological Society of America 93, 330–345. Cloos, M., 1985. Thermal evolution of convergent plate margins: Thermal modeling and reevaluation of isotopic Ar‐ages for blueschists in the Franciscan Complex of California. Tectonics 4(5), 421-433.
ACCEPTED MANUSCRIPT Cloos, M., Shreve, R., 1988. Subduction channel mode of prism accretion, mélange formation, sediment subduction, and subduction erosion at convergent plate margins: 2. Implications and discussions. Pure and Applied Geophysics 128, 501–541. Coggon, R., Holland, T.J.B., 2002. Mixing properties of phengitic micas and revised garnet‐phengite thermobarometers. Journal of Metamorphic Geology 20(7), 683-696.
SC RI PT
Davis, P.B. Whitney, D.L., 2008. Petrogenesis and structural petrology of high-pressure metabasalt pods, Sivrihisar, Turkey. Contributions to Mineralogy and Petrology 156, 217– 241.
Diener, J.F.A., Powell, R., White, R.W. Holland, T.J.B., 2007. A new thermodynamic model
NU
for clino- and orthoamphiboles in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2OO. Journal of Metamorphic Geology 25, 631–656.
MA
Fitzherbert, J. A., Clarke, G. L., Marmo, B.,Powell, R., 2004. The origin and P–T evolution of peridotites and serpentinites of NE New Caledonia: prograde interaction between
ED
continental margin and the mantle wedge. Journal of Metamorphic Geology 22(4), 327344. https://doi.org/10.1111/j.1525-1314.2004.00517.x.
PT
Ganguly, J., Cheng, W., Tirone, M., 1996. Thermodynamics of aluminosilicate garnet solid
CE
solution: new experimental data, an optimized model, and thermometric applications. Contributions to Mineralogy and Petrology 126, 137–151.
AC
Gardiner, N.J., Searle, M.P., Morley, C.K., Robb, L.J., Whitehouse, M.J., Roberts, N.M., Kirkland, C.L., Spencer, C.J., 2018. The crustal architecture of Myanmar imaged through zircon U-Pb, Lu-Hf and O isotopes: Tectonic and metallogenic implications. Gondwana Research 62, 27-60. doi:10.1016/j.gr.2018.02.008. Gerya, T.V., Stoeckhert, B., Perchuk, A.L., 2002. Exhumation of high-pressure metamorphic rocks in a subduction channel: a numerical simulation. Tectonics 21, 61-69, 1056. doi:10.1029/2002TC001406.
ACCEPTED MANUSCRIPT Goffé, B., Rangin, C., Maluski, H., 2000. Jade and associated rocks from Jade Mines area, northern Myanmar, as record of a polyphased high-pressure metamorphism. EOS Transactions AGU 81, Fall Meeting Supplement, F1365. Goffé, B., Rangin, C., Maluski, H., 2002. Jade and associated rocks from the Jade Mines area, northern Myanmar as record of a polyphased high-pressure metamorphism.
SC RI PT
Himalaya Karakoram-Tibet Workshop Meeting (Abstracts). Journal of Asian Earth Sciences 20, 16–17.
Green, E.C.R., Holland, T.J.B., Powell, R., 2007. An order disorder model for omphacitic pyroxenes in the system jadeite- diopside-hedenbergite-acmite, with applications to
NU
eclogite rocks. American Mineralogist 92, 1181–1189.
Groppo, C., Castelli, D., 2010. Prograde P-T evolution of a lawsonite eclogite from the
MA
Monviso metaophiolite (Western Alps): dehydration and redox reactions during subduction of oceanic FeTi-oxide gabbro. Journal of Petrology 51, 2489–2514.
ED
Groppo, C., Rolfo, F., Sachan, H.K., Rai, S.K., 2016. Petrology of blueschist from the Western Himalaya (Ladakh, NW India): Exploring the complex behavior of a
PT
lawsonite-bearing system in a paleo-accretionary setting. Lithos 252, 41-56.
CE
Guillot, S., Hattori, K., Agard, P., Schwartz, S., Vidal, O., 2009. Exhumation processes in oceanic and continental subduction contexts: a review. In: Lallemand, S., Funcinello, F.
AC
(Eds.), Subduction Zone Geodynamics: pp. 175–205. http://dx.doi.org/10.1007/978-3540-87974-9.
Guilmette, C., Hébert, R., Dostal, J., Indares, A., Ullrich, T., Bédard, É., Wang, C., 2012. Discovery of a dismembered metamorphic sole in the Saga ophiolitic mélange, South Tibet: Assessing an Early Cretaceous disruption of the Neo-Tethyan supra-subduction zone and consequences on basin closing. Gondwana Research 22(2), 398-414.
ACCEPTED MANUSCRIPT Guiraud, M., 1982. Géothermométrie du faciés schistes vert à glaucophane. Modélisation et applications (Afghanistan, Pakistan, Corse, Bohème). Ph.D. thesis, University of Montpellier, France. Hacker, B.R., Wang, X., Eide, E.A., Ratsbacher, L., 1996. The Qinling‐Dabie ultrahigh‐pressure collisional orogen. In: Tectonic Evolution of Asia (Yin, A.,
SC RI PT
Harrison, T.M. eds), Cambridge University Press, Cambridge, pp. 345-370. Hébert, R., Bezard, R., Guilmette, C., Dostal, J., Wang, C.S., Liu, Z.F., 2012. The Indus– Yarlung Zangbo ophiolites from Nanga Parbat to Namche Barwa syntaxes, southern Tibet: First synthesis of petrology, geochemistry, and geochronology with incidences
NU
on geodynamic reconstructions of Neo-Tethys. Gondwana Research 22, 377-397. Hermann, J., Müntener, O., Scambelluri, M., 2000. The importance of serpentinite mylonites
MA
for subduction and exhumation of oceanic crust. Tectonophysics 327(3-4), 225-238. Holland, T.J.B., 1980. The reaction albite = jadeite+ quartz determined experimentally in the
ED
range 600–1200 oC. American Mineralogist 65(1-2), 129-134. Holland, T.J.B., Powell, R., 1998. An internally consistent thermodynamic dataset for phases
PT
of petrological interest. Journal of Metamorphic Geology 16, 309–343.
CE
Holland, T.J.B., Powell, R., 2003. Activity–composition relations for phases in petrological calculations: an asymmetric multicomponent formulation. Contributions to Mineralogy
AC
and Petrology 145, 492–501. Honegger, K., Le Fort, P., Mascle, G., Zimmerman, J.L., 1989. The blueschists along the Indus Suture Zone in Ladakh, N.W. Himalaya. Journal of Metamorphic Geology 7, 57– 72. Htay, H., Zaw, K., Oo, T.T., 2017. The mafic–ultramafic (ophiolitic) rocks of Myanmar. In: Barber, A.J., Khin, Z., & Crow, M.J. (eds) Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs 48, 117–141.
ACCEPTED MANUSCRIPT Imchen, W., Thong, G.T., Pongen, T., 2014. Provenance, tectonic setting and age of the sediments of the Upper Disang Formation in the Phek District, Nagaland. Journal of Asian Earth Sciences 88, 11–27. Jan, M.Q., 1985. High-P rocks along the suture zone around Indo-Pakistan plate and phase chemistry of blueschists from eastern Ladakh. Geological Bulletin University of
SC RI PT
Peshawar 18, 1–40.
Krebs, M., Schertl, H.P., Maresch, W.V., Draper, G., 2011. Mass flow in serpentinite-hosted subduction channels: P-T-t path patterns of metamorphic blocks in the Rio San Juan melange (Dominican Republic). Journal of Asian Earth Sciences 42, 569–595.
NU
Lanari, P., Vidal, O., De Andrade, V., Dubacq, B., Lewin, E., Grosch, E.G., Schwartz, S.,2014. XMapTools: A MATLAB©-based program for electron microprobe X-ray
MA
image processing and geothermobarometry. Computers & Geosciences 62, 227-240. Leake, B.E., Wooley, A.R., Arps, C.E., Birch, W.D., Gilbert, M.C., Grice, J.D., Hawthorne,
ED
F.C., Kato, A., Kisch, H.J., Krivovichec, V.G. and Linthout, K., 1997. Nomenclature of amphiboles: report of the subcommittee on amphiboles of the International
PT
Mineralogical Association, commission on new minerals and mineral names. American
CE
Mineralogist 82, 1019–1037.
Liou, J.G., 1971. P-T Stabilities of laumontite, wairakite, lawsonite, and related minerals in
AC
the system CaAl2Si2O8-SiO2-H2O. Journal of Petrology 12(2), 379-411. Liu, C.Z., Chung, S.L., Wu, F.Y., Zhang, C., Xu, Y., Wang, J.G., Chen, Y., Guo, S., 2016a. Tethyan suturing in Southeast Asia: zircon U-Pb and Hf-O isotopic constraints from Myanmar ophiolites. Geology 44, 311–314. Liu, C.Z., Zhang, C., Xu, Y., Wang, J.G., Chen, Y., Guo, S., Wu, F.Y., Sein, K., 2016b. Petrology and geochemistry of mantle peridotites from the Kalaymyo and Myitkyina ophiolites (Myanmar): implications for tectonic settings. Lithos 264, 495–508.
ACCEPTED MANUSCRIPT Ludwig, K.R., 2003. User’s manual for a geochronological toolkit for Microsoft Excel (Isoplot/Ex version 3.0). Berkeley Geochronology Center, Special Publication 4, 1-71. Mahéo, G., Fayoux, X., Guillot, S., Garzanti, E., Capiez, P., Mascle, G., 2006. Relicts of an intra-oceanic arc in the Sapi-Shergol mélange zone (Ladakh, NW Himalaya, India): implications for the closure of the Neo-Tethys Ocean. Journal of Asian Earth Sciences
SC RI PT
26, 695–707.
Maruyama, S., Cho, M., Liou, J. G., 1986. Experimental investigations of blueschistgreenschist transition equilibria: Pressure dependence of Al2O3 contents in sodic amphiboles- A new geobarometer. Blueschists and Eclogites 164, 1-16.
NU
McDermid, I.R.C., Aitchison, J.C., Davis, A.M., Harrison, T.M., Grove, M., 2002. The Zedong terrane: A Late Jurassic intra-oceanic magmatic arc within the Yarlung-Zangbo
MA
suture zone, southeastern Tibet. Chemical Geology 187, 267-277. Metcalfe, I., 1995. Gondwana dispersion and Asian accretion. Journal of Geology 5-6, 223–
ED
266.
Mével, C., Kiénast, J.R., 1986. Jadeite-kosmochlor solid solution and chromian sodic
PT
amphiboles in jadeitites and associated rocks from Tawmaw (Burma). Bulletin de
CE
Mineralogie 109, 617–633.
Mitchell, A.H.G., 1986. Ophiolite and associated rocks in four settings: relationships to
AC
subduction and collision. Tectonophysics 125, 269-285. Mitchell, A.H.G., 1993. Cretaceous-Cenozoic Tectonic events in the western Myanmar (Burma)-Assam region. Journal of Geological Society, London 150, 1089–1102. Mitchell, A.H.G., Htay, M.T., Htun, K.M., 2015. The medial Myanmar suture zone and the western Myanmar Mogok foreland. Journal of the Myanmar Geosciences Society 6,73– 88.
ACCEPTED MANUSCRIPT Nyunt, T.T., Massonne, H.J., Sun, T.T., 2017. Jadeitite and other high-pressure metamorphic rocks from the Jade Mines Belt, Tawmaw area, Kachin State, northern Myanmar. Geological Society, London, Memoirs 48(1), 295-315. Oberhhänsli, R., 2013. High-pressure – low-temperature evolution in the Indus-Tsangpo Suture along the Kohistan Arc (Kaghan Valley, NE Pakistan). Episodes 36, 87–93.
SC RI PT
Oberhänsli, R., Bousquet, R., Moinzadeh, H., Moazzen, M., Arvin, M., 2007. The field of stability of blue jadeite: a new occurrence of jadeitite from Sorkhan, Iran, as a case study. Canadian Mineralogist 45, 1501–1509.
O'Brien, P.J., 2018. Eclogites and other high-pressure rocks in the Himalaya: a review.
NU
Geological Society, London, Special Publications 483,https://doi.org/10.1144/SP483.13 Okamoto, K., Maruyama, S., 1999. The high-pressure synthesis of lawsonite in the
MA
MORB+H2O system. American Mineralogist 84(3), 362-373. Page, F.Z., Armstrong, L.S., Essene, E.J., Mukasa, S.B., 2007. Prograde and retrograde
ED
history of the Junction School eclogite, California, and an evaluation of garnet-
153, 533–555.
PT
phengite-clinopyroxene thermobarometry. Contributions to Mineralogy and Petrology
CE
Peacock, S.M., Wang, K., 1999. Seismic consequences of warm versus cool subduction metamorphism: examples from southwest and northeast Japan. Science 286, 937–939.
AC
Plunder, A., Agard, P., Dubacq, B., Chopin, C., Bellanger, M., 2012. How continuous and precise is the record of P–T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustrés, W. Alps). Journal of Metamorphic Geology 30(3), 323-346. Plunder, A., Agard, P., Chopin, C., Pourteau, A., Okay, A. I., 2015. Accretion, underplating and exhumation along a subduction interface: From subduction initiation to continental subduction (Tavşanlı zone, W. Turkey). Lithos 226, 233-254.
ACCEPTED MANUSCRIPT Powell, R., Holland, T., 2008. On thermobarometry. Journal of Metamorphic Geology 26, 155–179. Qiu, Z.L., Wu, F.Y., Yang, S.F., Zhu, M., Sun, J.F., Yang, P., 2009. Age and genesis of the Myanmar jadeite: constraints from U-Pb ages and Hf isotopes of zircon inclusions. Chinese Science Bulletin 54, 658–668.
SC RI PT
Ravna, K., 2000. The garnet–clinopyroxene Fe2+–Mg geothermometer: an updated calibration. Journal of Metamorphic Geology 18, 211-219.
Ravna, E.J., Terry, M.P., 2004. Geothermobarometry of UHP and HP eclogites and schists– an evaluation of equilibria among garnet–clinopyroxene–kyanite–phengite–
NU
coesite/quartz. Journal of Metamorphic Geology 22, 57-592.
Rebay, G., Powell, R., Diener, J.F.A., 2010. Calculated phase equilibria for a MORB
MA
composition in a P–T range, 450–650°C and 18–28 kbar: the stability of eclogite. Journal of Metamorphic Geology 28, 635–645.
ED
Reinecke, T., 1998. Prograde high- to ultrahigh-pressure metamorphism and exhumation of oceanic sediments at Lago di Cignana, Zermatt-Saas Zone, western Alps. Lithos 42,
PT
147–189.
CE
Roddick, J.C., van Breemen, O., 1994. U-Pb zircon dating: A comparison of ion microprobe and single grain conventional analyses. In Radiogenic Age and Isotopic Studies,
AC
Report8. Geological Survey of Canada Current Res 1994-F, p 1-9. Rubatto, D., 2017. Zircon: the metamorphic mineral. Reviews in Mineralogy and Geochemistry 83, 261-295. Rubatto, D., Gebauer, D., 2000. Use of cathodoluminescence for U-Pb zircon dating by ion microprobe: some examples from the Western Alps. In Cathodoluminescence in geosciences. Springer, Berlin, Heidelberg. pp. 373-400.
ACCEPTED MANUSCRIPT Rubatto, D., Hermann, J., 2003. Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones. Geochimica et Cosmochimica Acta 67, 2173-2187. Rubatto, D., Hermann, J., 2007. Zircon behaviour in deeply subducted rocks. Elements 3, 3135.
SC RI PT
Sambridge, M.S., Compston, W., 1994. Mixture modeling of multi-component data sets with application to ion-probe zircon ages. Earth and Planetary Science Letters 128, 373–390. Sarkar, A., Datta, A.K., Poddar, B.C., Bhattacharyya, B.K., Kollapuri, V.K., Sanwal, R.,1996. Geochronological studies of Mesozoic igneous rocks from eastern India.
NU
Journal of Southeast Asian Earth Sciences 13, 77–81.
Schmidt, M.W., 1995. Lawsonite: upper pressure stability and formation of higher density
MA
hydrous phases. American Mineralogist 80 (11-12), 1286-1292. Searle, M.P., Kahn, M.A., Fraser, J.E., Gough, S.J., 1999. The tectonic evolution of the
ED
Kohistan-Karakoram collision belt along the Karakoram Highway transect, north Pakistan. Tectonics 18, 929–949.
PT
Searle, M.P., Noble, S.R., Cottle, J.M. et al., 2007. Tectonic evolution of the Mogok
CE
Metamorphic belt, Burma (Myanmar) constrained by U-Th-Pb dating of metamorphic and magmatic rocks. Tectonics 26, TC3014. https://doi:10.1029/2006TC002083.
AC
Searle, M.P., Morley, C.K., Waters, D.J., Gardiner, N.J., Htun, U. K., Nu, T.T., Robb, L.J., 2017.Tectonic and metamorphic evolution of the Mogok Metamorphic and Jade Mines belts and ophiolitic terranes of Burma (Myanmar). In: Barber, A.J., Khin, Z., & Crow, M.J. (eds) Myanmar: Geology, Resources and Tectonics. Geological Society, London, Memoirs 48, 261–293, https://doi.org/10.1144/M48.12.
ACCEPTED MANUSCRIPT Setiawan, N.I., Osanai, Y., Nakano, N., Adachi, T., Yonemura, K., Yoshimoto, A., 2016. Prograde and retrograde evolution of eclogites from the Bantimala Complex in South Sulawesi, Indonesia. Journal of Mineralogical and Petrological Sciences 111, 211–225. Shi, G.H., Cui, W.Y., Tropper, P., Wang, C.Q., Shu, G.M., Yu, H.X., 2003. The petrology of a complex sodic and sodic-calcic amphibole association and its implications for the
SC RI PT
metasomatic processes in the jadeitite area in northwestern Myanmar, formerly Burma. Contributions to Mineralogy and Petrology 145, 355–376.
Shi, G.H., Cui, W.Y., Cao, S.M., Jiang, N., Jian, P., Liu, D.Y., Miao, L.C., Chu, B.B., 2008. Ion microprobe zircon U–Pb age and geochemistry of the Myanmar jadeitite. Journal of
NU
the Geological Society, London 165, 221–234.
Singh, A.K., Chung, S.-L., Bikramaditya, R.K., Lee, H.Y., 2017. New U–Pb zircon ages of
MA
plagiogranites from the Nagaland-Manipur Ophiolites, Indo-Myanmar Orogenic Belt, NE India. Journal of the Geological Society, London 74, 170-179.
ED
Sorensen, S. S., Barton, M. D., 1987. Metasomatism and partial melting in a subduction complex: Catalina Schist, southern California. Geology 15(2), 115-118.
PT
Thet Tin Nyunt., 2009. Petrological and geochemical contribution to the origin of jadeitite
CE
and associated rocks of the Tawmaw area, Kachin State, Myanmar. Doctoral dissertation, Institut für Mineralogie und Kristallchemie der Universität Stuttgart.
AC
Vance, D., Mahar, E., 1998. Pressure-temperature paths from P-T pseudosections and zoned garnets: potential, limitations and examples from the Zanskar Himalaya, NW India. Contributions to Mineralogy and Petrology 132(3), 225-245. Von Blanckenburg, F., Davies, J. H., 1995. Slab break off: a model for syncollisional magmatism and tectonics in the Alps. Tectonics 14(1), 120-131.
ACCEPTED MANUSCRIPT Wang, W.L., Aitchison, J.C., Lo, C.H., Zeng, Q.G., 2008. Geochemistry and geochronology of the amphibolite blocks in ophiolitic mélanges along Bangong-Nujiang suture, central Tibet. Journal of Asian Earth Sciences 33, 122-138. Wei, C.J., Clarke, G.L., 2011. Calculated phase equilibria for MORB compositions: a reappraisal of the metamorphic evolution of lawsonite eclogite. Journal of
SC RI PT
Metamorphic Geology 29 (9), 939-952.
White, R.W., Powell, R., Holland, T.J.B., 2007. Progress relating to calculation of partial melting equilibria for metapelites. Journal of Metamorphic Geology 25, 511–527. Whitney, D.L., Evans, B.W., 2010. Abbreviations for names of rock-forming minerals.
NU
American Mineralogist 95, 185-187.
Xu, Y., Liu, C.Z., Guo, S., Wang, J., Sein, K., 2017. Petrogenesis and tectonic implications
MA
of gabbro and plagiogranite intrusions in mantle peridotites of the Myitkyina ophiolite, Myanmar. Lithos 284-285, 180–193.
ED
Yang, J.S., Xu, Z.Q., Duan, X.D., Li, J., Xiong, F.H., Liu, Z., Cai, Z.H., Li, H.Q., 2012. Discovery of a Jurassic SSZ ophiolite in the Myitkyina region of Myanmar. Acta
PT
Petrologica Sinica 28, 1710–1730 (in Chinese with English abstract).
CE
Yamato, P., Agard, P., Burov, E., Le Pourhiet, L., Jolivet, L., Tiberi, C., 2007. Burial and exhumation in a subduction wedge: mutual constraints from thermo-mechanical
AC
modelling and natural P-T-t data (Schistes Lustres, W. Alps). Journal of Geophysical Research 112, B07410. doi:10.1029/2006JB004441. Yin, A., Harrison, T.M., 2000. Geologic evolution of the Himalayan Tibetan orogen. Annual Review of Earth and Planetary Sciences 28, 211–80. Yu, Y-P., Xie, C-M., Fan, J-J., Wang, M., Dong, Y-C, Wang, B., Hao, Y-J., 2018. Zircon U– Pb geochronology and geochemistry of Early Jurassic granodiorites in Sumdo area,
ACCEPTED MANUSCRIPT Tibet: Constraints on petrogenesis and the evolution of the Neo-Tethyan Ocean. Lithos 320-321, 134-143. Yui, T.F., Fukoyama, M., Iizuka, Y. et al., 2013. Is Myanmar jadeitite of Jurassic age? A result from incompletely recrystallized inherited zircon. Lithos 160–161, 268–282. Zhang, J., Xiao, W., Windley, B.F., Cai, F., Sein, K., Naing, S., 2017. Early Cretaceous
SC RI PT
wedge extrusion in the Indo-Burma Range accretionary complex: Implications for the Mesozoic subduction of Neotethys in SE Asia. International Journal of Earth Sciences 106, 1391–1408.
Zhang, J., Xiao, W., Windley, B., Wakabayashi, J., Cai, F., Sei, K., Wu, H., Naing, S., 2018.
NU
Multiple alternating forearc- and backarc-ward migration of magmatism in the IndoMyanmar Orogenic Belt since the Jurassic: Documentation of the orogenic architecture
MA
of eastern Neotethys in SE Asia. Earth Science Reviews 185, 704-731. Zhang, Y.X., Jin, X., Zhang, K.J., Sun, W.D., Liu, J.M., Zhou, X.Y., Yan, L.L., 2018. Newly
ED
discovered Late Triassic Baqing eclogite in central Tibet indicates an anticlockwise West–East Qiangtang collision. Scientific Reports. 8(1), 966.
PT
https://doi:10.1038/s41598-018-19342-w.
CE
Zhu, D-C., Zhao, Z-D., Niu, Y., Dilek, Y., Hou, Z-Q., Mo, X-X., 2013. The origin and pre-
AC
Cenozoic evolution of the Tibetan Plateau. Gondwana Research 23, 1429-1454.
Figure Captions
Fig. 1: A tectonic map of eastern Tibet, SE China, Myanmar and NE India showing the occurrences of different Tethyan sutures (LCS, Longmu Co-Shuanghu suture; BNS, Bangong-Nujiang suture; YTS, Yarlung-Tsangpo suture (after Liu et al., 2016a) and locations of oceanic blueschists and amphibolites (1-4) along the YTS zone. N/M: Nagaland/Myktyina ophiolites. Other abbreviations used: IMR, Indo-Myanmar Ranges;
ACCEPTED MANUSCRIPT WMB, West Myanmar Block; MCT, Main Central thrust; STDS, South Tibetan Detachment System. Fig. 2: (a) Locations of the Indo-Myanmar Ranges (IMR) and western, central and eastern ophiolite belts (WOB, COB and EOB) in the tectonic framework of Indian, Asian and Burmese plates (modified after Searle et al., 2007). The outlined box in the northern margin
SC RI PT
of the IMR marks the location of the present study in the Nagaland Ophiolite Complex (NOC), shown in detail in Fig. 2b. Geochronological data sources for magmatic and metamorphic events are superscripted and subscripted as follows: Superscript:1. Shi et al.(2008); 2. Yui et al.(2013); 3. Goffé et al. (2002); 4. Singh et al. (2017); 5. Liu et al.
NU
(2016a); 6. Yang et al. (2012); 7. Zhang et al. (2017). Subscript: a, Jadeitite; b, Eclogite; c, Plagiogranite; d, Diorite and Gabbro; e, Rodingites; f, Amphibolite; g, Andesitic basalt,
MA
Leucogabbro and Plagiogranite; h, Trondhjemite. (b) Geological map of the NOC (modified after Anon., 1986; Ao and Bhowmik, 2014). Also shown are the distributions of different
ED
metamorphic facies and locations of eclogite samples [(Mokie eclogite, N22F and Thewati eclogites, N23B and N40B, this study and of previous study, abbreviated as CG10 (after
PT
Chatterjee & Ghose, 2010), along with their peak P-T estimates], blueschist occurrences (BS-
CE
A86, after Anon., 1986) and dated basalt sample (S-96, after Sarkar et al., 1996).
Eclogite.
AC
Abbreviations used: GS, Greenschist; PD, Pumpellyite-diopside; BS, Blueschist, EC,
Fig. 3: (a-b) Outcrop-scale maps showing the occurrences of Mokie (Sample N22F) (a) and Thewati (Samples N23B and N40B) (b) eclogites in a blueschist facies ensemble of metabasalt, marble and metachert. Photographs of hand specimens (c, e, g), photomicrograph (f) and false-colour thin section map (d) of Thewati(c-f) and Mokie (g) eclogite samples showing the mineralogy, texture and micro-structure of these rocks. See text for
ACCEPTED MANUSCRIPT more details. Mineral abbreviations in this and other figures and tables are after Whitney & Evans (2010). Fig. 4: Back-scattered electron images (BSEI, a-b, e, h, n-q, s-u) and photo-micrographs (c-d, f-g, i-m, r) of Thewati (a-q) and Mokie eclogites showing textural features of prograde, peak and multi-stage retrograde metamorphism. (a-c): Porphyroblastic Grt (a-b) and omphacitic
SC RI PT
Cpx (c) contain prograde inclusions of Ep1, Gln1 and Ph. (d) Relics of Omp and Rt occur within Brs porphyroblast. Brs is rimmed by a thin corona of Gln2. (e-f) The Thewati eclogite sample N40B shows a second textural variety of Grt (cf. Grt-B) (e-f), which is smaller in size compared to the larger Grt porphyroblasts (cf. Grt-A) (f). Grt-A shows inclusion-rich core
NU
and inclusion-poor rim. (g-h) The occurrence of Ep2 as very coarse aggregates in layer parallel segregations (g) or in the matrix foliation domain (h). Ep2 contains relics of Grt and
MA
Omp (g-h). (i-l) Syn- (i) to post-tectonic (j-l) growth of Brs. Brs includes Rt, Ep2, Grt and Ttn. Thin idioblastic corona of Gln2 occurs around Brs porphyroblast. (m) Composite corona
ED
of Chl + Ph occurs around Grt porphyroblast. (n-q) The developments of late Lws-bearing assemblages with or without Ab, (F)Wnc, Agt and Spn as veins in Grt (n-p) and Brs (q)
PT
porphyroblasts. (r-v) Sample N22F showing peak minerals of Omp porphyroblast and small
CE
Rt and Grt (r-s). Grt crystals, with an idioblastic habit and often in clusters occur in a matrix of retrograde Brs, Gln and Spn. Prograde inclusion of Gln1 occurs within the Omp
AC
porphyroblast. An intergrowth of Brs + Spn occurs around Omp and includes relics of Grt, Rt and Ep (s-u). Brs is later rimmed by Chl and Gln2 (t-v). Fig. 5: Mn-, Mg-, Fe-, and Ca- X-ray element images of representative porphyroblastic Grt crystals in Thewati eclogite samples N23B (a-h) and N40B (Grt-A in Fig. i-l and Grt-B in Fig. m-p) and corresponding compositional profiles along selected traverses (q-aj). The compositional features are attributed to growth zoning in Grt.
ACCEPTED MANUSCRIPT Fig. 6: (a-c) Ternary quadrilateral pyroxene-jadeite-aegirine diagram showing the compositional features of three textural varieties of Na-Ca Px crystals in the investigated samples: (i) Type 1 (T1) pyroxene as inclusions within Grt (a); (ii) Type 2 pyroxenes in the matrix (T2A type) or as relics in barroisite (T2B type) (b); (iii) T3 pyroxenes in veins (c). (df) Compositional varieties of Na-Ca (d-e) and Na amphiboles (f). (g) Binary NaB vs.
SC RI PT
(Al+Fe3+)VI diagram showing coupled substitution vectors in Na-Ca (dark-grey field) and Na(grey field) amphiboles.
Fig. 7: Colour coded mineral maps of two selected compositional layers in the banded Thewati eclogite sample N40B: Pyroxene-garnet-rich layer (a) and Barroisite-epidote-rich
NU
layer (b) using the software XMapTools (after Lanari et al., 2014). These two compositional layers are chosen for the computations of effective bulk rock compositions for phase
MA
equilibria modelling.
Fig. 8:(a) NCKFMASHTO P-T pseudosection for the Px-Grt-rich bulk rock compositional
ED
domain in sample N40B, showing stability of equilibrium mineral assemblage fields and key index minerals. The P-T estimate (with 2σ error) plotted in the diagram refers to the peak Ep-
PT
absent, eclogite facies metamorphism for the Thewati eclogite. (b-e) The mineral assemblage
CE
fields are contoured with isopleths of Grt compositions [x(g) and z(g) parameters, see Table 4 for the nomenclature] and modes of Grt [abbreviated as (Grt(m)], Omp [O(m)] and Amp
AC
[Amp(m)]. The reconstructed metamorphic reaction pathways in the Thewati eclogites are evaluated with compositional (e.g. Grt composition) and modal variations of these minerals along three representative metamorphic P-T paths of evolution (P1, P2 and P3). (f) A summary P-T diagram showing the stability fields of key index minerals, the P-T condition of formation
of
the
prograde
Grt
core
and
the
possible
sequences
of
mineral
appearances/disappearances along the three model P-T paths (see text for details). The numbers in Fig. 9a refer to different mineral assemblage fields, listed as: (1) Cpx Ph Ep Grt
ACCEPTED MANUSCRIPT Lws Amp Qz Rt, (2) Cpx Ph Ep Grt Lws Amp Rt, (3) Cpx Chl Ph Ep Grt Lws Amp Rt, (4) Cpx Ph Ep Grt Amp Rt, (5) Cpx Chl Ph Ep Amp Rt, (6) Cpx Chl Ph Amp Lws Spn Rt, (7) Cpx Ph Ep Grt Lws Qz Rt, (8) Cpx Ph Grt Amp Lws Qz Rt, (9) Cpx Ph Grt Amp Lws Rt, (10) Cpx Chl Ph Amp Lws Spn, (11) Cpx Ph Grt Lws Rt, (12) Cpx Pl Ph Grt Qz Rt, (13) Cpx Chl Ph Ep Lws Amp Rt, (14) Cpx Ph Grt Lws Coe Rt, (15) Cpx Ph Grt Amp Qz Rt, (16) Cpx
SC RI PT
Chl Ph Grt Lws Rt.
Fig. 9: (a) NCKFMASHTO P-T pseudosection for the Amp-Ep-rich bulk rock compositional domain in sample N40B, showing the stability of equilibrium mineral assemblage fields and key index minerals. (b-e) The mineral assemblage fields are contoured with compositional
NU
isopleths of Grt and Amp (NaB values at 0.5 and 1.5, where the values represent cations per formula unit) and modes of Grt, Omp and Amp. The prograde P-T path P1 predicts a Grt
MA
compositional zoning with progressive depletion in Grs + Adr content from core to inner rim of Grt followed by its enrichment toward the Grt outer rim, consistent with the measured Grt
ED
composition. The Amp compositional isopleths reveal the P-T windows at which Na-Ca (cf. Brs) and Na- (cf. Gln) amphiboles are likely to become stable along the retrograde path, P3.
PT
(f) The summary P-T diagram showing the stability fields of key index minerals and the
CE
possible sequences of mineral appearances along the two model retrograde P-T paths (see text for details). The numbers in Fig. 9a refer to different mineral assemblage fields, listed as:
AC
(1) Cpx Chl Ph Grt Lws Ep Spn, (2) Cpx Chl Ph Amp Lws Ep Spn, (3) Cpx Chl Ph Grt Amp Lws Ep Rt, (4) Cpx Chl Ph Grt Lws Ep Rt, (5) Cpx Ph Amp Pl Ep Rt, (6) Cpx Ph Amp Grt Pl Ep Rt, (7) Cpx Ph Grt Lws Qz Rt, (8) Cpx Ph Grt Lws Coe Rt, (9) Cpx Ph Grt Ep Lws Qz Rt, (10) Cpx Ph Grt Ep Lws Rt, (11) Cpx Ph Grt Amp Ep Rt, (12) Cpx Ph Amp Ep Rt, (13) Cpx Ph Amp Ep Qz Rt, (14) Cpx Ph Amp Pl Ep GrtQz Rt, (15) Cpx Chl Ph Amp Ep Lws Rt, (16) Cpx Chl Ph Ep Lws Rt, (17) Cpx Chl Ph Lws Ep Spn, (18) Cpx Ph Grt Ep Coe Rt, (19) Cpx Ph Grt Ep Lws Coe Rt.
ACCEPTED MANUSCRIPT Fig. 10: Reconstruction of the metamorphic P-T path of evolution of the Thewati eclogite using the following approaches: (a) plotting the P-T locus of the first appearance (shown by the symbol +) or disappearance (symbol -) of key minerals calculated through phase equilibria modelling, (b) calculating the P-T positions of activity corrected mineral reactions R3 to R6 (see text for details for the calculation procedures), (c) placing the mineral
SC RI PT
equilibria, R7 (after Maruyama et al., 1986), R8 (after Bohlen and Boettcher, 1982), R9 (after Holland, 1980) and experimentally determined Lws-out curve after Liou (1971) and Schmidt (1995) (Curve R10) in the P-T space and (d) the estimated P-T conditions of formation of prograde Grt core following the isopleth thermobarometry method and peak metamorphism
NU
through conventional thermobarometry. The deduced metamorphic evolutionary pathways when evaluated with (a) to (d) indicate a CW P-T path of evolution involving a prograde
MA
segment (MPR stage) in the epidote blueschist facies, and culminating in peak metamorphism in the eclogite facies (cf. MP stage) and three successive stages of retrograde evolution in
ED
amphibole eclogite facies (cf. MR1 stage), epidote blueschist facies (cf. MR2 stage) and transitional between greenschist and lawsonite blueschist facies (cf. M R3 stage). Mineral
PT
reactions: R3, 10(Jd)Omp+ (4Gr+5Prp)Grt+ 11Qtz + 8H2O → 5(Gl)Brs+6(Cz)Ep; R4, 5(Gl)Brs+
CE
12(Cz)Ep+ 14Qtz + 28H2O → 3(Tr)Fe-Wnc+ 10 Ab + 18 Lws; R5, 3(Dph)Chl + 5(Prp)Grt= 3(Clc)Chl + 5(Alm)Grt; R6, Ab = Jd17 + Qtz; R7, 25Gln + 6Cz + 7 Qtz + 14H2O = 6Tr + 9Chl
AC
+ 50Ab; R8; Qtz = Coe; R9; LAb = Jd + Qtz. Fig. 11: (a-b) BSE images showing the textural locations of zircon in N22F. (c-d) BSE images showing the distribution patterns of mineral inclusions and shape of host zircon crystal in N22F (c) and N23B (d). (e-f) Cathodoluminescence (CL) (e) and BSE (f) images of zircon showing the presence of Grt and Omp inclusions. (g-x) CL structures of representative analysed zircon grains in samples N23B (g-n) and N22F (o-x). 1-2-3 represent different CL domains (see text for details).
ACCEPTED MANUSCRIPT Fig. 12: Binary plots of Th vs. U compositions (in ppm) (a) and Th/U ratio vs. age (in Ma) (b) of analysed zircon grains in the two eclogite samples. Fig. 13: Concordia plots (a, c-d, f-g) and probability distribution diagrams (b, e) of 206Pb/238U ages (in Ma) in zircon grains from samples N22F (a-d) and N23B (e-g). Compositional ellipses in the concordia diagrams are linked with individual zircon grains (abbreviated as Z-
SC RI PT
10 etc.).
Fig. 14: Peak P-T conditions of metamorphism and metamorphic P–T path of the Nagaland eclogites (e.g. the Thewati eclogite path 1, this study) are compared with those from the Mokie blueschist (path 2) and Satuza eclogite (Path 3), all from the NOC, Himalayan
NU
blueschists (paths 4-6) and jadeitite and associated HP metamorphic rocks in the Central Ophiolite Belt (COB) of Myanmar. The P–T results/paths are plotted in the metamorphic
MA
facies diagram (after Okamoto and Maruyama, 1999). The apparent peak thermal gradients of the Nagaland eclogites are compared with model geotherms. The P-T field of global LT
ED
eclogites, and outlined by a red circle is adopted from Fig. 3a of Agard et al. (2016). See text for details. Abbreviations used: AM, Amphibolites facies; Ep-EC, Epidote eclogite facies;
PT
Amp-EC, Amphibole eclogite facies; BS, Blueschist facies; Dry-EC, Dry eclogite facies; EA,
CE
Epidote amphibolites facies; GS, Greenschist facies. References for metamorphic P-T paths and P-T conditions of metamorphism: 1-6: Metamorphic P-T paths of Neo-Tethyan HP/LT
AC
rocks; 1, Thewati EC (N40B), NOC (this study); 2, Mokie BS, NOC (after Ao & Bhowmik, 2014); 3, Satuza EC, NOC (Chatterjee & Ghose, 2010); 4,Sapi-Shergol-Zildat BS (Mahéo et al., 2006); 5, Sapi-Shergol LBS (Groppo et al., 2016); 6, Shangla BS (Guiraud, 1982, Jan, 1985); 7-11: P-T conditions of metamorphism of jadeitite and associated HP metamorphic rocks in the COB; 7, Mével et al., 1986; 8, Goffé et al., 2000; 9, Shi et al., 2003; 10, Oberhänsli et al., 2007; 11a/11b, Gln schist/Grt-Mica schist (after Thet Tin Nyunt, 2009). (b) P-T diagram comparing the peak P-T conditions of pumpellyite-diopside (PD), BS and
ACCEPTED MANUSCRIPT eclogite facies metamorphic rocks from the Mokie-Satuza (red circles) and Thewati (green circles) areas in the NOC. For a comparison, the P-T gradient of the subducted oceanic crust in a thermo-mechanical model-simulated intra-oceanic subduction system (after Gerya et al., 2002) is also plotted. The Nagaland HP/LT metamorphic rocks lie on the P-T gradient predicted by the thermo-mechanical model.
SC RI PT
Fig. 15: P–T path of the Nagaland eclogite (Path 1, NOC, this study) is compared with that from oceanic eclogites in metamorphic belts of the world. The grey P-T band refers to the average low T/P gradients for mature subduction (after Agard et al., 2018). The curve G02 (after Gerya et al., 2002) is the numerical simulation of prograde paths computed for
NU
subduction interface rocks. References for other paths: 2, Zermatt-Saas, Western Alps (Reinecke, 1998; Angiboust et al., 2009); 3, Dominican Republic (Krebs et al., 2011); 4,
MA
Monviso, Alps (Groppo and Castelli, 2010, Angiboust et al., 2012); 5: Rio San Juan mélange Complex (Krebs et al., 2011); 6: Sulawesi, Indonesia (Setiawan et al., 2016).
ED
Fig. 16: A simplified geological map of the NOC showing a progressive metamorphic sequence from GS through PD, BS to EC facies ensemble of basalt-limestone and chert. Also
PT
shown are the locations of Middle-Upper Jurassic-aged basalt and radiolarian chert samples
CE
of previous studies and PD, BS and eclogite facies samples with peak P-T estimates (Ao and Bhowmik, 2014 and this study). Ages of protolith formation and peak eclogite facies
AC
metamorphism (abbreviated as P and M respectively) of the eclogite samples (this study) are also indicated. To the east, the ophiolite terrane is bounded by Early Paleozoic Naga Metamorphics, a Lhasa/Tengchong-equivalent continental slice. Fig. 17: Cartoon diagrams showing the different tectonic models that correlate the Western, Central and Eastern Ophiolite belts in NE India and adjoining Myanmar and SE China with the Tibetan sutures (modified after Zhang, J.et al., 2018). We suggest that the HP metamorphic rocks of the Nagaland Ophiolite Complex and Jade Mines Belt together
ACCEPTED MANUSCRIPT constitute once continuous Early-Middle Jurassic accretionary complex within the NeoTethys.
Table 1: Representative mineral chemical analyses of garnet in eclogites from the NOC. Table 2: Representative mineral chemical analyses of pyroxene in eclogites from the NOC.
sphene, lawsonite, albite and rutile from the NOC.
SC RI PT
Table 3: Representative mineral chemical analysis of phengite, epidote, amphibole, chlorite,
Table 4: Results of thermobarometry of peak eclogite facies metamorphism in the NOC. Table 5: Calculations of effective bulk rock compositions of two representative chemical
NU
domains in sample N40B.
Table 6: SHRIMP U-Pb analyses of representative zircon grains in Mokie and Thewati
MA
eclogites from the NOC.
AC
CE
PT
ED
Table 7: A summary of characteristics of zircon from the Nagaland eclogite samples.
ACCEPTED MANUSCRIPT Table 1. Representative mineral chemical analysis of garnet in eclogite from the Nagaland Ophiolite complex, India N23B-1 133_I1 Grt C 37.14 0.17 21.03 0.01 27.08 2.90 2.21 9.56 0.09 b.d.l. 100.18 12 2.95 1.95 0.01 — 0.04 1.74 0.19 0.26 0.81 0.01 — 7.96 0.06 0.09 0.58 0.25 0.02 0.13
N23B-1 40_23_I1 Grt IR 37.13 0.06 21.13 0.13 30.11 0.49 3.34 7.99 0.04 0.02 100.42 12 2.93 1.94 — 0.01 0.05 1.91 0.03 0.39 0.67 0.01 — 7.93 0.01 0.13 0.64 0.20 0.02 0.17
N23B-1 57_1F2 Grt OR 37.78 0.12 21.13 b.d.l. 28.39 0.19 3.81 8.43 0.02 0.01 99.88 12 2.98 1.96 0.01 — 0.04 1.83 0.01 0.45 0.71 — — 7.98 b.d.l. 0.15 0.61 0.22 0.02 0.20
N23B-1 10_1F1 Grt OR 38.57 0.05 21.52 0.08 28.24 0.37 4.17 8.96 0.03 b.d.l. 101.98 12 2.97 1.95 — — 0.05 1.76 0.02 0.48 0.74 — — 7.96 0.01 0.16 0.59 0.22 0.02 0.21
N23B-3 1_1_I2 Grt C 37.76 0.62 20.71 0.04 25.60 4.40 2.09 9.86 0.06 b.d.l. 101.14 12 2.97 1.92 0.04 — 0.05 1.63 0.29 0.24 0.83 0.01 — 7.98 0.10 0.08 0.54 0.25 0.02 0.13
N23B-3 1_34_I2 Grt IR 37.92 0.02 21.35 b.d.l. 28.75 0.46 4.06 8.13 b.d.l. 0.02 100.71 12 2.97 1.95 — — 0.04 1.82 0.03 0.47 0.68 — — 7.96 0.01 0.16 0.61 0.20 0.02 0.20
SC RI PT
N23B-1 8_1F2 Grt C 37.32 0.11 20.82 0.05 25.64 3.55 1.70 10.67 0.01 b.d.l. 99.84 12 2.97 1.95 0.01 — 0.05 1.65 0.24 0.20 0.91 — — 7.97 0.08 0.07 0.55 0.28 0.02 0.11
NU
N23B-1 40_2_I1 Grt C 37.85 0.16 21.35 b.d.l. 27.87 3.14 2.17 9.29 0.03 b.d.l. 101.85 12 2.96 1.95 0.01 — 0.04 1.77 0.21 0.25 0.77 — — 7.96 0.07 0.08 0.59 0.24 0.02 0.12
MA
Sample Anal no. Mineral Site SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XSps XPrp XAlm XGrs XAdr XMg
N23B-3 1_39_I2 Grt OR 38.15 0.06 21.44 b.d.l. 27.73 0.40 3.97 9.57 0.08 b.d.l. 101.41 12 2.95 1.94 — — 0.05 1.73 0.03 0.45 0.79 0.01 — 7.96 0.01 0.15 0.58 0.24 0.03 0.21
N40B-4 5 _I4 GrtA C 37.85 0.25 20.81 b.d.l. 26.02 3.62 2.56 9.27 0.04 b.d.l. 100.42 12 2.99 1.94 0.01 — 0.05 1.67 0.24 0.30 0.78 0.01 — 8.00 0.08 0.10 0.56 0.24 0.02 0.15
N40B-4 30_I4 GrtA C 38.06 0.25 20.81 0.02 25.66 4.06 2.48 9.47 0.05 b.d.l. 100.86 12 3.00 1.93 0.02 — 0.05 1.64 0.27 0.29 0.80 0.01 — 8.00 0.09 0.10 0.55 0.24 0.03 0.15
N40B 35_I4 GrtA OR 38.03 0.11 21.17 b.d.l 27.49 0.36 4.20 9.17 0.04 b.d.l 100.5 12 2.97 1.94 0.01 — 0.06 1.73 0.02 0.49 0.76 0.01 — 7.97 0.01 0.16 0.58 0.23 0.03 0.21
AC
CE
PT
ED
Calculation of Fe3+ in Grt after Ravna & Terry (2004) and in pyroxenes, phengite, amphiboles, epidote and chlorite are bas abbreviations are after Whitney and Evans (2010); other abbreviations used: C/IR/OR/R: Core/Inner Rim/Outer Rim/Rim; I/M: In Porphyroblast/Vein; Fol/Cor: Foliation domain/Corona; Garnet and Pyroxene end members are in mole fraction; X Mg= Mg/( b.d.l: below detection limit.
Table 1. Continued Sample Anal no. Mineral Site SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen
N40B-4 P114_I3 GrtB Edge 38.81 0.04 21.81 b.d.l. 27.11 0.32 3.87 9.16 b.d.l. b.d.l. 101.11 12
N40B-4 23_I3 GrtB OR 38.30 0.04 20.95 0.08 28.52 0.31 4.32 8.41 0.05 b.d.l. 100.99 12
N40B-4 66_I3 GrtB OR 37.89 0.07 20.82 b.d.l. 27.47 0.34 4.20 8.72 b.d.l. b.d.l. 99.53 12
N40B-4 P96_I3 GrtB OR 38.43 0.01 21.23 b.d.l. 27.00 0.43 4.30 8.64 0.03 b.d.l. 100.06 12
N40B-4 15_I3 GrtB OR 38.27 0.04 21.13 b.d.l. 26.42 0.38 4.16 9.02 b.d.l. 0.02 99.44 12
N40B-6 P3_I3 GrtB C 37.08 0.20 20.19 b.d.l. 26.00 4.72 2.10 9.09 0.09 b.d.l. 99.46 12
N40B-6 P56_I3 GrtB IR 37.19 0.17 20.00 b.d.l. 28.28 0.43 3.88 8.34 0.02 b.d.l. 98.31 12
N40B-6 58_I3 GrtB IR 37.73 0.04 20.82 b.d.l. 26.48 0.49 4.25 9.16 b.d.l. b.d.l. 98.96 12
N40B-6 P61_I3 GrtB OR 37.69 0.07 20.38 b.d.l. 26.35 0.43 4.22 9.50 0.01 b.d.l. 98.63 12
N22F 69_F2 Grt C 37.47 0.22 20.16 b.d.l. 21.05 11.27 1.81 7.77 0.10 0.01 99.86 12
N22F 53_F2 Grt OR 37.63 0.06 20.88 0.02 23.29 6.23 2.54 8.11 0.10 b.d.l. 98.85 12
N22F 3_I1 Grt I(R)^Ttn 37.25 0.35 20.77 b.d.l. 26.21 7.78 3.02 5.34 0.06 b.d.l. 100.78 12
ACCEPTED MANUSCRIPT 2.98 1.92 — — 0.07 1.78 0.02 0.50 0.70 0.01 — 7.98 0.01 0.17 0.59 0.20 0.04 0.22
2.99 1.93 — — 0.06 1.75 0.02 0.49 0.74 — — 7.99 0.01 0.16 0.58 0.21 0.03 0.22
3.01 1.97 — — 0.03 1.74 0.03 0.50 0.73 — — 8.00 0.01 0.17 0.58 0.23 0.02 0.22
3.02 1.97 — — 0.03 1.72 0.03 0.49 0.76 — — 8.00 0.01 0.16 0.57 0.24 0.01 0.22
2.97 1.90 0.01 — 0.09 1.65 0.32 0.25 0.78 0.01 — 7.99 0.11 0.08 0.55 0.22 0.04 0.13
2.99 1.89 0.01 — 0.10 1.79 0.03 0.46 0.72 — — 7.99 0.01 0.15 0.60 0.19 0.05 0.21
2.99 1.94 — — 0.06 1.69 0.03 0.50 0.78 — — 7.99 0.01 0.17 0.56 0.23 0.03 0.23
SC RI PT
3.01 2.00 — — — 1.76 0.02 0.45 0.76 — — 8.00 0.01 0.15 0.59 0.25 0.00 0.20
NU
Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XSps XPrp XAlm XGrs XAdr XMg
3.00 1.91 — — 0.09 1.66 0.03 0.50 0.81 — — 7.99 0.01 0.17 0.55 0.23 0.04 0.23
3.00 1.91 0.01 — 0.07 1.35 0.77 0.22 0.67 0.02 — 8.00 0.26 0.07 0.19 0.45 0.04 0.14
3.02 1.99 — — 0.01 1.57 0.43 0.31 0.70 0.02 — 8.00 0.14 0.10 0.23 0.52 0.00 0.16
2.96 1.93 0.02 — 0.05 1.68 0.52 0.35 0.45 0.01 — 7.96 0.17 0.12 0.13 0.56 0.03 0.17
Table 2. Representative mineral chemical analysis of pyroxenes in eclogite from the Nagaland Ophiolite Complex, India. N23B-1 9_F1 Omp I^Grt (R) 55.80 0.11 8.01 0.01 8.57 b.d.l. 7.93 13.68 6.45 0.01 100.57 6 2.00 0.34 — — 0.10 0.16 — 0.42 0.53 0.45 — 4.00 0.55 0.35 0.10 0.73
MA
N23B-1 10_1F2 Agt I^Grt (C) 53.62 0.04 4.50 0.03 11.01 0.17 9.10 16.20 4.82 0.01 99.49 6 1.98 0.20 — — 0.20 0.14 0.01 0.50 0.64 0.34 — 4.00 0.65 0.16 0.19 0.78
ED
N23B-1 7_1F2 Omp I^Grt (C) 53.67 0.08 4.26 0.01 10.17 0.26 9.67 16.71 4.17 b.d.l. 99.00 6 1.99 0.19 — — 0.13 0.19 0.01 0.53 0.66 0.30 — 4.00 0.70 0.17 0.13 0.74
PT
N23B-1 15_F1 Omp I^Grt (C) 54.89 0.04 5.32 b.d.l. 10.85 0.13 8.73 16.15 5.05 0.02 101.16 6 1.99 0.23 — — 0.15 0.18 — 0.47 0.63 0.35 — 4.00 0.64 0.21 0.15 0.73
CE
Site SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XQuad XJd XAeg XMg
N23B-1 14_F1 Omp I^Grt (C) 52.57 0.01 5.53 0.01 12.95 0.19 9.30 14.47 4.38 0.08 99.48 6 1.95 0.24 — — 0.18 0.22 0.01 0.51 0.57 0.31 — 4.00 0.67 0.16 0.16 0.70
AC
Sample Anal no. Mineral
N23B-1 53_1F2 Omp I^Grt (R) 55.59 0.16 8.58 0.03 8.42 0.03 7.26 12.76 6.83 b.d.l. 99.66 6 2.01 0.37 — — 0.09 0.17 — 0.39 0.49 0.48 — 3.99 0.52 0.39 0.09 0.70
N23B-1 13_4F1 Omp
N23B-1 16_4F1 Omp
P(C)
P(R)
55.81 0.08 6.68 0.04 10.47 0.31 7.39 11.82 7.39 0.01 99.99 6 2.02 0.28 — — 0.20 0.12 0.01 0.40 0.46 0.52 — 4.00 0.48 0.30 0.21 0.77
55.51 0.15 5.78 b.d.l. 11.31 0.23 8.03 12.06 6.94 0.01 100.03 6 2.01 0.25 — — 0.21 0.13 0.01 0.43 0.47 0.49 — 4.00 0.51 0.26 0.23 0.99
N23B-1 59 _IF2 Omp Relics^ Brs(P) 54.45 b.d.l. 7.97 0.04 7.75 b.d.l. 8.45 13.93 6.16 0.02 98.76 6 1.98 0.34 — — 0.13 0.11 — 0.46 0.54 0.44 — 4.00 0.56 0.32 0.12 0.81
N40B-4 4_I4 Omp I^GrtA (C) 54.90 0.06 6.04 0.06 9.61 0.18 8.66 15.09 5.62 b.d.l. 100.21 6 1.99 0.26 — — 0.15 0.14 0.01 0.47 0.59 0.40 — 4.00 0.60 0.25 0.15 0.77
N40B-4 29_I4 Omp I^GrtA (C) 54.69 0.05 6.46 0.01 10.10 0.15 8.45 14.97 5.85 0.01 100.74 6 1.97 0.28 — — 0.19 0.12 0.01 0.45 0.58 0.41 — 4.00 0.58 0.24 0.18 0.80
N40B40_I4 Omp I^GrtA (R) 54.19 0.08 8.37 0.05 9.57 0.13 7.36 12.99 6.77 0.01 99.50 6 1.97 0.36 — — 0.18 0.11 — 0.40 0.51 0.48 — 3.99 0.52 0.31 0.17 0.79
ACCEPTED MANUSCRIPT
Table 2. Continued N40B-4 42_I3 Omp
Fol(C)
Fol(R)
54.94 0.01 8.16 b.d.l. 6.81 b.d.l. 8.64 14.45 5.94 0.03 98.98 6 2.00 0.35 — — 0.08 0.13 — 0.47 0.56 0.42 — 4.00 0.58 0.34 0.08 0.79
55.23 0.10 7.93 0.06 7.17 0.08 8.43 14.37 6.01 b.d.l. 99.37 6 2.00 0.34 — — 0.07 0.14 — 0.46 0.56 0.42 — 3.99 0.58 0.35 0.08 0.76
N40B-4 84_I3 Agt V(+Lws)^ GrtB 53.23 0.06 3.79 b.d.l. 19.92 0.33 3.81 6.77 9.47 b.d.l. 97.38 6 2.01 0.17 — — 0.50 0.13 0.01 0.21 0.27 0.69 — 4.00 0.31 0.17 0.52 0.63
N40B-4 23_I4 Agt V(+Ab)^ GrtA 53.74 0.22 4.27 0.02 21.82 0.33 2.23 4.84 10.97 0.01 98.44 6 2.00 0.19 0.01 — 0.59 0.10 0.01 0.12 0.19 0.79 — 4.00 0.21 0.19 0.60 0.57
N40B-4 74_I3 Agt V^MOmp 53.92 0.03 5.32 0.01 16.25 0.25 4.63 7.91 9.21 0.03 97.56 6 2.01 0.23 — — 0.41 0.10 0.01 0.26 0.32 0.67 — 4.00 0.33 0.24 0.42 0.73
SC RI PT
N40B-4 41_I3 Omp
NU
N40B-4 14_I3 Omp I^GrtB (OR) 54.67 b.d.l. 7.77 0.07 8.70 0.06 7.73 13.77 6.25 b.d.l. 99.01 6 2.00 0.33 — — 0.12 0.15 — 0.42 0.54 0.44 — 4.00 0.56 0.33 0.12 0.74
MA
Site
N40B-6 11_I3 Omp I^GrtB (OC) 54.78 0.10 7.51 b.d.l. 8.57 0.20 8.08 14.38 6.08 0.02 99.72 6 1.99 0.32 — — 0.13 0.13 0.01 0.44 0.56 0.43 — 4.00 0.57 0.30 0.13 0.77
N40B-6 32_I3 Omp I^GrtB (IR) 54.34 0.08 8.40 b.d.l. 8.99 0.19 7.12 13.18 6.71 0.01 99.02 6 1.98 0.36 — — 0.15 0.13 0.01 0.39 0.52 0.47 — 4.00 0.52 0.33 0.14 0.75
N40B-6 39_I3 Omp I^GrtB (IR) 54.82 0.05 9.25 b.d.l. 8.58 0.10 7.27 12.56 6.92 b.d.l. 99.54 6 1.98 0.39 — — 0.13 0.13 — 0.39 0.49 0.48 — 4.00 0.51 0.37 0.12 0.75
N40B-6 48_I3 Omp Fol 54.55 0.01 7.63 0.01 9.31 0.02 7.75 13.37 6.55 0.03 99.23 6 1.98 0.33 — — 0.17 0.12 — 0.42 0.52 0.46 — 4.00 0.53 0.30 0.16 0.78
CE
PT
SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XQuad XJd XAeg XMg
N40B-4 65_I3 Omp I^GrtB (OR) 55.23 0.06 9.21 0.02 7.90 0.07 7.46 12.58 6.93 b.d.l. 99.46 6 1.99 0.39 — — 0.10 0.14 — 0.40 0.49 0.49 — 4.00 0.51 0.38 0.10 0.75
ED
Sample Anal no. Mineral
AC
Table 3. Representative mineral chemical analysis of phengite, epidote, amphiboles, chlorite, sphene, lawsonite, albite and Nagaland Ophiolite Complex, India. Sample Anal no. Mineral
N23B-1 103_I1 Ph
N23B-1 56_2_I2 Ph
N40B-4 26 _I4 Ph
N40B-4 67_I3 Ph
N22F 44_I1 Ph
N22F 1_I4 Ep
N23B-1 2_4F1 Ep1
N40B-4 3_I4 Ep1
N40B-4 29_I3 Ep2
N40B-6 49_I3 Ep2
N23B-1 42_2_I2 Brs
N40B-4 93_I3 Brs
Site
I^Grt(R)
M
I^GrtA(C)
M(C)
M
I^Omp
I^Grt(C)
I^Grt(C)
Fol
I^Brs(P)
Fol
P
SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen
53.57 0.10 23.25 0.18 5.06 0.05 4.13 0.04 0.17 10.93 97.48 11
52.12 0.25 23.25 0.02 3.74 b.d.l. 4.22 0.01 0.14 11.12 94.87 11
51.85 0.07 22.11 b.d.l. 5.32 0.15 3.49 0.08 0.12 11.09 94.28 11
52.66 0.43 26.03 b.d.l. 3.87 b.d.l. 3.18 b.d.l. 0.67 10.49 97.33 11
55.59 0.02 21.86 b.d.l. 2.55 0.09 5.87 b.d.l. 0.12 11.60 97.70 11
38.13 0.17 26.62 0.13 8.35 0.21 0.11 23.23 0.01 0.00 96.95 12.50
37.80 0.07 23.86 0.01 11.51 0.24 0.04 22.78 b.d.l. b.d.l. 96.31 12.5
38.27 0.04 24.03 0.02 11.50 0.31 0.03 23.37 0.05 b.d.l. 97.62 12.5
38.06 0.16 24.46 b.d.l. 9.51 0.11 0.04 23.77 0.02 b.d.l. 96.13 12.5
38.26 0.06 25.13 b.d.l. 8.74 0.18 0.01 24.16 b.d.l. b.d.l. 96.54 12.5
51.70 0.17 7.43 b.d.l. 10.72 0.16 14.99 9.42 2.85 0.29 97.70 23
50.97 0.12 6.80 b.d.l. 12.89 0.15 13.36 8.18 3.22 0.20 95.88 23
ACCEPTED MANUSCRIPT 3.54 1.81 0.01 0.01 0.05 0.23 — 0.41 — 0.02 0.92 6.99 0.64 –
3.53 1.86 0.01 — — 0.21 — 0.43 — 0.02 0.96 7.02 0.67 –
3.57 1.79 — — — 0.31 0.01 0.36 0.01 0.02 0.97 7.03 0.46 –
3.46 2.02 0.02 — — 0.21 — 0.31 — 0.09 0.88 6.99 0.59 –
3.63 1.68 — — — 0.14 0.01 0.57 — 0.02 0.97 7.02 0.80 –
2.99 2.46 0.01 0.01 0.52 0.03 0.01 0.01 1.95 0.00 0.00 8.00 – 0.17
3.01 2.24 b.d.l. b.d.l. 0.76 0.01 0.02 b.d.l. 1.94 b.d.l. b.d.l. 7.99 – 0.25
3.01 2.23 — — 0.75 0.01 0.02 — 1.97 0.01 — 8.00 – 0.25
3.05 2.31 0.01 — 0.44 0.20 0.01 0.01 2.04 — — 8.07 – 0.16
3.03 2.34 — — 0.57 0.01 0.01 — 2.05 — — 8.01 – 0.20
7.34 1.24 0.02 — 0.22 1.06 0.02 3.17 1.43 0.78 0.05 15.33 0.75 –
7.42 1.17 0.01 — 0.30 1.27 0.02 2.90 1.28 0.91 0.04 15.31 0.70 –
NU
SC RI PT
Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XMg XPs
I^Grt(C)
COR
55.81 b.d.l. 7.21 0.13 15.88 0.21 9.33 1.90 6.30 0.01 96.79 23 7.95 1.21 — 0.01 0.39 1.51 0.03 1.98 0.29 1.74 — 15.11 0.57 –
57.52 0.01 6.22 b.d.l. 14.52 0.01 11.23 0.96 6.81 b.d.l. 97.27 23 8.00 1.02 — — 0.81 0.88 — 2.33 0.14 1.84 — 15.01 0.73 –
ED
N23B-1 50_I2 Gln2
PT
SiO₂ TiO₂ Al₂O₃ Cr₂O₃ FeO MnO MgO CaO Na₂O K₂O Total Oxygen Si Al Ti Cr Feᵌ⁺ Fe²⁺ Mn Mg Ca Na K Sum XMg XPs
N23B-1 12_I1 Gln1
CE
Site
N40B-4 101_I3 Fwn V(+Lws) ^Brs 52.88 0.03 1.41 b.d.l. 25.59 0.57 7.21 2.88 5.19 0.04 95.79 23 7.95 0.25 — — 1.17 2.05 0.07 1.62 0.46 1.51 0.01 15.10 0.44 –
AC
Sample Anal no. Mineral
N40B-2 72_I3 Gln2
N22F 101_I2 Gln1
N22F 63_I2 Gln2
N23B-1 38_I2 Chl
70_I1 Chl
N40B-4 6_I4 Spn1
N23B-1 85_I1 Spn2
COR
I^Agt
Cor
Cor
Cor
I^Grt(C)
Cor
56.63 0.05 7.67 0.06 12.79 0.21 11.17 1.85 6.08 0.02 96.53 23 7.97 1.27 0.01 0.01 0.31 1.20 0.03 2.34 0.28 1.66 — 15.06 0.66 –
57.58 0.02 9.79 0.02 11.09 0.24 11.52 1.09 7.13 0.02 98.49 23 7.87 1.58 — — 0.30 0.97 0.03 2.35 0.16 1.89 — 15.13 0.71 –
57.95 b.d.l. 8.80 b.d.l. 10.71 0.16 12.05 1.28 6.97 0.05 97.97 23 7.95 1.42 — — 0.28 0.95 0.02 2.46 0.19 1.85 0.01 15.13 0.72 –
28.37 0.02 18.48 b.d.l. 18.33 0.61 21.65 0.04 0.05 0.01 87.56 14 2.88 2.21 — — 0.03 1.53 0.05 3.28 0.01 0.01 — 10.00 0.68 –
29.06 0.05 18.97 0.00 14.75 0.33 24.86 0.05 0.00 0.00 88.09 14.00 2.87 2.21 — — 0.04 1.18 0.03 3.66 0.01 0.00 0.00 10.00 0.76 –
30.10 38.43 1.07 0.04 1.12 0.14 b.d.l. 28.35 0.03 0.02 99.30 5 0.99 0.04 0.95 — — 0.03 — — 1.00 — — 3.02 – –
30.70 38.25 1.43 b.d.l. 0.54 0.11 0.01 29.45 0.06 0.01 100.57 5 1.00 0.05 0.94 — — 0.01 — — 1.03 — — 3.03 – –
MA
Table 3. Continued
N22F
ACCEPTED MANUSCRIPT
ts of thermobarometry of peak eclogite facies metamorphism in the NOC. Grt
Cpx
Simultaneous solution of Grt-Cpx thermometry and Grt-Cpx-Phe barometry
Phe
Thermocalc average Cpx-Ph ass
t)
z(grt)
f(grt)
m(grt)
x(cpx)
j(cpx)
f(cpx)
Si(mu)
x(mu)
y(mu)
na(mu)
PRT , TRAV
PTC , TTC
PTC(Av.)(SdP),TTC(Av.)(S
0
0.24, ¹0.24
0.02
0.00
0.30
0.48
0.19
3.54
0.36
0.39
0.02
29.0±2, 650±60
28.7±3.7, 665±110
29.8±3.7, 685±70
29.1±3.7, 665±110
29.2±3.5, 650±65
0.24, ¹0.24
0.02
0.00
0.30
0.48
0.19
3.53
0.33
0.39
0.02
0
0.24, ¹0.24
0.02
0.00
0.27
0.45
0.23
3.54
0.36
0.39
0.02
27.3±2, 600±60
27.1±3.5, 615±90
29.3±3.6, 675±65
0
0.24, ¹0.24
0.02
0.00
0.27
0.45
0.23
3.53
0.33
0.39
0.02
27.6±2, 605±60
27.5±3.5, 620±90
28.7±3.5, 645±60
0.39
0.02
28.5±2, 640±60
28.1±3.4, 655±100
30.5±3.7, 710±70
0.33 0.39 Weighted average
0.02
28.8±2, 640±60
28.5±3.4, 660±100 28.1±2.8, 645±80
29.7±3.5, 675±65
28.4±1.6, 630±50 0.09
26.5±2, 675±60
26.0±3.3, 695±110
28.6±3.5, 770±75
0.09
25.6±2, 605±60
25.0±3.6, 625±100
30.2±4.7, 735±85
0.09
26.6±2, 645±60
26.0±3.1, 660±100
28.9±3.3, 765±75
9
0.25, ¹0.25
0.02
0.01
0.27
0.45
0.23
3.54
9
0.25, ¹0.25
0.02
0.01
0.27
0.45
0.23
3.53
0.36
8
0.24, ¹0.25
0.02
0.01
0.29
0.45
0.22
3.46
0.41
0.49
8
0.23, ¹0.24
0.04
0.01
0.26
0.51
0.25
3.46
0.41
0.49
8
0.26, ¹0.26
0.01
0.01
0.26
0.44
0.26
3.46
0.41
0.49
8
0.25, ¹0.25
0.03
0.01
0.25
0.49
0.21
3.46
0.41
0.49
Weighted average
7
0.26, ¹0.26
0.03
0.01
0.25
0.48
0.25
7
0.26, ¹0.26
0.03
0.01
0.25
0.47
0.30 Weighted average
0.15, ¹0.18
0.03
0.17
0.23
0.50
0.38
3.63
0.20
0.31
3
0.15, ¹0.18
0.03
0.17
0.24
0.46
0.41
3.63
0.20
0.31
4
0.16, ¹0.19
0.02
0.16
0.24
0.46
0.41
3.63
0.20
0.09
0.31
Weighted average
25.6±2, 605±60
24.8±3.4, 620±100
29.7±4.6, 739±90
25.5±3.3, 650±100
29.2±3.8, 755±80
²635±60
²645±95 ²640±90 ²640±130
0.02
²630±85 24.2±2, 550±60
24.1±4.8, 550±80
0.02
23.6±2, 565±60
23.4±4.4, 570±80
0.02
29.5±2.9, 670±50
26.1±2.0, 635±60 ²625±60
NU
3
SC RI PT
0
29.4±2, 650±60
24.0±2, 540±60 23.9±2.3, 550±70
24.0±4.3, 545±75 23.8±5.1, 555±90 2+
2+
#
24.1±4.2, #475±80
#
22.9±4.1, #470±80
#
23.1±3.6, #455±75 23.3±4.5, 465±90
AC
CE
PT
ED
MA
used: Chemical compositions of garnet (grt), clinopyroxene (cpx) and phengite (mu), x(mineral)=Fe /(Fe +Mg); z(grt)=Ca/(Ca+M m(grt)=Mn/(Ca+Mg+Fe2++Mn); j(cpx)=Na(total); f(cpx)=Fe3+/(Fe3++Al(VI)); y(mu)=XAlM2A; na (mu)=XNaA ; TRAV/PRT , temperatur ed using Grt-Cpx thermometry of Ravna (2000) & Grt-Cpx-Ph barometry of Ravna and Terry (2004); TTC/PTC: temperature (⁰C)/ g Grt-Cpx thermometric and Grt-Cpx-Ph barometric reactions calculated by using Thermocalc software (v.3.31); T TC(Av)/PTC(Av): Therm C)/pressure (kbar) calculated using Thermocalc software (v.3.31) (see text for details); #Average P-T computations with Grt activity mo , z(grt)=Ca/(Ca+Mg+Fe2+); 2,temperature calculated at a PRef=26 kbar.
ACCEPTED MANUSCRIPT
Table 5: Calculations of effective bulk rock compositions of two representative chemical domains in sample N40B NOC. XMapTools Data
Mole %
52.44±1.57 0.11±0.07 5.97±1.96 — 15.09±3.01 0.22±0.15 12.31±1.39 6.81±2.09 3.88±0.83 0.18±0.08 —
50.31±0.88 0.24±0.08 12.96±1.66 0.03±0.03 12.77±2.52 0.19±0.17 6.29±0.93 10.76±1.26 4.04±0.58 1.38±0.05 —
52.56 0.19 7.98 0.00 11.27 0.17 9.79 12.04 4.09 0.92 0.99
97.00±4.73
98.97±3.70
100.00
Calculated Bulk
Mole%
48.50±0.45 0.37±0.22 13.57±0.67 0.02±0.04 11.97±0.84 0.36±0.26 6.40±0.39 13.77±0.47 3.34±0.21 0.71±0.07 — 99.01±1.56
50.48 0.29 8.34 0.00 10.52 0.32 9.93 15.36 3.37 0.47 0.92 100.00
Grt
Ep
Ttn
Ph
Omp
Brs
17.04 4.00 0.34 15.40 57.11 6.11 100.00
SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O O2
38.07±0.46 0.07±0.05 20.82±0.34 0.02±0.03 27.49±0.88 0.53±0.17 3.88±0.32 8.80±0.46 0.04±0.03 0.00±0.01 —
38.40±0.30 0.09±0.03 24.77±0.64 0.04±0.04 10.11±1.03 0.06±0.06 0.10±0.11 23.40±0.50 0.03±0.02 0.00±0.00 —
30.96±0.09 36.92±1.64 2.09±0.97 — 0.86±0.47 0.04±0.06 — 29.09±0.11 0.02±0.03 0.02±0.01 —
52.84±1.25 0.34±0.12 25.12±1.80 0.01±0.02 3.96±0.21 0.03±0.03 3.56±0.51 0.01±0.02 0.45±0.22 10.68±0.29 —
54.78±0.93 0.07±0.07 7.40±2.13 0.03±0.04 9.65±3.68 0.12±0.22 7.65±1.25 13.20±1.80 6.56±0.87 0.01±0.01 —
Total
99.73±1.20
97.00±1.35
100.00±1.97
96.99±2.30
99.46±4.96
N40B-6
SC RI PT
Mineral
N40B-6
Sample
Average mineral composition(EPMA)
Mineral
Grt
Ep
Ttn
13.48 20.94 0.62 7.81 37.39 19.76 100.00
SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O O2 Total
37.80±0.45 0.24±0.39 20.29±0.38 0.02±0.03 27.01±1.17 1.97±1.42 3.19±0.66 9.38±0.52 0.05±0.06 0.05±0.19 — 100±2.15
38.60±0.32 0.23±0.64 24.70±0.65 0.04±0.08 10.34±0.92 0.10±0.08 0.08±0.08 24.38±0.28 0.03±0.05 0.01±0.01 — 98.50±1.37
29.85±0.15 39.00±0.33 1.26±0.10 0.04±0.05 0.72±0.29 0.01±0.01 — 29.05±0.16 0.05±0.03 0.02±0.01 — 100.00±0.51
PT CE
Ph
Omp
Brs
51.92±1.93 0.35±0.25 23.72±2.73 0.01±0.02 4.27±0.16 0.06±0.04 3.89±0.80 0.07±0.10 0.43±0.27 11.28±0.50 — 96.00±3.50
54.96±0.34 0.07±0.04 7.93±0.67 0.02±0.03 8.48±0.99 0.08±0.07 8.08±0.45 14.13±0.65 6.26±0.35 0.01±0.01 — 100.00±1.52
51.24±0.38 0.15±0.04 6.67±0.21 0.03±0.05 13.53±0.30 0.11±0.08 13.41±0.18 8.35±0.29 3.27±0.14 0.24±0.03 — 97.00±0.65
NU
Mode
AC
Grt Ep Spn Ph Omp Brs Total
Calculated Bulk
Mode
XMapTools Data Phases
Average mineral composition(EPMA)
MA
Grt Ep Spn Ph Omp Brs Total
N40B-4
Sample
ED
Phases
N40B-4
ACCEPTED MANUSCRIPT Table 6: SHRIMP U-Pb analyses of representative zircon grains in Mokie and Thewati eclogites from the Nagaland Ophiolite Complex ²⁰⁴Pb /²⁰⁶Pb
% comm 206 Pb
4corr ²⁰⁷Pb* /²⁰⁶Pb*
% err
4corr ²⁰⁷Pb* /²³⁵U
% err
4corr ²⁰⁶Pb* /²³⁸U
% err
204corr ²⁰⁶Pb* /²³⁸U Age(Ma)
31.23
4.6E-4
0.84
.0477
5.5
0.177
5.7
.0269
1.4
171
0.15
35.05
7.0E-4
1.28
.0701
5.8
0.519
6.0
.0536
1.5
337
66
0.03
60.67
5.0E-4
0.92
.0476
4.9
0.212
5.1
.0322
1.4
204
1267
30
0.02
40.33
2.1E-4
0.38
.0600
7.5
0.305
7.6
.0369
1.4
234
Zrn-13(O)
1110
36
0.03
26.37
9.0E-4
1.64
.0446
9.3
0.167
9.4
.0272
1.6
173
Zrn-22(N1)
1705
13
0.01
46.00
2.3E-4
0.42
.0491
2.9
0.212
3.3
.0313
1.6
199
Zrn-28(N1)
1380
271
0.20
52.09
3.1E-4
0.57
.0528
5.2
0.318
5.5
.0437
1.6
276
Zrn-33(N1)
1091
94
0.09
41.31
1.4E-3
2.58
.0566
13.3
0.335
13.4
.0429
1.8
271
Zrn-35(N1)
1511
88
0.06
44.94
8.2E-4
1.51
.0496
7.9
0.233
8.1
.0341
1.7
216
Zrn-36(N1)
2387
15
0.01
63.24
1.0E-3
1.90
.0495
7.9
0.207
8.1
.0302
1.6
192
Zrn-40(N1)
1197
10
0.01
33.80
6.8E-4
1.24
.0565
6.4
0.253
6.6
.0325
1.7
206
Zrn-45(N1)
1939
24
0.01
50.06
3.5E-4
0.65
.0480
3.4
0.198
3.8
.0299
1.7
190
Zrn-46(N1)
2478
8
0.003
63.27
1.1E-4
0.19
.0501
1.8
0.205
2.4
.0297
1.6
188
Zrn-47(N1)
1469
40
0.03
38.31
1.9E-3
3.57
.0591
11.6
0.238
11.8
.0293
1.8
186
Zrn-49(N1)
1561
29
0.02
57.64
1.3E-4
0.24
.0555
3.4
0.328
3.8
.0429
1.7
271
Zrn-50(N1)
2051
36
0.02
52.05
1.8E-4
0.34
.0497
2.8
0.202
3.2
.0294
1.6
187
Zrn-55(N1) Sample N23B
1765
163
0.09
61.25
3.5E-3
6.49
.0553
16.6
0.288
16.7
.0378
1.7
239
Zrn-10
106
0.88
0.01
3.03
4.8E-3
8.77
.0339
91.2
0.142
91.4
.0305
5.6
194
Zrn-16
97
0.85
0.01
2.48
1.2E-3
2.11
.0488
21.1
0.196
21.3
.0291
2.9
185
Zrn-21
45
0.16
0.004
1.19
7.7E-3
14.05
.0694
84.9
0.253
85.3
.0265
7.8
168
Zrn-22
200
0.57
0.003
6.22
1.2E-2
22.72
‒
‒
‒
‒
.0280
5.3
178
Zrn-23
92
0.24
0.003
2.98
1.4E-2
25.27
‒
‒
‒
‒
.0281
7.0
178
Zrn-34
136
0.39
0.003
3.78
6.7E-4
1.22
.0479
19.0
0.212
19.3
.0320
3.5
203
Zrn-46
61
0.002
1.97
6.5E-3
11.20
.0756
65.2
0.347
65.6
.0333
7.0
211
²⁰⁴Pb /²⁰⁶Pb
% comm 206
4corr ²⁰⁷Pb* /²⁰⁶Pb*
% err
4corr ²⁰⁷Pb* /²³⁵U
% err
4corr ²⁰⁶Pb* /²³⁸U
% err
204corr ²⁰⁶Pb* /²³⁸U Age(Ma)
U (ppm)
Th (ppm)
Th/U
²⁰⁶Pb* (ppm)
Zrn-8(O)
1340
27
0.02
Zrn-9(O)
751
113
Zrn-10(O)
2172
Zrn-11(O)
Spot No.
NU
MA
ED
PT
CE
AC
Table 6: contd.
0.14
SC RI PT
Sample N22F
U (ppm)
Th (ppm)
Th/U
²⁰⁶Pb* (ppm)
57
0.18
0.003
1.84
6.9E-3
12.72
.0235
79.5
0.106
79.6
.0328
4.1
208
Zrn-53
126
0.23
0.002
3.43
1.0E-3
1.87
.0408
29.2
0.175
29.4
.0311
3.7
198
Zrn-54
55
0.12
0.002
1.52
3.6E-3
6.60
‒
‒
‒
‒
.0298
5.2
189
Zrn-62
59
0.11
0.002
1.70
6.4E-3
11.80
‒
‒
‒
‒
.0297
5.5
189
Zrn-64
42
0.15
0.004
1.32
1.5E-2
27.27
‒
‒
‒
‒
.0266
10.8
169
Zrn-66
29
0.04
0.001
0.88
7.9E-3
14.41
‒
‒
‒
‒
.0299
9.7
190
Zrn-66.2
13
0.06
0.004
0.47
2.2E-2
41.12
‒
‒
‒
‒
.0239
27.7
152
Zrn-69
55
0.24
0.004
1.70
7.8E-3
14.28
‒
‒
‒
‒
.0307
6.8
195
Zrn-71
83
0.27
0.003
2.52
1.4E-3
2.66
.0828
19.4
0.391
19.8
.0343
4.0
217
Zrn-76
93
1.62
0.02
2.48
2.4E-3
4.37
.0411
62.6
0.168
62.7
.0297
4.8
188
Spot No.
Zrn-49
ACCEPTED MANUSCRIPT
AC
CE
PT
ED
MA
NU
SC RI PT
Notes: Pb* indicate the radiogenic Pb corrected using measured ²⁰⁴Pb & ²⁰⁷Pb . The “-” means undetected or imponderable.
ACCEPTED MANUSCRIPT Table 7: A summary of characteristics of zircon from two Nagaland eclogite samples Stage-1
Stage-2
Stage-3
Dark luminescent, faintly banded elongated domain in Zrn cores (cf. Type 1 Zrn); Euhedral external shape, often with pyramidal termination
Only in N22F; Irregular bright luminiscent domain that replaces stage 2 Zrn cores (cf. Type 3 Zrn)
Composition
Low Th (0.12-0.27 ppm) and U (55-126 ppm) concentrations in N23B; More uraniferous (15112387 ppm) and thoriferous (15-88 ppm) in N22F; Low Th/U ratios in both sample groups : <0.01 (N23B); 0.01-0.06 (N22F) ~205 Ma
Commonly structure-less dark luminescent core, locally with patchy, irregular zones of bright luminescence (cf. Type 2 Zrn); Change of regular shape of Type-1 Zrn; Bright luminescent narrow rim around Zrn cores; Contains inclusions of Grt, Omp and Rt Low Th (0.05-1.62 ppm) and U (29-200 ppm) concentrations in N23B; uraniferous (1110-2478 ppm) and thoriferous (8-66 ppm) in N-22F; Low Th/U ratios in both sample groups : ≤0.02 (N-23B); ≤0.03 (N22F) ~189-185 Ma
Older 206Pb/238U dates between 337 and 239 Ma; Discordant in concordia plot Localised fluid-mediated alteration of recrystallised Zrn core
Statistically Th-rich (94271) and U-poor (751-1765 ppm) than other textural domains in N22F; High Th/U ratios (0.09-0.20)
MA
Recrystallization/replacement of growth-zoned Zrn due to in situ dissolution–reprecipitation in sub-solidus metamorphic conditions Metamorphic/ Minimum age of protolith Peak eclogite facies Timing unknown; Older Magmatic formation metamorphism at ~189-185 discordant dates context Ma geologically meaningless Mineral abbreviations after Whitney & Evans (2010); Other abbreviations used: CL, Cathodoluminescence; Soln, Solution.
CE
PT
ED
Primary growth due to precipitation from fluid/melt
AC
Origin
NU
Age
SC RI PT
Zrn evolution stages CL Zoning and texture
ACCEPTED MANUSCRIPT Highlights Nagaland eclogites record the oldest evidence for Neo-Tethyan subduction in the YTSZ. Minimum age of Nagaland eclogitic protolith is Late Triassic (≥ c. 205 Ma). Peak eclogite facies metamorphism occurred between 189 and 185 Ma.
SC RI PT
Metamorphic peak P-T conditions range from 25.5-28.1 kbar, ~650 oC to 23.8 kbar, ~555oC.
Eclogites record low thermal gradient (~7-8 oC/km) at metamorphic peak and a CW
AC
CE
PT
ED
MA
NU
P-T path of evolution.