Diversity and evolution of suboceanic mantle: Constraints from Neotethyan ophiolites at the eastern margin of the Indian plate

Accepted Manuscript Diversity and evolution of suboceanic mantle: constraints from Neotethyan ophiolites at the eastern margin of the Indian plate Biswajit Ghosh, Sarmishtha Mukhopadhyay, Tomoaki Morishita, Akihiro Tamura, Shoji Arai, Debaditya Bandyopadhyay, Soumi Chattopadhaya, Thungyani N Ovung PII: DOI: Reference:

S1367-9120(18)30132-9 https://doi.org/10.1016/j.jseaes.2018.04.010 JAES 3464

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

14 June 2017 26 March 2018 13 April 2018

Please cite this article as: Ghosh, B., Mukhopadhyay, S., Morishita, T., Tamura, A., Arai, S., Bandyopadhyay, D., Chattopadhaya, S., Ovung, T.N., Diversity and evolution of suboceanic mantle: constraints from Neotethyan ophiolites at the eastern margin of the Indian plate, Journal of Asian Earth Sciences (2018), doi: https://doi.org/ 10.1016/j.jseaes.2018.04.010

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Diversity and evolution of suboceanic mantle: constraints from Neotethyan ophiolites at the eastern margin of the Indian plate Biswajit Ghosh1,2*, Sarmishtha Mukhopadhyay1, Tomoaki Morishita 2, Akihiro Tamura2, Shoji Arai2, Debaditya Bandyopadhyay1, Soumi Chattopadhaya 1, Thungyani N Ovung1 1 2

Department of Geology, University of Calcutta, 35 Ballygunge Circular Road, Kolkata, India School of Natural System, College of Science and Engineering, Kanazawa University, Japan ABSTRACT

The mantle sections of four Neotethyan ophiolite bodies viz. Nagaland, Manipur, Andaman island and Rutland island lie along the eastern margin of the Indian plate. Among the at least two existing ophiolite belts in this region, all of these four ophiolite bodies belong to the western ophiolite belt. The major and trace element signatures of the constituent minerals of mantle peridotites of these ophiolites suggest that the samples from Nagaland, Manipur bear unequivocal signatures of abyssal peridotites. The compositional spectrum observed in them is within the limit of anhydrous melting of a MORB mantle source under reasonable melting condition. The Andaman ophiolite although overall characterizes the same, however, some mantle peridotites from this ophiolite might have experienced a hydrous melting event. The Rutland samples are distinct, showing signatures attesting to their forearc origin. The compositional spectrum observed in Nagaland, Manipur and a group of Andaman samples are grossly similar with samples recovered from Philippine Sea basins. Accordingly, interpretation regarding the geotectonic setting for the origin of these ophiolites straddles between mid-ocean ridge (MOR) and back-arc. If they truly represent their origin at MOR setting then the plagiogranites in these places showing arc affinity likely represent a later arc-related event, without having much connotation to the age of these ophiolites. Alternatively this study establishes a back-arc origin of these ophiolites. Keywords: Ophiolite; Mid-ocean ridge; Supra-subduction zone; Nagaland; Manipur; Andaman *Corresponding Author. E-mail:[email protected] Tel: +91-33-2461-4891

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1. Introduction The petrology and geochemistry of suboceanic mantle documented from ophiolites demonstrates their diversity, inherited in course of their evolution. Suboceanic lithospheres preserved in ophiolites may form in any geodynamic setting during their evolution, from seafloor spreading to terminal closure (Dilek, 2003). Nevertheless, forearc, embryonic arc, and back-arc settings in suprasubduction zones (SSZ) are the most common tectonic environments of their origin. Subduction zone magmatism is a direct response to tectonic and chemical processes operating at convergent margins (Macpherson and Hall, 1999). The magmatic and geochemical evolution of SSZ ophiolites is controlled, therefore, by the mode and nature of partial melting of the mantle above the subduction zone, and the dehydration of and element flux from the subducted slab into the overlying mantle. Hamilton (1994) argued that most subduction zones are characterised by retreat of the hinge with time, or rollback. Periodic decompressional partial melting at the initial stage causes a progressive change in the composition of the mantle, from fertile lherzolite to ultra-refractory harzburgite, and consequently the mantle source become depleted in incompatible elements. At a later stage, dehydration of the subducting oceanic slab and partial melting of subducted sediments lead to the incorporation of the light rare earth elements (LREEs) and the mobile elements into the mantle wedge (Hawkesworth et al., 1997; Pearce, 2014), and the depleted mantle becomes progressively enriched in these elements. Thus a MORB mantle at the initiation of subduction gradually changes to a depleted mantle below the forearc and to an ultra-depleted mantle at its maturity and consequently to a re-enriched mantle at its final stage. Continuing subduction and magmatism may open up back-arc basin with underlying mantle geochemically grossly similar to MORB mantle. This lithological variation may be preserved laterally across the subduction system (Dilek and Furnes, 2009, 2011) and a

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similar sequence may be manifested vertically in an accretionary prism setting at the time of its terminal closure. The Jurassic-Cretaceous ophiolites of the Tethyan ocean systems extend from the Betic mountain range in the west through the Alpine-Himalayan orogenic belts in the centre to the Indonesian region in the east (Pubellier et al., 2004). The internal structure and geochemistry of the Tethyan ophiolites all along this belt show a multifaceted pattern of igneous accretion in SSZ environments that involves subduction initiation, followed by rapid slab rollback, leading to extension and seafloor spreading in the extending upper plate within relatively short time spans (Shervais, 2001; Dilek and Flower, 2003). The SSZ ophiolites generated during these arc-trench rollback cycles commonly have structurally and geochemically heterogeneous crustal components attesting to the progressive evolution of their mantle melt sources. Tectonic accretion of these SSZ Tethyan ophiolites involved upper plate extension and advanced melting of previously depleted asthenosphere, showing a progressive evolution from MORB-like to IAT and boninitic geochemical affinities (Furnes et al., 2014). However, there are some distinct differences in the geochemical evolution of these Tethyan ophiolites that appear to have resulted from variations in their subduction zone geodynamic setting. In a regional scenario the oblique convergence between northeasterly moving Indian plate (Jagoutz et al., 2015) and nearly stationary Eurasian plate (Dubey, 2014) has brought out several geotectonic features, two most important being the Indus–Tsangpo suture zone (ITSZ) in collisional Himalaya and the Indo-Myanmar (Burma) Ranges (IMR). The first one defines the northwestern and northern boundary of the Indian plate, whereas the second one defines eastern boundary extending south from the eastern Himalayan syntaxis at Namche Barwa (Tibet), near Arunachal Pradesh (India) through the IMR to the Indonesian arc system, where the Indian plate is presently undergoing subduction under the Myanmar sub-plate, a part of the much larger 3|P a ge

Eurasian Plate (Khin et al., 2017)(Fig. 1). Discontinuous occurrences of Cretaceous ophiolitic rocks within Indian Territory are reported along its eastern boundary from Nagaland, Manipur (Agarwal and Ghose, 1986; Singh, 2013), Andaman island (Pal, 2011; Ghosh et al., 2013), and Rutland island (Ghosh et al., 2009; Bhattacharya et al., 2013). They were described as dismembered bodies in reviews of “Ophiolites in SE Asia” (Hutchison, 1975). With reference to the tectonic framework of SE Asia these ophiolites provide an important window into the composition and structure of Neotethyan oceanic lithosphere. And because of this, coupled with the active dynamism all along this convergent boundary, the ophiolite sequences of this belt have received attention of many workers, particularly in recent times, and various aspects of these ophiolites have been addressed. Unfortunately, they have been studied in isolation, making the available information sporadic, and several issues regarding their origin, emplacement and geodynamic evolution are still being debated (Acharyya, 2007; Pal, 2011; Ghosh et al., 2017a). Further, no attempt to correlate these bodies which have appreciable differences in age and geochemical characteristics over a strike distance of nearly 2000km has yet been made. Consequently, a holistic evolutionary scenario of Neotethyan ophiolites from this region is lacking yet. Here we focus on mantle sequences of these ophiolites and describe their diversity in terms of major and trace element mineral chemistry and finally we attempt to link them all in the context of evolution of Neotethys Ocean.

2. Regional geological setting The Indonesian arc system in SE Asia includes the Banda arc towards the east and the Sunda arc towards the west which extends further north into the IBR. In attempting to fit the ophiolites into a regional geotectonic framework, Ray et al. (1988) described two parallel Mesozoic ophiolitic belts occurring along the Indonesian arc system. The ophiolites at NagalandManipur and Andaman and Rutland islands all belong to the outer belt and lie on the east of the 4|P a ge

present subduction system with an aerial distance over 1800 km apart (Fig. 1). This outer belt passes through the Nagaland-Manipur Hills region in eastern India and adjoining western Myanmar (together Indo-Myanmar Ranges) and joins this on-land exposure of the late Mesozoic collision front to the north with a modern trench-arc system in the Andaman Sea region to the south that continues to Nias, western coast of Sumatra (Moore and Karig, 1980) and further extends to Timor, Indonesia (Audley-Charles, 1968).The Andaman and Rutland islands are located at the northern segment of the Sunda subduction zone and forms the outer-arc high. To the west it is bordered by an oceanic trench and to the east there occurs two volcanic islands (Barren and Narcondam) that belong to a volcanic arc extending from south of Sumatra to central Myanmar Basin in north Myanmar (Acharyya, 2007). Ophiolites of the inner belt lie east of the IMR and passes through central Myanmar (Bender, 1983), Sumatra (Page et al., 1979) and Java (Hamilton, 1979). The ophiolites of this belt are poorly preserved and ill exposed. It essentially follows a Neogene-Quaternary magmatic arc line juxtaposing ophiolitic rocks only in central Myanmar (Acharyya, 2007). These two ophiolitic belts (outer and inner) along the eastern margin of the Indian plate were named as the western and eastern belts respectively (Sengupta et al., 1990). Further, the western belt ophiolites have been described as rootless subhorizontal sheetlike bodies occurring as nappes propagated from the eastern belt. Nevertheless, recent geochronological data from Myitkyina and Kalaymyo areas of Myanmar suggest that these two ophiolite belts, the eastern and the western belong to two different sutures of the Meso-Tethys and Neo-Tethys respectively (Liu et al., 2016a). The former belonging to the eastern belt has a Middle Jurassic age (ca. 173 Ma) and represents the southern continuation of ophiolites along the Bangong-Nujiang suture in the Tibetan Plateau, whereas, the latter belonging to the western belt has an Early Cretaceous age (ca. 127 Ma), coeval with ophiolites along the Yarlung Tsangpo suture in Southern Tibet. However, recent study shows existence of three ophiolitic belts trending 5|P a ge

nearly north – south parallel with each other (from west to east): the Western Ophiolitic Belt (WOB); the Central Ophiolitic Belt (COB); and the Eastern Ophiolitic Belt (EOB) (Htay et al., 2017). The geology of Andaman and Rutland islands includes four major stratigraphic units, viz. Cretaceous Ophiolite Group, Eocene Mithakhari Group, Oligocene Andaman Flysch Group and Mio-Pliocene Archipelago Group. The Mithakhari Group comprises the trench sediments, whereas the Andaman Flysch Group is interpreted to represent deep sea fan deposits while the Archipelago Group represents shallow marine shelf deposits (Bandopadhyay, 2012). Ophiolites in these islands are mostly exposed on the eastern side of Andaman accretionary ridge that began to take its present geomorphic configuration in relation to a late Oligocene subduction (Ghosh et al., 2017a). They occur as dismembered bodies with a pronounced variation all along the ridge. The structural setting of the ophiolites is highly debatable. Pal et al. (2003) described these ophiolites interleaved with Eocene trench-slope sediments (Mithakhari Group), as accretionary thrust slices from different structural levels. However, Acharyya (2007) described the ophiolites and the closely associated sediments as flat-lying nappes overriding the Paleogene flysch (Andaman Flysch), and thrust up against the Neogene pelagic cover which overlies the Andaman Flysch unconformably. Though these ophiolites are tectonically dismembered, petrographic and mineral chemistry data suggest that most of the ophiolite sequence is preserved (Pal, 2011). The ophiolite consists of a tectonised, restitic mantle sequence (~700 m thick in north-Andaman) hosting chromitite pods, progressively overlain by a crustal cumulate sequence, intrusive (together up to 150 m) and volcanic rocks (maximum 400 m in south-Andaman)(Ghosh et al., 2013). However, there exists a thin zone at the boundary between the mantle and crustal sequences that has been identified as the Moho transition zone (MTZ) (Ghosh et al., 2014a).

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The geology of the Indo-Mayanmar Ranges consists of a variety of Mesozoic and Cenozoic magmatic, metamorphic and sedimentary rocks which are broadly classified into three distincttectono-stratigraphic units, viz. the Nimi Formation consisting of low- to medium-grade accretionary wedge metasediments of possible Mesozoic age, the Nagaland-Manipur ophiolite (NMO) of Early Cretaceous age, andthe Disang Formation consisting of a thick pile of folded Late Cretaceous-Eocene flysch-type sediments. A fourth tectono-stratigraphic unit known as the Jopi Formation is a post-orogenic molasses-type sediments consisting dominantly of ophiolitederived sediments belonging to a paralic sedimentary facies (Ghose et al., 2010). The NMO, comprising of discontinuous exposures of dismembered bodies of serpentinized peridotite tectonite, ultramafic-mafic cumulates, volcanics and marine sediments is tectonically bounded between Disang meta-sedimentary rocks to the west and the Naga metamorphic assemblage to the east (Acharyya, 2010). Mantle peridotites consist dominantly of lherzolite and occasionally harzburgite that host chromitite pods at places (Ghosh et al., 2014b).

3. Sample descriptions The mantle peridotites at Andaman and Rutland islands are variably serpentinised (40-80 vol.%) (Ghosh and Morishita, 2011), however, the least serpentinised samples preserve their primary mineralogy. Compositionally a pronounced variation exists between Rutland island to the south and middle-/north-Andaman to the north (Ghosh et al., 2013). Harzburgites dominate in Rutland island (Ghosh et al., 2009), whereas in middle- and north-Andaman the peridotites are mostly represented by lherzolite occasionally grading into clinopyroxene-bearing harzburgite with development of dunite pods (Ghosh et al., 2013). These dunite pods are commonly irregular in

shape (Ghosh and Bhatta, 2014). Dunite also occurs as subconcordant dykes/lenses enveloping chromitites within harzburgite in Rutland island and also within lherzolite in north-Andaman (Fig. 2a). 7|P a ge

The mantle peridotites are either massive or occasionally crudely foliated with subparallel arrangement of stretched/elongated pyroxene grains. The overall texture is porphyroclastic where large pyroxene grains are surrounded by smaller olivines. Olivines and pyroxenes are commonly strained and show features of high-temperature plastic deformation like undulose extinction, deformation twins, bending of cleavage. Pyroxenes are mostly anhedral with embayed margins, surrounded by aggregates of olivines protruding into them. Accessory chromian spinel grains in peridotites occur in two distinct modes; (a) anhedral grains often exhibiting holly-leaf structures (Pal, 2011), indicating their residual origin, and (b) symplectitic intergrowths mainly with clinopyroxene, depicting their origin by unmixing from non-stoichiometric Ca–Al–Cr rich clinopyroxenes (Fig. 2b). All these textures are typical of chromian spinels from residual mantle peridotites in ophiolites and from modern ocean ridge settings (Ghosh et al., 2009; Morishita and Arai, 2003). The mantle peridotites of Naga-Manipur ophiolite are more serpentinised (60 – 95 vol.%) which sometimes poses challenges to identify them by microscopic observations, obliterating almost all the primary mineralogy and fabric in the rock. However, the relict mineralogy coupled with mineral chemistry of the most alteration-resistant minerals like chromite in serpentinised samples suggests that the mantle section of NMO is lherzolite dominated. Extremely serpentinised harzburgites with depleted mineral chemical signatures, localised around chromitite pods represent the wall rock along the melt channels (Fig. 2c). Mesh-textured serpentine and pseudomorphic bastite is the most common alteration mineralogy. Their residual origin after partial melting is evident, from features like amoeboid shape of chromite grains, embayed margin of pyroxenes filled with secondary olivines (Fig. 2), and also from mineral chemical support. Ghosh et al. (2017b) recently reported an extensive Ca-metasomatism event during the evolution of Neotethyan ophiolite from this region. 8|P a ge

4. Analytical methods Quantitative Electron Probe Micro Analysis (EPMA) were carried out at Kanazawa University, Japan on polished sections with an Electron Probe Micro Analyzer (JEOL JXA-8800 Superprobe) using an accelerating voltage of 15–20 kV, a beam current of 15–20 nA and a beam diameter of 3 μm. Both natural and synthetic minerals were used as standards. The standards were analyzed at regular intervals to check the precision of analysis. Raw data were corrected using ZAF online correction program. Trace element concentrations of clinopyroxenes were determined at the same institute by laser ablation (193nmArF excimer: MicroLasGeoLas Q-plus) inductively coupled plasma mass spectrometry (Agilent 7500S) (LA-ICP-MS). Each analysis was performed by ablating 40–60 μm in diameter at 6 Hz with energy density of 8 J cm−1 per pulse. Signal integration times were 50 s for a gas background interval and another 50 s for an ablation interval. The element concentration of the glass standard (NIST SRM612) was selected from the preferred values of Pearce et al. (1997). Data reduction was facilitated using 29Si as the internal standard for clinopyroxenes, based on SiO2 contents determined by microprobe, following a protocol essentially identical to that outlined by Longerich et al. (1996). The accuracy of measurements estimated from analyses of glass reference material (NIST SRM 614) is better than 4% in relative standard deviation for all elements. The analytical details and data quality are the same as those reported in Morishita et al. (2005).

5. Results While this section deals with the mineral chemistry of all the four ophiolites viz. Rutland island, Andaman island, Manipur and Nagaland, the major element data (EPMA)have been generated mainly from the NMO samples and these have been compared with published data from

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Andaman and Rutland islands (Ghosh et al., 2013). The trace element data (LAICPMS) are new to all these four ophiolites. 5.1. Major elements In terms of forsterite content and NiO wt.%, the olivines of all the four ophiolite bodies (Supplementary Table A.1)mostly occupy the compositional field of mantle olivine (Fig. 3a). Few plots from Andaman and Rutland have higher NiO values. The Cr# [=Cr/(Cr + Al) atomic ratio] Mg# [=Mg/(Mg + Fe) atomic ratio] systematics of accessory chromites from these ophiolites clearly show a wide range of composition (Fig. 3b)(Supplementary Table A.2). Most of the Nagaland and Andaman samples have the lowest Cr# values occupying the most fertile end of the compositional field for abyssal peridotites. The Manipur samples also occupy the same field but they have little higher Cr# in comparison. Samples from Rutland island are distinct and they have markedly higher Cr# values occupying the compositional field for forearc peridotites. However, accessory chromites, the sole residual minerals preserved in almost completely serpentinised samples around chromitite pods from Nagaland have the highest Cr# values. Their unusually high Mg# values in comparison to those with similar Cr# from other forearc settings drag them to plot mostly outside the compositional field for forearc peridotites, indicating these grains are completely reequilibrated with the melts responsible for the chromitite pods. The composition of these chromites have not been plotted elsewhere in this manuscript. Orthopyroxenes from all these ophiolites are of ‘enstatite’ variety with a Mg# range of 0.89-0.91(Supplementary Table A.3). A correlation between Al2O3 content of them and the Cr# of the coexisting spinels clearly differentiates Rutland ophiolite from the rest three (Fig. 3c). The latter group has higher Al2O3 content comparable to that from abyssal peridotites, in contrast to the former which occupies an overlapped field with those from forearc peridotites.

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Clinopyroxenes from all the four ophiolites are of ‘diopside’ variety with Mg# ranging between 0.91 and 0.94(Supplementary Table A.4). The Rutland samples are distinguished from rest of the samples by their higher Mg# and lower Na2O, TiO2 and Al2O3 contents (Fig. 4). In terms of concentration of these elements samples from Andaman, Manipur and Nagaland occupy almost the entire range of abyssal peridotite field. The samples from Andaman and Nagaland are nearly indistinguishable, whereas, Manipur samples mostly occupy the lower end of this compositional field. A compositional correlation between Al2O3 content of the clinopyroxenes and the Cr# of the coexisting spinels establishes a similar relationship (Fig. 5a). This is also consistent with an increase in the degree of partial melting of peridotitic clinopyroxenes in the TiO2 vs. Al2O3content bivariate diagram (Fig. 5b). 5.2. Trace elements Trace elements of clinopyroxenes from all the four ophiolites are listed in Supplementary Table A.5. The chondrite-normalised (McDonough and Sun, 1995) rare earth element (REE) pattern of clinopyroxenes (Fig.6a) from these ophiolites in general display a depleted light rare earth element (LREE) and flat heavy rare earth element (HREE) pattern. The concentration of La is below the detection limit in all the cases. The Rutland samples are distinct from all the rest. They have the lowest concentration of total REE (∑REE) and many of the medium rare earth elements (MREE) and all the LREEs except Ce are not detected in them. The Nagaland, Manipur and one subset of Andaman samples display a similar pattern with variation in degree of depletion. Among them the Manipur samples are highest depleted and Ce is not detected in them. When compared to Kalaymyo ophiolite of Myanmar (Liu et al., 2016b) which is another ophiolite body on the Indo-Myanmar Ranges, located south of Manipur (Fig. 1) the REE pattern of clinopyroxenes closely matches with that from these three ophiolites. However, another subset of Andaman samples with lower HREE than the earlier show enrichment in LREEs. The chondrite11 | P a g e

normalised (McDonough and Sun, 1995) trace element pattern of clinopyroxenes (Fig. 6b) also clearly distinguishes Rutland samples from all the rest. The samples of the latter group are marked by strong Zrnegative and moderate Ti negative anomalies. Among them there is a close resemblance between Andaman and Nagaland samples. The normalised bivariate diagrams of Yb versus Dy and Ti contents of clinopyroxenes from these ophiolites demonstrate strong positive correlations (Fig. 7). Here too, plots for Nagaland, Manipur and Andaman occupy the compositional field for abyssal peridotites whereas, those for Rutland are akin to forearc peridotites.

6. Discussion Mineral chemical signatures of mantle peridotites give immense information of various mantle processes including partial melting-its type and extent. Partial melting of the mantle is controlled by factors like pressure, temperature and fluid/water content, which may be used as diagnostic indicators of tectonic settings (Dick et al., 2010). Partial melting below mid-ocean ridges occurs under anhydrous condition, whereas the mantle wedge above a subducted slab may undergo hydrous melting due to the slab dehydration. This is attested by the fact that forearc mantle tends to experience higher degrees of partial melting relative to the oceanic mantle beneath the mid-ocean ridges (Parkinson and Pearce, 1998; Pearce et al., 2000). Therefore, chemical compositions of mantle peridotites may provide effective constraints on geodynamic evolution of the ophiolites. 6.1. Nature and extent of mantle melting The petrography and mineral chemistry of peridotites from the mantle sequence of the investigated ophiolites demonstrates their residual origin after partial melting. Major and trace element signatures of the constituent minerals clearly suggest that the Andaman, Manipur and Nagaland samples are always akin to abyssal peridotites whereas, Rutland samples are secluded 12 | P a g e

and they show signatures attesting to their forearc origin. Following the empirical equation outlined by Hellebrand et al. (2001) that describes the extent of melting (F) as a function of spinel Cr#, i.e., F = 10 × ln(Cr#) + 24, we have estimated F for mantle peridotites from each of these areas(Supplementary Table A.2). The calculation yields low values (1-5%)for Nagaland samples, low to moderate values (2-10%) for Andaman samples, moderate values(7-10%) for Manipur samples and high values(14-18%) for Rutland samples. Serpentinised samples around chromitite pods from Nagaland have not been assessed since we consider that the accessory chromites in them have been completely reequilibrated with the melts responsible for the formation of chromitite pods. The estimated extents of melting for each of these ophiolites closely match with the corresponding modeled REE pattern of residual clinopyroxenes, starting with a depleted MORB mantle (DMM) source (Workman and Hart, 2005) (Fig. 6). Though clinopyroxene has a low abundance in abyssal peridotites, it is a good proxy for bulk composition because >85% of the trace element budget of peridotites, in general is contained within this mineral (Warren et al., 2009). The relatively less incompatible trace elements in clinopyroxenes, HREEs in particular preserve information on extents of partial melting experienced by mantle peridotites. A compositional correlation between normalized Yb content in clinopyroxenes from the investigated samples and the Cr# of the coexisting spinels establishes a relationship following the trajectory of perfect fractional melting of a DMM source (Fig. 8a). Further, this relationship for Nagaland, Andaman and Manipur samples is within the compositional spectrum of abyssal peridotites whereas, the Rutland samples are isolated, occupying the forearc peridotite field. Mantle melting in hydrous condition consumes more orthopyroxenes relative to clinopyroxenes in comparison to anhydrous melting (Gaetani and Grove, 1998). This allows survival of clinopyroxenes even in higher extents of melting. Consequently, this results in extreme 13 | P a g e

depletion of Ti and HREEs in clinopyroxene from the SSZ peridotites than the abyssal peridotites (Bizimis et al., 2000).The Ti – Zr relationship in residual clinopyroxenes from the investigated ophiolites show a systematic relationship with increasing extent of melting (Fig. 8b). The Nagaland, Manipur and one subset of Andaman samples clearly follow a dry melting trend of a DMM source. The Rutland samples deviate completely from this trend and plot outside the compositional field for abyssal peridotites, close to the SSZ peridotite field, implying that they have been subjected to hydrous melting in subduction-related settings. A critical observation of trace element pattern of Rutland samples (Fig. 6b) shows that Pr and Nd are below the detection limit whereas, Ce has been detected in them. This indicates that Sr and Zr in them actually shows a positive anomaly which is in contrary to the samples from other three areas, supporting the addition of slab-derived components (McCulloch and Gamble, 1991; Pearce and Peate, 1995).Another subset of Andaman samples with lower HREE than the other also deviate a little from the dry melting trend, indicating involvement of water during partial melting. 6.2. Constraints on geodynamic evolution The magmatic mechanisms that construct the ophiolites and control their petrological and geochemical architectures depend on the geodynamic settings of their formation (Dilek, 2003). Subduction-related ophiolites are by far the most dominant ophiolite-type constituting nearly 75% of the Phanerozoic ophiolites. On the other hand, MOR ophiolites are relatively lower in abundance, but not rare, making up ~20% of the population (Furnes et al., 2014).Tectonic discrimination of ophiolites is commonly based on geochemical and structural evidence from the crustal rock assemblages (Metcalf and Shervais,2008; Pearce, 2014). Nevertheless, it has been demonstrated that the mantle sequence of ophiolites may also provide critical information on the tectono-magmatic settings where ophiolites formed (Barth et al.,2003; Bizimis et al., 2000; Choi et al., 2008; Jean et al., 2010; Liu et al.,2012; Uysal et al., 2012). The nature and extent of mantle 14 | P a g e

melting are the key processes that distinguish between peridotites from these two broad geodynamic settings. In comparison to mid-ocean ridge setting which is characterized by anhydrous melting, fluid influx from the downgoing slab to the mantle wedge in subduction setting lower the solidus of mantle peridotites and thereby enhance the melting rates (Hirose and Kawamoto, 1995). SSZ peridotites are therefore, compositionally more refractory than the abyssal peridotites (Parkinson and Pearce, 1998; Pearce et al., 2000). The major and trace element mineral chemical characteristics from the investigated ophiolites clearly suggest that Nagaland and Manipur ophiolites bear unequivocal signatures of abyssal peridotites. The compositional spectrum observed in them is within the limit of anhydrous melting of a MORB mantle source under reasonable melting condition. The Andaman ophiolite although overall characterizes the same, however, some mantle peridotites from this ophiolite might have been experienced a hydrous melting event. In comparison, Rutland ophiolite is distinct and is more akin to forearc peridotites. Noteworthy to mention here is that back-arc basins are characterized by lithologic associations and geochemical systematics grossly similar to MORB; ophiolites formed in this setting are for the most part indistinguishable from MORB geochemically and can only be linked to a back-arc origin by careful study of the regional geologic setting (Harper, 1984). To test the validity of this statement we have attempted to compare the REE signatures of Nagaland, Manipur and Andaman peridotites with the same of the spinel peridotites from the Philippine Sea back-arc basin (Ohara et al., 2002, 2003) (Fig. 8c) and we find a close resemblance between them. This opens up several possibilities how this diversity over a strike distance of nearly 2000km may be explained. Despite of having great diversity from petrological point of view the emplacement of all these four ophiolites together with the Kalaymyo ophiolite of Myanmar are linked with same geodynamic event, i.e., closure of Neotheys Ocean. Tethyan ophiolites are generally regarded as subduction-related ophiolites 15 | P a g e

(Wakabayashi and Dilek, 2003; Dilek and Furnes, 2009) and ophiolites of IMR, Andaman and Rutland are no exception. It is also attested by the great diversity of volcanic rocks from these areas (Ray et al., 1988; Jafri et al., 1990; Jafri and Sheikh, 2013; Pal, 2011; Khogenkumar et al., 2016; Ovung et al., 2017). Previous studies (Parkinson and Pearce, 1998; Pearce et al., 2000) have shown that peridotites now exposed in current forearc region also have different origins, varying from abyssal peridotites to SSZ peridotites. The Indonesian orogenic belt reveals a complex magmatic and tectonic history that evolved during the closure of the Meso-Tethyan ocean (Hall, 2012). The internal structure and geochemistry of the Phanerozoic ophiolites in this orogenic belt show a complex pattern of igneous accretion that involved multiple stages and sources of melt evolution and life cycles in suprasubduction zone environments (Shervais, 2001; Dilekand Flower, 2003). Coexistence of MOR- and SSZ-type peridotites are increasingly common in recent literatures, viz. the Troodos ophiolite (Batanova and Sobolev, 2000), the Coast Range ophiolite (Choi et al., 2008; Jean et al., 2010), ophiolites in SW Turkey (Uysal et al., 2012) and it is also not scarce for ophiolites along the Yarlung-Tsangpo Suture in the Tibetan Plateau (Liu et al., 2012). Presence of MOR-type peridotite signatures in Subduction-related ophiolite setting is linked to the evolution of the SSZ lithosphere from its birth to resurrection. With this argument in mind the mantle section of presently studied Nagaland, Manipur, Andaman and Kalaymyo as well could either be vestiges of abyssal mantle that had their origin in Neotethyan mid-ocean ridge system at geological past, later upwelled from the region below the sinking slab at subduction initiation stage and finally got trapped during terminal closure (Parkinson and Pearce, 1998), or incorporation of MOR mantle into arc lithosphere after the slab delamination/break-off. It can even be an incident of arc-ridge collision (ridge subduction) (Shervais, 2001).Alternatively, the peridotites of these areas represent a back-arc mantle that finally cropped out during terminal closure. The recent report of pillow 16 | P a g e

basalts, geochemically akin to back-arc basin basalts from Bompoka island (Jafri and Sheikh, 2013) which the authors correlate with the volcanic stratigraphy of Andaman ophiolite may substantiate this proposition.

7. Conclusions Among the two possibilities that exist in connection with the geotectonic setting for the origin and evolution of the mantle sections of Nagaland, Manipur and Andaman ophiolite i.e., MOR versus back-arc, the former has a major implication on the assigned ages for some of these ophiolites. The ophiolites at Andaman and Nagaland-Manipur have been dated as ~95 Ma (Pedersen et al., 2010; Sarma et al., 2010) and 116-118 Ma (Singh et al., 2016) respectively from U-Pb zircon geochronology of plagiogranites. The plagiogranites in both the places show intrusive relationships with gabbros-basalt and they geochemically have a volcanic arc affinity. Therefore, if the plagiogranites in these places are not genetically related with the ophiolites thereof which they truly represent their origin at MOR setting, these arc-related plagiogranites likely have intruded a MOR lithosphere. In such a scenario, the zircon ages of the intruding plagiogranites may represent a later arc-related event. Alternatively, these ophiolites had their origin in subduction-related setting (back-arc) where the age difference between them reflects a migration of the subduction system from north to south as a result of counter clockwise rotational convergence between the Indian plate and the Eurasian plate (Jagoutz et al., 2015).

Acknowledgements This research is an outcome of the Invitation Fellowship (FY2015) of Japan Society for the Promotion of Science (JSPS), awarded to BG. SM acknowledges financial support received from the Department of Science and Technology, Government of India under Women Scientist Scheme (No. SR/WOS-A/EA-1023/2015). DB acknowledges the support of DST INSPIRE 17 | P a g e

Fellowship (IF 130148) for fieldwork.The comments from two anonymous reviewers have significantly improved the manuscript. Editorial handling and helpful suggestions to improve the manuscript by Khin Zaw are highly appreciated.

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Figure captions Fig. 1. Color-shaded relief map showing the major geological and tectonic features of the northeastern Indian Ocean and southeastern Asia. Red triangles represent Holocene volcanoes. Relief data are from ETOPO1 Global Relief Model (http://www.ngdc.noaa.gov/mgg/global/). Fig. 2. (a) Field photograph showing chromitite pod within dunite melt channel (bordered by dashed white line) from north-Andaman. (b) BSE image showing symplectitic chromian spinel from middle-Andaman. (c) Field photograph of chromitite pod with serpentinite around it from Nagaland. Microphotographs showing embayment in clinopyroxene (Cpx) filled with olivines (d) and amoeboid spinel (e) (white arrows) from Manipur sample. (f and g) BSE images showing olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx) and spinel (Spl) in serpentinised peridotite from Manipur samples. Note the development of bastite serpentine (Srp) after Opx. Fig. 3. Mineral chemical variations of olivine, orthopyroxene and accessory spinel from mantle peridotites of the investigated ophiolites. The compositional field of spinel peridotites of Kalaymyo ophiolite is drawn using data from Liu et al. (2016b).(a) NiO vs. Forsterite (Fo) contents of olivine. The mantle olivine array is from Takahashi et al. (1987). (b) Cr# [=Cr/(Cr + Al) atomic ratio] - Mg# [=Mg/(Mg + Fe) atomic ratio] systematics of accessory chromites. Compositional fields for abyssal and forearc peridotites are from Tamura and Arai (2006).(c) Compositional correlation between Al2O3 content of orthopyroxenes and the Cr# of the coexisting spinels showing fields for abyssal and forearc peridotites after Morishita et al. (2015). For Rutland and Andaman plots in all the diagrams data taken from Ghosh et al. (2013). Fig. 4. Major element compositional variations against Mg# [=Mg/(Mg + Fe) atomic ratio] of clinopyroxenes from mantle peridotites of the investigated ophiolites. (a) TiO2, (b) Na2O, (c) Al2O3 and (d) Cr2O3 contents. The compositional field of spinel peridotites of Kalaymyo ophiolite is drawn using data from Liu et al. (2016b).Compositional field for abyssal peridotites in a - c are from Dick (1989) and Seyler et al. (2011), and that in d is from Johnson et al. (1990). Compositional field for forearc peridotites in a, c and d are from Ishii et al. (1992) and that in b is from Parkinson and Pearce (1998).For Rutland and Andaman plots in all the diagrams data taken from Ghosh et al. (2013).

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Fig. 5. (a) Compositional correlation between Al2 O3 (wt.%) of clinopyroxenes and Cr# [Cr/ (Cr+Al) atomic ratio] of coexisting spinelsfrom mantle peridotites of the investigated ophiolites showing trend of increasing degree of depletion.(b) Bivariate Al2O3 vs. TiO2 content of clinopyroxenes showing trend of increasing partial melting. Compositional fields for abyssal and forearc peridotites are from Uysal et al. (2012).For Rutland and Andaman plots in all the diagrams data taken from Ghosh et al. (2013). Symbols are same as in Fig. 3.The compositional field of spinel peridotites of Kalaymyo ophiolite is drawn using data from Liu et al. (2016b). Fig. 6. (a) Chondrite-normalized REE concentrations in clinopyroxenes from mantle peridotites of the investigated ophiolites. Compositional range of the same in spinel peridotites from Kalaymyo ophiolite, Myanmar (Liu et al. 2016b) is also shown. Modeled clinopyroxene-REE after 5%, 10% and 15% fractional melting from a depleted MORB mantle (DMM) source (Workman and Hart, 2005) is taken from Liu et al. (2016b). Note that a subset of Andaman samples shows little LREE enrichment. (b) Chondrite-normalized trace element concentrations in clinopyroxenes from mantle peridotites of the investigated ophiolites. Normalizing values for all the elements are from McDonough and Sun (1995). Fig. 7. Chondrite-normalized logarithm plots of Dy vs. Yb (a) and Ti vs. Yb (b) of clinopyroxenes from mantle peridotites of the investigated ophiolites. The compositional field of spinel peridotites of Kalaymyo ophiolite is drawn using data from Liu et al. (2016b).Compositional fields for abyssal peridotites are from Hellebrand et al. (2005) and Seyler et al. (2011), and for forearc peridotites are from Parkinson et al. (1992). Fig. 8. (a) Compositional correlation between chondrite-normalized Yb in clinopyroxenes and Cr# [Cr/ (Cr+Al) atomic ratio] of coexisting spinels from mantle peridotites of the investigated ophiolites showing fractional melting trend and field of abyssal peridotites after Liu et al. (2008). (b) Bivariate clinopyroxene Ti vs. Zr relationship showing compositional fields of abyssal and SSZ peridotites, and melting trends after Bizimis et al. (2000).Note that a subset of Andaman samples (pointed with red arrow) deviates a little from the dry melting trend. The compositional field of spinel peridotites of Kalaymyo ophiolite is drawn using data from Liu et al. (2016b). (c) Comparison of chondrite-normalized REE patterns of clinopyroxenes from Nagaland, Manipur and Andaman peridotites with that of spinel peridotites from Philippine Sea back-arc basins (data from Ohara et al., 2002, 2003). 29 | P a g e

Figure 1

100˚E

90˚E 30˚N

TIBET

30˚N

N.E. Himalayan Syntaxis

Indus-Tsangpo Suture

EURASIAN PLATE

Myitkyina

Nagaland

INDO-MY ANM A RANG ES R

INDIA Kalaymyo

20˚

CHINA Shan Plateau

Fault

Manipur

20˚

g Sagain

Popa

BAY OF BENGAL Irawaddy Fan

Narcondam Andaman Island

10˚

Barren

Rutland Island

10˚ MA

RIDGE

AR r

6cm/y

I N D I A N P L A T E

INDIAN OCEAN

10˚S

10˚S 90˚E

Figure 1

0˚

RA AT

M

SU

NINETYEAST

LA

SU

IN

NM YA M E AT

PL

EN

T

ES

YP LA

W

0˚

100˚E

Figure 2

a

ite

Chromit

c

b

d ite

Chromit

Cpx

ite

tin pen

Ser

e

f Ol Srp Opx Cpx

g Spl Srp Opx

Figure 2 Ol

Figure 3

0.5

1.0

a

Ma

Oliv

ine

0.3

Arr

Nagaland Manipur Andaman Rutland

0.2 0.92

0.88

0.84

0.80

Fo content in olivine 6

0.6

Abyssal peridotites

0.4

partia

c

Abyssal peridotites

4

asing

Spinel peridotite (Kalaymyo)

0.2

Incre

Al2O3 (wt.%) (Opx)

Forearc peridotites

0.8

ay

ting

ntle

l mel

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2

Nagaland Manipur Andaman

Forearc peridotites

Nagaland Manipur Andaman

Spinel peridotite (Kalaymyo)

0.0

Rutland 0 0

b

Nagaland chromitite

Cr# [= Cr/(Cr + Al)]

NiO (wt.%)

Spinel peridotite (Kalaymyo)

0.2

0.4

0.6

0.8

Cr# = Cr/(Cr + Al) (Spl)

Figure 3

1

1.0

0.8

0.6

Rutland

0.4

0.2 2+

Mg# [= Mg/(Mg+Fe )]

0.0

Figure 4 0.6

a

TiO2 wt.%

1.2

b

Na2O wt.%

Spinel peridotite (Kalaymyo)

0.4

Spinel peridotite (Kalaymyo)

Abyssal peridotites

Forearc peridotites

0.2

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Abyssal peridotites

0.4 Forearc peridotites

Nagaland Manipur Andaman

Nagaland Manipur Andaman

Rutland

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c

Al2O3 wt.%

Spinel peridotite (Kalaymyo)

Cr2O3 wt.% 1.6

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6

Rutland

0.0

d

Forearc peridotites

Spinel peridotite (Kalaymyo)

1.2 Abyssal peridotites

4 0.8

2 Nagaland Manipur Andaman

0.4

Forearc peridotites

Rutland

0 0.75

0.80

Rutland

0 0.85

0.90

Mg# = Mg/(Mg + Fe)

Figure 4

Nagaland Manipur Andaman

0.95

1.00

0.75

0.80

0.85

0.90

Mg# = Mg/(Mg + Fe)

0.95

1.00

Figure 5

0.8

8

a

b 0.6

Spinel peridotite (Kalaymyo)

2

Forearc peridotites

Abyssal peridotites

Forearc peridotites

0.2

ing s a Incre

0

0

0

0.2

0.4

0.6

Cr# = Cr/(Cr + Al) (Spl)

Figure 5

0.8

1.0

g

Spinel peridotite (Kalaymyo)

0.4

ial m eltin

n

4

part

TiO 2 (wt.%) (Cpx)

tio

le

ep

D

Al 2O3 (wt.%) (Cpx)

6

0

2

4

Al 2O3 (wt.%) (Cpx)

Abyssal peridotites

6

Figure 6

10

0.1 15%

Nagaland Manipur Andaman

10%

5%

Kalaymyo, Myanmar

0.001 La

Ce Pr

Figure 6

b

05)

an and Hart 20

DMM (Workm

1

0.01

100

a

Nd Sm Eu Gd Tb

Dy Ho Er

Rutland Tm Yb Lu

Sample/CI Chondrite

Sample/CI Chondrite

100

10

1

0.1

0.01

0.001

Nagaland Manipur Andaman Rutland Nb Ta La Ce Sr Pr Nd Zr Hf SmEu Gd Ti Tb Dy Y Ho Er TmYb Lu

Figure 7

100

100

a

Dy (n)

b

Ti (n)

10

Spinel peridotite (Kalaymyo)

10

Spinel peridotite (Kalaymyo)

Abyssal peridotites

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Abyssal peridotites

1 Forearc peridotites

Forearc peridotites

Nagaland Manipur Andaman Rutland

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Yb (n) in Cpx Figure 7

Rutland

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Nagaland Manipur Andaman

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Yb (n) in Cpx

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Figure 8 10000

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a

b

Spinel peridotite (Kalaymyo) 5%

Spinel peridotite (Kalaymyo)

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Abyssal

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Ti in Cpx (ppm)

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peridotites

15%

g

ltin

e ry m

D 100

Cpx out SSZ peridotites

Abyssal peridotites Nagaland Manipur Andaman

Nagaland Manipur Andaman

Forearc peridotites

Rutland

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0.1 0.0

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0.3

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Hydrous melting

Rutland

0.01

0.1

Sp Cr# [Cr/(Cr+Al)]

1

10

Figure 8

10

100

Zr in Cpx (ppm)

100

Sample/CI Chondrite

Yb (n) in Cpx

DMM

c an DMM (Workm

)

and Hart 2005

Philippine Sea back-arc basins (Ohara et al., 2002, 2003)

1

0.1 Nagaland, Manipur and Andaman

0.01

0.001 La

Ce Pr

Nd Sm Eu Gd Tb

Dy Ho Er

Tm Yb Lu

10

100

05)

an and Hart 20

DMM (Workm

1

0.1 15%

0.01

5%

Nagaland Manipur Andaman

10%

Kalaymyo, Myanmar

0.001 La

Ce Pr

Nd Sm Eu Gd Tb

Dy Ho Er

Sample/CI Chondrite

Sample/CI Chondrite

100

Rutland Tm Yb Lu

Forearc Trench basin Outer arc/ Forearc ridge

Oceanic Crust

Accretionary prism/wedge

10

rt 2005)

an and Ha DMM (Workm

Philippine Sea back-arc basins (Ohara et al., 2002, 2003)

1

0.1 Nagaland, Manipur and Andaman

0.01

0.001 La

Ce Pr

Nd Sm Eu Gd Tb

Arc volcano

Dy Ho Er

Tm Yb Lu

Back-arc basin

Mantle doming and formation of back-arc lithosphere

Highlights



Diversity of mantle peridotites from Neotethyan ophiolites are investigated



The compositional spectrum ranges between abyssal and forearc peridotites



Those akin to abyssal peridotites are grossly similar to modern back-arc samples



Clinopyroxene trace elements are modeled under reasonable mantle melting condition

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