Zircon geochronology of deformed alkaline rocks along the Eastern Ghats Belt margin: India–Antarctica connection and the Enderbia continent

Accepted Manuscript Zircon geochronology of deformed alkaline rocks along the Eastern Ghats Belt margin: India–Antarctica connection and the Enderbia continent Sameer Ranjan, Dewashish Upadhyay, Kumar Abhinay, Kamal L Pruseth, Jayanta K Nanda PII: DOI: Reference:

S0301-9268(17)30280-2 https://doi.org/10.1016/j.precamres.2018.04.005 PRECAM 5055

To appear in:

Precambrian Research

Received Date: Revised Date: Accepted Date:

23 May 2017 15 March 2018 5 April 2018

Please cite this article as: S. Ranjan, D. Upadhyay, K. Abhinay, K.L. Pruseth, J.K. Nanda, Zircon geochronology of deformed alkaline rocks along the Eastern Ghats Belt margin: India–Antarctica connection and the Enderbia continent, Precambrian Research (2018), doi: https://doi.org/10.1016/j.precamres.2018.04.005

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Zircon geochronology of deformed alkaline rocks along the Eastern Ghats Belt margin: India–Antarctica connection and the Enderbia continent

1

Sameer Ranjan, 1Dewashish Upadhyay*, 1Kumar Abhinay, 1Kamal L Pruseth, 2Jayanta K

Nanda

1

Department of Geology and Geophysics, Indian Institute of Technology (IIT), Kharagpur–

721302, India 2

S–4/113, Niladrivihar, Chandrasekharpur, Bhubaneshwar 751021, India

*Corresponding author: Dewashish Upadhyay ([email protected]) Address: Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, 721302 Kharagpur, West Bengal, India. Phone: +91–3222–283398 Fax: +91–3222–282268

Abstract Linear chains of Deformed Alkaline Rocks and Carbonatites (DARCs) mark sutures where continents had rifted apart and later amalgamated. Since DARCs are products of two well– defined components of Wilson cycle, i.e., continental rifting and subsequent collision, geochronological constraints from DARCs along the Singhbhum/Bastar Craton–Eastern Ghats Belt contact in eastern India is used to unravel the history of continental breakup and amalgamation in the Indo–Antarctic Enderbia continent. Proto India and East Antarctica were involved in several episodes of collision and breakup during the assembly of past supercontinents. The Napier Complex of East Antarctica collided with the Dharwar Craton and Ongole Domain at 1.60 Ga forming the central-eastern Indian shield. Zircon U–Pb ages from DARCs at the craton–Eastern Ghats Belt margin show that the alkaline complexes (Kamakhyanagar: 1350±14 Ma; Rairakhol: 1379±6 Ma; Koraput: 1387±34 Ma; Kunavaram: 1360±5 Ma; Jojuru: 1352±6 Ma) were emplaced between 1320 Ma and 1370 Ma. The alkaline magmatism marks an episode of rifting in the Indo–Antarctic continental fragment, correlatable with the breakup of the Columbia supercontinent. The Khariar alkaline complex was emplaced at 1478±5 Ma during an earlier phase of crustal extension coeval with the formation of the Mesoproterozoic Chattisgarh rift basin. Metamorphic zircons from the alkaline rocks furnish age populations at 953–930 Ma, 808–795 Ma, and 661–563 Ma. The 953–930 Ma ages are correlated with oceanic closure between Ruker Terrane of East Antarctica and India during Rodinia assembly. The resultant collision of the Ruker Terrane with India–Napier Complex composite produced the Grenville–age Eastern Ghats Province–Rayner Complex orogen. The 808–795 Ma ages record disintegration of Rodinia when India broke away from East Antarctica. In the early Paleozoic, India redocked with East Antarctica and Australia during Gondwanaland assembly.

The 661–563 Ma zircon ages date the resulting collisions during Pan–African orogenesis. The assembly of the Eastern Ghats Province and the Rayner Complex with the Bastar–Singhbhum Craton occurred in the early Neoproterozoic during the final stages of Rodinia assembly, supporting the existence of Enderbia continent comprising crustal units from India and East Antarctica.

Keywords Supercontinent; Rodinia; Eastern Ghats Belt; East Antarctica; Deformed Alkaline Rocks and Carbonatites; Zircon

1. Introduction The major landmasses on Earth have come together several times in the geologic past to form supercontinents (e.g., Rogers and Santosh, 2004; Bradley, 2011). The record of the episodic amalgamation and breakup of past supercontinents are dispersed amongst the present day continents and can be used to correlate crustal fragments, reconstruct their paleogeographic positions, and identify global rifting and orogenic events (Harley et al., 2013). The Rodinia supercontinent, comprising almost all the landmasses on Earth, is thought to have assembled at ca. 1.0 Ga (Li et al., 2008 and references therein). In most paleogeographic reconstructions of Rodinia, the continents of Australia, India, and East Antarctica are inferred to girdle Laurentia in the northeast (e.g., Li et al., 2008) with India sharing a conjugate margin with East Antarctica (Fig. 1). The two continents were involved in several episodes of collision and breakup during the assembly of supercontinents since the early Mesoproterozoic. The Eastern Ghats Belt along the eastern Indian margin and the Napier–Rayner Complex of East Antarctica were at the forefront of these collisions and amalgamations (Dasgupta and Sengupta 2003; Upadhyay et al., 2009; Bose et al., 2011; Dasgupta et al., 2013, 2017). It is thought that the Napier Complex or an Archean craton of similar age characteristics collided with the Dharwar Craton of India to form the central and eastern Indian Shield at ca. 1.60 Ga (Harley et al., 2003; Dasgupta et al., 2013, 2017) (Fig. 1). Rifting in this composite continental fragment at ca. 1.50–1.30 Ga (Upadhyay et al., 2006a, b; Upadhyay and Raith, 2006a, b; Upadhyay, 2008) opened an oceanic basin. The closure of this basin at ca. 1.0 Ga resulted in the collision of the Ruker Terrane and Rayner Complex of East Antarctica with the combined India–Napier Complex Shield producing the Grenville–age Eastern Ghats Province–Rayner Complex orogen (Harley, 2003; Upadhyay, 2008; Dasgupta et al., 2013; Chattopadhyay et al., 2015). The resulting India–Napier–Rayner–Ruker

continent has been called Enderbia (Harley, 2003; Upadhyay, 2008; Dasgupta et al., 2013; Chattopadhyay et al., 2015), which was later involved in collision/accretion with the East Africa/Maud Province continental blocks on its western margin at 600–500 Ma and with the Rauer Terrane and other crustal blocks on its eastern margin at 550–500 Ma (Harley, 2003) (Fig. 1). While most authors support such a paleo–geodynamic scenario, some paleomagnetic studies suggest India either was not a part of Rodinia or was adjacent to the northwestern margin of Australia and therefore north of its Gondwana fit by ca. 45° of latitude at 750 Ma (Torsvik et al., 2001a, b; Powell and Pisarevsky, 2002) (Fig. 1). This requires India to have moved ~5000 km (Harley et al., 2013) southwest to attain its Gondwana position in the early Paleozoic, and has led to controversial proposals that the Eastern Ghats Province and Rayner Complex crustal block constituted an exotic terrane that first collided with India during the Pan–African orogeny at ~0.5 Ga and not during Rodinia assembly (Dobmeier et al. 2006; Biswal et al. 2007; Das et al., 2008; Bhattacharya et al., 2016). Clearly, the timing of amalgamation of the Eastern Ghats Province with the Indian shield is central to the correlation between India and East Antarctica in Rodinia reconstructions (e.g., Chattopadhyay et al., 2015). A Grenville–age for the juxtaposition would support the conventional paleogeographic reconstruction models that position India adjacent to East Antarctica while an early Paleozoic amalgamation of the two terranes would be consistent with the hypotheses that India was not a part of Rodinia. The latter would mean that the Enderbia continent never existed. In this study, we report zircon U–Pb ages and trace element chemistry as well as metamorphic conditions of several Deformed Alkaline Rocks and Carbonatites (DARCs) along

the contact between the Eastern Ghats Belt and the Singhbhum–Bastar Cratons (Fig. 2). DARCs are associated with ancient collisional plate boundaries and are markers of suture zones where continents have rifted apart and later amalgamated (Burke et al., 2003). The alkaline complexes that were studied include Kamakhyanagar, Rairakhol, Khariar, Koraput, Kunavaram, and Jojuru from north to south. The age data is used to reconstruct the history of breakup and amalgamation events within the Indo–Antarctic Enderbia continent. The results establish that the docking of the Eastern Ghats Province with India first took place during the Grenvillian and not during the Pan– African orogeny, supporting conventional paleogeographic reconstruction models that position India adjacent to East Antarctica in Rodinia reconstructions. The ages from the Kamakhyanagar and Rairakhol nepheline syenite bodies are being reported for the first time.

2. Regional geologic setting 2.1 The Eastern Ghats Belt The Eastern Ghats Belt is a polycyclic granulite facies terrane of Neoarchean to Proterozoic age (e.g., Dobmeier and Raith, 2003; Simmat and Raith, 2008; Dasgupta et al., 2013), widely considered to have formed during orogenic collisions between India and East Antarctica (Dobmeier and Raith, 2003; Harley, 2003; Bose et al., 2011; Dasgupta et al., 2013, 2017). On the basis of Nd and Pb isotope mapping (Rickers et al., 2001) and geochronological data, Dobmeier and Raith (2003) have identified several crustal blocks with distinct geological histories, and accordingly sub–divided the Belt into four major units, namely, the Rengali Province, Jeypore Province, Ongole Domain and the Eastern Ghats Province (Fig. 2). The Jeypore and Rengali provinces represent Archean crustal units accreted to the Bastar and Singhbhum Craton margins in the Neoarchean (Fig. 2). The Jeypore Province is a poorly

studied Neoarchean crustal fragment consisting mostly of enderbite, charnoenderbite, and basic granulite. The orthogneisses with Archean protolith ages (TDM [Nd]: 3.9–3.0 Ga; Rickers et al., 2001) have undergone high–grade metamorphism at 2.8ȂʹǤ͹ Ga (U–Pb zircon; Kovach et al., 2001). The Rengali Province is a Meso– to Neoarchean fault bounded lithostructural domain at the southern margin of the Singhbhum Craton. The Province is dominated by medium to high– grade migmatitic hornblende gneisses and charnockite/enderbite gneisses overlain by the Deogarh, Tikra, and Malayagiri supracrustal successions (Crowe et al., 2003). The protoliths of the gneisses formed at 3.06–2.78 Ga (Misra et al., 2000; Chattopadhyay et al., 2015; Bose et al., 2016a) and were metamorphosed at ca. 2.82 Ga and ca. 2.50 Ga (Bose et al., 2016a). The supracrustal sequences comprise quartzite and schist interlayered with amphibolite (e.g., Mahalik, 1994; Crowe et al., 2003; Chattopadhyay et al., 2015). The Malayagiri supracrustal group of rocks were affected by four major tectonothermal events at ca. 2.42 Ga, 0.98–0.94 Ga, ca. 0.82 Ga and 0.57–0.54 Ga (Crowe et al., 2001; Chattopadhyay et al., 2015; Bhattacharya et al., 2016). The Rengali Province rocks are juxtaposed against the low–grade granite–greenstone terrane of the Singhbhum Craton along the northerly verging E–W trending Sukinda Thrust. The Ongole Domain and the Eastern Ghats Province are Proterozoic orogens within the Eastern Ghats Belt (Fig. 2). The Ongole Domain comprises calc–alkaline enderbites and charnockites, mafic–ultramafic rocks, anorthosites, diatexitic metapelites, quartzites, psammitic gneisses and calc–silicate granulites. It is interpreted to be a magmatic arc with widespread volcanism and plutonism between 1.75–1.71 Ga, ultra–high temperature metamorphism along an anticlockwise P–T trajectory at 1.68–1.60 Ga (Simmat and Raith, 2008; Upadhyay et al., 2009; Bose et al., 2011; Henderson et al., 2014; Sarkar and Schenk, 2014) culminating in accretion to

the eastern margin of India during collision of the Napier Complex with the Dharwar Craton at 1.60–1.55 Ga (Upadhyay et al., 2009; Sarkar et al., 2014, 2015). The Eastern Ghats Province is a polycyclic Grenville–age orogen (Upadhyay and Raith, 2006a, b; Upadhyay et al., 2006a, b; Simmat and Raith, 2008; Upadhyay, 2008; Bose et al., 2011; Dasgupta et al., 2013) (Fig. 2). It comprises charnockitic gneisses, megacrystic garnet– and orthopyroxene–bearing granitoid plutons (0.98–95 Ga; Paul et al. 1990; Kovach et al., 1998) and massif–type anorthosite complexes intrusive into a granulite facies supracrustal package (TDM [Nd]: 2.6–2.0 Ga; Rickers et al. 2001) dominated by garnet–sillimanite gneiss (khondalite), migmatitic quartzofeldspathic gneiss and mafic granulite. The earliest metamorphism (M1) at ultra–high temperature conditions (peak P–T conditions of 8–10 kbar and ca. 1000°C) took place between 1.25–1.0 Ga (Simmat and Raith, 2008; Upadhyay et al., 2009; Das et al., 2011; Bose et al., 2011). The second metamorphic overprint (M2), also at high temperature conditions (peak P– T conditions of 8.0–8.5 kbar, 800–850°C) is associated with extensive reworking of the granulites during the Grenville–age orogeny at 980–930 Ma (Simmat and Raith, 2008; Bose et al., 2011; Das et al., 2011; Korhonen et al., 2013; Bose et al., 2016b). This event was broadly synchronous with the emplacement of megacrystic granitoid plutons at 985–955 Ma (Paul et al., 1990; Kovach et al., 1998; Krause, 1998; Dharma Rao et al., 2014) and massif–type anorthosites between 984 Ma and 918 Ma (Krause et al., 2001; Dharma Rao et al., 2014; Chatterjee et al., 2008; Raith et al., 2014). The province witnessed continued tectonothermal activity into the late Neoproterozoic and early the Paleozoic. The prominent events in this period include 850–780 Ma high–grade thermal metamorphism/decompression well documented in the Chilka lake region (Upadhyay et al., 2009; Bose et al., 2016b) and 620–500 Ma tectonism and metamorphism related to the Pan–African orogeny (Crowe et al., 2001; Dobmeier and Raith,

2003; Simmat and Raith, 2008; Upadhyay et al., 2006b, 2009; Bose et al., 2016b). The Pan– African orogenesis led to significant internal reorganization of the belt and reactivation of Grenville–age terrane boundaries.

2.2 DARCs at the craton–Eastern Ghats Belt contact The contact between the cratons and the Ongole Domain/Eastern Ghats Province (henceforth referred to as the craton–Eastern Ghats Belt contact) is a terrane boundary and a crustal suture (Gupta et al., 2000; Dobmeier and Raith, 2003; Bhadra et al., 2004) (Fig. 2). The suture zone is curvilinear with a WNW–ESE strike and strike–slip character in the north and NNE–SSW strike with a thrust character in the west (Biswal et al., 2002; Gupta and Bose, 2004; Gupta et al., 2005). The shear zones assume listric geometry at depth forming décollement for the fold–thrust belt developed on the northwestern front of the Eastern Ghats Province (Biswal et al., 2007). The shears are pervasive and continuous across the contact, have an east dipping shear fabric with northwest vergent thrust movement, and are interpreted as resulting from the westward thrusting of the Eastern Ghats Province granulites (Gupta et al., 2000; Biswal et al., 2002; Dobmeier and Raith, 2003; Bhadra et al., 2004). Gupta et al. (2000) identified three phases of deformation in migmatitic gneisses and mafic granulites of the Eastern Ghats Province at the contact with the Bastar Craton rocks. Thermobarometric studies on the rocks gives near–peak P–T conditions of ≥900°C, 9.5 kbar during D3 deformation followed by a period of cooling and isothermal decompression to 800–850 °C and ~7 kbar. The cratonic gneisses also record three deformation events with peak P–T conditions of ~ 700°C and 6.5 kbar during the D2 event. The craton– Eastern Ghats Belt contact hosts a chain of DARCs that outcrop as linear belts from Kamakhyanagar in the north to Elchuru and Uppalapadu in the south. The Rairakhol and

Kamakhyanagar alkaline bodies outcrop at the contact of the Rengali Province and the Eastern Ghats Province whereas the Khariar alkaline complex is located at the contact of the Bastar Craton and the Eastern Ghats Province. In contrast, the Koraput alkaline body outcrops at the contact of the Jeypore Province and the Eastern Ghats Province. The Kunavaram and Jojuru alkaline complexes occur at the contact between the Ongole Domain and the Eastern Dharwar Craton (Fig. 2). Most of the alkaline complexes are dominated by miaskitic nepheline syenite, syenite, and monzosyenite with minor occurrences of mafic alkaline rocks (Fig. 3). The geochemistry of these rocks are indicative of a rift–related tectonic setting for the alkaline magmatism (Upadhyay et al., 2006a, b; Upadhyay and Raith, 2006a, b; Upadhyay, 2008) (Fig. 3). The alkaline rocks have been overprinted by multiple high–grade tectonothermal events and often display gneissic fabric (Upadhyay and Raith, 2006a) (Fig. 3). The Kamakhyanagar alkaline complex outcrops as a narrow E–W trending belt at the contact between garnetiferous granite gneisses of the Rengali Province and garnetiferous ortho– and para–gneisses of the Eastern Ghats Province. The rocks of the complex are mostly fine– grained to pegmatitic nepheline syenites and are characterized by a pervasive tectonic foliation (S1) defined by aligned amphibole and biotite which is axial planar to tight/isoclinal folds on magmatic layering (S0). The igneous layering is defined by alternate mafic mineral–rich and feldspar/nepheline–rich layers. The S0–S1 composite is refolded during a second phase of deformation. The Rairakhol alkaline complex comprises amphibole– and biotite–bearing fine–grained to pegmatitic nepheline syenites/syenites/alkalifeldspar syenites that outcrop along ca. 0.5 km wide and ca. 15 km long linear E–W stretch. The alkaline rocks have a penetrative tectonic foliation that is sub parallel to igneous layering defined by amphibole/biotite–rich layers

alternating with feldspar/nepheline–rich layers. At places, the nepheline syenites are strongly sheared and boudinaged along mylonitic shear bands. The Khariar alkaline complex is exposed along ca. 30 km long NE-SW trending sinuous belt at the contact between granite gneisses of the Bastar Craton and garnet sillimanite gneisses of the Eastern Ghats Province. The complex is dominated by nepheline syenites in the core and syenites along the marginal zones. The alkaline rocks are inter-layered with tholeiitic amphibolites. They are deformed and display a gneissic foliation that overprints the igneous layering. The earliest folds (F1) on magmatic layering (S0) are isoclinal, with the development of a penetrative axial planar foliation (S1) defined by amphibole and biotite which have replaced clinopyroxene. The S0–S1 composite is refolded by a set of open to tight westerly overturned asymmetric F2 folds. At the contact with the Eastern Ghats Province, a shear fabric (S2) draws into parallelism all earlier structures. A northwest vergent movement on the shear fabric is inferred from C–S fabric and other shear sense indicators (Upadhyay et al., 2006a). The Koraput alkaline complex is a NE-SW trending lenticular body comprising alkaline gabbro in the core fringed by syenodiorite and nepheline syenite. The rocks have undergone high–grade deformation and metamorphism with the development of penetrative tectonic foliations. Gupta et al. (2005) have identified three phases of deformation in the complex. The earliest fabric (S1) defined by aligned amphibole needles and plagioclase is rotated parallel to a NE–trending, east–dipping S2 foliation. In localized domains, the D2 deformation is followed by D3 shearing. The Kunavaram alkaline complex forms a NE-SW trending elongate body along the Sileru Shear Zone. The complex is dominated by leucocratic nepheline syenite with volumetrically minor proportions of syenite and nepheline bearing monzonitic rocks (Upadhyay

and Raith, 2006a). The alkaline rocks are deformed and metamorphosed with the first phase of deformation (D1) producing a pervasive segregation banding (S1) which was isoclinally folded along subvertical axial planes with shallow plunging axes during D2 deformation (Gupta and Bose, 2004). The last deformation event (D3) produced a NNE–SSW trending dextral strike–slip shear zone which truncates the earlier structures in the rocks. The deformation is inferred to have taken place under amphibolite to granulite facies conditions (Gupta and Bose, 2004). The Jojuru alkaline complex is dominated by monzosyenitic rocks hosted in garnetiferous quartzo-felspathic gneisses of the Eastern Dharwar Craton. The host rocks are in contact with the nearby Kondapalle granulite package of the Ongole domain. The monzosyenite body is affected by N–S shearing with west vergent movement along its eastern margin. Away from the shear zone, it is almost massive without any fabric. Metamorphic garnet is present in both the sheared and the undeformed zones (Upadhyay and Raith, 2006b).

3. Analytical technique Uranium–lead (U–Pb) isotope dating of zircon was done using a Laser Ablation–Inductively Coupled Plasma Mass Spectrometer (LA–ICPMS) at the Department of Geology and Geophysics, Indian Institute of Technology (IIT), Kharagpur. Zircon grains were separated using bromoform,

mounted,

and

polished

using

standard

sample

preparation

techniques.

Cathodoluminescence (CL) and Back Scattered Electron (BSE) images of the grain interiors were obtained using a JEOL JSA 6490 Scanning Electron Microscope at the Department of Geology and Geophysics, IIT Kharagpur. The images were used as guides for spot selection. The U–Pb isotope measurements were done on a Thermo–Fisher Scientific iCAP–Q quadrupole ICPMS coupled to a New Wave 193 ArF Excimer laser ablation system. The laser was operated

at 5 Hz repetition rate, ca. 5 J/cm2 beam energy density and 25–40 μm spot size. The ICPMS was optimized for maximum sensitivity on Pb, Th, and U using the NIST 612 reference glass. The oxide production rate monitored on

232

Th16O was found to be in the range of 0.6–0.8%. The

analyses were performed in time–resolved mode with each analysis consisting of 30 seconds background measurement and 40 seconds peak signal measurement. External standardization was done by bracketing groups of ten unknowns with three measurements of the GJ–1 reference zircon (Jackson et al. 2004). The data quality was monitored by analyzing the 91500 and Plešovice reference zircon (Wiedenbeck et al. 1995; Sláma et al., 2008) as unknowns interspersed with the sample measurements. The data were reduced offline using an in–house Excel© spreadsheet that corrects for instrumental and gas backgrounds, laser–induced elemental fractionation, as well as instrumental mass–bias and drift. The uncertainty on each analysis was estimated by quadratic addition of the 2SE (standard error) internal run statistics of each analysis and the 2SD (standard deviation) of isotopic ratios measured in the bracketing GJ–1 reference zircon. The measured 2σ; 207

206

Pb/238U and

207

Pb/206Pb ratios for the 91500 (206Pb/238U=0.1791±1.9%,

207

Pb/206Pb=0.0748±0.85%, 2σ, n=60) and Plešovice (206Pb/238U=0.0535±1.8%, 2σ;

Pb/206Pb=0.0536±2.1%, 2σ, n=4) reference zircons match published values within analytical

errors. All uncertainties are reported at the 2σ–level. The U contents were estimated relative to the GJ–1 reference zircon. Concordia diagrams and age probability/histogram plots were constructed using Isoplot 4.15 (Ludwig, 2003). The Rare Earth Element concentrations in zircon were measured using the same LA– ICPMS setup, but the ablation parameters were 10 Hz repetition rate, 5 J/cm2 beam energy density and 40 μm spot size. The ICPMS was optimized using the NIST 612 reference glass. The analyses were performed in time–resolved mode with each analysis consisting of 30 seconds

background measurement and 40 seconds peak signal measurement. External standardization was done by bracketing groups of ten unknowns with two measurements of the NIST 612 reference glass. The data quality was monitored by analyzing the NIST 610 reference glass as unknown interspersed with the measurements of the samples. Accuracy and precision as determined from repeat analyses of the NIST 612 and NIST 610 glasses are better than 7% (2σ) and 10% (2σ) respectively over several analytical sessions. The major element compositions of minerals were measured using a Cameca SX–100 Electron Probe Micro Analyzer (EPMA) at the Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur. The operating conditions were 15 kV accelerating voltage, and 15 nA beam current. Dwell times were set at 10 s on the peak and 5 s on the background. Calibration was done using natural and synthetic mineral standards.

4. Results 4.1 Sample details and petrography Zircons were separated from six nepheline syenite samples collected from the Kamakhyanagar, Rairakhol, Khariar, Koraput, Kunavaram and the Jojuru alkaline complexes along the craton– Eastern Ghats Belt contact. The GPS coordinates of the samples are listed in Table S1in the online supplementary material and the petrographic descriptions of the samples are summarized in Table 1.

4.2 Metamorphic conditions of the nepheline syenites The pressure–temperature condition of metamorphism of the nepheline syenites is difficult to constrain due to the high variance of the assemblage and the lack of proper activity–composition

relationship for phases such as taramitic amphibole, aegirine, and nepheline. An attempt was made to retrieve meaningful metamorphic temperatures using the Ti–in–amphibole thermometer of Otten (1984), the amphibole–plagioclase thermometer of Holland and Blundy (1994), and the Ti–in–biotite thermometer of Henry (2005). For the amphibole–plagioclase thermometry, only the edenite + albite = richterite + anorthite calibration is suitable for the alkaline rocks as the edenite + 4 quartz = tremolite + albite calibration requires unit SiO2 activity. The EPMA analyses of the minerals and the estimated metamorphic temperatures are summarized in Table S2 in the online supplementary material. The Ti–in–amphibole thermometer furnishes temperatures of 715–832°C whereas the amphibole–plagioclase thermometer gives temperature of 602–755°C for the nepheline syenites and the monzosyenite. The Ti–in–biotite thermometer furnishes temperatures of 424–649°C.

4.3 Textures, trace element chemistry and U-Pb ages of zircons Textural relations documented by BSE and CL imaging reveal the presence of both magmatic as well as metamorphic zircons in the alkaline rocks. The textures, trace element chemistry and the U-Pb ages of the zircons from the six alkaline complexes are described below and listed in the online supplementary material. The ages from the Kamakhyanagar and Rairakhol nepheline syenite bodies are being reported for the first time.

4.3.1 Kunavaram nepheline syenite (KUN 3B) In the Kunavaram nepheline syenite, zircons are usually associated with amphibole or the alkalifeldspar–nepheline matrix (Figs. 5a–f). They are subhedral to round with complexly zoned interiors representing at least three textural events. Most grains are characterized by the presence

of subhedral to anhedral relict core mantled by metamorphic zones (Figs. 5a–f, 6a). The cores show clear evidence of recrystallization with featureless irregular reequilibration and resorption fronts proceeding inward from the margins (Figs. 5a–f, 6a). The interiors of the cores are also strongly recrystallized as seen from the presence of irregular patches which appear as dark or moderately luminescent zones in CL images (Figs. 5a–f, 6a). In BSE images, the CL–dark patches appear as brighter porous zones (Fig. S1 online supplementary material). The presence of small domains with faint traces of relict oscillatory zoning within the cores attests to their magmatic origin (Figs. 5b, 6a). The cores are mantled by thin to broad metamorphic overgrowth (Figs. 5a–f, 6a). This zircon forming event involved growth along grain boundaries, changing the grain shape and producing lobate overgrowths protruding into the neighboring minerals (Figs. 5a–f). The presence of resorption fronts and mantling overgrowths are interpreted as evidence of zircon reequilibration, dissolution and reprecipitation. Primary zircon growth in the rock was followed multiple events of metamorphism leading to resorption and reequilibration of the magmatic grains and the formation of overgrowths. The paragenetic association of zircon with amphiboles suggests that in addition to the dissolution of preexisting igneous zircon, amphiboles or their precursors, i.e., aegirine augite may have been a probable source of zirconium for the formation of metamorphic zircon (e.g., Möller et al., 2003; Söderlund et al., 2004; Harley et al., 2007). The majority of U–Pb spot analyses of the patchily zoned cores gave discordant ages. However, concordant ages could be extracted from some spots which define a concordia population at 1373±6 Ma (Fig. 6). These spots are also characterized by strongly fractionated HREE–enriched REE patterns with positive Ce anomalies, similar to igneous zircons (Fig. 6). The CL–dark (brighter zones in BSE images) patches within the recrystallized core domains are

enriched in U–Th, the light REEs, Na, Ca, Sr, Al, Fe, Y, Nb, and Ta, and often gave discordant ages (Fig. 6; Table S3, S4 online supplementary material). The reequilibrated rims or the metamorphic overgrowths wherever wide enough to be analyzed furnished concordant ages of 942±16 Ma, 801±18 Ma, and 567±12 Ma (Fig. 6).

4.3.2 Khariar nepheline syenite (DU 37) Zircons in nepheline syenite from the Khariar alkaline complex record a complicated paragenetic history similar to those from the Kunavaram complex. The mineral is often associated with amphibole and have euhedral to subhedral shapes (Figs. 5g–l, 7a). The subhedral ones are found mostly near amphiboles and nepheline. Most grains have cores surrounded by multiple generations of metamorphic zones of variable thicknesses (Figs. 5g–l, 7a). The cores are strongly recrystallized producing chaotic lobate patchy zones which are CL–dark or display moderate cathodoluminescence (Figs. 5g–l, 7a). The CL–dark patches correspond to bright, interconnected porous domains in BSE images (Fig S1, online supplementary material). The presence of relict oscillatory growth zones in some of the patches confirms their magmatic origin (Figs. 5j, 7a). In other grains, the igneous domains are pseudomorphically replaced by CL–bright zones along irregular fronts (Fig. 7a). Texturally, at least three to four events involving zircon forming or reequilibration can be identified. The growth of primary igneous zircon in the nepheline syenites was followed by reequilibration and recrystallization of the magmatic grains during metamorphism. The recrystallized cores are mantled by poorly luminescent metamorphic overgrowths

which

are

in

turn

rimmed

or

replaced

by

relatively

brighter

overgrowths/recrystallized zones (Figs. 7a). Here again, the paragenetic association of zircon with amphiboles suggests that the mineral or its precursor aegirine augite may have been a

probable source of zirconium for the formation of metamorphic zircon, in addition to the dissolution of preexisting zircon. A large number of the analyzed spots give discordant ages (Table S3, online supplementary material). The concordant ones define a prominent concordia population at 1471±4 Ma and a minor cluster at 563±20 Ma (Fig. 7). The

206

Pb/238U ages of the concordant

points give probability/histogram peaks at 1470±5 Ma and 562±20 Ma, identical to the concordia ages (Fig. 7). The discordant analyses are mostly from the recrystallized patchily zoned cores. Such zones are also characterized by higher REE contents and enrichment in the light REEs, Na, Ca, Sr, Al, Fe, Y, Nb, and Ta compared to the relict igneous domains Table S4, online supplementary material).

4.3.3 Jojuru monzosyenite (JJ 2) In the Jojuru monzosyenite, zircons are often associated with ilmenite and biotite (Figs. 5m–q). Most grains are euhedral and preserve their oscillatory growth zoned interiors (Fig. 8a). Rare metamorphic grains with irregular shapes are found associated with patches of ilmenite and pyroxenes (Figs. 5n, q). Some of the magmatic grains are characterized by narrow lobate metamorphic overgrowths near their pyramidal terminations (Figs. 5m, o, p, 8a). These lobate overgrowths protrude into the neighboring biotite or ilmenite (Figs. 5m, o, p). The textural association of zircon with ilmenite and biotite suggests that these two minerals were the source of zirconium for the formation of the metamorphic overgrowths (e.g. Vavra et al., 1996). The zircon spot ages define a prominent concordia age peak at 1352±6 Ma (Fig. 8). These ages are from domains showing oscillatory growth or sector zoning. A large number of analyses gave discordant ages (Table S3, online supplementary material). These spots were

usually from featureless zones or from the narrow lobate overgrowths. However, concordant ages of 930±23 Ma and 569±34 Ma could be extracted from a few spot analyses in such domains (Fig. 8). The 206Pb/238U ages of the concordant points define identical age populations at 1352±6 Ma, 930±24 Ma and 569±35 Ma (Fig. 8). The metamorphic zircons have higher REE contents than the magmatic ones (Fig. 8; Table S4, online supplementary material).

4.3.4 Rairakhol nepheline syenite (RAK 7) Two textural types of zircons can be identified in nepheline syenite from the Rairakhol alkaline complexes (Figs. 5 r–t). The larger grains (ca. 100–500µm across) are usually euhedral or subhedral and characterized by complex internal structures (Fig. 9a). They have a core which is either featureless or display lobate, chaotic, and patchy domains, a clear evidence that they represent recrystallized igneous zircon (Fig. 9a). The CL–dark patches can be correlated with bright, interconnected porous domains in BSE images (Fig S1, online supplementary material). Irregular reequilibration fronts progressing into their interiors document an event of zircon recrystallization (Fig. 9a). The reequilibrated cores are surrounded metamorphic overgrowths of variable thicknesses (Fig. 9a). In addition to the above textural variety, smaller zircon grains (ca. <100 µm across) that usually tend to be anhedral can be identified. These appear as numerous tiny grains or their clusters scattered throughout the rock matrix but usually growing within or near amphibole and nepheline grain boundaries (Figs. 5r–t). Some of these grains are associated with Nb–U–REE phases or allanite (Fig. 5s, t). Their irregular to globular shapes indicate that they are of metamorphic origin. They are characterized by protruding lobate margins and are interpreted to have grown along grain–boundaries during metamorphism (Figs. 5r–t). The

zirconium was probably sourced from amphibole/biotite or the Nb–U–REE–Zr bearing accessory phases. The U–Pb spot ages in zircons from the Rairakhol nepheline syenite define two concordia populations at 1372±5 Ma and 953±16 Ma (Fig. 9). The

206

Pb/238U ages from the concordant

data points define identical age peaks at 1371±6 Ma and 950±15 Ma respectively in age probability/histograms plot (Fig. 9). The CL–dark patches within the cores are enriched in U–Th and the REEs as well as Na, Ca, Sr, Al, Fe, Y, Nb, and Ta (Table S4, online supplementary material). These domains often give discordant ages. The concordant 1372±5 Ma population corresponds to domains having REE patterns similar to magmatic zircon. The 953±16 Ma ages are either from the recrystallized core or from the metamorphic overgrowths.

4.3.5 Kamakhyanagar nepheline syenite (KNS 3) The zircons in the Kamakhyanagar nepheline syenite are also recrystallized with grains having core which is either featureless or display lobate, chaotic, and patchy domains rimmed by metamorphic rims or irregular reequilibrated zones (Fig. 10a). The U–Pb isotope ratios of zircons from the Kamakhyanagar nepheline syenite furnish both concordant as well as discordant spot ages (Fig. 10; Table S3, online supplementary material). The concordant analyses define three concordia populations at 1344±12 Ma, 930±9 Ma, and 795±8 Ma (Fig. 10). The

206

Pb/238U ages of concordant domains define three relative

probability/histogram peaks at 1335±15 Ma, 926±9 Ma, 792±8 Ma that are identical to the concordia ages (Fig. 10). The recrystallized and patchily zoned core regions furnish both concordant as well as discordant spot ages. The 1344±12 Ma concordant ages are often from the moderately cathodoluminescent patches within the cores. These domains have REE patterns

typical of magmatic zircon, i.e., showing depletion of the light REEs, enrichment of the heavy REEs and positive Eu anomaly (Fig. 10). The discordant analyses from the cores have high but variable concentrations U–Th, and higher REE abundances with strong enrichment of the light REEs. They are also characterized by higher abundances of non–formula cations such as Na, Ca, Sr, Al, Fe, Y, Nb, and Ta (Table S4, online supplementary material). In contrast, the 930±9 Ma and 795±8 Ma ages are commonly from rims or marginal zones of grains. Such domains are also often characterized by higher REE concentrations and strong enrichment of the light REEs.

4.3.6 Koraput nepheline syenite (KP 1) Zircons in nepheline syenite from the Koraput alkaline complex show extreme recrystallization. In most grains, growth zoning is completely lacking, having been replaced by recrystallized patchy domains or metamorphic overgrowths (Fig. 11a). The concordant spot ages define two major concordia clusters at 947±10 Ma and 808±3 Ma, also recognized in

207

Pb/206Pb age probability and histogram plot (Fig. 11). Two spot

analyses gave an older age of 1383±23 Ma while a few analyses cluster close to 661±8 Ma (Fig. 11). Several of the analyses gave discordant ages (Table S3, online supplementary material. Such spots were largely in the patchily zoned parts of the grains

5. Discussion 5.1 Discordance of U–Pb ages and enrichment of REE as well as non–formula cations in zircon The zircon textural relations described and illustrated above clearly reveal that the grains have complicated internal structure consisting of igneous as well as recrystallized, reequilibrated and newly grown metamorphic domains. A majority of the discordant analyses were from the

recrystallized core regions of the grains or from the irregular reequilibration fronts proceeding inward from the core margins or from the featureless metamorphic domains. The recrystallized cores and the irregular reequilibration fronts are often characterized by the presence of irregular patches of CL–dark and CL–bright regions. The CL–dark domains form an interconnected network of patches which appear as brighter zones in BSE images. These zones are enriched in U–Th, the REEs, and nonformula cations like Na, Ca, Sr, Al, Fe, Y, Nb, and Ta. Such domains are interpreted to have formed by diffusion–controlled recovery–recrystallization of severely radiation damaged zircon in the presence of an aqueous fluid phase. The process involves inward diffusion of hydrous species into radiation–damaged zircon, activating structural recovery processes while the zircon is still in the solid state (Geisler et al. 2007, and references therein). In this mechanism of zircon reequilibration, the structural recovery process is accompanied by partial loss of radiogenic Pb and gain of solvent cations such as Ca, Al, and Fe. The U and Th have been seen to remain in the altered areas (Nasdala et al. 2010), while the LREE were found to be enriched (Hoskin, 2005). Long et al. (2012) have also reported metamorphic zircon showing LREE enrichment from Paleoproterozoic anatectic granite in the northern Tarim Craton, NW China. These authors also noted a positive correlation between REE enrichment and increasing discordance of the zircons and attributed it to recrystallization in an open system with the participation of aqueous fluids or hydrous melts. For the zircons in the alkaline rocks along the craton–Eastern Ghats Belt contact, the discordance of the U–Pb ages from the reequilibrated zircon domains can be attributed to two reasons: (1) partial loss of radiogenic Pb during diffusion–controlled recovery–recrystallization, (2) mixing of material from adjacent domains during their ablation. The second possibility is very likely given that the reequilibrated and relict zones are finely intergrown and both would

have been invariably ablated during analyses. Mixing would also explain the variable U–Th and REE concentrations of the spot analyses depending on the amounts of reequilibrated and pristine zircon sampled during the ablation process. Therefore, the discordant ages may not have any geological meaning and are not used for further interpretation.

5.2 Emplacement ages of the alkaline complexes The emplacement ages of the complexes were constrained using CL internal structures and trace element characteristics of the zircon grains, and the assumption that the oldest concordant age population corresponds to magmatic domains. The latter premise is justified given that nepheline syenite parental magmas are invariably mantle–derived and therefore unlikely to have any significant zircon inheritance. For the Kamakhyanagar nepheline syenite complex, the 1344±12 Ma ages retrieved from domains with REE pattern similar to igneous zircon are interpreted to be dating the emplacement of the alkaline complex. In the Rairakhol alkaline complex, the 1372±5 Ma ages obtained from zircon domains with magmatic REE patterns corresponds to igneous crystallization of the alkaline rocks. This age is broadly similar to the 1413±23 Ma Rb–Sr whole rock isochron age reported by Sarkar et al. (1994) for the complex. For the Khariar nepheline syenite complex, the 1471±4 Ma age population is interpreted to date igneous emplacement. This age is identical to the 1480±17 Ma zircon ages reported earlier by Upadhyay et al. (2006a). For zircons from the Koraput alkaline complex, the Mesoproterozoic concordant age of 1387±34 Ma retrieved from a few spots closely matches the emplacement ages of the other alkaline complexes along the craton–EGB suture. Therefore, we interpret the 1387±34 Ma age to be dating the intrusion of the Koraput nepheline syenite body. These results are in contrast to the U–Pb zircon ages reported

by Hippe et al. (2015) for the Koraput alkaline rocks. These authors obtained age clusters at 869±11 Ma, 801±9 Ma, and 728±11 Ma and interpreted the 869±11 Ma age to represent the timing of the emplacement of the Koraput alkaline body. However, the authors reported Depleted Mantle Hf model ages clustering at 1.4–1.2 Ga for the zircons, an indication that the nepheline syenite magmas were extracted from the mantle much before than what the zircon ages indicate. It appears that the ages reported by Hippe et al. (2015) correspond to recrystallized/partially reset metamorphic domains and the authors may have missed analyzing the scarcely preserved magmatic domains in the highly recrystallized zircons. For the Kunavaram nepheline syenite complex, the 1372±8 Ma age cluster from zircon domains with igneous trace element signatures dates the emplacement of the alkaline magma. This age is identical to the upper intercept age of 1384±63 Ma obtained earlier by Upadhyay and Raith (2006a). Zircons from the Jojuru monzosyenite body furnish an igneous crystallization age of 1352±6 Ma. Upadhyay and Raith (2006b) had earlier reported an intrusion age of 1263±23 Ma for the monzosyenite complex. The discrepancy is possibly a result of inadvertent analyses of mixed igneous and metamorphic domains by Upadhyay and Raith (2006b). The revised intrusion age is in conformity with the emplacement ages of the other alkaline bodies along the contact. The U–Pb zircon ages of the Elchuru (1321±17 Ma: Upadhyay et al., 2006), Uppalapadu (1356±7 Ma: Vijaya Kumar et al., 2007), and Chhatabar–Lodhajhari–Baradangua (1322±8 Ma: Sheikh et al., 2017) alkaline complexes published earlier together with those from the Kamakhyanagar, Rairakhol, Koraput, Kunavaram and Jojuru nepheline syenite bodies reported in this study date a phase of Mesoproterozoic continental rifting and alkaline magmatism between 1320 Ma and 1370 Ma. The Khariar alkaline complex emplaced at 1471±4 Ma is significantly older than the alkaline bodies marking the 1320-1370 Ma rift and may correspond

to an earlier phase of crustal extension. This earlier extensional event may have been coeval with rifting in the Bastar Craton which formed the Mesoproterozoic Chattisgarh rift basin at 1.5–1.4 Ga (Bickford et al., 2011).

5.2 Timing of deformation and metamorphism of the alkaline complexes The temperatures estimated from the Ti–in–amphibole thermometer possibly represents near peak metamorphic conditions. The lower temperatures recorded by the amphibole–plagioclase thermometer maybe due to post–peak reequilibration and compositional readjustment of the plagioclase as evidenced from the extensive mobilization and redistribution of perthite exsolution lamellae. These temperature estimates clearly establish that the nepheline syenites have undergone high–grade metamorphism under granulite facies conditions. Zircons from the Kamakhyanagar

nepheline

syenite

complex

record

two

episodes

of

metamorphic

recrystallization at 930±9 Ma and 795±8 Ma. The Grenville–age tectonothermal event is also seen in zircon from the Rairakhol nepheline syenite body where an age cluster at 953±16 Ma is obtained from recrystallized zircon domains having high REE concentrations. In the Khariar alkaline complex, Grenvillian ages are found to be lacking in the analyzed zircons which instead record a Pan–African tectonothermal overprint at 563±20 Ma. Zircons from the Koraput alkaline complex record two major Neoproterozoic tectonothermal events, a Grenville–age overprint at 947±10 Ma and a mid–Neoproterozoic overprint at 808±3 Ma. There is also a hint of Pan– African high–grade metamorphism at ca. 661 Ma. In the Kunavaram alkaline complex, zircons furnish several metamorphic age populations. An age cluster at 942±17 Ma corresponds to the Grenville–age tectonothermal overprint; the 801±18 Ma age population dates a mid– Neoproterozoic overprint, while the 566±12 Ma ages can be correlated with a Pan–African

tectonothermal overprint. Zircons from the Jojuru monzosyenite complex were also affected by the Grenvillian and Pan–African tectonothermal overprints as seen from the presence of 930±24 Ma and 569±35 Ma ages. In summary, the U–Pb zircon age data indicate that DARCs along the craton–Eastern Ghats Province contact were variably affected by three major high–grade tectonothermal events at 953–930 Ma, 808–795 Ma, and 661–563 Ma.

5.3 Constraints on the history of break–up and amalgamation events within the Indo–Antarctic Enderbia continent and link to supercontinent cycles Nepheline syenites and carbonatites are emplaced in intracontinental rifts (Bailey, 1974). Later deformation/metamorphism of the alkaline rocks during continent–continent or continent–arc collisions gives rise to DARCs. Linear chains of DARCs thus mark sutures where oceans had opened and then closed in the geologic past (Burke et al., 2003). Since DARCs are the products of two well–defined phases of the Wilson cycle, i.e., continental rifting and subsequent collision, geochronological constraints from the chain of DARCs along the craton–Eastern Ghats Belt contact can be used to unravel the history of rifting and amalgamation in the Indo–Antarctic Enderbia continent and its predecessor and link it to the assembly and break–up of past supercontinents. It is widely accepted that the major land masses on Earth have been involved in the assembly and dispersal of at least two Precambrian supercontinents, namely Columbia and Rodinia (Rogers and Santosh, 2002; Zhao et al., 2002, 2004; Li et al., 2008; references therein; Meert, 2002, 2012) (Fig. 1). The Columbia supercontinent was assembled by ca. 1.8 Ga and underwent subduction–related magmatic accretionary growth at continental margins between 1.8

Ga and 1.3 Ga. The supercontinent started fragmenting at 1.6 Ga and finally dispersed by 1.3–1.2 Ga (Zhao et al., 2004). In most reconstructions of Columbia (e.g., Rogers and Santosh, 2002; Zhao et al., 2004; Meert, 2002), India and East Antarctica share a conjugate margin with the Napier Complex juxtaposed against the Dharwar Craton (Fig. 1). The two continents first amalgamated at ca. 1.60 Ga (Harley, 2003; Dasgupta et al., 2013) following a period (ca. 1.75– 1.60 Ga) of subduction and ocean closure between East Antarctica/Napier Complex and the Dharwar Craton. The Ongole Domain arc accreted to the eastern margin of the Dharwar Craton (Dasgupta et al., 2013; Sarkar et al., 2015) during this collision forming the composite Dharwar Craton–Ongole Domain–Napier Complex/East Antarctic shield (e.g., Dasgupta et al., 2017). The Paleoproterozoic crustal accretion and collision processes involving India and East Antarctica is in conformity with the growing body of evidence supporting long–lived accretionary growth of Columbia along continental margins (Zhao et al., 2004; references therein). The history of Enderbia began with the Mesoproterozoic breakup of the composite Dharwar Craton–Ongole Domain–Napier Complex/East Antarctica continent which was the precursor to Enderbia. Rifting in this continent occurred at 1471–1321 Ma as documented by the zircon ages from the linear chain of DARCs along the craton–Eastern Ghats Belt contact. The precise configuration of the rifted margin is difficult to constrain due to later modification by Neoproterozoic and Paleozoic high–grade tectonothermal events (discussed below). However, it would have broadly corresponded with the Ongole Domain–Dharwar Craton, and Jeypore Province/Rengali Province–Eastern Ghats Province contact. The event can be correlated with widespread continental rifting in several landmasses (e.g., western margin of Laurentia, southern margin of Baltica, southeastern margin of Siberia, northwestern margin of South Africa, northern

margin of North China; Zhao et al., 2004) during the fragmentation and dispersal of the Columbia supercontinent. The rifting reopened an oceanic basin between India and parts of East Antarctica. It is in this basin that the sedimentary sequences of what would later become the Eastern Ghats Province were deposited between 1.42 Ga and 1.1 Ga (Upadhyay et al., 2009; Dasgupta et al., 2017). The sediments underwent an early ultra–high temperature metamorphism on an anticlockwise P–T path at approximately 1.1–1.0 Ga (Dasgupta et al., 2013). Inversion and closure of the oceanic basin that existed between the Ruker Terrane of East Antarctica and the Indian Shield led to the oblique collision of the Ruker Terrane with the rifted southeastern margin of India giving rise to the Eastern Ghats Province–Rayner Complex orogen (Harley, 2003; Upadhyay, 2008; Dasgupta et al., 2013; Chattopadhyay et al., 2015). The Eastern Ghats Province granulites were reworked along a clockwise P–T path (Das et al. 2011) during the event. This amalgamation resulted in the formation of the India–Napier–Rayner–Ruker Enderbia continent and can be correlated to collisions leading to the final assembly of the Rodinia supercontinent (Fig. 1). The event is recorded as the 953–930 Ma tectonothermal overprint on zircon from the DARCs along the craton–Eastern Ghats Belt contact. Li et al. (2008) proposed that the sinking of stagnated slabs accumulated at the mantle transition zone surrounding the Rodinia supercontinent, coupled with thermal insulation by the supercontinent, caused periodic superplumes and associated continental rifting between 825 Ma and 750 Ma. The zircons from the DARCs along the craton–Eastern Ghats Belt contact record an event of metamorphic recrystallization at 808–795 Ma. Similar ages have been obtained from other parts of the Eastern Ghats Belt and have been interpreted as dating high–temperature metamorphism (Dobmeier and Simmat, 2002; Simmat and Raith, 2008; Upadhyay et al., 2009)

or decompression (Bose et al., 2016b). Broadly similar ages documenting regional high–grade metamorphism and emplacement of A–type granite and anorthosite have been documented in the northern part of the Southern Granulite Terrane (Kooijman et al., 2011; Brandt et al., 2014) as well as in the Rayner Complex (Black et al., 1987; Shiraishi et al., 2008), and Prydz Bay (Tong et al., 1995) of East Antarctica. The 808–795 Ma event recorded in the DARC zircons can be correlated with plume–induced heating of Enderbia during Rodinia breakup (see Chatterjee et al., 2017). This event led to the separation of East Antarctic Block from the India Block (e.g., Bose et al., 2016b; Dasgupta et al., 2017). The palaeopole of India from the 771–751 Ma Malani Igneous Suite in north western India (Torsvik et al., 2001a; Gregory et al., 2009; Meert et al., 2013) diverge greatly from the 755 Ma pole of the Mundine Well dykes in Australia (Wingate and Giddings, 2000), indicating that India had drifted away from Australia–East Antarctica by ca. 755 Ma (Li et al., 2008). Fitzsimons (2003) suggest that by ca. 550 Ma India had moved closer to its Gondwanaland position along the western margin of Australia. The final assembly of Gondwanaland took place by 540–530 Ma during the Malagasy Orogeny that closed the Mozambique Ocean (Meert, 2003; Jacobs and Thomas, 2004; Collins and Pisarevsky, 2005). The event led to the re–docking of India to Australia–East Antarctica along the Pinjarra/Kuunga Orogen (Fitzsimons, 2003; Meert, 2003; Boger and Miller, 2004; Li et al., 2008). The 661–563 Ma ages from the zircons in the DARCs along the craton–EGB contact can be correlated with tectonothermal events during Gondwanaland assembly (Fig. 1).

Summary and concluding remarks The major findings of this study can be summarized as follows:

(1) The continents of India and East Antarctica were involved in several episodes of collision and breakup during the assembly and dispersal of past supercontinents. (2) DARCs at the contact–between the cratons and the Eastern Ghats Belt preserve the record of these amalgamation and breakup events. (3) The rift–related alkaline magmas were emplaced between between 1471 and 1321 Ma and mark an episode of rifting in the Indo–Antarctic continental fragment, correlatable with breakup of the Columbia supercontinent. (4) Metamorphic zircons from the alkaline rocks furnish age populations at 953–930 Ma, 792– 806 Ma and 661–563 Ma. (5) The 953–930 Ma ages are correlated with the closure of an oceanic basin between the Ruker Terrane of East Antarctica and the Indian Shield during the assembly of the Rodinia supercontinent. This led to the collision of the Ruker Terrane with the combined India–Napier Complex producing the Grenville–age EGP–Rayner Complex orogen in the Enderbia continent. (6) The 808–795 Ma ages record the plume–induced disintegration of Rodinia and associated high–grade metamorphism when Greater India started to break away from East Antarctica. (7) In the early Paleozoic, India redocked with Antarctica and Australia during Gondwanaland assembly. The 661–563 Ma zircon ages date the resulting collisions during Pan–African orogenesis. The zircon age data from the DARCs at the craton–Eastern Ghats Belt contact also shed light on the controversy related to the timing of docking of the Eastern Ghats Province with the Singhbhum and Bastar Cratons. Some studies have favored an early Paleozoic amalgamation of the Eastern Ghats Province with India (Dobmeier et al., 2006; Biswal et al., 2007; Das et al., 2008; Bhattacharya et al., 2016). Dobmeier et al. (2006) suggested that the Eastern Ghats

Province–Rayner Complex crustal block constituted an exotic terrane that became a part of peninsular India only during the Paleozoic Pan–African orogeny. The inference was based primarily on the apparent lack of Grenvillian ages along the craton–Eastern Ghats Province contact. However, Chattopadhyay et al. (2015) reported early Neoproterozoic ages (0.98–0.94 Ga) from monazite and zircon in schists and quartzites of the Rengali Province at the southern margin of the Singhbhum Craton, and argued for Grenville–age suturing of the Eastern Ghats Province with the Singhbhum Craton. This was contested by Bhattacharya et al. (2016) who argue against early Neoproterozoic accretion of the two terranes and put forth among others the following reasons: (1) the early Neoproterozoic zircons in the schists and quartzites of the Rengali Province may be of detrital origin, (2) the Rengali supracrustal rocks escaped the 800– 700 Ma high–grade metamorphism widespread in the Eastern Ghats Province, (3) lack of Grenvillian–age mantles around Paleoproterozoic and older cores in monazites in the Rengali schists. All these arguments are untenable and refuted below. First, while it is possible that Chattopadhyay et al. (2015) may have misidentified detrital zircons in the Rengali metasedimentary rocks as metamorphic, the 953–930 Ma ages from zircon in the DARCs along the craton–Eastern Ghats Belt contact cannot be detrital or inherited because the nepheline syenites are mantle–derived igneous rocks. Rather, they date a high–grade tectonothermal event shared by the craton margin and the Eastern Ghats Province. Second, the statement by Bhattacharya et al. (2016) that the Rengali supracrustal rocks escaped the 800–700 Ma high–grade metamorphism widespread in the Eastern Ghats Province is untrue and misleading. Chattopadhyay et al. (2015) have clearly reported a prominent peak at ca. 0.81 Ga in the monazite age spectrum from the Rengali supracrustal rocks (Figure 13, Chattopadhyay et al., 2015) and argued that the ages correspond to the 0.85–0.80 Ga high–grade metamorphism, well

documented in the Chilka lake region of the Eastern Ghats Province (page 526, Chattopadhyay et al., 2015). Clearly, the Rengali Province and the Eastern Ghats Province share not just the early Neoproterozoic Grenville–age but also the mid Neoproterozoic high–T event. Third, the argument put forth by Bhattacharya et al. (2016) on the lack of Grenvillian–age mantles around Paleoproterozoic and older cores in monazites of Rengali schists is also not true. Chattopadhyay et al. (2015) in their figure 12 show representative X–ray element maps of monazite in which the grain labelled as M88b–1 from a Rengali Province garnet–kyanite–sillimanite quartzite has a small 1.64 Ga relict core surrounded by a broad 0.97–0.93 Ga mantle. In summary, the assembly of the Eastern Ghats Province and the Rayner Complex with the Bastar–Singhbhum Craton occurred in the early Neoproterozoic during the final stages of Rodinia assembly, supporting the existence of Enderbia continent comprising crustal units from India and East Antarctica.

Acknowledgements The U–Pb isotope data were generated at the newly established Diamond Jubilee Radiogenic Isotope Facility of the Department of Geology and Geophysics, IIT Kharagpur. Financial support from IIT Kharagpur for setting up the laboratory is gratefully acknowledged. Biswajit Mishra is thanked for access to the DST funded EPMA National Facility of the Department. Financial support from the Ministry of Earth Sciences, Government of India through a research grant (sanction no. MoES/P.O. (Geosci)/37/2014, Dt. 11–03–2015) is duly acknowledged.

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Figure captions Figure 1: (a) Simplified reconstruction of the Columbia supercontinent (modified from Meert, 2012, originally from Zhao et al., 2004) [SCB=South China Block; SAM=South America; WAfr=West Africa; NCB=North China Block]. India and East Antarctica shared a conjugate margin which was the result of the collision of the Napier Complex of East Antarctica with the Dharwar Craton of India to form the central and eastern Indian Shield at ca. 1.60 Ga; (b) Reconstruction of the Rodinia supercontinent at 750–755 Ma (from Torsvik et al., 2001a) showing the probable positions of India [ST=South Taimyr; MDS=Mundine Well dyke swarm]. Based on paleomagnetic data from the 771–751 Ma Malani Igneous suite in western India, Torsvik et al. (2001) suggest that India was located at latitudes comparable with those of Western Australia (position–I). However, in conventional paleogeographic reconstructions of Rodinia, India is juxtaposed against East Antarctica (position–II, stippled outline), the two cratons being welded together along the Grenville–age Eastern Ghats Province–Rayner complex orogen. This amalgamation resulted in the formation of the India–Napier–Rayner–Ruker Enderbia continent and can be correlated to collisions leading to the final assembly of the Rodinia supercontinent; (c) Reconstruction of the Gondwana supercontinent (modified from Meert and Lieberman, 2008, originally from Gray et al., 2008) with the eastern Indian margin juxtaposed along the northern margin of the East Antarctic Shield [RP=Rio Plata; SF= Sao Francisco Craton]. The re–docking of India to Australia–East Antarctica took place along the Pinjarra/Kuunga Orogen.

Figure 2: Geological map of the Eastern Ghats Belt and its contact with the Archean Cratons. The major alkaline bodies forming a linear chain of Deformed Alkaline Rocks and Carbonatites

(DARCs) along the craton–Eastern Ghats Belt contact are marked with stars. The emplacement ages of the alkaline complexes are also given (yellow stars mark alkaline bodies whose emplacement ages were determined in this study while white stars mark alkaline complexes whose emplacement ages are taken from previous studies: Upadhyay et al., 2006b for Elchuru, Vijaya Kumar et al., 2007 for Uppalapadu and Sheikh et al., 2017 for Chhatabar–Lodhajhari– Baradangua). The inset is a map of peninsular India showing the major lithotectonic divisions of India (modified from Ratheesh Kumar et al., 2014). [SC=Singhbhum Craton; RP=Rengali Province; CG=Chhattisgarh Basin; BSC=Bastar Craton; JP=Jeypore Province; EGP=Eastern Ghats Province; GPG=Godavari Pranhita Graben; OD=Ongole Domain; EDC=Eastern Dharwar Craton; CGC=Chhotanagpur Gneiss Complex; SP=Shillong Plateau; NSMB=North Singhbhum Mobile Belt; BC=Bundelkhand Craton; A=Aravalli Delhi Fold Belt; DT=Deccan Traps; CB=Cuddapah Basin; WDC=Western Dharwar Craton].

Figure 3: (a–c) Field photographs of deformed nepheline syenites at the contact between the Archean Cratons and the Eastern Ghats Belt. Note the strong gneissic fabric in the rocks (a, c). Leucocratic patches containing alkalifeldspar and nepheline rimmed by biotite selvages can be seen in nepheline syenite from the Rairakhol complex (b). These are suspected to be water– fluxed partial melts; (d) Total alkali vs. SiO2 diagram of the nepheline syenites documenting their alkaline character; (e) Primitive mantle normalized multielement spider diagram of the rocks showing positive Nb–Ta anomalies, characteristic of rift–related alkaline rocks.

Figure 4: Photomicrographs of the deformed alkaline rocks showing the textural and mineral paragenetic relations. (a-c) Crossed polar images of nepheline syenite from the Kunavaram

alkaline complex. (a) Aligned amphibole and biotite define the tectonic foliation. The amphiboles are anhedral to subhedral and replace titanite. The perthite exsolution lamellae in the alkalifeldspars have coarsened to veins and patches. Their alteration gives the feldspars a clouded appearance. The amphiboles and alkalifeldspars show high–temperature annealing to a granoblastic polygonal mosaic grain; (b) Skeletal amphibole and biotite replacing calcite and feldspar; (c) Lobate grain boundaries of alkalifeldspar and nepheline due to grain boundary migration recrystallization; (d–e) Crossed polar images of nepheline syenite from the Khariar alkaline complex; (d) Taramitic amphibole replacing aegirine augite, alkali feldspar, and calcite and containing them as inclusions. The perthitic exsolution lamellae in alkalifeldspar have coarsened to flame and patch perthite; (e) Lobate grain boundaries of alkalifeldspar and nepheline due to grain boundary migration recrystallization; (f-g) Plane polarized images of Jojuru monzosyenite; (f) Polygonal granoblastic mosaic of clinopyroxene documenting high temperature recrystallization. Coronal garnet has developed at the interface between plagioclase and aggregate of clinopyroxene, orthopyroxene and ilmenite. Amphiboles have partially replaced clinopyroxene and ilmenite; (g) Fan shaped aggregate of biotite growing into alkalifeldspar; (h– i) Crossed polar images of nepheline syenite from the Rairakhol alkaline complex; (h) Anhedral amphibole replaces titanite and defines the tectonic fabric together with biotite. The nepheline and alkalifeldspar show grain boundary migration recrystallization and high–temperature annealing to a granoblastic polygonal mosaic; (i) Alkalifeldspar showing extensive sub–solidus fluid–assisted reequilibration forming vein and patch perthite and migration of albitic plagioclase to alkalifeldspar and nepheline grains boundaries; . All mineral abbreviations used here and elsewhere in the text are after Whitney and Evans (2010).

Figure 5: Zircon textural relations in the deformed and metamorphosed nepheline syenite gneisses along the craton–Eastern Ghats Belt contact. (a–f) Pairs of BSE and CL images depicting zircon textures in the Kunavaram nepheline syenite. The zircons are subhedral to rounded with complexly zoned interiors. Most grains have a subhedral to anhedral relict core mantled by metamorphic zones. The cores are recrystallized with featureless irregular reequilibration fronts proceeding inward from the margins. The interiors of the cores are also recrystallized and are characterized by irregular patches which appear as dark or moderately luminescent zones in CL images. Relict oscillatory zoning is seen in some patches within the cores. The cores are mantled by thin to broad lobate metamorphic overgrowths that have grown along grain boundaries or into the neighboring minerals; (g–l) BSE and CL image pairs illustrating zircon textural relations in the Khariar nepheline syenite. Most grains have cores surrounded by multiple generations of metamorphic zones. The cores are strongly recrystallized producing chaotic lobate patchy zones

which are

CL–dark or display moderate

cathodoluminescence. Relict oscillatory growth zones are present in some of the patches. The igneous domains are pseudomorphically replaced by CL–bright zones along irregular fronts. The recrystallized cores are mantled by a poorly luminescent metamorphic overgrowth which is in turn rimmed or replaced by relatively brighter overgrowths/recrystallized zones; (m–q) Representative BSE and CL images of zircons from the Jojuru monzosyenite depicting their usual textural association. The zircons are often associated with ilmenite and biotite. Most grains are euhedral and preserve their oscillatory growth or sector zoning (e.g., image p). Anhedral metamorphic grains with irregular shapes are found associated with patches of ilmenite and pyroxenes (e.g., images n, q). Some of the magmatic grains are characterized by narrow lobate metamorphic overgrowths near their pyramidal terminations (e.g., image m, o, p); (r–t) BSE

images documenting zircon textural relations in the Rairakhol nepheline syenite. Metamorphic zircons appear as numerous tiny grains or their clusters scattered throughout the rock matrix but usually grow within or near amphibole and nepheline grain boundaries. Some of these grains are associated with Nb–U–REE phases or allanite. The grains have irregular to globular shapes and are characterized by protruding lobate margins.

Figure 6: (a) Cathodoluminescence images of zircon internal structure from Kunavaram nepheline syenite. Most grains have a subhedral to anhedral relict core mantled by metamorphic zones. The cores are recrystallized with irregular reequilibration fronts proceeding inwards from the margins. The interiors of the cores are also strongly recrystallized and characterized by the presence of irregular patches which appear as dark or moderately luminescent zones in CL images. Small domains with relict oscillatory zoning can be seen within the cores. Thin to broad lobate metamorphic overgrowth mantle the reequilibrated cores; (b) REE patterns of magmatic and reequilibrated domains in the zircons. The recrystallized domains are characterized by higher light REE concentrations; (c) Concordia diagram of zircon U–Pb ages and, (d)

206

Pb/238U age

probability density/histogram plot of concordant zircon spot ages from the Kunavaram nepheline syenite.

Figure 7: (a) Cathodoluminescence images depicting internal structure of zircon from Khariar nepheline syenite. Most grains have cores surrounded by multiple generations of metamorphic zones of variable thicknesses. The cores are strongly recrystallized producing lobate patchy zones which are CL–dark or display moderate cathodoluminescence. Oscillatory growth zone is seen in relict patches within the cores. In some grains, the igneous domains are

pseudomorphically replaced by CL–bright zones along irregular fronts. The recrystallized cores are mantled by metamorphic overgrowths; (b) REE patterns of magmatic and recrystallized domains in the zircons. The recrystallized parts are characterized by higher REE contents and enrichment in the light REEs; (c) Concordia diagram of zircon U–Pb ages and, (d) 206Pb/238U age probability density/histogram plot of concordant zircon spot ages from the Khariar nepheline syenite.

Figure 8: Representative cathodoluminescence images of zircon from a monzosyenite sample of the Jojuru alkaline complex. Most grains are euhedral and preserve their oscillatory growth zoned interiors. Some of the magmatic grains are characterized by narrow lobate metamorphic overgrowths near their pyramidal terminations; (b) REE patterns of magmatic and metamorphic domains in the zircons. The metamorphic domains have higher REE concentrations; (c) Concordia diagram of zircon U–Pb ages and, (d)

206

Pb/238U age probability density/histogram

plot of concordant zircon spot ages from the Jojuru monzosyenite. Three concordant age populations at 1352±6 Ma, 930±23 Ma, and 569±34 Ma can be seen.

Figure 9: (a) Representative cathodoluminescence images of zircon interiors from nepheline syenite of the Rairakhol alkaline complex. The grains are usually euhedral or subhedral and characterized by complex internal structures. They have a core which is either featureless or display lobate, chaotic, and patchy domains. Irregular reequilibration fronts progress into the interiors of the cores. The reequilibrated cores are surrounded metamorphic overgrowths of variable thicknesses; (b) REE patterns of magmatic and metamorphic domains in the zircons. The metamorphic zircon domains have higher REE concentrations compared to the magmatic

ones; (c) Concordia diagram of zircon U–Pb ages and, (d)

206

Pb/238U age probability

density/histogram plot of concordant zircon spot ages from the Rairakhol nepheline syenite.

Figure 10: (a) Representative cathodoluminescence images of zircon from a Kamakhyanagar nepheline syenite. The internal structures of the grains are similar to those in the Rairakhol nepheline syenite with most grains having a reequilibrated core region surrounded by metamorphic overgrowths; (b) REE patterns of magmatic and metamorphic domains in the zircons. Note the strong enrichment in the light REEs in the metamorphic zircons; (c) Concordia diagram of zircon U–Pb ages and, (d)

206

Pb/238U age probability density/histogram plot of

concordant zircon spot ages from Kamakhyanagar nepheline syenite.

Figure 11: (a) Representative cathodoluminescence images of zircon internal structure in nepheline syenite from the Koraput alkaline complex. The zircon grains show extreme recrystallization. Growth zoning is completely lacking in most grains, having been replaced by recrystallized patchy domains; (b) Concordia diagram of zircon U–Pb ages and, (c)

207

Pb/206Pb

age probability density/histogram plot of concordant zircon spot ages from the Koraput nepheline syenite.

Highlights · · · · · ·

Deformed Alkaline Rocks & Carbonatites (DARCs) at craton-Eastern Ghats Belt contact DARCs preserve history of amalgamation & breakup events in Indo–Antarctic continent DARC zircons used to date magmatic and tectonothermal events Rifting in Indo–Antarctic continent at 1471–1321 Ma related to Columbia breakup Grenvillian (953–930 Ma) ages record continental collision during Rodinia assembly 808–795 Ma age relates to Rodinia breakup & 661–563 Ma age to Gondwanaland assembly

a

Columbia SCB

Siberia

Tarim

South Africa

Baltica An Ea tar st cti ca

Australia ia Ind

Laurentia

NCB Madagascar

WAfr

SAM Congo

Mozam

bique O

cean

?

S.CHINA

Rift

?

INDIA (I)

Rift

ST

?

MDS

?

30º N

SIBERIA

AUSTRALIA

Proto-Barents Sea (Sea-floor spreading)

? CT

lia East Antarctica BALTICA

IA NT

E UR A L

~ 1.0 Ga collisional belts

Future “Iapetus” Rifts

AMAZONIA

Arabian Nubian Shield

South Metacraton

East Gondwana

c

1000 km

MA

Indian Shield

East Antarctic Shield

Kalahari Craton West Gondwana

30º S

Gondwana

C Cr ong at o on

Mesozoic-tertiary Orogen PalaeozoicMesozoic Orogen Palaeozoic West Orogen African Kuungan Craton Orogen Brasiliano-Damara Orogen East African Amazonian Orogen Craton Proterozoic Orogen Precambrian Shield

SF

NEOPROTEROZOIC ( c. 750-755 Ma )

Au stra

India

RP

(II)

AN E TA AS RC T TI CA

? Equator

b

Subduction Zone

Ranjan et al. Figure 1

Australia

70°E

80°E

90°E

N

Rairakhol (RAK-7) 1372±5 Ma

30°N

Scale 0

Delhi

320

HIMA

EDC CB Chennai

20°N

RP Lodhajhari

Kolkata

22° N

Kamakhyanagar

Chhatabar Baradangua

1344±12 Ma (KNS-3)

1322±8 Ma

Proterozoic basins Young sediment cover

10°N

WDC

CG

RNSMB

SC

BSC

DT

1280 km

SP CGC CITZ

Mumbai

960

LAYA

BC

A

640

SC

BSC

Bhubaneshwar

Khariar (DU-37) 1471±4 Ma

Koraput (KP-1)

Bay of Bengal

1387±34 Ma

Jeypore

GP G

JP

0

EGP

Kunavaram (KUN-3b)

100

200 km

Scale

1373±6 Ma

Jojuru (JJ-2) 1352±6 Ma

Cuddapah Basin

Elchuru

1321±17 Ma

OD Uppalapadu 1356±7 Ma

Nellore

EDC

Chennai 80° E

INDEX Proterozoic Basins Proterozoic/Phanerozoic cover Alluvium Rengali Province (SC) Jeypore Province (BSC) Eastern Ghats Province (EGP) Ongole domain (OD) Major alkaline bodies Craton-EGP thrust contact

Ranjan et al. Figure 2

a

c

b

d

e

Ranjan et al. Figure 3

b

Afs Amp

Bt

Cal Bt

Pl 500µm

Afs

500µm

d Pl Aeg

Cal

g Bt

Kfs Grt

Amp

Afs h

200µm

Afs

Amp Ttn Bt

Ilm

Amp Zrn Opx+Cpx+Ilm 500µm

Kfs Amp Bt

500µm

Amp

i

Grt Pl

Ranjan et al. Figure 4

Cpx Amp

Afs

Qz

Pl Afs

Nph 500µm

Ttn

500µm

Nph

Afs

Afs Zrn Amp

Nph

f

Afs

Cpx

Nph

Amp Zrn

e

Amp 500µm

Cal

Ttn

Ttn

Afs

c

Cpx

a

500µm

Nph

Pl

a

Afs

Amp

Zrn

a b

Afs

Amp

Zrn

e a

100µm

a f

Afs

Pl 100µm

g a

Afs

Zrn Afs

Zrn

Pl Afs

100µm

i

Amp

Amp

Amp

Nph

Zrn

m a

Cpx

n a Grt Pl

Bt 500µm

q a ZrnCpx Qz

Ilm

Bt

Zrn Kfs

Nph

100µm

Grt

Qz

Cpx+Opx Grt 200µm

ka

Zrn Pl

Nph Afs

r a

Amp

100µm

a h

Amp Zrn

Afs

Zrn

Pl Amp

Amp

Ilm

Afs

Ilm Ap

Pl

p a

500µm

500µm

Nph

Grt

Zrn

100µm

Amp 200µm

Ranjan et al. Figure 5

100µm

Ap Zrn Ilm

sa (Nb,U,REE Phase) at Nph

Zrn

Amp

Amp

Grt Qz Zrn

Zrn

Nph Zrn

200µm

Cpx

100µm

al

Nph

o a

Opx

Afs

100µm

Afs

100µm

Pl

Afs Amp

100µm

aj

Amp

Zrn

Afs

Afs

Zrn Amp

Nph

Zrn Afs

100µm

a d

c Nph a

Afs

200µm

Aln

Amp Zrn

200µm

Afs Nph Amp

1r. 567±12

(a)

2c. 1388±45 1c. 1326±42d 30c. 1384±54

30c. 1328±67d

12r. 942±17

(c)

12c. 1331±32

(b)

Ranjan et al. Figure 6

(d)

(a) 2r1. 1469±57 2c. 13r. 1478±74d 1437±78 33r. 551±40

13c. 1484±47

33c. 1417±74

25c. 1458±52

(c)

25rc. 1477±73d

(d)

(b)

(b)

Ranjan et al. Figure 7

6r2. 1331±42

1r1. 1367±46

(a)

6c. 1358±73d 10c. 934±26

34r. 569±35

(c)

10r. 1328±149d

(b)

Ranjan et al. Figure 8

(d)

27r. 941±50

(a)

27c. 1368±60

43c. 1372±40 1c. 957±22

(c)

2c. 1384±20

(b)

Ranjan et al. Figure 9

(d)

(a)

2c. 1347±53

33c. 1361±53

29c. 940±36

40c. 915±36

(c)

(b)

Ranjan et al. Figure 10

(d)

28r. 804±16

(a)

35c. 905±31

21r. 1389±24 28c. 986±47d

21c. 1015±39d 38rc. 804±18

41c2. 35r. 904±24 917±14

41r. 809±13

43rc. 812±14

49r. 930±22

49c. 1325±77

43c. 905±25

16c. 959±15

(c)

(b)

Ranjan et al. Figure 11

Table 1 Summary of petrographic characteristics of the alkaline rocks used in the study Alkaline complex

Rock type/sample number

Mineralogy*

Kunavaram alkaline complex

Amphibole-bearing nepheline syenite (KUN-3b)

Afs, Nph, Fprg/Hst, Cal, Pl, Bt, Ttn, Zrn, Ap

Khariar alkaline complex

Amphibole-bearing nepheline syenite (DU-37)

Afs, Nph, Amp, Pl, Cal, Ttn , Zrn, Ap

Jojuru alkaline complex

Garnet-Pyroxene monzosyenite (JJ-2)

Pl, Kfs, Cpx, Opx, Amp, Gt, Qz, Ilm, Zrn, Ttn, Ap

Rairakhol alkaline complex

Amphibole and biotitebearing nepheline syenite (RAK-7)

Afs, Nph, Pl, Ttn, Fe-Ti oxide, Amp, Bt, Zrn

Kamakhyanagar alkaline complex

Biotite-bearing nepheline syenite (KNS-3)

Afs, Pl, Nph, Bt, Amp, Ttn, Ap, Fe-Ti oxides, Zrn, Cal

Koraput alkaline complex

Amphibole and biotitebearing nepheline syenite (KP-1)

Afs, Pl, Bt, Amp, Nph, Ilm, Ap, Zrn, Rt

Petrographic description Amphiboles subhedral to anhedral and replace aegirine augite, calcite and metamorphic titanite. They show preferred orientation defining a tectonic fabric Alkalifeldspars recrystallized to vein and patch perthites and nepheline altered to cancrinite Alkalifeldspar and nepheline show lobate boundaries and grain boundary migration recrystallization (Figs. 4a to c) Taramitic amphiboles have replaced aegirine augite, plagioclase, alkalifeldspar and nepheline and define a weak tectonic foliation Alkalifeldspars are clouded and porous with vein and flame perthite Feldspars and nepheline show extensive bulging and grain boundary migration recrystallization with boundaries defining 120° triple junction (Figs. 4d, e) Mildly deformed with feldspars, pyroxenes, and amphiboles partially recrystallized but lacking any preferred orientation Ferromagnesian minerals (Cpx, Opx and Ilm) together with zircon and titanite occur as aggregates with coronal garnet developing at interface between plagioclase and mafic minerals Ferropargasite forms thin rims around Ilm, relic Cpx and Opx Late stage intergrowth of biotite and quartz overgrows orthopyroxene and large alkalifeldspar grains (Figs. 4f, g) Gneissic layering defined by amphibole- and biotite-rich layers alternating with feldspars- and nepheline-rich layers Taramitic amphiboles overgrow Fe-Ti oxides and titanites Alkalifeldspars perthitic whereas plagioclases are clouded, porous and partially altered to sericite Alkalifeldspars and nepheline show high temperature recrystallization and annealing to granoblastic polygonal mosaic (Figs. 4h, i) Medium– to coarse–grained, with biotite- and amphibole-defined tectonic foliation Biotites and amphiboles have replaced clinopyroxene, calcite, titanite, apatite and also contain them as inclusions Perthitic alkalifeldspars have undergone extensive fluid–assisted reequilibration forming vein, patch and flame perthites. Nepheline altered along fractures to cancrinite Amphibole-rich layers alternate with felsic layers containing perthitic alkalifeldspars and nephelines Feldspars and nepheline have annealed into a granoblastic mosaic

* Minerals are listed in decreasing order of abundance and mineral abbreviations after Whitney and Evans (2010)