Abstract

There is consensus that, at 1.0–0.9 Ga, the granulites in the Eastern Ghats Province (EGP), Eastern India, and the Rayner Complex, Antarctica, were parts of a coherently evolved crustal block. Paleogeographic reconstructions suggest that in the Neoproterozoic/Early Paleozoic, India and Antarctica were closely positioned at equatorial latitudes in two periods at 1.0–0.9 Ga and 0.6–0.5 Ga. The question is, in which of these periods did the EGP–India vis-à-vis India–Antarctica accretion occur. Top-to-the-west thrusts juxtaposed the EGP with the Bastar Craton, a part of the Greater India landmass. The eastern fringe of the craton underwent anatexis (750–780 °C; 8–9 kbar) and high deformation strain that demonstrably weakened westward. Zircon in the anatectic migmatites at the EGP margin and in the weakly-deformed and non-migmatite granite in the hinterland in the west yields U–Pb upper intercept ages of 2.5–2.4 Ga whereas titanite, hosted in the leucosome of a metatexite and in a granite, has an age of 502 ± 3 Ma coinciding with the lower intercept ages of zircon discordia lines. The lack of 1.0–0.9 Ga dates in the cratonic margin suggests that the EGP accreted with the Bastar Craton and the Greater India landmass at 0.5 Ga during the Gondwanaland assembly, and not in the Early Neoproterozoic. It is within the realms of possibility that the EGP had already separated from the Rayner Complex during the disintegration of Rodinia, and therefore, the 0.5 Ga accretion of the dismembered EGP with Greater India may not be symptomatic of India–Antarctica accretion, in spite of the proximity of the two landmasses inferred from paleogeographic reconstructions.

INTRODUCTION

Paleogeographic reconstructions (Fig. 1A) suggest that India and Antarctica were closely positioned at equatorial latitudes in the Early Neoproterozoic (1.0–0.9 Ga) and in the Late Neoproterozoic/Early Paleozoic, 0.6 and 0.5 Ga (Torsvik, 2003; Li et al., 2008). In the intervening period, broadly overlapping with the dispersal of crustal fragments of the Rodinia supercontinent, the Greater India landmass was positioned at northern latitudes, whereas the Australo-Antarctic Block was broadly stationary near the equator (Torsvik, 2003; Li et al., 2008; Fig. 1A). Several researchers (Black et al., 1987; Dalziel, 1997; Mezger and Cosca, 1999; Halpin et al., 2005; Morrissey et al., 2015) argue that the metamorphic histories of the granulites in the Eastern Ghats Province, India (EGP) and in the Rayner Complex (Antarctica) are similar, and the two crustal domains probably constituted a coherently evolved landmass by 1.0–0.9 Ga (Fig. 1B). This leads to the question whether the Greater India landmass and Antarctica (inclusive of the EGP) were juxtaposed in the Early Neoproterozoic (Dalziel, 1997; Black et al., 1987; Mezger and Cosca, 1999; Chattopadhyay et al., 2015), or whether the accretion occurred in the Late Neoproterozoic/Early Cambrian (Biswal et al., 2007) or Middle/Late Cambrian coinciding with the final phase of Gondwanaland assembly (Bhattacharya et al., 2016; Chatterjee et al., 2017). We address this vexed issue based on field observations, petrological studies, and U–Pb zircon and titanite geochronology using Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) of granitoids and anatectic migmatites in the cratonic granites and gneisses. The results are crucial for delineating the assembly of crustal fragments in the East Gondwanaland.

GEOLOGICAL BACKGROUND

Early Paleoproterozoic/Neoarchean granitoids (Fig. 1C; Data Repository File DR11 for geochemical data) of the Bastar Craton (Mohanty, 2015, and references there in) are juxtaposed with the ensemble of Early Neoproterozoic high-T granulites, massif anorthosite and syenite plutons, and expansive megacrystic orthopyroxene-bearing granitoids of the EGP along its northwestern and northern margins (Rickers et al., 2001). At Ranmal, Orissa, the cratonic granites (quartz, K-feldspar, plagioclase, biotite, hornblende, ilmenite, titanite ± epidote, with apatite and zircon as accessory minerals) experienced polyphase anatexis (Figs. 1D–1F) in response to top-to-the-west thrusting of the EGP over the Bastar Craton (Bhadra et al., 2007).

In the anatectic granites of the craton, stromatic biotite-rimmed leucosome layers (S1) in alternations with biotite-hornblende defined mesosomes (File DR2A, DR2B) are folded into asymmetric west-vergent tight/isoclinal folds (F2; Figs. 1D–1F) with variable plunges. The axial planes of F2 folds are filled by leucosome layers that form networks of N-trending channels that coalesce to form coarse-grained diatexite melt pods (Figs. 1D–1F). The axial planar leucosomes (S2) are continuous with the folded S1 leucosome layers (Figs. 1E–1F). The S1 stromatic layers are folded near the S2 diatexite segregations, but the folds die out progressively distal from the diatexite melt pods (Figs. 1E–1F).

In the leucosomes, trains of tiled plagioclase grains, and rational faces of euhedral feldspars impinging into neighboring quartz films attest to syn-deformational melt emplacement. In the metatexite mesosomes, the dominance of dispersed weakly-strained/unstrained grains with high-energy boundaries, weak alignment among subhedral feldspar crystals subparallel to shape-preferred biotite (XMg = 0.35, TiO2 ∼ 4.4 wt%, File DR3A) aggregates, plumose margins of quartz grains, and quartz films against subhedral feldspar boundaries (File DR2C) attest to deformation of the anatexites at high-temperature. The features imply that the polyphase anatexis in the craton was broadly contemporaneous with progressive deformation/thrusting EGP over the Bastar Craton. Hornblende- (XMg = 0.31 − 0.32, File DR3A) plagioclase (XCa = 0.19 − 0.20, File DR3A) thermometry (Holland and Blundy, 1994) and hornblende-plagioclase-quartz barometry (Bhadra and Bhattacharya, 2007) in the S1 mesosomes indicate that melting in the migmatites occurred at ∼750–780 °C and 8–9 kbar (File DR3B).

Westwards and progressively more distal from the EGP–Bastar Craton interface the granitoids in the cratonic footwall vary from mylonitic in the east, through foliated and then to massive (lacking mesoscopic fabrics) varieties in the hinterland. The critical minerals in deformed and anatectic granites defining the thrust fabric vary from biotite-amphibole in the east (this study) through epidote-amphibole to chlorite-epidote in the weakly-deformed granites in the west (Bhadra et al., 2007). In these zones, the eastern margins of the Khariar basin (basin initiation at 1.455 ± 0.047 Ga; Das et al., 2009) and the Chattisgarh basin (basin initiation at ca. 1 Ga; Das et al., 2009) unconformably overlying the Bastar granites are sheared along the eastern margin (Fig. 1G), but are undeformed farther west (Chakraborty et al., 2015). This testifies to the westward decrease in the strain intensity as a consequence of the EGP–BC accretion.

U–Pb GEOCHRONOLOGY

ID-TIMS U–Pb analyses in zircon and titanite in the Bastar granitoids were carried out in anatectic migmatites located within 200 m (sample 04) and ∼2 km (sample 113) from the Bastar Craton–EGP contact, non-anatectic foliated granite (sample 9 and 96) located around 2 and ∼4 km west of the thrust contact (Fig. 1C).

Zircon was separated by crushing, and subsequently separated by Wilfley table, heavy liquid and magnetic procedures. Zircon grains were hand-picked and subjected to chemical abrasion (Mattinson, 2005) before dissolution, chemical separation (for grains >1 µg), and measurement following the technique of Krogh (1973). Detailed analytical procedures for the Oslo laboratory are given in Corfu (2004). Titanite was dissolved on a hotplate and processed with a single stage HCl-HBr procedure. Blanks are <2 pg for Pb and 0.1 pg for U. The initial Pb in titanite was corrected using a composition calculated with the Stacey and Kramers (1975) model at 0.5 Ga. Decay constants are from Jaffey et al. (1971). The plotting was done with the Isoplot program (Ludwig, 2009).

Selection of zircon grains targeted the youngest growth phase detectable in the population such as tips or visible overgrowths. The populations of the two anatectic granites (samples 4 and 113) are dominated by generally subhedral grains, locally with still preserved euhedral shapes, but mostly somewhat subhedral to anhedral (File DR4). Three zircon tips were analyzed for each of the samples (Table 1). The analyses are between 4% and 40% discordant (Fig. 2), and approximate a line with an upper intercept at ca. 2.46 Ga (mean square weighted deviation [MSWD] = 29). Two grains of the abundant U-rich (300 ppm) titanite, however, plot close to the lower intercept of the zircon discordia line (i.e., at ca. 0.5 Ga).

The foliated granite sample 96 contains similar zircon as the migmatites, with locally visible cores and overgrowths. The selection of tips and some prisms turned into very brittle fragments after chemical abrasion and the three analyses are between 6 and 15% discordant. These again approximate a line (MSWD = 7.1) and yield an upper intercept age of 2.387 ± 0.045 Ma (Fig. 2).

The other non-anatectic foliated granite sample (9) exhibits a variety of morphologies including prismatic subhedral to euhedral grains, but also anhedral grains and prisms with clearly visible cores and overgrowths (File DR4). Some of these overgrowths were mechanically separated from the cores, and two of them were analyzed. Other analyses were done of two tips and two fragments of long prismatic grains (File DR4). These four analyses are discordant and plot close to a line (MSWD = 129) with an upper intercept at ca. 2.45 Ga (Fig. 2). The two separated overgrowths, however, plot distinctly to the left of the discordia indicating growth or (and) isotopic disturbances during younger Proterozoic events. A line projected through the youngest data point from the titanite (at 0.5 Ga, see below) intersect the concordia curve at 2.23 Ga (Fig. 2), but it is not possible to know how reliable this intercept age is because of the nature of the discordia. The four titanite analyses together define a line with a lower intercept age of 0.502 ± 0.003 Ga, and an upper intercept at around 2.4 Ga, though imprecise due to the long extrapolation.

DISCUSSION

The results obtained from the zircon populations indicate growth at ca. 2.45–2.46 Ga in the migmatites (sample 4 and 113), and somewhat later in granite (96). The granite (9) has a main population suggesting the same age as the migmatites, but also younger overgrowths reflecting later growth or disturbances. The general morphological and isotopic features of the zircon data are consistent with a period of granite emplacement at around 2.45 Ga, similar to those recorded elsewhere in the Bastar Craton (cf. Mohanty, 2015; Bhattacharya et al., 2016; Chatterjee et al., 2017).

Several aspects of the U–Pb data need to be discussed for understanding the evolution of the Ranmal migmatites and foliated granites. The first notable aspect is the large degree of discordance in zircon ages, in spite of the generally good quality of the analyzed grains (Fig. 2), and the use of chemical abrasion (CA), which effectively reduces the effects of secondary Pb loss. The second important issue is the near-concordant titanite age at near 0.5 Ga. It is unclear whether the small discordance in titanite ages reflects old cores, incomplete resetting of old titanite, or incorrect common Pb correction. The fact that analyses of separate grains are identical for each sample suggests interferences due to the presence of older cores may not be a factor (rather it would be a coincidence), and this would also argue against resetting, although similar features have been seen, for example, in the Western Gneiss Complex (Tucker et al., 1987). The exact reason notwithstanding, the age of the titanite implies growth or a very strong overprint at 0.5 Ga, a view that is reinforced by the high U content and high Th/U values. Based on these data, the most likely conclusion is that the granites are Paleoproterozoic emplacements, but they were reactivated and partially re-melted during tectonic events at 0.502 ± 0.003 Ga.

The lack of concordant Cambrian zircon grains is a point for concern. The Zr contents in the Ranmal migmatites and non-migmatic, yet deformed granites (this study; Bhadra et al., 2007) plot below the Zr saturation curves at 750 °C and 850 °C (File DR5). This implies that new crystallization of zircons at 0.502 ± 0.003 Ga, especially in the high-T leucosomes was impeded due to Zr under-saturation at the temperature (∼750 °C) of melting in the Ranmal migmatites.

In this context, the U–Pb zircon SHRIMP ages (Chatterjee et al., 2017) obtained in migmatitic quartzofeldspathic gneiss from the Bastar Craton and Eastern Ghats granulites in the Dharamgarh-Deobhog area (located ∼50 km NW of Ranmal) across the EGB–Craton tectonic contact (Gupta et al., 2000) are instructive. As in this study, the cratonic gneiss in the area yielded discordant U–Pb isotope ratios that were resolved into 2.425 ± 0.032 Ga and 0.545 ± 0.034 Ga upper and lower intercept ages respectively (Chatterjee et al., 2017). By contrast, the EGP granulites yielded single-population concordant 238U–206Pb zircon ages between 0.534 ± 0.029 Ga and 0.510 ± 0.024 Ga, with no evidence of early Neoproterozoic dates that characterize the EGP (Chatterjee et al., 2017).

The 2.5–2.4 Ga upper intercept ages in the Bastar Craton granites (Fig. 2) are similar to the Late Neoarchean/Early Paleoproterozoic emplacement ages reported for granites elsewhere in the Bastar Craton (summary in Mohanty, 2015; Bhattacharya et al., 2016). The ca. 0.5 Ga lower intercept ages in zircon and the 0.502 Ga age of titanite in leucosomes in the anatectic migmatites document the first report of Pan African crustal anatexis experienced by the Bastar Craton synchronous with the top-to-the-west thrusting of the Neoproterozoic EGP (Fig. 3). The noteworthy feature of the age determinations in this study and those of Chatterjee et al. (2017) is the lack of 1.0–0.9 Ga ages in the Bastar Craton. By implication, the EGP did not juxtapose with the cratonic nucleus of the Greater India landmass, of which the Bastar Craton is an integral part, at least in the Early Neoproterozoic (Dalziel, 1997; Black et al., 1987; Mezger and Cosca, 1999; Dobmeier and Raith, 2003; Halpin et al., 2005; Chattopadhyay et al., 2015).

In the Rayner Complex, several authors provide compelling evidence for Pan African high-T metamorphism strongly overprinting the Grenvillian-age granulites (Harley and Buick, 1992; Hensen and Zhou, 1995; Fitzsimons et al., 1997; Phillips et al., 2009; Morrissey et al., 2016). By contrast, no demonstrable evidence has been put forth in favor of widespread high-T Pan African overprinting in the EGP, barring its western and northern margins. In few studies based on chemographic and P–T pseudosection analyses, the near-isothermal decompression and retrograde segment of the reconstructed P–T path in the Grenvillian-age EGP granulites is tacitly ascribed to Pan African decompression (Das et al., 2008). But unequivocal structural and textural evidence for Pan African decompression-induced metamorphism—such as syn-tectonic mineral growth or juvenile magma emplacements along ductile shear zones in tensional or contractional setting (Block and Royden, 1990; Teyssier et al., 2005)—in support of the assumption are lacking. Toward this end, this study provides the first geochronological evidence of high-T Pan African metamorphism leading to anatexis in the cratonic rocks fringing the EGP.

Integrating the aforementioned arguments with the paleogeographic reconstructions leads to the following scenarios. First, the Greater India landmass (excluding the EGP) accreted with the composite 1.0–0.9 Ga EGP–Rayner Complex as late as ca. 0.5 Ga (model A; Fig. 3) coinciding with the final assembly of the landmasses south of the equator into Gondwanaland, and not during the Early Neoproterozoic as has been suggested by some workers (Dobmeier and Raith, 2003; Chattopadhyay et al., 2015). Alternatively the EGP may have been detached from the composite EGP–Rayner Complex during the Rodinia break up at 0.75 Ga and was subsequently accreted to the Greater India landmass at ca. 0.5 Ga (model B; Fig. 3) as the landmass migrated from its position at northern latitudes it held at ca. 0.75 Ga (Fig. 1A). In the second model, the 0.5 Ga accretion of the EGP with the Bastar Craton vis-à-vis the Greater India landmass does not represent Indo-Antarctica accretion (model B; Fig. 3) in spite of the proximity suggested in the paleogeographic studies (Fig. 1A).

In the last decade and a half, several studies have highlighted the existence of Mid-Neoproterozoic ages (0.8–0.7 Ga) along the northern and western margins of the EGP, e.g. in the Chilka Lake anorthosite complex (Bose et al., 2016), along the Mahanadi shear zone (Veevers and Saeed, 2009; Bhattacharya et al., 2016), and in the Koraput syenite-anorthosite complex (Hippe et al., 2016; Saikia et al., 2018). Bose et al. (2016) suggest the age corresponds to high-grade metamorphism related to extensional tectonism in the EGP. This high-T extensional tectonic event is, however, not recorded in the Archean cratonic blocks neighboring the EGP (Biswal et al., 2007; Bhattacharya et al., 2016). This absence of Mid-Neoproterozoic ages in the cratonic nucleus in Eastern India provides tacit support to the proposition that the EGP–Bastar Craton accretion occurred at ca. 0.5 Ga, and not during the 1.0–0.9 Ga events. Moreover, the 0.8–0.7 Ga extensional tectonism maybe a vestige of the breakup of the EGP–Rayner Complex composite suggested in the model B in Figure 3.

CONCLUSIONS

Thrusting of the Grenvillian-age (1.0–0.9 Ga) granulites of the EGP over the Bastar Craton induced polyphase anatexis and deformation in granites of the cratonic footwall. U–Pb ages in the suite of anatectic migmatites, traceable to un-deformed granites in the hinterland, indicate that anatexis (750–780 °C; 8–9 kbar) in the 2.5–2.4 Ga cratonic granites at the EGP– Bastar Craton interface occurred at ca. 0.5 Ga. The lack of 1.0–0.9 Ga ages in the cratonic granites/gneisses suggest the EGP did not accrete with the Bastar Craton vis-à-vis the Greater India landmass in the Early Neoproterozoic. We suggest that the EGP—presumably coherently evolved with the Rayner Complex (Antarctica) at 1.0–0.9 Ga—welded with India at ca. 0.5 Ga culminating with the final assembly of the East Gondwanaland. Alternately the EGP may have dismembered from the EGP–Rayner Complex composite during the Rodinia break up and accreted with the Greater India landmass in the Cambrian. Therefore, India and Antarctica may not have accreted until late in the Cambrian, in spite of the close positioning of the two landmasses at 1.0–0.9 Ga and 0.6–0.5 Ga, inferred from paleogeographic considerations.

ACKNOWLEDGMENTS

Dicton Saikia is acknowledged for his help during field work. PN acknowledges financial support from Science and Engineering Research Board, Government of India through research project no. EMR/2014/000538. Dr. A.K. Singh, Wadia Institute of Himalayan Geology, and Dr. M. Satyanarayanan, National Geophysical Research Institute, are acknowledged for the geochemical analysis. We thank Tamer Abu-Alam and Kurt Stüwe for their comments that helped to improve the scientific rigor of the manuscript.

1GSA Data Repository Item 2018166, File DR1: Whole rock major element oxides (in wt %), trace and rare earth element oxides (in ppm) of Ranmal migmatite complex; File DR2: Back-scatter electron images of textural relations in Ranmal migmatites; File DR3: Electron probe microanalysis data, structural formulae of minerals (A) and (B) P-T conditions in Ranmal migmatite complex; File DR4: Photomicrographs of representative zircon grains selected for analyses; File DR5: Plot of zirconium concentrations in anatectic and foliated granites from Ranmal (whole rock data, this study and Bhadra et al., 2007), is available at http://www.geosociety.org/datarepository/2018, or on request from editing@geosociety.org.
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