Magmatic and metamorphic history of Paleoarchean tonalite–trondhjemite–granodiorite (TTG) suite from the Singhbhum craton, eastern India
Introduction
Archean tonalite–trondhjemite–granodiorite (TTG) associations and granitoids represent the oldest archetypical juvenile felsic components of cratons and mark the transition from a dominantly mafic to a more felsic crust (Glikson, 1979, Smithies, 2000, Martin et al., 2005, Moyen and Martin, 2012). The growth, evolution, and stabilization of this silicic crust are thought to have occurred in short-lived episodes involving magmatic accretion, tectonic thickening and high-grade metamorphism (e.g., Wells, 1981, De Wit, 1998, Almeida et al., 2011) over a protracted period. Archean TTG rocks are therefore expected to preserve the geological record of the early continent building-stage of crustal evolution (e.g., Barker, 1979).
The Singhbhum craton in eastern India has extensive occurrences of greenschist- to amphibolite-facies TTGs and granites of Paleoarchean to Neoarchean age (e.g., Sarkar et al., 1979, Basu et al., 1981, Moorbath et al., 1986, Moorbath and Taylor, 1988; Saha et al., 1988, Paul et al., 1991, Saha, 1994, Goswami et al., 1995, Basu et al., 1993, Misra et al., 1999, Acharyya et al., 2010, Prabhakar and Bhattacharya, 2013). Published geochronological data on these rocks mostly comprise whole-rock Rb–Sr, Sm–Nd, or Pb–Pb isochron ages (e.g., Saha and Rao, 1971, Saha, 1972, Sarkar et al., 1979, Basu et al., 1981, Moorbath et al., 1986, Paul et al., 1991, Sharma et al., 1994, Saha, 1994, Ghosh et al., 1996) or K–Ar and Ar–Ar mineral (biotite and hornblende) ages (Sarkar et al., 1979, Iyengar et al., 1981, Vohra et al., 1991). Because the whole rock isotope systems may potentially get disturbed due to open system behavior during metamorphism, the Rb–Sr, Sm–Nd or K–Ar/Ar–Ar ages may have been compromised and often yield contradictory and inconsistent age information (e.g., Basu et al., 1981 vs. Moorbath et al., 1986) which makes them difficult to interpret in geological context. More recent studies using ion probe U–Pb or Pb–Pb isotope dating of zircon from these rocks revealed the presence of several Paleoarchean to Neoarchean age populations (e.g., Moorbath et al., 1988; Goswami et al., 1995, Misra et al., 1999, Acharyya et al., 2010, Tait et al., 2011). However, most of these studies have not documented the internal structures of the dated zircon grains, which quite commonly record episodes of zircon formation/consumption, strain- and fluid-induced recrystallization and reequilibration as well as chemical alteration during magmatic or metamorphic processes affecting the rocks. The textural relations of zircon interiors imaged through Back Scattered Electron (BSE) and Cathodoluminescence (CL) techniques can be used to link the zircon internal structure to particular rock forming and modifying processes (e.g., Corfu et al., 2003). As many of the earlier studies have not utilized this powerful tool, the interpretation of the published U–Pb age data, although spatially resolved, is ambiguous, and robust constraints on the timings of TTG magmatism and the subsequent metamorphic overprints on the rocks is still lacking. This has been a major hindrance to understanding the sequence of crustal evolution in this important Archean craton.
This contribution reports Laser Ablation-Sector Field-Inductively Coupled Plasma Mass Spectrometer (LA-SF-ICPMS) U–Pb ages of zircon grains from the Archean TTG suite of the Singhbhum craton. We have used BSE and CL imaging to document the internal structures of the dated grains. The texturally controlled dating of zircon grains from the TTGs is used to constrain the timing of TTG magmatism and metamorphism of the rocks. This age information has implications for temporal relationships among the TTGs, granites and neighboring crustal units as well as the regional crustal evolution. This study establishes for the first time that the Archean TTGs and granites of the Singhbhum craton form an expansive and correlatable suite of rocks emplaced in two pulses at ∼3.44 Ga and ∼3.33 Ga.
Section snippets
Regional geologic setting
The oval Archean Singhbhum craton is bordered by the North Singhbhum Mobile Belt (NSMB) to the north, the Eastern Ghats Belt to the southeast and the Bastar craton to the southwest (Saha, 1994). The major crustal units making up the craton include greenschist- to amphibolites-facies supracrustals of the Older Metamorphic Group (OMG), TTGs of the Older Metamorphic Tonalite Gneisses (OMTG), granitoids and tonalities of the Singhbhum Granite (SG) batholith and the greenschist facies platformal
Analytical techniques
Zircon grains from TTGs, granitoids and a micaschist sample were dated using LA-SF-ICPMS at the Institut für Mineralogie, Westfälische Wilhelms-Universität, Münster. The grains were separated from the rocks using routine techniques involving crushing/grinding followed by magnetic and heavy liquids separation. Representative grains of all sizes and morphologies were handpicked under a binocular microscope, mounted on epoxy disks, and polished to reveal their interiors. The internal structures
Sample details and petrography
Samples were collected from three major Paleoarchean crustal units of the Singhbhum craton viz., the OMG, the OMTG and the SG. The majority of them are from the TTG suites of the OMTG and the SG. Together with one micaschist from the Paleoarchean OMG metasediments, zircon grains from thirteen samples were separated and analyzed for their U–Pb isotope compositions and ages. The OMTG samples include tonalite–trondhjemite (CHM-10A, CHM-28B) and granite (CHM-3B, CHM-7, CHM-18C). The SG samples
Results and discussion
The U–Pb isotope data and calculated ages for the analyzed zircon grains are listed in Appendix A, and illustrated on concordia diagrams (Fig. 4) and 207Pb/206Pb age (90–110% concordance only) probability density plots (Fig. 5). Representative CL images of the analyzed grains with the corresponding spot ages marked are shown in Fig. 3.
The important results of the study are described and discussed in the following section in the context of the existing subdivisions of the major Archean crustal
Conclusions
The zircon age data shows that major crust formation in the Singhbhum craton occurred in a narrow time interval between 3.46 and 3.32 Ma with minor contributions of material as old as 3.6 Ga. The Singhbhum TTGs therefore represent quite pristine Paleo-to-Mesoarchean material with possibly little recycled material, an inference supported by their low initial 87Sr/86Sr (0.701; Moorbath et al., 1988) and chondritic to positive initial ɛNd (Moorbath et al., 1988; Sharma et al., 1994). This is in
Acknowledgements
This project was supported by grant from the Deutsche Forschungsgemeinschaft (Leibniz Award) to KM. DU acknowledges financial support from IIT Kharagpur (ISIRD research grant). We thank Udo Zimmermann and an anonymous reviewer for their constructive comments that helped to improve the paper significantly. Editorial handling by R.R. Parish is gratefully acknowledged.
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