Magmatic and metamorphic history of Paleoarchean tonalite–trondhjemite–granodiorite (TTG) suite from the Singhbhum craton, eastern India

Highlights

• Polycyclic evolution of Paleoarchean felsic crust in Singhbhum craton, India.

• Tonalite–trondhjemite–granites emplaced in two pulses at 3.44 Ga and 3.33 Ga.

• Amphibolite-facies metamorphism at 3.3 Ga, four low-grade events between 3.2 and 1.1 Ga.

• Magmatism synchronous with 3.3 Ga metamorphism of older supracrustal sequence.

• Major crust formation occurred in a narrow time interval between 3.46 and 3.32 Ma.

Abstract

Texturally controlled dating of zircon from Paleoarchean tonalite–trondhjemite–granodiorites of the Older Metamorphic Tonalitic Gneisses and the Singhbhum Granite batholith (Phases I, II, and III) from the Singhbhum craton in eastern India reveals a polycyclic evolution of the Archean crust. The granitoid suites were emplaced in two pulses at 3.45–3.44 Ga and 3.35–3.32 Ga. Tonalites and trondhjemites of the Older Metamorphic Tonalitic Gneisses were emplaced at ca. 3.45–3.44 Ga together with Phase III of the Singhbhum Granite pluton while granites belonging to the Older Metamorphic Tonalitic Gneisses were emplaced at ca. 3.35–3.32 together with Phase I and Phase II of the Singhbhum Granite pluton. Both crustal units underwent an early phase of relatively high-grade metamorphism at 3.30–3.28 Ga followed by extensive fluid-induced alteration during low-grade metamorphism at 3.19–3.12 Ga, and 3.02–2.96 Ga. The two units have also been marginally affected at ca. 2.52 Ga and 1.06 Ga by major metamorphic events in the North Singhbhum Mobile Belt and the Singhbhum shear zone at the northern margin of the craton. The zircon grains in granites have inherited cores with ages of ca. 3.61 Ga and 3.46–3.41 Ga and with well-developed oscillatory growth zonation which suggests the granitic magmas were derived by partial melting of an igneous precursor or sedimentary rocks derived from an igneous source. The emplacement of the expansive granitoids belonging to the Older Metamorphic Tonalitic Gneisses and the Singhbhum Granite was synchronous with the amphibolite-facies metamorphism (ca. 3.32 Ga) of older meta-igneous and metasedimentary rocks belonging to the Older Metamorphic Group. Major felsic crust formation in the 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 complex polycyclic evolution of the Paleoarchean crust in the Singhbhum craton can account for the wide range of often disparate ages obtained using whole rock isochron dating techniques with some of the isochron dates being geologically meaningful while others representing mixing lines or disturbance of the isotopic systems during metamorphism.

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.