Juvenile Crust Formation in the Precambrian Singhbhum, Dharwar Cratons and the Southern Granulite Terrain, India and Geodynamic Transitions: Evidence from Zircon U-Pb age-Hf Isotope Systematics

Zircon age-Hf isotopic data on the Archean Singhbhum and Dharwar cratons and the Archean-Proterozoic Southern Granulite Terrain (SGT) obtained at the CSIR-NGRI and by others elsewhere are in focus here. These data are used to decipher episodes of juvenile crust formation in the protracted (collectively spanning ∼3.7 billion years) geologic history of the three terranes in the light of their regional geology, structure and deep-crustal architecture based on recent geophysical experiments as well as current perspectives on early Earth crust forming processes and geodynamics. Our important observations and inferences include: (1) the Hf-isotopic compositions of the Hadean-Eoarchean aged (ca. 4.2–3.6 Ga) zircon grains from the Singhbhum craton have distinctly unradiogenic Hf-isotopic compositions quite similar to the Jack Hills Hadean-Eoarchean detrital zircons, suggesting derivation from TTG-like melts generated by the internal reworking of a long-lived, geochemically enriched mafic reservoir formed around ca. 4.5 Ga; (2) a shift to strongly radiogenic zircon Hf isotope compositions during the early Paleoarchean around ca. 3.6–3.5 Ga (Singhbhum craton) and ca. 3.5–3.4 Ga (Western Dharwar craton) is conspicuous. This may relate to the time of development of depleted mantle reservoirs, the source of the voluminous Paleo-Mesoarchean juvenile felsic magmatism and crust formation events that extended for ca. 400–300 million years; (3) in the entire Dharwar craton and the northern parts of the SGT there is clear evidence for widespread juvenile magmatic episodes during the Neoarchean, around ca. 2.7 Ga and ca. 2.55 Ga, the latter being predominant and widespread; (4) in the southernmost part of the SGT, prominent juvenile magmatic episodes are also evident during the Paleoproterozoic (ca. 2.0 Ga, Trivandrum block) and early Neoproterozoic (ca. 1.0–0.9 Ga, in parts of the Madurai block); (5) onset of plate tectonic processes in the Singhbhum and Western Dharwar cratons during early Paleoarchean (ca. 3.6–3.5 Ga) cannot be ruled out, but there is clear evidence for the operation of plate tectonics, significant crustal growth and terrane amalgamation only after ∼3.0 Ga in the Dharwar craton and the SGT and (6) regional dome and basin structural pattern of the pre-3.0 Ga crust attests to the role of internal differentiation processes (Rayleigh-Taylor Inversions) and vertical tectonics for the Paleo-Mesoarchean crust of the Singhbhum and Dharwar cratons. Together with other lines of evidence; changes in bulk crustal composition, deep crustal architecture, zircon age-Hf isotope distribution etc., we infer a transition to plate tectonics around 3.0 Ga in the Singhbhum and Dharwar cratons.