Formation of Paleoarchean-Mesoarchean Na-rich (TTG) and K-rich granitoid crust of the Singhbhum craton, eastern India: Constraints from major and trace element geochemistry and Sr-Nd-Hf isotope composition

Upadhyay, Dewashish; Chattopadhyay, Sabyasachi; Mezger, Klaus (2019). Formation of Paleoarchean-Mesoarchean Na-rich (TTG) and K-rich granitoid crust of the Singhbhum craton, eastern India: Constraints from major and trace element geochemistry and Sr-Nd-Hf isotope composition. Precambrian research, 327, pp. 255-272. Elsevier 10.1016/j.precamres.2019.04.009

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The Singhbhum craton in eastern India is underlain by Paleo- to Mesoarchean Tonalites-Trondhjemites-Granodiorites (TTGs) and granites that formed in two pulses at 3.45–3.44 Ga and 3.35–3.32 Ga, followed by potassic granite activity at 3.1 Ga. The TTGs are metaluminous to slightly peraluminous and highly evolved with low Ni, Cr, and MgO. Low heavy-REE concentrations, Sr/Y and Eu/Gd, and high Nb, Ta, and Y concentrations indicate they belong to the low HREE, medium- to low-pressure TTG type. Their compositions are similar to TTGs derived from chondritic source with amphibolitic residue, consistent with rutile-free and garnet-bearing metabasic source. The granites have similar REE and multielement patterns as the TTGs, albeit with pronounced negative Eu, Sr, and Ti anomalies. High K2O/Na2O, higher heavy-REE concentration, low Sr/Y and Eu/Gd, and negative Eu anomalies of the 3.05 Ga Mayurbhanj potassic granites are consistent with shallow plagioclase-bearing crustal sources. The increasing K2O/Na2O of the granitoids from 3.45 Ga to 3.05 Ga is related to gradual thickening of the continental crust and increasing contributions of intracrustal melts in younger granitoids. Neodymium and Hf isotope data indicate the 3.45–3.44 Ga granitoids have juvenile as well as Eoarchean TTG crust-like signature. The 3.35–3.32 Ga granitoids were produced by partial melting of the same mafic crust that gave rise to the 3.45–3.44 Ga TTGs with some components requiring up to 50% crustal contribution. The Mayurbhanj granites represent intracrustal melts of more potassic members of the Paleoarchean TTGs. The trace-element characteristics of the granitoids reflect melt generation at different depths in a tectonic setting producing both TTGs as well as granites contemporaneously. Plausible geodynamic settings could be very hot subduction or “dripduction” regime involving shallow melting of delaminated mafic crust producing the 3.45 Ga juvenile TTGs and minor granites. Large scale delamination and melting of mafic lower crust and felsic proto-crust in a thickened crustal pile in a subduction-like regime could have produced granitoids with juvenile as well as recycled isotopic signatures. The Nd-Hf isotope data indicate that the Paleoarchean upper mantle was moderately depleted in response to a spurt in Paleoarchean continental growth.

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