Continental growth of Bastar craton, central Indian shield during precambrian via multiphase subduction and lithospheric extension/rifting: evidence from geochemistry of gneisses, granitoids and mafic dykes

Abstract

Two major tectono-magmatic episodes can be recognized within Bastar craton. The first episode is represented by granitic-gneisses and the second one is represented by bi-modal granitoids-basic magmatic events. The multi-element patterns for the gneisses are fractionated (LILE enriched, HFSE depleted) with negative anomalies at P and Ti. REE patterns are moderately to highly fractionated (La_N/Yb_N=8-51). The gneisses have calc-alkaline trend, K/Rb ratios ranging from 117 to 325 (average 225) and the Rb/Sr values averaging 0.9. These geochemical features coupled with average concentration of Y (23ppm), Nb (17ppm), Rb (145ppm) and Th (31ppm) of the Bastar gneisses indicate an arc related tectonic setting for the protoliths. In general the geochemistry of the granitoids shows resemblance with that of the gneisses in terms of their fractionated multi-element patterns (elevated LILE in comparisons to the HFSE with strong depletion at P and Ti) and also points to volcanic arc tectonic setting. Two groups of mafic dykes are recognized within the craton; one is metamorphosed amphibolites while the other is unmetamorphosed dolerites. The geochemical features of the dolerites indicate contamination of the mantle source by sediment, whereas those of the amphibolites have been interpreted to represent variations in source characteristics in the underlying mantle. Our study indicates change in the nature of subduction related magmatism from dominantly felsic during Archaean to bimodal (acidic-basic) magmatism during Proterozoic as observed in many well-studied terrains all over the world.