Petrogenetic evolution of Ramagiri greenstone terrane, Central Dharwar craton, Andhra Pradesh, India: Unravelling ancient oceanic basin of Archean Earth

Abstract

The Ramagiri greenstone terrane (RGT) of Central Dharwar craton, India, is recognized as remnants of Archean oceanic crust squeezed between ancient proto-continental terranes. The granites and gneisses formed as a result of partial melting, resisted subduction and collided, sinking the intervening ocean basin. The geochemical characteristics of the bimodal metavolcanics of RGT were carefully assessed using suitable proxy. An insight into them suggests that the mafic rocks bear characteristics of island arc tholeiites belonging to suprasubduction zone setting, as corroborated by their low TiO₂/Yb (avg 443) and high Th/Nb (avg 0.8). Their high V/Ti (avg 0.3) ratio points towards a mantle source that has been influenced by subduction, whereas low Ti/Yb echoes high degrees of shallow melting with little residual garnet. The felsic rocks on the other hand are metaluminous–peraluminous, mostly calc-alkaline and geochemically akin to I- and S-type syncollisional and volcanic arc granite emplaced during intercontinent collision. Their immobile element plot with low Nb + Y (avg 25) concentration also points towards a volcanic arc environment.