Paleoproterozoic Cordilleran-style accretion along the south eastern margin of the eastern Dharwar craton: Evidence from the Vinjamuru arc terrane of the Krishna orogen, India

•Krishna orogen at southeastern margin of the Eastern Dharwar craton classified into Vinjamuru and Ongole arc terranes•Vinjamuru continental and Ongole oceanic arc terranes formed by Cordilleran-style accretion during Late Paleoproterozoic•Vinjamuru arc graniotoids generated by a multi-phase petrogenetic mechanism•Granodiorite magma forms by 10% partial melting of a lower crustal hydrous, high K2O-bearing gabbro•Magma underwent fractional crystallization, mixing and crustal assimilation at middle crust

Accretion along continental or island arcs at cratonic margins was responsible for most Paleoproterozoic crustal growth. For the development of the Krishna orogen, India, at the southeastern margin of the Eastern Dharwar craton (EDC), two contrasting models, one by long-lived accretion between ~ 1.85 Ga and 1.33 Ga terminating in continental collision with the Napier Complex and the other involving continental collision with the Napier Complex at ~ 1.6 Ga have been proposed. Here we report the geology and geochemistry of the granitoid rocks grouping them into the Vinjamuru arc terrane. These comprise biotite ± hornblende high-silica granite which are mostly calc-alkaline, weakly metaluminous to peraluminous with normalized trace and rare earth element plots resembling those derived from arc sources as seen by negative Nb, Ti, Zr anomalies, enriched LREE and moderate Eu anomalies. On (La/Yb)CN vs YbCN and Sr/Y vs Y discrimination diagrams these rocks plot in the field of liquids from mantle-derived melts resembling Cordilleran type granitoids. Petrography, major oxide and trace element concentrations suggest formation in an arc tectonic setting during convergent tectonics at the active continental margin of the EDC with evidence for crustal assimilation. To generate the observed high-silica granite, using selected trace and REE, we modeled 10% aggregate continuous melting of a lower crustal hydrous, high K2O-bearing gabbro yielding a granodiorite magma that underwent fractional crystallization at mid-to lower crust followed by mixing with country rock tonalite and minor assimilation with metasedimentary crustal rocks resulting in the observed heterogeneity in trace elements from the granite. We interpret Paleoproterozoic paleopostions of component Indian cratons leading to their Mesoproterozoic assembly and in that context relate the crustal growth along the southeastern margin of the EDC. In contrast to the existing two models, we propose an alternative Cordilleran-style accretion involving development of an intra-oceanic arc due to ocean-ward migration following the earlier choking of the subduction zone at an active continental margin, caused probably by the North China crustal ribbon that had by ~ 1.78 Ga accreted to the EDC margin. The formation of the outboard intra-oceanic Ongole arc terrane occurred thereafter and was eventually accreted (and metamorphosed) to the Vinjamuru arc terrane between ~ 1.63 and 1.55 Ga to form the two arc terranes of the Krishna orogen; we discount any continental collision at this stage as tectonics along this margin, post 1.5 Ga, switched to an extensional regime.

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