Neoarchean magmatism in Shimoga greenstone belt, India: Evidence for subduction-accretion processes in the evolution of the western Dharwar stratigraphy

Abstract

New geochemical and geochronological data are presented for a suite of metavolcanic rocks from Shimoga greenstone belt to understand their petrogenesis and to constrain the timing of final mafic magmatic activity in western Dharwar craton. We report an Sm-Nd isochron age of 2638 ± 66 Ma for metabasaltic and metaandesitic rocks of the stratigraphically uppermost Medur Formation of Dharwar Supergroup exposed in the belt, which is the youngest age obtained yet for Neoarchean mafic magmatism in western Dharwar craton. The emplacement of the basalt-andesite-dacite-rhyolite suite of rocks took place in a magmatic arc culminating in the deposition of vast greywacke-argillites in a continental margin-type setting. The magmatic precursors of the metavolcanic rocks were generated by partial melting of depleted mantle by slab-derived fluids with variable input from the subducting slab. Assimilation-fractional crystallization processes may also have contributed to the magmas to a significant extent. The younger mafic lithologies of the Dharwar stratigraphy represent accretionary orogens and the date obtained in this study points towards the final stages for the assembly of western Dharwar craton by subduction processes during Neoarchean. The thickness and extent of the greywacke-argillite suite in SGB could be attributed to the protracted deposition during amalgamation of western Dharwar craton with a northern Archean continental crust that might underlie Deccan volcanic rocks.