Geochemical Characteristics of Archean Clastic Metasediments of Gadag Gold Field, Southern India: Implications for Provenance and Tectonic Setting

Abstract

The Gadag Gold Field in the western Dharwar craton consists of a thick pile of sediments, apart from the volcanic rocks. These rocks have undergone metamorphism of greenschists to lower amphibolite facies. The clastic metasediments (metagreywacke, chlonte phyllite and quartz-sericite phyllite) constitute the prominent lithounits in the eastern half of the area. Metagreywackes consist of abundant lithic fragments and are designated as lithic greywackes. Their mineralogical maturity index is very low (0.4). Metagreywackes have higher SiO, and Ba contents, lower Al, O, FeO', MgO and CaO contents than chlorite phyllites. Compared to chlorite phyllites, the quartz-sericite phyllites have higher SO, YO, Ba, Zr and Sr contents, and lower FeO', MgO, CaO, Ni, Co, Cr and V contents. YO/Na, O ratios of metagreywackes and chlorite phyllites are typically less than 1, and are chemically immature. Further, these sediments are characterised by higher concentrations of SiO, and Zr, higher SiOJMgO and Zr/Y ratios, suggesting felsic component in the provenance. While higher abundance of Cr and Ni with K, O/MgO ratios less than 1 and low O/MgO ratios suggest a more basaltic nature of the source area. High L a m ratios (42), negative Eu anomaly and overall REE enrichment in metagreywackes indicate a component of granite-granodioritic rocks in the provenance. The polymictic metaconglomerate with pebbles of gneiss, granite, quartzite, andesite, rhyolite and the metagreywackes with lithic fragments like quartzites, chert, andesite and rhyolite suggest a sialic dominant, mixed felsic and mafic source in the provenance. The geological setting, associated rock types, their field relationships and primary sedimentary structures coupled with petrographic and geochemical characteristics plus their modeling through various tectonic discrimination diagrams indicate that metagreywackes have continental island arc + active continental margin, chlorite phyllites have continental island arc + oceanic island arc and quartz-sericite phyllites have active continental margin + passive margin tectonic settings. It is suggested that these clastic metasediments of Gadag Gold Field, with different tectonic settings, were probably brought together (juxtaposed ?) by accretion processes during convergence of plates. However, geochronolgic and rare earth element data on a large number of samples of volcano-sedimentary rocks of this area is required to test this accretionary model.