Geology, geochemistry and tectonic setting of Archaean greywackes from Karnataka nucleus, India

Abstract

More than 80% of the area covered by the Chitradurga and Shimoga schist belts (3.0-2.6 Ga) of Karnataka nucleus, India, is made up of greywackes. Feldspathic, lithic and quartzose greywackes are deposited, respectively, along the western, northern and the eastern margin of the Chitradurga schist belt. Greywackes of the Shimoga schist belt show a large compositional scatter between feldspathic and lithic types. Compositional differences indicate variable provenance for the different localities. Tonalite-trondhjemite gneisses (TTG), mafic-ultramafic schists, basic-intermediate-acid volcanic rocks, sedimentary rocks like banded iron-formation, quartzites and cherts, and K-granites in minor quantity, were exposed in the source area. Volcanic debris was also contributed from contemporaneous volcanism in the basin. Greywackes in this region are invariably interbedded with fine-grained, biotite-rich phyllites and schists. Marked compositional differences are observed between the greywackes and the interbedded phyllites. The greywackes have K 2O/Na 2 O < 1, whereas phyllites show large variation in K 2O/Na 2O ratios ranging from 1.5 to 15.0. High Cr-Ni content of the greywackes and phyllites of the Chitradurga belt are indicative of an important contribution from ultramafic rocks. Light REE enrichment and negative Eu anomalies are particularly pronounced in the mudpacks of the greywackes represented by phyllite. The REE and other chemical data, such as La/Th, Ti/Zr and K 2/MgO ratios, indicate that these greywackes were supplied from a juvenile continental nucleus and were laid down on a simatic and converging active continental margin. An Archaean miniplate model with low-angle subduction and sedimentary basins of moderate size is proposed to explain the geochemical and tectonic features of the schist belts.