Basement-cover relationships of the Peninsular Gneiss with the greenstone belts of Karnataka, India

Abstract

The Archaean terrain of Karnataka is composed of greenstone belts and high-grade schists set in a sea-of-gneiss complex (Peninsular Gneiss). The existence of several greenstone belts and continuous transitions between gneiss and greenstone terrains in Karnataka provide unique insight into the evolution of the Archaean crust. The older mafic and ultramafic enclaves in the Peninsular Gneiss probably represent relicts partially of early oceanic crust and partially of reworked continental crust. Over this polycyclic gneiss complex were deposited supercrustal rocks in small linear basins, which are now represented by high-grade schists. A major tectonothermal event (Pantectogenesis) around 3000 Ma ensued throughout the Karnataka Craton, and involved the high-grade schist and gneiss complex. Subsequent to the Pantectogenesis, greenstone belts evolved on a basement of the gneissic complex. Under a relatively unstable regime, the older greenstones (i.e., Keewatin type of eugeosynclinal greenstone belts — Kolar, Hutti, and Mangalur belts) comparable to the Archaean greenstone belts evolved. These are also widely known as the Kolar-type greenstone belts. A more stable regime witnessed the evolution of younger greenstones (Dharwar type of greenstone belts), the Chitradurga Group and the Bababudan Group, which are similar to the Early Proterozoic basins and geosynclines. The Kolar-type greenstone belts are mainly metavolcanic belts with predominant mafic to andesitic volcanic rocks, subordinate acid volcanic rocks, ultramafites, graphitic and sulfidic schist, ironstones, cherts, and immature sediments like wackes and polymictic conglomerates. The Dharwar-type greenstone belts are predominately metavolcano-sedimentary belts, with shallow water “shelf facies” sediments at the base. The metavolcanic rocks of the older greenstone belts are geochemically comparable to oceanic tholeiites. A marginal-basin tectonic setting is the most favourable environment for their formation. Among the Dharwar-type greenstone belts, the Chitradurga Group metavolcanic rocks are marked by ocean-floor basalts followed by a volcanic-arc series made up of low-K tholeiites and calcalkaline basalts formed in a submarine tectonic environment. The Bababudan Group metavolcanic rocks are of tho-leiitic to calcalkaline affinity formed in a shallow basin stable tectonic environment.