Basement-Cover Relationships of Peninsular Gneiss With High Grade Schists and Greenstone Belts of Southern Karnataka

Abstract

The low grade Archaean terrain of southern Karnataka is composed of greenstone belts and high grade schists set in a sea of gneiss complex (Peninsular gneiss). Peninsular gneiss contains three major components which are inter-gradational, namely: (a) a trimodal macrolayered unit of amphibolites, ultramafites and granite gneiss (b) normal migmatites with characteristic megascopic structures and (c) nebulitic, schlieric and homophanous granitoids. On the whole it is a polymigmatite encompassing several episodes. The ubiquitous enclaves of mafic-ultramafic rocks within Peninsular gneiss probably represent residues of repeated reworking of the crustal rocks. The composition of the gneiss complex varies in the indestructible range of tonalite through granodiorite to granite. The high grade schists or supracrustals consist of shelf sediments, volcanics and ironstones with profuse emplacements of ultramafites. Their contacts with the gneiss complex are concordant due to intense deformation, high metamorphic grade and extensive migrnatisation. This high grade thermal event (Pantectogenesis) dated at 2900-3000 m.y. has acted as an effective smokescreen blurring earlier episodes. Subsequent to the pantectogenesis, greenstone belts were evolved on a basement of gneiss complex. Two types of greenstone belts of mutually exclusive geographic distribution are seen in the craton: (i) dominantly volcanic belts, lacking shelf facies rocks and ultramafics at the base, called the Keewatin type, which are broadly comparable to the Archaean greenstone belts and (ii) Platformal volcanic and protogeosynclinal belts of Dharwar type, which are similar to the early Proterozoic basins and geosynclines. The contacts of the Dharwar type greenstone belts with the gneiss complex are almost always unconformable, whereas in the case of Keewatin type of belts, the contacts are obscured either by soil cover or by massive invasion of later granites. This major granitic event possibly corresponds to the 2500-2600m.y. isochron in the gneiss complex. This evolutionary picture reconstructed mainly from newer field data is consistent with the available limited information on geochronology, geochemistry and mineralisation trends.