The generation and emplacement of closepet granite during late archaean granulite metamorphism in southeastern Karnataka

Abstract

In the amphibolite-granulite facies transition zone of southeastern Karnataka a typical metamorphic-anatectic sequence is exposed. In this zone large scale melting of basement gneisses during late Archaean granulite metamorphism has led to the generation of the Closepet granite. Field evidence suggests a space-time relationship between charnockite development, migmatite formation and generation of the Closepet granite. This is also substantiated by U-Pb and Pb-Pb isotope data. At the southern end of the Closepet granite, conventional petrological data and oxygen isotope work suggest that CO₂-rich fluid phase of subcrustal origin was instrumental in granulite metamorphism and anatexis. The Southern Closepet granite is highly heterogeneous and displays a very complex polyphase internal structure. Based on texture and cross cutting relationships, four major granite phases are recognised. The P-T conditions inferred from mineralogic geothermometry and geobarometry indicate that granite melts were generated at midcrustal levels. The spatial association of migmatites and gradational contacts suggest that the granite in the transition zone did not rise far from its source region. The occurrence of ductile mylonites and augen gneisses further north of the study area indicates a major shear control in the generation of the Closepet granite. A model involving collision of eastern and western crustal blocks along a N-S dividing line is the possible tectonic set-up for evolution of the Closepet granite.