Mineral thermobarometry and fluid inclusion studies on the Closepet granite, Eastern Dharwar Craton, south India: Implications to emplacement and evolution of late-stage fluid

摘要

The Closepet granite (CPG), a spectacularly exposed magmatic body along with other intrusive bodies (to the east of it) typifies the late Archean granitic activity in the Eastern Dharwar Craton (EDC), south India. In the present study, the P-Y-ƒO_2 conditions of emplacement and physico-chemical environment of the associated magmatic-hydrothermal regime of CPG have been retrieved on the basis of mineral chemical and fluid inclusion studies. Amphibole-plagioclase Ti-in-amphibole and Ti-in-biotite geothermometers along with Al-in-amphibole geobarometer have been used to reconstruct the emplacement temperature and pressure conditions in the majority of the pluton. Estimated temperatures of emplacement of CPG vary from to 740 to 540 ℃. A variation of pressure from 4.8 to 4.1 kilo bars corresponding to this temperature range was obtained. While there is a faint south to north negative gradient in temperature, the variation of pressure does not seem to follow this trend and indicates more or less same crustal level of emplacement for the body between Ramanagaram-Kalyandurga segment extending for about 230 km. Mineral chemistry of biotite indicates crystallization of CPG under high oxygen fugacity conditions (mostly above QFM buffer) with no clear spatial variation in the fugacity of halogen species in the late-stage magmatic fluid. It may be surmised that barring the southernmost part of CPG, there is no perceptible variation in the physicochemical environment of emplacement. Fluid Inclusion studies in the granitic matrix quartz and pegmatite/vein quartz show dominance of H_2O and H_2O-CO_2 fluids respectively in them. The difference in the fluid characteristics is interpreted in terms of the initial loss of CO_2 rich fluid from granitic magma and aqueous-rich nature during the later stages of crystallization of quartz. The exsolved CO_2-rich fluid was responsible in formation of the later quartz and pegmatitic veins at different crustal levels and also possibly was responsible in CO_2-induced metamorphism (char-nockitization) preserved in the Kabbaldurga area in the southernmost part. The fluid characteristics in the vast tract of CPG revealed from mineral chemical and fluid inclusion studies make a case for its involvement in gold mineralization in the schist belts in EDC.