Metamorphic evolution of a garnetiferous mafic granulite from north-eastern segment of chotanagpur granite gneissic complex, India

Abstract

The studied area Massanjore near Dumka district lies in the north-eastern part of the Chotanagpur Granite Gneissic Complex where mafic granulites occur as enclaves within the host felsic gneisses that form the major country rock. These mafic granulites develop a planar fabric mainly defined by the diagnostic mineral assemblage of high pressure granulite metamorphism (garnet + clinopyroxene+ amphibole + plagioclase + quartz + rutile+ ilmenite). The earliest discernable mineral assemblage includes amphibole + clinopyroxene + plagioclase + quartz + rutile occurring as inclusions in garnet porphyroblasts which from the early M0 stage. Subsequently the assemblage garnet + clinopyroxene + plagioclase + quartz + rutile formed during the peak condition of metamorphism (M1) defines the pervasive fabric. Conventional geothermobarometric calculations from the data of mineral chemistry yield 820 ± 30 ° C and 12 ± 0.8 kbar for the M1 stage. The P-T estimations of the peak metamorphic assemblage suggest a steep geothermal gradient with dP/dT ˜71°C/kbar that corresponds with the geothermal gradient of collisional orogeny. During the early retrograde stage (M1R1) garnet breaks down to from symplectic intergrowth of clinopyroxene + plagioclase which form along the grain boundaries of garnet grains. In the late retrograde part amphibole is seen to replace the garnet and clinopyroxene grains along boundaries and fractures. These textural studies supportive with geothermobarometric calculations show a steep decompressional early retrograde path (M1R1 dP/dT ˜ 50°C/kbar) from the peak followed by a late retrograde event (M1R2 dP/dT ˜ 40°C/kbar) showing consistent cooling along with hydration. From the above information a CW P-T path is generated and the calculated peak condition 20°C/km plots in the field of high pressure granulite metamorphism. All these results are consistent with a continent continent collisional set up and this leads to the fact that a large part of CGGC behaved as a coherent lower plate during the globally extensive convergent margin orogenesis at the time of formation of the supercontinent Rhodinia