EXSOLUTION TEXTURES IN PYROXENES FROM METAMORPHOSED BANDED IRON FORMATION FROM PARTS OF THE SOUTHERN GRANULITE TERRANE, INDIA: EVIDENCE OF UHT METAMORPHISM DURING LATE ARCHAEAN TIME

Metamorphosed ~2.9 Ga old banded iron formation (BIF) from parts of the Southern Granulite Terrain (SGT) shows rare garnet and two pyroxene bearing assemblages. Orthopyroxene (Opx) in this rock contains lamellae of clinopyroxene (Cpx) along the (100) plane. In places granules of Cpx are noted around the Opx porphyroblasts that can be best explained by ‘granular’ exsolution. The Cpx porphyroblasts also show lamellae of Opx along the (100) planes. Width of the lamellae are generally thin (mostly <4µm), but coarser lamellae (up to ~ 100µm) are also observed in few grains. Integration of the mineral chemistry of host and exsolved phases yielded pigeonite for Opx hosted Cpx and subcalcic ferroaugite for Cpx porphyroblast. The reintegrated pyroxene composition indicates ultra high temperature (UHT) metamorphism (>900°C) from the graphical methods of Lindsley geothermometer. Distinctly lower temperature (~ 750°C) is obtained with Fe-Mg exchange thermometry for garnet-pyroxene and two pyroxenes. This temperature is in agreement with the regional granulite facies metamorphism that occurred at the beginning of Proterozoic Eon (~2.46 Ga). The UHT metamorphism is reported for the first time from unequivocal Archaean rocks of the SGT and can be interpreted as a product of contact metamorphism preceding the regional high grade metamorphism. Potential source of heat for this metamorphism could be the felsic magmatism that intruded the BIF and its host rock at ~2.51 Ga.