Pressure-temperature-deformation history for a part of the mesoproterozoic fold belt in north Singhbhum, eastern India

Abstract

The supracrustal rocks in the easternmost part of the Proterozoic fold belt of North Singhbhum, eastern India, are folded into a series of large upright folds with variable plunges. The regional schistosity is axial-planar to the folds. The folds were produced by a second phase of deformation (D2) and were preceded by D1 deformation, which gave rise to isoclinal folds (mapped outside the study area) and the locally preserved, bedding-parallel schistosity. A shearing deformation during D2 was responsible for the sheath-like geometry of a major fold. The axial planes were curved by D3 warping. The first metamorphic episode (M1) of low-pressure type produced andalusite porphyroblasts prior to, or in the early stage of, D1 deformation. The main metamorphism (M2), responsible for the formation of chloritoid, kyanite, garnet and staurolite porphyroblasts, was late- to post-D2 in occurrence. The Staurolite isograd separates two zonal assemblages recorded in the high-alumina and the low-alumina pelitic schists. Geothermobarometric calculations indicate the peak metamorphic temperature to be 550 °C at 5.5 kb. Fluid composition in the rocks before and during M2 metamorphism was buffered and fluid influx, if any, was not extensive enough to overcome the buffering capacity of the rocks. From M1 to M2, the P-T path is found to have a clockwise trajectory, that is consistent with a tectonic model involving initial asthenospheric upwelling and rifting, followed by compressional deformation leading to loading and heating.