Prograde polyphase regional metamorphism of pelitic rocks, NW of Jamshedpur, eastern India: constraints from textural relationship, pseudosection modelling and geothermobarometry.

The study area belongs to the Singhbhum metamorphic belt of Jharkhand, situated in the eastern part of India. The spatial distribution of the index minerals in the pelitic schists of the area shows Barrovian type of metamorphism. Three isograds, viz. garnet, staurolite and sillimanite, have been delineated and the textural study of the schists has revealed a time relation between crystallization and deformation. Series of folds with shifting values of plunges in the supracrustal rocks having axial-planar schistosity to the folds have been widely cited. Development of these folds could be attributed to the second phase of deformation. In total, two phases of deformation, D1 and D2, in association with two phases of metamorphism, M1 and M2, have been lined up in the study area. Chemographic plots of reactant and product assemblages corresponding to various metamorphic reactions suggest that the pattern of metamorphic zones mapped in space is in coherence with the temporal-sequential change during prograde metamorphism. The prograde P–T evolution of the study area has been obtained using conventional geothermobarometry, internally consistent winTWQ program and Perple_X software in the MnNCKFMASHTO model system. Our observations suggest that the progressive metamorphism in the area is not related to granitic intrusion or migmatization but that it was possibly the ascending plume that resulted in the M1 phase of metamorphism followed by D1 deformation. The second and prime metamorphic phase, M2, with its possible heat source generated by crustal overloading, was preceded by D1 and it lasted until late- to post-D2 deformation. [ABSTRACT FROM AUTHOR]