Arkasani Granophyres from the Singhbhum Shear Zone, Bihar

The granophyre bodies have massive monzo- to syenogranitic core and gneissose to schistose borders where the composition varies from granodioritic to tonalitic. These bodies appear to have crystallised from granitoid melts generated within the shear zone during an earlier phase and were replaced by albitic liquids, around their borders, during a later phase. The evolution of these rocks seems to be related to the tectonic history of the shear zone which represents the boundary between a southerngranitic platform and a northern mobile belt and evolved as the platform progressively underthrust below the mobile belt during the course of four successive phases of deformation. It is suggested that the granitoid melt was derived through the partial melting of the metapelitic rocks of the shear zone during the first phase under pressure of about 2 Kbar while the albitic liquid was derived during a later phase when pressure exceeded 5 Kbar. Temperature, well above 650°C, required for such melting could have been provided by the higher geothermal gradient along the shear zone, which primarily represents a zone of deep seated fracture, possibly extending up to the upper mantle; frictional heat generated during thrusting and shearing, rise of isogeotherms on either side and within the shear zone during concomitant regional metamorphism, and radioactive heat as indicated by the occurrence of uraniferous minerals along this zone.