Paleomagnetism and U-Pb geochronology of easterly trending dykes in the Dharwar craton, India: feldspar clouding, radiating dyke swarms and the position of India at 2.37 Ga

Abstract

A U-Pb baddeleyite age of 2367 ± 1 Ma from a diabase dyke together with previously published age data, suggests that a major early Proterozoic dyke swarm cuts across the structural grain of the Archean Dharwar craton in India. Paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30° with convergence to the west. It was originally emplaced at high latitudes, and together with the Widgiemooltha dykes of the Yilgarn block of Australia, may have been a segment of a larger radiating swarm related to a long-lived plume event that was active for about 50 My from 2418 to 2367 Ma. A regional change in the intensity of brown feldspar clouding in the dykes suggests that the Dharwar craton was tilted northwards, in harmony with previous observations on the structure and metamorphism of the Archean rocks. Towards the south the brown feldspar clouding becomes more intense and locally assumes a blacker, more "sooty" appearance. The black clouding, whose precise origins remain unknown, is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at ˜800 Ma, and with a shear zone and change in structural trend related to Pan-African deformation at ˜550 Ma. Paleomagnetic studies suggest that (i) the high coercivity part of the remanent magnetization is carried by magnetite exsolved within either brown or black clouded feldspars and (ii) as a more general observation, diabase with brown clouded feldspar can carry a primary magnetization but not if the clouding is black.