Peninsular Gneissic Complex of the Dharwar Craton - A Suggested Model for its Evolution

Abstract

The relationship between the gneisses and the greenstone belts of the Dharwar 'Craton is reviewed. Arguments for and against considering the gneisses as forming the basement, are presented and the evidences critically evaluated. Stages in the evolution of the craton are traced. The suggested model envisages the recognition of an older greenstone sequence, largely mafic and ultramafic in character followed by a major differentiation episode in the earth's upper mantle giving rise to a tonalite magma. The bulk of the gneisses forming the Peninsular gneissic complex is assigned to this major event which has not been repeated subsequently. The variations shown by the Peninsular gneisses are indicated to be the result of intimate mixing and interaction of the tonalite magma with the pre-existing mafic and ultramafic lavas and associated sediments. The banding and migmatitic character of the Peninsular gneisses is ascribed to plastic flow in response to relatively small directed stresses.

The suggestion is made that the greenstone belts of the Dharwar craton be split into two major stratigraphic units - an older, composed of mafic and ultramafic rocks of distinct chemical character, and a younger geosynclinal pile with a lower volcanic phase and an upper well-developed sedimentary phase. While the gneisses show an intrusive relationship with the older group, they form the basement for the younger group of lavas and sediments deposited in long linear basins. The suggested model explains a large number of related facts, reconciles the different views, and takes into account the prevailing conceptions of the evolution of the early crust in other parts of the world.