Structural evolution of the Peninsular Gneiss — an early Precambrian migmatitic complex from South India

Abstract

The Peninsular Gneiss, which is considered by a number of workers to be the basement on which the supracrustal rocks of the Dharwar Group were deposited, is a composite gneiss formed by migmatization of pre-existing metasedimentary and meta-igneous rocks. These gneisses show the same style and sequence of superposed deformation as those in the enclaves of metamorphic rocks and in the linear Dharwar schist belts outside. The main migmatization is broadly coeval with the isoclinal first folding, which is followed by near-coaxial refolding and non-coaxial upright folding. Small inclusions of migmatized amphibolite and granodioritic to dioritic gneiss, with a fabric athwart to, and overprinted by, the earliest deformation affecting the Dharwar Group of rocks in a large part of the gneissic terrane, point to at least one deformation, a metamorphic event and one episode of migmatization antedating the isoclinal first folds in the rocks of the Dharwar Group. The Peninsular Gneiss in its present state, therefore, represents an extensively remobilized basement.