Boudinage and composite boudinage in superposed deformations and syntectonic migmatization

The banded gneisses of the Jasidih–Deoghar area of East India behaved as a complex geological multilayer, which evolved through the combined effects of deformation and leucosome emplacement. The early foliation and banding were transposed and retransposed by two generations of isoclinal folds. The intensity of foliation development, the layer-thickness ratios and the rheological contrasts within the gneisses changed both in space and in time. Under layer-normal compression these complex multilayers gave rise to a variety of boudins and of their pegmatite-filled separation zones. The geometrical relations with successive generations of folds can distinguish the closely associated pre-F1, syn-F1 and syn-F2 boudins. D2 boudinage occurred at an advanced stage of F2 folding so that boudinage-induced subhorizontal extension was considerably less than the total subhorizontal extension during D2. Composite boudinage occurred during progressive syntectonic leucosome emplacement in the course of which a group of boudinaged layers with a profusion of coarse quartz--feldspar leucosome became more competent than the adjoining rocks and underwent boudinage as a whole.