Structure and metamorphism of the precambrian rocks of the area between Deoli and Rangshai Goalpara district Assam

Abstract

This thesis embodies the results of (i) the structural study of the rocks belonging to the Precambrian Gneissic Complex of the Assam-Meghalaya plateau in the areas between Deoli and Rangshai in Goalpara district, Assam ; (ii) the petrographical as well as chemical analyses of the rocks ; (iii) the mineral chemistry analyses of the rocks and (iv) the determination of the pressure (P) - temperature(T) condition of metamorphism prevailed during the formation of these rocks, and also the nature and behaviour of fluid regime operative during metamorphism. The rocks types in the study area in order of abundance are : quartzofeldspathic gneisses (occasionally migmatised), metacalcareous rocks (calc-silicate gneisses and hornblende gneisses), metabasic rocks (amphibolites), banded iron formations, basic dykes and quartzofeldspathic veins, pods and lenses. The structural study has revealed that all the major rock types of the area except basic dykes suffered multiple deformations, and three distinct phases of folding, a number of brittle-ductile shear zones and late minor structures including three sets of joints have been recognised. F1 folds are preserved only as hooks in their type-3 interference structure with F2 folds and penetrative as well as pervasive S1 foliations are axial planar to F1 folds. F2 folds are the most dominant folds in the area and are vertical to steeply plunging upright folds. F1 and F2 folds are coaxial as well as coaxial planar with NE-SW striking F2 axial planes. Geometrical analyses of F2 folds indicate that the folds mostly belong to class 1C and they experienced different degrees (0.1-0.7) of flattening. F3 folds are warps with axial planes, striking NW-SE and effected S1 foliations and at some places, the limbs of pre-existing folds. Deformation related to brittle-ductile shear zones effected S1 foliations and at some places, the limbs of F2 folds. This shearing episode was either syn- or post- F2 folding episode. The three sets of joints represent the brittle deformation episode in the...