Field and Petrographic Studies of the Guwahati Metapelites, Assam, NE India: Evidence for Post- Metamorphic K-Metasomatism

Abstract

In this study, we report mineralogical features of post-metamorphic K-metasomatic assemblage of high-grade metapelites from Guwahati, the northern extension of the central part of the Shillong Plateau, NE India. Several reaction textures of metasomatic metapelites are shown and they offer clues for relevant reactions for metasomatic modification of pre-metasomatic metapelites. Two sets of mineral reactions are recognized, which include, the alteration of high-temperature of all kinds of aluminium-silicate phases of pre-metasomatic metapelites into low-grade minerals and the replacement of coexisting metamorphic plagioclase and quartz by K-feldspar due to the involvement of K-rich fluids at the end of all deformational events. Our structural study indicates the rocks of the area are intensely deformed with multiple folding events and post-kinematic shearing and faulting which cross-cut the dominant foliation of the rocks. The deep-seated K-rich metasomatic fluids moved into shear zones and migrated along the foliation plane, grain boundaries of minerals transforming the parent metapelites into metasomatic rocks. As a result of metasomatic changes, all pre-existing features disappear and metapelites turn into K-feldspar -rich granitic rock.