Origin of trondhjemite and albitite at the expense of A-type granite, Aravalli orogen, India: Evidence from new metasomatic replacement fronts

Abstract

The A-type Harsora-Dadikar granites in the Alwar complex of northern Aravalli orogen, NW India provide evidence for subsolidus-requilibration of feldspars. They record three new discrete stages of albitisation, producing trondhjemite and albitite sequentially at the expense of original granite. Stage-I metasomatism deanorthised the magmatic oligoclase and transformed the grey least-albitised granite to pinkish grey microcline-oligoclase granite. Stage-II converted the latter to trondhjemite by replacement of microcline to oligoclase. Stage-III metasomatism led to the formation of albitite/albite granite from trondhjemite, where the metasomatically formed oligoclase was replaced by albite. This stage of metasomatism resulted in nearly complete disappearance of amphibole and biotite, producing a monomineralic rock (albitite), which is consistent with Korzhinskii theory of infiltration metasomatism. The reaction fronts delineating the Stage-II and Stage-III are sharp and easily discernible by their prominent color differences in Harsora on the outcrop scale. Chemically, the mineral transformations during three stages are manifested by the differential gains/losses in Na, K, Ca, Rb, Ba, Sr, Fe and Mg. The formation of albite, Cl-rich marialitic scapolite and Cl-rich amphibole in the albitised granites are suggestive of Na- and Cl-brines as the metasomatising fluids. The fluid-rock interactions, which can significantly transform the pristine mineralogy of granitoids, should be carefully considered to avoid any misinterpretations about their petrological history.