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

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. (C) 2019, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V.