Experimental study on soda granites of the Singhbhum Craton, eastern India, and its petrogenetic significance

Abstract

The soda granite (~Paleoproterozoic) of the Singhbhum Craton (SC) of eastern India occurs along the entire stretch of the Singhbhum Shear Zone (SSZ; or Copper Belt Thrust Zone). The SSZ (marking a thrust contact between the adjacent North Singhbhum mobile belt and SC) serves as the important locale to host apatite‐magnetite, copper sulphide, and uraniferous mineralization. Typical field occurrence of soda granite is either as tongues (or patches) within a shear zone or as lit‐per‐lit style injections into surrounding country rocks which are generally highly sheared and mylonitized. Experimental studies on granite bodies can potentially throw light on their mineralogical and petrogenetic aspects; however, these studies are hitherto unreported in case of soda granites for almost the last 60 years. In this view, during the present research, experimental studies on soda granites have been carried out and these decipher the nature of the stability relationship of K‐feldspar (and transgression to more disordered anorthoclase), amphibole (both Ca‐amphibole and Fe–Mg–Mn amphibole and breaking down of Ca‐amphibole to Fe–Mg–Mn amphibole + non‐quad pyroxene + quartz at an appropriate temperature), biotite, muscovite, and albite (high albite‐ low albite displacive transformation). Whole‐rock chemistry (deduced from homogenized glass compositions) indicates its peraluminous, S‐type nature with an affinity towards Caledonian post‐collisional plutons. Characteristic geochemical plots suggest a pelitic source material for melting (and subsequent internal differentiation of the melt) and this has given rise to both migmatite as well as patchy soda granite exposures in the field. It is suggested that partial melting of mainly metapelitic (and minor mafic magmatic rocks) source (belonging to Singhbhum Series) gave rise to soda granites in this region.