Ultra-potassic and ultra-aluminous hybrid volcanic rock in the Proterozoic of Bastar craton, Madhya Pradesh, India

Late Proterozoic intracratonic Indravati basin of Bastar has been dissected by Silekjhodi–Tirathgarh fault zone marked by deep gullies and escarpments which afford access to an unconformable junction between basement crystallines and the overlying basinal sediments. This junction exposes the presence of fine-grained green coloured volcanic rock in certain locations. The rock is aphanitic with phenocrysts of potash feldspars and leucite(?) set in a ground-mass which is pervasively sericitized. The rock displays unique chemical characters in having high K2O (≃11.5%) and Al2O3 (≃32%) contents whereas CaO, MgO, TiO2 and Na2O contents are low (≃1%). This ultra-potassic character indicates a deep-seated source for the rock material, whereas its ultraaluminous character indicates crustal contamination during its ascent.