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 diss ected by Silekjhodi-Tirathgarh fault zone marked by deep gullies and e scarpments which afford access to an unconformable junction between ba sement 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 fe ldspars and leucite(?) set in a groundmass which is pervasively serici tized, The rock displays unique chemical characters in having high K2O (similar or equal to 11.5%) and Al2O3 (similar or equal to 32%) conte nts whereas CaO, MgO, TiO2 and Na2O contents are low (similar or equal to 1%), This ultra-potassic character indicates a deep-seated source for the rock material, whereas its ultra-aluminous character indicates crustal contamination during its ascent.