Mineral compositions of the Deccan Trap volcanic rocks of India: an overview.

An overview of the mineral chemical variations observed in the Deccan Trap igneous rocks of western India indicates a very wide and continuous compositional range for olivine (Fo(92)-Fo(11)), pyroxene (augite to ferroaugite and pigeonite with rare orthopyroxene in the tholeiitic rocks; diopside to saute with no low-Ca clinopyroxene in slightly more alkaline basalts), spinel (from aluminous to chromiferous and to Ti-magnetite), ilmenite (Mg- to Mn-rich), feldspar (from An(83) to albite and anorthoclase to K-rich sanidine), and minor phases (biotite, amphibole, pseudobrookite, etc.). These compositions reflect a large variation in temperature and chemical composition of the host rocks (from about 1,200-1,300 degrees C in picritic basalts to 850-800 degrees C in the rhyolites or in the groundmass of basalts). The range of mineral compositions indicates parental magmas with varying degrees of silica saturation (mildly alkaline to tholeiitic), and very similar (low-pressure) crystallization environment for all the tholeiitic magmas throughout the Deccan, regardless of their widely different geochemical and isotopic compositions