Chemistry of clinopyroxenes from subalkaline and alkaline rocks of Phenai Mata Igneous Complex, Baroda District, Gujarat, Western India

Abstract

The predominant tholeiitic gabbro-microgranite/granophyre suite has been found in close association with subordinate alkali gabbro-syenite suite at Phenai Mata Igneous Complex (PMIC) (65 Ma). The pyroxene compositions from the selected tholeiitic and alkaline lithotypes of PMIC have been classified and compared to those from alkaline rocks of adjoining complexes. The results indicate that pyroxenes were crystallized in diverse tholeiitic and alkaline parental magmas undergoing differential oxygen fugacity (fO2) and silica activity (aSiO2). The fractionation trend of pyroxenes from tholeiitic rocks of PMIC follows the trend transitional between Ca-poor and Ca-rich augite-ferroaugite trends, though limited iron- enrichment, formed under low fO2. Whereas pyroxenes in alkaline rocks follow the Ca-rich salite-ferrosalite trend at relatively higher fO2, and can be related to those of volcanic and subvolcanic alkaline rocks of adjacent alkaline complex probably representing an analogous alkaline pulse. The pyroxenes of alkaline rocks of PMIC and adjoining area can also be distinguished from those of tholeiitic rocks, having higher Ti and Al contents. An early and strong partitioning of Cr in crystallizing diopside, magnetite and probably chromspinel of tholeiitic and alkaline parental magmas impoverished the pyroxenes (Mg/Mg+Fe′<0.700) of microgranite and syenite in this element during the late stage of crystallization.