Open System Fractional Crystallization Process in the Evolution of Deccan Basalts of Pratapgarh Region, Western India

The northwestern fringe of the Deccan Flood Basalt sequence is characterized by isolated occurrences of sub-horizontal tholeiitic flows. Because the flow units in this region are not traceable over large distances, the petrographic and geochemical characters have been utilized to establish nine flow units of olivine tholeiite (OT), quartz tholeiite (QT) and low calcium quartz tholeiite (LCT). The flows show striking mineralogical and chemical similarity. Low-Mg numbers and moderate to high silica content indicate the derivative nature of the magma. The flow sequence has evolved through predominant plagioclase fractionation and less significant clinopyroxene separation from a non-primary olivine tholeiite melt from which olivine had been removed earlier. Cyclic variation in geochemical signatures and prominent chemical breaks within the flow sequence indicate a periodically replenished, open system, shallow level magma chamber with gabbroic fractionation.