Picrite basalts and related lavas from the Deccan Traps of Western India

Detailed chemical and mineralogical data are given for three sequences of basalts and picrite basalts from bore-holes in Western India. The picrite basalts show bulk compositional variation generated by the fractionation of olivine and chromite. Evolved picrite basalt magma appears to have given rise to basalt by the fractionation of olivine+clinopyroxene, despite the presence of abundant plagioclase phenocrysts. It is suggested that a slow settling rate for plagioclase relative to clinopyroxene and olivine is sufficient to account for this feature. The high degree of equilibrium crystallisation which many of the lavas have apparently undergone is interpreted in terms of the mechanism of compensated crystal settling (Cox and Bell, 1972). Experimentally determined atmospheric pressure phase relations are used to model dyke-like magma chambers in some detail. Finally volumetric and age relationships are used to argue that the picrite basalts, despite their porphyritic nature, crystallised from ultramafic liquids containing in some cases at least 16% MgO.