Petrology, geochemistry and petrogenesis of the mafmatic rocks of pachcham island, kutch, northwestern India

Abstract

Western most on-land exposure of basalt of Deccan affinity occurs along the southern coastal belt in Kutch, Abundant magmatic rocks also occur in the Pachcham Island in northern Kutch. These include olivine-clinopyroxene-plagioclase-phyric under saturated basalt (the Sadara sill), gabbro (Kuran), a differentiated alkaline igneous complex comprising pyroxenite, gabbro, diorite and lamprophyre at Nir Wandh and a swarm of basaintic dykes at Kaladongar. Magmatic rocks of Nir Wandh and Kaladongar are characterised by the presence of abundant kaersutite phenocrysts. These rocks are LILE - enriched and have a fractionated REE pattern. Compared to the primitive mantle, the alkaline rocks of Pachcham Island are enriched in K, Ti, alkalis and LREE but depleted in Sc, V, Ni, Cr, Y. There are measureable differences in the major and trace element abundances in the rock types. Paleomagnetic data suggest that the magmatic activity in northern Kutch preceded the Deccan Volcanic eruption, which is generally belived tot be in the range 64-67 Ma. Alkaline rocks generally occur in the rift zones in the Deccan Volconic Province. In Kutch, however, alkaline rocks are common. This is belived to reflect an enriched lithospheric source. The chemical composition was subsequently modified by shallow level fractional crystallisation.