Magmatic Differentiation in Mount Girnar

Abstract

The Girnar laccolith was intruded into a thick covering of basalt of late Cretaceous or early Eocene age in Kathiawar, India, which was lifted up in the form of a dome. Slow cooling of the magma gave rise to olivine-gabbro, diorite, monzonite, syenite, nepheline-syenite, and granophyre. This paper deals principally with the mechanism of intrusion and differentiation. Three phases of movement are represented by olivine-gabbro, diorite-monzonite, and granophyre, respectively. Diorite and monzonite occur in the center and are surrounded by olivine-gabbro. Granophyre is intruded in the marginal hills of basalt in the form of large dikes. This structural feature is explained by a circular system of fractures in basalt due to arching. The differentiation is shown to be the result of fractional crystallization; sediment-assimilation or immiscibility in the liquid state is regarded as improbable. The physical conditions which prevailed indicate quiet crystallization and absence of convection currents and crystal settling. Field evidence has been noticed supporting the origin of nephelite-syenite by the dissociation of the polysilicate feldspar.