Petrology of Olivine Basalt Dyke of Lamprophyre Affinity at Uppalapadu, Prakasam District, Andhra Pradesh

The basic dyke combines the mineralogical assemblage of an olivine basalt and textural characteristics of a lamprophyre. This porphyritic rock contains few euhedral phenocrysts of olivine set in a fine-grained groundmass, constituted by plagioclase and clinopyroxene. The olivine crystals are invariably mantled by a thin reaction rim of Ca-poor pyroxene and in spite of this, the rock has developed nepheline in the norm. The composition of the basaltic liquid can be expected to lie very close to the Fo-Di-Ab plane (or the Ol-Cpx-Pl thermal divide) within the basalt tetrahedron. Petrogenetic considerations regarding (a) the crystallization of olivine basalt magma and (b) the role played by volatiles in modifying its composition (corresponding to that of olivine lamprophyre) elsewhere in the Prakasam district are briefly discussed.