Petrology of the Elchuru Alkaline Pluton, Prakasam District, Andhra Pradesh, India

The oblong alkaline pluton at Elchuru is constituted of shonkinite, malignite, nepheline syenite (foyaite and pulaskitic foyaite) and nepheline gneiss (including litchfieldite and mariupolite); the bulk of the pluton is, however, made up of nepheline syenite and nepheline gneiss. These alkaline rocks are emplaced conformably and permissively by a simple mechanism of dilation of the surrounding country rocks under tensional stress. During emplacement, the pluton deformed its own nearly solid periphery producing an outer ring of nepheline gneiss with subsolvus assemblage which surrounds the central core of nepheline syenite with hypersolvus assemblage.

The field, petrographic and geochemical studies, when viewed together, suggest that all the rocks of the pluron arc comagmatic and that nepheline syenites (and nepheline gneisses) are the differentiated products of a basic alkaline magma. The present study has shown that the alkaline magma remained strongly undersaturated throughout the differentia tion process.

The agpaitic coefficient of these rocks is very near to, but invariably less than unity. In tune with their miaskitic chemistry, these rocks show low Na₂O/K₂O and Fe₂O₃/FeO ratios and a generally lower content of MgO, TiO₂ and MnO. The contents of Sr and Ba are very high and with differentiation the ratios Ba/Sr and Sr/Ca fall whereas K/Ba ratio rises. The uniformly higher KJRb ratios of these rocks suggest a deep seated source for the magma.