Petrography of Serra da Gruta Nepheline Syenite Stock, Southern Bahia Alkaline Province

Abstract

The Serra da Gruta Nepheline-Syenite Stock is located in the southeast part of the South Bahia Alkaline Province. It has an area of 4km2 and intrudes Precambrian Granulites. The stock's rounded shape combined with abrupt geological contacts suggest an epizonal emplacement. Serra da Gruta Syenite consists of rocks with a varied mineralogy (nepheline-syenites, cancrinite-nepheline-syenites, cancrinite-nepheline-sodalite-syenites, nepheline-biotite-syenites, and alkali feldspar-syenites). The complex gradational contacts between these syenites suggest that fractional crystallization was involved in the Serra da Gruta Stock evolution. The textural relationships and mineralogical evolution show that magma fractionation was controlled by mafic minerals and alkali-feldspar crystallization. In this context, the co-existence of cancrinite, carbonate and sodalite is interpreted, in face of the identified textures and experimental data reported in the literature, as indicative of the presence of fluids rich in CO2 and chlorine that interacts with nepheline and alkali-feldspars.