Petrology and geochemistry of mafic dyke and sills in Cumbum Formation, of the Proterozoic Nallamalai fold belt, Rajampet area, Andhra Pradesh, India

Abstract

Intensely deformed Proterozoic Nallamalai fold belt (NFB) occupies the eastern part of undeformed, Proterozoic Cuddapah basin and is separated from Cuddapah basin by easterly dipping Rudravaram thrust line. Igneous activity in NFB is represented by intrusive syenite and lamproite dykes. This study brings out the petrological and geochemical character of unreported intrusive dolerite dykes and sills within the Cumbum Formation of NFB. The dykes are undeformed, whereas the sills suffered D1 phase of deformation, although both are petrologically and geochemically similar. The rocks are highly fractionated sub-alkaline tholeiite, consist mainly of clino-pyroxene and plagioclase. Chondrite-normalised rare earth element (REE) plot shows enriched light rare earth element (LREE) and flat heavy rare earth element (HREE). A primitive mantle-normalised multi-element spider diagram shows troughs in Nb, Ta, Ti and Zr indicating subduction zone character. Tectonic discrimination plots indicate both within-plate and subduction zone environment of formation. Geochemical modelling also indicates enriched spinel peridotite as a probable source for dykes and sills. We suggest that the lithospheric mantle beneath NFB got enriched by subduction-derived fluid around 1.6 Ga, and in later stage, these dykes and sills were generated by extension of the enriched lithosphere at different phases in turn inheriting the subduction zone geochemical characters.