Geological setting of the black granite deposits (mafic dykes) in the parts of villupuram district, southern granulitic terrain, Tamilnadu, India

The study area forms the part of the Southern Granulitic terrain of the peninsular India. It is represented by charnockites and gneisses. These terrain was found intersected by the younger mafic dykes commonly known as Black granites. The economic importance and its massive nature of the rock brought name and fame to the locality all over the world. Based on the nature of the trend these dyke bodies were characterized into three groups namely Group – I, Dykes having trends NW-SE, WNW-ESE and E-W. Group – II, Dykes having trends NNW-SSE, N-S and NNE-SSW. Group –III, Dykes having trends NE-SW. Based on mineralogical composition they can be classified into two types namely dolerite and gabbro. These bodies are medium to coarse grained in nature with fine grained border zone at the contact with the country rocks. These dyke bodies exhibits typical spheroidal weathering. The rocks are essentially composed of plagioclase, clinopyroxene and opaques. Olivine and micro perthite constitute the accessory mineral phases while minor biotite, amphibole and quartz rarely occur as secondary components. All the samples exhibit ophitic to sub ophitic intergrowth of plagioclase and pyroxene.