Geochemistry and Petrogenesis of Shonkinite and Associated Alkaline Rocks of Tiruppattur Carbonatite Complex, Tamil Nadu

Abstract Shonkinite occurring in the carbonatite complex of Tiruppattur, Tamil Nadu is essentially composed of equal proportions of anorthoclase and augite with accessories of olivine, hastingsite, biotite, apatite and magnetite. The carbonatite complex comprises alkaline rocks emplaced in two adjacent structural basins of Sevvattur and Jogipatti. The chemical composition of shonkinite is intermediate between melanocratic rocks, alkali syenites and soda-rich alkali syenites of the Sevvattur structural basin and highly potassic alkaline rocks of the Jogipaui structural basin. Both these basins constitute the carbonatite complex of Tiruppattur. The study of geochemistry of clinopyroxenes in the co-magmatic alkaline series of the carbonatite complex reveals that both the groups of alkaline rocks were derived from shonkinitic parent magma by convection current aided differentiation and fractionation of mafic cumulates with intermittent immiscible separations of carbonate and silicate liquids.