Mineralogy and Geochemistry of the Carbonatites of the Sevathur and Samalpatti Complexes, Tamil Nadu

The Proterozoic carbonatites and associated pyroxenites and syenites of Tamil Nadu are emplaced in the Precambrian gneissic rocks. The carbonatite complexes are located along a major NE trending lineament. In Sevathur, dolomitic carbonatite is predominant over sovite and ankeritic carbonatite whereas in Samalpatti, sovite and silico-sovite form the major bulk of carbonatite mass with small dykes of dolomitic and ankeritic carbonatites. In different types of carbonatites, in addition to calcite, dolomite and ankerite, phlogopite, amphibole, magnetite, and apatite are found in varying amounts between 2% and 10% with accessory amounts of pyrochlore, perovskite, monazite, NbiImenorutile, zircon, baddeIeyite, pyrite, ilmenite, galena and thorite. Both mica and amphibole show change in composition from Fe-rich in syenite and sovite to Mg-rich in the dolomitic carbonatite. The carbonatites are enriched in Ba, Sr, Nb, REE's, Th and U in comparison to the primitive mantle values. Pyroxenites are characterized by high abundances of Mg, Cr and Ni and, low abundances of Nb and Zr. The petrographic evidence, with globules silicate rocks surrounded by carbonatitic matrix is sufficiently clear to suggest a genetic link between syenite and carbonatite by liquid immiscibility.