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 carb onatite complexes are located along a major NE trending lineament. In Sevathur, dolomitic carbonatite is predominant over sovite and ankerit ic carbonatite whereas in Samalpatti, sovite and silico-sovite form th e major bulk of carbonatite mass with small dykes of dolomitic and ank eritic carbonatites. In different types of carbonatites, in addition t o calcite, dolomite and ankerite, phlogopite, amphibole, magnetite, an d apatite are found in varying amounts between 2% and 10% with accesso ry amounts of pyrochlore, perovskite, monazite, Nb-ilmenorutile, zirco n, baddeleyite, pyrite, ilmenite, galena and thorite. Both mica and am phibole 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, Grand Ni and, low abundances of Nb aid Zr. The petrographic evidence, with g lobules silicate rocks surrounded by carbonatitic matrix is sufficient ly clear to suggest a genetic link between syenite and carbonatite by liquid immiscibility.