GEOCHEMISTRY, AGE AND ORIGIN OF ALKALINE AND ULTRAMAFIC ROCKS OF SALEM, TAMIL-NADU, SOUTH-INDIA

Alkaline rocks spatially associated with magnesite bearing dunites occ ur in the vicinity of Salem, Tamil Nadu. This association occurs withi n the gneiss-charnockite terrrain and is confined to the intersection of two deep seated lineaments. Based on mineralogy the alkaline rocks are recognised as missourite, shonkinite and syenite. The alkaline roc ks have high Mg numbers and are enriched both in compatible and incomp atible trace elements. Majority of the samples show ultrapotassic char acter and are enriched in REE (30-200 ppm) with LREE enrichment and no Euanamoly. The Rb-Sr isochron gives an age of 808+/-18 Ma and shows f airly high Sr initial ratio of 0.705+/-1. The unusual chemistry of the se rocks suggests that they are derived from an enriched mantle source , with the dunite representing the mantle residua