Carbonatite Complex near Borra, Visakhapatnam District Andhra Pradesh

Abstract

The carbonate rocks near Borra, hitherto considered as crystalline limestones forming part of the khondalite group of metasediments of the Eastern Ghats mobile belt, have been identified as carbonatites. Genetically associated rocks include pyroxenite and syenite, characteristic of alkalic carbonatite complexes. Phlogopitization, local domingup and other features characterstic of forceful intrusion of carbonatites have been noticed. Characteristic rare-earths include Ce and La. The rare earth mineral identified is bastnaesite. The typical zirconium mineral is badleyite. Relative rarity of magnetite and pyrochlore makes the complex' atypical'. The Borra carbonatite is the oldest known occurrence in the country; the K-Ar dating of the phlogopite has given an age of 1490± 50 million years. It is surmised that the apatite-magnetite veins associated with a possible carbonatite at Kasipatnam and the Borra carbonatite are localised along a deep seated NW-SE fault system in the Eastern Ghats mobile belt.