Occurrence of Fluorapatite in Granitic Veins from the Kerala Khondalite Belt, Southern India

This paper presents the first description of fluorapatite in rocks of the Kerala Khondalite Belt in southern India. The fluorapatites occur either dispersed or aggregated in granitic and pegmatitic veins which intruded into high-grade metamorphic gamet-biotite gneisses. Microtextures indicate an exclusively magmatic origin for the fluorapatites and the strong zoning of F and OH points to a change in fluid/melt composition during apatite crystallization. The P-T conditions of melt crystallization are estimated to be in the range of 750-800°C and ≤5 kbar. The origin of the fluorapatite bearing melts is poorly known. It is suggested that they formed either through fluid-absent incongruent melting of a F-rich biotite or during a late stage of fractional crystallization of a granitic parent magma. Additionally, we report a single occurrence of a monazite-rich quartzmonzodiorite vein. Field observations indicate an emplacement subsequent to the fluorapatite bearing granites and pegmatites. The occurrence of magmatic andalusite and cordierite in this rock points to a crystallization at low pressures (P ≤ 3 kbar).