Generation of leucogranites in the Kerala Khondalite belt, southern India

Psammitic and pelitic gneisses of the Kerala Khondalite belt in southern India were subjected to granulite-facies metamorphism and migmatisation through fluid-absent biotite dehydration-melting which led to the generation of garnet-bearing leucogranites. Geochemical investigations provide support that the leucogranites were generated from gneisses compositionally similar to those into which they were emplaced. The intrusion of apatite-rich granites subsequent to the garnet-bearing leucogranites is recognized predominantly in leptynitic gneisses. A comparison of the chemical composition of both garnet- and apatite-bearing varieties displays a systematic shift to intermediate compositions (increase in CaO, FeO, LREE, Zr, Y; decrease in SiO(2) and delta(18)O) which coincides with higher modal abundances of apatite, garnet and biotite and decreasing contents of quartz and alkalifeldspar. The observation that apatite occurs in aggregates together with biotite and/or garnet as well as the extreme concentrations of FeO, LREE and Zr in some of the apatite-beraing granites which clearly exceed the maximum solubility values for these elements in granitic melts indicates that dehydration-melting alone, as it has been proposed for the generation of garnet-bearing leucogranites, cannot have been responsable for generation of apatite-rich leucogranites but that additional processes like crystal entrainment and accumulation or liquid removal must also have played a role. (C) 1999 Elsevier Science Ltd. All rights reserved.