PRELIMINARY U-PB ZIRCON AGES OF HIGH-GRADE ROCKS IN SOUTHERN KERALA, INDIA

South Kerala in southwestern India offers interesting khondalite-charn ockite assemblages suitable for detailed multidisciplinary case studie s on granulite petrogenesis. Arrested charnockite formation is also ch aracteristic of the terrain. Absolute ages of the khondalites are poor ly constrained; we have obtained limited U-Pb zircon data for various rock types of south Kerala khondalite belt. Zircons from khondalite se nsu-stricto and garnet-biotite gneiss of possible sedimentary origin g ive a concordia upper-intercept apparent age of ca. 2200 Ma, suggestin g that the khondalites contain detrital material that is atleast Early Proterozoic in age; the data yield a lower intercept apparent age of ca. 400-600 Ma, but it is not clear if this represents new zircon grow th and episodic Pb loss at that time (preferred) or is an artifact of normal diffusive loss of radiogenenic Pb. Zircons from retrograded gra nulitic gneisses have zircon upper-intercept apparent ages of ca. 1550 Ma, indicating either a younger protolith age or, more probably, grea ter radiogenic Pb loss during subsequent migmatite-grade metamorphism that caused retrogression of the granulite. Since all the zircon popul ations are complex, more detailed analyses will need to be pursued to resolve age relationships of these rocks with greater certainty. Monaz ite from a retrograded granulitic gneiss, yields a concordant age of c a. 520 Ma; this age is probably related to late Proterozoic-Early Camb rian regional thermal event in south Kerala and is correlative with a similar event in Sri Lanka.