THERMOBAROMETRIC AND AR-40 AR-39 GEOCHRONOLOGICAL CONSTRAINTS ON EOHIMALAYAN METAMORPHISM IN THE DINGGYE AREA, SOUTHERN TIBET

Mineral assemblages in the Dinggye area of southern Tibet (28-degrees- N; 88-degrees-E) provide new insights regarding the poorly understood ''Eohimalayan'' metamorphic event in the eastern Himalayan orogen. Maj or element partitioning thermobarometry of pelitic rocks indicates tem peratures of 750-830 K at depths of 14 +/- 3 km, consistent with the p resence of kyanite, sillimanite, and andalusite schists in the area. L aser and resistance furnace Ar-40/Ar-39 analyses of hornblendes from i ntercalated amphibolites yield closure ages of 25 Ma. Overlap between the probable range of Ar closure temperatures for these hornblendes an d the metamorphic temperatures estimated through thermobarometry sugge sts that Eohimalayan metamorphism in the Dinggye area occurred in Late Oligocene time, no more than about 10 million years before the main o r ''Neohimalayan'' phase of metamorphism in Early to Middle Miocene ti me. Muscovite, biotite, and K-feldspar Ar-40/Ar-39 ages indicate an im portant episode of rapid cooling between 16 and 13 Ma, which is interp reted as a signature of tectonic denudation related to movement on N-d ipping extensional structures of the South Tibetan detachment system.