THERMOBAROMETRIC AND 40AR 39AR 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. Major element partitioning thermobarometry of pelitic rocks indicates temperatures of 750-830 K at depths of 14 +/- 3 km, consistent with the presence of kyanite, sillimanite, and andalusite schists in the area. Laser and resistance furnace Ar-40/Ar-39 analyses of hornblendes from intercalated amphibolites yield closure ages of 25 Ma. Overlap between the probable range of Ar closure temperatures for these hornblendes and the metamorphic temperatures estimated through thermobarometry suggests that Eohimalayan metamorphism in the Dinggye area occurred in Late Oligocene time, no more than about 10 million years before the main or ''Neohimalayan'' phase of metamorphism in Early to Middle Miocene time. Muscovite, biotite, and K-feldspar Ar-40/Ar-39 ages indicate an important episode of rapid cooling between 16 and 13 Ma, which is interpreted as a signature of tectonic denudation related to movement on N-dipping extensional structures of the South Tibetan detachment system.