Metamorphic Rocks of the 1985 Tibet Geotraverse, Lhasa to Golmud

Two examples of uplifted basement have been studied in the Lhasa Terrane of the Tibetan Plateau. The Nyainqentanglha orthogneisses are bounded by staurolite--garnet schists to the north which record prograde metamorphism at 5.0 $\pm $ 1.3 kbar, 610 $\pm $ 70 degrees C Garnet--sillimanite xenoliths within the orthogneiss suggest that peak temperatures reached at least 700 $\pm $ 70 degrees C at 5.1 $\pm $ 2.5 kbar. These P/T fields reflect high T/low P metamorphism during Eocene subduction, and indicate that the syntectonic Nyainqentanglha orthogneiss was emplaced at depths greater than 10 km. Sillimanite-bearing assemblages from the Amdo gneisses in the northern Lhasa Terrane provide evidence of crustal anatexis at temperatures > 680 degrees C. This event is poorly constrained in time but is probably Cambrian or earlier. Within the Kunlun Terrane, biotite and garnet isograds north of the Xidatan Fault indicate an increase in metamorphic grade from north to south, reaching peak metamorphism at 470 $\pm $ 30 degrees C, 4.3 $\pm $ 1.5 kbar synchronous with the emplacement of the Triassic batholith. Regional metamorphism was followed by uplift of at least 2 km before emplacement of post-tectonic, early Jurassic granites.