Isotope Geochemistry of the 1985 Tibet Geotraverse, Lhasa to Golmud

Geochronological data from the Golmud--Lhasa section across the Tibetan Plateau indicate progressively younger periods of magmatism from north to south associated with successively younger ocean closures. Pre-collision Eocene magmatism (50-40 Ma) exposed along the southern margin of the Lhasa Terrane in the Gangdise Belt resulted from anatexis of mid-Proterozoic crust ($\sim $ 1000 Ma) at depths greater than 10 km, but at higher crustal levels subduction-related intrusions were predominantly mantle-derived with $\sim $ 30% crustal assimilation. Intrusions from the northern Lhasa Terrane are early Cretaceous in age (130-110 Ma). These form a bimodal suite comprised of two-mica granites derived from anatexis of Mid-Proterozoic crust and of biotite-hornblende granodiorites from about 60% crustal assimilation by mantle magmas above a post-collision subduction zone. They place a minimum constraint on collision between the Lhasa and Qiangtang Terranes of 130 Ma. Granite magmatism from the Kunlun Mountains is late Permian--early Jurassic in age (260-190 Ma). The Kunlun batholith represents reworked mid-Proterozoic crust (1400-1000 Ma) at an active continental margin from 260-240 Ma. Post-tectonic granites were emplaced in a post-collision setting (200-190 Ma). Collision between the Qiangtang and Kunlun Terranes is dated as end-Triassic. Nd model ages of sediments from across the plateau record uplift and erosion of young source regions throughout the Phanerozoic confirming that the Tibetan Plateau is the site of multiple continental collision through time. Phanerozoic magmagenesis throughout the plateau requires considerable crustal reworking and limited crustal growth which suggests thickened continental crust in the region may predate the most recent Eocene collision