Plutonic Rocks of the 1985 Tibet Geotraverse, Lhasa to Golmud

Two large east-trending granitic batholiths are exposed on the plateau of Central Tibet. In the southern Lhasa Terrane, north of the Zangbo Suture, the Gangdise Belt is a calc-alkaline composite batholith dominated by monzodiorites, tonalites, granodiorites and monzogranites. Trace elements indicate that strongly fractionated melts were emplaced at an active continental margin; deeper crustal levels of the batholith are exposed in the crustally-derived Nyainqentanglha orthogneiss. Along the northern edge of the plateau, a syn-tectonic calcic to calc-alkaline suite of tonalites, granodiorites and monzogranites forms the Kunlun batholith with posttectonic post-tectonic granites emplaced to the south. The Kunlun intrusions are derived from anatexis of a garnet-bearing source at intermediate crustal depths above an active or recently active continental margin. Between these two batholiths, a bimodal suite of metaluminous tonalite-granodiorite and peraluminous two-mica granite is exposed in the northern Lhasa Terrane, indicative of melting both in the upper crust and at deeper levels in the crust or upper mantle. This association suggests a post-collision setting.