Volcanic rocks of the 1985 Tibet Geotraverse: Lhasa to Golmud

Abstract

Volcanic rocks encountered during the Tibet Geotraverse have been studied in the field, in thin section and by major and trace element geochemistry in order to determine their most probable original eruptive environment. Rocks from a total of eleven distinct volcanic provinces were studied in this way. They provide evidence for: an active continental margin or post-collision province of probable Devonian/early Carboniferous age in the northern Kunlun mountains; an active continental margin of late Carboniferous age in the southern Lhasa Terrane; Permian continental rifts in the central Qiangtang and central Kunlun Terranes; Triassic volcanic arcs in the southern Lhasa and northern Qiangtang Terranes; a Triassic active continental margin dyke swarm in the northern Kunlun mountains; a Jurassic post-collision or back-arc rifting province in the southern Qiangtang Terrane; a Jurassic island arc in the northern Lhasa Terrane; a Cretaceous post-collision province in the northern Lhasa Terrane possibly extending in to the southern Qiangtang Terrane; and a Palaeogene active continental margin in the southern Lhasa Terrane. An Oligocene trachyte plug in the northern Qiangtang Terrane was the only evidence encountered during the Geotraverse of volcanism post-dating the Palaeogene India-Eurasia collision. However, the composition of this plug, coupled with new and published analyses from Miocene volcanics in the southern Lhasa terrane and from the Pliocene-Recent volcanic province of northwest Tibet, places important constraints on models for post-collision underplating of Tibet by continental lithosphere: any underplating is likely to have been (a ) much later than the start of collision, ( b ) directed beneath Tibet from the north as well as the south, and ( c ) limited in extent.