Magmatic history of the northeastern Tibetan Plateau

Abstract

The northeastern margin of the Tibetan Plateau is underlain by the Qaidam and Qilian terranes, which consist primarily of mid‐Proterozoic through lower Paleozoic oceanic and arc‐type assemblages that have been accreted to the southern margin of the Tarim/Sino‐Korean craton. Most previous models suggest that these assemblages formed along a northeast dipping subduction system constructed along the margin of the Tarim/Sino‐Korean craton during early Paleozoic time. The main components are interpreted to have formed either as an archipelago of volcanic arcs and back arc basins, or as a broad expanse of accretionary complexes. Our geochronologic data support a model, suggested by Sobel and Arnaud [1999], in which the Qaidam and Qilian terranes are separated from the Tarim/Sino‐Korean craton by a mid‐Paleozoic suture that closed along a southwest dipping subduction zone. The basement to these terranes consists of oceanic assemblages that were amalgamated into a coherent crustal fragment prior to emplacement of ∼920–930 Ma granitoids. Early Paleozoic arc‐type magmatism occurred between ∼480 and ∼425 Ma, apparently sweeping southwestward across much of the Qilian and Qaidam terranes. Accretion‐related magmatism along the inboard margin of the Qilian terrane occurred between ∼423 Ma and ∼406 Ma. Following Silurian‐Devonian accretion, the region has experienced late Paleozoic and Mesozoic uplift and erosion and has been severely overprinted by Tertiary thrusting, uplift, and strike‐slip motion along the Altyn Tagh fault. Correlation of geologic features and magmatic histories between the Altun Shan and the Nan Shan suggests that the eastern Altyn Tagh fault has a total left‐lateral offset of ∼375 km.