Subduction initiation of the Bangong–Nujiang Tethys Ocean, Tibetan Plateau

Abstract

• The northward subduction initiation of the meso -Tethys Ocean was diachronous from the east (∼240–230 Ma) to the west (213–194 Ma). • The collision and slab breakoff during the closure of the Paleo-Tethys Ocean were vital to the northward subduction initiation. • The southward subduction of the meso -Tethys Ocean was initiated no later than the Middle Triassic . • The southward subduction was attributed to the closure of the Paleo-Tethys Ocean and the spreading of the NeoTethys Ocean. The initiation of subduction is widely recognized as a critical process associated with the evolution of the meso -Tethys Ocean. Here, we present zircon U–Pb, Lu–Hf isotope, and whole-rock geochemical data for a suite of Triassic–Jurassic magmatic rocks from the central Tibetan Plateau, with the aim of gaining insights into the subduction initiation of the Bangong–Nujiang Tethys Ocean (BNTO). Early Triassic trachyandesite (248 Ma) from the South Qiangtang terrane resemble intraplate magma and probably resulted from magmatic activity at the passive continental margin during the BNTO opening. Late Triassic S-type granodiorite (220 Ma) and Middle Jurassic SSZ-type gabbro (∼165 Ma) from the South Qiangtang terrane may result from the northward subduction of the BNTO oceanic lithosphere. Jurassic granitoids (I and S type; 193–177 Ma) and metamorphic rocks (179–174 Ma) from the Amdo and Jiayuqiao subterranes are related to the northward subduction of the BNTO oceanic lithosphere. Middle–Late Jurassic diorites (164–162 Ma) from the Central Lhasa subterrane have geochemical features of continental margin arc magma with a sediment component, and these are derived from the southward subduction of the BNTO oceanic lithosphere. Finally, this Triassic–Jurassic magmatic suite of various geochemical features was combined with the results of previous studies, and we suggest that the initiation of northward BNTO subduction was probably diachronous from the east (∼240–230 Ma) to the west (213–194 Ma). This initiation resulted both from collision and slab breakoff during the closure of the Longmuco–Shuanghu–Lancangjiang Paleo-Tethys Ocean and considerable density and rheology contrasts between the thick oceanic lithosphere and continental lithosphere. The southward subduction was initiated no later than the Middle Triassic and was attributed to the closure of the Sumdo Paleo-Tethys Ocean and the spreading of the Indus–Yarlung Zangbo NeoTethys Ocean.