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The early Mesozoic evolution of the Lhasa terrane, which represents a major component of the Himalayan–Tibetan orogen, remains highly controversial. In particular, geological units and events documented either side of the eastern Himalayan syntaxis (EHS) are poorly correlated. Here, we report new petrological, geochemical and geochronological data for cogenetic peraluminous S-type granites and metamorphic rocks (gneiss and schist) from the Motuo–Bomi–Chayu region of the eastern Lhasa terrane, located on the eastern flank of the EHS. Zircon U–Pb dating indicates that these units record both Late Triassic magmatic (216–206 Ma) and metamorphic (209–198 Ma) episodes. The granites were derived from a Paleoproterozoic crustal source with negative zircon εHf(t) values (–5·5 to –16·6) and TDM2 model ages of 1·51–1·99 Ga, and are interpreted to have formed by crustal anatexis of nearby metasediments during collisional orogeny and crustal thickening. The gneisses and schists experienced similar upper amphibolite-facies peak metamorphism and associated partial melting, followed by decompressional cooling and retrograde metamorphism. These rocks were buried to lower-crustal depths and then exhumed to the surface in a collisional orogenic setting during plate convergence. From comparison of these data with those for other metamorphic belts with similar grades and ages, and association of coeval granitic magmatism widespread in the central–east Lhasa terrane, we propose that the studied cogenetic magmatism and metamorphism in the Motuo–Bomi–Chayu region records Late Triassic accretion of the North Lhasa and South Lhasa terranes, which represents the first evidence of the Paleo-Tethys Ocean closure in this part of Asia. These data provide new constraints on the spatial and temporal evolution of the Paleo-Tethyan Wilson Cycle and provide a 'missing link' to correlate the geology and tectonic history of the Lhasa terrane continental crust on either side of the EHS. [ABSTRACT FROM AUTHOR]

The tectono-magmatic evolution of the Bangong-Nujiang Tethyan Ocean (BNO) plays a crucial role in understanding the Tibetan Plateau accretion before the Cenozoic India-Asia collision. The microcontinents and oceanic plateau recognized recently within the BNO provide important clues for constructing the subduction process between BNO and the Qiangtang terrane. In this contribution, we provide a comprehensive overview of the multidisciplinary information from magmatism, metamorphism, stratigraphy, geochronology, geochemistry, Hf isotopic data, ophiolite suites, and paleomagnetism to gain insights into the spatially and temporally related geodynamic processes of the BNO. A compilation of geochronological data indicates that the emplacement of the diverse igneous suites occurred mainly during three stages: stage 1 (190–150 Ma), stage 2 (130–110 Ma), and stage 3 (110–70 Ma). The igneous suites, including granitoids, mafic intrusions and volcanic rocks, formed two arcuate E–W trending belts that parallel the Bangong-Nujiang suture zone and are correlated to the double sided subduction of the BNO. We infer that the BNO initially opened at ~270Ma between the Lhasa terrane and Qiangtang terrane, although the relationship between the BNO and Longmu Tso-Shuanghu paleo-Tethyan ocean is still unclear. Age data related to metamorphism, arc magmatism, supra-subduction zoneophiolites, and sedimentary strata indicate that the northward subduction of the BNO started at ~190 Ma, followed by the subduction of microcontinents beneath the Qiangtang terrane at 195 Ma. In contrast, the subsequent southward subduction of the BNO started at ~170 Ma, resulting in the opening of the Shiquanhe-Namuco ocean. As indicated by the high Mg volcanic suite, intra-ocean subduction within the BNO also occurred during 170-145Ma, possibly related to the resistance from subduction of ocean plateau and slab retreat. Based on evidence from petrology, isotope geochronology, and isotopic geochemistry, we also infer that the magmatic lull during 145-130Ma is related to the slab subduction of ocean plateau beneath the southern Qiangtang subterrane. The closure of the BNO was most likely diachronous, which started as early as Late Jurassic and continued to the late Early Cretaceous (120-110Ma). • Initial opening of BNO at ~270 Ma and northward subduction at ~190 Ma. • Southward subduction started at ~170 Ma, leading to the opening of SNO. • Ocean plateau mechanism for intra-ocean subduction of BNO and magmatic lull during 145–130 Ma. • Diachronous closure of the BNO initiated at 150 Ma and the final closure during 110–100 Ma. [ABSTRACT FROM AUTHOR]