Subduction of the Bangong Meso-Tethys and collision between the Lhasa and Qiangtang blocks were important for the growth of Tibetan crust, as well as the development of super-large porphyry copper deposits in central Tibet. However, the initiation and closure timing, and nature and structure of the Bangong Meso-Tethys are still poorly constrained. Petrology, geochronology, and geochemistry of the Rebang Co and Julu ophiolites in the western part of the suture zone are used to constrain the tectonic history and structure of the Bangong Meso-Tethys. These two suites consist of ultramafic rocks, gabbro and diabase dikes, pillow basalts, and radiolarian cherts. Zircon U–Pb dating of zircon grains of gabbros yields well-defined weighted mean ages of 161.5 ± 1.5 Ma and 103.8 ± 3.9 Ma for the Rebang Co and Julu suites, respectively. Both of ophiolitic suites have mid-oceanic ridge basalt and island-arc tholeiite affinities, similar to back-arc basin crust. We suggest these suites formed above an intra-oceanic subduction zone at least during Middle Jurassic through mid-Cretaceous times. Along with coeval subduction beneath the southern Qiangtang margin, as well as fore-arc basin development, the Bangong Meso-Tethys has a complicated structure and history. Clearly, subduction in the Bangong Meso-Tethys was still active during mid-Cretaceous time and the Meso-Tethys did not close until Late Cretaceous time.

The nature of the Namco–Renco ophiolites in the northern Lhasa subterrane is widely disputed. To investigate their formation age, petrogenesis, and tectonic setting, the harzburgites, basalts, and metagabbros of the Namco ophiolite and the harzburgites, lherzolites, gabbros, and diabasic dikes of the Renco ophiolite were selected for whole-rock geochemical and zircon U-Pb dating and in situ Lu-Hf isotopic analyses. The geochemical and geochronological data indicate that the Namco metagabbros were generated at 178.0 ± 2.9 Ma, along with the Namco–Renco peridotites formed in the initial stage of a continental margin basin; whereas the Renco gabbros were developed at 149.7 ± 1.6 Ma, along with the Renco diabasic dikes and Namco basalts formed later in a mature back-arc basin. The Namco–Renco ophiolites were derived from a depleted mantle source with involvement of minor older crustal materials. Combined with the regional geological background, the Namco–Renco ophiolites were likely formed mainly associated with the southward subduction of the Bangong–Nujiang oceanic lithosphere beneath the Lhasa terrane. This study provides new constraints on the formation ages of the Namco–Renco ophiolites and the tectonic evolution of the Namco–Renco Ocean.

The Jurassic–Early Cretaceous Yilashan mafic–ultramafic complex is located in the middle part of the Bangong–Nujiang suture zone, central Tibet. It features a mantle sequence composed of peridotites and a crustal sequence composed of cumulate peridotites and gabbros that are intruded by diabases with some basalts. This article presents new whole-rock geochemical and geochronological data for peridotites, gabbros, diabases and basalts to revisit the petrogenesis and tectonic setting of the Yilashan mafic–ultramafic complex. Zircon laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) U–Pb ages of three diabase samples are 169.6 ± 3.3 Ma, 132.5 ± 2.5 Ma, and 133.6 ± 4.9 Ma, respectively. These ages together with previous studies indicate that the Yilashan mafic–ultramafic complex probably formed during the Jurassic–Early Cretaceous. The peridotites exhibit nearly U-shaped REE patterns and are distinct from abyssal peridotites. The diabase and basalt samples show arc features with selective enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILEs; e.g. Rb, U, and Sr) and depletion in high field strength elements (HFSEs; e.g. Nb, Ta, and Ti). The gabbro samples display cumulate features with selective enrichment in LILEs (e.g. Rb, Ba, and Sr) but depletion in LREEs and HFSEs (e.g. Nb, Zr, and Ti). Combing the positive ε_Nd(t) values (+6.1 to +10.0) and negative zircon ε_Hf(t) values (–16.5 to –11.7 and –13.6 to –0.4) with older Hf model ages for the mafic rocks, these signatures suggest that the Yilashan mafic and ultramafic rocks likely originated from an ancient lithospheric mantle source with the addition of asthenospheric mantle materials and subducted fluids coupled with limited crustal contamination in a continental arc setting as a result of the southward subduction of the Bangong–Nujiang Tethys Ocean beneath the Lhasa terrane during the Jurassic–Early Cretaceous.

The Mesozoic Xigaze ophiolite is a key to understanding the tectonic evolution of the Yarlung Zangbo suture zone. Although many studies have been reported, the formation age and petrogenesis of the Xigaze ophiolite remain controversial. In this paper, new geochronological and geochemical data for mafic dikes (diabase, dolerite), lavas, and gabbros of the Xigaze ophiolite are provided to constrain the origin of the Xigaze ophiolite. Combined with previous studies, three new zircon U–Pb ages of samples from two gabbro and one dolerite samples show that the Xigaze ophiolite was produced at two distinct stages of 174–149 Ma and 137–123 Ma. Whole-rock geochemical data indicate that these rocks exhibit N-MORB-like features, but the gabbros are more depleted in trace elements and belong to cumulates. Geochemical characters, combined with their positive ε_Nd(t) values (+3.2 to +9.6), suggest that these samples originated from depleted mantle sources with minor influence of slab-derived fluids. Considering the previous studies on the Yarlung Zangbo suture zone, the Xigaze ophiolite was likely generated in an active continental margin fore-arc basin with a multistage model associated with the northward subduction of the Yarlung Zangbo Neo-Tethys Ocean beneath the Lhasa terrane. The Middle–Late Jurassic ophiolitic massifs (174–149 Ma) were produced as the result of slab rollback and were followed by subsequent slab break-off at ~ 150 Ma. The fore-arc lithosphere may be frozen at ~150–137 Ma, consistent with the termination of the Gangdese arc magmatism during this period. The Early Cretaceous ophiolitic massifs (137–123 Ma) were developed in relation to the reinitiation of the Neo-Tethyan oceanic lithosphere subduction, the retreat of the subduction zone, and the creation of a fore-arc basin with strong hyperextension in a new cycle.