The Acite ophiolitic mélange represents a remnant of the Proto-Tethys forearc oceanic lithosphere. Two gabbros yield zircon ²⁰⁶Pb/²³⁸U ages of 510-512 Ma. The magmatic rocks are divided into three subtypes: (1) Mid-ocean ridge basalt (MORB) -like forearc basalts (FABs), (2) Low-titanium tholeiitic gabbros (LTGs), and (3) Normal calc-alkaline dacites (CADs). The FABs have comparatively higher TiO₂ concentrations (1.13–1.42 wt%) and show almost flat REE patterns. In the NMORB normalized trace element patterns, the FABs display flat distributions of high field strength element (HFSE). The ε_Nd(t) values of FABs range from +4.0 to +4.8. These features are similar to the composition of Izu-Bonin-Mariana forearc basalts (FABs). LTGs are characterized by higher contents of MgO (8.85–9.95 wt%) and lower concentration of TiO₂(0.29–0.50 wt%). They show LREE-depleted patterns, however having comparatively lower total REE contents than those of FABs. The ε_Nd(t) values of LTGs range from +7.4 to +8.4. These features show that LTG magmas originated from a progressively depleted mantle source. In contrast, the CADs having higher SiO₂ contents (63.58–70.92 wt%) have higher total REE contents. In the NMORB normalized trace element patterns, CADs are characterized by enrichment of LILEs and depletion of HFSEs. CADs have negative ε_Nd(t) values ranging from -9.8 to -10.5, which are likely suggestive of a crust-derived source. The rock association of FABs and LTGs, together with tectonic discrimination plots and regional data, suggest a forearc setting above the SSZ for Acite ophiolitic mélange. Graphical Abstract

The East Kunlun Orogen on the northern margin of the Tethyan orogenic system records a history of Gondwana dispersal and Laurasian accretion. Uncertainties remain regarding the detailed histories of northern branches of the Paleo-Tethys Ocean in East Kunlun Orogen (Buqingshan Ocean). Based on a synthesis of sedimentary, structural, lithological, geochemical, and geochronological data from the East Kunlun Orogen and adjacent regions, this paper discusses the spreading and northward consumption of the Paleo-Tethys Ocean during Late Paleozoic–Early Mesozoic times. The main evolutionary stages are: (1) during Carboniferous to Middle Permian, the Paleo-Tethys Ocean (Buqingshan Ocean) was in an ocean spreading stage, as suggested by the occurrence of Carboniferous MORB-, and OIB-type oceanic units and Carboniferous to Middle Permian Passive continental margin deposits; (2) the Buqingshan Ocean subducted northward beneath the East Kunlun Terrane, leading to the development of a large continental magmatic arc (Burhan Budai arc) and forearc basin between ~270–240 Ma; (3) during the late Middle Triassic to early Late Triassic (ca. 240–230 Ma), the Qiangtang terrane collided with the East Kunlun–Qaidam terranes, leading to the final closure of the Buqingshan Ocean and occurrences of minor collision-type magmatism and potentially inception of the Bayan Har foreland basin; (4) finally, the East Kunlun Orogen evolved into a post-collisional stage and produced major magmatic flare-ups and polymetallic mineral deposits between Late Triassic to Early Jurassic (ca. 230–200 Ma), which is possibly related to asthenospheric mantle upwelling induced by delamination of thickened continental lithosphere and partial melting of the lower crust. In this paper, we propose that the Wilson cycle-like processes controlled the Late Paleozoic–Early Triassic tectonic evolution of East Kunlun, which provides significant implications for the evolution of the Paleo-Tethys Ocean.

The subduction process of the Paleo-Tethyan Ocean is still under controversy. We report zircon U-Pb ages and geochemistry (major and trace elements, and Hf isotopic compositions) for the Xiangjiananshan granitic plutons in the eastern segment of the East Kunlun Orogenic Belt, northern Tibetan Plateau, to constrain the subduction-related granitoids in its petrogenesis and to reconstruct the subduction prosess of the Buqingshan-A’nyemaqing Ocean. Zircon U-Pb dating yields coeval ages of 246.6 ± 4.1 Ma for granodiorite and 246.4 ± 3.9 Ma for biotite monzogranite of the Xiangjiananshan granitoid pluton. The granitoids include granodiorites, biotite monzogranites, and pseudo-porphyritic monzogranites. The granitoids have high contents of SiO₂ (66.60–73.19 wt.%) and Al₂O₃ (13.28–16.38 wt.%), consistent with the high-K calc-alkaline series. The A/CNK ratios range from 0.98 to 1.02, indicating that the granodiorite is metaluminous and weakly peraluminous. The mafic microgranular enclaves (MMEs) in granodiorite and biotite monzogranite have a restricted SiO₂ range (57.24–60.03 wt.%), and have higher Fe₂O₃^T, and MgO contents than the host granitoids, as well as higher total rare earth element (REE) concentrations. All samples are depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Ti and P) and heavy rare earth elements (HREEs), and are enriched in large ion lithophile elements (LILEs; e.g., Rb, Nd and La) and light rare earth elements (LREEs). Zircons from the pseudoporphyritic monzogranite yield two-stage model ages of 1137.32 to 1588.12 Ma, together with ε_Hf(t) values from −5.01 to +2.11. These results indicate that the parental magma of pseudoporphyritic monzogranite was generated by partial melting of Paleoproterozoic to Mesoproterozoic rocks of the lower crust with contamination of ancient crustal material. Field and petrological observations, together with elemental and Hf isotopic data, indicate that the Xiangjiananshan granitic plutons was formed by magma mixing of mafic and felsic magmas. The felsic magma was derived from partial melting of the lower crust, while the mafic magma was derived from partial melting of an enriched lithospheric mantle metasomatized by slab-derived fluid due to the subduction of the Buqingshan-A’nyemaqing Ocean. Therefore, we propose that the northward subduction of the Buqingshan-A’nyemaqing ocean north to the Bayan Hara block under East Kunlun initiated in the Middle Permian (ca. 270 Ma), yielding calcium metaluminous arc magmatic rocks with I-type granite characteristics at East Kunlun, including the XG pluton in this study. Subsequently, the Buqingshan-A’nyemaqing ocean closed and resulted in collision of the Bayan Hara and East Kunlun blocks from 240 to 230 Ma.