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The subduction of the Proto-Tethyan Ocean is still a controversial subject. Here we report zircon U-Pb ages, geochemistry, and Hf isotopic compositions of the zircon in the Mengkete quartz diorite in the Buqingshan–A'nyemaqen Tectonic Mélange Belt, which lies at the southern margin of the East Kunlun orogenic belt on the northern Tibetan Plateau, to constrain the petrogenesis of subduction-related granitoids and to reconstruct the evolution of the Buqingshan–A'nyemaqen Ocean. Zircons U-Pb dating yields coeval ages of 441–436 Ma for Mengkete quartz diorite. Mengkete plutons have variable SiO2 (56.63–65.22%) and high Al2O3 (16.09–17.79%) contents and aluminous saturation indexes (A/CNK ratios) ranging from 0.77 to 0.96, which define their metaluminous and medium-K2O calc-alkaline signatures. The plutons have a low total rare-earth element (REE) content (45.49–168.31 ppm) and slightly positive Eu anomalies (Eu/Eu* in the range of 0.96 to 1.32). They are also enriched in large-ion lithophile elements (LILEs), such as Rb, Th, and Ba, but are depleted in high-field-strength elements (HFSEs), such as Nb, Ta, Zr, Hf, and Ti. Their zircon εHf(t) values ranging from 7.79 to 13.02, and the two-stage Hf (TDM2) model ages are in the range of 1130–657 Ma. These geochemical signatures indicate that the Mengkete quartz diorite was derived from partial melting of the mafic Meso-Neoproterozoic lower crust during the northward subduction of the Proto-Tethyan Ocean. The evolution of the East Kunlun Belt can be divided into five stages from the Neoproterozoic to late Middle Triassic: (1) an opend Buqingshan‒A'nyemaqen ocean during the Neoproterozoic up to 516Ma; (2) the continuous expansion of the Buqingshan‒A'nyemaqen ocean during the period 516 to 441Ma, along with the beginning of northward subduction in the Late Cambrian; (3) further subduction from 441 to 400Ma, accompanied by the production of large volumes of arc magmatic rocks; (4) the long period during which the Buqinshan–A'nyemaqen Ocean existed (400–240 Ma); and (5) the final closure of the Buqinshan–A'nyemaqen Ocean in the late Middle Triassic. [ABSTRACT FROM AUTHOR]

The East Kunlun Orogenic Belt (E‐KOB) of the northern Tibetan Plateau is an accretionary orogenic belt, which has experienced complicated tectonic evolutionary processes of the Proto‐ to Palaeo‐Tethys Ocean. The Muztagh‐Buqingshan‐Anemaqen ophiolitic mélange zone (MBAM) is an accretionary complex related to the northward subduction of the Tethyan Ocean. Numerous intrusions outcropped in MBAM have witnessed the long‐term subduction‐accretionary process and thus site the key to comprehending the tectonic evolution of the E‐KOB, as well as the Tethyan Ocean. In this study, petrological, geochronological, and geochemical studies of the Marzheng diorites in the MBAM were carried out to explore the Early Palaeozoic tectonic evolution of the E‐KOB. Zircon U–Pb dating analysis suggests that the diorites were formed in the Early Ordovician at ca. 470 Ma. Furthermore, the εHf(t) values (5.3–14.1), Mg# values (39.7–51.2), and Nb/Ta (12.8–14.8), Zr/Hf (33.8–44.8), and Lu/Yb (0.15–0.16) ratios are generally close to those of depleted mantle‐derived rocks and the diorites were originated from a depleted asthenosphere mantle source. In addition, these diorites are characterized by enriched light rare earth elements (LREEs) (La and Nd) and depleted high‐field‐strength elements (HFSEs) (Nb, Ta, P, and Ti), similar to the fingerprints of arc‐related gabbroic‐dioritic rocks. This is also consistent with their high H2Omelt (6.0–8.3%) and oxygen fugacity (logfO2 = −14.3 to −13.5) features, indicating that they were formed in a subduction‐related setting. Our new investigations suggest that the Proto‐Tethys Ocean in the East Kunlun was still actively subducting northward during the Early Ordovician. [ABSTRACT FROM AUTHOR]