Widespread evidence for ultrahigh-pressure (UHP) metamorphism is reported in the Dulan eclogitebearing terrane, the North Qaidam-Altun HP-UHP belt, northern Tibet. This includes: (1) coesite and associated UHP mineral inclusions in zircon separates from paragneiss and eclogite (identified by laser Raman spectroscopy); (2) inclusions of quartz pseudomorphs after coesite and polycrystalline K-feldspar + quartz in eclogitic garnet and omphacite; and (3) densely oriented SiO 2 lamellae in omphacitic clinopyroxene. These lines of evidence demonstrate that the Dulan region is a UHP metamorphic terrane. In the North Dulan Belt (NDB), eclogites are characterized by the peak assemblage Grt + Omp + Rt + Phn + Coe (pseudomorph) and retrograde symplectites of Cpx + Ab and Hbl + Pl. The peak conditions of the NDB eclogites are P ¼ 2.9-3.2 GPa, and T ¼ 631-687°C; the eclogite shows a near-isothermal decompression P-T path suggesting a fast exhumation. In the South Dulan Belt (SDB), three metamorphic stages are recognized in eclogites:(1) a peak eclogite facies stage with the assemblage Grt + Omp + Ky + Rt + Phn at P ¼ 2.9-3.3 GPa and T ¼ 729-746°C; (2) a high-pressure granulite facies stage with Grt + Cpx (Jd < 30) + Pl (An24-29) + Scp at P ¼ 1.9-2.0 GPa, T ¼ 873-948°C; and (3) an amphibolite facies stage with the assemblage Hbl + Pl + Ep ⁄ Czo at P ¼ 0.7-0.9 GPa and T ¼ 660-695°C. The clockwise P-T path of the SDB eclogites is different from the near-isothermal decompression P-T path from the NDB eclogites, which suggests that the SDB was exhumed to a stable crustal depth at a slower rate. In essence these two sub-belts formed in different tectonic settings; they both subducted to mantle depths of around 100 km, but were exhumed to the Earth's surface separately along different paths. This UHP terrane plays an important role in understanding continental collision in north-western China.

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The Central Orogenic Belt (COB) of China is a major continental collision zone that contains extensive outcrops of deeply subducted and exhumed rocks at both the eastern and the western end of the belt. Here we report discovery of microdiamonds from both eclogites and felsic gneisses in the North Qinling zone in the central portion of the COB. This discovery demonstrates that the country rocks of continental affinity shared in the ultra‐high‐pressure metamorphic (UHPM) event and provides a bridge connecting the two previously recognized UHPM terranes, thereby establishing the existence of a UHPM belt extending more than 4000 km. Geochronological dating yields Early Palaeozoic ages in the west and Early Mesozoic ages in the east, recording two separate continental collisions overprinted within the COB. Occurrence of UHP metamorphism during recurrent continental collision here and in the Alps suggests that deep subduction of continental material during such collisions is probably common rather than exceptional, with significant implications for processes of plate tectonic reorganization and mantle mixing over time.

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