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The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan plateau separates Gondwana‐ from Eurasia‐derived continental blocks and marks the main suture of the Paleo‐Tethys, as evidenced by a variety of oceanic basalt‐derived eclogites. However, it is uncertain whether the belt contains high‐pressure rocks derived from gabbro, which is a key component of oceanic ophiolite. Here, we present a study of newly discovered gabbroic eclogites from the CMOB. These eclogites preserve relic gabbroic crystals (diopside, bytownite/anorthite and ilmenite) that survived metamorphism and occur in the form of inclusions within porphyroblasts. The eclogites have positive εNd(t) values of +1 to +8 and have an affinity to N‐MORB, with positive Eu anomalies and no depletions in high‐field‐strength elements (e.g. Nb, Ta, Zr and Hf). Cumulate gabbros generated in a mid‐ocean ridge setting are possible protoliths for the studied samples. The eclogite facies mineralogy is defined by the assemblage of garnet + omphacite + kyanite + talc + phengite + rutile, which was followed by the post‐kinematic crystallization of winchite and clinozoisite, and a later symplectite assemblage (diopside + sodic plagioclase + calcic amphibole + clinozoisite). Phase equilibrium modelling, average P–T thermobarometry and conventional mineral geothermobarometry constrain the P–T conditions for the peak‐stage and initial post‐peak‐stage metamorphism and symplectite formation to 25.6–27.1 kbar/595–637°C, 15.3–17.9 kbar/563–605°C and 5.5–7.3 kbar/470–500°C, respectively, consistent with a subduction depth of 75–85 km. Metamorphic zircons yielded a Triassic mean U–Pb age of 223.7 ± 2.9 Ma, which is interpreted to record the early‐stage decompressive overprinting. The similar paragenetic sequences, mineral evolution, peak P–T conditions and P–T–t paths for the gabbro‐ and basalt‐derived eclogites in the CMOB indicate that these rocks formed in the Paleo‐Tethys subduction regime. The lack of deformation, and the cold and rapid subduction history, contributed to the local preservation of gabbroic minerals and igneous textures under high‐pressure conditions in the studied rocks. The gabbroic eclogites provide insights into the detailed metamorphic evolution during the burial–exhumation cycle of ophiolites in the Paleo‐Tethyan regime. [ABSTRACT FROM AUTHOR]

The Ailao Shan (ALS) metamorphic belt separates the Indochina Block to the west from the South China Block and is an important region for understanding the history of evolution of the Palaeo‐Tethys in relation to the collision of the Indian and Eurasian plates and the formation of the Tibetan Plateau. The multiple metamorphic events and P–T–t paths of the ALS belt remain debated. Here, we present the results of an integrated study of the petrography, mineral chemistry, phase equilibria modelling, and zircon U–Pb geochronology of high‐grade metasedimentary rocks from this belt. Abundant metamorphic zircon grains with mineral inclusions of Sil/Ky + Bt + Pl + Qz ± Mnz record two metamorphic events, the first one during the Triassic (227–220 Ma), although intense shearing overlapped during the Cenozoic and mineral assemblages of this thermal event were not preserved. The second metamorphic event occurred during the Palaeogene (49 –23 Ma) and involved three stages: (a) a peak high‐pressure (HP) granulite‐facies metamorphism (M1) with a mineral assemblage of Ky + Grt1 + Bt1 + Pl + Qz ± Kfs; (b) a postpeak near‐isothermal decompression (M2) characterized by Sil1–2 + Grt2 + Bt1 + Pl + Qz + Kfs + Ilm and the decompression reactions of Grt + Ky + Qz = 3An and Ky = Sil; and (c) a late decompressional cooling (M3) with a mineral assemblage of Sil3 + Grt3 + Bt2 + Pl + Qz + Mnz ± Kfs, followed by a series of hydration reactions. The metamorphic evolution occurred along a clockwise P–T–t path with peak P–T conditions of 13.5–14.0 kbar and 810–820°C. Our new data, combined with those from previous studies, reveal that the Diancang Shan–Ailao Shan Complex Belt is not a single suture that represents the closure of the Palaeo‐Tethyan Ocean during the Triassic, but that it also represents a typical continent–continent collisional orogenic belt that formed during the Eocene–Oligocene prior to the strong left‐lateral, ductile deformation along the Ailao Shan–Red River shear zone. [ABSTRACT FROM AUTHOR]

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau, is considered as the main suture zone marking the closure of the Palaeo‐Tethys Ocean between the Indochina and Sibumasu blocks. Here, we investigate the recently discovered retrograded eclogites from this suture zone in terms of their petrological, geochemical and geochronological features, with the aim of constraining the metamorphic evolution and protolith signature. Two types of metabasites are identified: retrograded eclogites and mafic schists. The igneous precursors of the retrograded eclogites exhibit rare earth element distribution patterns and trace element abundance similar to those of ocean island basalts, and are inferred to have been derived from a basaltic seamount in an intra‐oceanic tectonic setting. In contrast, the mafic schists show geochemical affinity to arc‐related volcanics with the enrichment of Rb, Th and U, and depletion of Nb, Ta, Zr, Hf and Ti, and their protoliths possibly formed at an active continental margin tectonic setting. Retrograded eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, white mica, lawsonite and rutile, and underwent five‐stage metamorphic evolution, including pre‐peak prograde stage (M1) at 18–19 kbar and 400–420°C, peak lawsonite‐eclogite facies (M2) at 24–26 kbar and 520–530°C, post‐peak epidote–eclogite facies decompression stage (M3) at 13–18 kbar and 530–560°C, subsequent amphibolite facies retrogressive stage (M4) at 8–10 kbar and 530–600°C, and late greenschist facies cooling stage (M5) at 5–8 kbar and 480–490°C. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb spot analyses of zircon show two distinct age groups. The magmatic zircon from both the retrograded eclogite and mafic schist yielded protolith ages of 451 ± 3 Ma, which is consistent with the ages of Early Palaeozoic ophiolitic complexes and ocean island sequences in the CMOB reported in previous studies. In contrast, metamorphic zircon from the retrograded eclogite samples yielded consistent Triassic metamorphic ages of 246 ± 2 and 245 ± 2 Ma, which can be interpreted as the timing of closure of the Palaeo‐Tethys Ocean. The compatible peak metamorphic mineral assemblages, P–T–t paths and metamorphic ages, as well as the similar protolith signatures for the eclogites in the CMOB and Longmu Co–Shuanghu suture (LCSS) suggest that the two belts formed part of a cold oceanic subduction system in the Triassic. The main suture zone of the Palaeo‐Tethyan domain extends at least 1,500 km in length from the CMOB to the LCSS in the Tibetan Plateau. The identification of lawsonite‐bearing retrograded eclogites in the CMOB provides important insights into the tectonic framework and complex geological evolution of the Palaeo‐Tethys. [ABSTRACT FROM AUTHOR]