The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is an important link between the Longmu Co–Shuanghu suture (LCSS) in the northern Tibetan Plateau and the Chiang Mai–Inthanon and Bentong–Raub sutures in Thailand and Peninsular Malaysia. These belts and sutures are generally regarded as containing the remnants of the oceanic crust of the Palaeo-Tethys that formed by seafloor spreading as a result of the separation of Gondwana- and Eurasia-derived blocks during the Middle Cambrian. In this paper we report the first discovery of abundant unaltered and retrograde eclogites that occur as irregular lenses and blocks in metasedimentary rocks of the CMOB, and these eclogites form an elongate and almost north–south-trending high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belt that is ∼200 km long and ∼50 km wide. The newly discovered phengite/talc/epidote–glaucophane eclogites, lawsonite–talc–phengite eclogites, dolomite/magnesite–kyanite eclogites and phengite–kyanite-bearing retrograde eclogites have enriched (E-) and normal mid-ocean ridge basalt (N-MORB)-like affinities and mainly positive as well as some negative whole-rock ε_Nd values (–4·34 to +7·89), which suggest an enriched and depleted oceanic lithosphere source for their protoliths. Magmatic zircons separated from the epidote–glaucophane, magnesite–kyanite and (phengite–kyanite-bearing) retrograde eclogites gave protolith ages of 317–250 Ma, which fit well within the time frame of the opening of the Palaeo-Tethys during the Middle Cambrian and its closure during the Triassic. Abundant metamorphic zircons in the eclogites indicate a Triassic metamorphic event related to the subduction of the Palaeo-Tethys oceanic crust from 235 to 227 Ma. Taking into account previous isotopic age data, we now establish the periods of Early–Middle Triassic (246–227 Ma) and Late Triassic (222–209 Ma) as the ages of subduction and exhumation of the Palaeo-Tethyan oceanic crust, respectively. Thermodynamic modelling revealed that the eclogites record distinct HP–UHP peak metamorphic conditions of 23·0–25·5 kbar and 582–610 °C for the phengite–glaucophane eclogites, 24·0–25·5 kbar and 570–586 °C for the talc–glaucophane eclogites, 29·0–31·0 kbar and 675–712 °C for the dolomite–kyanite eclogites, and 30·0–32·0 kbar and 717–754 °C for the magnesite–kyanite eclogites. These P–T estimates and geochronological data indicate that the Palaeo-Tethys oceanic slab was subducted to different mantle depths from 75 km down to 95 km, forming distinct types of eclogite with a variety of peak eclogite-facies mineral assemblages. The eclogites consistently record clockwise metamorphic P–T–t paths characterized by a heating–compression prograde loop under a low geothermal gradient of 5–10 °C km^–1, indicating the rapid subduction of cold oceanic crust at a rate of 4·5–6·0 km Ma^–1, followed by isothermal or cooling–decompressive retrogression and exhumation at an average rate of 3·2–4·2 km Ma^–1. The newly discovered eclogites of the CMOB with their signatures of ocean-crust subduction are petrologically, geochemically and geochronologically comparable with those of the LCSS, providing powerful support for the idea that a nearly 2000 km long HP–UHP eclogite belt extends from the northern Tibetan Plateau to the southeastern Tibetan Plateau, and that it represents the main boundary suture of the Palaeo-Tethyan domain. These results have far-reaching implications for the tectonic framework and complex metamorphic evolution of the Palaeo-Tethyan domain.

中文翻译：

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在青藏高原东南部发现了一条新的HP-UHP榴辉岩带：追溯了古特提斯缝合带的延伸

青藏高原东南部的长宁－孟连造山带（CMOB）是青藏高原北部的龙木科－双湖缝合线（LCSS）与泰国和马来西亚半岛的清迈－茵他侬缝合线和本通－劳布缝合线之间的重要纽带。这些带和缝合线通常被认为是古特提斯洋壳的残余物，该残余物是由于中寒武纪期间冈瓦纳和欧亚大陆衍生的块体分离而导致海底扩散而形成的。在本文中，我们报告了首次发现大量未改变和逆行的榴辉岩，它们以不规则晶状体和块体的形式出现在CMOB的沉积岩中，这些榴辉岩形成了细长的，几乎是南北向的高压（HP）-超高压高压（UHP）变质带，长约200 km，宽约50 km。_钕值（–4·34到+ 7·89），表明它们的原始岩体富集和枯竭。岩浆锆石与附生辉石-葡甲硫烷，菱镁矿-蓝晶石和（辉石-蓝晶石）逆冲榴辉岩分离，原生石年龄为317-250 Ma，非常适合中寒武纪古特提斯开放时期。及其在三叠纪时期的关闭。榴辉岩中大量的变质锆石表明是三叠纪变质事件，与古特提斯洋壳从235 Ma到227 Ma的俯冲有关。考虑到以前的同位素年龄数据，我们现在将早中三叠世（246-227 Ma）和晚三叠纪（222-209 Ma）的时期分别确定为古特提斯洋壳俯冲和掘出的年龄。热力学模型表明，榴辉岩记录的辉石-葡甲壳榴辉岩的HP-UHP峰值变质条件分别为23·0-25·5 kbar和582–610°C，24·0-25·5 kbar和570-586°C对于滑石-葡甲壳榴辉岩，白云岩-蓝晶岩榴辉岩为29·0-31·0 kbar和675-712°C，对于菱镁矿-蓝晶岩榴辉岩为30·0-32·0 kbar和717-754°C。这些P – T估计和年代学数据表明，古特提斯洋板俯冲到从75 km到95 km的不同地幔深度，形成了不同类型的榴辉岩，并形成了各种峰榴辉岩相矿物组合。榴辉岩始终记录着顺时针变质P – T – t路径，其特征是在5–10°C km ^–1的低地热梯度下，加热-压缩前进循环，表明冷洋壳以4·5的速度快速俯冲。 –6·0 km Ma ^–1，随后以等温或冷却-压缩逆行和回火，平均速度为3·2–4·2 km Ma ^–1。CMOB的新发现的洋壳具有俯冲作用，在岩石学，地球化学和地质年代学上与LCSS的岩石学，地球化学和地质年代学相当，为以下观点提供了强有力的支持：从藏北北部延伸了将近2000 km长的HP–UHP榴辉岩带。青藏高原东南部的高原，它代表了古特提斯地区的主要边界缝合线。这些结果对古构造-特提斯构造域的构造框架和复杂的变质作用具有深远的意义。