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Understanding the relationship between ultra-high-pressure (UHP) metamorphic rocks and their surrounding country rocks is crucial for interpreting the tectono-metamorphic dynamics within orogenic belts. This study focuses on the ChangningMenglian orogenic belt in Southeast Tibet to delineate the subduction history of the Paleo-Tethys Ocean. We integrate garnet SmNd and LuHf dating, rutile/zircon UPb geochronology, phase equilibria modeling, and thermobarometry to examine high-pressure eclogites and surrounding metasediments. Pseudosection modeling and conventional thermobarometry suggest a clockwise pressure-temperature (PT) path for the eclogite, from ~1.4 GPa/~505530 C to ~2.42.8 GPa/~600640 C, followed by decompression to <~0.7 GPa. The micaschists reached peak conditions of 2.02.2 GPa and 570620 C. LuHf dating yields ages of around 236241 Ma for the eclogites and 249251 Ma for the micaschists. SmNd dating indicates a similar age range for the eclogites (236242 Ma) but yields slightly younger ages for the micaschists (~240 Ma). These results imply a brief period of garnet growth in the eclogites, suggested by Rayleigh-fractionation-style Lu zoning and a rimward increase in Sm concentration within garnet. In contrast, the micaschists exhibit a more prolonged garnet growth period of approximately 10 Myr. The comparable peak metamorphic conditions and high-pressure metamorphic ages in both rock types suggest they underwent a concurrent subduction-exhumation cycle. Magmatic zircon UPb dating establishes the protolith age of the metabasaltic rocks at ca. 247 Ma. The multi-mineral geochronological data indicate a rapid transition from oceanic divergence to convergence within the orogenic belt around 250 Ma. These findings, along with previous geochronological data on continental subduction, point to a brief ~20 Myr cycle involving the formation, deep burial (exceeding 75 km), and subsequent ascent to the shallow crust of the Paleo-Tethys oceanic rocks. This evidence supports a continuous transition from oceanic to continental subduction within the region.
Zhou et al. (Fri,) studied this question.