Metamorphic P–T-t path and tectonic implications of the eclogite from Changning–Menglian orogenic belt, southeastern Tibetan Plateau

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 Changning–Menglian orogenic belt in Southeast Tibet to delineate the subduction history of the Paleo-Tethys Ocean. We integrate garnet Sm–Nd and Lu–Hf dating, rutile/zircon U–Pb geochronology, phase equilibria modeling, and thermobarometry to examine high-pressure eclogites and surrounding metasediments. Pseudosection modeling and conventional thermobarometry suggest a clockwise pressure-temperature (P–T) path for the eclogite, from ~1.4 GPa/~505–530 °C to ~2.4–2.8 GPa/~600–640 °C, followed by decompression to <~0.7 GPa. The micaschists reached peak conditions of 2.0–2.2 GPa and 570–620 °C. Lu–Hf dating yields ages of around 236–241 Ma for the eclogites and 249–251 Ma for the micaschists. Sm–Nd dating indicates a similar age range for the eclogites (236–242 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 U–Pb 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.