Garnet and zircon petrochronology reveal coherent subduction and exhumation of eclogites and associated schists in the Changning–Menglian orogenic belt, southeast Tibet

Abstract

Determining whether high-grade metamorphic eclogites and surrounding metasediments underwent a shared subduction-exhumation cycle or distinct metamorphic paths before their juxtaposition at shallow levels is crucial for deciphering metamorphic processes in orogenic belts. Constructing pressure-temperature-time (P–T-t) paths for both rocks is the most straightforward approach. However, current time constraints overwhelmingly rely on dating accessory mineral like zircon, rutile, and monazite, which may only record a portion of the entire metamorphic history. Here we present an integrated petrochronological analysis using garnet Lu–Hf and Sm–Nd, and zircon U–Pb on two eclogites and their surrounding micaschists from the Changning–Menglian orogenic belt in southeast Tibet. Pseudosection modelling and conventional thermobarometers suggest a clockwise P–T path for the eclogites, from ∼1.4 GPa/∼520 °C to ∼2.5 GPa/∼600 °C, followed by decompression to <∼0.7 GPa, and peak conditions of 2.0–2.2 GPa/570–620 °C for the micaschists. Lu–Hf dating for eclogites yields ages of ca. 238–241 Ma, indistinguishable from the Sm–Nd dates of ca. 239 Ma. However, the Lu–Hf ages for micaschists (ca. 249–251 Ma) are older than the corresponding Sm–Nd dates (ca. 240 Ma). The well-preserved growth zoning in garnet indicates a short growth period in eclogites and at least ∼10 Myr in micaschists, bracketing the timing of protracted the two distinct zircon growth phases at around 238 Ma and 250 Ma. The similarity in peak metamorphic conditions and the consistent high-pressure metamorphic ages suggest a coherent subduction and exhumation process for both eclogites and micaschists. The Lu–Hf ages of the micaschists constrain the subduction of the Lancang Group dates back to ca. 250 Ma. The narrow temporal gap between the initiation of subduction and the age of the protoliths implies a rapid shift from the formation of the protoliths to the subduction of oceanic slabs and the scraped-off sediments to depths exceeding 80 km during the Triassic—a hypothesis that warrants further exploration.