Consistent garnet Lu-Hf and Sm-Nd ages indicate short-lived high-pressure metamorphism and rapid subduction in oceanic subduction belt: An example from the Changning-Menglian orogenic belt, southeastern Tibetan Plateau

The integration of garnet-based petrologic constraints with multimineral geochronologic data in eclogites and blueschists allows the timing and rate of subduction zone metamorphism to be constrained. We present a combined garnet Lu-Hf/Sm-Nd and zircon/rutile U-Pb geochronology study on three eclogites, a garnet-bearing blueschist, and a micaschist from the Changning-Menglian orogenic belt, a newly discovered ultrahigh-pressure metamorphic belt in southeast Tibet, in order to characterize tectono-metamorphic events and determine the duration of Paleo-Tethys oceanic subduction. Integration of phase equilibrium modelling and conventional thermobarometry for the eclogites defines a clockwise P-T path evolving from blueschist facies conditions at similar to 1.4 GPa and similar to 505-530 degrees C to peak eclogite facies conditions at similar to 2.8 GPa and similar to 630-640 degrees C, followed by isothermal decompression to amphibolite facies at similar to 1.0 GPa and similar to 630-650 degrees C. The Lu-Hf ages of c. 239-236 Ma obtained for the eclogites and the blueschist are indistinguishable from the rutile U-Pb age of c. 239 Ma obtained for the eclogites and, combined with the observation of well-preserved Rayleigh-fractionation-style Mn and Lu zoning profiles in garnet, reflect the timing of early prograde garnet growth. The Sm-Nd ages of c. 242-236 Ma reflect a later period of garnet growth, evidenced by flat and/or M-shaped Sm zoning profiles. Each of the Sm-Nd ages overlaps, within uncertainty, with its corresponding Lu-Hf age (i.e., from the same garnet fraction). The consistency of the Lu-Hf and Sm-Nd ages indicates a short overall duration of garnet growth from blueschist to eclogite facies metamorphism, reflecting rapid subduction of the oceanic slab. The magmatic zircon U-Pb dates of c. 247 Ma constrain the protolith age of these metabasaltic rocks. The close protolith and the high-pressure metamorphic ages, together with the consistent garnet Lu-Hf and Sm-Nd ages and the overlapping youngest and oldest metamorphic ages of the oceanic-type and continental-type eclogites, respectively, suggest a fast tectonic transition from divergence to convergence highlighted by rapid oceanic subduction, continuous transition from oceanic to continental subduction, and a rapid cooling of the subduction interface.