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

Abstract

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 ~1.4 GPa and ~505–530°C to peak eclogite facies conditions at ~2.8 GPa and ~630–640°C, followed by isothermal decompression to amphibolite facies at ~1.0 GPa and ~630–650°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.