Pressure–Temperature–Time Evolution of a Polymetamorphic Mica Schist With Two‐Stage Garnet Growth From the Baoyintu Group of Central Inner Mongolia, China

ABSTRACT Understanding metamorphic overprinting is crucial for identifying transitions in tectono‐thermal regimes during orogenesis. Here, we report the first definitive example of polymetamorphism in metapelites of the Baoyintu Group, central Inner Mongolia. Our analyses included petrographic observations, phase equilibrium modelling, as well as U–Pb and Rb–Sr geochronology. Garnet–white mica schist exhibits two stages of garnet growth: an early‐stage assemblage of chlorite + garnet (I) with epidote + phengite + quartz + rutile (I) inclusions and a late‐stage assemblage of chlorite + garnet (II) + muscovite + quartz + ilmenite + staurolite + rutile (II) in the matrix. Phase equilibrium modelling yields pressures of 16–22 kbar and temperatures of 400°C–500°C for the early‐stage assemblage corresponding to blueschist facies metamorphism, and 6–7 kbar and ~550°C for the late‐stage assemblage corresponding to amphibolite facies metamorphism. A garnet–two mica schist contains a mineral assemblage similar to the late‐stage assemblage in garnet–white mica schist, yielding peak pressures of 4–5 kbar and temperatures of 570°C–580°C. Zircon and rutile U–Pb geochronology of the metapelite samples yielded two groups of metamorphic ages (c. 384 and c. 259–236 Ma). In situ mica Rb–Sr geochronology of the garnet‐two mica schist yielded an age of 215 Ma interpreted to record cooling after the Permian–Triassic metamorphic peak temperature. These results reveal that medium‐ and low‐dP/dT metamorphism occurred during the late Permian–early Triassic that overprinted Devonian high‐dP/dT metamorphism. Based on previous studies of magmatism in the region, we relate the two stages of metamorphism to subduction of Paleo‐Asian oceanic lithosphere (Devonian, c. 384 Ma) and back‐arc extension (Permian–Triassic, 259–236 Ma). This contribution fills a critical gap in our understanding of the tectono‐thermal transition history of subduction within the Paleo‐Asian Ocean and provides a robust framework for interpreting polymetamorphism in accretionary orogens.