Low-temperature eclogite-facies meta-felsic rocks in the Beishan Orogenic Collage, NW China: Implications for Middle Permian subduction-to-accretion orogenesis

We report the first-known low-temperature eclogite-facies meta-felsic gneisses from the Beishan Orogenic Collage in the Central Asian Orogenic Belt in NW China. Petrological examination, mineral compositions and phase equilibrium modeling reveal that these gneisses experienced three metamorphic stages, with peak-pressure assemblage of garnet, phengite, paragonite, epidote, hornblende, plagioclase, and rutile at eclogite-facies conditions of ∼14 kbar and ∼580−590 °C, representing an apparent geothermal gradient of ∼11−13 °C/km. A clockwise pressure−temperature path involving decompression heating to amphibolite-facies at ∼9 kbar and ∼630−650 °C during exhumation is indicative of a transition from subduction to accretion orogenesis. Zircon and garnet U-Pb geochronology indicate that the low-temperature eclogite-facies metamorphism occurred at ca. 279 Ma, followed by amphibolite-facies metamorphism at ca. 273 Ma. This study provides key evidence supporting subduction in the Beishan Orogenic Collage during the Permian, addressing the long-standing controversy over its tectonic environment during the late Paleozoic. We demonstrate that closure of the Paleo-Asian Ocean at the Beishan Orogenic Collage occurred after the Middle Permian.