Reconstructing the tectonic evolution of the Mudanjiang Ocean: Metamorphic insights from the Heilongjiang Complex, NE China

The Heilongjiang Complex in northeast China marks the suture zone recording the closure of the Mudanjiang Ocean between the Jiamusi and Songliao blocks. This study presents the first integrated analysis of garnet amphibolite, epidote-barroisite schist, and garnet-staurolite-mica schist from this complex, combining petrology, mineral chemistry, thermodynamic modelling, whole-rock geochemistry, and geochronology to constrain their metamorphic evolution and protolith origins. The garnet amphibolites display a peak mineral assemblage of garnet + amphibole + clinopyroxene + plagioclase + sphene + quartz, corresponding to pressure–temperature (P–T) conditions of 10.9–13.1 kbar and 712–760 °C. These conditions indicate an intermediate geothermal gradient, reflecting high-temperature metamorphism associated with early warm subduction of basaltic oceanic crust. E-MORB-like geochemical signatures and zircon U–Pb ages suggest a middle Permian oceanic crust protolith (ca. 272 Ma), with subsequent amphibolite-facies metamorphism occurring around 235 Ma. The epidote-barroisite schists are characterized by a peak assemblage of sodium amphibole + epidote + chlorite + phengite + sphene + rutile + quartz, and record peak P–T conditions of 14.9–16.4 kbar and 472–505 °C. These values correspond to a low geothermal gradient of 9–11 °C/km, consistent with mature, cold subduction environments. Their OIB-like geochemical affinities and zircon ages of 244–241 Ma indicate a basaltic seamount origin within the Mudanjiang Ocean during the middle Triassic. The garnet-staurolite-mica schists record a clockwise P–T path, marked by decompression coupled with heating from blueschist- to amphibolite-facies conditions. This thermal evolution supports a diapiric exhumation model into the warm middle to lower crust of the overriding plate, rather than exhumation along a cold subduction channel, and is interpreted as a response to the collision between the Jiamusi and Songliao blocks. Integrating these findings with previous research, we propose a revised tectonic model for the Mudanjiang Ocean. Its initial opening occurred in a back-arc extensional setting in the western Jiamusi Block, driven by westward subduction of the Mongol–Okhotsk Ocean during the latest Carboniferous to Permian. Final closure and continental amalgamation took place during the late Triassic to middle Jurassic, resulting from westward subduction and compression of the Paleo-Pacific Plate. The Mudanjiang Ocean is interpreted as a branch of the Panthalassa or Paleo-Pacific Ocean, with an estimated lifespan of approximately 116 million years, spanning from the early Permian to middle Jurassic (ca. 288–172 Ma).