Metamorphic evolution and tectonic implication of the Heilongjiang Complex: Unraveling a complete Wilson cycle, Mudanjiang Ocean, Northeast China

The Heilongjiang Complex in northeast China (NE China), located at the primary suture zone between the Jiamusi and Songliao Blocks, formed by the closure of the Mudanjiang Ocean. Understanding the opening and closing processes of the Mudanjiang Ocean is of fundamental importance deciphering the tectonic evolution of the Paleo-Asian Ocean's closure and the subsequent subduction of the Paleo-Pacific Ocean. In this paper, garnet amphibolite, epidote blueschist and garnet mica schist of the Heilongjiang complex are comprehensively studied in order to constrain their metamorphic evolution, protolith characteristics. P-T pseudosection modelling of the garnet amphibolite reveals peak metamorphic P–T conditions of 10.5–12.7 kbar and 696–728 °C, suggesting an intermediate geothermal gradient associated with the warm subduction of basaltic oceanic crust during the early stages of the Mudanjiang Ocean's subduction. The garnet amphibolite shows a N-MORB-like geochemical signature, with zircon U–Pb ages indicating a late Permian protolith age of the oceanic crust about 255–254 Ma. Subsequent amphibolite-facies metamorphism occurred around 197 Ma. The epidote blueschist records peak P–T conditions of 13.2–14.7 kbar and 488–505 °C with geochemical affinities similar to those of OIB. Zircon U–Pb ages of 263–260 Ma suggest their formation within a basaltic seamount setting within the Mudanjiang Ocean during the middle Permian. The modelled peak P–T conditions of garnet-mica schist are 13.3–14.6 kbar and 487–507 °C, indicating a low geothermal gradient similar to that of the epidote blueschist. The 40Ar/39Ar phengite dating (187–165 Ma) of the meta-sedimentary rocks from the Heilongjiang Complex reflects a tectonic development ranging from the subduction of the Mudanjiang Ocean crust to its involvement in an eventual collision between the Jiamusi and Songliao Blocks. Integrating these findings with previous research, we propose a new tectonic framework for the evolution of the Mudanjiang Ocean. The Heilongjiang Complex represents a distinct orogenic rock sequence that recorded a complete and continuous Wilson cycle. The initial opening of the Mudanjiang Ocean is attributed to a back-arc extensional environment in the western Jiamusi Block, driven by westward subduction of the Mongol-Okhotsk Ocean during the latest Carboniferous to Permian. The eventual closure of the Mudanjiang Ocean and the subsequent amalgamation of the Jiamusi and Songliao Blocks occurred during the late Triassic to middle Jurassic, resulting from the westward subduction and compression of the Paleo-Pacific Plate beneath the Jiamusi Block. The Mudanjiang Ocean existed as a branch of the Panthalassa or Paleo-Pacific Ocean between the early Permian and middle Jurassic, with a lifespan of approximately 116 Myr (ca. 288–172 Ma).