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

Abstract 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 closure of the Mudanjiang Ocean is fundamental to deciphering the tectonic transition from the closure of the Paleo-Asian Ocean to the subsequent subduction of the Paleo-Pacific Ocean. This study conducted a comprehensive investigation of garnet amphibolite, epidote blueschist, and garnet-mica schist from the Heilongjiang Complex to constrain their protolith characteristics and metamorphic evolution. Phase equilibrium thermodynamic modeling of the garnet amphibolite revealed peak metamorphic pressure-temperature (P-T) conditions of 10.5–12.4 kbar and 693–724 °C, suggesting an intermediate geothermal gradient associated with the warm subduction of basaltic oceanic crust during the early stages of Mudanjiang Ocean subduction. The garnet amphibolites exhibit normal mid-ocean-ridge basalt–like geochemical signatures, with zircon U-Pb dating indicating a Late Permian protolith age of the oceanic crust ca. 255–254 Ma. Subsequent amphibolite-facies metamorphism occurred ca. 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 ocean-island basalt. Zircon U-Pb ages of 263–260 Ma suggest its formation from a basaltic seamount setting within the Mudanjiang Ocean during the Middle Permian. The peak P-T conditions modeled for garnet-mica schist (15.2–16.0 kbar and 512–540 °C) also indicate a low geothermal gradient, comparable to that of the epidote blueschist. The 40Ar/39Ar phengite ages (187–165 Ma) of the metasedimentary rocks in the Heilongjiang Complex record the tectonic evolution from subduction of Mudanjiang Ocean crust to 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 records a complete and continuous Wilson cycle. The initial opening of the Mudanjiang Ocean is attributed to a backarc extensional environment in the western Jiamusi block, driven by the 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 Panthalassa or the Paleo-Pacific Ocean from the Early Permian to Middle Jurassic, with an estimated lifespan of ~116 m.y. (ca. 288–172 Ma).