Final closure of the eastern Mongol-Okhotsk Ocean: First evidence from Barrovian metamorphism and multistage deformation in the Luomahu Group, NE China

Formation of the Mongol-Okhotsk suture belt represents the terminal orogenic event that constructed the modern tectonic framework of NE Asia. However, the timing and mechanisms of the final closure of the eastern Mongol-Okhotsk Ocean remain controversial. Based on integrated field investigations and analyses of microstructures, petrography, mineral chemistry, whole-rock geochemistry, phase equilibrium modeling, quartz electron backscatter diffraction fabric, and zircon U-Pb geochronology, we document for the first time a complete Middle to Late Jurassic Barrovian metamorphic sequence within the Luomahu Group of NE China, providing critical constraints on the timing and geodynamic processes of the final closure of the eastern Mongol-Okhotsk Ocean. Our study reveals that the Luomahu Group constitutes a tectonic collage of Lower Devonian to Lower Carboniferous metasedimentary strata intruded by early Mesozoic magmatic rocks, subsequently affected by Middle Jurassic to earliest Cretaceous syncollisional magmatism, metamorphism, and deformation. The prograde Barrovian metamorphism preserves a clockwise pressure-temperature-time-deformation path superposed by two deformation phases. Phase equilibrium modeling indicates progressive burial to amphibolite-facies peak conditions followed by retrograde cooling in both garnet-staurolite mica schist and garnet-sillimanite gneiss. Both deformation phases correlate with the peak metamorphic stage. D1 deformation is characterized by top-to-the-SW/SSW thrusting. D2 deformation is marked by sinistral strike-slip shearing along NNE-SSW−trending shear zones. Zircon U-Pb dating constrains the peak metamorphic age to ca. 165−145 Ma, with the two deformation phases occurring at ca. 162−150 Ma and ca. 150−144 Ma, respectively. We interpret the prograde Barrovian metamorphism and associated D1 deformation as direct evidence of crustal thickening during the closure of the eastern Mongol-Okhotsk Ocean. The subsequent D2 deformation is attributed to the far-field effects of NNW-directed low-angle subduction of the Paleo-Pacific plate, while a potential contribution from the ongoing ocean closure remains plausible. Together with regional data, we integrate our results to establish a west-to-east multistage closure model involving slab break-off, which conclusively demonstrates that the eastern Mongol-Okhotsk Ocean finally closed at the Jurassic to Cretaceous boundary.