Metamorphic evolution of blueschists in the Heilongjiang Complex:Implications for the subduction history of the Mudanjiang Ocean

The Heilongjiang Complex in northeast China (NE China) separates the Jiamusi and Songliao blocks and marks the suture zone of the former Mudanjiang Ocean, as evidenced by a variety of oceanic basalt-derived blueschists. Understanding the closure history of the Mudanjiang Ocean is crucial to unravelling the tectonic transition from the final amalgamation of the Central Asian Orogenic Belt (CAOB) to the onset of the Paleo-Pacific subduction. In this study, we investigate epidote-ferroglaucophane (Ep-Fgl) and garnet-ferrobarroisite (Grt-Fbrs) schists from the Yilan area of the Heilongjiang Complex through petrological, mineralogical, thermodynamic modelling, whole-rock geochemical, and geochronological analyses. The Ep-Fgl schists preserve a peak assemblage of ferroglaucophane + epidote + chlorite + clinopyroxene + phengite + titanite with peak P-T conditions of 13.5-15.8 kbar and 458-495 ℃. On the other hand, the Grt-Fbrs schists exhibit a peak assemblage of garnet + glaucophane/ferroglaucophane + lawsonite + chlorite + phengite + rutile ± clinopyroxene ± titanite, deriving peak P-T conditions of 16.4-18.3 kbar and 457-475 ℃. Both types of schist record similar clockwise P-T paths, with three metamorphic stages:a peak epidote-to-lawsonite blueschist-facies stage, a post-peak decompression stage in the epidote amphibolite-facies, and a late greenschist-facies overprint stage. The Ep-Fgl schists display alkaline OIB-like geochemical affinities, while the Grt-Fbrs schists show tholeiitic MORB-like characteristics, suggesting that the protoliths represent fragments of the Mudanjiang oceanic crust. Magmatic zircon grains from Ep-Fgl schists yield protolith ages of 276 ± 1 Ma and 280 ± 1 Ma, whereas zircon of Grt-Fbrs schists document protolith ages of 249 ± 2 Ma and 248 ± 2 Ma, indicating that the Mudanjiang Ocean existed since at least the early Permian. Reconstruction of the metamorphic P-T evolution, combined with previous magmatic and metamorphic age data from rocks of the Heilongjiang Complex and of adjacent tectonic units suggests that the subduction and eventual closure of the Mudanjiang Ocean occurred between the late Triassic and middle Jurassic, driven by a regional stress regime shift caused by the westward subduction of the Paleo-Pacific Plate beneath Eurasia.