Tectonic nature, subduction, and closure of the Mudanjiang Ocean: Insights from newly discovered oceanic fragments in the Luobei Heilongjiang Complex

The Mudanjiang Ocean was a paleo-ocean located between the Jiamusi and Songnen blocks in the eastern Central Asian Orogenic Belt (CAOB). Understanding its opening and closure is crucial to unravelling the subduction of the Paleo-Pacific oceanic crust and the final amalgamation of Northeast (NE) China. However, the tectonic nature and duration of the subduction in the Mudanjiang Ocean have long been debated. We present new zircon SIMS UPb ages, whole-rock major and trace element compositions, and SrNd isotopic data for the metabasic rocks of the Heilongjiang Complex in the Luobei area, NE China, to determine their rock types, protolith and metamorphic ages, petrogenesis, and tectonic setting. Geochemically, the metabasic rocks can be divided into two types: tholeiitic N-MORB-like metabasalt with high positive εNd(t) values (+8.7 to +9.3), and alkaline OIB-like metagabbro and metabasalt with relatively low positive εNd(t) values (+7.4 to +7.7). Zircon SIMS UPb dating and mineral electron microprobe analyses show that the metabasic rocks crystallized during the late Permian-Early Jurassic (256–181 Ma) and experienced high-pressure metamorphism starting in the Late Triassic (ca. 222 Ma). Whole-rock geochemical and SrNd isotopic data indicate that the tholeiitic N-MORB-like rocks were derived from low-degree (5%–20%) partial melting of a depleted mantle source in the spinel stability field, whereas the alkaline OIB-like rocks were generated from the partial melting of a relatively enriched mantle source in the garnet stability field. Their primary magmas underwent olivine and clinopyroxene fractionation with negligible crustal contamination during ascent. Our new data suggest that the OIB- and MORB-like rocks formed on the Mudanjiang oceanic plate rather than in a continental rift setting between the Songnen and Jiamusi blocks as previously thought. Integrating pre-existing data on magmatic arc migration and high-pressure metamorphism, we propose that the Mudanjiang Ocean opened as a branch of the Paleo-Pacific Ocean no later than the early Permian, experienced westward subduction between ca. 222 and 172 Ma, and eventually closed at 172–165 Ma.