Provenance and Metamorphic Records of Forearc Sediments from the Heilongjiang Complex, NE China: Implications for Subduction Erosion Associated with the Mudanjiang Ocean

Subduction erosion is widely thought to play a significant role in crustal recycling at modern convergent plate margins. However, identifying subduction erosion in fossil accretionary margins is difficult due to multiple episodes of tectonic superimposition and an absence of direct geophysical evidence. By assessing provenance and metamorphic records in this study, we are the first to document the record of subduction erosion in Late Triassic to Middle Jurassic meta-sedimentary rocks from the Luobei Heilongjiang Complex, NE China. Most detrital zircons from four meta-sedimentary samples have typical core-rim textures. Approximately 90% of the detrital zircon cores yielded Phanerozoic ages with εHf(t) values of +12.8 to −17.6. Geochronological and isotopic comparisons indicate that the meta-sedimentary rocks have a provenance in the eastern Songnen Block and were deposited in a forearc basin. The youngest ages of detrital zircon cores span from 229 to 176 Ma, while the metamorphic ages obtained from detrital zircon rims range from 209 to 185 Ma. Combined with the 186−165 Ma phengite 40Ar-39Ar ages of the meta-sedimentary rocks, this suggests that their protoliths were deposited during the Late Triassic−Middle Jurassic. Mineral inclusions define peak blueschist-facies metamorphism at pressure-temperature conditions of 0.9−1.0 GPa and 359−365 °C; this indicates that forearc sediment from the overriding plate was abraded into the subduction channel and underwent high-pressure metamorphism. Metamorphic ages (209−165 Ma) obtained from phengite and detrital zircon rims suggest that a period of subduction erosion occurred during slab subduction. These results reveal rapid progression from early forearc sedimentation (229−165 Ma) to subduction erosion (209−165 Ma) during westward subduction of the Mudanjiang Ocean.