Sedimentological Evidence for Pre-Early Permian Continental Subduction in the Dabie Orogen, Central-East China

How to constrain the onset of continental subduction and prograde metamorphism in an orogen remains a fundamental question. The widespread Triassic ultrahigh-pressure (UHP) metamorphic rocks in the Dabie orogen in central-east China are generally attributed to continental subduction. The earliest Triassic peripheral basins (the Huangshi, Yueshan and Nanjing basins) around the Dabie orogen represent an ideal archive to reconstruct the early evolution of the orogen. Here we present a multidisciplinary provenance study on these Triassic strata, including framework petrography and heavy mineral analyses combined with U-Pb age and trace-element analyses of detrital zircon, rutile and apatite. The abundant metapelitic lithics and muscovite grains, a heavy mineral population dominated by metamorphic apatite, and a significant Permian (270–290 Ma) age peak from U-Pb age spectra of detrital zircon, rutile and apatite imply an Early Permian medium-high grade metamorphic source in the uppermost structural levels of the Dabie orogen. Our provenance data indicate that the onset of northward continental subduction of the South China Block commenced no later than the Early Permian (c. 290 Ma). This event is clearly earlier and distinct from the more widespread Triassic UHP metamorphism of the Dabie orogen, suggesting that continental subduction and mountain-basin interaction were protracted processes (>55 Myr, c. 290–235 Ma). When combined with other geological evidence, our results show that the prolonged continental subduction is not always preceded by subduction of oceanic crust.