Back-arc heterogeneity and final closure of the Hegenshan-Nenjiang Ocean: Constraints from Paleozoic successions of the Xing’an accretionary terrane, NE China

Northeast China consists of several microcontinents and accretionary terranes, providing valuable insights into how different tectonic units amalgamate during orogeny. The Hegenshan-Nenjiang Ocean (HNO) is a branch of the Paleo-Asian Ocean located between the Xing’an and Songliao accretionary terranes. However, the geodynamic setting of these terranes, as well as the timing of the final closure of the HNO, remains uncertain. Here, we present an integrated study of field and microstructural observations, electron backscatter diffraction (EBSD), zircon U-Pb dating, Lu-Hf isotopes and geochemistry of the hitherto poorly dated low-grade metamorphic Xiqiuhe and Tongshan formations located in the northeastern Xing’an Accretionary Terrane (XAT). The results indicate that the Xiqiuhe Formation was deposited in the Late Silurian (ca. 425 Ma), constrained by the crystallization age of an interbedded rhyolite layer and the youngest detrital zircon population, whereas the Tongshan Formation has a maximum depositional age of the Early Carboniferous (ca. 330 Ma). The detrital zircons in both formations show similar U-Pb ages and ɛ_Hf(t) value distribution patterns. The primary sources are the Erguna Massif and the Paleozoic magmatic arcs within the XAT. Rhyolites from the Xiqiuhe Formation are characterized by high SiO₂ and Al₂O₃ concentrations and low MgO, Fe₂O₃^T, Cr, Co and Ni concentrations. Their positive ɛ_Hf(t) values (9.47-12.79) and two-stage model ages (808-596 Ma) indicate that they were derived from partial melting of a juvenile accreted lower crust. In addition, two deformation phases have been identified in the Xiqiuhe and Tongshan formations. Microstructures and quartz EBSD fabrics demonstrate that they developed at mid-crustal levels with deformation temperatures ranging from 400 to 500 ℃. The early deformation (D₁) is characterized by top-to-the-NNW thrusting, followed by the development of open folds and crenulation cleavage (D₂) resulting from NW-SE shortening. Both deformation phases are associated with the final closure of the HNO. Our new data alongside with previously published data on the HNO, suggest that the Xiqiuhe and Tongshan formations formed in a Paleozoic back-arc basin setting related to the slab roll-back of the HNO. The back-arc basin initiated prior to the Late Silurian (ca. 425 Ma), showed along-strike variations, and began closing between the Late Devonian and Early Carboniferous. The HNO finally closed between the late Early Carboniferous and middle Late Carboniferous (ca. 330-310 Ma), accompanied by transpressional structures in the XAT.