Lingli Guo

During the Mesozoic, NE Asia experienced intense tectonic and magmatic activity, including the closure of the Mongol–Okhotsk Ocean (MOO), the subduction and demise of the Mudanjiang Oceanic Plate (MOP), and the continuous westward subduction of the Paleo-Pacific Plate (PPP). The evolution of the MOP remains highly contentious, particularly regarding its final closure timing and subduction polarity, and warrants further investigation. The Heilongjiang Complex (HLC), primarily distributed within the Mudanjiang Suture Zone (MSZ), which separates the Jiamusi and Songnen blocks, preserves key geological records of the Mudanjiang Oceanic subduction and closure. By employing detailed structural analysis, zircon U-Pb dating, and tomographic imaging, we reconstruct the tectonic history of the HLC and propose its five stages of deformation since the Mesozoic. The first stage, which occurs from the Late Triassic to the Early Jurassic, is characterized by SE-dipping schistosity within the HLC. Integrating the identification of the east-dipping remnants of the oceanic lithosphere west of the MSZ by tomographic imaging suggests an eastward subduction of the MOP underneath the Jiamusi Block during the Early Mesozoic. The second stage, spanning the Early Jurassic to the Late Jurassic, is characterized by E–W-striking tight folds associated with the MOO’s SSE-ward subduction and PPP’s NNW-ward subduction. The third stage of deformation, occurring from the Late Jurassic to Early Cretaceous, features S–N-striking box folds, indicative of the final amalgamation of the Jiamusi, Songnen, and Nadanhada terranes. The fourth stage, taking place in the late Early Cretaceous, is marked by near E–W-striking thrust faults that are associated with the closure of the eastern segment of the MOO. Finally, the fifth stage, which follows the Early Cretaceous, involves nearly NE-striking thrust faults related to NW-ward subduction of the PPP.

The Xinchenggou area is located in the northeast part of the eastern segment of the Xingmeng orogenic belt (EXOB), NE China and has been demonstrated to be a promising exploration target for epithermal deposits. Although previous studies have shown that syenogranite and monzogranite occurring in the Xinchenggou area are promising in forming epithermal mineralization, the petrogenesis and geodynamic settings in which these granites were emplaced are still unclear. To address these problems, in this study detailed whole-rock major and trace element analyses for these granites were conducted. Combined with previously published data, we show that both syenogranite and monzogranite in the Xinchenggou area are high-K calc-alkaline and peraluminous with high SiO₂. Their rare earth element concentrations are low (ΣREE = 72.35 × 10⁻⁶–217.64 × 10⁻⁶) and show obvious differentiation between LREE and HREE (La_N/Yb_N=2.74–11.37), with apparent Eu negative anomalies (δEu = 0.14–0.83) and indistinctive Ce anomalies (δCe = 0.96–1.11). Combined with petrographical observations, it is suggested that both syenogranite and monzogranite are (slightly fractionated) I-type granite. Nb/Ta ratios of syenogranite and monzogranite range from 6.18 to 26.33, indicating that the granitic magma was derived from the upper mantle or the lower crust. Both syenogranite and monzogranite were emplaced in a continental arc setting, which was related to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate during the Late Triassic to Early Jurassic.