Late Carboniferous–Middle Triassic Tectonic Evolution of the Hongshishan Ocean in the Beishan Orogenic Belt: Insights from the Hongling and Xiaocaohu Granites

Rock formed in the Hongshishan Ocean—a branch of the Paleo-Asian Ocean (PAO) in the Beishan area—is distributed throughout the Northern Beishan Orogenic Belt (NBOB). Widespread Late Paleozoic–Early Mesozoic tectono-magmatic activity occurred in the Queershan and Mingshui–Hanshan units on the both sides of the Hongshishan ophiolite belt, generated by subduction and closure of the Hongshishan Ocean. The magmatic record provides critical constraints on progress and completion of the ocean–continent tectonic transition in this area. Compared with the well-studied Southern Beishan Orogenic Belt, the tectonic setting and magma source of Carboniferous–Triassic igneous rocks in the NBOB (especially in the Queershan and Mingshui–Hanshan units) remain debated controversies, hampering our understanding of the ocean–continent transition, the final closure of the Hongshishan Ocean, and even the PAO in the Beishan area. We present zircon U-Pb ages, in situ Hf isotope data, and whole-rock major oxide and trace element data for the Hongling and Xiaocaohu granites from the Queershan unit, located north of the Hongshishan ophiolite belt. The Hongling granites (Ca. 315 Ma) and Xiaocaohu granites (Ca. 236 Ma) were formed in the Late Carboniferous and Middle Triassic, respectively, and are both I-type granites. The Hongling granites are characterized by high Na2O content, low K2O and MgO contents, and variable Na2O/K2O ratios, with metaluminous to weakly peraluminous composition. The Xiaocaohu granites exhibit high SiO2 and total alkali contents, showing metaluminous characteristics. Both granites are all enriched in light rare earth elements and large ion lithophile elements, depleted in heavy rare earth elements and high-field-strength elements (e.g., Nb, Ta, and Ti), and have weakly negative Eu anomalies ((Eu/Eu*)N = 0.39–1.38 and 0.64–0.81; except for 2022IXYQ-2, with a value of 1.38). Additionally, the Hongling and Xiaocaohu granites all have positive εHf(t) values (12.2–13.4 and 8.60–10.8) and young TDM2 ages (549–472 Ma and 718–577 Ma), indicating that they originated from partial melting of juvenile crustal material and that they formed in subduction and within-plate settings, respectively. Through integrated analyses of Carboniferous–Triassic magmatic associations, geochemical signatures, and sedimentary stratigraphic variations in the Queershan and Mingshui–Hanshan unites, it is determined that the Hongshishan Ocean underwent a bidirectional subduction-accretionary process during the Early Carboniferous to early Late Carboniferous. The ocean–continental transition of the Hongshishan Ocean was completed during the period covering the late Late Carboniferous to early Early Permian. Subsequently, the NBOB was in a within-plate position in the Early–Middle Permian to the Triassic, and it was influenced by the strongly intracontinental compression during the Early–Middle Triassic, as the crust thickened in the NBOB.