Insights into the final closure of the Mongol-Okhotsk Ocean (eastern segment): Implications from Mesozoic granitoids in the Northern Great Xing'an Range, NE China

The Mongol-Okhotsk suture signifies the closure of the Mongol-Okhotsk Ocean between the Siberian Craton and the Erguna Block (also known as the Amuria Block), a process that was pivotal in shaping the Mesozoic tectonic evolution of East Asia. While a scissor-like, west-to-east diachronous closure mechanism is widely accepted, considerable debate persists regarding the exact timing of final suturing in the eastern segment and the underlying geodynamic processes. Granitic rocks provide critical insights into crustal growth and reworking, and supercontinental cyclicity. Notably, magmatic activity in the Mongol-Okhotsk suture zone is relatively subdued compared with the adjacent Northern Great Xing'an Range (NGXAR), which is widely attributed to the evolution of the Mongol-Okhotsk Ocean. To elucidate the evolution of the Mongol-Okhotsk Ocean, we investigate the Tayuan granitoids in the Erguna Block of the NGXAR and conduct a comparative analysis with the published Early Jurassic–Early Cretaceous granitoid data from the region. Zircon U–Pb geochronology constrains the Tayuan granitoid emplacement to the Late Jurassic (ca. 152–148 Ma). They are characterized by high Sr content and Sr/Y ratio, together with low Mg# and Y-Yb contents, showing low-Mg# adakitic affinity. Given its weakly depleted Sr–Nd–Hf isotopic signatures (⁸⁷Sr/⁸⁶Sr(t) = 0.70516–0.70624; ε_Nd(t) =  −1.28 to + 1.4; ε_Hf(t) = +2.1 to +7.3), the Tayuan adakitic granitoids are inferred to have sourced from the juvenile, thickened lower crust.

The Early to Late Jurassic low Mg# adakitic granitoids in the NGXAR suggest a substantial crustal thickening event, which is attributed to the southward subduction of the Mongol-Okhotsk oceanic plate. Our calculations reveal that the crust attained its average thickness (∼60 km) during the Jurassic (200–155 Ma). The Late Jurassic witnessed the coeval emergence of not only low-Mg# adakitic granitoids but also A-type granites and high-Mg# adakitic granitoids. This petrogenetic shift tightly constrains the final suturing to 155 Ma and marks a pivotal tectonic transition from compressional to extensional regimes. Subsequently, during the Early Cretaceous, the widespread emplacement of calc-alkaline, non-adakitic volcanic and granitic rocks signifies regional lithospheric extension accompanied by significant crustal thinning, with estimated thicknesses of approximately 40 km. This extensional phase and associated thinning were primarily driven by slab rollback and subsequent break-off following the final closure of the Mongol-Okhotsk Ocean.