This study focuses on the Permian strata and volcanic rocks in the Ondor Sum area, Inner Mongolia. By integrating zircon U–Pb geochronology, whole-rock geochemistry, and paleontological analysis, we constrain their formation age, provenance characteristics, and depositional environment, aiming to elucidate the Permian to Early Triassic tectonic evolution along the southern margin of the central Xing–Meng Orogenic Belt (XMOB). The meta–basalts of the Ondor Sum Group exhibit geochemical signatures consistent with an OIB source, including enrichment in LILEs (Rb, Ba, K, Sr), moderate enrichment in HFSEs (Nb, Ta, Zr), and the absence of a Nb–Ta negative anomaly. These features collectively indicate derivation from an enriched deep mantle source, possibly related to a mantle plume. The protolith was emplaced during the Late Permian. Detrital zircon data from sericite–bearing quartz schist within the group yield a maximum depositional age of 244 Ma. The lower part of the Amushan Formation yields a maximum depositional age of 411 Ma, while the upper part is dated at 252 Ma, indicating a substantial time gap between the two units. Combined with fossil assemblages and zircon provenance analysis, the unit is interpreted to have been deposited in a back–arc basin setting during the Late Carboniferous to Early Permian under a convergent plate regime. Integrating regional magmatic and sedimentary records, the Permian–Early Triassic tectonic evolution of the southern margin of the XMOB can be subdivided into three stages: (1) Early Permian (∼285 Ma), characterized by the development of typical calc-alkaline island arc volcanics and crustal thickening, marking the continued subduction of the Paleo-Asian Ocean (PAO); (2) Middle Permian (285–260 Ma), when slab break-off induced a shift in the magma source from juvenile arc crust to older basement, leading to peak crustal thickness and transition from subduction to collision; (3) Late Permian to Early Triassic (260–240 Ma), characterized by a post-collisional extensional regime evidenced by A–type granites, intraplate basalts, and crustal thinning. This extensional onset at ~260 Ma marks the termination of collision, constraining final closure of the PAO to the Early Triassic. This study, by integrating previous research, reconstructs the spatiotemporal pattern of PAO closure and provides new constraints on the tectonic evolution of the XMOB.
Zhang et al. (Fri,) studied this question.