Late carboniferous to early Permian granitoids from central Inner Mongolia, north China: petrogenesis and tectonic implications

Traditional models suggest that the final closure of the Paleo-Asian Ocean (PAO) occurred during the Late Permian to Triassic. However, the timing of the closure remains a topic of considerable debate. The widespread distribution of A-type granites dating from the Carboniferous to Permian periods indicates that closure may have occurred earlier, potentially as far back as the Late Carboniferous to Early Permian or even the Late Devonian. The granitoids with ages from the Late Carboniferous to Early Permian in northeastern Erenhot, within the Xing’an-Mongolian Orogenic Belt (XMOB), offer insightful perspectives on the complicated tectonic history related to the eventual closure of the PAO. By studying zircon U-Pb ages, Hf isotopic composition, and whole-rock geochemical composition of granitoids from the Zhunsujihua Granitic Belt (ZGB) and Baolage Granitic Belt (BGB) in northeastern Erenhot, we aim to gain a better understanding of their petrogenesis and tectonic settings while also constraining the closure time of the PAO. Zircon U-Pb ages of the ZGB and BGB granitoids span from 288 to 317 Ma and from 275 to 295 Ma, respectively. These granitoids have compositional ranges of 70.16–78.04 wt% SiO₂, 3.73–5.02 wt% K₂O, and 10.48–15.65 wt% Al₂O₃, and show light rare earth element (LREE) enrichment and negative Eu anomalies. The ZGB alkali-feldspar granite exhibits adakitic characteristics, whereas other ZGB granitoids show I-type features. The BGB granitoids display A-type signatures. Zircon ε_Hf(t) values vary from + 10.05 to + 15.48, with young two-stage Hf model ages (T_DM2) spanning from 332 to 656 Ma, suggesting that both the ZGB and BGB granitoids have originated from partial melting of juvenile crustal materials. These findings suggest that the granitoids formed during the transition from subduction to post-collisional extension, which implies that the PAO in the northeastern region of Erenhot had closed at the latest by the Early Permian.