Last-stage southward subduction of the central segment of the Paleo-Asian Ocean: Insights from Middle-Late Permian granites in the Siziwangqi area in Inner Mongolia, China

Arc magmatism in collisional orogens presents persistent challenges in discriminating terminal subduction signatures due to potential overlap with slab underthrusting processes during continental convergence. This ambiguity is particularly pronounced in multi-arc accretionary systems such as the Central Asian Orogenic Belt (CAOB), where the cessation timing of Paleo-Asian Ocean (PAO) southward subduction remains contentious. We address this debate through integrated geochronological and geochemical investigations of Middle-Late Permian granitoids (252–260 Ma) from the Bainaimiao Arc Belt, a critical component of the North China Collage System. Zircon U[sbnd]Pb data resolve two distinct magmatic phases: (1) 258–260 Ma calc-alkaline granites exhibiting arc-like signatures (enriched LILEs, depleted HFSEs), and (2) 252–254 Ma highly fractionated alkaline granites with elevated SiO₂ (∼66.49–77.71 wt%), total alkalis (Na₂O + K₂O = ∼6.88–9.55), and diagnostic trace element ratios (Nb/Ta = ∼4.4–16.8; Zr/Hf = ∼13.5–39.2). Our findings redefine the tectonic evolution of the western Bainaimiao Arc: Andean-style continental arc magmatism persisted through the Carboniferous-Early Permian (>280 Ma), transitioning to hybrid arc/crustal sources with addition of the CAOB juvenile crusts during 260–280 Ma, as evidenced by increased zircon ε_Hf(t) (∼ − 18 to 12), and whole-rock ε_Nd(t) (∼ − 13 to −4) values with elevated SiO₂ (most >70.00 wt%). These 260–280 Ma arc-resembling magmas, coeval with regional A-type granites and bimodal volcanism, constrain terminal PAO subduction to ∼260 Ma. Such arc-resembling magmatism has been well documented in other orogens worldwide, suggesting oceanic slab consumption during collisions as a global phenomenon. In addition, we highlight that subduction-driven magmatism is an important supplement to crustal anatexis during collisions.