## 5 Discussion
### Tectonic Implications  
The Late Permian-Early Triassic was a key stage in the tectonic evolution of the Central Asian Orogenic Belt (CAOB), in which the paleo-Asian Ocean closed and changed from subduction accretion to collisional orogen. Previous research on the sedimentology and paleogeography of the Late Permian Linxi Formation in the southern segment of the Daxing'anling Mountains show that the region was marine sediment in the early-middle stage of the Late Permian as a remnant basin, and it was not until the late stage that it transitioned to a terrestrial environment. In the Early Triassic, river lake facies of sedimentary layers with red layer properties appeared [56], indicating that the surface of the paleo-Asian Ocean had closed.  
In the Early Triassic, although the surface of the Paleo-Asian Ocean was closed, a large number of adakitic igneous rocks were derived from the melting of the subducted slab in the area north of the Xar Moron suture zone. For example, in the Early Mesozoic (244 Ma) high-Mg adakitic andesite exposed in the Linxi region was derived from the partial melting of the residual oceanic crust preserved in the mantle beneath the paleo-subduction zone, followed by interaction with mantle peridotite [46]. [PERSON] et al. (2020) [17] concluded that the volcanic rock of the Lower Triassic Xingfuzhilu Formation (247 Ma) in the Linxi area was a typical adakite derived from the melting of oceanic slabs and that subducted oceanic slabs still existed during the Early-Middle Triassic after the closure of the paleo-Asian Ocean surface. According to a detailed zircon mineral study, [PERSON] et al. (2007) [57] concluded that the adakitic rocks of the Longtoushan pluton (241 Ma) were hydrous mafic rocks of the subducted oceanic crust. [PERSON] et al. (2002) [58] analyzed the Sr-Nd isotopic characteristics of various igneous rocks and combined them with geophysical data, suggesting that the Mesozoic \"soft collision, weak orogenesis\" tectonic environment allowed a considerable portion of the oceanic plates to remain in the crust-mantle transition zone rather than completely returning to the asthenosphere, indicating that lithospheric-scale subduction continued.  
Additionally, in the Xilinhot-Xi Ujjmqin region, the Early Triassic dicate-rhylolite has been discovered in the volcanic rock sections of the Dalinuoer Formation and the Baoeraobao Formation. These rocks' ages of petrogenesis were 245.7 \(\pm\) 1.8 Ma and 242.7 \(\pm\) 4.0 Ma (K-Ar whole-rock ages, [59]), and they exhibit geochemical characteristics similar to A-type granites. They belong to the extrusive phase of the contemporaneous A-type calc-alkaline series of peraluminous-type granites, closely related to the continental crust extension process resulting from the underplating of mantle-derived magma.  
During the Late Permian to Early Triassic, the subduction plate segment of the Paleo-Asian Ocean broke off, and the remnant oceanic crust preserved in the mantle of the Paleo-subduction zone still underwent partial melting, generating adakitic magma. Concurrently, the asthenospheric mantle upwelling occurred, causing the melting of materials at different depths within the lithosphere, resulting in the petrogenesis of both adakitic rocks with island arc characteristics and rock assemblages under an extensional setting in the subduction zone.