## 5 Discussion
### Petrogenesis of Baiyinhushuo Granitoid
The adakitic rocks were enriched in large ion lithophile elements (LILEs, such as Th and Rb) and depleted in Nb, Ta, Ti, and P, reflecting the geochemical characteristics of the subduction zone. In the (Yb + Ta)-Rb diagram and (Y + Nb)-Rb diagram, all the samples also fall into the field of volcanic arc granite, indicating that the adakites were most likely emplaced in the island arc background (Figure 9). The igneous rocks derived from the melting of the lower crust exhibit distinct crustal characteristics in terms of isotopes, while the magma rock derived from the melting of subducted oceanic crust inevitably carries the signature of the oceanic crust. It is generally believed that the magma with Sr-Nd isotopic characteristics close to MORB is directly derived from the melting of the oceanic crust. Figure 7: Diagrams of (**a**) Sr/Y–Y [15] and (**b**) (La/Yb)N–(Yb)N for granitoids in Baiyinhushuo (after [37]). Generally, the Sr-Nd isotopic signatures of magmas close to MORB are directly derived from the melting of the mantle-derived oceanic crust (MORB), which is related to the island arc setting of the subduction zone. If (\({}^{87}\)Sr/\({}^{86}\)Sr)i \(>\) 0.704, typically between 0.706 and 0.710, it is considered that the magma derived from the partial melting of the basalt of the juvenile lower crust is caused by underplating, which is closely related to the intracontmental post-collision tectonic environment during the thickening lithosphere delamination and mantle magma underplating [48; 49; 50]. The Baiyinhushuho adakite has low (\({}^{87}\)Sr/\({}^{86}\)Sr), (0.703382) and high \({}^{143}\)Nd/\({}^{144}\)Nd (0.512721) and \(\varepsilon_{\rm Nd}\)(t) values (+7.72). In the Sr-Nd isotope diagram (Figure 10), the samples fall in the field of subducted oceanic adakite, far from the fields that are derived from the thickened lower crust. The Sr-Nd isotopes show that the source region is close to the depleted mantle, with a two-stage model age of 387 Ma, It exhibits a close affinity to the Sr-Nd isotopic values of the ophiolite in the adjacent Xi Ujimqin region, suggesting that the isotopic characteristics are similar to those of the subducted oceanic crust. Therefore, the Baiyinhushuho adakite is derived from the partial melting of the subducted oceanic crust, with its source potentially being the break-off subducted oceanic crust of CAOB. Figure 8: SiO\({}_{2}\)-Mg\({}^{\#}\) (**a**) diagram and SiO\({}_{2}\)-MgO diagram (**b**) for the in Baiyinhushuho area (after [15; 45]). Figure 9: (**a**) Rb-(Y + Nb) and (**b**) Rb-(Yb + Ta) tectonic discriminant diagrams of the Baiyinhushuho adakite (modified from [15; 47]). The high stability of zircon, along with its high Hf mass fraction and very low \({}^{176}\)Lu/\({}^{177}\)Hf, and the fact that it crystallizes with little or no radiogenic Hf accumulation, make zircon Hf isotope studies one of the most important means of tracing magma source at present [34,54]. The \(\epsilon_{\rm Hf}\)(t) values in the Baiyinhushuo adakite are all positive, with a small range of variation between 11.3 and 13.8, and all the points are located between the evolutionary line of depleted mantle and the chondrite (Figure 10); in addition, the Hf isotopic signature is consistent with the evolution of Hf in the mantle-derived magma in the eastern region of the Xing-Meng Orogenic Belt (Figure 11) [8,55], showing similar isotopic composition characteristics to Phanerozoic magmatic rocks in the eastern part of the Xing-Meng Orogenic Belt. The two-stage model age of zircon Hf isotopes is 323-459 Ma, which is relatively young, implying that the magma may have originated from the partial melting of the Early Paleozoic subducted oceanic crust. Figure 11: \(\epsilon_{\rm Hf}\)(t)-T diagram of zircon (after [55]). Figure 10: (\({}^{87}\)Sr/\({}^{86}\)Sr)—\(\epsilon_{\rm NM}\)(t) isotope composition of Baiyinhushuo (after [51]). Date sources: intra-oceanic adakite of Tibet is from [49]; adakite of modern intra-oceanic arc is from [52,53]; adakite in Lixix is from [8].