## 6 Discussion
### Tectonic Setting  
The tectonic setting of the Chinese Altay during the Devonian period remains a subject of debate. [PERSON] et al. [87] linked the abrupt rise in the heat flux and geothermal gradient during the Early Devonian to the activation of thermal subduction mechanisms. [PERSON] et al. [88] proposed that the Devonian volcanic rocks in the Ashele mining district  
Figure 13: (La/Sm)\({}_{\rm CN}\) versus (Tb/Yb)\({}_{\rm CN}\) diagram showing the calculated compositions of basaltic magmas produced by variable degrees of decompression fractional melting of DM, FM, EM-20, and EM-40 mantle sources for the volcanic rocks in the southern margin of the Chinese Altay [78]. (CN, chondrite-normalized; EM-40, EM-20 are enriched mantle-40 and enriched mantle-20, the compositions of them are detailed in [78]; PM, primitive mantle; N-MORB, normal mid-ocean ridge basalt; DM, depleted mantle; OIB, ocean island basalt; the legends are the same as in Figure 7). were generated by mantle plume upwelling and slab break-off. Other researchers have argued that the Chinese Altay was situated in an arc environment, though the specific type of magmatic arc is still contested. Proposed models include the island arc [20; 21], continental marginal arc [19; 89; 90], back-arc basin [22; 91], frearc extension [92], and ridge subduction [2; 3; 23; 24; 93; 94; 95]. As mentioned earlier, the rock assemblages and geochemical characteristics of the volcanic rocks from the Early-Middle Devonian pointed to the features of arc rocks, indicating that there should be a tectonic setting and crust-mantle structure of the subduction zone. Furthermore, four of the collected samples exhibited (La/Yb)CN values of 1.9, 2.64, 2.94, and 2.97 (ranging from 1.3 to 3.4, indicating enrichment in LREEs compared to N-MORB), and (La/Nb)CN values of 2.02, 2.04, 1.85, and 1.83 (ranging from 1.2 to 2.35, indicating a depletion in HFESs relative to LILEs and LREEs). These findings indicated that they belonged to E-MORB with a subduction-related imprint [26]. The remaining samples displayed a significant enrichment in LREEs and LILEs, along with a depletion in HFESs, characteristic of arc magmatic rocks ((La/Yb)CN ranging from 4.20 to 9.04; (La/Nb)CN ranging from 2.09 to 7.66). In the Ti/100-Zr-YY3 and Hf/3-Th-Ta triangle diagrams (Figure 14), the sample points predominantly fell within the CAB and MORB, as well as the CAB and E-MORB domains, further underscoring the dual characteristics of the Early-Middle Devonian volcanic rocks. Numerous models have been proposed to explain the tectonic setting of igneous rocks exhibiting dual geochemical characteristics of both arc-like and MORB-like signatures. These include fore-arc basins, back-arc basins, decompression melting, asthenospheric upwelling due to slab break-off, and ridge subduction [3; 17; 22; 23; 26; 95]. Fore-arc basalts are typically characterized by low LREE contents [74], with REE patterns resembling those of MORB. However, the samples in this study display LREE enrichment. Additionally, fore-arc basalts are generally associated with the initial stages of subduction, whereas subduction-related arc magmatism had already emerged during the Early Paleozoic in the Chinese Altay [46; 98]. Furthermore, the presence of adakites in the Fuyun-Qinghe area during the same period suggests the partial melting of the young and hot oceanic crust [83], likely occurring on both sides of a slab window. The Kurt ophiolite on the southern margin of the Chinese Altay has been interpreted as a product of a back-arc basin [22]. However, distinctive rock types such as adakite, boninite, HMA, NEAB, picrite, and A-type granite also formed in this region during the Late Paleozoic [17; 25; 45]. Such  
Figure 14: **(a)** Ti/100-Zr-YY3 [96] and **(b)** Hf/3-Th-Ta diagrams for the volcanic rocks in the Fuyun-Qinghe area [97].