## 6 Discussion
### Magma Sources and Petrogenesis  
As mentioned earlier, the Early-Middle Devonian volcanic rock assemblages in the study area are hawaite, mugearite, potassic trachybasalt, basaltic andesite, Bendmoreite, etc., which contain special labeled rocks such as adakite, sub-boninite, NEAB, HMA, and MA. Boninite forms through the high-degree partial melting of mantle peridotite under low-P and high-T conditions, facilitated by fluids enriched in incompatible elements derived from subducting oceanic crust [74]. In contrast, NEAB originates from the partial melting of a mantle wedge metasomatized by adakitic melts [75; 76]. When subducting slab melts interact with mantle peridotite, they increase the Nb abundance and result in higher La/Yb ratios [2]. HMA, on the other hand, is generated by the partial melting of the mantle wedge above a subduction zone, driven by the release of H\({}_{2}\)O from the subducted oceanic crust [61]. In the P\({}_{2}\)O\({}_{5}\) versus TiO\({}_{2}\) and Nb/Yb versus Nb diagrams (Figure 10), samples of NEAB are present and fall within the slab melt metasomatism field. Similarly, in the primitive mantle-normalized Hf/Sm versus Ta/La diagram (Figure 11), some sample points align with fluid-related metasomatism or lie near the boundary between fluid- and melt-related metasomatism, indicating that the mantle peridotite was influenced by subducting fluids or melts. Furthermore, the subalkaline rocks are predominantly low-K to medium-K calc-alkaline series, characterized by a relatively high MgO content. Their enrichment in large-ion lithophile elements (LILEs) and depletion in high-field-strength elements (HFSEs) are consistent with the features of island arc magmatic rocks. In the primitive mantle-normalized trace element spider diagram (Figure 8a), the patterns of these samples closely resemble those of the island arc basalts (IABs). Consequently, it is evident that the Early-Middle Devonian volcanic rocks were formed in an arc environment associated with ocean subduction. Figure 10: P\({}_{2}\)O\({}_{5}\) versus TiO\({}_{2}\) (**a**) and Nb/Yb versus Nb (**b**) diagrams for the volcanic rocks in the Fuyun–Qinghe area [77] (the legends are the same as in Figure 7). Figure 9: Variation diagrams for some major oxides and Cr and Ni versus MgO contents for the volcanic rocks in the Fuyun–Qinghe area. (Data of black circles are from [45; 49]). Experimental studies indicate that garnet preferentially retains heavy rare earth elements (HREEs) over middle rare earth elements during melting. As a result, melts derived from garnet-facies peridotite exhibit higher middle rare earth element/HREE ratios compared to those from spinel-facies peridotite [79]. The Sm/Yb ratios of basaltic rocks (including NEABs), HMAs, and sub-boninite from the Early-Middle Devonian volcanic rocks align with the melting curves of garnet-spinel mantle peridotite (Figure 12), suggesting that their precursor magma originated from 5-20% partial melting of enriched garnet-spinel lherzoolite mantle. This is consistent with the relatively flat chondrite-normalized REE patterns, which are influenced by the presence of both spinel and garnet. Additionally, in a chondrite-normalized La/Sm versus Tb/Yb plot (Figure 13), basaltic samples and HMAs fall between the 5 and 10% partial melting curves of the primitive mantle and enriched mantle-20 fields [78], further supporting the idea that the precursor magma was generated by the partial melting of the enriched mantle. While the oceanic lithosphere typically exhibits depletion characteristics, the enriched mantle in this context likely reflects asthenospheric materials. This enrichment may have been triggered by the upwelling of asthenospheric materials through a slab window formed by the subduction of a spreading ridge. Figure 11: (Hf/Sm)\({}_{\rm FN}\) versus (Ta/La)\({}_{\rm FN}\) diagram for the volcanic rocks in the Fuyun–Qinghe area [78]. (PN, primitive mantle-normalized; DM, depleted mantle; the legends are the same as in Figure 7).