## 5 Discussion
### Potential Magma Source  
All of the samples formed within Stages 2 and 3 of [PERSON] et al. (2005) [16], during which basaltic lavas (Pyosunri basalt) formed the bulk of the exposed volcanic rocks as a lava plateau, and volcanic rocks (basalts, trachyandesites, and trachytes) formed the Halla shield volcano. Geochemical studies regarding continental alkaline suites have shown that alkaline rocks evolve along a lineage of strong silica-undersaturated basanite to phonolite, or along a lineage of a more silica-saturated alkali basalt to trachyte [56; 57]. The major and trace-elemental compositions of the evolved rocks can be attributed to the fractional crystallization of mafic rocks dominated by olivine, clinopyroxene, apatite, and Fe-Ti oxides, as well as to plagioclase fractionation in intermediate rocks, while the fractionation of plagioclase, alkali feldspar, zircon, and sphene may have been controlled in the highly evolved rocks [58].  
By studying seismic-wave velocities, [PERSON] et al. (2018) suggested that edge-driven convective flow at sub-lithospheric depths at the lithospheric thickness transition below the island led to decompression melting [31]. The complex and dispersed intraplate volcanism  
Figure 8: Plots of Zr vs. Nb contents (**a**) and La vs. Nb contents (**b**) for the volcanic rocks from Jeju Island by the FC–AFC–FCA and mixing modeler [47]. A 10% increment is marked in the trajectory of AFC (\(\mathrm{r}>0\)) and FC (\(\mathrm{r}=0\)).  
on Jeju Island can be attributed to the intense interaction between the ascending magma and the lithosphere.  
This study obtained geochemical signatures of volcanism due to FC, with crustal contamination from the lower crust, based on the REE and trace-elemental patterns of volcanic rocks and mantle xenoliths. The chondrite-normalized REE diagrams (Figure 5) indicate an OIB source.  
The volcanic rocks had an enriched-mantle-2-like ocean island basalt signature and the basalt was formed by partial melting of the upper mantle, which is represented by the xenolith samples of our study. The upper mantle of Jeju was affected by arc magmatism, which is associated with the subduction of the Pacific Plate beneath the Eurasian Plate.  
Comparisons of incompatible and trace-elemental ratios in the basalts and different mantle and crustal sources, including primitive mantle, N-MORB, E-MORB, continental crust, HIMU-OIB, EM1-OIB, and EM2-OIB on Jeju Island are shown in Table 2. The EM1-OIB endmember component was assumed to be related to an ancient SCLM [59; 60; 61] and/or to a metasomatized lower portion of an old SCLM [61; 62]. Alternatively, it may be attributable to the metasomatic enrichment of ancient oceanic lithospheric mantle from long-term storage in the deep mantle [63; 21]. The EM-2 OIB endmember component was generally regarded as originating from an enriched-mantle source that was metasomatized by subduction-related melt and/or fluids [64; 61], or from terrigenous sediment-bearing subducted oceanic lithosphere [64; 65; 66].  
The trace-elemental ratios of the Jeju Island volcanic rocks were similar to those of an EM2-OIB source. Their Th/Nb and Th/La ratios were also within the range of values for the EM2-OIB source (Table 2). Nearly all of the samples plotted within or near the EM-2 OIB field in the Ba/Nb-Th/Nb diagram and were scattered toward the continental crust field (Figure 7). This indicates that the Jeju Island volcanic rocks are mainly the result of AFC of an EM2-OIB source.  
The isotopic ratios of Jeju volcanic rocks can be explained as a mixture of DMM and EM2, but the analyses of source mantle reservoirs vary [17]. The volcanic rocks of this study show that the Jeju alkali rocks formed from a partially melted upper-mantle source.