## 4 Results
### Trace and Rare-Earth Elements  
The chondrite-normalized rare-earth-element (REE) plots for the volcanic rock samples from Jeju Island showed a notable enrichment in light REEs (LREEs) and a depletion of heavy REEs (HREEs) (Figure 5). The HREEs displayed a flat trend. The pattern resembles that of oceanic island basalts (OIBs) [39], and the (La/Lu)\({}_{\rm N}\) and (La/Sm)\({}_{\rm N}\) ratios were 8.85-20.22 and 2.20-6.10, respectively. The trachytte sample (sample no. 4) exhibited a pronounced negative Eu anomaly (Eu/Eu\({}^{\star}\) = 0.78), whereas the other samples exhibited slightly negative and positive Eu anomalies (Eu/Eu\({}^{\star}\) = 0.93-1.37; Table 1). The (Ce/Ce\({}^{\star}\)) anomalies were steady and negative, with values of 0.92-0.97, indicating that the volcanic rocks were not affected by oxidation. The two most-evolved rocks (tephriphonolite and trachytte) contained higher concentrations of incompatible elements, in the form of the large-ion lithophile elements (LILEs) Rb and Ba and the high-field-strength elements (HFSEs) Nb and Zr, than the other rock types analyzed in this study (Table 1 and Figure 6). The two xenolith samples displayed depletion of the trace elements and REEs compared to the alkaline volcanic rocks, but the patterns look similar to those of the volcanic rocks (Figures 5 and 6), which indicates their origins are linked. Figure 3: AFM diagram of late Cenozoic volcanic rocks on Jeju Island, including the boundary between the calc-alkaline and the tholeiitic fields (after Irvine and Baragar, 1971) [38]. Symbols are the same as in Figure 2. FeO\({}^{\star}\) is total iron as FeO assuming Fe\({}^{2+}\)/Fe\({}_{\rm total}\) = 0.9. Figure 4: Variations in major oxide contents vs. SiO\({}_{2}\) content of the volcanic rock samples from Jeju Island, Al\({}_{2}\)O\({}_{3}\) vs. SiO\({}_{2}\) (**a**), Na\({}_{2}\)O vs. SiO\({}_{2}\) (**b**), Fe\({}_{2}\)O\({}_{3\rm total}\) vs. SiO\({}_{2}\) (**c**), CaO vs. SiO\({}_{2}\) (**d**), K\({}_{2}\)O vs. SiO\({}_{2}\) (**e**), TiO\({}_{2}\) vs. SiO\({}_{2}\) (**f**), MgO vs. SiO\({}_{2}\) (**g**), P\({}_{2}\)O\({}_{5}\) vs. SiO\({}_{2}\) (**h**) and MnO vs. SiO\({}_{2}\) (**i**). Published data of other researchers are from [PERSON] et al. (2019) and [PERSON] et al. (2012b) [21,28]. Figure 5: C1-chondrite-normalized [39] rare-earth-element patterns of alkaline volcanic rock and mantle xenolith samples from Jeju Island. The E-MORB, N-MORB, and OIB data are from [PERSON] and [PERSON] (1989) [39]. The compiled xenolith data are from [PERSON] et al., 2012a; [PERSON] et al., 2012b; [PERSON] et al., 2014 and [PERSON] et al., 2005 [11; 12; 13; 34]. Figure 6: Primitive-mantle-normalized [39] multi-elemental spidergram. The E-MORB, N-MORB, and OIB data are from [PERSON] and [PERSON] (1989) [39]. The compiled xenolith data are from [PERSON] et al., 2012a; [PERSON] et al., 2012b; [PERSON] et al., 2014 and [PERSON] et al., 2005 [11; 12; 13; 34]. The Zr/Nb ratios of the Jeju alkaline volcanic rocks were within the range of OIB mantle sources and were far lower than those of continental crust (16.2), primitive mantle (14.8), and normalized-MORB (N-MORB) (30) [40,41] (Table 2). However, contamination from the continental crust cannot be ruled out, as the plots of Ba/Nb vs. Th/Nb indicate that the samples are within or close to the EM2 range but are slightly scattered in the direction of the continental crustal field (Figure 7).