## 5 Discussion  
### Fractional Crystallization and Crustal Contamination  
TiO\({}_{2}\) was low in the tephriphonolite and trachyte, indicating that Fe-Ti minerals (titanomagneticite) were removed by fractional crystallization (Table 1; Figure 4f). Basaltic trachyandesite, tephrite, and trachybasalt are believed to have formed from basaltic magma. The Fe\({}_{2}\)O\({}_{3}\) and MgO contents were low in tephriphonolite and trachyte, as they are not directly related to basaltic magma and can be related to the fractional crystallization of basaltic magma (Table 1; Figure 4c,g). In addition, the tephriphonolite and trachyte had higher LREE and HREE concentrations than the basaltic trachyandesite, tephrite, and trachybasalt (Figure 5). The (La/Sm)\({}_{\rm N}\) and (La/Lu)\({}_{\rm N}\) ratios were also higher in the tephriphonolite and trachyte, which suggests that they were produced by a later stage of fractional crystallization from the parent magma (Table 1). The REE and multi-elemental diagrams (Figures 5 and 6) indicate that the volcanic rock samples were derived from OIB. The negative anomalies of Sr and Ti observed in the samples of the most-evolved rocks (tephriphonolite and trachyte) were caused by the fractional crystallization of plagioclase and titanomagneticite, respectively, rather than by a particular magmatic environment. Except for Sr and Ti depletions in those two samples, all of the samples display similar trends on the spidergram (Figure 6), which indicates that they were derived from the same magma source. The Th/Nb vs. Ba/Nb ratios also indicate that the volcanic rocks originated mainly from an enriched-mantle (EM1, EM2) source and were slightly affected by the continental crust (Figure 7). No depletion of HFSEs (as shown in Figure 6) and no negative Eu anomaly except in the trachyte sample (as shown in Figure 5) were observed. Therefore, the magma of Jeju volcanism is thought to originate from the spinel peridotite in the upper mantle. The two mantle xenolith samples displayed relatively depleted REE and trace-elemental patterns but similar trends to the volcanic rocks, which indicates that the xenoliths and the volcanic rocks are related in their origins. The basaltic melt most likely originated from partial melting of the upper mantle, represented by the xenoliths. However, the patterns of the xenoliths obtained in this study on the spidergram (Figure 6) and REE diagram (Figure 5) were different from those of xenoliths described in previous studies of [PERSON] et al., 2012a; [PERSON] et al., 2012b; [PERSON] et al., 2014 and [PERSON] et al., 2005; [PERSON] et al., 2011; [PERSON] et al., 2013; [PERSON] et al., 2014; [PERSON] et al., 2014; [PERSON] et al., 2014; [PERSON] et al., 2014) divided xenoliths from Jeju Island into three groups ([PERSON] et al., 2014): the first group comprises Cr-diopside and spinel-bearing ultramafic xenoliths that correspond to fragments of peridotitic mantle, corresponding to Group I of [PERSON] and [PERSON] (1978) ([PERSON] and [PERSON], 1978); the second group, which contain Al-Ti-rich black clinopyroxenes, are magmatic cumulates reported to have directly crystallized near the Moho ([PERSON] et al., 2014) and correspond to Group II of [PERSON] and [PERSON] (1978) ([PERSON] and [PERSON], 1978); and the third group comprises gabbroic xenoliths reported to have crystallized in the reservoir-roof environment within the crust ([PERSON] et al., 2014). The xenolith samples of this study fall into the first group of [PERSON] et al. (2014) ([PERSON] et al., 2014), and the rock types are dunite and harzburgite of spinel peridotite. However, they exhibited larger depletions in MREE, HREE, and some HFSEs (Figures 5 and 6) compared to the Group I xenoliths of [PERSON] et al. (2012a) ([PERSON] et al., 2014) and the two other xenolith groups of Jeju Island (shaded gray areas in Figures 5 and 6), which needs to be further investigated.