## 1 Introduction  
Jeju Island lies approximately 90 km south of the Korean Peninsula and has a maximum elevation of 1950 m above sea level. It is an elliptical island, 73 km in length and 31 km in width, and is characterized by thick voluminous basaltic lava flows, minor pyroclastic rocks, and numerous small monogenetic volcanoes [1]. The volcanic rocks form a continuous series of alkali/sub-alkaline basalt-trachtype associations. The geology of Jeju Island has been studied by many researchers since 1925 [2, 3, 4, 5, 6, 7, 8, 9, 10]. Two separate magmatic events occurred on Jeju Island. The first was associated with the subduction of the Pacific Plate beneath the Eurasian Plate during the late Cretaceous and early Tertiary, while the other was associated with an extensional tectonic setting and is represented by Quaternary intraplate magmatism [11, 12, 13, 14].  
Over the last two decades, the Quaternary volcanic rocks on Jeju Island, comprising alkaline and sub-alkaline rocks formed by intraplate volcanism in a continental margin setting, have been studied [15, 16, 17, 18]. Based on stratigraphic relationships, the volcanic activity of Jeju Island can be divided into four stages [16, 19, 20, 21]: the first stage that formed the Seoguipo Formation and basal basalts, the second 'lava plateau' stage, the third'shield' stage that created Mt. Halla and formed the bulk of the island, and the final'scoria cone' stage that generated the dispersed small-volume vents visible on the surface [9]. In contrast, based on the geochemical development, [PERSON] et al. (2015) divided the volcanism into three main stages associated with changes in the depth and degree of partial melting of magma, with two sub-stages based on chemical differences observed in drill cores. Stage 1 produced high-Al alkaline basalts (stage 1b) and derivative evolved trachytic lavas (stage 1t), stage 2 produced high-Al and low-Al alkaline basalts, and stage 3 included the contemporaneous extrusion of primitive low-Al alkaline and sub-alkaline magmas (stage 3b) and evolved trachytes (stage 3t) [14].  
The volcanic activity on Jeju Island began at <1.9 Ma and persisted until ~1000 C.E. [9; 16; 17; 18]. Geochemical studies using Sr-Nd-Pb radiogenic isotope ratios can help determine the genesis of these volcanic rocks [17; 22]. Based on Sr-Nd-Pb isotopic systematics, the mantle source for Jeju volcanism has been characterized as a mixture of depleted mid-ocean ridge basaltic (MORB) mantle (DMM) and enriched-mantle-type-2 (EM2) components [16; 17].  
[PERSON] et al. (2014) proposed that the isotopic signatures of the enriched-mantle (EM1-EM2) source beneath East Asia are from recycling of the sub-continental lithosphere, oceanic crust, and/or subducted sediment [13]. Moreover, the recycling of those source materials is related to late Cretaceous to early Tertiary magmatism caused by the subduction of the Pacific Plate beneath the Eurasian Plate prior to the opening of Cenozoic back-arc basins [22; 23; 24].  
Two chemically distinct magma series (alkaline and sub-alkaline) have accumulated on Jeju Island during the Quaternary period to form voluminous volcanic deposits [9; 16; 25].  
Previous studies on Jeju volcanic rocks have focused on the history of volcanism, the magma sources, and the tectonic settings [14; 16; 18; 21; 26; 27; 28]. Our study, additionally, aimed at investigating the underlying mechanism in the generation of the evolved volcanic rocks (trachybasalt to trachyte) on the island.