## 2 Geologic Setting  
Jeju Island is a symmetrical shield-shaped volcanic orifice, the peak of which is located at Mt. Halla (1950 m). Topographically, the volcanic landforms can be divided into the lava plateau, the shield-shaped Halla volcanic edifice, and more than 360 monogenetic volcanoes [1; 29]. Quaternary volcanism occurred on 25-35 km-thick continental crust [30], which is located ~650 km behind the Ryukyu Trench/Nankai Trough where the Philippine Sea Plate subducts below the Eurasian Plate. The lithosphere beneath Jeju Island is estimated to be ~60 km thick [21; 31].  
The geology of the island is characterized by lavas ranging from basaltic to trachytic. The lavas occur extensively on the island together with diverse volcanic landforms, including Mt. Halla at the center of the island and about 360 volcanic cones that are scattered throughout the island. In the subsurface, pherotamagmatic volcanoes (tuff rings and tuff cones) produced by explosive hydrovolcanic activity occur extensively, together with intervening volcaniclastic sedimentary deposits [1; 32; 33].  
Researchers have previously studied the petrography of volcanic rocks from Jeju Island [16; 28]. On Jeju Island, lava flows display a compositional spectrum from alkali basalt/basanite to trachyte. Less-evolved rocks (basalt to trachy basalt) generally consist of phenocrysts of clinopyroxene and olivine in modal abundances of <15%; the groundmass comprises plagioclase, olivine, and clinopyroxene microlites, with or without dispersed titanomagnetic. The more evolved rocks (basaltic brachydesite) contain phenocryst phase of plagioclase and groundmass of alkali-feldspar microlites [28]. On examining the samples from deep cores (400-500 m) of the central Jeju Island, dominant phenocryst assemblages were visible in the hand specimens. These varied systematically from the olivine-and-pyroxene-dominated base to a plagioclase-rich top. [PERSON] et al. (2005)reported that, in Jeju basalts, spinel crystals were included in olivine phenocrysts. This is indicative of the equilibrium of basalt magmas with the residual spinel in the mantle [16].  
The volcanism of Jeju Island comprises the syn-depositional (~1.88-0.5 Ma) and post-depositional (0.5 Ma) Holocene Seoguipo Formation, which is composed of basaltic volcaniclastics and fossiliferous deposits. The syn-depositional volcanism is characterized by hydrovolcanism with minor alkali basaltic lava effusions (~1 Ma), domed extrusions with evolved alkaline lavas, and alkaline lava effusions that erupted locally and sporadically during the deposition of the Seoguipo Formation. The post-depositional volcanism is characterized by extensive terrestrial volcanism after the termination of the deposition of the Seoguipo Formation at ~0.5 Ma [18].  
The basement rocks of Jeju Island include granites and silicic volcanic rocks of Jurassic to Cretaceous age, as well as engisess of unknown age that were observed in drill cores as xenoliths in the basalts and tuffs [9; 16; 17; 20; 21]. Mantle xenoliths are observed within volcanic rocks distributed extensively on Jeju Island and include spinel peridotites and pyroxenites [21; 27; 34], as well as gabbrocic and anorthite xenoliths [11; 12; 13; 20; 21; 34; 35].