ABSTRACT

Daisen volcano is located on the Quaternary volcanic front in southwestern Japan. The volcano is composed mainly of andesite and dacite, which chemically resemble adakites, with high Al₂O₃ and Sr/Y, steep REE patterns, and no negative Eu anomaly. (²³⁸U-²³⁰Th) disequilibrium (herein, a ratio in parentheses denotes the activity ratio) and trace element analyses of adakites from two volcanic domes, Karasugasen and Misen, indicate two adakite types. Adakite from Karasugasen is characterized by excess (²³⁰Th) over (²³⁸U), typical of most adakites, whereas adakite from Misen is characterized by excess (²³⁸U) over (²³⁰Th). The latter is consistent with enrichment in fluid-mobile elements relative to fluid immobile elements compared to rocks from Karasugasen. The values of (²³⁰Th/²³²Th) of adakites from Karasugasen and Misen are, respectively, around 0.75 and 0.81. These low (²³⁰Th/²³²Th) ratios result from the incorporation of subducted sedimentary material. The ratios, nevertheless, are higher than that for the estimate of lower crustal material suggesting significant incorporation of lower crust is unlikely. As adakites from Misen have (²³⁸U) excess over (²³⁰Th), adakite magma must have interacted with wedge mantle metasomatized by a slab-derived fluid, confirming the presence of a fluid-metasomatized mantle beneath Daisen volcano.