Abstract
Late Miocene to Pliocene volcanic rocks occur in the Wakurayama and Dakesan areas of Matsue city, San-in district, Southwest Japan. We present new petrographic and geochemical data for the volcanic rocks. Previous studies refer to the volcanic rocks as the “Wakurayama Andesite.” However, on the basis of bulk rock compositions (SiO2: 64.47–65.84 wt.%) determined in this study, we propose to rename the volcanic package the “Wakurayama Dacite.” The Wakurayama Dacite is divided petrographycally into three types: Type I, Type II, and Type III lavas, reflecting the order of eruption. The Wakurayama Dacite lavas have adakitic compositions (i.e., higher Al2O3 and Sr, and lower Y). This adakitic magma may have been generated by partial melting of the subducting Philippine Sea Plate. The Wakurayama Dacite was erupted at ∼5 Ma, the earliest Tertiary to Quaternary adakitic activity along the southwestern portion of the Japanese arc. The eruption of the Wakurayama Dacite suggests that melting of the Philippine Sea Plate occurred at ∼5 Ma.
