K–Ar ages and geochemistry of Quaternary frontal arc andesites from southern Kyushu, Japan

To characterize frontal arc volcanism in the northern Kyushu–Ryukyu arc, we conducted K–Ar dating and whole-rock geochemical analyses of andesites from southern Kyushu Island, Japan. We collected three andesites (Kitsubae, Nagaoyama, and Takanomine andesites). The Nagaoyama and Takanomine andesites yield Early Pleistocene ages (1.8–1.2 Ma), but a reliable K–Ar age was not obtained from the Kitsubae andesite. The Kitsubae and Takanomine andesites are calc-alkaline, and the Nagaoyama andesite has a high-magnesian andesite-like FeO*/MgO ratio. The TiO2, FeO*, MnO, and Na2O contents of the andesites do not show substantial variation on variation diagrams. In contrast, the Al2O3, MgO, CaO, and P2O5 contents vary markedly among the andesites. All andesite samples yield negative Nb and Ta anomalies on a trace element spider diagram, but only the Nagaoyama and Takanomine andesites yield negative Zr and Hf anomalies. Large negative Ce anomalies and positive Y anomalies are remarkable characteristics of the studied andesites. These chemical characteristics are consistent with those of the Late Miocene to Middle Pleistocene Hisatsu volcanic rocks in the southern Kyushu and northern Kyushu–Ryukyu arc.