Tectonic setting of high-Mg andesite magmatism in the SW Japan arc: K-Ar chronology of the Setouchi volcanic belt

The Setouchi volcanic belt in SW Japan is characterized by the occurrence of andesites with an unusually high Mg concentration (high-Mg andesites, HMAs). Such HMAs may have formed under unusual tectonic settings, as basaltic magmas are produced dominantly in the mantle wedge of modern subduction zones. A total of 50 new K–Ar ages for the Setouchi rocks confirmed that intermediate to felsic magmati...