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

Abstract

Summary 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 magmatism including the eruption of HMAs took place in this volcanic belt within a short period of 13.2 ± 0.4 Ma. This is synchronous with the clockwise rotation of the SW Japan arc sliver that was associated with the opening of the Japan Sea back-arc basin. Such rotation of an arc sliver forced the initiation of subduction and partial melting of a young and hot lithosphere of the Shikoku Basin beneath the arc. Interaction between slab melts and mantle wedge peridotites may be a possible generation process of unusual HMA magmas. When the SW Japan arc sliver ceased to drift, the Setouchi magmatism was switched off by the cessation of subduction. The eruption of HMA magmas may also be attributed to an extensional stress field in the Setouchi region, in contrast to the compressional stress regime documented for other regions of the SW Japan arc.