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K–Ar ages, and petrographical and geochemical characteristics of high-magnesium andesites and plagioclase-phyric andesites from the NE Kyushu region, Japan, are presented. K–Ar ages range from 10·7 ± 0·3 to 14·4 ± 0·4 Ma, overlapping those reported for lavas of the Setouchi Volcanic Belt in other regions (11–16 Ma). This, together with major and incompatible trace element, and Sr–Nd–Pb isotopic characteristics, confirms that the Setouchi Volcanic Belt, which is characterized by the occurrence of high-magnesium andesites, extends for ∼600 km along the SW Japan arc. Thus, it may be suggested that the unusual tectonic setting required for high-magnesium andesite magma generation was attained only for this limited region, possibly in association with subduction of the young, hence hot, lithosphere of the Shikoku Basin. Two types of high-magnesium andesite, possessing identical bulk-rock compositions, are recognized: one contains olivine phenocrysts with chromian spinel inclusions showing compositional characteristics consistent with their crystallization as liquidus phases, whereas the other contains Ni- and Fe-rich olivine and Fe 3+ -rich spinel. One of the possible causes for such unusual mineral compositions could be effective elemental diffusion within and through olivine crystals associated with a long residence time in a crustal magma chamber and the slow rate of cooling of the host magma. The compositions of liquidus chromite and olivine, as well as whole-rock major element compositions, suggest that the NE Kyushu high-magnesium andesite magmas were produced by higher degrees of partial melting than those in other regions of the Setouchi Volcanic Belt.

The Petrology and Geochemistry of High-Magnesium Andesites at the Western Tip of the Setouchi Volcanic Belt, SW Japan