The petrology and geochemistry of high-magnesium andesites at the western tip of the Setouchi Volcanic Belt, SW Japan

Abstract

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 in compatible trace element, and Sr Nd Pb isotopic characteristics, confirms that the Sctouchi 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 chromain spinel inclusions showing compositional characteristics consistent with their crystallization as liquidus phases, whereas the other contains Ni-and Fe-rich olivine and Fe3+-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 liquids 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.