Genetic relationships between basalts and andesites in the Taradake volcanic area, Northwest Kyushu.

The Pleistocene volcanic activities in the Taradake volcanic area are divided into four stages older basalts (OB), older andesites (OA), younger basalts (YB) and younger andesites (YA). Basalts and calc-alkaline andesites as well as tholeiitic and alkaline basalts are erupted in close spatial and temporal proximity. The basalts (OB and YB) and the andesites (OA and YA) show distinct trends on the variation diagrams and 87Sr/86Sr ratios, suggesting that both have derived from different parental magmas. The relationship between the tholeiitic and the alkaline basalts in the YB stage can be explained by different degree of melting of the same source mantle. Incompatible element compositions of these basalts show within-plate type signature. The andesites appear to have derived from a high-magnesian andesite (HMA) magma that was generated in the upper mantle, because they have magnesian olivine phenocrysts coexisting in equilibrium with mantle peridotite and high-Cr spinels similar to those in the Setouchi HMAs. The HMA magma in the studied area may be produced by reaction between peridotite and ascending within-plate basalt magmas or by partial melting of hydrous peridotite induced during asthenospheric upwelling.