Geochemistry of Neogene Nansatsu volcanic rocks in South Kyushu, Japan

Abstract

ABSTRACT We report the whole-rock major and trace element and Sr – Nd – Pb isotopic compositions of volcanic rocks from the Nansatsu area in South Kyushu. We analysed samples from the Oldest stage (middle Miocene), Older stage (late Miocene), and Middle stage (early Pliocene) Nansatsu volcanic rocks, which are andesite to dacite in composition, with an island-arc trace element signature. The compositional variabilities within the Older and the Middle stage rocks are relatively small compared to those in the Oldest stage and Quaternary volcanic rocks. Isotopic characteristics suggested that the Oldest stage volcanic rocks had magma sources similar to regional ilmenite-series granitoids of later ages (Outer Zone granitoids), which incorporated a substantial amount of graphite-bearing accretionary sediments (Shimanto Supergroup). The sources for the Older and Middle stage magmas were similar to those of primary Quaternary basalts in South Kyushu, indicating that Pliocene and Quaternary volcanic rocks in South Kyushu share a common mantle source, despite the change in the nature of volcanic activity from andesite-dominated to more variable volcanism of basalt to rhyolite.