Contemporaneous ascent of within-plate type and island-arc type magmas in the Beppu–Shimabara graben system, Kyushu island, Japan

Within-plate (WP) type and island-arc (IA) type magmas have erupted in north Kyushu and south Kyushu, respectively, during Quaternary time. The former is related to the eastern end of the East Asian extensional province and the latter to the subduction zone of the Philippine Sea plate. The Beppu–Shimabara graben system in the central part of Kyushu island is located in the area of overlap between them. Quaternary volcanic rock samples from the graben system can be chemically classified as WP- and IA-types by their multi-element patterns and positions on a Zr–Nb–Y discrimination diagram. Rocks of both type and of similar ages occur throughout the graben system. Furthermore, the rocks of WP-type give systematically higher 87Sr/86Sr ratios (0.7048–0.7049) than those of the associated IA-type (around 0.7040). The relations between 87Sr/86Sr and Nb/Y ratios of the former are clearly distinguished from those of the latter. These results demonstrate that this graben system is characterized by the contemporaneous ascents of WP- and IA-type magmas to the same Quaternary volcanic areas. Such rivalrous ascents of WP- and IA-type magmas seem to be a common feature of magmatism associated with graben in extensional provinces related to subduction zones of island arcs and their boundary areas.