Origin of xenoliths within the Hime–shima volcanic group, Kyushu, southwestern Japan Arc

Abstract

Granitic and gabbroic xenoliths have been found within dacitic lavas in the Hime – shima volcanic group (HVG) of northeastern Kyushu, Japan. The HVG is located near the boundary between the Ryoke and Sangun belts, suggesting that the HVG and associated crustal xenoliths may provide insights into the subsurface distribution of the Ryoke and Sangun belts in Kyushu. This study focuses on xenoliths obtained from the coastal boulders near the Kane Lava of the HVG. The HVG xenoliths consist of gabbro, gabbroic diorite, amphibolite, gneiss, basaltic andesite, and tu ff aceous sandstone, with the latter two types resembling those found in the basement rocks of the HVG. The gabbroic xenoliths are geochemically similar to gabbros in the Ryoke belt. The U – Pb dating for zircon in the gneiss xenoliths yielded a metamorphic age of ~ 111 Ma with Th/U values <0.1, similar to the age obtained for metamorphic rocks in or of the Ryoke belt. The new data presented in this study indicate that the xenoliths in the HVG were derived from basement units associated with the Ryoke belt, which in turn, means that the HVG is tectonically underlain by the Ryoke belt. This also suggests that the Ryoke belt extends further north in Kyushu than was previously considered, as implied by the presence of this belt directly below the HVG.