Petrology of the Kuju Volcanic Group, northeast Kyushu Island, Japan

Abstract

We present detailed petrographic observations, modal compositions of phenocrysts, and whole-rock major and trace element data (Ba, Rb, Nb, Sr, Zr, and Y) for volcanic rocks of the Kuju Volcanic Group (KVG), northeast Kyushu Island, southwest Japan Arc. We classified these volcanic rocks into three groups based on the mineral assemblage and modal abundance of phenocrysts: (1) a group containing ≥2.9 vol% olivine and lacking hornblende (Ol-g); (2) a group containing hornblende (Hbl-g); and (3) a group containing ≥2.0 vol% orthopyroxene with a small amount of olivine (≤1.3 vol%) and lacking hornblende (Opx-g). In addition, based on the characteristics of plagioclase (Pl) and clinopyroxene (Cpx) phenocrysts, Hbl-g can be subdivided into three types: samples containing abundant Pl with dusty cores are defined as Hbl-g1, whereas those poor in Pl with dusty cores are divided into Cpx-rich Hbl-g2 and Cpx-poor Hbl-g3. The petrographic features of Opx-g, Hbl-g1, and Hbl-g2 indicate they experienced mixing between mafic and silicic magmas, as indicated by the occurrence of Pl phenocrysts with honeycomb and dusty cores, as well as the coexisting olivine and quartz. Fractional crystallization cannot explain the geochemical relationship between Ol-g and the other groups. In addition, the geochemical characteristics of each group show that Opx-g, Hbl-g1, and Hbl-g2 cannot be explained by simple binary mixing between the most mafic Ol-g and most silicic Hbl-g3 samples from the KVG. Furthermore, Opx-g, Hbl-g1, and Hbl-g2 are not related to each other by magma mixing. The variations in Y contents and Sr/Y ratios suggest that Hbl-g1 and Hbl-g3 were derived by partial melting of a subducting slab (i.e., slab melts), and that Hbl-2 may be petrogenetically related to a bajaitic magma that was generated by interactions between slab melt and mantle material. The variations in SiO₂, TiO₂, P₂O₅, and Nb contents, and the trend defined by Opx-g towards the compositional field of high-Nb basalts, similar to volcanic rocks erupted in areas around the KVG, are ascribed to interactions between slab melt and the mantle above the subducting slab.