A rare basanite distribution in the northern part of the Izu–Bonin volcanic arc, Japan: Petrological and geochemical constraints

Petrological and geochemical investigations were performed on the uniquely distributed Nanzaki basanite (0.43Ma) in the northern part of the Izu–Bonin volcanic arc, Japan, to clarify its original magma chemistry, and to constrain the source mantle and formation process of the magma. The Nanzaki basanite (monogenetic volcano) is mainly composed of nepheline-bearing basanite lava and scoria. The mineral chemistries are characterized by high forsterite (Fo) contents of olivines, high Mg# (=Mg/(Mg+Fe)) values of clinopyroxenes, and low Cr# (=Cr/(Cr+Al)) values of spinels. Whole-rock major element contents have narrow variation ranges as follows: SiO2 (41.5–44.1%), MgO (10.2–13.1%), CaO (11.9–13.3%), and K2O (0.4–1.9%). Combined with these mineral and whole rock chemistries, the low FeO*/MgO (0.81–1.09) values, high Ni and Cr contents, and narrowly distributed rare earth element (REE) patterns of the Nanzaki basanite represent the primary (undifferentiated) chemical features of the magmas. The incompatible trace element characteristics, especially the high Sr, Ba, and REE contents and low K, Rb, Zr, Hf, and Ti contents, suggest that the basanite magmas were generated from an enriched mantle that was affected by metasomatism with carbonatite magma (or carbonate-melt). In addition, the slight enrichment of Pb, Cs and other alkaline elements in the basanites indicates the close concern of fluids, and the Sr–Nd isotope characteristics of the basanites (low 87Sr/86Sr and 143Nd/144Nd ratios relative to those of basaltic rocks in the volcanic front) are consistent with across arc isotopic variations of the Izu–Bonin volcanic arc.