High-silica rhyolites of Niijima volcano in the northern Izu–Bonin arc, Japan: Petrological and geochemical constraints on magma generation and supply

Abstract

Niijima volcano in the northern part of the Quaternary Izu–Bonin volcanic arc, Japan, consists predominantly of rhyolitic volcanic units associated with minor basaltic and andesitic units. Most of the rhyolites are characterized by a phenocryst assemblage of orthopyroxene (Opx), cummingtonite (Cum), biotite (Bt), and hornblende, which are classified into four types: Opx–Cum, Cum, Cum–Bt, and Bt (in eruption order). The major element compositions of the rhyolites are typified by high SiO2 (73–78 wt%) and K2O (1.5–3.3 wt%) contents, and the trace elements are indicative of a highly differentiated nature, which differs significantly from rhyolites at other sites in the Izu–Bonin arc. The presence of cummingtonite phenocrysts, and mineralogical and whole-rock chemical compositions of the rhyolite lavas clearly indicate that these magmas were produced under low temperatures (<800 °C) and low pressures (<3 kbar). The decreasing anorthite content (An mol.%) of plagioclase and Mg number (Mg#) of cummingtonite and biotite with time suggest progressive eruption from shallower (low-T and -P) magma chambers. The small volumes of coeval basaltic and andesitic volcanic units and mafic–intermediate magmatic inclusions in the rhyolitic lavas played an important role in rhyolite generation and evolution. The parental rhyolitic magmas were produced by partial melting of lower to middle crust by basaltic underplating, after which the magmas ascended and fractionated into >13 units in a shallow magma system. At the same time and at deeper levels, basaltic and rhyolitic parental magmas mixed to produce andesitic magmas between the basaltic and rhyolitic parental magmas. In some cases, basaltic and andesitic magmas in deeper level ascended and mingled with shallow rhyolitic magmas before eruption. The eruption of rhyolitic magmas from shallow levels might have been triggered by the injection of basaltic and andesitic magmas from deeper levels. The complex magmatic system beneath Niijima volcano formed within a relatively thick crustal sequence on the rear-arc side of the Izu–Bonin volcanic arc.