Anorthosites in Nishiyama volcanic products from the Hachijo–jima island, Izu–Bonin arc The direct evidence for ‘plagioclase control’ in shallow magma reservoir

摘要

The Nishiyama volcano is a Quaternary stratovolcano consisting of the northwestern part of the Hachijo–jima island, located in a volcanic front of the Izu–Bonin arc. The Holocene activity of the Nishiyama volcano began at ~ 10–13 ka and has mainly produced basaltic lava flows and scoria fall deposits. While gabbroic and doleritic enclaves are generally found in scoriaceous pyroclasts, we discovered anorthositic enclaves in the lava flows. Here, we report the petrographical and petrological characteristics of the anorthositic enclaves. In the basaltic lava flows, the amount of plagioclase phenocrysts positively correlated with the whole–rock content of Al_(2)O_(3), CaO, and Sr with these elements preferentially contained in the feldspar. In addition, an ideal anorthite content obtained from the whole–rock chemistry of the basaltic lava flows (An_(88)) was largely consistent with the measured anorthite content obtained from anorthosite (An_(84)). These results suggest that the plagioclase fractionation and/or accumulation controls the whole–rock chemical composition of basaltic lava flows and that the accumulated plagioclases represent part of anorthositic enclaves. Anorthosite was divided into three types of textures: 1) Comb texture, 2) adcumulate texture, and 3) radial texture. These textures (from the comb texture to the radial texture) reflected the change in the undercooling state of the magma. Based on the petrography of the anorthosite and lava flows, the plagioclase was in a liquidus phase in the Nishiyama basaltic magma. Additionally, the anorthosite was formed by the effects of adiabatic ascent and the degassing of near–H_(2)O–saturated magma at the shallow magma reservoir (~ 5 km in depth) beneath the Nishiyama volcano.