Petrology of adakitic rocks from Quaternary Bishamon-dake volcano, central Japan.

Abstract

Quaternary Bishamon-dake volcano, central Japan, is composed of calc-alkalic clinopyroxene-orthopyroxene andesite and dacite with subordinate hornblende andesite having Sr/Y>40, SiO2 ∼63%, and Al2O3 ∼18%. The volcanic rocks are characteristically poorer in incompatible elements such as K2O and Rb than rocks from the adjacent volcanoes (Dainichi-dake, Eboshi and Washiga-take, Gankyoji, Haku-san, Kyoga-take, and Tomuro). They are essentially mixing products between two magmas; an andesite having SiO2 ∼60% and higher in FeO.t/MgO (FeO.t denotes total iron as FeO), and an adakite richer in SiO2 and lower in FeO.t/MgO carrying phenocrysts of orthopyroxene with Mg/(Mg+Fe) ∼0.80 and clinopyroxene with Mg/(Mg+Fe) ∼0.82 and presumed mantle-derived olivine (Fo ∼90) xenocrysts. The end-member andesite magma may have been formed by regional volcanism or lower crustal anatexis, while the end-member adakite magma is likely to have been locally generated beneath Bishamon-dake. The end-member adakite magma had SiO2>65%, Na2O≤3.2%, K2O≤1.2%, Rb≤20 ppm, Y<10 ppm, Nb≤6 ppm, FeO.t/MgO≤1.6 and Sr/Y>60, and was presumably generated by melting of the subducting Philippine Sea plate.