High-Magnesian Andesite Produced by Two-Stage Magma Mixing: a Case Study from Hachimantai, Northern Honshu, Japan

Abstract

High-magnesian andesite occurs at Hachimantai, northern Honshu, Japan. Disequilibrium zoning features indicate that the phenocryst minerals were derived from three different magmas. Chemical compositions and zoning profiles are accounted for by two-stage magma mixing: the first mixing occurred between a crystal-free basalt magma and a more differentiated olivine basalt magma; the second stage occurred by mixing between the resultant of the first-stage mixing and a hypersthene–augite andesite magma. Mass balance of phenocryst crystals shows that end-member compositions were c. 52·0 wt % SiO2 and 10·1 wt % MgO for the mafic end-member and 57·0 wt % SiO2 for the felsic end-member of the second-stage mixing. Phenocryst minerals of the first-stage mixing end-member indicate the similarity of the end-member composition to that of basalts from nearby volcanoes. The counterpart aphyric magma in the first-stage mixing was more magnesian than the estimated mafic end-member. Calculations of the phase equilibria of similar basalts from nearby volcanoes and comparison of results with previous phase equilibrium experiments showed that the olivine basalt end-member of the first stage was hydrous and situated at a depth where the pressure was less than 2 kbar. Two-pyroxene thermometry estimates are about 1050°C for the pyroxenes derived from the felsic end-member of the second-stage mixing, and about 1180°C for groundmass pyroxenes. Crystallization temperatures of 1170–1230°C are estimated for minerals from the mafic end-member of the second-stage mixing based on phase equilibrium calculations. These similar temperature estimates between the groundmass and the mafic end-member imply achievement of thermal equilibrium between end-members preceding crystallization. The magma plumbing system of the eastern Hachimantai is illustrated by a recent volcanic event, involving lateral dike intrusion toward a pressure source. The encounter of a laterally migrating basalt dike and an andesite magma chamber triggered the magma mixing that produced the high-magnesian andesite. The model can account for the relation between the petrological model and surface distribution of volcanic rocks. The infrequency of such mixing-derived high-magnesian andesite stems from the rarity of high-magnesian basalt as a potential mixing end-member in northern Honshu.