Eruption history and magma plumbing system of Futamatayama Volcano, southern Fukushima, Japan

Abstract

Futamatayama volcano is a Quaternary stratovolcano located 6 km north of the Nasu volcano group in southern Fukushima Prefecture, NE Japan. We investigated the eruption history and magmatic processes of the volcano using geological, petrological, and geochronological [thermoluminescence (TL) dating] analyses. The eruptive activity of the volcano can be divided into two stages. The lava flow stage (Stage 1: 3.56 km³ DRE) included at least seven lava flows, and the lava dome stage (Stage 2: 0.09 km³ DRE) involved the formation of two lava domes and a small pyroclastic flow. We obtained TL ages of 163±7 ka, 93±3 ka, and 79±3 ka from the lava flows (Stage 1) and 56±4 ka from a lava dome (Stage 2), which indicate that the volcano formed over a period of >100 ky. The eruption products of Futamatayama volcano commonly consists of felsic rocks (SiO₂ = 56.2-68.4 wt.%) that contain mafic enclaves (SiO₂ = 50.6-59.3 wt.%), indicating magma mixing. On an FeO*/MgO-SiO₂ (whole-rock) diagram, data from Stage 1 and Stage 2 form subparallel linear trends (Stage 1: FeO*/MgO = 1.9; Stage 2: FeO*/MgO = 2.2; at SiO₂ = 64.1 wt.%). These trends suggest that the magmatic system changed between Stage 1 and Stage 2, with different end-member magmas in each stage. Compositional variations between the mafic end-member magmas can be explained by olivine and pyroxene fractionation from a common basaltic magma. Variations in Rb/Ba ratios between the mafic and felsic end-members suggest that the latter could not have been derived from the former by simple fractional crystallization. The felsic end-member magmas of Stage 1 and Stage 2 were produced by different degrees of partial melting of crustal materials in response to heating by mafic end-member magmas.