Petrological characteristics and K-Ar ages of Miocene intrusive rocks in western part of Fukui City, Fukui Prefecture, central Japan

Abstract

We report the petrographic characteristics, major and trace element compositions, and K-Ar ages of Miocene intrusive rocks from western Fukui, central Japan. The intrusive rock samples were basaltic to dacitic (SiO₂ = 51.1-69.6 wt.%) and were divided into four different rock types based on their modal and major element compositions: basalt and basaltic andesite, andesite, pyroxene dacite, and hornblende dacite. The compositions of the samples showed a tholeiitic trend and were plotted in the medium-K field. The MORB-normalized trace element patterns of the intrusive rock samples were similar to those of the Kunimidake Volcanic Rocks and Ichinose Rhyolite, which are located in the same area. The K-Ar ages of the plagioclase in the samples from the NNW-SSE-trending dike (pyroxene dacite) and stock (andesite) were 14-12 Ma and were slightly younger than those of the Ichinose Rhyolite and Kunimidake Volcanic Rocks (15-14 Ma). These observations indicate that multiple supplies of basaltic to dacitic magmas occurred subsequent to the activities of the Ichinose Rhyolite and Kunimidake Volcanic Rocks after the opening of the Japan Sea.