Geology and petrography of the Abukuma granites in the Funehiki area, Fukushima Prefecture, NE Japan

Abstract

In the Funehiki area, Fukushima Prefecture of the Abukuma granitic terrain of NE Japan, intermediate to felsic granitic rocks and gabbroic rocks are complexly distributed. The gabbroic rocks occur as small xenoblocks (1 to 3 km) within the surrounding granitic rocks. Based on geological relations and petrography, the granitic rocks have been divided into older and younger types. The older type is composed of weakly foliated granodiorite to tonalite with euhedral to subhedral hornblende. In contrast, the younger type comprises massive granodiorite to granite with K-feldspar phenocrysts or minor muscovite. The older type is represented by Nagaya, Shikayama, and Ishimori bodies, and the younger type comprises Oishizawa, Miharu, and Hatsumori plutons based on their lithofacies and intrusive relationships. Although all the granitic plutons are geochemicaly classified as calcalkaline I-type granitoids in volcanic arcs, each body has a different compositional trend on the Harker diagram. The gabbroic rocks belong to the tholeiitic rock series and their chemical compositions do not define a continuous trend typical of granitic rocks in the Harker diagram. Though magnetic susceptibility of the granitic rocks show generally low values of 0.2 to 1.0 × 10^-3 SI unit corresponding to ihnenite-series granites, the Ishimori and Oishizawa granites have higher values of 1.0 to 30.0 × 10^-3 Si unit corresponding to magnetite-series and/or ilmeniteseries granites. The gabboic rocks generally show magnetic susceptibility of 30.0 to 60.0 × 10^-3 SI unit. The above characteristics of individual plutons and the gabbroic rocks suggest that all of the granitic and gabbroic rocks formed from discrete magmas. To study the petrogenesis of the granitic rocks, we have calculated the chemical composition of trace elements in the gabbroic rocks using partial melting model. This calculation shows that the granitic melt was formed by melting of a basaltic rock simultaneously forming remaining solid phase of amphibole or garnet + plagioclase.