茨城県大子地域に産するホルトノライト安山岩

Abstract

Hortonolite andesites of middle Miocene age have been found in the Daigo district of the northeastern part of Ibaraki Prefecture, Northeast Japan. These andesites (the Nantaisan andesites) occur as subaqueous autobrecciated lavas and massive lavas which are also erupted in water bottom. The Nantaisan andesites are mostly aphyric, but some andesites contain small amount of phenocrysts of hortonolite, ferrohypersthene and plagioclase. An andesite specimen has ferropigeonite, hortonolite and ferrohortonolite in the groundmass. Ferropigeonite of the groundmass is lower in Fe/Mg ratio than the coexisting hortonolite and ferrohortonolite. The latter minerals bear no reaction relation to the former mineral. Bulk chemical compositions of the Nantaisan andesites are similar to those of icelandites from eastern Iceland. Differences in petrologic characters in the Quaternary and Miocene tholeiitic volcanic rocks occurring near the Quaternary volcanic front and its neighbouring Miocene volcanic field in the northeast Japan arc are described in the paper.