Petrology of Hachimantai and Adjacent Volcanoes

Abstract

The basement rocks of this volcanic area consist of the so-called green tuff formation of Neogene age. These are overlain by thick welded rhyolitic tuffs of the latest Pliocene or earliest Pleistocene age. Covering these tuffs there are many kinds of basalts and andesites (pyroclastics and flows), which can be grouped into six volcanic groups such as Hachimantai, Yakeyama, Moriyoshi-yama, Iwate-san, Komagatake and Kayodake. From the lithologic characters both mineralogic and chemical, the six groups are divided into the northern type of the hypersthenic rock series (calc-alkali rock series) and the southern type of the pigeonitic rock series (tholeiitic rock series). The forty seven chemical analyses from the mentioned six volcanic groups show that the rocks are rich in CaO and poor in Na₂O and K₂O; this is the characteristics of the Japanese volcanic rocks. In the differentiation trend of the pigeonitic rock series in MgO-FeO+Fe₂O₃-Na₂O+K₂O triangular diagram, the enrichment of iron in the middle stage is noticed; whereas in the trend of the hypersthenic rock series the enrichment of Na₂O + K₂O is marked from the early to later stage without concentration of iron. These differentiation trends coincide well to that of the Izu and Hakone volcanic rocks and to the other districts of the Nasu volcanic zone.