東北表日本における中新世の火山作用 : 北上川低地帯に分布する火山岩類

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In the basin of the Kitakami river situated in the Pacific side of northeast Japan are widely developed the lowest Green Tuff formations which have the characteristics of marginal facies of Green Tuff region. The main formations are Inase formation, Ishikoshi andesite and Nonodake volcanics. The Inase formation distributed in the area of 15km from west to east, and 25km from north to south, is composed of basaltic and andesitic lava flows, tuff breccia, sandy tuff and conglomeratic tuff. The basement rocks of the Inase district comprise the Motai metamorphic rocks and granodiorite, and these rocks are overlain by the above mentioned sedimentary rocks. The early volcanic activity in the Inase formation started with the eruption of basalt and andesite belonging to pigeonitic rock series proposed by Kuno (1953), and was followed in turn by the eruption of calc-alkalic andesite. The chief volcanic rocks are as follows. augite hypersthene olivine basalt, augite hypersthene basalt bytownite basalt, augite hypersthene andesite, hornblende andesite. In the east of the Ishikoshi area is distributed Paleozoic Toyoma formation, and the relationship between the Ishikoshi andesite and its formation has not been made clear yet. Ishikoshi andesite is composed mainly of lava flows which consist of andesite, dacite and a little basalt. The volcanic rocks are as follows. augite hypersthene andesitic basalt, hornblende augite hypersthene andesite, augite hypersthene dacite (phenocrystic quartz free), hornblende augite hypersthene dacite (phenocrystic quartz free), biotite augite hypersthene dacite (phenocrystic quartz free),. Neogene Tertiary system in the Nonodake and Asahiyama area is divided, in ascending order, into Nonodake volcanics, Oido formation, Koganehazama formation and Pleiocene series. Nonodake volcanics are composed of tuff breccia, sandy conglomerate and lava flow of augite hypersthene andesite, the total thickness of which is over 110 m. The chemical compositions of Inase formation, Ishikoshi andesite and Nonodake volcanics are plotted on the MgO-FeO+Fe_20_3-K_20+Na_20 and K_20+Na_20-SiO_2 diagrams. The Ishikoshi andesite and Nonodake volcanics indicate iron-poor fractionation trend, but the rocks of Inase formation are distributed in the comparatively wider field than the former (Fig. 7). The total alkali of the rocks of the Ishikoshi andesite and Nonodake volcanics, as shown in the diagram are distributed in the field below line CD (Fig. 8). It may be considered that the origin of their volcanic rocks differs from that of Inase formation.
1979-03-30

東北表日本における中新世の火山作用 : 北上川低地帯に分布する火山岩類

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