Chemical Composition of Younger Volcanic Rocks of Oshima Volcano, Izu

Abstract

Volcanic products deposited during and after the caldera formation (about 400 A.D.) in the Oshima Island are called the younger Oshima group which is divided into twelve unit members, each corresponding to a major cycle of eruption (Fig. 1). Thirty-seven samples of lavas, bombs and scoria were collected (Fig. 2) from nine horizons of the younger Oshima group, and their chemical analyses are given in Table 1～6. Although the whole samples are, in a broad sense, similar in chemical composition and almost are nearly aphyric basalt, but several differences in chemical composition are recognizable. Chemical homogeneity of the lavas in the same horizon has been verified in three horizons (Table 1, 3 and 5). There is fairly regular change in composition of the five groups of volcanic rocks successively extruded during the last seven hundred years (Fig. 10). Linear relations between two of the components, SiO₂, TiO₂, Fe₂O₃, MnO, MgO, CaO and K₂O + Na₂O are shown in variation diagrams Figs. 11～14, in spite of the small range of their variation. Some of the relations are held good not only within the younger Oshima group but, the same trends expand out to the somma and the basement volcanics, the latter probably of Pliocene (Fig. 15). Fe/Mg ratio, which was proposed by KATSURA as a good measure of magmatic differentiation in his previous paper, increases gradually from the base to the top of the younger Oshima group (Table 7).