Volcanic Geology of the Yokoate-jima, the Tokara Islands, Kagoshima Prefecture, Japan (II)

Abstract

The results of the petrochemical study of the volcanic rocks from the Yokoate-jima of Tokara islands, Kagoshima Prefecture are summarized. Constituent minerals and their optical properties of the volcanic rocks from the Yokoate-jima are shown in Table 1. As seen from the table, essential constituent minerals are olivine, orthopyroxene (bronzite and hypersthene), augite, plagioclase (labradorite) and magnetite phenocryst and microphenocryst, and in addition to these minerals the groundmass anorthoclase, phlogopite, ilmenite, apatite and silica minerals. They are olivine-bearing augite hypersthene andesite and two pyroxene andesite. Mineral assemblages of these rocks belong to KUNO'S Vd type, namely hypersthenic rock series. Chemical and normative mineral compositions are shown in Table 2. Generally, they are rich in Al₂O₃ and MgO contents, while poor in Na₂O and K₂O. Petrochemical character are shown in the normative or-ab-an, wo-en-fs, wo-fo-Q, Q-fo-fa and MgO-(FeO+Fe₂O₃)-(Na₂O+K₂O) diagrams (Figures 2～6). As shown in these figures, the evolution of normative feldspar has been traced to be of early stage than that of normative mafic minerals. The same evolutional tendencies can be recognized in the so-called high alumina basalts of post-Miocene time from northern Kyusyu (Y. MATSUMOTO 1964) In the normative Q-fo-fa diagram, these rocks are plotted in the hornblende area, although they belong to pyroxene andesite.