Chemical Compositions of the Lavas from the Chokai Volcanic Zone, Japan

Abstract

In this paper, the chemical compositions of the lavas from the Chokai volcanic zone along the inner are of the Japanese Islands is described., The Chokai volcanic zone includes volcanoes of Oshima-oshima, Iwaki, Kanpu, Chokai, Gassan and Yudonosan, which are characterized by the common occurrence of hornblende in lavas., The lavas vary from augite-olivine basalt to hypersthene-biotite-hornblende dacite and range from 48., 89 % to 66., 98 % in silica content., The most outstanding chemical characteristic of the lavas is the richness in alkalies, especially in K₂O, also the undersaturation in sillica in most of basic lavas., The alkali-lime index is about 60, indicating an alkalic content greater than that of the average Japanese effusive rocks (63., 7)., The ration of Fe₂O₃/FeO, total FeO/MgO, CaO /Na₂O + K₂O and Na₂O/K₂O are representative of those of hornblende-bearing lavas from Japan., Qz and at-alk of NIGGLI'S values are low, and c-(al-alk), k and mg are high, while normative Or, En and Wo are high, and Q and An are low., Such chemical characteristics are closely related to modal compositions., A striking similarity in the chemical characteristics is found between the lavas of the Chokai and the Daisen zones, showing a contrast to those of the Nasu zone and Izu-Hakone region., It may be concluded that lavas of the Chokai zone have an intermediate character between the extremely calcic rock suite of the Nasu zone and alkalic rock suite of the Circum-Japan Sea province, not only from their geographical distributions but also from their petrographical characteristics.,