抄録
The rocks from Asakusa volcano, Northeast Japan, are described, and the representative 76 samples were chemically analyzed. Asakusa Volcano Collaborative Research Group (1991) divided the geologic history of the volcano into main two stages, early and late stages. The late stage activity formed the principal edifice of the volcano and is further subdivided into four stages. The rocks of the volcano are composed mostly of the rock suites ranging from basalt to andesite with minor amounts of dacite and all the samples are porphyritic with 20-50 vol. % of phenocrysts. The early stage rocks are composed of andesite of Kuno's (1950) hypersthenic rock series (H series), containing phenocrysts of hornblende together with pyroxene. The rock suite of the first stage consists mainly of andesite and basalt, belonging to both Kuno's pigeonitic rock series (P series) and H series, with minor amounts of dacite of H series. Hornblende phenocrysts are also found in the dacite. The second stage volcanic products are composed largely of andesite and basalt, corresponding to both P and H series. Hornblende phenocrysts are present in andesite erupted in the early period of this stage, but phenocrystic hydrous minerals are absent in one formed in the late period. The volcanic products of both the third and fourth stages are made up basalt and andesite without phenocrystic hydrous minerals. They are of both P and H series. The major and trace element chemical characteristics of the rocks from Asakusa volcano are as follows: 1) In the FeO^*/MgO-SiO_2 diagram, the rocks of each stage mostly plot in the field of the tholeiitic series regardless of P or H series and represent similar differentiation trend. 2) They generally plot on around the boundary between the two fields of the low-alkalitholeiite and high-alkalitholeiite series by Kuno (1968) in total alkalies vs. SiO_2 diagram. 3) All of the rocks also plot in the midway region of the field of Gill's (1981) medium-K andesites and its extention in K_2O vs. SiO_2 diagram. 4) The basaltic rocks from the volcano show the incompatible element characteristics typical of island are basalts, indicating the positive Rb, K and Rb anomalies and the negative Zr, Ti and Y anomalies on the N-type MORE normalized pattern for incompatible elements. Although Asakusa volcano is situated in the Moriyoshi volcanic zone, the volcano has the petrographical and chemical characteristics similar to those of both Moriyoshi and Chokai volcanoes according to the comparative examination of LIL element compositions (Na_2O+K_2O, K_2O, Rb, Sr) and petrography among three volcanoes.
