open access

https://hdl.handle.net/2115/36726

Origin of Calc-Alkaline Andesite from Oshima-Ōshima Volcano, North Japan

英語

1984-02

Hokkaido University

Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy

21(1)

77-131

Oshima-Ōshima volcanic island, a triple stratovolcano, is situated off the Japan sea coast, north Japan. The volcanic edifice consists of lavas and pyroclastics, about 30 vol% of calc-alkaline andesite and 70% of alkali olivine basalt. The andesite and basalt are intimately associated with each other, even within a single cycle of eruption as in the recorded eruption in 1741-1742. Most of andesite and some basalt contain ultramafic and mafic inclusions which show a typical cumulate texture. The occurrence, texture and chemistry of minerals of these inclusions may have played an important role in fractional crystallization of the basaltic magma. A gradual change in modal contents of minerals is observed in the inclusions. Mg/Mg + Fe ratio of olivine, clinopyroxene and amphibole decreases from dunite to hornblende gabbro. Consequently, the crystallization sequence of the inclusions is inferred as follows: dunite → wehrlite → olivine clinopyroxenite → clinopyroxenite → hornbledite → hornblende gabbro. The variation in major and trace elements of the volcanic rocks seems to result from successive fractionation of these inclusions from the basaltic magma. The trend from picritic basalt to felsic basalt is interpreted as due to the successive removal of dunite, wehrlite and clinopyroxenite (first removal), whereas that from the felsic basalt to andesite as due to the successive removal of hornblendite and hornblende gabbro (second removal). The first removal results in a considerable increase in Al2O3 content with a little SiO2 enrichment of residual liquid, whereas the second removal leads to a constant Al2O3 content with a considerable SiO2 enrichment. These chemical varialions in the residual liquid reflect on the chemistry of clinopyroxenes in the inclusions. The phase relationships in the basalt-andesite magma and the comparison of mineral chemistry between the inclusions and their host rocks suggest that the dunite fractionation from the primary magma took place under hydrous condition at higher pressure, whereas the subsequent fractionation proceeded under hydrous condition at low pressure. The variation in the volcanic rocks from alkaline basalt to calc-alkaline adnesite, therefore, is considered to be caused by amphibole fractionation from the felsic basalt under hydrous condition at shallower place in the continental crust. The Quaternary volcanic rocks in the Notheast Honshū Arc vary markedly in the chemistry from the Nasu Volcanic Zone (Pacific side) to the Chōkai Volcanic Zone (Japan Sea side). The rocks of the Chōkai Zone including Oshima-Ōshima volcano are higher in alkali content than those or the Nasu Zone. Alkali content as well as H2O content are considered to be important factors for the stability of amphibole. High alkali and H2O contents are favorable to the fractionation of amphibole from the basaltic magma at the early stage of differentiation. In the Nasu Zone, common hornblende is rarely found in dacite whereas in the Chōkai Zone, pargasite occurs frequently in andesite, even in basalt. Thus, the calc-alkaline andesite from the Chōkai Zone may be originated from the basaltic magma through amphibole fractionation.

紀要論文

Version of Record

0018-3474