北海道産玻璃質岩の研究II

北見国紋別郡白滝地域の玻璃質岩

抄録

The authors have investigated the petrochemical characters of holohyaline rocks from Shirataki district, Hokkaido, and the following results were obtained.
(1) X-ray studies on the spherulite showed that they are mainly composed of cristobalite and anorthoclase (and albite).
(2) Fifteen specimens of holohyaline rocks were chemically analyzed, and it is found that they are generally poor in CaO, richer in alkalines, especially in K₂O and range from 68.52 to 76.72 per cent in silica content. On the other hand, spherulite is richer in CaO and its alkali content is distinct, in Na₂O.
(3) The diagramatic reprensentation of the system Na₂O+K₂O-SiO₂ shows that most of the holohyaline rocks except spherulite have an intermadiate character between the tholeiitic rock suite of the Nasu-Fuji volcanic zone and alkalic rock suite of Circum Japan Sea province.
(4) The analytical compositions of obsidian, perlite and pitchstone group close to the minimum in the system NaAlSi₃O₈-KAlSi₃O₈-SiO₂ (Tuttle and Bowen, 1958), and obsidian and perlite correspond closely with the minimum at 500kg/cm² and/or 1000kg/cm² water vapour pressure.