1986 年 40 巻 5 号 p. 322-336b
Neogene formations distributed in the northwestern part of the Shimokita Peninsula, Northeast Japan, are divided into the Kinpatizawa Formation, Oma Formation, Metakigawa dacites and Okoppe andesites in ascending order. This paper mainly presents the results of studies of occurence and chemical character of the Okoppe andesites. The Okoppe andesites are subdivided into two members viz. the western Okoppe andesites and the eastern Okoppe andesites, which are interfingeied each other. The eastern Okoppe andesites are composed mainly of andesitic tuff breccia with intercalations of rhyolitic lavas and shales. The western Okoppe andesites are composed of submarine volcanic products which contain massive lavas, hyaloclastite, pillow lavas and dykes. Massive lavas are largely divided into two types; One type is jointed lava with regular columnar joints, while another type consists of angular, polyhedral blocks, and corresponds to subaqueous auto-brecciated lava by KUNO (1968) or in-situ breccia by YAMAGISHI (1985). Hyaloclastites are also classified into two types, A and B, according to whether they include pillow or not. The former contains abundant concentrically jointed pillows (concentric pillows) and the latter rarely includes pillow-like large blocks which correspond to pseudo-pillow lava by WATANABE and KATSUI (1976). Basalts and andesites of the western Okoppe andesites belong to the tholeiitic series, whereas the Metakigawa dacites, and basalts and andesites of the eastern Okoppe andesites are classified as the calc-alkalic series in FeO*/MgO-SiO2 diagram. All the chemical data of these volcanic rocks are plotted in the low alkali tholeiitic field defined by KUNO (1968b). Pillow lavas are composed of hypersthene augite basalt and hypersthene augite basaltic andesite, having SiO2 contents ranging from 52.7 to 56.3wt%. On the other hand, massive lavas and angular fragments in hyaloclastite B type mainly of hypersthene augite andesite with SiO2 contents ranging from 54.2 to 65.6 wt%. This evidence suggests that basaltic magmas with lower silica contents tend to produce pillow lavas, whereas andesitic magmas with higher silica contents yield hyaloclastite B type or lavas with angular, polyhedral blocks, or columnar joints. Pseudo-pillow lavas were possibly formed from high viscous feeder-dykes exposed on water bottom.