伊予大洲市に分布するみかぶ緑色岩類の地向斜火成活動

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The Mikabu green rocks exposed on the hills of Tomisuyama (Figs. 1, 2), Ozu City, Ehime Prefecture, lie conformably on the crystalline schists of the Sambagawa and Chichibu Belts. They are layered rocks consisting of basic tuff, lapilli tuff, pillow-lava, basalt, diabase and gabbro. Quartz-diorite intrudes diabase and gabbro. The core of these green rocks is occupied by coarse-grained rocks (gabbro), and fine-grained rocks form a carapace around the coarse-grained rocks. Medium-grained rocks (diabase) grade imperceptibly into the fine-grained facies on the one hand and into the coarse-grained facies on the other. There are no clear-cut boundaries between the different petrographic types, excepting quartz-diorite. About 30 samples of various rock-types of the Mikabu green rocks from Tomisuyama (Fig. 3) were collected and their major chemical constituents have been analysed (Table 1). The silica content of these samples is almost 47-51% and falls within the range of basaltic rocks. On the (Na_20+K_20)-SiO_2 diagram (Fig. 4) and Al_20_3-(Na_20+K_20) diagram (Fig. 5) after KUNO (1960, 1966), these green rocks belong to high alkali and low alkali tholeiite. From particular poverty of K_20 (Figs. 6, 7) and titanium contents, it may be said that these green rocks are oceanic tholeiite. The Mikabu green rocks in this area were found to show remarkable magmatic differentiation. On the MgO-(FeO+Fe_20_3)-(Na_20+K_20) diagram (Figs. 9, 10), these green rocks show a wide range of composition and the plotted points are elongated parallel to the MgO-(FeO+Fe_20_3) side, that is, in the direction of tholeiitic trends of differentiation. Successive increases in (FeO+Fe_20_3)/MgO ratio are recognized towards the fine-grained rocks from the coarse-grained rocks. Moreover, oxide variation diagrams plotted against solidification index (SI) (Fig. 11), TiO_2-P_20_5 diagram (Fig. 12) and Al_20_3-TiO_2 diagram (Fig. 8), show also remarkable differentiation. These facts show that the fine-grained rocks are products of more progressive crystallization differentiation than the medium- and coarse-grained rocks. The fine-grained rocks at the top are entirely similar to those at the bottom, with regard to petrographic and petrochemical characters. On the other hand, magmatic differentiation by crystal accumulation in the coarse-grained rocks is also recognized, namely, with higher SI towards the bottom and lower SI towards the top. From the mode of occurrence and petrochemical data, a new model of emplacement of the Mikabugreen rocks is discussed (Fig. 13); that is, these green rocks underwent magmatic differentiation at great depth including perhaps the upper mantle. The magma formed at shallower levels first extruded near the eugeanticlinal ridge of the geosynclinal domainand formed the carapace of the fine-grained facies. The magma formed at greater depth was continuously uplifted, and in the core of this green rock mass cooled and solidified slowly, undergoing gravity differentiation again and fractional crystallization, and medium- and coarse-grained rocks were formed. It is conceivable that quartz-diorite is a final product of the Mikabu igneous activity.
日本地質学会の論文
1971-07-31

伊予大洲市に分布するみかぶ緑色岩類の地向斜火成活動

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