奈良県十津川地域における日高川帯の緑色岩類

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(1) Shimanto terrain in the Kii peninsula can be divided into three belts: Obamine belt (northernmost belt), Hitakagawa belt (northern belt) and Muro belt (southern belt). The Hitakagawa belt can be further divided into two sub-belts: northern sub-belt and southern one. Greenstone, the subject of the present paper, occurs predominantly in the southern sub-belt of the Hitakagawa belt. Rock group of the southern sub-belt is designated as the Hiradani Group, which is divided into two formations: the Hatenashi formation (lower) rich in sandstone and the Kohara (upper) rich in greenstone and shale. The Kohara formation is considered to be Late Cretaceous in age. (2) The greenstone in the Kohara formation commonly occurs in intimate relation with red shale and greenish grey shale, whereas it does not occur so frequently with sandstone. And the greenstone body seems to thin out abruptly and the body does not extend horizontally so much. In the Totsukawa area there are several distribution centres of the greenstone body with shale, showing isolate steep hill- or Guyot?-shape in their outline. In these distribution centres, the eruption activities of the greenstone possibly continued through some duration in Late Cretaceous period. (3) The greenstones in this area are divided into three types : basaltic lava, basaltic pillow lava and basaltic lapilli tuff. And these greenstones are aphanitic basaltic rocks, in which the original volcanic textures, e.g., glassy-, variolitic-, subophitic- and intergranular-ones, are well preserved. (4) Metamorphic minerals such as prehnite, pumpellyite, leucoxene, chlorite, quartz, calcite, epidote, sodic plagioclase, and potassium feldspar are found to occur both in groundmass and in veins through the greenstones. In this area the low-grade regional metamorphism took place in the period later than Late Cretaceous. (5) The greenstones of this area are of low potassium and have high ratio of K/Rb and are chemically similar to the abyssal basalt. The chemical features of the shale of this area are similar to those in the Permian shale of Japan. It may be suggested that the environmental condition of eruption of these basalts in the Shimanto geosyncline is similar to that of the Palaeozoic Chichibu geosyncline. (6) Most Mesozoic basalts of this area belong to high alkali tholeiite and few of them are of alkali basalt. On the contrary, nearly half of basalts in the Japanese Palaeozoic geosyncline is of alkali basalt and the rest is of high alkali tholeiite. (7) The solidification index of these greenstones suggests that the differentiation of the basalt magma did not proceed so much. The molar diagram of the chemical composition of these basalts indicates pigeonite-augite fractionation in the process of the magmatic differentiation.
日本地質学会の論文
1971-07-31

奈良県十津川地域における日高川帯の緑色岩類

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