火成活動および変成史よりみた東北日本

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The igneous activity and metamorphic history of pre-Cenozoic deposits in Northwest Japan are summarized and briefly discussed. The nature of the igneous activity shows distinct differences between Northeast and Southwest Japan, two provinces which are divided by the Tanakura Tectonic Zone. In the Abukuma plateau, regional metamorphism of kyanite-sillimanite type took place during the Silurian-Devonian or earlier and that of andalusite-sillimanite type in the Cretaceous superposed the early metamorphism. Glaucophanitic metamorphism in the area from the eastern margin of the Abukuma plateau to the southwestern part of the Kitakami mountains probably occurred during the pre-Middle Silurian. Isotope ages of the granitic rocks in the Abukuma plateau are grouped roughly into two stages and of about 400 m.y and 150 m.y. In the southern Kitakami, the Unoki and the Tsubonosawa metamorphic rocks are parts of the basement complex of the district. Discrepancies between K-Ar age (350-315 m.y.) and geologic evidences of the Hikami granites indicate that a part of the body may have a possibility of re-intrusion or that the values of K-Ar age must be evaluated as the results of various effects after intrusion, thus their detailed field, petrological and chronological studies are expected to solve the problem. The volcanicity of the southern Kitakami mountains which are characterized by many acid volcanic rocks intercalated in the basic volcanic ones of the Silurian to Permian was different from the geosynclinal volcanism in Southwest Japan. On the contrary, volcanicity and metamorphism in the northern Kitakami mountains are similar to those of Southwest Japan. The volcanic rocks of the Lower Cretaceous show wide compositional range from moderately K_20 rich basalt to rhyolite. They are distributed along the Pacific Coast and closely associated with the Kuki, Taro, Oura, Hirota, and Orikabe granitic masses which are characterized by shallow intrusive facies arranged in order from north to south. Chemically, they show no difference between those of the northern and southern Kitakami mountains, and most of them belong to the high-alumina basalt series in chemistry. These Early Cretaceous volcanism and plutonism may construct a series of volcano-plutonic formations. The Cretaceous granitic rocks in the Kitakami mountains show higher oxidation ratio (mol. Fe_2O_3×100/Fe_2O_3+FeO) and lower total FeO/MgO ratio than those of the inner belt of Southwest Japan (mainly the Ryoke Belt). Similar relation in the total FeO/MgO ratio is recognized in the associated volcanic rocks in these two districts. These differences between the two districts may reflect the differences of oxygen partial pressure in magmatic differentiation and intrusion.
日本地質学会の論文
1974-07-31

火成活動および変成史よりみた東北日本

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