東北日本の白堊紀花崗岩II
スポンサーリンク
概要
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Northeast Japan is divided into the two zones, which are the outer and inner zones. The outer zone consists of the Kitakami and Abukuma mountainland, and the inner zone consists of the so-called " green tuff regions." Pre-Tertiary rocks are mainly distributed in the Kitakami and Abukuma mountainland.In the green tuff regions, they expose as the basement complex of the Neogene formation. These pre-Tertiary rocks are composed of the Paleozoic and Mesozoic formations and metamorphic rocks and granites. Paleozoic formations, especially Permian formations, are mainly distributed in the northern Kitakami, southern Kitakami mountainlands, and the Joetsu, Tide, Asahi mountainlands of the inner zone. Boundary between the northern and southern Kitakami mountainlands is represented by the Hayachine tectonic line. Permian formations of northern Kitakami are composed of clayslate, partly phyllite, sandstone, limestone, chert and schalstein, and resemble the rocks of the Chichibu zone of Southwest Japan. The same sediments are also distributed in southwestern Hokkaido. Permian formations of southern Kitakami are composed of clayslate, sandstone, conglomerate, limestone, and are devoid of chert. These rocks are also distributed in the eastern margin of the Abukuma mountainland. Permian formations of the Joetsu, Tide, Asahi mountainlands are composed of clayslate, sandstone, limestone, schalstein and chert, and resemble the rocks of the Mino-Tanba zone of Southwest Japan. Regional metamorphic rocks such as the Gosaisho-Takanuki metamorphics, occur mainly in the southern part of the Abukuma mountainland and form the one metamorphic belt, which is named the Abukuma metamorphic belt. The metamorphic rocks are accompanied with the so-called " older granites." Such metamorphic rocks and granites also expose as the basement complex of the green tuff in the eastern margin of the Shinjyc basin and the Taiheizan area. And so, the Abukuma metamorphic belt extends from the Abukuma mountainland to the Taiheizan area and occupies the central part of Northeast Japan. Cretaceous granites, which are intruded into the Paleozoic and Triassic, Jurassic and early Cretaceous formations, metamorphic rocks and older granites, are widely distributed in Northeast Japan. But these rocks of the green tuff region are separated into blocks and has not been studied in detail, compared to those of the Kitakami and Abukuma mountainlands. The writer studied geological and petrographical character of granites which occur in the Asahi and Tide mountainlands and other areas of the green tuff region and discussed the relation with the granites of the Kitakami and Abukuma mountainlands. The intrusive belts of granites in Cretaceous age of Northeast Japan are divided into the following three belts, from east to west : the Kitakami, and Abukuma and Joetsu intrusive belts. These three belts arrange in the direction of NNW-SSE and the boundary lines between them are the tectonic lines, which were formed by the tectonic movement in the early Cretaceous age. The former lines probably pass from Futaba through Yakeishi and Wagasennin to the Oshima peninsula and the latter probably runs from Tanakura to the Oga peninsula. Granodiorite and quartz diorite of the main bodies of the Kitakami intrusive belt and granodiorites and pink granites of the Abukuma belt are intruded elongatedly and arrange parallel to these tectonic lines. Stocks of grey granites and two mica granites also occur in the Abukuma belt, on the other hand, stocks of quartz monzonites occur in the Kitakami belt. Granites of the Taihezan area situated in the southern part of the line presumed to be the extension of the Hayachine line resemble the Abukuma granite proper, but some of the granites of Iwadate and southwestern Hokkaido are different from them. These granites are porphyritic granodiorite and are unique in Northeast Japan. Granitic rocks of the Joetsu belt are composed of hornblende-biotite granodiorite and biotite granite, which are respectively named the Sanekawa granodiorite and the Kusozu-Kogawa granite by Sugiyama and Chihara. Biotite granite is correlated to the Hiroshima granite of SouthwestJapan from its petrography and attending wolframite greisen vein. Lastly, the writer briefly discussed mutual relation between the above-mentioned geologic structure of the pre-Tertiary rocks and the fracture movement in the early Miocene, and the subsidence of the Neogene sedimentary basin.
- 1964-05-30