群馬県西部に分布する上部中新統霧積累層の緑色化変質と岩石磁気
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概要
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Upper Miocene Kirizumi formation is distributed in the western part of Gunma prefecture, Central Japan. The Kirizumi formation is composed of andesitic pyroclastic rocks subjected to green coloured alteration. Yunosawa lavas intercalated of the Kirizumi formation is distributed from non-alterd area to intensely alterd area successively, so that they are fit to study research the influence of green coloured alteration on rockmagnetism. The author has been studied green coloured alteration of the Kirizumi formation and investigated the influence of green coloured alteration on rockmagnetism. The results are as follows. (1) The Kirizumi formation is divided into four alteration zones. Zone I; Montmollilonite-Saponite zone. Zone II; Chlorite-smectite mixed layer clay mineral zone. Zone III; Chlorite-Sericite zone. Zone IV; Epidote-Chlorite-Sericite zone. Those zones are arranged from zone IV to zone I arround the Nakaoyama body. The distinct feature of alteration products in this district is rich in clay minerals and carbonates in stead of zeolites. (2) In Fe-Ti oxide minerals of slightly alterd lavas, ilmenite is produced in addition to hematite and maghemite. And former has a tendency to be much more than the later two. The chemical composition of ilmenite as alteration products under low temperature is plotted to intermediate range of ilmenite-hematite solid solution line, so that those ilmenite is able to be distinguished from high-temperature ilmenite on the chemical composition. Furthermore, the microscopic feature of this low-temperature ilmenite are higher reflectance than Titanomagnetite and anisotropism. In the violently alterd lavas, hematite, maghemite and rutil are produced in Titanomagnetite. (3) In the non-alterd area, the concentration of directions of magnetization is good and the stability against AC-demagnetization is strong. In the slightly alterd area, the concentration is disorderd and the stability is weak. On the other hand, in the intensely alterd area, the concentration is very good and the stability is extremely strong.
- 地学団体研究会の論文
- 1984-05-25