原地山層堆積時期の北上山地の火成活動および北部北上山地の古・中生層砂岩について
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This paper presents preliminary notes on the igneous activities of the Early Cretaceous and on the petrography of the eugeosynclinal sandstones in the Kitakami Mountains. IGNEOUS ACTIVITIES : The Kitakami Mountains are constructed mainly of Upper Paleozoic to Mesozoic sediments. The Harachiyama and its corresponding formations are Lower Cretaceous in age and their major rock facies is dacitic to basaltic pyroclastic rocks. Igneous activities before deposition of the Harachiyama Formation are known in various districts as synsedimentary volcanisms although they were not so strong. On the contrary, the igneous activites of the Harachiyama and its succeeding period were very violent: the related igneous rocks are wide spread over the Kitakami Mountains. These activities are characterized, by the association of both volcanism and plutonism. Although the present distribution of the volcanic rocks are limited to the eastern and southernmost areas of the mountains, the volcanisms probably and originally prevailed in a wider area. Investigating the combination of the volcanism and plutonism, it is ascertained that volcanic groups chiefly of dacitic composition are harmoniously associated with the intrusive groups of granitic composition, while those of andesitic to basaltic compopition, with the groups of gabbroic to granitic complex masses. Some plutonic rocks along the western margin of the mountains are shoshonitic in chemical composition. SANDSTONES :- The northern Kitakami Mountains are geologically divided into three belts from northwest to southeast, the Northern Kitakami Belt (in narrow sense), Iwaizumi Belt and Taro Belt. They consist of Permian, Triassic to Jurassic and Jurassic to Early Cretaceous eugeosynclinal deposits, respectively. The sandstones of the three belts are characteristic in possessing common constituents, such as Na-feldspar and fragments of dacitic rocks. This suggests that the volcanic rocks mainly of dacitic nature and granitic rocks rich in Na-feldspar cropped out in the source area of the sandstones. The Northern Kitakami Belt of Paleozoic age is clearly different in volumetric ratios of the constituents from the Iwaizumi and Taro Belts of Mesozoic age. The sandstones of the former belt are richer in plagioclase and fragments of dacitic rocks than the latter two belts but are poorer in quartz and alkali feldspars. Inferring from these facts, the volcanic rocks were certainly dominant in the source area of the Paleozoic sandstones but granitic rocks in those of the Mesozoic sandstones.
- 日本地質学会の論文
- 1974-07-31
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