奥羽脊梁山脈焼石岳南麓の先第三系基盤岩類について

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The pre-Tertiary rocks which consists of the basement of the Neogene strata around the Ou Backbone Range are distributed along the upstream of the Isawa River at the foot of Mt. Yakeishidake, Iwate Prefecture. The pre-Tertiary rocks are classified into three formations and several intrusives. The oldest formation called the Chinzanbashi Formation is composed of grayish green sericite quartz schist of acidic volcanic origin and is overlain by the Jonokurazawa Formation. The Jonokurasawa Formation which occupies a stratigraphic position intermediate between the Chinzanbashi and the Arasawa formations, is composed of melanocratic hornblendite and hornblende schist of basic volcanic origin, occasionally with the intercalation of thin biotite schist, derived from slate. The Arasawa Formation is composed of non-metamorphosed black slate and is in fault contact with the Neogene Oarasawa Formation. The stratigraphical relation between the Jonokurasawa Formation and the Arasawa Formation is uncertain owing to the interruption of the granodiorite. These three formations mentioned above are intruded by quartz-diorite, ultra-basic rocks and granodiorite. The occurrence of the oldest intrusive rocks, melanocratic quartz diorite, is confined within the area of distribution of the Jonokurasawa Formation. The former intrudes the latter with concordance. The ultra-basic intrusive rocks are distributed along a major tectonic zone of NNW-SSE trend in the central part of this area. The intrusives are composed of several lenticular bodies of hornblendite and serpentine. The granodiorite which occupies almost half of this area intrudes the Chinzanbashi Formation discordantly and is bounded by the major fault of NNW-SSE trend on the east. Potash feldspar bearing pink coloured granodiorite occurs only along the fault of NW-SE trend at the western part of this area. From the lithological similarities and grade of metamorphism the Jonokurasawa Formation is considered to be a correlative of the Motai Formation in the Kitakami massif and the Matsugatai Formation in the Abukuma massif, though no fossil evidence for determination of the geological age has been found from this area. Both the Motai and the Matsugatai formations are considered to have been formed as the products of the initial basic volcanic activity at the axial zone in the Honshu eugeosyncline during Devonian time. It is highly probable from the structural point of view that the Jonokurasawa Formation in this area is the representative of the northern extension of the Matsugatai Formation which is distributed along the eastern margin of the Abukuma massif. These facts seem very important for further studies on the geological structure of the basement beneath the Green Tuff Region in Northeast Japan.
東北大学の論文
1971-03-31

奥羽脊梁山脈焼石岳南麓の先第三系基盤岩類について

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