北上・阿武隈山地に分布する中・古生界の砂岩組成  [in Japanese] Compositions of the Paleozoic to Mesozoic sandstones distributed in the Kitakami and Abukuma Mountains, Northeast Japan  [in Japanese]

北上・阿武隈山地に分布する中・古生界の砂岩の組成を検討した。その結果, 砂岩組成は当時の造構環境をよく指示し, シルル系, 上部ジュラ系〜下部白亜系先宮古統, 阿武隈山地の上部白亜系の砂岩はQ-F-R図のQ-F辺に, デボン系, 下部石炭系の砂岩はF-R辺に, 下部白亜系宮古統, 北上山地の上部白亜系, 古第三系の砂岩はQ-R辺に集中し, それぞれ花崗岩質岩(大陸性), 火成活動を伴う島弧および下位堆積岩類からの再食によることを示し, 地質調査の結果から得られている造構環境と調和する。北部北上山地を構成する付加体中の砂岩は鉱物組成と化学組成から大きく東西に二分できることが示され, 今後の構造帯区分の指標となることが明らかになった。

In the Kitakami and Abukuma Mountains, Paleozoic to Mesozoic sandstones are widly distributed with peculiar compositions reflecting the tectonic movement and geology of the hinterland during each geologic age. Q-F-R diagram of the sandstones of each geologic age shows that the Silurian and Jurassic sandstones of the South Kitakami and Abukuma Mountains, and Upper Cretaceous sandstones of the Abukuma Mountains are concentrated along the Q-F side (quartzose and feldspathic), that the Permian sandstones of the South Kitakami Mountains, Lower-Upper Cretaceous and Paleogene sandstones of the Kitakami Mountains are concentrated along the Q-R side (lithic), and that the Devonian and Carboniferous sandstones of the South Kitakami Mountains are concentrated along the F-R side. The three types of sandstones are indicative of their derivation from a continental setting (including granitic rocks), underlying sedimentary rocks (chert and argillite) and an arc setting with volcanics, respectively. Their geologic setting nearly coincides with the tectonic environments recognized from geologic surveying. The Triassic sandstones of the South Kitakami Mountains and Paleogene sandstones of the Abukuma Mountains plot in the central field of the Q-F-R diagram, suggesting a complex geologic setting. The Jurassic sandstones of the South Kitakami and Abukuma Mountains change in composition with time, implying that both regions belong to the same petroprovenance. The accretionary complex in the North Kitakami Mountains with exotic blocks of Carboniferous to Triassic limestones and Permian to Triassic cherts are inferred to be Jurassic to Early Cretaceous inage. At present, the tectonic division of the North Kitakami Mountains is under discussion. The sandstones of the "Northern Kitakami Belt" are rich in plagioclase in contrast to this, those of the "Iwaizumi Belt" and "Taro Belt" are rich in K-feldspar. The differences in both sandstones are also clear in chemical composition. For example, in the diagram K_2O/Na_2O-SiO_2, the sandstones of the "Northern Kitakami Belt" plot in the fields of oceanic island arc and active continental margin, and those of the "Iwaizumi Belt" and "Taro Belt" plot in the fields of active continental and passive margins. The Jurassic to Early Cretaceous rocks in the North Kitakami Moutains can be divided into two areas in terms of sandstone composition.