北海道,北部日高帯の"古第三紀"砂岩の組成  [in Japanese] Modal and chemical compositions of the "Paleogene" sandstones from the northern Hidaka belt, Hokkaido  [in Japanese]

北海道, 北部日高帯の中央部および東縁部の"古第三系"砂岩について, 鏡下観察・モード組成・全岩化学組成の予察的な検討を行った。北部日高帯の砂岩は, 一般に岩片(特に火山岩岩片)を主とする石質砂岩であり, 全岩化学組成では比較的Fe_2O_3+MgOやTiO_2に富み, SiO_2に乏しい傾向にある。モード組成の特徴からこれらは火成弧起源の砕屑物を主体とすると考えられる。古第三系暁新統を主とするとみられる中央部の上興部-滝上地域, および始新世前後の地層からなるとみられる東縁部の立牛-丸瀬布地域の砂岩について比較した。前者は安山岩質岩片に著しく富む火山岩質砂岩を含み, 後背地として活動的な火成弧が推定される。それに比べて東縁部の砂岩は全体として火山岩片の含有量が少なくなり, 安山岩質な岩片より酸性火山岩起源の岩片に富む傾向がある。また, チャートなどのリサイクル起源とみられる岩片や酸性深成岩起源の岩片を含み, 前者の砂岩に比べてより発達した島弧からの供給が推定される。

This paper discribes the modal and chemical compositions of sandstones from Paleocene (?) formations in the Nishiokoppe-Takinoue (N.-T.) area, and from Eocene (?) formations in the Tatsuushi-Maruseppu (T.-M.) area, the northern Hidaka belt, Hokkaido. The sandstones from the N.-T. area are poor in quartz and rich in andesitic volcanic rock and argillaceous rock fragments. The sandstones from the area are charactrized by low SiO_2 (56-67%), high TiO_2 and Fe_2O^<3+> MgO contents, and plot in the oceanic and continental island arc fields in TiO_2-(FeO+MgO), K_2O/Na_2O-SiO_2 and Al_2O_3/SiO_2-(FeO+MgO)/(SiO_2+K_2O+Na_20) diagrams. Therefore, most of the clastic sediments in the N.-T. area were derived from an active magmatic arc. The characteristics of the sandstone from the N.-T. area are similar to those of Paleocene clastics in the Tokoro and Nemuro belts, eastern Hokkaido. The sandstones from the T.-M. area, the eastern marginal zone of the northern Hidaka belt, have a low content of volcanic rock fragments, and some what high content of quartz, chert, schists, and plutonic rock fragments, in comparison with the sandstones from the N.-T. area. Most volcanic rock fragments are acidic to intermediate in composition on the basis of their texture. These sandstones plot in the active continental margin or continental island arc fields of the discrimination diagrams described above. These compositional features strongly resemble the characteristics of rocks from the Eocene Yuyanbetsu and Rurochi Formations in the eastern marginal zone of the Hidaka belt. Clastic sediments of this zone seem to have been derived from an evolved arc and older accretionary complex provenances.