火成弧に由来する砂岩の化学組成

Abstract

火成弧に由来する日本列島の中・古生界砂岩の主成分化学組成を検討した。火成弧起源の砂岩の化学組成は, その源岩構成に強く規定されていると推定される。Al₂O₃/SiO₂-(FeO+MgO)/(SiO₂+K₂O+N₂O)図を用いることにより, これらの砂岩の供給源であった火成弧の発達程度やタイプの相違をよく識別することができる。未発達な島弧から発達した島弧, さらに陸弧へと移行するに従い, Al₂O₃/SiO₂と(FeO+MgO)/(SiO₂+K₂O+Na₂O)は, 次第に増加する。解析された火山弧に由来すると推定される砂岩は, この図中において陸弧起源の砂岩とほぼ同じ領域を占める。日本列島のペルム系, ジュラ系および白亜系の砂岩をこの判別図にプロットし, これら各時代の後背地の性格を概観した。

We propose a new diagram using major element compositions of the Paleozoic and Mesozoic sandstones derived from magmatic arcs in Japan for discrimination of the degree of growth and nature of magmatic arcs. Al₂O₃/SiO₂ and (FeO+MgO)/(SiO₂+K₂O+Na₂O) are adopted as the most discriminating parameters in general, Al₂O₃/SiO₂ is considered to correspond to the feldspar/quartz ratio in sandstones. (FeO+Mgo)/(SiO_2+K_2O+Na_2O) (basicity index=B.I.) is a rough measure of basicity of the sandstones derived from magmatic arcs. Al₂O₃/SiO₂ and B.I. increase progressively in sandstones from continental arc through evolved island arc to immature island arc. The dissected-arc type sandstones fall in the same field as the continental-arc type sandstones in our diagram, although both sandstones are clearly distinguishable from each other by their modal composition. Most of the Permian sandstones from the Ultra-Tamba Belt, Maizuru Belt and Chichibu Belt (Kyushu) plot in the immature and evolved island-arc fields. The Jurassic sandstones from the Oshima Belt, Northern Kitakami Belt, Soma Group, Tetori Group and Tamba Group display a fairly good concentration in the Al₂O₃/SiO₂-B.I. diagram, and fall in the continental-arc/dissected-arc field. Considering their modal composition, they are inferred to be of the dissected-arc type. The composition of the Cretaceous sandstones from the Middle Yezo Group, Hidaka Supergroup, Yubetsu Group, Nemuro Group, Shimanto Supergroup (Kii Peninsula and Shikoku) and Izumi Group spread in a wide area from the immature island-arc to continental-arc/dissected-arc fields, and a real differences in composition are distinct.