日本列島の代表的砂岩のモード組成と造構場  [in Japanese] Modal compositions of representative sandstones from the Japanese Islands and their tectonic implications  [in Japanese]

日本列島を代表する7つの砂岩(標準砂岩)を選び, それを素材としてモード組成の測定方法の基礎的問題を詳しく検討し, 伝統的な方法とGazzi-Dickinson methodとの比較を行った。Gazzi-Dikinson methodは, 伝統的な測定方法に比べて, 粒度による影響を受けにくく, また, 個人差も小さくなるという利点があきらかとなった。標準砂岩の属する地層群の平均的砂岩モード組成と造構場との関係を基に, 後背地としての火成弧の発展過程とモード組成との対応関係を示すQ-F-Rダイアグラムの試案を提案した。未発達の島弧, 成熟した火成弧, 削剥された火成弧, 深く削剥された火成弧, 火山活動の再開した火成弧という後背地の識別ができるが, 深く削剥された火成弧を後背地をもつと認定した砂岩の組成には堆積環境も強く反映していると考えられる。

We have selected seven specimens of Jurassic to Paleogene sandstones from the Japanese Islands, which represent different sedimentary basins and different tectonic settings. Firstly, some methodological problems concerning modal composition analysis have been checked by analysing the same specimens by nine of the above authors. As a result, the data show that the compositional variations of sandstones are quite high. Thus, we have to be careful when comparing the modal compositional data of different workers. It is also concluded that, in order to evaluate the provenance factor controlling the sandstone composition, the Gazzi-Dickinson point-counting method is better than the traditional one, although the latter is still useful for the description of sandstone. Secondly, we have examined the relationship between the modal composition of sandstone and its tectonic setting. The following five tectonic settings for magmatic arc provenance have been successfully discriminated: primitive volcanic island arc, evolved and mature magmatic arc, dissected magmatic arc, deeply dissected magmatic arc, and renewed magmatic arc.