西南日本内帯の中・古生界砂岩

モード解析と広域比較

抄録

The sandstones from a large area (the Tamba Belt, the Ultra-Tamba Zone, the Maizuru Belt and the Akiyoshi Belt) in the Inner Zone of Southwest Japan were analysed on the modal compositions of major minerals. Based on the petrographical features and detrital modes, the Permian-Jurassic sandstones are classified into three distinctive groups, that is, lithic, arkosic and quartz-rich arkosic sandstones. The lower to middle Permian sandstones are lithic and the upper Permian sandstones are arkosic. Both of them generally include felsic volcanogenic fragments. Sedimentary environment of the Permian sandstones is assigned to deep sea floor. The Triassic sandstones are lithic, including acidic to intermediate igneous rock fragments. They are derived from a evolved and matured magmatic (continental) arc provenance. The Jurassic sandstones of the Tamba Belt change from lithic to quartz-rich arkosic through time. At the same time, their paleocurrents are changed from south-southwestward into north-northeastward. The detritus of the lower to middle Jurassic sandstones (Type II suite) are derived from a evolved and matured continental arc provenance. On the other hand, the detritus of the upper Jurassic sandstones (Type I suite) are derived from a dissected continental arc provenance. The Relationship deduced from multivariate analyses on the modal composition may reflect similarities of the depositional setting and geological provenance. The dendrograms of the sandstones made by cluster analysis, for example the phylogenetic tree of an animal or a plant, probably show the genetical relationships.