日本列島のペルム紀〜白亜紀砂岩の改訂BI図と後背地の造構場  [in Japanese] Revised BI diagram and provenance types of the Permian-Cretaceous sandstones in the Japanese Islands  [in Japanese]

砂岩の改訂版BI(Basicity Index) ダイアグラムと構成物の特徴に基づき, 日本列島のペルム紀〜白亜紀供給源の発達過程や主要な地質区間の組成的関連を検討した.南部北上帯や舞鶴帯, 秩父累帯のペルム紀〜ジュラ紀砂岩は, BIダイアグラム上でIIA(immature island arc) 領域からEIA(evolved island arc)領域をへて, CA (continental arc) & DA(dissected arc)領域へと変化する.砂岩構成物の特徴も考慮すると, この変化は, 未発達な島弧から, 花崗岩を伴ったより発達した島弧をへて, 花崗岩と高度変成岩から構成される後背地へ変化したことを示している.これは, これらの地質区のペルム系〜ジュラ系が同じような後背地を有していたことを暗示している.秋吉帯や飛騨外縁帯のペルム系および北部北上帯のジュラ系などのデータもこの結論と調和的である.白亜紀初期に後背地は, 安山岩質〜石英安山岩質の火山岩に覆われた.

On the basis of revised BI (Basicity Index) diagram [Al_2O_3/SiO_2-(TFeO+MgO)/(SiO_2+K_2O+Na_2O)] and modal compositions of sandstones from the Permian to Cretaceous in the Japanese Islands, we discuss the evolution of provenance areas and the compositional relationships of sandstones among the main geologic provinces. The Permian to Jurassic sandstones in the South Kitakami Belt, Maizuru Belt and Chichibu Composite Belt migrate from IIA (immature island arc) field through EIA (evolved island arc) field to CA (continental arc) & DA (dissected arc) field in the BI diagram with time. Considering modal compositions, the geochemical change is indicative of provenance transition from immature island arc composed mainly of andesitic to rhyolitic volcanic rocks to continental arc made up mainly of granitic and high-grade metamorphic rocks in each geologic province. This suggests that the Permian to Jurassic clastic rocks in the South Kitakami, Maizuru and Chichibu Composite Belts were supplied from the same source. Permian to Jurassic sandstones from other geologic provinces such as the Akiyoshi, Hida-Gaien, Oshima and North Kitakami Belts also support this conclusion. The source was covered by andesitic to dacitic volcanic rocks from the beginning of the Cretaceous onward. The nature of volcanism in the source during the Cretaceous changed from andesitic to rhyolitic in the Turonian-Coniacian.