09:30 〜 09:45
[MIS13-03] 中国南西部昌寧―孟連帯三畳系牡音河層の堆積場の再検討
キーワード：地球化学、三畳紀、珪質岩、放散虫、パレオテチス、昌寧―孟連帯
Siliceous rocks of the Triassic Muyinhe Formation in the Changning–Menglian belt in southwestern Yunnan Province in Southwest China had been considered to be pelagic deposits. We observed them and analyzed their geochemistry, and recosidered the sedimentary place.
The observation revealed that the siliceous rocks are characterized by inclusion of abundant radiolarian test (e.g., Triassocampe Dumitrica, Kozur, and Mostler, Pseudostylosphaera Kozur and Mostler, Eptingium Dumitrica, and Paroertlispongus) and the lack of rhythmical bedding. The geochemical results are as follows: the samples have high concentrations of SiO2; most of the samples were plotted in the non-hydrothermal field on the Al–Fe–Mn diagram; most of the samples were plotted in the continental margin field on the Fe2O3/TiO2–Al2O3/(Al2O3+Fe2O3) and (La/Ce)N–Al2O3/(Al2O3+Fe2O3) diagrams. In addition, the samples show a flat rare earth element pattern normalized to North America shale composite.
These observational and geochemical results strongly suggest that the siliceous rocks are unlikely to represent pelagic deposits, indicating that the extent of the pelagic ocean basins in the Paleotethys during the Triassic is probably less than previously believed. These non-pelagic deposits may represent the closure stage of the Paleotethys.
The observation revealed that the siliceous rocks are characterized by inclusion of abundant radiolarian test (e.g., Triassocampe Dumitrica, Kozur, and Mostler, Pseudostylosphaera Kozur and Mostler, Eptingium Dumitrica, and Paroertlispongus) and the lack of rhythmical bedding. The geochemical results are as follows: the samples have high concentrations of SiO2; most of the samples were plotted in the non-hydrothermal field on the Al–Fe–Mn diagram; most of the samples were plotted in the continental margin field on the Fe2O3/TiO2–Al2O3/(Al2O3+Fe2O3) and (La/Ce)N–Al2O3/(Al2O3+Fe2O3) diagrams. In addition, the samples show a flat rare earth element pattern normalized to North America shale composite.
These observational and geochemical results strongly suggest that the siliceous rocks are unlikely to represent pelagic deposits, indicating that the extent of the pelagic ocean basins in the Paleotethys during the Triassic is probably less than previously believed. These non-pelagic deposits may represent the closure stage of the Paleotethys.