抄録
Geochemical criteria for the discrimination of origins and depositional environments of siliceous sedimentary rocks are reviewed. Biogenic siliceus rocks containing radilarians and sponge spicules are extremely enriched in SiO2 and show an inverse correlation between SiO2 and hydrolyzates such as TiO2, Al2O3, and Zr. This suggests that biogenic siliceous rocks are regarded as a mixture of clays and biogenic silica. Volcanogenic siliceous rocks, derived from acidic volcanoclastics, are characterized by low Fe2O3/SiO2 and high Al2O3/TiO2 ratios that change during magma processes. Hydrothermal siliceous-rocks are characterized by high concentrations of Fe, Mn, and low ratio of Al/(Al + Fe + Mn). Diagrams of SiO2/TiO2-Al2O3/TiO2 and Al-Fe-Mn are useful tools for distinguishing origins of siliceous rocks. Hydrogeneous elements such as MnO and Co can be an important key to depositional marine environments. The intensity of hydrothermal activity calculated by fraction analysis may also be a criterion for depositional environment of siliceous sedimentary rocks with hydrothermal emanations. The results show that the bedded cherts in the Mino terrane, central Japan, had deposited in hemipelagic region and those in the Franciscan terrane, California, had deposited on or near some active ocean ridge.
