西南日本の舞鶴層群の地球化学的検討 : とくに中/上部ペルム系境界における組成変化について

Voluminous black mudstone deposition occurred in the narrow and elongated Maizuru Belt of Southwest Japan during the Permian. Geochemistry of these mudstones reflects the provenance of the Maizuru Belt across the Middle and Upper Permian boundary. Proposed provenance of the mudstones suggests derivation from a mature island arc or continental arc. Mudstones of the Maizuru Group show similar composition to PAAS (post-Archean Australian shales), but are enriched in Na and Cs, and other alkaline elements (K, Rb). Among the rare earth elements (REE), negative Eu anomaly is common in samples, and heavy REE are enriched rather than light REE. Although organic carbon contents are less than 1 wt%, the ratios C/N and N/P coincide with Red-fields ratio for marine planktonic organisms. Abrupt change in chemistry occurred at the Middle and Upper Permian boundary with high concentration of sulfur, As and Sb, and depletion in organic carbon and nitrogen, and lower ratio of Th/U and V/(V+Ni). This corresponds to enrichment in Na, and depletion in Eu, Cs, Rb and K. The environmental change across the boundary may be caused by active volcanism, which resulted in an anoxic basin with consequent decline in radiolarian productivity.