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[MIS20-05] Age and depositional environment of the strata-bound manganese deposits in the Jurassic accretionary complex in southwest Japan.
Keywords:Jurassic accretionary complex, stratiform manganese deposits
The stratiform manganese deposits range in thickness from 30 to 150 cm, and occur intercalated within the bedded cherts. The red-bedded chert above the Takahira manganese ores and gray-bedded chert above the Tamaiwa manganese ores contain Late Triassic (Carnian) radioralians Trialatus longicornutus and Trialatus megacornutus. In constract, radiolarian fossils from the Takahama and Kubodomari deposit indicate Early Jurassic age (possibly Toarcian), based on the occurrence of Parvicingula nanoconica with Parahsuum and Trillus species. These results show that two manganese ore forming events occurred in the pelagic Panthalassa Ocean during the Late Triassic and Early Jurassic.
Chemical compositions of the Upper Triassic manganese deposits are characterized by the enrichments in Mn and the depletions of Co, Ni and Zn. These geochemical features are similar to those in modern submarine hydrothermal manganese deposits. In contrast, the enrichments in Cr, Ni and Zn were recognized just below the Lower Jurassic manganese deposits, suggesting an anoxic depositional environment. It is likely that the Lower Jurassic manganese deposits were formed by an oceanic anoxic event in the Early Jurassic.