Isotopic ages of siliceous shale from Hida-Kanayama, central Japan

抄録

Whole-rock samples of siliceous shale, assigned to lower Tithonian by radiolarian fossils, from Hida-Kanayama give a Rb-Sr isochron age of 128±3Ma. K-Ar ages for these samples range from 125 to 133 Ma except two much older ages, which are perhaps caused by the presence of detrital material. On the basis of the concordance between Rb-Sr and K-Ar ages and the lack of significant thermal event, the age of 128Ma is interpreted as indicating the time when the whole-rock became closed with respect to Rb-Sr system, thus there should be a period of about 20Ma between the deposition and chemical diagenesis. The initial ⁸⁷Sr/⁸⁶Sr ratio for the shale is determined to be 0.7155 ± 0.0004 and much higher than the contemporaneous marine ⁸⁷Sr/⁸⁶Sr ratio. This fact suggests the existence of older rocks in the provenance.