ABSTRACT

Isotopic age determinations were carried out for Jurassic siliceous shale (assigned to Bajocian by radiolarian fossils) and for, Triassic bedded chert (assigned to Anisian by radiolarians and conodonts) in Unuma, Gifu Prefecture, Central Japan. Whole-rock samples of the Jurassic shale give a Rb-Sr isochron age of 179.8 ± 5.7 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.71011 ± 0.00036, whereas two of these Jurassic samples have K-Ar whole-rock ages of 151 and 152 Ma. For the Triassic chert, the Rb-Sr whole-rock isochron shows 211.9 ± 4.7 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.71325 ± 0.00020, and a K-Ar whole-rock age for one of these Triassic samples is 207 ± 7 Ma. Two Rb-Sr whole-rock isochron ages indicate the time when the respective sedimentary rocks became chemically closed with regard to the Rb-Sr system. The initial ⁸⁷Sr/⁸⁶Sr ratios in both sets of rocks are higher than the contemporaneous marine ⁸⁷Sr/⁸⁶Sr ratio, suggesting the existence of an older landmass. The K-Ar ages of the Jurassic siliceous shale presumably represent the time of a later geologic event.