Rb-Sr Isochron of the Cretaceous Acid Volcanic Rocks of Himeji District, Hyogo Prefecture, Japan

Rb-Sr isochrons were obtained on the cretaceous acid volcanic rocks of Himeji district, Hyogo prefecture, Japan. The isochron of the Tenkadaiyama forInation, younger rocks in this area, yielded T=67.5±5. 9m. y. and (87Sr/86Sr) . =0.7105±0.0006by both whole rock specimens and separated fractions. On the contrary, it was determined two ages on the Aioi formation, older extrusive rocks. One age was obtained by the whole rock isochron which gave T=105±16m. y. and (87Sr/86 Sr) 1= 0. 7057±0.0008. The other was obtained by internal isochrons. Two rock specimens of the Aioi formation were separated for internal isochrons. One gave T=70±7m.y. and (87Sr/86Sr) =0.7079±0.0007, and the other gave the same age and higher initial strontium ratio, respectively. This indicates that the Aioi formation erupted at about100m. y. BP and was influenced at about70m. y. BP by the eruption of the Tenkadaiyama formation which caused re-equilibration of the Rb-Sr system in the mineral parts of the Aioi formation to give a younger internal isochron age.