Rb–Sr isochron ages of the Cretaceous granitoids in the Ryoke belt, Kinki district, Southwest Japan

Abstract Granitoids are widely distributed in the Ryoke belt and have been divided into four main igneous stages based on their field setting. In this paper, we present Rb–Sr isochron ages for the younger Ryoke granitoids (second stage to fourth stage) in the Kinki district. The Yagyu granite (second stage) gave a Rb–Sr whole‐rock isochron age of 74.6 ± 10.9 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70938 ± 0.00016, and a Rb–Sr mineral isochron age of 71.8 ± 0.1 Ma. The Narukawa granite (second stage) yielded a Rb–Sr mineral isochron age of 79.5 ± 0.4 Ma. A Rb–Sr whole‐rock isochron age of 78.3 ± 3.0 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70764 ± 0.00014 was obtained for the Takijiri adamellite (third stage). The Katsuragi quartzdiorite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 85.1 ± 18.3 Ma (initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70728 ± 0.00006), and mineral isochron ages of 76.9 ± 0.5 Ma and 74.8 ± 0.5 Ma. The Minamikawachi granite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 72.8 ± 2.0 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70891 ± 0.00021. These age data indicate that the igneous activity in younger Ryoke granitoids of Kinki district occurred between 80 and 70 Ma, except for the Katsuragi quartz diorite. The isotopic data on the various igneous stages in Kinki district correspond with the relative timing from field observations. Based on the initial ⁸⁷Sr/⁸⁶Sr ratios, the granitoids of the Ryoke belt in Kinki district are spatially divided into two groups. One is granitoids with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.707–0.708, distributed in the southern part of the Ryoke belt. The other is granitoids with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.708–0.710 distributed in the northern part of the Ryoke belt. The initial ⁸⁷Sr/⁸⁶Sr ratios of granitoids increase with decreasing (becoming younger) Rb–Sr whole‐rock isochron ages.