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

Granitoids are widely distributed in the Ryoke belt and have been divided i nto four main igneous stages based on their field setting. In this paper; w e present Rb-Sr isochron ages for the younger Ryoke granitoids (second stag e 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 Sr -87/Sr-86 ratio of 0.70938+/-0.00016, and a Rb-Sr mineral isochron age of 7 1.8+/-0.1 Ma. The Narukawa granite (second stage) yielded a Rb-Sr mineral i sochron age of 79.5+/-0.4 Ma. A Rb-Sr whole-rock isochron age of 78.3+/-3.0 Ma with an initial Sr-87/Sr-86 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 Sr- 87/Sr-86 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 Sr-87/Sr-86 ra tio 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 M a, except for the Katsuragi quartz diorite, The isotopic data on the variou s igneous stages in Kinki district correspond with the relative timing from field observations. Based on the initial Sr-87/Sr-86 ratios, the granitoid s of the Ryoke belt in Kinki district are spatially divided into two groups . One is granitoids with initial Sr-87/Sr-86 ratio of 0.707-0.708, distribu ted in the southern part of the Ryoke belt. The other is granitoids with in itial Sr-87/Sr-86 ratio of 0.708-0.710 distributed in the northern part of the Ryoke belt. The initial Sr-87/Sr-86 ratios of granitoids increase with decreasing (becoming younger) Rb-Sr whole-rock isochron ages.