ABSTRACT

Ryoke granitoids on Ikoma-yama and Katsuragi-yama in the Kinki District, and on Yashiro-jima in the Yanai District, have Late Cretaceous zircon U/Pb isotopic crystallization ages. Older and Younger granites on Ikoma-yama were emplaced within a short time interval between 81 and 82 ± 1 Ma. Younger Ryoke granite on Katsuragi-yama, located south of Ikoma-yama, crystallized at 75 ± 3 Ma. The Gamano granodiorite and Kita-Oshima granite-gneiss on Yashiro-jima have overlapping crystallization ages of 95 to 96 ± 1 Ma. This result supports earlier ideas that the Gamano and Kita-Oshima rocks are cogenetic. An inherited component of Precambrian (1855 to 1953 Ma) radiogenic lead is present in some of the zircons from the Ryoke granitoids analyzed in this study. This result implies that older continental materials were assimilated into the granitic magmas. Possible sources include Precambrian basement or younger upper crustal rocks that contain recycled Precambrian detritus.