LATE CRETACEOUS U PB ZIRCON AGES AND PRECAMBRIAN CRUSTAL INHERITANCE IN RYOKE GRANITOIDS, KINKI AND YANAI DISTRICTS, JAPAN/

Ryoke granitoids on Ikoma-yama and Katsuragi-yama in the Kinki Distric t, and on Yashiro-jima in the Yanai District, have Late Cretaceous zir con U/Pb isotopic crystallization ages. Older and Younger granites on Ikoma-yama were emplaced within a short time interval between 81 and 8 2 +/- 1 Ma. Younger Ryoke granite on Katsuragi-yama, located south of Ikoma-yama, crystallized at 75 +/- 3 Ma. The Gamano granodiorite and K ita-Oshima granite-gneiss on Yashiro-jima have overlapping crystalliza tion ages of 95 to 96 +/- 1 Ma. This result supports earlier ideas tha t the Gamano and Kita-Oshima rocks are cogenetic. An inherited compone nt 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 detritu s.