Along-arc lateral variation of Rb−Sr and K−Ar ages of Cretaceous granitic rocks in Southwest Japan

Abstract

Cretaceous granitic rocks were emplaced over a distance of 700 km along arc in Southwest Japan. Rb−Sr and K−Ar ages of a major group of these granitic rocks, with ilmenite series ore mineralogy, were examined. Rb−Sr whole rock ages of 92.8±4.0 Ma and Rb−Sr and K−Ar biotite ages of 80–88 Ma were obtained on one group of these granitic rocks from Kamo-Sera area of central Hiroshima Prefecture. The K−Ar ages of various minerals, combined with the Rb−Sr whole-rock age, give a smooth cooling curve, which suggests a 5 to 10 Ma time-lag between intrusion and cooling at 300° C for the Cretaceous granitic rocks. The Rb−Sr whole-rock and Rb−Sr/K−Ar biotite ages of these granitic rocks become younger eastward along the Southwest Japan arc, and the time-lag between the two systems remains constant at 5 to 10 Ma over the entire area. The along-arc age variation does not support the genetical relationship of the Cretaceous granitoids with steady-state subduction. The Cretaceous granitic province at the eastern margin of Eurasian continent was, at least partly, formed by an episodic event such as ridge subduction.