A Rb-Sr geochronological study on the Early Cretaceous Monai volcanic rocks from Okushiri Island, Northeast Japan Sea

Abstract

Rb-Sr whole rock isochron age has been determined on the Early Cretaceous Monai volcanic rocks occurring in Okushiri Island, Northeast Japan Sea. Five samples of dolerite, andesite and rhyolitic welded tuff give a well defined whole rock isochron with an age of 101.3±4.8 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70532±0.00007 (2σ). These results almost agree with the mineral and whole rock isochron ages and the initial Sr isotopic ratios determined by the previous workers for the pre-Tertiary granitoids occurring in Okushiri Island and the western margin of southwest Hokkaido. This facts indicate the concurrence of the pre-Tertiary volcanism and plutonism in these areas and the similarity in isotopic compositions of the source materials for the Monai volcanic rocks and these granitoids. The initial Sr isotopic ratio of the Monai volcanic rocks are much higher than those of the Early Cretaceous volcanic rocks from the region to the west of Hidaka Mountains and the eastern margin of the Kitakami Mountains.
Comparative investigations of initial Sr isotopic ratios reported by the previous and present workers for the Early Cretaceous volcanic rocks and granitoids from the NE Japan arc and Hokkaido indicate that initial Sr isotopic ratios for these igneous rocks increase from the eastern tholeiitic volcanic zone to the western calc-alkaline volcanic-plutonic zone, in contrast to the across-arc variation of the Sr ratios of the Quaternary volcanic rocks from the NE Japan arc.