Temporal variation of Sr isotopic compositions of the Cretaceous to Tertiary volcanic rocks from Okushiri Island, Northeast Japan Sea.

Abstract

Sr isotopic compositions have been determined for the Tertiary volcanic rocks from Okushiri Island, Northeast Japan Sea, together with the temporal variation of compositions of magma sources beneath Okushiri Island. The Cretaceous to Tertiary volcanics from Okushiri Island have the following SrI (initial 87Sr/86Sr) ratios; Cretaceous Monai volcanics: 0.70532 (Shuto et al., in preparation), Oligocene, Lower Matsue basalts: 0.70344-0.70346, Upper Matsue basalts: 0.70378-0.70419, Early Miocene andesites in the Aonaegawa Formation: 0.70448-0.70475, Early Miocene andesites in the Tsurikake Formation: 0.70432-0.70442, Middle Miocene dolerite: 0.70325, Pliocene volcanics: 0.70294-0.70331. Significant Sr isotopic variations recognized in the Upper Matsue basalts may be explained by assimilation of the Cretaceous granitic rocks from Okushiri Island. Thus the uncontaminated Upper Matsue basalts may yield SrI ratio around ?? 0.7037. Comparative examinations of SrI ratios reported by the previous and present workers for the volcanic rocks after Cretaceous from the back-arc side in the NE Japan arc and the Japan Sea suggest that the primary magmas for the Cretaceous and the early Miocene volcanics from Okushiri Island were generated from the sub-continental mantle enriched in Sr isotopic ratio, whereas those for the middle Miocene and Pliocene volcanics from the same island were derived from the depleted asthenosphere, probably injected into the sub-continental mantle during the opening of the Japan Sea. The data on the Matsue basalts indicate a possibility of the generation of basalt magmas from the depleted asthenosphere before the spreading of the Japan Sea.