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.

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Petrological characteristics of greenstones from the Sanchu belt were determined. The rocks are closely associated with serpentinite masses along the south marginal fault of the belt. A preliminary K-Ar age for the least metamorphosed rock is 120 Ma. They commonly have phenocrysts of plagioclase and olivine, which is totally altered into chlorite and has small euhedral inclusions of chromian spinel. Major-element bulk chemistry indicates they are tholeiitic basalts of MORB affinity. They especially resemble E-type MORB on the basis of chondrite-normalized rare-earth element distribution patterns as well as of Hf-Ta-Th ratios. Relic chromian spinels are enriched with Ti and Fes³⁺, showing a derivation from within-plate basalt or E-type MORB.Geochemical and mineralogical data indicate that the Sanchu greenstones are most likely E-type MORB.

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The Hashigami plutonic mass, northern Kitakami, Iwate prefecture, Japan, has been dated by Rb-Sr method. The plutonic mass consists of three successive intrusives of tonalitic to granodioritic composition. The Rb and Sr abundances of the rocks from the first intrusive are within limited ranges, and thus their Sr isotopic compositions do not change much. Although the rocks from the second intrusive do not vary much in Sr and Rb concentrations, they yielded an isochron of 135±25 Ma with a low initial ⁸⁷Sr/⁸⁶Sr ratio (=0.70355±0.00006). The ⁸⁷Rb/⁸⁶Sr ratios of the last intrusive rocks are low and similar to each other. Their ⁸⁷Sr/⁸⁶Sr ratios are also low and do not change much. None of the rocks from the first and the last intrusives is on the isochron. All intrusives might have originated from the common parental material, but crustal assimilation during upwelling possibly disturbed their Rb-Sr systematics.

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Thermoluminescence (TL) dating was made for volcanic rocks and pyroclastic deposits around Toya Caldera, Southwest Hokkaido. TL ages of Nakajima lava dome in the caldera are 0.040, 0.041 and 0.045 Ma, and those of Toya Pyroclastic Flow Deposit are 0.103-0.134 Ma (average: 0.119 Ma for 6 samples). Both data coincide well with the ages which were estimated indirectly from ¹⁴C dating of the related pyroclastic deposits and a previously reported fissiontrack age, respectively. TL ages of the Takinoue Pumice Flow Deposit (0.90 Ma) and the Sobetsu Pumice Flow Deposit (1.59 Ma) indicate them to be middle and early Pleistocene, respectively, although they were vaguely considered as Pliocene or Pleistocene in age. A TL age for Miocene Osarugawa Formation showed only 2.57 Ma, which is interpreted as an age started from the time when the temperature of the formation decreased below 40-50°C. In case of a low density pumice sample, a reasonable age was obtained in terms of weight correction.

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