奥尻島に産する白亜紀～第三紀の火山岩類のSr同位体比を測定し,その結果に基づきこれらの火山岩類の起源物質の性質の経年変化を検討した。火山岩類の⁸⁷Sr/⁸⁶Sr初生値 (SrI値) は以下の通り。白亜紀火山岩類: 0.70532, 漸新世松江玄武岩(下部): 0.70344-0.70346, 松江玄武岩(上部): 0.70378-0.70419, 前期中新世安山岩(青苗川層): 0.70448-0.70475, 前期中新世安山岩(釣懸層): 0.70432-0.70442, 中期中新世ドレライト: 0.70325, 鮮新世火山岩類: 0.70294-0.70331。 このうち上部松江玄武岩のSrI値の大きな変異幅は,奥尻島の白亜紀花崗岩類の汚染作用によって説明され,汚染作用を受ける前の上部松江玄武岩のSrI値は0.7037前後の値と見積もられる。中期中新世以降の低いSrI値をもつ火山岩を形成したマグマの起源は,日本海拡大に伴い,背弧側に貫入したとみられるアセノスフェアに求めることができ,前期中新世以前のSrI値の高い火山岩の起源は大陸性マントルから生成されたマグマに求めることができるが,上記アセノスフェア起源のマグマ(松江玄武岩)は漸新世にも活動したものとみられる。

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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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