New K-Ar ages of four volcanoes; Mutsu-Hiuchidake, Osoreyama, Nanashigure, and Aoso volcanoes are presented. All of which are located in the Aoso-Osore volcanic zone along the Quaternary volcanic front of the Northeast Japan arc, and characterized by relatively low-K contents.
The obtained ages are as follows: 0.73±0.05 Ma for Mutsu-Hiuchidake, 0.74+0.06, 0.81±0.05 Ma for Osoreyama, 0.96±0.09, 1.06±0.07 Ma for Nanashigure, and 0.38±0.04, 0.40±0.02, 0.38±0.03, 0.38±0.04, 0.38±0.05 Ma for Aoso volcano.
Lateral and temporal variation of silica-normalized potassium contents (SiO₂=57.5%) of the rocks from volcanoes, which are distributed in the volcanic front side during the past 4 m. y., is investigated. Lavas with relatively low potassium content have erupted continuously during past 4 m.y. in the whole area, whereas lavas with relatively high potassium content have erupted from volcanoes of the Sekiryo zone with ages younger than 0.7-1.0 Ma which is the formation age of the present volcanic zonation.

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Cenozoic volcanic rocks are widely distributed in the south of Uglegorsk in the western part of South Sakhalin. In the north of the town, large numbers of dolerite sheets and dykes, which are represented by the Morotsu dolerite sheet, intrude the Miocene formations. K-Ar ages of two basalts, a dolerite and an andesite are 11.3, 11.8, 9.2 and 4.7 Ma respectively and these are consistent with the stratigraphical succession. However, one basalt dated at 18.0 Ma is older than the others.
Based on major element compositions, the basalts belong to the high alkali tholeiite series and the andesite to the calc-alkali rock series. On the MORB-normalized spidergram patterns of minor element compositions including REE, the volcanic rocks are divided into two groups. Those of the first group are similar to those of E-MORB and BABB and are different from that of high alkali tholeiite of the Japan arc because of enrichment of Nb. On the εNd-⁸⁷Sr/⁸⁶Sr diagram, two basalts and an andesite lie on the mantle array and are considered to have been derived from enriched source material which is different from that of the Quaternary volcanic rocks of NE Japan. The basalt and dolerite having higher εNd and ⁸⁷Sr/⁸⁶Sr may have been affected by interaction with sea water or post-magmatic alteration.

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The early to middle Cretaceous mélange complex crops out along the eastern part of the Nakanogawa group, central Hokkaido. We found lamprophyre (camptonite) from the Tachiiwa mélange complex in the southern most part of the Nakanogawa group. This camptonite occurs as dyke form in the pelagic sediment, which is surrounded by the basic tuff matrix. The camptonite is composed of alkali feldspar, kaersutite, salite, biotite, opaque minerals, apatite and aegirine-augite. The K-Ar dating for the camptonite indicates an age of 95 Ma. The mineralogical and bulk chemical features of the camptonite shows the alkali-lamprophyre, e. g. the MORB-normalized pattern of the camptonite does not show negative Nb-anomaly. The camptonite would be emplaced in the environment of oceanic islands or sea mount during the early to middle Cretaceous age (Cenomanian) based on mode of occurrences and chemical characteristics.

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