The Fuji, Hakone and Izu Quaternary volcanic area has been divided petrographycally into three zones which lie parallel to the Izu-Mariana arc. From east to west they are characterized respectively by the presence of the low-alkali tholeiite, high-alkali tholeiite, and alkali basalt (Fig.1). A marked contrast is seen geochemically between the rocks on the volcanic front (the east zone), and those of the western zones in that low-alkali tholeiite of the east zone is low in Sr content (150-300ppm), and high K/Rb ratio (1, 000). There is a good positive correlation between the Sr content and the distance from the volcanic front (Fig.5). The initial ⁸⁷Sr/⁸⁶Sr rations in the volcanic rocks from the Fuji, Hakone, and Izu area, lie in the range 0.703 to 0.704 (average very close to 0.7035). These geochemical evidences and isotopic ratios are characteristic of the rocks from most of the young island arcs. In the case of the in situ differentiation of lava flow from Fuji volcano, the chondrite-normalized REE pattern for the segregation vein lies above and nearly parallel to that for host basalt lava (Fig.8). No distinct difference in the value of the ⁸⁷Sr/⁸⁶Sr ratio, with the difference of 0.0003 can be detected. Formation of the calc-alkali rock series by cotamination of basaltic magmas with crustal materials is not required by the isotopic evidence, but it may take place to the extent that the contaminating radiogenic Sr in the crustal material can be diluted beyond detection by magmatic Sr. The possible origin of these volcanic rocks is, therefore, the melting of the upper mantle, of the oceanic crustal rocks, or of both. If the lavas was formed by the melting of the upper mantle and (or) oceanic crustal rocks, their initial ⁸⁷Sr/⁸⁶Sr ratios should be close to 0.7035, the most common value found in oceanic volcanic rocks.

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The preferred orientations of dike swarms of four areas formed from Late Miocene to Pliocene are investigated in order to know the age of change of regional tectonic stress orientation in the inner belt of Southwest Japan. As the mechanism of dike formation is considered to be a natural magma-fracturing experiment and dikes are regarded as fossil tension cracks formed parallel to the direction of horizontal maximum compression stress (σ_Hmax) of regional tectonic origin, the preferred direction of σ_Hmx could be determined from the most concentrated trend of dikes of a swarm. The most concentrated trend of dikes at northern Tango Peninsula, Shimane Peninsula, Susa area and Myoken area are N-S, N-S, N10°W and N30°W, respectively. The directions of σ_Hmax obtained from the preferred orientation of dike swarms of the four areas coincide with the direction of the maximum compressional principal tectonic stress obtained from the strike and the sense of motion of faults and folds formed in the same age. This result gives the estimation that the average direction of σ_Hmax in the inner belt of Southwest Japan was N15°W±15° from Late Miocene to Pliocene.

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