島弧の火山活動と安山岩マグマの発生  [in Japanese] Arc volcanism and genesis of andesite magma in Japan  [in Japanese]

Researches on volcanic geology and magma genesis of andesites in Japan during 1970〜83 are reviewed. In the field of volcanic geology, many works concerning stratovolcanoes, pyroclastic flows and their source calderas, and subaqueous volcanisms were carried out. Systematic arrangement of parasitic vents on stratovolcano were explained as an expression of regional stress field (NAKAMURA, 1977). The sliding model of rock-slide debris avalanche deposit was supported by paleomagnetic study at the pre-historic deposit of Yatsugatake volcano (MIMURA et al., 1982). Wide spread co-ignimbrite tephra layer associated with caldera formation was clarified by MACHIDA & ARAI (1976). ONO & WATANABE (1983) pointed out the possible mechanism of pyroclastic flow traversed over the sea. Depositional ramps structure at the surface of the Ata pyroclastic flow deposit was proposed by SUZUKI & UI (1982). Steeply sloping caldera floor was confirmed by drilling data at Nigorikawa caldera which diameter is only 3km (AND0, 1983). 1783 nuee ardente deposit of Asama volcano has been examined at its distal part. Most of the materials consisting the pyroclastic flow have replaced to accidental lithic materials captured during the flowage (ARAMAKI, 1980). Various kinds of surface structures of Tertiary pillow lavas were compared with those of young oceanic ridge basalts (YAMAGISHI, 1983). Internal structure of Tertiary small-scale andesitic sea mount now exposing at the southern tip of Izu peninsula was described by ARAMAKI et al. (1983). Among petrogenetic problems of andesite, the origin of calc-alkalic andesite was discussed by many authors. SAKUYAMA (1981, 1983) proposed a mixing origin of calc-alkalic andesite, in which the components of mixing are products of fractional crystallization of a parental basaltic magma. This model neatly explains many of petrographic, chemical, and mineralogical features of calc-alkalic andesite. In several cases, however, some chemical and mineralogical discrepancies have been recognized between tholeiitic and calc-alkalic volcanic rocks within a volcano (TOGASHI, 1977; FUJINAWA, 1980; WADA, 1981). In this respect, MASUDA & AOKI (1979) effectively discerned tholeiitic and calc-alkalic volcanic rocks in Cr-Th diagram. ISHIKAWA et al. (1980) showed that variation of F content in a volcanic rock suite can elucidate the role of hydrous minerals in magmatic differentiation. SEKINE et al. (1979) determined water content of calc-alkalic magmas by gas-media high pressure apparatus. Concerning high-magnesian andesite, petrographic and chemical description showed that sanukitoid and boninite represent primary andesite magmas from mantle peridotite (SHIRAKI & KURODA, 1977; TATSUMI & ISHIZAKA, 1982). Experimental studies on these high-magnesian andesites verified physical conditions of their formation (TATSUMI, 1982). Other extensive studies on andesite include those of zonal arrangement of chemistry and petrography (SAKUYAMA, 1979; NOHDA & WASSERBURG, 1979; FUJIMAKI & KURASAWA, 1980; NOTSU, 1983), and those of statistical data handling of the volcanic rocks of Japan and surrounding areas (MIYASHIRO, 1974: ARAMAKI & UI, 1978; UI & ARAMAKI, 1978).