1994 年 103 巻 3 号 p. 207-220
The Myoko volcano group, consisting of five stratovolcanoes, Myoko, Kurohime, Iizuna, Yakeyama and Madarao, is situated in the northern part of central Japan. Twenty one volcanic rocks from the early stage of volcanic successions of Myoko (5 samples), Kurohime (4 samples), Iizuna (9 samples) and Madarao (3 samples) were dated with K-Ar methods. On the basis of previously well studied geology and petrology of the volcano group and chronology of marker tephra layers in this area together with newly obtained K-Ar ages, volcanic history of the Myoko volcano group was discussed in detail. The results confirmed the genetical story of the volcanoes by Hayatsu (1985), i. e., the Myoko volcano has grown by four stages of active volcanisms with three distinctive pauses and each stage has a chemical fractionation trend from basalt to dacite through andesite during a volcanism, and other volcanoes were also in the same story though number of stages was variable : three for Kurohime and two for Iizuna and Madarao, and the fractionation trend was little changeable. This paper calls this type of volcano “poly-generation volcano”.
Growth mechanism of the poly-generation volcano was also revealed in detail : life span of each generation, that is time span of each stage of activity, is nearly constant, 20-50 kyrs though some exceptions but total amount of volcanic ejecta decreases with time, e. g., 40-20-7-5 km3 for Myoko, and pause period between any two active volcanic stages decreases with time from 100-160 kyrs of early pause to ca. 10 kyrs of late one.
This poly-generation volcano is common in Japan and should be studied in further detail from view points of igenous petrology and tectonics of magmatism as well as geothermal energy and volcanic hazards.