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(1959, 1966), the ophiolite sequence is com posed of plutonic members which have been derived in solid state from the upper mantle, and of subvolcanic and extrusive members which have crystallized from a basaltic magma. Many geologists are still reluctant to accept this model because field observa tions do not suggest such genetic relation, that is, often there are gradual transitions between the "plutonic" and the hypabyssal and extrusive rocks. In such cases the ophiolite sequence shows what may be called "stratigraphy" , ranging from ultramafic or basic rocks with coarse-grained granular texture at the base to successively basic rocks with fine-grained texture at the top. Aubouin (1965) defined the ophiolite as complex masses of ultramafic and basic rocks which were extruded on the eugeosyn clinal domain at the earlier geosynclinal stage. After current conception the ophio litic magmatism is a type of powerful sub marine fissure-eruptions of basaltic magma, during which the outer surface was chilled to form a fine-grained crust enclosing the fluidal magma, in which gravitational differ entiation proceeded. Reinhardt (1969) discussed the origin and emplacement of the Oman ophiolites and concluded that the gradual transition between the group of gabbroic-basaltic suite and that of ultramafic rocks was caused by the assimilation of gabbroic injection into peridotites at a great depth. In any way, there are no established theories on the genetic relation between plutonic members and subvolcanic to ex trusive ones of the ophiolitic magmatism. In recent years, many works have been done about Late-Paleozoic submarine vol canic rocks in Japan from the viewpoint of geosynclinal volcanism (Kanmera, 1968, 1969; Tanaka, 1970; Hashimoto, Kashima and Saito, 1970; Sugisaki, Tanaka and Hattori, 1970; Kanisawa, 1971). The "Mikabu" green-rocks, which occur along the boundary between the belt of Sambagawa crystalline schists and the Chi chibu Belt, consisting of Paleozoic forma tions, are now thought to be a product of initial basic submarine volcanism on the Late-Paleozoic Chichibu geosyncline (Suzu ki, 1966, 1967; Iwasaki, 1969). Suzuki and others have described remarkable magmatic

GEOSYNCLINE VOLCANISM OF THE MIKABU GREEN-ROCKS IN THE OKUKI AREA, WESTERN SHIKOKU, JAPAN