四万十帯緑色岩類の産状を中心とした成因的考察(<特集>日本列島のオフィオライト-地団研第31回総会テーマ別討論会-)

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The green rocks in the Shimanto belt are distributed at many places in the Cretaceous and Paleogene formations in Kyushu and Shikoku. They show a succession of massive basaltic lava, pillow lava, pillow breccia, hyaloclastite, red chert with or without radiolaria, red tuff, and greenish tuff. The following common occurrence is recognized: the green rocks are always conformably surrounded by "pebbly shales", which are intensively sheared and contain pebbles of dismembered sandstones and cherts. Fragments of sandstones in "pebbly shales" are contrasted with sandstones in neighbouring turbiditic layers of fine alternation of sandstones and shales, which are always adjacent to "pebbly shales" with a tectonic contact. Sandstone fragments in "pebbly shales" are lithic arenite type and those in turbiditic layer are "greywacke" type. Therefore, it is conceivable that "pebbly shales" were originally pelagic and deformed to "tectonic melange" after deposition within deep sea realm. Ultramafics are found in the green rocks of the Sakihama body in the Muroto Formation. The green rocks of the Sakihama body are more than 1,000 m in thickness and composed mainly of pillow lava at the upper and middle parts, whereas they consist of dolerite, microgabbro, picritic rocks and serpentinized ultramafic rocks at the lower part. These green rocks are considered to show a kind of "ophiolitic" igneous sequence. Many dykes of dolerites, which were probably feeder channels of surrounding pillow lavas, are found along the Aki River in the Terauchi area, Aki City. They intruded into the "pebbly shales" parallel or oblique to their foliations. Therefore, it is concluded that the green rocks and "pebbly shales" in the Shimanto belt were formed in the same deep sea realm, whereas turbiditic layers of alternated sandstones and shales, which are in tectonic contact with the green rocks and "pebbly shales", were deposited in shallow sea environment of the continental margin. This conclusion will throw light on the geodynamics and developmental mechanism of the "Shimanto geosyncline".
地学団体研究会の論文
1978-11-25

四万十帯緑色岩類の産状を中心とした成因的考察(<特集>日本列島のオフィオライト-地団研第31回総会テーマ別討論会-)

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