小豆島および讃岐東部の領家帯

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In the Shodo-shima Island, granitic rocks and related rocks of the Ryoke belt occur beneath the Miocene sediments and volcanics. The Ryoke belt occupies the southeastern part of the Island. The Ryoke granites are classified into the Yoshino granite and the Nanpudai gneissose granite. The Nanpudai gneissose granite probably belongs to the older granite, intimately associated with the Ryoke regional metamorphism. The metamorphic rocks, which are characterized by such mineral and mineral associationas andalusite and cordierite-K-feldspar in pelitic ones, belong to the upper cordierite zone of the Ryoke regional metamorphism. Metadiabase and gabbros form a composite mass on the southeastern border of the Island. The massive Shodo-shima granite, a member of the so-called "Hiroshima granite," is widely distributed in the northwestern part of the Island. It is divided into the Nadayama-type, the Fukuda-type, and the Ikeda-type, from northtosouth. In the northwestern limit of the Island, nearly non-metamorphic 'Palaeozoic' formation occurs. Therefore, a considerable gap of metamorphic grade exists between the 'Palaeozoic' formation and the metamorphic rocks in the southeastern part. In the eastern Sanuki region, the northern half is occupied by the Ryoke granites and metamorphic rocks, and the southern half by a massive granite. The main Ryoke granites are the Shido granite and the Aji granite. The Shido granite can be correlated to the Inagawa granite of the Chubu district as well as the Yoshino granite. Fine-grained and massive Aji granite has very similar characters to the Kadoshima granite of the Chubu district. The metamorphic rocks are sporadically found in the granites, and belong to the first sillimanite zone of the Ryoke regional metamorphism. The presence of metamorphosed rhyolitic tuff is worthy of special attention for its rare occurrence in the Ryoke belt. The southern massive granite, Shirotori granite, is divided into seven rock types. It is not settled so far whether it belongs to the ″Hiroshima granite″or the Ryoke granite. Correlations of petrographical and geological characters are made of the granites and metamorphic rocks in this region with those of the Awaji-shima, the western Kinki district and the Takanawa peninsula, and the following conclusions are got. In the Setouchi district, the Ryoke belt is relatively narrow, and is essentially devoid of the second sillimanite zone and the granite s of the oldergroup. The temperature axis of the metamorphism runs from east to west at the northern margin of the Sanuki, and the metamorphic grade decreases both northward and southward. On both sides of the belt, the younger massive granite occurs. Especially, the presence of the massive granite on the southern side near to the Median Tectonic Line, is characteristic to this district. Simllar feature to this is recently found in the western Kinki, Sennan area.
日本地質学会の論文
1979-09-20

小豆島および讃岐東部の領家帯

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