Granitic masses around Lake Biwa, southwest Japan : Part 6. The Yasu Granite pluton

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  • 琵琶湖周辺の花こう岩質岩体 ―その6.野洲花こう岩体
  • ビワコ シュウヘン ノ カコウガンシツ ガンタイ ソノ 6 ヤス カコウガンタイ

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The Yasu Granite pluton, 10km×7km, is one of the Cretaceous granites around Lake Biwa, southwest Japan. It was emplaced within the Jurassic accretionary complex of the Tanba-Mino Belt, and is situated within the Cretaceous Biwa-ko Cauldron (60km×40km). The Yasu Granite can be lithologically divided into the following three main rock-types: the coarse-grained, medium-grained and fine-grained types. All of them are frequently porphyritic. They are essentially biotite granites, and sometimes hornblende is contained. The coarse-grained granite occurs partry in the southern part of the pluton, and represents a deep-seated lithology. The medium-grained granite mainly occupies the southern part, and should be the predominant constituent of this pluton. The fine-grained granite is distributed in the northern half and represents the roof-zone lithology. Granite porphyry occurs only as thin dykes at several places within the pluton. The lithological variations should have been formed during the crystallization and also at the cooling stages of the magma. The fine-grained granite was formed by quenching due to a rapid degassing, which led to the transfer of solidus to a higher-temperature side in consequence of the decrease of water-vapor pressure. The coarse-grained granite was reconstructed and/or reequilibrated under long durated high-temperature conditions, followed by both the pneumatolitic and hydrothermal activities. The medium-grained granite was formed under an intermediate condition between the above two. The geological situation of the Yasu Granite is not yet confirmed whether it belongs to the ring-complex associated with the formation of the Biwa-ko Cauldron or not, despite of its lithological and geochemical similarities to those granites forming the ring complex.

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