Stratigraphical Sequence of the Meta-basic Rock Complexes in the Shizunai-MitsuishiDistrict, Hokkaido(Original Report and Comment,Ophiolites in the Japanese Islands)

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  • 北海道,静内-三石地域の塩基性岩類の層序(<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-)
  • 北海道,静内--三石地域の塩基性岩類の層序
  • ホッカイドウ シズナイ サンセキ チイキ ノ エンキセイガンルイ ノ ソウジョ

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In the Shizunai-Mitsuishi district, two zones in which geosynclinal basic rocks develop are well known, i.e., the Kamuikotan and Hidaka Western Marginal Zones. No fossils are found in this district, though a few fossils indicating Triassic to Early Cretaceous ages are reported in other districts of the Kamuikotan and Hidaka Western Marginal Zones. In the both zones, original structures such as pillow lava, massive lava, pillow breccia, sheet, dyke, hyaloclastic rocks and reworked volcanic sediments are recognized. However, the discrimination of original rocks in Hidaka Western Marginal Zone is difficult due to effects of shearing. Scarce vesicles are found in the lava in the Hidaka Western Marginal zone. The following volcanic sequence from lower to upper is found at some localities in the Kamuikotan Zone : volcanic breccia (autobrecciated lava ?) → irregular alternation of massive lava and pillow lava → autobrecciated lava or pillow breccia → hyaloclastic rocks and reworked volcanic sediments associated with cherts and shales. The thickness of one cycle mentioned above is several meters in minimum scale. In the Kamuikotan Zone, basic rocks along the Mitsuishi river and Pirashuke river have suffered glaucophane schist facies metamorphism and those along the Futamata river and Menashibetsu river prehnite-pumpellyite metagraywacke facies metamorplism. Therefore, it is estimated that the latter two masses represent the upper horizon, judging from their .lower pressure metamorphism.

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