北米コーディレラにおける中生代酸性火成活動の性格(特集 濃飛流紋岩およびそれに関連する酸性火成作用-地団研第24回総会シンポジウム-)

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概要

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• The Mesozoic large batholiths in the North American Cordillera have been considered to be the product of the so-called Nevada orogeny in the late Jurassic age. On the other hand, our studies on the late Mesozoic acid igneous activities in the Inner Belt of Southwest Japan made clear that the activities are exceptional from the concept of geosyncline and orogenic belt and that they are plutonism intimately connected with acid volcanism in the geanticlinal regions. The late Mesozoic acid igneous activities of this nature has been known also in the southeastern margin of the Asiatic Continent, occupying the area of 2,000 km wide and 4,000 km long, including the Inner Belt ot South-western Japan as a part. In this paper, the writer attempted to reconstruct the Mesozoic geohistory immediately with the activities of granite batholiths in North AmericanCordillera, located in the same geological situationof the Circum-Pacific region from the above viewpoint. A belt of Cordillera occupied by large batholiths of granite (the so-called Nevada orogenic belt) is situated almost only within the Paleozoic eugeosynclinal and miogeosynclinal regions. In the Mesozoic age, the belt was in geanticlinal circumstance and here subaqueous or subaerial acid volcanisms were violent, especially in the Jurassic age. Granitic activities preceded side by side with these volcanisms. On the other hand, the eugeosynclinal belt of the Mesozoic age was situated in the Western Jurassic belt of the Klamath Moutains, the western half of the Western metamorphic belt of the Sierra Nevada and Coast Ranges. This belt underwent a regional metamorphism of the glaucophanitic type. There is granite in the eastern margin of the belt. But in the the Western Juras, -sic belt of the Klamath Mountains the intrusive relation to the Mesozoic sediments is questionable, and only in the restricted area of the Sierra Nevada granite is in intrusive relation to the Mesozoic sediments. American geologists have generally distinguished the eastern margin of the belt from Coast Ranges. They have considered the former to be pre-Tithonian and the latter to be post-Kimmeridgian. The gap represents the so-called Nevada orogeny. The writer can not accept the idea from the following facts. Firstly, the activities of granite were not restricted in this age and ranged from the Triassic(?) to the Cretaceous age. Moreover, the activities were mainly restricted in the Mesozoic unsubsiding regions, where were in geosyncline during the Paleozoic age. Secondly, it was suggested recently that the Galice Formation is continuous to the Franciscan Group through the South Fork Mountain schist, at least in some stratigaraphic horizon. Lastly, the metamorphism in the Western Jurassic belt of the Klamath Mountains is not a type taking a granite plutonism. The writer regards the Mesozoic acid igneous activities in Cordillera as a substancially same type of those in the Inner Belt of Southwest Japan. The term "Nevada orogeny" of American geologists should be used as a synonym with our Hiroshima disturbance or Yenshan movement.

• 地学団体研究会の論文
• 1971-03-25

著者

• 端山 好和

  東京支部

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