Geochemical study of cherts and mudstones in the Chichibu Southern Belt and the Shimanto Belt in Shikoku

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In the Southern Chichibu Terrane of the western part in Kochi Prefecture, the Mesozoic strata are widel-V distributed. The Togano Group (middle Triassic - late Jurassic) in the studied area consists mainly of chert, mudstone, silicious claystone, sandstone and acidic tuff. This Group divided into three belts, namely, the Ogawa belt, the Kobiura belt and Nishiyama II belt in order to the lower unit. The purpose of this study is to analyse bedded cherts and mudstones in their units chemically based on their paleontological data and to elucidate their sedimentary environments in detail. From theiy. samples, 10 major and 9 minor elements are analysed by the X-ray fluorecence. The authors investigated their chemical compositions within each columnar section, and behavior and relationship among each element bythe clustering technique, and composed their characteristics of the chemical. composition-of the various siliceous rocks with those of modern marine sediments detected from various environments.From the results, it is considered that the rocks of the Ogawa belt are of pelagic type with influence of oceanic tholeiite and twice times hydrothermal environment in early Early Jurassic and late Early Jurassic, and those of Nishiyama II belt are hemipelagic type from Middle Jurassic to Late Jurassic in age, and the bed ded cherts of the Kobiura belt are of pelagic to hemipelagic type with weak hydrothermal environment in early Early Jurassic.The cherts of the Kobiura belt might have been formed under the deeper condition than those of the Ogawa and Nishivama II belts from late Triassic to Early jurassic. In the Shimanto belt, field survey and sampling of cherts and mudstones were finished, and chemical analyses are in proaress at present.Age determination of mudstone by K-Ar method was finished from Cretaceous to Tertiary strata. It is tendency that the ages are younger towards the south gradually.