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

在高知县西部南部赤部地体中，中生代地层广泛分布。研究区中三叠世-晚侏罗世Togano群主要由硅质岩、泥岩、硅质粘土岩、砂岩和酸性凝灰岩组成。该群按下部单元划分为三个带，即小川带、小浦村带和西山二号带。本项研究的目的是根据层状硅质岩和泥岩的古生物资料，对其单元中的层状硅质岩和泥岩进行化学分析，并详细阐明其沉积环境。从他们那里。用X-射线荧光光谱分析了样品中的10种主量元素和9种次要元素。用聚类法研究了它们在柱状剖面中的化学组成，以及各元素之间的行为和关系，并合成了它们的化学特征。结果表明，小川带的岩石在早侏罗世和早侏罗世晚期受大洋拉斑玄武岩和两次热液环境的影响为远洋型，西山II带的岩石时代为中侏罗世至晚侏罗世的半远洋型，小浦村带的层状硅质岩为远洋-半远洋型硅质岩，早侏罗世水热环境较弱。小浦村带硅质岩的形成时代可能比小川和西山II带的硅质岩更深。在岛曼托带，完成了硅质岩和泥岩的野外调查和取样，目前正在进行化学分析，并完成了白垩纪至第三系泥岩的K-Ar法年龄测定。年龄逐渐向南方年轻化是一种趋势。

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.