砂岩と頁岩の化学組成に基づく四国東部秩父累帯南帯・四万十帯北帯の岩石相ユニット  [in Japanese] Petrofacies units on the basis of chemical composition of sandstone and shale in the Southern Chichibu Belt and the Northern Shimanto Belt, eastern Shikoku, Japan  [in Japanese]

四国東部の秩父累帯南帯〜四万十帯北帯の砂岩・頁岩の化学組成から, 岩石相ユニットの区分を試みた.砂岩の化学組成に基づくと, 秩父累帯南帯は, 単一の岩石相ユニット(J-KCユニット)であり, 四万十帯北帯は, 年代的に古いKS Iユニットと新しいKS IIユニットに区分される.KS Iユニットは, 南側に向かいKS Ia・Ib・Icのサブユニットに細分できる.J-KCユニットとKS IIユニットの化学組成は類似しているが, SiO_2-Zr/TiO_2図を用いることにより両者を区別できる.さらに, この図上では各岩石相ユニット(サブユニット)がそれぞれ独自の領域を占める.一方, 頁岩の化学組成に基づくと, 四万十帯北帯は, 砂岩と同様にKS IユニットとKS IIユニットに区分される.しかし, 頁岩ではKS Iユニットの細分が困難である.秩父累帯南帯の頁岩では, 一部の試料において放散虫化石が含まれているため, 主成分・微量成分元素の多くがかなりばらつき, 四万十帯北帯の岩石相ユニットとは一定した関連を示さない.

The Southern Chichibu Belt and the Northern Shimanto Belt in eastern Shikoku are divided into some petrofacies units and subunits on the basis of the chemical compositions of sandstone and shale. The Southern Chichibu Belt is composed of a single petrofacies unit (J-KC unit) in terms of sandstone chemistry. The Northern Shimanto Belt can be divided into KSI unit (older) and KS II unit (younger) on the basis of sandstone chemistry. KS I unit is subdivided into KS Ia, KS Ib and KS Ic north to south. Geochemical features of the sandstones from J-KC unit are similar to those of KS II unit, although J-KC unit is distinguishable from KS II unit by SiO_2-Zr/TiO_2 diagram. This daigram was also used for discrimination of all units and subunits.The Shimanto Belt is divided into KS I and KS II units on the basis of shale chemistry as same as in the case of sandstone. The chemical compositions of shales, however, can not discriminate among the subunits of KS I. The chemical compositions of shales from the Southern Chichibu Belt vary to some extent due to presence of radiolarian fossils, and do not show consistent relationships to the petrofacies units as in the Northern Shimanto Belt.