瀬戸内火山岩類の年代測定 ＩＶ 四国北東部および北西部の瀬戸内酸性火山岩類フィッショントラック年代とＫ‐Ａｒ年代

Abstract

Fission track ages were determined on two zircon separates from the Setouchi acidic volcanics occurring in Takamatsu and Shodo-Shima, northeastern part of Shikoku. They yielded concordant ages of 13.9±1.4 Ma and 14.0±1.2 Ma, respectively. A glassy, densely welded acidic volcanic rocks occurring in the northwestern part of Shikoku, referred to the Matsuyama welded tuff, yielded a K-Ar age of 14.2±0.8 Ma. These results indicate, together with the reported identical K-Ar ages for the Setouchi acidic volcanics in the southern part of Oita Prefecture, that the acidic volcanic activity occurred approximately 14 million years ago and was contemporaneous with the acidic igneous activity occurred 14±1 million years ago in the Outer Zone of Southwest Japan. In the Setouchi volcanic belt, mafic volcanic activity, represented by the extrusion of Sanukitoid magma, occurred slightly later than or almost contemporaneously with the acidic volcanic activity.