K-Ar ages of hornblendes from gabbroic rocks in Southwest Japan.

Abstract

K-Ar age determinations have been carried out on 15 hornblendes from the Yakuno Basic Rocks and their affinities in the Inner Zone of Southwest Japan. Gabbroic rocks from the Yakuno Basic Rocks and those from the Sangun Metamorphic Terrane in eastern Chugoku gave ages ranging from 228 to 275 m.y.; more than half of them are concordant with the geologically estimated age of the rocks. Remaining older ages may indicate either the time of plutonism in Early Permian, which might have been contemporaneous and comagmatic with the basic volcanic activity in the same areas, or the existence of inherited argon in hornblendes. Ages of gabbroic rocks from the Sangun Metamorphic Terrane in northern Kyushu are 296-372m.y., all older than those from eastern Chugoku. Hornblendes from gabbroic rocks in the Nagasaki Metamorphic Terrane gave still older ages of 449-472m.y., suggesting that these rocks may be a part of old basement rocks in the Japanese Islands. Graphical presentation of argon and potassium data indicates that most of the hornblende samples contain no significant amount of excess argon.