Phengite K-Ar ages of the Yamagami Metamorphic Rocks in the Motai-Matsugataira Belt

－Probability of the extension of the Renge Metamorphic Belt to the Northeastern Japan Arc－

Abstract

The Motai-Matsugataira Belt (MMB) in the eastern margin of the northeastern Japan arc comprises Paleozoic metamorphic complexes. The high-P Yamagami metamorphic rocks in the belt are subdivided into the high-grade Yamagami I and low-grade Yamagami II units. The Yamagami I metamorphic rocks consist mainly of pelitic schist, epidote amphibolite, and garnet amphibolite. The occurrence of paragonite in rutile and the chemistry of amphiboles indicate that the Yamagami I metamorphic rocks underwent the highest-grade metamorphism in the MMB. Phengite K-Ar ages of the pelitic schists and amphibolitesindicateexhumation during 322 to 287 Ma. These petrological and geochronological results for the Yamagami metamorphic rocks indicate that part of the MMB is probably an extension of the Renge Belt of southwestern Japan.