K-Ar ages of granitic inclusions from the Futamata and Tomuraushi pyroclastic flow deposits in central Hokkaido, Japan

The pliocene Futamata and Tomuraushi pyroclastic flow deposits and their granitic inclusions in central Hokkaido, form a series of volcano-plutonic association. The newly obtained K-Ar ages of the granitic inclusions are 1.1-1.2 Ma for whole rocks. These K-Ar ages are much younger than the ages which are expected from stratigraphic correlation. This discrepancy in age suggests loss of radiogenic 40Ar from the pyroclastic flow deposits and their granitic inclusions during thermal event of subsequent volcanism. (author)