K-Ar ages of feldspathization at the Yabuhara feldspar mine, Nagano Prefecture, central Japan

摘要

To determine feldspathization age of the Yabuharafeldspar deposit, we have performed K-Ar age dating for anorthoclase-rich feldspar ore and a biotite fraction form fine-grained biotite granite of the Nagawa granite. The feldspar ore andbiotite fraction yielded ages of 57.5+-2.9 Ma and 61.2+-3.1 Ma,respectively. These ages suggest that the feldspathization occurredin early Paleogene by hydrothermal fluids form the fine-grained biotite granite.