Fission track and K-Ar dating on some granitic rocks of the Hida Mountain Range, Central Japan

Fission track (FT) zircon ages for eight samples, and K-Ar ages using bioti te and hornblende for two samples were determined on four granitic rock bod ies and a dyke in the Hida Mountain Range, Central Japan. FT zircon ages we re determined <3 Ma for six samples, and >40 Ma for two samples. K-Ar ages were always older than the FT ages, reflecting the difference in closure te mperature. For a sample of the Okukurobe Granite, the evidence of secondary thermal overprinting was found according to the statistical test for singl e grain FT ages and FT length distribution aspect. The apparent FT age of 4 4 +/- 5 Ma (2 sigma level) should not correspond to any geological events. On the basis of the radiometric age data, the emplacement order of the rock bodies could be reconstructed as follows: The Kitamatadani Tonalite intrud ed at similar to 90 Ma, the Tsurugidake Granite at similar to 60-70 Ma, the Okukurobe Granite prior to similar to 60 Ma, the Kurobegawa Granite (KG) p rior to similar to 7 Ma, and a dyke intruded into KG at similar to 1 Ma. Be cause a simple cooling process is not applicable to the rock bodies in this region, it is necessary to analyze systematically the cooling histories of relatively small parts of each rock body prior to the thermo-tectonic hist ory analysis.