Abstract
The Iyari Formation crops out on the faces of the eastside of the Hida Mountain Range, central Japan. As previously reported, reversed paleomagnetic polarity of this formation is correlated to the Matsuyama Chron. Hence, the estimated depositional age of the Iyari Formation is sometime between 1.65 Ma and 0.78 Ma. The Iyari Formation mainly consists of a gravel bed containing clasts (long axis over 128 mm) of brown or gray welded-tuff (70 %), granitoids (25 %), and green or dark-green welded-tuff (5 %). From the petrographic viewpoints, origin of these clasts is considered to be the Hida Mountain Range. Furthermore, clasts of fine-grained biotite-granite containing mafic microgranular enclave in the Iyari Formation are derived from the Kurobegawa Granite, the northern Hida Mountain Range. Because radiometric ages of the Kurobegawa Granite (1.5-1.0Ma: zircon FT age and biotite K-Ar ages) are almost similar to the estimated depositinal age of the Iyari Formation, it is strongly indicated that the northern Hida Range uplifted and eroded rapidly after ca.1.5-1.0 Ma.
