Abstract
Fission-track dating of volcanic, hypabyssal, and plutonic rocks of the Miocene Shionomisaki Igneous Complex exposed at the southern tip of Kii Peninsula was carried out to delimit the age of volcanic activity. Applying the external detector method to internal and external surfaces of zircon crystals (ED1 and EP2 methods respectively), ages from both methods were compared for individual samples. Three samples from felsic extrusive and hypabyssal rocks showed that the ages by the ED2 method (13-18 Ma) were older than those by ED1 (13-15 Ma). Lengths of semi-tracks on external surfaces of zircons in these samples were abnormally short, whereas confined tracks had a normal-length distribution and short tracks were lacking. Experiments with chemical solutions revealed that the older ED2 ages were very likely caused by an external effect: spontaneous tracks of uranium existing around or adhering to the external surfaces of zircon may have created the apparently older ages. This external effect may thus occur not only in plutonic rocks but also in some extrusive and hypabyssal rocks. The ED 1 ages are considered to represent cooling ages on the basis of confined-track length data, which leads to the conclusion that the Shionomisaki igneous activity commenced about 15 Ma and ceased about 13 Ma. In addition, some rhyolite samples contained colorless zircon grains as well as reddish ones. Such character is generally uncommon, but has also been noted from other Middle Miocene felsic bodies in the Kii Peninsula.
