This paper reports fission track FT ages on apatite from Miocene igneous rocks of the Kii Peninsula. A total of twelve ages was determined, of which nine data passed a statistical test. Ages of around Ma were obtained for the Muro Pyroclastic Flow Deposit, the Sekibutsu Tuff, and the granite porphyry of the Kumano Acidic Rocks, and were indistinguishable from the zircon FT ages we reported previously Iwano et al., , Jour. Geol. Soc. Japan, ,-, indicating instantaneous cooling. On the other hand, two plutonic bodies of the Omine Granitic Rocks yielded apatite FT ages of about Ma that were significantly younger than the zircon FT ages approximately Ma , suggesting relatively slow cooling.

In order to understand compositional characteristics of zircon crystals with various appearance and to relate them with evolution of Stype, felsic magmatism of the Kumano Acidic Rocks in the Outer Zone of southwest Japan, U-Pb ages and trace element compositions of colorless and reddish zircons were measured using LA-ICPMS technique.

Using the double-dating approach, we have conducted fission-track (FT) and U-Pb age dating on the same zircon grain to evaluate the reliability of the resulting age data. The zircon grains (OD-) used for the investigation were collected from the Mihara Body of the Kawamoto Granodiorite, Shimane Prefecture, Japan. The zircon crystals yielded a FT age of Ma, with a spontaneous FT density of -cm . The U-Pb age data were obtained using inductively coupled plasma-mass spectrometry (ICP-MS) combined with a frequency quintupled Nd-YAG (λ = nm) laser ablation (LA) sample introduction system. The U-Pb age determinations on the zircon samples were performed after chemical leaching using % HF for hrs at room temperature, or after FT etching using a KOH-NaOH eutectic solution for hrs at °C. The resulting U-Pb age of Ma was concordant with the FT age data, suggesting that there was no significant loss of Pb during etching or chemical preparation, thus indicating the feasibility of FT and U-Pb double dating of single zircon grains. The results indicate that the zircon grains (OD-) from the Kawamoto Granodiorite (Mihara Body) can become a new working standard for precise U-Pb age determinations of young zircons.

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