Zircon Systematics of the Shionomisaki Volcano–Plutonic Complex (Kii Peninsula, Japan): A Potential Tool for the Study of the Source Region of Silicic Magmas

The Shionomisaki Igneous Complex is part of the Mid Miocene igneous province developed within the Shimanto Accretionary Complex in front of the volcanic front in SW Japan. The igneous rocks in this province mostly have silicic compositions. New U-Pb ages obtained for two samples from the Shionomisaki Complex at the southern tip of the Kii Peninsula (14.6 ± 0.4 Ma and 14.9 ± 0.4 Ma) fall into the range of previous age determinations (14.6 ± 0.2 to 15.4 ± 0.3 Ma). Hf isotopic compositions obtained for co-magmatic zircon (εHf(t) = −0.7 to +4.8) lie between typical values obtained for mantle-derived magmas and values obtained for old crustal rocks. They are thus consistent with previous interpretations that the magmas are mixtures of mantle and crustally derived magmas. In the modelling of the isotopic characteristics of the magmas, the sediments of the Shimanto belt are taken as the protolith of the silicic magmas. Xenocrystal zircon (i.e., zircon picked up during ascent and emplacement of the magma) found in the silicic igneous rocks exhibits a similar age pattern as detrital zircon of the Shimanto sediments. However, the age pattern obtained in this study for zircon cores, which are considered to be restitic zircon (i.e., zircon derived from the source of the anatectic melt), shows little semblance with the age pattern of Shimanto sediments. It is, therefore, tentatively suggested that the source area of the silicic magmas may not be identical with the sediments of the Shimanto Accretionary Complex.