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Concentrations of uranium, and concentrations and isotopic ratios of thorium in zircon and glass were determined for pyroclastic dacite pumices in southern Kyushu, Japan, and lo-ages were calculated. Uranium and thorium distributions in other minerals were also determined. Uranium and thorium contents of minerals were highest for zircon and decreased in the order, zircon-apatite-glass-mafic minerals-plagioclase. Zircons showed the highest U/Th activity ratios compared with other minerals. Io-ages obtained by glass-zircon pair for pyroclastic flows in southern Kyushu, except for Ata pyroclastic flow were consistent with 14C and fission track ages and the stratigraphic sequence. Comparison of uranium and thorium concentrations in mineral constituents of acidic volcanic rocks showed that the glass-zircon pair is the most preferable for ionium dating.

Io-ages obtained for glass-zircon pairs which were not consistent with expected ages were discussed on the basis of uranium and thorium distribution between zircon and glass. Uranium and thorium distribution between zircon and host rocks from dacites and granites were also discussed. Discordant Io-ages obtained for glass-zircon pairs with normal (Th/U)zircon/(Th/U)glass ratio are explained by early stage crysallization of the zircon in the magma. Discordant Io-ages with abnormal (Th/U)zircon/(Th/U)glass ratios suggest that the zircon was captured in the magma as xenoryst. The discrepancy between (Th/U)zircon/(Th/U)glass ratios (about 0.19) for dacites and (Th/U)zircon/(Th/U)whole rock ratios (about 0.12) for granites could be explained by crystallization of granitic zircon from liquid having chemical composition different from that of the whole rock of granite.