Zircon Morphology on Borecores from Well NE-4 in the Takigami Geothermal Field, Oita Prefecture, Kyushu, Japan

Abstract

The morphology of zircon crystals contained in borecores from Well NE-4, drilled in the Takigami field, Oita Prefecture, is described. As expected, zircon is common in dacitic rocks, but rare in andesitic ones. The prism indices for andesite are large while those for dacite are small, suggesting a higher temperature of andesitic magma. The elongation indices are large in rocks containing pumice. In lava flows, the values of the two indices for each sample are concentrated, whereas they are scattered in volcanoclastics. According to these morphological data, the stratigraphic boundaries of this well have been revised as follows : the Ajibaru Formation above 975m, the Takigami Formation from 975m to 2008m, and the Usa Group, below 2008m in depth. The comparison of zircon morphology between these borecores and the rocks from surrounding areas will lead to a better geological correlation.