Zircon U-Pb Ages of Ohno Volcanic Rocks in eastern Kyushu

Abstract

The Ohno Volcanic Rocks in eastern Kyusyu occur at the western tip of the Setouchi Volcanic Rocks. Previously reported K-Ar ages of the Ohno Volcanic Rocks range from 10.5 to 15.9 Ma. Here we present the results of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb analyses of three felsic rocks from the Ohno Volcanic Rocks. We obtained mean ²³⁸U-²⁰⁶Pb ages as follows: 14.71±0.20 Ma (2σ) for the Shiraiwasan pyroclastic flow deposit at the lowermost horizon within the sequence, 15.14±0.43 Ma for the Koguraki Tuff, and 14.58 ± 0.21 Ma for the Miyakeyama pyroclastic flow deposit at the uppermost horizon within the sequence. Our results indicate that the Ohno Volcanic Rocks, except for the high-Mg andesite found only as dikes, were formed over a shorter period than previously considered (ca. 1 m.y.).