[S-GL22] Geochronology and Isotope Geology

[SGL22-P02] Preliminary ⁴⁰Ar/³⁹Ar dating results for the younger stage of Kirishima Volcanic Complex, SW Japan

*Seiko Yamasaki¹, Teruki Oikawa¹, Daniel P. Miggins², Anthony A.P. Koppers² (1.Geological Survey of Japan, Advanced Industrial Science and Technology, 2.Oregon State University)

Keywords:40Ar/39Ar dating, geochronology, Kirishima Volcanic Complex

Kirishima volcanic complex is located soutern part of Kyushu Island, and one of the most active volcanoes in Japan. It consists of more than 20 volcanic bodies, including Mt. Karakunidake (the highest peak at 1,700 m), Shinmoedake, Ohachi, and Takachiho-no-mine, spread over an area of ca. 30 km by 20 km in a northwest-southeast direction. Numerous eruptions have been recorded in historical times, mainly from Shinmoedake and Ohachi, and active fumarolic activity continues at Ioyama. The activity of Kirishima volcanic complex began ca. 600 ka and is divided into two stages: the older stage, preceding the formation of Kakuto Caldera ca. 330 ka, and the younger stage, spanning from ca. 330 ka to the present. The relative ages of the volcanic edifices formed during the younger stage have been constrained based on their stratigraphic relationships with Iwato Pumice (ca. 60 ka), Ito ignimbrite (ca. 30 ka) and Kikai-Akahoya Ash (ca. 7.2 ka). However, direct geochronological data of the volcanic units have been scarcely reported.
We conducted ⁴⁰Ar/³⁹Ar geochronological study for nine samples from eight units (Yunotanidake, Shiratoriyama, Ryuodake, Ohnamiike, Maruokayama, Karakunidake, Koshikidake, and Shinmoedake) of the younger stage of Kirishima Volcanic Complex. Two samples yielded no age results as their ages were younger than the detection limit, but six samples yielded ca. 160 to 1.2 ka, which are mostly consistent with the stratigraphy. Further geochronological studies are needed to reconstruct the growth history of Kirishima Volcanic Complex.