Unspiked K-Ar Geochronology of Kuju Volcano, Kyushu, SW Japan

Abstract:

Kuju volcano is an active volcano located on the volcanic front on Kyushu Island and also in the Beppu-Shimabara Graben. It consists of small stratovolcanoes and lava domes, and the volcanic edifices are surrounded by gently sloping skirts composed of pyroclastic flow, debris avalanche and debris flow deposits.

In order to reconstruct the evolutional history of Kuju volcano under the geological mapping project (Kawanabe et al., 2015), we measured unspiked K-Ar ages for 10 lava samples from 7 volcanic units. Unspiked K-Ar dating procedure enables the correction for mass fractionated initial ⁴⁰Ar/³⁶Ar ratios, and has been successfully applied to young volcanic samples, especially younger than 0.5 Ma. The ages obtained were evaluated by the fraction of non-radiogenic ⁴⁰Ar, the volume of ³⁶Ar, and in comparison with already reported conventional K-Ar, TL, FT and ¹⁴C ages.

Based on the ages and stratigraphic relationships, the volcanic stages of Kuju volcano can be reconstructed as follows; Stage I: small andesitic edifices were formed in the western and central part during ca. 200-90 ka. Unspiked K-Ar ages show that some edifices were simultaneously formed around ca. 150 ka (Kutsukakesan volcano and Iozan lava) and ca. 90 ka (Kuroiwasan volcano and Narukoyama volcano). Stage II: the largest eruption in the volcano associate with the Handa pyroclastic flow was occurred (ca. 54 ka; ¹⁴C age, Okuno et al., 2013). Stage III: several dacitic lava domes and andesitic lava flows were formed in the central part during ca. 54-15 ka. Unspiked K-Ar age data for Nakadake volcano (46 ± 12 ka) and Ogigahana volcano (34 ± 3 ka) are agree well with stratigraphic relationships. During the stage III, large debris avalanche deposit formed within the evolutional process of the north-central region. Stage IV: the latest stage started with mafic magma in the eastern part (<15 ka; Kawanabe et al.,2015).