Bull. Volcanol. Soc. Japan
Vol. /0 (,*++) No. +, pp. +3ῌ,-Letter

Zircon Fission-track Dating of the Hiyoriyama Cryptodome
at Kuttara Volcano, Southwestern Hokkaido, JapanYoshihiko GOTOῌ and Tohru DANHARAῌῌ
(Received, October ,+, ,*+* ; Accepted, February ,,, ,*++)
Zircon ﬁssion-track dating was applied to determine the age of the Hiyoriyama Cryptodome in theNoboribetsu Geothermal Field, Kuttara Volcano, southwestern Hokkaido, Japan. We dated two rock samples(HY-+* and HY-++) collected from the wall of an explosion crater at the summit of the cryptodome. A total of,*./ kg of dacite (HY-+*) and ,/./ kg of dacite (HY-++) was crushed, and +**. (HY-+*) and +**2 (HY-++)zircon grains were used to determine the relatively young ﬁssion-track ages. Dating was performed using theexternal detector method, and the ages were calculated from the densities of spontaneous and induced tracks in thewhole zircon grains. The obtained ages are +/ῌ. ka (HY-+*) and +.ῌ. ka (HY-++), which are the same withinerror. The dating results suggest that the cryptodome formed at ca. +/ ka.
Key words : Fission-track dating, zircon, Hiyoriyama Cryptodome, Noboribetsu Geothermal Field, Kuttara Volcano+. Introduction
Fission-track dating is a radiometric dating method
based on counting the number of damage trails left byﬁssion fragments in uranium-bearing minerals, such as
zircon and apatite, and in glasses (Fleischer et al., +31/ ;Hurford, +33* ; Wagner and Van den haute, +33,).
This method is commonly used for samples in the age
range of +*/ to +*2 years, but may be applied to younger
samples in the range of +*- to +*. years by measuring alarge number of mineral grains (Danhara, +33/ ; Wagner,+332 ; Kameyama et al., ,**/ ; Takagi et al., ,**1).
In the present study, we applied ﬁssion-track zircon
dating to a Quaternary subaerial dacite cryptodome at
Hiyoriyama (the Hiyoriyama Cryptodome) in the Noboribetsu Geothermal Field, Kuttara Volcano, southwestern Hokkaido, Japan. No previous study has reported
geochronological data for the cryptodome, and ﬁssiontrack dating of zircon from the cryptodome provides aninsight into the history of dome formation and theevolution of the Kuttara Volcano.
,. Hiyoriyama Cryptodome
The Hiyoriyama Cryptodome is located in the northern part of the Noboribetsu Geothermal Field, in thewestern part of Kuttara Volcano (Fig. +). The Kuttara
Volcano consists mainly of an andesitic stratovolcanothat reaches an elevation of /.3 m above sea level, with
῏College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology,
Mizumoto-cho ,1῍+, Muroran, Hokkaido */*῍2/2/,Japan.

a small caldera (Lake Kuttara) on the summit. Thevolcano evolved over the period 2*῍./ ka, involving
early silicic explosive activity and subsequent stratovolcano building associated with caldera collapse at .*ka (Katsui et al., +322 ; Yamagata, +33. ; Moriizumi, +332 ;Moriya, ,**-). The Noboribetsu Geothermal Field isinferred to have formed after the collapse of the caldera(Katsui et al., +322). The geothermal ﬁeld is approximately + km wide (northeast-southwest) and +./ km long(northwest-southeast).
The Hiyoriyama Cryptodome (Fig. ,) is elliptical inplan view, ranging in diameter from -/* m (northeastsouthwest) to //* m (northwest-southeast). It rises +-*m above the surrounding area, with the highest point
being -11 m above sea level. The surface of thecryptodome is covered with sediments up to +/ m thick(Katsui et al., +322). An explosion crater occurs at
the summit (Fig. -). The crater is //῍3/ m in size
(elongate northwest-southeast) and ,* m deep, and contains active fumaroles.
The Hiyoriyama Cryptodome consists of coherent
dacite that is well exposed on the wall of the summit
explosion crater, where it appears massive with columnar
joints spaced at intervals of +**῍+/* cm. The dacite isgrey and porphyritic, containing phenocrysts of plagioclase (῎. mm long, ,+῍,/ vol.ῌ), quartz (῎/ mm, 0῍2 vol.ῌ), hypersthene (῎, mm, .῍0 vol.ῌ), trace
῏῏Kyoto Fission-Track Co. Ltd, Minamitajiri-cho, Omiya,Kita-ku, Kyoto 0*-῍22-,, Japan.
Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp