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Page 1: K-Arkazan.or.jp/cgi-bin/dpdf.cgi?q=51.1.3.pdf · Fig. +. Topography and geologic map of Kimpo volcano. (a) Location of Kimpo volcano in Kyushu. (b) Bird’s eye view of Kimpo volcano

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K-Ar Dating of Kimpo Volcano, Northern Ryukyu Arc

Kiyoshi TOSHIDA�, Kozo UTO��and Akikazu MATSUMOTO���

Kimpo volcano is a distinct volcano in central Kyushu, western Japan, and is located -* km west of the activevolcanic front of Ryukyu arc. In order to determine the growth history of the volcano, systematic K-Ar dating

has been performed.

The activity of Kimpo volcano has been divided into three stages. The oldest activity formed the andesitic

stratovolcano in the southern part (Older stage). Six ages are concentrated between +.-2 and +.+/Ma. The

horseshoe shaped caldera was formed before the middle stage activity. The middle stage activity formed the

andesitic volcano in the northern part (Sannotake-Ninotake). Ages between *./2 and *./*Ma are obtained fromsix samples. The youngest activity formed a dacite lava dome (Ichinotake), which is located within the caldera

of the first stage activity. Two samples gave consistent ages of *.,Ma.The total duration of the volcanism at Kimpo volcano exceeds one million years. However, long periods of

repose existed between each active stage : about 0**,*** years between the first and the second stages, and about-**,*** years between the second and the third stages. These inactive periods are much longer than the durations

of volcanic activities of each stage. It is therefore important to determine the duration of each active and inactive

period of the volcanoes that have longer than a few thousand years of growth history.

Key words : Kimpo volcano, K-Ar dating, Ryukyu arc, Quaternary, long term eruption rate

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 ,1*�++3. ¡¢£N¡¢£ +0.0�¤�¥�K¦§¨© ��ª�§¨©�«¬��­Geosphere Science Sector, CRIEPI, +0.0 Abiko,

Abiko-shi, Chiba ,1*�++3., Japan. -*/�2/02 ®¯°�z±N�² +�+�+ �z±K¦� ,³´µ¶·¸§¨©¹º��Planning Headquarters, AIST, Tsukuba Central ,, +�+�+Umezono, Tsukuba-shi, Ibaraki -*/�2/02, Japan.

��� -*/�2/01 ®¯°�z±N7 +�+�+�z±K¦� 1³´µ¶·¸§¨© �»¼�§¨��Institute of Geology and Geoinformation, Geological

Survey of Japan, AIST, Tsukuba Central 1, +�+�+Higashi, Tsukuba-shi, Ibaraki -*/�2/01, Japan.

Corresponding author : Kiyoshi Toshida

e-mail : [email protected]

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Fig. +. Topography and geologic map of Kimpo volcano. (a) Location of Kimpo volcano in Kyushu. (b)

Bird’s eye view of Kimpo volcano with the dip angle of ,/ degrees. Vertical enlargement ratio is , : +. (c)

Geologic map of Kimpo volcano (after Hoshizumi et al., ,**.). K-Ar ages by mass fractionation correction

method are shown (average, in Ma). Contour line interval is +** m.

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Table +. K-Ar ages of Kimpo volcano.

����������� 36

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Takai et al. (+32.)�� ��������� - ����� *.3.�*.+/Ma, +.+,�*.,*Ma, +.*+�*.*3Ma

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)��YZ:(& [ �����S�$%:()�0*�OPQTRPQ�G /*5�6�\5�JE]�^;J_)`($a�3478NKb�(��M#X��BC*��& '(� 12�34��J�� cdNefgh�NK�ij� G 0*5�J�k�lm2�no��)��p*0�q`(&

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et al. (+32.)� FT����� !"#$%:� UVW#X�)���YZ:(&7`��$PQ�G ,*5�6NKb�(�BC*�� OPQTRPQ��34;2

Fig. ,. Three active stages of Kimpo volcano and comparison of K-Ar ages. K-Ar ages of this study are plotted

by their measurement methods. Error bars represent + sigma for K-Ar ages and , sigmas for FT ages. The

radiometric ages are concentrated into three groups ; +..�+.+Ma (Older stage), *./Ma (Middle stage ;

Sannotake-Ninotake), and *.,Ma (Younger stage ; Ichinotake).

st���K-Ar�� 37

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