/+ (,**0) + -+.* K-Ar,**/ 0,1,**0 + +* 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 active volcanic 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 from six 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 + -* km (Fig. +) +** km -** km,1* ++3. +0.0 Geosphere Science Sector, CRIEPI, +0.0 Abiko, Abiko-shi, Chiba,1* ++3., Japan. -*/ 2/02 + + +, Planning Headquarters, AIST, Tsukuba Central,, + + + Umezono, Tsukuba-shi, Ibaraki -*/ 2/02, Japan. ( +33, ( +320,/ km - Takai et al. (+32.) (FT) +** +/ 2* -*/ 2/01 + + + 1 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: toshida@criepi.denken.or.jp
32 (+33+) (+32-) A (+33+) 0* /*.* km - Tsukui et al. (+320) / +*. / +* 0 m - y Takai et al. (+32.) - +*. m - y Tsukui et al. (+320) Takai et al. (+32.) K-Ar + + 00/ m (Fig. +) Takai et al. (+32.) FT - + Ma ( *.3. *.+/ Ma +.+, *.,* Ma +.*+ *.*3 Ma *.+/ *.*/ Ma FT (,**.) - (,**.) Takai et al. (+32.) ( +332 ( ( +332 ( +3/0 ; Iwauchi and Hase, +33, ( +332 Takai et al. (+32.) Aso-. ( +332 +, km - / km, /** m +, km - ( - km, 0/* m, ( *.. km - ( +., km,./* m,/ km - +, K-Ar Matsumoto and Kobayashi (+33/) ( ( (,, +. K-Ar -, + -, (Fig. +) +
ῒ ῌ K-Ar ῑῐ 33 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.
34, +, + (KUM23*-) -, 0*, / g - K-Ar (0 (- + -, XRF (, l b..30, +* +* y, l e *./2+ +* +* y,.* K K *.*++01 atm (Steiger and Jager, +311) - + Matsumoto et al. (+323) Micromass +,** ( +,/**,/ Micromass -0** (,**-.* Ar -0 Ar -2 Ar -0 Ar SORI3- biotite (Sudo et al., +332) Matsumoto et al. (+323), Sudo et al. (+330) (+32*) (+33/) (,..3 +* 0 ml STP g; +32* +,.** +* Micromass0*- -, XRF, (+323) PIF--D *./ ( +323 JB-+, JG-+ K,O +..,- *.**. wt -.33/ *.*+* wt n - ; Imai et al., +33/ +..- wt -.32 wt XRF (+333) XRF+/** (,**- 1/* + K-Ar K, O XRF ( +333 H,O( ) JG-+a, JH-+, JA--, JGb-,, JB-,, JGb-+, JR-+, JR-- 2 Imai et al. (+33/, +333) XRF + (,**- JSl-, K,O -.*+ *.*+ wt (n. ; Imai et al., +330 -.**2 wt (-.+. *.*+ wt H,O ( ) (..+/2 wt ; Imai et al., +330)
K-Ar 35.. + 0 / -2 Ar -0 Ar (*.+203 ; Nier, +3/*) (,s) (Table +) (KUM23*-) 01.3 +.. +* 3 ml STP/g, /1 / +* 3 ml STP g 3- -2 Ar -0 Ar + (TAM23*+ ; *.+2, *.**,) - (Table +) (KUM23*-) 03., +.- +* 3 ml STP g -2 Ar -0 Ar (TAM23*+) -2 Ar -0 Ar + (KUM23*+) (Table +), - (,s)., XRF, K, O XRF *.0 +.0 (Table +) (,s). - K-Ar (,s) K-Ar (,s) (Fig.,, Table +) (KUM23*-), (+.-2 *.*- Ma, +..* *.*- Ma) (+.+0 *.*3 Ma) +.. Ma +.* -2 Ar -0 Ar + (KUM32*-) -2 Ar -0 Ar (Table + ;, *.+2/. *.***/ *.+2/1 *.***., (*..3 *.*2 Ma, *./2 *.*1 Ma).* Ar -0 Ar (,3/./ ; Nier, +3/*) ( *.,- *.*- Ma, *.-1 *.*- Ma 3 +* 1 ml STP g (+.* +* 1 ml STP g +* / / + + (KUM23*-) +.-3 *.*- Ma, +.,* *.*- Ma, +.+/ *.*- Ma, +.,- *.*- Ma, +.+0 *.*. Ma, +.,/ *.*- Ma +.,/ +.+/ Ma +* (Takai et al., +32.) KUM 23*-, +.. Ma +.*
36 Table +. K-Ar ages of Kimpo volcano.
K-Ar 37 Takai et al. (+32.) - *.3. *.+/ Ma, +.+, *.,* Ma, +.*+ *.*3 Ma FT K-Ar K-Ar ++* +.* ++* -* /, *./, *.*+ Ma, *./. *.*+ Ma, *./* *.*+ Ma, *./. *.*1 Ma, *./0 *.*, Ma, *./, *.*+ Ma *./ Ma (Takai et al., +32.) /* 0* / - *.,* *.*0 Ma, *.,+ *.*- Ma *., Ma Takai et al. (+32.) FT,* 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).
38 /. +.* -* 0* /*,* 2* - +,* -* 0* -* 3*,/ km - +,*, +*. m - y Tsukui et al. (+320) + +* + - -* +, km -. +*. m - y, +, km - +* +., +* / m - y (+33+) Tsukui et al. (+320) - (+3/0) 3 3/ +**. (,**.),* + Imai, N., Terashima, S., Itoh, S. and Ando, A. (+33/) +33. compilation of analytical data for minor and trace elements in seventeen GSJ Geochemical Reference Samples, Igneous Rock Series. Geostandard Newsletter, +3, +-/,+-. Imai, N., Terashima, S., Itoh, S. and Ando, A. (+330) +330 compilation of analytical data on nine GSJ Geochemical Reference Samples, Sedimentary Rock Series. Geostandard Newsletter,,*, +0/,+0. Imai, N., Terashima, S., Itoh, S. and Ando, A. (+333) +332 compilation of analytical data for five GSJ Geochemical Reference Samples: the Instrumental Analysis Series. Geostandard Newsletter,,-,,,-,/*. (+33,)././ /+. Iwauchi, A. and Hase, Y. (+33,) Late Cenozoic vegetation and paleoenvironment of northern and central Kyushu, Japan. Part / Yoshino area (Middle Pleistocene). J. Geol. Soc. Japan, 32,,*/,,+. (+323) K-Ar.* 0/ 1*. Matsumoto, A. and Kobayashi, T. (+33/) K-Ar age determination of late Quaternary volcanic rocks using the mass fractionation correction procedure : application to the Younger Ontake Volcano, central Japan. Chem. Geol., +,/, +,- +-/. Matsumoto, A., Uto, K. and Shibata, K. (+323) K-Ar dating by peak comparison method new technique applicable to rocks younger than *./ Ma. Bull. Geol. Surv. Japan,.*, /0/ /13. (+32-) +-/ p. (+320), -+ 3/ ++*. (+333) U33*10 pp / 0. Nier, A. O. (+3/*) A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon
K-Ar 39 and potassium. Phys. Rev., 11, 123 13-. Steiger, R. H. and Jager, E. (+311) Subcommision on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett., -0, -/3-0,. Sudo, M., Tagami, T., Sato, K., Hasebe, N. and Nishimura, S. (+330) Calibration of a new Ar analytical system for the K-Ar dating method and analytical results of K-Ar age known samples. Mem. Fac. Sci. Kyoto Univ., Ser. Geol. and Mineral., /2,,+.*. Sudo, M., Uto, K., Anno, K., Ishizuka, O. and Uchiumi, S. (+332) SORI3- biotite : a new mineral standard for K-Ar dating. Geochem. J., -,,.3 /2. Takai, M., Miyachi, M. and Hirano, I. (+32.) Paleomagnetism and fission-track ages of Kimbo volcano, southwest Japan. Rock Magnetism and Paleogeophysics, ++,,+,/. (+33+) -0,++,,+. (,**-) K-Ar U*-*,.,2 p. Tsukui, M., Sakuyama, M., Koyaguchi, T. and Ozawa, K. (+320) Long-term eruption rates and dimensions of magma reservoirs beneath Quaternary polygenetic volcanoes in Japan. J. Volcanol. Geotherm. Res.,,3, +23,*,. (+32*) K-Ar -+,01,1-. R. M. (+33/) K-Ar.0,-3,.3. (+332) + pp 12 +*2. A+ A+ + KUM23*- (-,.1,- N, +-* -2 -- E), mm + mm KUM32*+,+ N, +-* -3 2 E) (-,.1 +., mm, mm KUM23*, (-,.1 +0 N, +-* -3 -* E) +.0 mm + mm, mm KUM23*0 (-,.2 / N, +-* -0 -+ E), mm, mm - mm KUM23*+ (-,.1-3 N, +-*.* /, E), mm,./ mm KUM**+2 (-,.3 +* N, +-*.* +1 E) +./ mm + mm A+, TAM23*+ (-, /-.1 N, +-* -1 -- E) + mm -./ mm, mm KUM**+* (-, /+.- N, +-* -3,* E) + mm, mm KUM ***. (-, /+ /0 N, +-* -1./ E) + mm, mm
40 KUM32*- (-, /* -2 N, +-* -0 1 E), mm *.. mm KUM23*1 (-,.3,1 N, +-* -1 +3 E), mm +./ mm KUM***/ (-, /* // N, +-* -1,. E), mm *./ mm, mm +./ mm A+ - ( KUM23*2 (-,.3 / N, +-* -2 -, E), mm + mm KUM23*3 (-,.2.* N, +-* -2 +- E) KUM23*2,./ mm