長野県大門峠周辺に分布する流紋岩質溶岩ドームのK-Ar年代：諏訪‐八ヶ岳火山地域の流紋岩類の活動時期について

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(2) K-Ar : ῌ !"#$% &'( P Q R SOT U V WOX Y,O3 4 Z [ \ ] ^ _O_ ` a b ,**2 J 2 K 2 LMN ,**3 J - K 3 LM:. K-Ar Ages of the Rhyolitic Lava Domes in the Daimontoge Area in Nagano Prefecture, Central Japan: Duration of the Rhyolitic Activity in Suwa-Yatsugatake Volcanic Province Kuniaki NISHIKIῌ, Teruki OIKAWAῌ, Yasushi OHTAῌ,ῌῌ, Akikazu MATSUMOTOῌ, ῌῌῌ Kozo UTOῌῌῌ and Yasuyuki MIYAKEῌ The voluminous Quaternary volcanic rocks in the Suwa-Yatsugatake volcanic province (SYVP) of central Japan are known as Enrei volcanic rocks, Yabashira and Yatsugatake volcanic products. These volcanic rocks consist mostly of basalt and andesite, with a small amount of rhyolite. We report on the revised stratigraphy and K-Ar ages of rhyolitic lava domes in the Daimontoge area of the central part of the SYVP, and discuss the duration of rhyolitic volcanism in the SYVP. Our work clarifies the fact that the two rhyolitic lava domes studied were formed independently, ca. *.,. Ma (Utsukushimatsu lava dome) and ca. +.*1 Ma (Daimontoge lava dome), and the age of the basement volcanic rocks predates ca. + Ma. The duration of rhyolitic volcanism in the SYVP is divided into +.,ῌ*.0 Ma and *.-ῌ*., Ma, based on the findings of previous studies and this study. The previous studies show that volcanic activity had not occurred post-*.0 Ma in the central part of the SYVP. However, our data show that the rhyolitic lava dome (Utsukushimatsu lava) was formed much later, ca. *.,. Ma. The spatiotemporal distributions and whole-rock composition of the rhyolite suggest that rhyolitic volcanism prior to *.0 Ma was related to the formation of Oiwake volcanic graben in the central SYVP. At the time of the e#usion of the Utsukushimatsu lava dome, the activity of the Yatsugatake volcanoes that adjoined the eastern side of the Daimontoge area was the most vigorous in their history. Therefore, we suggest that the formation of the Utsukushimatsu lava dome in the Daimontoge area resulted from the increment and expansion of activity of the Yatsugatake volcanoes. Key words : Rhyolite, K-Ar ages, Oiwake volcanic graben, Suwa-Yatsugatake volcanic province, Enrei Volcanic Rocks and Yatsugatake volcanic products. -*/ῌ2/01 +ῌ+ῌ+ ! 1 "#$%&'()*+,-.()/0 AIST, GSJ, Institute of Geology and Geoinformation, Tsukuba Central 1, +ῌ+ῌ+ Higashi, Tsukuba, Ibaraki -*/ῌ2/01, Japan *-+ῌ20+1

(3) /ῌ, 8<=>?@ABCDE/ Ore Purchasing & Mining Engineering Department, Pacific Metals Co. Ltd., /ῌ, Toyama-shinden, Kawaragi, Hachinohe, Aomori *-+ῌ20+1, Japan -*/ῌ2/02 FG +ῌ+ῌ+ ! , "#$%&'()*HI/0. AIST, International A#airs Department, Tsukuba Central ,, +ῌ+ῌ+ Umezono, Tsukuba, Ibaraki -*/ῌ2/02, Japan -3*ῌ20,+ 12 345 -ῌ+ῌ+ 6789:9/ +,;9; Department of Geology, Faculty of Science, Shinshu University, -ῌ+ῌ+ Asahi, Matsumoto, Nagano -3*ῌ 20,+, Japan Corresponding author : Kuniaki Nishiki e-mail : [email protected].

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(139) . 70. ! "# ! "12# 3$45%6,7&5'8 @ 5 CDEF6G 31 $45./,2 # *./ Ma HI ,J76KLM282#1K UV3WXY $45%6KZ[Z2#F28" .2#6F_3 3à $45./,9b<4 ,3?%6:@85 CDEFK6 3fL NFjkYl ῏. ῎. ?,S @XtY'-Z#u, [w\]x 8@3y@3 123#,^'_\ 7"#d:@8:eV3 Q{ mjkKUlAB3#,m E\ nH%& S,rs \ gatu,3\,vw;V 8 3Qr}l3 WW 8~ v ῌ ῑ ῐ ῍ Cox, K.G., Bell, J.D. and Pankhurst, R. J. (+313) The interpretation of igneous rocks.George Allen and Unwin, ./*p. ^¡9()* (+331) #c./¢=,+,,. /, K-Ar 0 123#AB#4- 3, ++1ῌ +,2. ^¡>W § (+33.) #c./¢,2 # ¡e0./

(140) %#¨ +*- ,*1ῌ,,*. ^¡>W § ¬l&9()* (+330) <94$¥5%®&./¦, K-Ar 0 123#AB#4s¥AB%&¦ -- +ῌ0. Imai, N., Terashima, S., Itoh, S. and Ando, A. (+33/) +33. compilation values for GSJ reference samples, “Igneous rock series”. Geochemical Journal, ,3, 3+ῌ3/. '&'¨>W § (+33+) 0+, v«¤²¤e./, K-Ar 0 ./ -0 +3-ῌ+3/. Kaneoka, I. and Suzuki, M. (+31*) K-Ar and fission track ages of some obsidians from Japan. Jour. Geol. Soc. Japan, 10, -*3ῌ-+-. Kaneoka, I., Mehnert, H., Zashu, S. and Kawachi, S. (+32*) Pleistocene volcanic activities in the Fossa Magna region, central JapanK-Ar age studies of the Yatsugatake volcanic chain.Geochem. Jour., +., ,.3ῌ,/1. Kaneoka, I. and Kawachi, S. (+32-) K-Ar ages of volcanic rocks from the northern area of the Yatsugatake volcanic chain, central Japan.Jour. Geol. Soc. Japan, 23, -/3ῌ-0+. 9()* (+31.) $/%6,%B %6%B%&+ / O, + %BPQ %BRmS ++3p. 9()* (+311) $45%6,%B %6%B%&+ / O, + %BPQ %BRmS 3,p 9()* (+32/) $45./,7"# ·. /1/3 $#%&' (%& A %&)* + ,-./,0 -+ῌ-0. 9()* (+332) $45:;<=,+,,./>? -.$/ / , K-Ar 0 123 #AB# 4 3- +.3ῌ+0*. 9:;&<=NOPQR> (+33-) ?S @ @3 ITP-FT 0 K-Ar 0,AT % B#\ +** 3\]^C -+0. D Ec

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(142) W>F (+333) $/%6 v9$45./, K-Ar 0 ./#\ +333 `z3\]^{a ,*. (,**0) ./2#b|}c./2#R~ D,f %B#\g %h%Bi . H4%h opq /0.p., .0,ῌ.0-. ^'_'? xyz{ (+330) | @X,, X d DC, d, 123##4 - -+ +*/ῌ++1. <?QQ Z ** (,**0) ! ?"4 %6,./ 0 -#\Ca -0 30ῌ31. H?(/ £/ 39()* (,***) "4 ./ y,¤/p,%B@ 2c 123#AB#4s¥AB%&¦ -1 ,-ῌ-/. H¢5£{ (+33+) 9$45./¤1,@#©ª« ./ -0 3-ῌ++,.

(143) c¯ (,**1) H4 $45%6,./2#1,°±§ %B¨ ++- +3-ῌ,++. Oikawa, T. and Nishiki, K. (,**/) K-Ar ages of the lavas from Kirigamine Volcano, central Japan. Bull. Volcanol. Soc. Japan, /*, +.-ῌ+.2. ©ª ,**- H4 :n./2# ,0 (¬ ® 94³´Z[µ 1 ¶·¯/%%6,./2#)°,±²XY3¸1 «¤eHI,%¹º³,±² ./#\ ,**`z3\]^{a +0-. [w (,**/) $45ῌ %6 ,./2#0 ./,2#0H´ %B#\ ++, 3\]^C +/.. Peccerillo, A. and Taylor, S.R. (+310) Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area, northern Tukey. Contrib. Mineral. Petrol., /2, 0-ῌ 2+. ¤J& H?*µ ¶h»· ¬¼ª Q (,**1) "4%½:nS @¸%,¹º ;»¼ S+@½"%& / 1+ῌ2+. Steiger, R.H. and J¨ ager, E. (+311) Subcommission on geochronology : convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett., -0,.

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