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 ,  0+ῌ1+ ῐ ῏
 K-Ar  :ῌ  !"#$% &'(P Q R SOT U V WOX 
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 MIYAKEῌThe 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 theduration 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, ourdata 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#usionof 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"#$%&'()*+,-.()/0
AIST, GSJ, Institute of Geology and Geoinformation,
Tsukuba Central 1, +ῌ+ῌ+ Higashi, Tsukuba, Ibaraki-*/ῌ2/01, Japan  *-+ῌ20+1 /ῌ,8<=>?@ABCDE/
Ore Purchasing & Mining Engineering Department,
Paciﬁc Metals Co. Ltd., /ῌ, Toyama-shinden, Kawaragi, Hachinohe, Aomori *-+ῌ20+1, Japan -*/ῌ2/02  FG +ῌ+ῌ+ ! ,"#$%&'()*HI/0

AIST, International A#airs Department, Tsukuba Central ,, +ῌ+ῌ+ Umezono, Tsukuba, Ibaraki -*/ῌ2/02,
Japan -3*ῌ20,+ 12 345 -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 : k-nishiki@aist.go.jp

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. 7 'H, ,**-, H+, ,**. ; H+H,$yEBzBG. 7"J¡ ¢, ?9E,**1(- H+ (,**.) J9?@l#$£¤¥{"ihCz sFB .J|¦z".01123456&- , 

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EBzB**.1 Ma 'Oikawa and Nishiki, ,**/ ; L"+, ,**/(, .  - n/qrnJ, :;x5° , 0«1¬j5zB% n, ?91!±²4ῌN³P (K-Ar)  . 7- K ?91! K-Ar  J +.0

k?@l#$"BJK ': , +31.( "7891 ': , +31.( #;<=>?@ ': ,

5° , 11234hC5zB

+31.(  K-Ar  J,  ´*.,.)*.*+ Ma '&'+

- , 1!1¯¬}CE5° ,

, ,**1(, +10)0 ka './+, +333( n67- ::;$"m5°G- µ¶, :;(+332) J, I11234 '/qrI11234J, IBsEM.

.1B(  K-Ar  5° , E

7O,;1"·7}C5¸"5zB

5 *.--)*.+. Ma "A,  B7CD- ., 11234EJ, '/.

I?9E (*.1 Ma);$ Y *.,. Ma n67"5zBn
BA7-

, E 57BFG.  - 9?@l#$%MNOPQRSTUP+VWSX (Zr-FT) 
J *.,1)*.*- Ma 'H+H, ,**1( n67-

B, 1?9E"56ῌ¹ºU":;I11234J,

7ab?9I?9E"»jsB .

, A¬j® , E ¼. 7- 

$½7-J, O,;BsEM7O,;1Z3"
7/'9v+, +310( " "J1"

 K-Ar  : ῌ  !"#$% &'(Fig. +. a) Geological map of Quaternary volcanic products in Suwa-Yatsugatake volcanic province. The
circled areas show where rhyolitic rocks are distributed. b) Geological map around Daimontoge.
Abbreviations : S, Lake Suwa ; Ak, Akadake ; Ta, Tateshinayama ; Yo, Yokodake ; Ku, Kurumayama ; M,
Mugikusatoge ; In, Inagodake ; ISTL, Itoigawa-Shizuoka Tectonic Line active fault system ; Sh, Lake
Shirakaba ; ta, Takayama river ; da, Daimon river ; ot, Otonashi river.

63

:;<=>? @ABCDE64

FG H(IJK +31/ ; LM +323 N 45
%,!(FG(O PQLR (+310) 

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

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ST

$ '(U V  ( 'WXYL.
 YL. Z[ !. "\#]^_^`$#$ +! !.%a(Y

!"#$ %#&!.X./01b
bA((FG'(!. &c.' #$

b( H(IJK (+31/)

LM (+323) de!, H(IJK (+31/) ,! !.X./01fghi+,YL h
5jk(l)YL.*Q&*m+no- "!,! !. l)YL. ,(Yip-(.q/# +.-* m . rs(0#@.
tu$b(v1

.

2#$ $ %(p-34

5\wYL.q/fgx b( # 5\wYL(y6.(A7ST$ (.q/

+.-* m .

2#$89*z+,& -  (.'hi#(:{9 FG!, #"# (;|(#(:{ }-g +! ( +.-* m
.  h $(~< /01 t+$ h -# Pf%,,(# i.8( .8+=>b}- LM (+323) ?(~< /01(b(i. @ !. A

Fig. ,. Topographical map of two rhyoritic lava domes(Utsukushimatsu dome and Daimontoge dome)around Daimontoge. Contour line interval is +* m.
DEM data (/* m mesh) used in this map was published by Geographical Survey Institute.

. Z[ B l)YL.

7(C\#YL. &(.q/ # 5\wYL

 (Fig. ,) B.'(' "! 0#@.

(y6*.+/ km-

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X

#&

g 8+,& - 
4(f
-L !,&

`  +,&  rs % a( @ (  .GLHkf&Kj+,& -X+[ ? (

0#@./01

(Fig. +b)./01(M(k ¡)* +,& -  #"#+.0+.3 m :{N < ./01 i Xq+, $ 0#@.(?F@YL (?: hiG
LHk I +31. (7 ('( !. &JA+-

. *.+. km-

./012¢O(l)YL.f&i+,L'£,` 0#@. n$2# A( +/*..0 m&<,) %($T 0#@./01( h./01(+323)i

Q¥#X` LM

.  (Fig. +b)P 0#@./01(b(fg¤(Fig.

./01b(fgbn# fG¦ .8+=> &PQ §.¨©i- 

 

n$R°#$0#@.

.( hf± .8+=> PQ

PW,.**  (+330) $ - B. b Cox et al. (+313) 2 Wilson (+323)(f !.(©i z

0#@. STU:V +., km, A?V , km, *{ ,.* m (./01 hi.' 0#@.
/01 N (Fig. ,)

 ª« 87(¬­ X ® ¯D PHILIPS

0#@.
g :(

8WXY#$67 !.N=² 

8²³´(6Z8. 0#@.(Aµ¶+, [· #&Ih- #\Ih(¸8 STU:V *.2 km, A?V +.+ km, *{ ,** m

3 +./ mm 45 67 +./ mm 45 89 *./ mm 4

(./01 hi.' 5./01 N

S¹ 8yº»¼X ½·X-7Ng º
Table +.

 K-Ar :  ῍ 



65

Chemical compositions of volcanic rocks in Suwa-Yatsugatake volcanic province.

=> $?>@A*=B81C& DE $

4FGH IJ'3( ,**1( KLM

?OE PQR( RS( TU( V W=

X( +33+( YZ> /ῌ ab c.$ & [ \]^_?( SiO, 1 10.03

defgh#&N

NX( +33+ [ \]^_`

wt.ῌ( Na,O 1 ..,. wt.ῌ( K, O 1 -.30 wt.ῌ (Table +(Fig. - ?iBjB8^_kd+l78m/

Fig. -) Pn &1(  ?( QREPCom/ d? SiO,

1 1* wt.ῌ abp 1/ wt.ῌ 2q^_1oPr(h#stR Pn & Ephuv$huv, )*+,-./ & 0#( SiO,

QR w ,./ mm 2x( stR ,./ mm 2x yq)*+z{|( stR?}~jB8\h & stR?V QR *.+ mm 2x( QR *.+ mm 2x y$,( TiO,!

1 1/ wt.ῌ 2d TiO,( Fe,O-( K,Op( m. , )*+m/ &
%

Fe,O-

 |( K, Op1n & RL?OE PTU( Q"h#h ( YZ p/2$

R( RSc.$ & [ \]^_?( SiO, 1 03.0+ wt.ῌ(( 4FGH-./ & 0( #h

Na,O 1 -.2. wt.ῌ( K, O 1 -.+- wt.ῌ(Table +, Fig. -)p$ \]^_q?$ &Pn &/ K-Ar 
.ῌ  K-Ar

(P?( )

<3

[ \]^_y*#( $$  p ῍c. / k( -YZ c. . k 3 k[\]^_yh# (Table +)& m.( $p¢£¤$

¥'3( +310 om/

2%iBjB8 , kd*#?( +.0+.3 m %& +-.* m '#h

(Ut-En+ ; Fig. +b)( +/*..0 m %& +.** m ')+

Fig. +b)( %


Da+ %

(Da-Sm+ ; Fig. +b)(

,L.p$x+(Da-Da+ ;¡/(Ot-Otb, ; Fig. +b) !"#& $%( DaOt-Otb , R¦§?01¦ & Ot-Otb ,c¨.¨RE13 y4" 1( /2

y!"#& /.k©ª «?01g & 0#(?¬/k%d­®xP3 U$¯?°

Fig. - ?( /0PP5±m/#

./$& RL?¬/kd3 U?°./

[ \]^_`($,( ²³´µ¶e·¬( Ut-En + 2?DEPn& Ut-En + RL?> R&¸k JR-+( JR-, ¬/d$ ¹@A*=B8Ch#W=> $pDE $

6R º»` (Imai et al, +33/)( $

IJ'

1¼½

5678 , 9:';<( +33.(7#&3( ,**1( 41( k?RL1DE $|yC

66

DEFGHIJ

KLM>?NOOPQR

Fig. -. Variation diagrams for major elements of rhyolites in the Suwa-Yatsugatake volcanic province. The
boundary lines in K,O vs. SiO, diagram are after Peccerillo and Taylor (+310) and Na,OK,O vs. SiO,
diagram are after Cox et al. (+313).

K-Ar #$ %& '()*+,-./0 *.,/*./* mm 123456 7

 +*+* ῌ y lb  ..30,  +*+* ῌ y .* K ῌ Ktotal  *.*++01ῌ
(Steiger and Jäger, +311) S4TAUVWXYZ[*\]/(^_`ab

#Zc /K'AU *.,. *.*+ Ma !defg *)h/ ijkl

mn#" op'AU +.*1 *.*+ Ma !de$ )*qU2rsk-mn#" tuv'AU +.*.

x/y*5 $ z  VG Isotopes +,**C 
*5
 

Ar b24

-2

 

 .* Ar

2894:;<=*.*- Ma, wF*.++ Ma h5{ K-Ar #|}U~

 (Table ,) Y/*){ op'#$ +s :;M/ M  /K'#$ ,s :;M/$ >?@A (+33/) 2B Ut-En+ !/K'" op' (Da-Sm+)uv'AU +.22

$ 2 , Y/ ij

0 



0ῌ+ 

kl$ z x 2}U~ K-Ar #2¡5¢£¤94 o FP---D ¥¦¦§*5 ¨ !©h/¢jªkp« tuv'2"¬~4wFuv'!K +323" *¯/ i# (+.22 *.++ Ma) $ tuv'°±#| +.0l*­®94¥¦¦§jkl2894:;$ K (+323) 24'(!

*.1 Ma !Oikawa and Nishiki, ,**/ ; EF² ,**/" ©)Y/ZcAUS4´hAU µ¶·2#¸/W5S4 op*./ῌ !+s ³©" h¹ºy K-Ar #<25»$ le*./2+¼½2$94tuv'# (+.*.

*.*- Ma)  K-Ar

Table ,.

CH-.9 :  ῍ 



67

K-Ar ages of the volcanic rocks around Daimontoge.

6 (+.*1  *.*+ Ma)IEJK "LD?& -.MD,-./.0123 4& 5' -./.

ND' +.*1  *.*+ Ma % (6%' -./.78 9:2;<=. &) ,LO5?& P

%QRSE-T'  !

%' O.= U V

> 6 (*.,.*.*+ Ma) 9: ?&G '

 *.1/ Ma CD?    EF ? (Oi-

kawa and Nishiki, ,**/)& -W .* ! X%OE

YZ[%' \]^_`ab"c ?d`\ 4#%"LO=4G5?&  *.,**.- Ma %eH

9$f' gh 4ij9YZ%?O2G=' kl>5? 9:  @A%B2;<=. &"' -.2op&qr'(stuv)

w?' 

xy9*<+,tuv)%' K-Ar

" *.,.**.+2 Ma (!+' +333 ; -+' ,**1) E' !

 6%-.23 4& =' !

'  *.,. Ma  !

 ; *.,1*.*- Ma : z{+|{' ,**1) "s(Zr-FT}~ ?&0ῌ, ῌ



"# $%' "mn  ῍ J

*K





"# ,2  2;<=.' -.2.

4%'  +.0**.1 Ma wE%*

6%3#E /IsI9 IG=0

K-Ar

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68

Fig. .. Radiometric ages of rhyolites in Suwa-Yatsugatake volcanic province. * : this study ; a : Kaneoka andSuzuki (+31*) ; b : Kitada et al. (+33-) ; c : Kitada et al. (+33-) ; d : Wadatsumi et al. (+33.) ; e : Kawachi(+332) ; f : Nagai et al. (,**0) ; g : Sugihara and Danhara (,**1).

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78 #uv¶yz#k#Ao[S(&,n&¼½dW? XY !"#῏E

Fig. 0. Spatiotemporal distribution of eruptive center in Yatsugatake volcanoes. The gray areas show distributionof Yatsugatake volcanic rocks. OVG : Oiwake volcanic graben ; N.Y : Northern Yatsugatake volcanoes ; S.Y :Southern Yatsugatake volcanoes ; Ta : Tateshinayama ; Yo : Yokodake ; In, Inagodake ; Ak : Akadake.

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\ gatu,3\,vw;V8 3Qr}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./  %#¨ +*- ,*1ῌ,,*.
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0 123#AB#4s¥AB%&¦ -- +ῌ0.
Imai, N., Terashima, S., Itoh, S. and Ando, A. (+33/) +33.
compilation values for GSJ reference samples, “Igneousrock 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 trackages 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 Magnaregion, 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,p9()* (+32/) $45./,7"# ·  /1/3 $#%&' (%& A %&)*
+  ,-./,0  -+ῌ-0.
9()* (+332) $45:;<=,+,,./>?
-.$/ /  , K-Ar 0 123 #AB# 4  3- +.3ῌ+0*.
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DEc F d%GH IeJghM (+330) ci%6,%B %6%B%&+/ O, + %BPQ %BRmS ++.p.
 (+323) K-Ar 0TU:nopqEUV,r %Rs .* 0/ῌ1*.
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+333 `z3\]^{a ,*.
  (,**0) ./2#b|}c./2#R~D,f  %B#\g  %h%Bi. H4%h  opq /0.p., .0,ῌ.0-.
 ^'_'? xyz{ (+330) |
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-#\Ca -0 30ῌ31.
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      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  (¬  ­®  94³´Z[µ  1¶·¯/%%6,./2#)°,±²XY3¸1«¤eHI,%¹º³,±²  ./#\ ,**`z3\]^{a +0-.
[w (,**/) $45ῌ %6,./2#0   ./,2#0H´  %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-ῌ2+.
¤J& H?*µ  ¶h»·  ¬¼ª  Q(,**1) "4%½:nS @¸%,¹º;»¼ S+@½"%& / 1+ῌ2+.
Steiger, R.H. and Jäger, E. (+311) Subcommission on geochronology : convention on the use of decay constants ingeo- and cosmochronology. Earth Planet. Sci. Lett., -0, 

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K-Ar

-/3ῌ-0,.
#=D$ (,**-) %&EFG'(H)P, -.+ G'/QRS* ,* 13ῌ3/.
#=D$,1= 2,O 3 (,**.) 4 E
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d,R.M. & ,\],Z e (+33/)
BhEFfmgh K-Ar  iY ¡-jW¢J¢k£Ek¤$@¥¦#§@\lmh nX-+ B¨.* .0*,-3ῌ,.3.
ª=r,G]s«¬,DEINO, A. (+33.) ­'@!®
 JAS-G+ Ar-Ar + !"#$%&,'#\23 @0 * 1* 1.
Wilson, M. (+323) Igneous petrogenesis. Unwin Hayman,.00p.
qx²y (+323) 6z{K[+ }J9t* .ῌ
.-.
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B +-.* m* Gs -0(*1῍.,)* "C +-2(+.῍-*)+
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mD : <=  > ?@'&A* !B +0.* m+
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