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0  +3 K-Ar Ages of Lavas from the Mount Eboshidake, Johshin District, Central Japan
Kou TAKAHASHI῍ and Yasuyuki MIYAKE῍῍Mt. Eboshidake stands at the western part of the Eboshi volcano group, in Johshin district, central Japan.
The K-Ar ages of *.11ῌ*.10 Ma are obtained for three lava samples collected from Ofuji, Kanahara lavas alongwith the lowermost part of Eboshidake lavas in the southwestern ﬂank of Mt. Eboshidake. These studied lavas
are stratigraphically situated between the Iwayakan’non lavas and the upper part of the Eboshidake lavas whichhave been dated to be *.31 Ma and *.-/ Ma, respectively. The Kanahara lavas which have previously been
regarded as a part of the Eboshidake lavas, are newly deﬁned because of their distinct inclination. Our results
indicate that the volcanic activity of the Eboshi volcano group has initiated at least *.2 Ma, and possibly + Ma,
instead of the formerly regarded age of ca. *.. Ma. On the basis of the new age data, the life span of the Eboshi
volcano group was longer and consequently its eruption rate was lower than they used to be estimated.
Key words : K-Ar age, Eboshi volcano group, Johshin district, central Japan, Quaternary+ῌ ῐ ῎ ῑ ῏\bz"t96 u{kw "|6N    +3/2   !"#$%& !!ῌ'(')",-+./01 3 45. CD

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Division of Environmental System Science, Graduate
School of Science and Technology, Shinshu University, Asahi -ῌ+ῌ+, Matsumoto -3*ῌ20,+, Japan.
 -3*ῌ20,+ ¡¢£ -ῌ+ῌ+¤XXO±¨X¨
Department of Geology, Faculty of Science, Shinshu

University, Asahi -ῌ+ῌ+, Matsumoto -3*ῌ20,+, Japan.
Corresponding author : Kou Takahashi
e-mail : th32.*/@gipac.shinsyu-u.ac.jp+} (+323)  (+3/2) \<²³<<´"  (+30,)
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b#,L TUV ',%;< +30, AB#Ngj'5,Fig. +. Index map of the Eboshi volcano group.
a : Location map of the Eboshi volcano group. Open triangles indicate Quaternary volcanoes
after Ono et al. (+32+).
b : Geomorphological map of the Eboshi volcano group and Asama volcano.
The areas of Eboshi volcano group and Asama volcano are quoted from Iijima (+30,) andAramaki (+302), respectively. The dotted area indicates the distribution of Eboshidake
lavas. The square is an extent of the Fig. ,. The contour interval is +** m.

»7  )3 K-Ar 4¼
 (Fig. ,).   209mm, -.+ῌ) "b (*.2 mm, *.- mm, ,.3ῌ)

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Fig. ,. Geological map of the southwestern ﬂank of Mt. Eboshidake. Sample localities are alsoshown. The contour interval is +** m.

¸ ¹º»¹¼210

Fig. -.

The occurrence of the Kanahara lavas exposed during the construction of the Kanahara dam.

Table +.

K-Ar ages for lavas from the southwestern ﬂank of the Mt. Eboshidake.

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Fig. .. Stratigraphic relations of the lavas in thestudied area. Stratigraphy and the name of thelavas are modiﬁed after Iijima (+30,). K-Ar
ages are from * Kaneko et al. (+323) and ** thisstudy.

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(+322) Itaya et al. (+33+)

   (+32.)

   K  ,,*** ppm Cs    +  /*+** mg   + !" , # $%&  Ar %'()

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,uH1ILMgGHch3< *.2 Ma

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( le6*./2+7+*8+*ῌyear, lb6..30,7+*8+*ῌyear, .*KῌK6+.+017+*8.

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V1 C^ H1IJ0BT_' `5#CD *.10E*.*/ Ma 3K02D23  H1IJ0B@&abWRFI (+323) ?< .* AB1230CDC c+- H1IJ0B D1./A0B3FG0B
 78 (+30,)  P=RS 0B+4 EF;123  ±r (+302) ]L b  no. +.
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 -1 .0ῌ/3.
Itaya, T., Nagao, K., Honjou, Y., Okada, T. and Ogata, A.
(+33+) Argon isotope analysis by a newly developed mass
spectrometric system for K-Ar dating. Mineralogical
Journal, +/, ,*-ῌ,,+.
@ABC$D EFGHI (+323) K-Ar JKLMNOPQR=S89+TU VW 2. ,++ῌ,,/.
@ABC$D EFGHI (+33+) P=SXYZ[\]^ 9+8 K-Ar JK:_`a b cd 00,33ῌ-,,.

efghFGHI (+322) K-Ar iXj\JKkl =mno ,3 /ῌ,+.
efghpqrsFGHItuv (+32.) K-AriXj\JKkl w+x?y+ z d{ 3 +3ῌ-2.
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Steiger, R. and Jäger, E. (+311) Subcommission on geochronology : convection on the use of decay constants ingeo- and cosmo- chronology. Earth Planet. Sci. Lett., -0,-/3ῌ-0,.
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+9+8 K-Ar JK =m 3+ ,13ῌ,22.
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