Bull. Volcanol. Soc. Japan
Vol. /* (,**/) No. ,, pp. +.-ῌ+.2Letter

K-Ar Ages of the Lavas from Kirigamine Volcano, Central Japan
Teruki OIKAWAῌ and Kuniaki NISHIKIῌῌ
(Received July ,-, ,**. ; Accepted January +0, ,**/)The Late Pliocene to Early Pleistocene volcanic rocks in the Suwa area, central Japan, comprising an
enormous quantity of volcanic rocks, are named the Enrei Volcanic Rocks. The authors will report two K-Ar
ages of volcanic rocks from the Kirigamine Volcano, which composes the uppermost part of the Enrei Volcanic
Rocks. Based on the newly obtained ages, previously reported radiometric ages, and volcanostratigraphic data,
we identify activity periods of +.- to *.1/ Ma for Kirigamine Volcano. We point out the possibility that the
Oiwake Volcanic Graben partly covered by Kirigamine Volcano was mainly formed at ca. *.2/ Ma. From theprevious studies and this study, the duration of the activity of the Enrei Volcanic Rocks is mainly from ca. +./ to*.1/ Ma. This duration of activity of the Enrei Volcanic Rocks almost corresponds to the active period in the
Early Pleistocene of the Yatsugatake Volcanoes that adjoined Suwa area. Thus, in the Suwa and Yatsugatake
areas in the Early Pleistocene, volcanic activity occurred with enormous volumes, covering a large area (2** km,).
Key words : K-Ar dating, Kirigamine Volcano, Enrei Volcanic Rocks, Yatsugatake Volcano, Early Pleistocene+. Introduction
The Late Pliocene to Early Pleistocene volcanic rocks
in the Suwa area, central Japan, comprising an enormous quantity of volcanic rocks covering over .** km,
area, are named the Enrei Formation (Momose et al.,+3/3) or the Enrei Volcanic Rocks (Makimoto et al.,+330). In the area adjoining the Suwa area, the Older
Yatsugatake Volcanoes (Kawachi, +31. and +311), also
comprising an enormous volume (ca. +** km-) of Early
Pleistocene volcanic rocks, are distributed (Fig. +).
Thus, the Suwa and Yatsugatake areas are the locationin which outstanding and enormous volcanism occurredin central Japan. However, in these areas, chronological studies were insu$cient and the relationship in timeand space of the enormous volcanism was not clear.
The chronology of the Enrei Volcanic Rocks was previously studied by Momose et al. (+300), Kaneoka andSuzuki (+31*), Kitada et al. (+33.), Kawachi (+331),
Uchiumi et al. (+332), Miwa and Hoshi (,**,) andSato (,**.). However, these studies were insu$cient
to clarify the volcanic history of the Enrei Volcanic
Rocks.
Because of the active structural movements in andaround the Suwa area, especially related to the ItoigawaShizuoka Tectonic Line active fault system (Fujimori,+33+) and the Oiwake Volcanic Graben (Kawachi,+31.), the Enrei Volcanic Rocks has been considerably,. Outline of Geology
The Kirigamine Volcano, with the highest summit
being Mt. Kurumayama ; (+,3,/ m), +* km (N-S)ῌ2
km (E-W), 3** m relative height and ca. +/ km- volume,
has gentle slopes of less than +* degree. The basement
rocks of the Kirigamine Volcano consist of Miocene
granite and the Lower Enrei Volcanic Rocks (Compilation Group of Natural History in Suwa, +31/). The
Kirigamine Volcanic Rocks, Takayama Volcanic Rocks,
Wadatoge Volcanic Rocks, and Mitsumine Volcanic
Rocks compose the upper Enrei Volcanic Rocks (Compilation Group of Natural History in Suwa, +31/).
These volcanic rocks are distributed in and around the
Oiwake Volcanic Graben (Fig. +). Kawachi (+31.)
considered that the Enrei Volcanic Rocks is the base-῍ Tono Geoscience Center, Japan Nuclear Cycle Deve-

University, -ῌ+ῌ+ Asahi, Matsumoto -3*ῌ20,+, Japan.

lopment Institute, 3/3ῌ-+, Jorinji, Izumi, Toki /*3ῌ/+*,, Japan.
῍῍ Graduate School of Science and Technology, Shinshu

Corresponding author : Teruki Oikawa
e-mail : oikawa.teruki@jnc.go.jp

deformed. Therefore, the chronological study of the
Enrei Volcanic Rocks is important to clarify not only
the volcanic history but also the relationship between
the structural movements and the volcanic activity.
In this paper, the authors will report two K-Ar agesof volcanic rocks from the Kirigamine Volcano (deﬁnition after Compilation Group of Natural History in
Suwa, +31/), which composes the uppermost part of the
Enrei Formation (Momose et al., +3/3), and will discuss the temporal relationships with the adjoining
volcanoes.

144

Teruki OIKAWA and Kuniaki NISHIKI

Fig. +. a) Simpliﬁed geological map of Quaternary volcanic products in and around the Suwa and Yatsugatake
areas. This map is simpliﬁed after Nakano et al. (+332), Ozaki et al. (,**,) and Yamada et al. (+323). b)
Simpliﬁed geological map of the Kirigamine Volcano, the Wadatoge Volcanic Rocks and the Takayama
Volcanic Rocks. These Volcanic Rocks compose the upper Enrei Volcanic Rocks (Compilation Group of
Natural History in Suwa, +31/). This map simpliﬁed after Compilation Group of Natural History in Suwa(+31/) and Teshima and Kawachi (+33.). Sampling localities for K-Ar dating are also shown. S : Suwako(Suwa Lake), U : Mt. Utsukushigahara, A : Yatsugatake (Mt. Akadake), T : Mt. Tateshinayama, K : Mt.
Kurumayama, ISTL : Itoigawa-Shizuoka Tectonic Line active fault system, MTL : Median Tectonic Line.

K-Ar Ages of the Lavas from Kirigamine Volcano, Central Japan

145

Fig. ,. Stratigraphic relationship of Quaternary volcanic products in the Kirigamine Volcano and the Wadatoge
Volcanic Rocks, modiﬁed from Compilation Group of Natural History in Suwa (+31/).

ment rocks of adjacent Yatsugatake Volcanoes.
Part of the Kirigamine Volcano ﬁlled in the Oiwake
Volcanic Graben (Fig. +). The Oiwake Volcanic Graben, some - km wide and +- km in length, has a verticaldisplacement of more than ,** m (Kawachi, +31.).
The Takayama Volcanic Rocks (Teshima and Kawachi,+33.) on the north of Kirigamine Volcano were deformed by activity of the Takayama Fault (Kawachi,+31.), a boundary of the Oiwake Volcanic Graben (Fig.
+). Kawachi (+331) shows an age of *.2.῎*.*, Ma
for the lava of the Takayama Volcanic Rocks. The
arrangement of the adjacent Yatsugatake Volcanoes,
especially the northwestern Yatsugatake Volcanoes, iscontrolled by this graben (Kawachi, +31.). However,
the origin and forming age of this graben are not clear.
Volcanic rocks of the Kirigamine Volcano are composed of hornblende or pyroxene andesite or dacite(Yamazaki, +30/ ; Compilation Group of Natural History in Suwa, +31/ ; Nakai et al., ,***). The ﬁrst stratigraphic study of the volcanic rocks of the Kirigamine
Volcano was made by Sawamura and Owa (+3/-).
After that, Yamazaki (+30/), the Compilation Group of
Natural History in Suwa (+31/), Yamazaki et al. (+310)
and Nakai et al. (,***) reported the stratigraphy.
Based on the stratigraphy of the entire Kirigamine
Volcano by the Compilation Group of Natural History
in Suwa (+31/), the Kirigamine Volcano is divided into
lower (K I) and upper (K II) parts, both of which arefurther subdivided into K Iaῌb and K IIaῌc units inascending order (Figs. + and ,). Two radiometric ages,+.--῎*.*, Ma (Uchiumi et al., +332) and +.+1῎*.*0
Ma (Sato, ,**.) for the K Ia unit have been obtained
for this volcano. The K Ib unit of this volcano covers

Wadatoge Volcanic Rocks (*.3ῌ+.+ Ma : Kaneoka andSuzuki, +31* ; Kitada et al., +33.).
-. Samples and Analyses
We analyzed the following samples for the K-Ar
ages : the lava of K Ib unit (Loc. + ; sample No. K-+)
in Takayama at the northern part of volcano, and thelava of K IIc unit (Loc. , ; sample No. K-,), from thesummit area of Mt. Kurumayama at the central part of
volcano. The K Ib unit at Takayama covers the
Wadatoge Volcanic Rocks and partly covers the Oiwake
Volcanic Graben. The K IIc unit in the summit areaof Mt. Kurumayama is the youngest geological unit in
the Kirigamine Volcano. Sampling localities, petrographic descriptions, and SiO, contents of the two
samples are shown in Table +. Localities are also shownin Fig. +. Both phenocrysts and groundmass are freshwithout any alteration products in these samples.
Samples were crushed and sieved to obtain 0* (,/*mm) to 2* mesh (+10.2 mm) fractions. Phenocrysts
were removed from the fractions using a hand magnet
and an isodynamic separator. The groundmass fraction was used for potassium and argon analysis. The
argon isotope was analyzed by isotope dilution at
Hiruzen Institute for Geology and Chronology Co.
Ltd.. The concentration of potassium was also determined by ﬂame spectrometric analysis at the same laboratory, too. The analyses and the age determination
method were based on the method described by Nagao
et al. (+32.) and Itaya et al. (+33+). For calculationof K-Ar age, leῐ*./2+῏+*῍+*/yr, lbῐ*..30,῏+*῍+*/y and .*K/Kῐ*.***++01 (Steiger and Jäger, +311)
were used. Average ages were calculated using formu-

146

Teruki OIKAWA and Kuniaki NISHIKI
Table +.

Table ,.

Localities and petrographic descriptions of dated lava samples.

K-Ar ages of lava samples determined by this study. Ages aremeasured for separated groundmass fraction.

la proposed by Tsukui et al. (+32/).
Analytical results are summarized in Table ,. Thesesamples have low atmospheric .*Ar ; lower than 0/ῌ.
Obtained K-Ar ages from K-+(K Ib) and K-,(K IIc)
are *.2/ῌ*.*, and *.1/ῌ*.*-, respectively (Table ,).
.. Result and Discussion
Fig. , compiles the stratigraphic relationship among
the lavas (Compilation Group of Natural History in
Suwa, +31/) and their K-Ar ages determined in this andprevious studies. All of the four determined ages from
fresh samples are consistent with the stratigraphy.
Therefore, all these age data represent the eruption ageof each lava. The duration of the activity of the
Kirigamine Volcano is concluded to be from +.- to *.1/
Ma (Fig. ,). Notably, the duration of the activity of
the upper Kirigamine Volcano, that is, above the K Ib
unit, is ca. *.+ my. ; between *.2/ῌ*.*, Ma and *.1/ῌ*.*- Ma. However, the duration of the activity of thelower Kirigamine Volcano (K I unit) is *./ Ma that islonger than the upper volcano. Taking the number ofunits in the lower volcano into consideration, there is
the possibility that the volcanism of lower Kirigamine
volcano had an interruption.
Kawachi (+331) suggested that the formative age of
the Oiwake Volcanic Graben is older than *.2 Ma basedon the K-Ar age for Takayama Volcanic Rocks.
Amount of the vertical displacement of Takayama Volcanic Rocks formed by activity of the Takayama Fault(a boundary of the Oiwake Volcanic Graven) is over,** m high (Kawachi, +31.). The present fault scarp
of Takayama Fault is ,** m relative high. Thus, theformation of the fault scarp topography of Takayama

Fault occurred after the formation of Takayama Volcanic Rocks. According to the age determined for the
K Ib unit that ﬁlled in the Oiwake Volcanic Graben andthe age of the Takayama Volcanic Rocks, the Takayama
Fault mainly deformed the Takayama Volcanic Rocks
at ca. *.2/ Ma. The present topography of Takayama
Fault is mainly formed ca. *.2/ Ma. It is consideredthat the deformation rate of Takayama Fault waslargest at ca. *.2/ Ma. From the result that the duration of the activity of the Takayama Fault and the
Takayama and the Kirigamine Volcano corresponds
closely, it is considered that structural movement accompanied by volcanism caused the formation of the
Takayama Fault. Based on above ages and stratigraphic data, we point out the possibility that the
Oiwake Volcanic Graben formed rapidly at ca. *.2/ Ma.
The Kirigamine Volcano composes the uppermost
unit of the upper Enrei Volcanic Rocks (Compilation
Group of Natural History in Suwa, +31/). The age of+...ῌ*.*, Ma for the lava of the lower unit of the lower
Enrei Volcanic Rocks (Compilation Group of Natural
History in Suwa, +31/ ; Kubota, +333) is obtained by
Uchiumi et al. (+332). The paleomagnetic stratigraphy of the Enrei Volcanic Rocks suggests that mostof the formation belongs to the Lower Pleistocene(Momose et al., +300 ; Miwa and Hoshi, ,**,). Sato(,**.) considered that the volcanic activity after Pliocene in Suwa and Yatsugatake areas was started suddenly at Pleistocene. Judging from the above studies, suchas Momose et al. (+300), Compilation Group of Natural History in Suwa (+31/), Uchiumi et al. (+332),
Kubota (+333), Miwa and Hoshi (,**,), Sato (,**.)and this study, the duration of the activity of the Enrei

K-Ar Ages of the Lavas from Kirigamine Volcano, Central Japan

Volcanic Rocks is mainly ca. +./ to *.1/ Ma. Thisduration of activity of the Enrei Volcanic Rocks almost
corresponds to the active period in the Early Pleistoceneof the Yatsugatake Volcanoes that adjoined Suwa area,such as * and + stages of the Older Yatsugatake
Volcanoes (ca. +.-ῌ*.12 Ma : Kawachi, +31., +311, +331 ;Kaneoka et al., +32* ; Kaneoka and Kawachi, +32- ;Akimoto et al., ,**,). Furthermore, the active periodof these volcanoes corresponds to the active period (ca.
+.. Ma : Kaneoka et al., +33-) of the Meshimoriyama
Volcanic Rocks (Kawachi, +311), too. Kawachi (+31.)
had shown that the Enrei Volcanic Rocks is older thanthe volcanic rocks of the Yatsugatake Volcanoes of the
Early Pleistocene (e.g. Kasuga Volcanic Rocks : Kawachi, +31.). Kaneoka and Kawachi (+32-) suggested
that the start of the volcanic activity of the Yatsugatake
volcanoes agreed with a part of the duration of theactivity of the Enrei Volcanic Rocks. Based on thisstudy, it has become clear that these volcanic rocks wereformed in the same period (ca. +./ῌ*.1/ Ma). According to estimations from the geological maps by Nakano
et al. (+332) and by Ozaki et al. (,**,), the covered
areas of these volcanic rocks are 2** km,. Thus, in the
Suwa and Yatsugatake areas in the Early Pleistocene,
volcanic activity occurred with enormous volumes, covering a large area.
Acknowledgments
We would like to thank Professor Y. Miyake of
Shinshu University and Dr. K. Umeda of Japan Nuclear
Cycle Development Institute for their constructive comments on an early version of this manuscript. Mr. G.
McCrank of ex-JNC International Fellow helped us
with English. Mr. H. Hayashi of PESCO Co. Ltd.
helped in the preparation of thin section. We also
thank an anonymous reviewer, Mr. S. Nakano of AIST
GSJ, and Associate Professor M. Tsukui of Chiba University for reviewing the manuscript.
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