J. Earth Planet. Sci. Nagoya Univ., Vol. 45, 17 to 27, 1998

An 85±5 Ma CHIME age for the Agigawa welded tuff sheet
in the oldest volcanic sequence of the Nohi Rhyolite,
central Japan

Kazuhiro SUZUKI,

Mineko NAKAZAKI

andMamoru ADACHI

Department of Earth and Planetary Sciences, Nagoya University,
Nagoya 464-8602, Japan(Received October 24, 1998 / Accepted November 21, 1998)

ABSTRACTCHIME ages were determined for zircon and allanite from the Agigawa weldedtuff sheet of the Nohi Rhyolite in the Kamado area, Gifu Prefecture. The CHIME
zircon age is 85±5 Ma and the CHIME allanite age is 86±7 Ma. The Agigawa
welded tuff sheet in the lowest volcanic sequence of the Nohi Rhyolite, is chronologically identical with the post- tectonic Shinshiro Tonalite (85.2±3.3 - 86.0±4.7Ma) and the Mitsuhashi Granodiorite (83.8±1.3 - 84.1±3.1 Ma) in the Ryoke
metamorphic belt to the south of the Kamado area. Acidic plutonism and volcanism started simultaneously and widely at about 85 Ma after the high T/P
Ryoke metamorphism at about 100 Ma.

INTRODUCTION
The Nohi Rhyolite, occupying an area of about 5000 km2 , is the largest
cluster of volcanic piles in central Japan. It extends from the northern partof the Ryoke metamorphic belt through the Mino and Circum-Hida terranes tothe Hida terrane over a 120 km long in a NW direction; the width is about 40
km. The Nohi Rhyolite consists of numerous welded tuff sheets with lavas andvolcaniclastic sediments, and is classified into 6 volcanic sequences defined as
the successive accumulation of welded tuff sheets with underlying volcanic
sedimentary layers (e.g. Yamada, 1977; Koido, 1991). Rocks of oldest sequence
I occurs in the southeastern part and those of younger sequences in the central
and northwestern parts.
Although the Nohi Rhyolite is inferred to have formed during a long period
from late Cretaceous to Paleogene(Koido, 1991) with a climax before 70 Ma(Yamada et al., 1992), the question of when the volcanism had started is left
open. The known fact is that rocks of sequence I is intruded by the Inagawa
Granodiorite (Sakai et al, 1965; Yamada, 1966). The emplacement age for the
Inagawa Granodiorite is still poorly understood, because the pluton have to agreat extent undergone thermal effects of successive granite intrusions. Recent
CHIME dating, however, discloses that the Inagawa Granodiorite formed atabout 82 Ma (Suzuki and Adachi, 1998). A question we address in this paper iswhether or not the Nohi Rhyolite volcanism overlapped with the peak Ryoke

17

K. Suzuki et al.

18

metamorphism (100-95 Ma, Suzuki et al., 1993, 1994b; Suzuki and Adachi.
1998) and the syntectonic plutonism (95-92 Ma, Nakai and Suzuki, 1996;
Suzuki and Adachi, 1998). This paper reports CHIME ages of zircon andallanite from a welded tuff sheet in the oldest volcanic sequence of the Nohi
Rhyolite in the Kamado area, Gifu Prefecture.
GEOLOGY
The geologic configuration of the Kamado area are given in Fig. 1. The area
is underlain mainly by the sedimentary rocks of the Mino terrane, a thickFig. 1.

Geologic map of the Kamado area, Gifu Prefecture, central Japan,
showing the sample locality. MTL: the Median Tectonic Line, ISTL:
the Itoigawa-Shizuoka Tectonic Line.

An 85±5 Ma CHIME age for the Agigawa welded tuff sheet

19

welded tuff sheet of the Nohi Rhyolite, granitoids, stocks and dikes of granite
porphyry and Cenozoic covers. The sedimentary rocks of the Mino terrane,
consisting predominantly of sandstone, shale and chert, underwent extensively
thermal metamorphism, and were converted into biotite- and cordierite-hornfels. The welded tuff sheet within the map area, named as the Agigawa weldedtuff, is a member of the oldest volcanic sequence of the Nohi Rhyolite (Yamada,1989). Rocks of this sheet, biotite rhyolite to hornblende-biotite rhyodacite incomposition, are densely welded, wholly devitrified and thermally metamorphosed. The Agigawa welded tuff sheet is intruded by small stocks and dikesof granite porphyry. Granitoids in the area are grouped into the Inagawa
Granodiorite and the Toki Granite. The Inagawa Granodiorite consists mainlyof coarse-grained hornblende-biotite granodiorite and biotite adamellite, and ischaracterized by the common occurrence of porphyritic K-feldspar. The Toki
Granite consists predominantly of medium-grained biotite granite and adamellite.
Sakai et al. (1965) and Yamada (1966) found that the Inagawa Granodiorite
had intruded into the Agigawa weldedtuff sheet at Kami-giri, Iwamura Town,
about 5 km southeast of the map area. The Inagawa Granodiorite, in turn, is
intruded by the Toki Granite (Yamada, 1967). Although most granite porphyrystocks are intruded by the Inagawa Granodiorite (Yamada, 1967), granite porphyry dikes in the map area remain immune to thermal metamorphism. We,
therefore, presume that granite porphyry dikes, if not all, formed after theemplacement of the Inagawa Granodiorite and Toki Granite.
Previous K-Ar biotite ages show that the emplacement of the Inagawa Granodiorite and the Toki Granite took place at 65-70 Ma (Shibata et al., 1962;
K a w a n o and Ueda, 1966). Subsequent Rb-Sr dating by Shibata and Ishihara(1971) reported a 72.3±3.9 Ma whole-rock isochron age as the emplacement
time of the Toki Granite. Recent CHIME dating yielded 81.9±1.4 and 82.6
±1 . 8 M a m o n a z i t e a g e s f o r t h e I n a g a w a G r a n o d i o r i t e a n d a 6 8 . 3 ± 1 . 8 M am ona z i t e a ge for t he T oki G ra ni t e (S uz uki a nd A da c hi , 1998).

SAMPLE DESCRIPTION AND CHIME DATINGThe sample (Sample 97121401) for the present CHIME dating was collectedoofrom a river-side outcrop of the Sasaragi River (Fig. 1, 32 24'6"N, 137 18'47"E).
The rock is dense and porphyritic with a fine-grained matrix of quartz andfeldspars (Fig. 2A). Phenocrysts, attaining up to 4 mm in size, make up 40 %of the total volume, and are dominated by quartz, plagioclase, K-feldspar andbiotite. They are partly corroded. Most biotite phenocrysts are highly distorted and recrystallized along the margin. Rare pumice lenses are completely
recrystallized into a fine-grained aggregate of quart and feldspars. Accidental
lithic fragments, up to 15 mm in size, constitute only a few volume percent of
the rock. They are mainly shale with a subordinate amount of sandstone.
Zircon and allanite occur mainly in close association with distorted biotite
phenocrysts, and sometimes as inclusions (Fig. 2B and Fig. 3); they seem to becogenetic with phenocrystic biotite. Zircon grains range in size from 0.03 toK. Suzuki et al.

20

Fig. 2.

(A) Photomicrograph of Sample 97121401 from the Agigawa welded tuff sheet
in the Kamado area. Phenocrysts of quartz, oligoclase, orthoclase and biotite
are seen in a fine-grained matrix of mainly quartz and feldspars. Note that the
sample contains little lithic fragments. One polar. (B) Photomicrograph ofallanite grains contained in and attached on a biotite phenocryst in Sample
97121401. The pleochroic halos (dark ring) around allanite is conspicuous. One
polar.

An 85±5 Ma CHIME age for the Agigawa welded tuff sheet

Fig. 3.

21

Backscattered electron and X-ray images of zircon (Z01) and allanite (A04) grains
attached on a biotite phenocryst in Sample 97121401. Scale bars are 0.1 mm.

0.18 mm, and show faceted forms of magmatic origin. This morphologicalcharacter contrasts with the rounded shape of detrital zircon grains in the lithic fragments. Magmatic zircon grains have no visible interior structure underthe microscope.
Allanite is pleochroic from deep reddish brown to pale
brown. Most allanite grains are homogeneous in color, but some show a concentric color zoning with central deep brown and marginal pale brown zones.
Zircon and allanite were analyzed on the JCXA-733 electron microprobe22

K. Suzuki et al.

analyzer equipped with four wavelength dispersive type spectrometers. The
instrument operating conditions were 15kV accelerating voltage, 0.35mA
probe current and 3mm probe diameter. The ThMα, UMβ, PbMα and YLα only
were m e a s ur e d w i t h a P E T c r ys t a l , a nd t he s pe c t r a l i nt e r f e r e nc e of Y L γ on
PbMa was corrected through the procedure described by Amli and Griffin (1975).
X-r a y i n t e n s i t i e s w e r e i n t e g r a t e d o v e r 4 0 0 s f o r t h e l i n e s a n d 2 0 0 s f o r t h ebackgrounds at two optimum positions on both sides of the lines. The peaka n d background measurements were repeated twice or three times on zircon,
whereas on allanite the damage by electron bombardment prevents repeatedmeasurement. Raw intensity data were converted into concentrations throught he m e t hod de s c ri be d by Be nc e a nd A l be e (1968) us i ng a na l ys e s of na t ura lzircon (SiO 2 30.5%, ZrO 2 58.8, HfO 2 3.51, Er 2 O 3 0.313, Yb 2 O 3 0.755% and P 2 O 5
1.28%) and allanite ( SiO 2 31.6%, Al 2 O 3 14.1%, La 2 O 3 4.34%, Ce 2 O 3 10.1%, Pr 2 O 3
0.949%, Nd 2 O 3 4.95%, Sm 2 O 3 1.01%, Gd 2 O 3 0.833%, Dy 2 O 3 0.622%, FeO 14.3%,
MnO 0 . 9 4 % a n d C a O 9 . 3 8 % ) a s t h e m a t r i x c o m p o s i t i o n s . T h e d e t e c t i o n
l i m i t s o f ThO 2 , UO 2 and PbO at 2σ confidence level are 0.008%, 0.010% and0.003% for zircon and 0.009%, 0.012, and 0.004% for allanite, respectively.
The relative error in the PbO determination of 0.01 wt.% concentration isabout 20% for zircon and 25 % for allanite. For the details of the CHIME
age calculation, readers are requested to refer Suzuki and Adachi (1991a,b,
1994 and 1998), Adachi and Suzuki (1992) and Suzuki et al. (1994a).
RESULTS
Most zircon grains are low in the Th, U and Pb concentrations, but threegrains were found to containmeasurable amounts of Pb as well as Th and U.
A total of 28 spots on these grains were analyzed, and the analytical results
are listed in Table 1 together with apparent ages and UO 2* (the measured UO 2
plus UO 2 equivalent to the measured ThO 2 ) values. The ThO 2 concentration
ranges from 0.073 to 0.898 wt.%, the UO 2 concentration from 0.271 to 1.313
wt.%, and the PbO concentration from 0.0041 to 0.0162 wt.%. The analyticald a t a a r e p l o t t e d l i n e a r l y o n t h e P b O - U O 2* d i a g r a m ( F i g . 4 A ) , a n d g i v e a nisochron of 85±5 Ma (MSWD=0.05) with an intercept value of 0.0002±
0.0004.
A total of 80 spots on 7 allanite grains were analyzed, and the results arelisted in Table 2 together with apparent ages and ThO 2 * (the measured ThO 2plus ThO 2 equivalents to the measured UO 2 ) values. Allanite is rich in ThO 2
but poor in UO 2 . The ThO 2 concentration ranges from 1.32 to 3.63 wt.%, while
the UO 2 concentration varies from 0.013 to 0.122 wt.%. The PbO concentration
ranges from 0.0050 to 0.0158 wt.%. All data define a linear array in plot of
P b O v e r s u s T h O 2* ( F i g . 4 B ) , a n d y i e l d s a n i s o c h r o n o f 8 6 ± 7 M a ( M S W D =0.21) with an intercept value of 0.0010±0.0009.

23

An 85±5 Ma CHIME age for the Agigawa welded tuff sheet

Table 1.

Electron microprobe analyses of ThO 2 , UO 2 and PbO of zircons (Z) and allanites(A) in Sample 97121401 from the Agigawa welded tuff sheet of the Nohi Rhyolitein the Kmado area, together with apparent ages, UO 2 * (the measured UO 2 plus
UO 2 equivalent of the measured ThO 2 ) and ThO 2 * (the measured ThO 2 plus ThO 2
equivalent of the measured UO 2 )

Spot
No.

ThO2 UO2 PbO
Age UO2*(wt.%) (wt.%) (wt.%) (Ma) (wt.%)

Spot
ThO2 UO2 PbO
Age ThO2*
No. (wt.%) (wt.%) (wt.%) (Ma) (wt.%)

Z01-01
Z01-02
Z01-03Z01-04
Z01-05
Z01-06
Z01-07
Z01-08Z01-09Z01-10Z01-11
Z01-12
Z01-13
Z01-14
Z01-15
Z01-16
Z01-17
Z01-18Z01-19Z02-01
Z02-02
Z02-03
Z02-04
Z03-01
Z03-02
Z03-03
Z03-04
Z03-050.205 1.313 0.0161 87 1.38
0.224 0.630 0.0095 101 0.700
0.073 0.319 0.0043 94 0.341
0.185 0.271 0.0042 95 0.329
0.185 0.553 0.0072 88 0.611
0.140 0.327 0.0046 92 0.371
0.135 0.334 0.0041 81 0.376
0.595 0.767 0.0110 86 0.953
0.129 0.338 0.0046 91 0.378
0.372 0.630 0.0091 91 0.747
0.188 0.392 0.0050 83 0.451
0.537 0.771 0.0108 86 0.939
0.220 0.374 0.0051 86 0.443
0.464 0.583 0.0079 81 0.728
0.832 0.905 0.0134 86 1.17
0.110 0.305 0.0041 90 0.339
0.112 0.371 0.0049 90 0.406
0.128 0.365 0.0044 81 0.405
0.177 0.361 0.0048 86 0.416
0.183 0.445 0.0063 93 0.502
0.145 0.463 0.0062 91 0.509
0.125 0.451 0.0062 94 0.490
0.109 0.393 0.0049 85 0.427
0.133 0.314 0.0047 98 0.355
0.668 0.479 0.0075 81 0.687
0.467 0.410 0.0065 87 0.556
0.485 0.417 0.0058 76 0.568
0.898 0.564 0.0097 86 0.844
ThO2 UO2 PbO
Age ThO2*(wt.%) (wt.%) (wt.%) (Ma) (wt.%)

A02-14
A03-01
A03-02
A03-03
A03-04
A03-05
A03-06
A03-07
A03-08
A04-01
A04-02
A04-03
A04-04
A04-05
A04-06
A04-07
A04-08
A04-09
A04-10
A05-01
A05-02
A05-03
A05-04
A05-05
A05-06
A05-07
A05-08
A05-09
A05-10
A05-11
A05-12
A05-13
A05-14
A05-15
A05-16
A05-17
A05-18A05-19
A06-01
A06-02
A06-03
A06-04
A06-05
A06-06
A06-07
A07-01
A07-02
A07-03
A07-04
A07-05
A07-06
A07-07
A07-08
A07-09
A07-10Spot
No.
A01-01
A01-02
A01-03
A01-04
A01-05
A01-06
A01-07
A01-08
A01-09
A01-10
A01-11
A01-12
A02-01
A02-02
A02-03
A02-04
A02-05
A02-06
A02-07
A02-08
A02-09
A02-10
A02-11
A02-12
A02-13

3.31
1.76
1.53
1.81
2.81
2.51
2.20
2.43
2.26
1.84
2.23
2.14
3.03
2.60
2.98
2.97
2.80
2.67
2.64
2.66
2.61
2.62
3.63
2.91
2.85

0.049 0.0133 91
0.020 0.0060 78
0.026 0.0064 94
0.026 0.0094 117
0.045 0.0121 97
0.042 0.0109 98
0.034 0.0088 90
0.042 0.0113 104
0.038 0.0104 103
0.043 0.0083 99
0.031 0.0089 90
0.046 0.0087 90
0.085 0.0137 98
0.078 0.0106 88
0.081 0.0125 91
0.089 0.0132 96
0.077 0.0127 99
0.073 0.0117 95
0.076 0.0118 97
0.081 0.0123 100
0.072 0.0104 87
0.082 0.0122 100
0.092 0.0158 95
0.079 0.0118 88
0.094 0.0123 92

3.47
1.82
1.61
1.89
2.95
2.64
2.31
2.56
2.38
1.97
2.33
2.28
3.30
2.85
3.24
3.26
3.04
2.90
2.88
2.92
2.83
2.88
3.93
3.16
3.15

1.32
3.53
2.83
3.42
2.51
2.48
2.46
3.58
3.24
2.43
2.74
2.88
2.69
2.70
2.77
3.40
3.13
2.80
2.91
2.02
1.75
1.97
1.97
1.59
1.63
1.63
2.06
2.32
2.34
2.37
2.58
2.38
2.27
2.05
1.47
1.55
2.10
1.81
3.10
2.90
3.61
3.58
3.37
2.21
3.22
2.66
3.30
2.81
3.58
2.48
3.11
2.85
2.82
2.76
2.98

0.013 0.0060 104
0.067 0.0145 91
0.064 0.0128 100
0.086 0.0139 89
0.052 0.0102 90
0.061 0.0107 95
0.056 0.0106 95
0.084 0.0141 86
0.076 0.0134 91
0.068 0.0104 93
0.087 0.0105 82
0.080 0.0118 89
0.076 0.0121 98
0.076 0.0133 107
0.081 0.0119 93
0.100 0.0141 90
0.098 0.0142 97
0.091 0.0116 89
0.082 0.0142 106
0.036 0.0091 101
0.035 0.0089 113
0.035 0.0081 92
0.044 0.0106 119
0.019 0.0069 99
0.019 0.0077 108
0.018 0.0050 70
0.042 0.0084 91
0.060 0.0099 93
0.062 0.0101 94
0.067 0.0100 91
0.069 0.0128 108
0.050 0.0091 85
0.054 0.0098 95
0.037 0.0108 118
0.017 0.0066 102
0.024 0.0055 80
0.048 0.0089 93
0.029 0.0080 100
0.078 0.0118 83
0.074 0.0110 83
0.090 0.0158 96
0.122 0.0152 91
0.091 0.0143 92
0.053 0.0109 108
0.081 0.0133 90
0.034 0.0104 89
0.062 0.0139 94
0.053 0.0110 87
0.057 0.0149 94
0.042 0.0126 114
0.051 0.0132 95
0.040 0.0109 87
0.037 0.0107 86
0.043 0.0099 81
0.044 0.0118 89

1.36
3.75
3.03
3.70
2.68
2.67
2.64
3.86
3.48
2.65
3.02
3.13
2.93
2.95
3.03
3.72
3.45
3.10
3.17
2.14
1.86
2.08
2.11
1.65
1.69
1.68
2.19
2.52
2.54
2.58
2.80
2.54
2.44
2.17
1.53
1.63
2.25
1.90
3.35
3.14
3.90
3.97
3.67
2.38
3.48
2.77
3.50
2.98
3.76
2.62
3.27
2.98
2.94
2.90
3.12

24

Fig. 4.

K. Suzuki et al.

Plots of PbO vs. UO 2 * of 28 analytical data on 3 zircon grains (A) and PbO vs.
ThO 2 * of 80 analytical data on 7 allanite grains (B) in Sample 97121401. Error
bars in the figures represent 2σ analytical uncertainty, and error given to the age
is of 2σ.

DISCUSSION AND CONCLUDING REMARKSThe CHIME zircon and allanite ages for the the Agigawa welded tuff sheet in
the oldest volcanic sequence of the Nohi Rhyolite are 85±5 and 86±7 Ma,r e s p e c t i v e l y . The ages coincide well with each other within the limit of analyticaluncertainty, and are likely to show the eruption time of the Agigawa welded tuff.
As mentioned earlier, the Agigawa welded tuff sheet is intruded by the Inagawa
Granodiorite (Sakai et al., 1965; Yamada, 1966). S i n c e t h e C H I M E m o n a z i t e
a ge s of t he Ina ga w a G ra nodi ori t e a re 81.9± 1.4 a nd 82.6± 1.8 M a ( S uz uki a nd
A da c hi , 1998), the ca. 85 Ma CHIME ages for the Agigawa welded tuff sheet are
in agreement with the geologic relation.
The present CHIME zircon and allanite ages are placed in the context of theAn 85±5 Ma CHIME age for the Agigawa welded tuff sheet

Fig. 5.

25

Comparison of CHIME zircon and allanite ages for the Agigawa welded tuff sheetof the oldest volcanic sequence of the Nohi Rhyolite with CHIME monazite ages for
granitoids and gneisses in the Ryoke metamorphic belt.

intrusive relation of granitoids in the Ryoke metamorphic belt (Fig. 5), together
with the previously reported CHIME ages for gneisses and granitoids (Suzuki etal., 1994a,b, 1995; Morishita and Suzuki, 1995; Nakai and Suzuki, 1996; Suzuki
and Adachi, 1998). The CHIME zircon and allanite ages of the Agigawa weldedtuff sheet are distinctly younger than the monazite ages for the Ryoke gneiss(98.3±11.1 - 101.9±5.8 Ma, Suzuki et al., 1994a,b), but are nearly identical

26

K. Suzuki et al.

with the CHIME monazite ages for the Shinshiro Tonalite (85.2±3.3 - 86.0±4.7
Ma, Morishita and Suzuki, 1995) and the Mitsuhashi Granodiorite (83.8±1.3 84.1±3.1 Ma, Suzuki et al., 1994a,b). These granitoids, occurring in higher
grade parts of the Ryoke metamorphic belt, have been regarded as the Older
Ryoke Granites (Ryoke Research Group, 1972). The Shinshiro tonalite, however,is known to have intruded discordantly into the Ryoke gneiss, and overprinted
the andalusite-orthoclase assemblage on the regional assemblage of sillimaniteorthoclase (Asami and Hishino, 1980: Miyake et al., 1992). The Shinshiro Tonalite
and the subsequent Mitsuhashi Granodiorite are of shallow emplacement than
the Tenryukyo Granodiorite with a contact assemblage of sillimanite-orthoclase,and appear to have been controlled by tectonics different from those for the
Tenryukyo Granodiorite and the syntectonic Kamihara Tonalite. Although weare uncertain whether the Agigawa welded tuff sheet has a petrogenetic relation
with the Shinshiro Tonalite and the Mitsuhashi Granodiorite or not, we can statethat post-tectonic acidic magmatism started simultaneously at about 85 Ma in
both non-metamorphosed and metamorphosed parts of the Mino terrane.

ACKNOWLEDGEMENTS
We would like to express our thanks to Dr. N. Yamada for a particularly
careful review, and to Mr. S. Yogo for his technical assistance. This work was
supported in part by Grant-in-Aid for fundamental Scientific Research (Nos.
09304048 and 09640568) from the Ministry of Education, Science and Culture,
Japan.

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Geol. Soc. Japan, No.38, 111-120.
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Ryoke metamorphic belt, central Japan. Jour. Geol. Soc. Japan, 86 , 581-591.
Bence, A.E. and Albee, A.L. (1968) Empirical correction factors for the electron microanalysis
of silicates and oxides. J. Geol., 76, 382-403.
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metamorphic rocks from the Nukata area, Aichi Prefecture. Jour. Min. Pet. Econ.
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Morishita, T. and Suzuki, K. (1995) CHIME ages of monazite from the Shinshiro Tonalite of
the Ryoke belt in the Mikawa area, Aichi Prefecture. J. Earth Planet. Sci., Nagoya
Univ., 42 , 45-53
Nakai, Y. and Suzuki, K. (1996) CHIME monazite ages of the Kamihara Tonalite and theTenryukyo Granodiorite in the eastern Ryoke belt of central Japan. Jour. Geol. Soc.
Japan, 102 , 431-439.

An 85±5 Ma CHIME age for the Agigawa welded tuff sheet

27

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