J . E ar th P l an et . S c i. N ag o ya U ni v. , Vo l. 4 2 , 1 7 t o 3 0, 1 9 95
C H IM E m o na zi te a g e s o f t h e O t a g i r i a n d I c h i d a G r a n i te s i n
t h e Komagane area, Nagano Prefecture
K a z uh i ro S U Z UK I , To s h i a k i N A S U a nd K en S H I B ATA
Departmen t o f Ea rth a n d Planetary Scien c es, Nagoya Unive r sit y,
Nagoya 464-01, Japan
( R ec e iv e d Septemb e r 1 8 , 1995 / A c cepted D e ce mb e r 1 5 , 1995)
ABSTRACT
Th e Otagi r i Granite c o mprises peralumino u s fine-grained two-mic a g r anites
t o granodiorites, a n d i n trudes into th e Ichid a Granit e w i th i n t h e easter n part o f
t he Ry ok e m e ta m orp hi c b e l t . M on a z i t e s fr om t hr e e s a m p l e s o f t he O t a g i ri
Granite a n d a s am p l e o f t h e Ichida G r anit e we r e dat e d b y the Chemic a l Th - U total P b I so chron M e th o d (CHIME). T h e a ge s a r e 7 7 .4±3.1 M a , 7 6 .3±2.8 Ma an d
77.2+3 . 5 M a f o r t h e Otagiri Granite an d 7 9 .4±3.1 M a f o r t h e I chida Gr anite.
T he CHIME monazite a g e s , interpr e t e d a s t he t i me o f t h e e mplacement, a g r e e
w it h t he i nt ru s i v e re l a t ion . T h e pr e s e n t d a t i n g s h o ws t ha t t he 1 0 0- 90 M a Rb S r w h ol e - ro c k i s o c h ro n a g e s ( Yu ha r a , 19 9 4) f or t he O t a g i r i Gr a n i t e i s fo r t ui t ou s . Th e CHIME monazi t e a g es support t h e previo u s correla t io n o f t he O t agiri
G r a n i t e wi t h t he Ka do s hi m a a n d B us et s u G r a n i t es of c a . 7 7 M a a g e ( K a g a m i ,
1972; Shibata and Ishihara, 1979 ; Nakai e t al., 1985 ; Suzuki e t al., 1994b).
INTRODUCTION
Pe r a l u mi n o us f i n e - g rai n e d t wo - m i ca g ran i t e s t o granodiorites o c cu r i n d i s c o nt inu ou s pl ut on s ov e r a n ar e a of a hu nd r ed ki l om e t e r l on g, f r om K om a ga ne
t o M i k aw a w i t h in t h e e as t e r n Ry o k e m et a mo rp h i c b el t ( e .g . Ry o k e R es e ar c h
G r ou p, 1 97 2) . T h ey a r e na m e d a s t he O t a g i r i G r a ni te i n t he K o m a ga n e a r ea ,
t he Ka d os hi ma G r a ni t e i n t h e K a do s h i m a a r e a a n d t h e B u s e t su G r a ni te i n t he
M i ka w a a r ea . R o c ks o f i nd i v i du a l pl ut on s , de spi t e t he w i d e ra n ge i n s p a t i a l
d i s t r i b u t i o n , ar e p e t r o g rap h i ca l l y q u i t e s i m i la r ; t h e f i n e- g r a i n ed t e x tu r e a n d
t h e p re s e n ce o f m u s c o v i t e , g ar n e t , m o n az i te , x en o ti m e , s il l i ma n it e a n d / o r
c o r d i e r i t e a r e t h e d i a g no s t ic f e a t ur e s of th e O t a g i r i, K a d o sh i ma a n d B u se ts u
G r a n i te s f r o m ot h e r g r a n it oi d s in t he e a s te r n Ryo k e b e l t. A l l p l ut o ns o f t he
O t a g ir i , K ad o s h im a a n d B u s e t su G r a n i te s i n tr u d e i nt o t h e I n a g a w a G r a n o d io r i te w h ic h is a c on ti n uo u s p lu t on e x te n d e d f r o m K o ma g a n e to M i ka w a a n d
i s r eg a rd e d a s t h e r ep r es e n t a t iv e o f t h e Yo u n g e r Ry ok e G r a n i t e ( i. e .p os t t e ct o n i c g r an it e s , Ry o k e R e s e ar c h G r o u p , 1 9 7 2 ) . T h e i n tr u s i v e r el a t i o n s
c o u pl ed w it h t h e p etr o gr a p h i c s i mi l a rit y l e ad ma n y sc h o la r s t o a c o n clu s i o n
t ha t th e O t a gi r i , Ka d o sh i m a a n d B u s e t s u G ra n i t e s fo rm e d s i m u lt an eo u s l y. T h e
K a d o sh i ma Gr ani te h a s be e n d a te d b y t he R b - S r w ho l e - r o c k is o c h r on m e th o d
t o b e 7 6 . 1 M a (K ag am i 1 9 7 3; o ri g i n a l l y 7 3 . 3 M a , 6 d at a w er e r e c al c u lat e d u si n g
t h e c u rr e n t l y a c c ep te d d ec a y c o n s t a n t o f 1 . 4 2 x 10 -11/ y ) . T h e B u s e t s u G ra n i t e
17
18
K . Suzuki e t al.
s h o w s R b - S r w h o l e- r o c k i s o c h r o n ag e s o f 8 2 . 5 ± 3 . 9 ( 1σ ) Ma ( Sh i b a t a an d I s h i h a r a , 1 9 7 9 ) an d 7 5 . 4 ± 10 .4 M a ( N a k ai e a l . , 1 9 8 5 ; o ri g i n al l y 7 1 . 4 ± 1 . 4 M a, i t w as
r e c a lc ul a t e d b y d i s c a r d i n g d a t a f o r m e t a b a si t e a nd h or n b l e n de g a b b r o) a n d
C H I M E m o n a zi t e a g es o f 7 5 . 0 ± 5 . 1 - 7 8 .9 ±5 . 3 M a ( Su z u k i et al . , 1 9 9 4 b ).
T h e O t a gi r i G r a ni t e w as r e c e nt l y d at e d b y Yu ha r a ( 19 9 4) b y t he R b - S r
w h o l e - ro ck i s o ch ro n m e th o d . T h e a g es ar e 7 1 .6 ±6 .8 , 9 9.0 ±1 0 .9 an d 9 2 . 7 ± 1 3 . 8
M a . T he se d at e s , at l e a st 9 9. 0 M a , a r e s u r pr i si n gl y o l d e r t h an t ho s e f o r t h e
K a do s h im a an d Bu s e t s u G r a n i t e . I f t h is i s t r u e , t he p r ev i o us c o r r el a t i on o f
t h e O t a g i r i G r a ni te w i th t h e K a d os h i ma a n d B u s e t su G ra n i t e s i s i n va li d. H ow e ve r, t h e R b - S r a g e s o f t h e Ot a g i r i Gr a n i t e a p p e a r t o b e i nco n c u l u si v e , s in c e t he
R b -S r i s oto p i c d a t a e x h i b i t a s i g nif i ca n t l y l a rg e s ca t t er. To ch e c k t h e e x i s t en ce o f t h e c a. 9 0- 1 0 0 M a p l u t o n s b y a n a l t er n at i v e m e t h o d , w e h a v e d e t er m i n en ed CH I ME ( c h emi c a l T h- U - t ot a l Pb isochron m e t h o d) m o naz i tes a g e s u si n g
a n e l ec t ro n m i cr o p ro be f o r s a mp l es o f th e O t ag ir i G r a n i t e. A l s o d at e d i s t h e
I c hi d a G r a ni te w i c h i s i nt r ud ed b y t he 9 2 . 7± 13 . 8 M a p l ut o n o f t h e O t a gi r i
G r an i t e . T h e m o n a zi t e a g e c a n b e r e g ar d e d a s r e p re s e n t i n g t e t i me f o r cr y s t a l l iz a t io n o f g r a ni ti c i n tr u si v e s, s i n c e t he b lo c ki n g t e m pe r at u r e f o r P b i n
m o n a zi t e ( ca . 7 0 0 o C ; P ar ri s h , 1 9 9 0 ; S m i t h an d B ar re i ro , 1 99 0 ; S u z u k i e t a l . ,
1 9 9 4 a ) i s c l o s e t o t h e t e m p er at u r e f o r c ry s t a l l i za t i o n o f g ra n i t i c m a g m as ( e. g .
Merril e t a l. , 19 70; R ob e rts o n a nd Wyll i e , 1 971 ) .
GEOLOGY AND EXPERIMENTS
T h e K o ma g a n e a r e a, s i t u at e d i n t h e n o r t h e as t e r n p ar t o f t h e Ry o k e b el t , i s
un de r l a in m a i nl y by t h e Ry ok e m e ta m or p hi c r oc k s , a s e r i e s of gr a ni t o i ds a n d
Q u at e r n a r y c o v er s ( Fi g . 1 , M u ray a ma an d K a t ad a , 1 9 5 7 ). T h e Ry o k e m et a m o rp h i c r o c k s c o n s i s t s m a i n l y o f p e l i t ic a n d p s am m i t i c s ch i s t s an d g n ei s s e s . T h e
m e t a mo r p h i c g r a d e i n c r e a s e s f r o m t h e b i o t it e z o n e i n t h e n or th w e st o f t h e
m a p p e d a r ea t o t h e s i ll i ma n i te - c o r d ie ri t e - o r t ho c l a s e z o n e i n t h e s o ut h e a s t
( O n o , 1 9 69 ).
F i v e g r a n i t i c i n t ru si v es ar e d i s t i n g ui s h ed i n th e ar ea , t h e I n ag aw a G ra n o d i o r i t e ( c o ar s e- g r a i n ed p o r p h y ri t i c g ranodiorite t o a d am e l li t e ) , t h e I c h i d a G rani t e ( me d i u m- g r ain e d b i o ti t e g r a n o d i or it e t o a da me l l it e ) , t h e O t a g i r i G r a n i te
( m ai n l y t w o - m i c a a d a m e l li t e ) , t he K i s o ko m a G r an o di o r i t e ( m ed i u m- g r a i ne d
h o r n bl e n de - b io t i te g r a no d i or i t e ) a nd t h e u n n a m e d b i o ti t e g r a n o di o r i t e a t
Mi ya ta . T h e i n tr u si v e s e q u e nc e o f t h e se g r a n it o id s ( F i g . 2 ) w a s sy n th e s iz e d
o n t h e b a s i s o f f i e ld e v i d e nc e a n d p e t r o gr a p hi c c or r e la t i o ns b y Ry o ke R e s e a rc h G ro u p ( 1 9 7 2 ) . T h e r el a t i o n b e t w e en t h e O t a g i ri G r a n i t e a n d I c h i d a
G r a ni te i s n ot s h ow n in t h e o r ig i na l c on t e x t gi v e n b y Ry o k e R e se ar c h G r o up ,
b ut Yu h ar a ( 1 9 9 4 ) f o u n d t h a t t h e f o r m e r i nt r u d es i n t o t h e l a t te r a t N a k a t ag i r i R i v er. A lt ho u gh t he g e o lo g ic r e l a t io n b e t w e e n th e O ta g i r i G r a n it e a n d
Ki s o ko m a G ra n od i o r i t e i s n e ve r d e t e c t a bl e , t h e K i s o ko m a G r a no di o r i t e i s c o ns i d e r e d to in t r ud e a f t e r th e e mp l a c e me nt of t he O t a g ir i G r ani te f r o m t he m o de
of oc cur r enc e ( Ts u c h iy a , 1 9 6 6) .
C H I ME m on a zi te a g es o f t he O t ag i ri a n d I ch i da G r an i te s
Fig. 1.
Geolpgi ca l map o f the Komagane a rea s howing sample localitie s
of granitoids (simplified fr o m Murayama a n d K a t ada, 1957).
Naegi G ran it e (Agemats u Granite )
Kisokoma Granodiorite
Takato Granite
Busetsu Granite (Kadoshima Granite)
O ta giri Granite
Seinaiji Granite
Ichida Granite
Inagawa Granodiorite
Mitsuhashi Granodiorite
Shinshiro Granodiorite
Nohi Rhyolite
Kiyosaki Granodiorite
Tenryukyo Granodiorite
Kamihara Tonali te (Hij i Tonalite)
Ryoke metamorphic rocks
Fig. 2.
I n t r u si v e r el a t io n s o f g r a n i to i d s i n t h e e as t er n Ry o k e me t am o r p h i c
b e l t ( s implified a n d modified f r o m Ryo k e Research Gr o up , 1 97 2 ) .
19
20
Ta b le 1 .
G r a in
No.
Sample
M01-01
M01-02
M01-03
M01-04
M02
M03-01
M03-02
M03-03
M03-04
M04-01
M04-02
M04-03
M05-01
M05-02
M05-03
M06-01
M06-02
M06-03
M07-01
M07-02
M08
M09-01
M09-02
M10-01
M10-02
M11-01
M11-02
M11-03
M12-01
M12-02
M13-01
M13-02
M14-01
M14-02
M14-03
M14-04
M14-05
M15-01
M15-02
M15-03
K . S u z uk i et a l.
M i croprob e a n al yses o f T hO 2 , UO 2 a nd P b O o f mo n az it es from t h e O t ag ir i Gra n i t e (sampl e s 2 9 03 , 2 9 05 and 2 90 9 ) a n d t h e Ichid a Granit e ( s ampl e 2 9 07 ) in t he
Komagane area. ThO 2 * : s u m o f th e m easured ThO 2 a n d ThO 2 equivalent o f th e
m e a su red U O2 .
ThO2
wt.%
UO2
wt.%
PbO
wt.%
2903: O ta gi r i G r an i te
10.9
0.287
0.038
6.86
0.163
0.024
10.7
0.261
0.038
10.8
0.338
0.040
6.34 0.168 0.023
6.82
0.227
0.024
7.47
0.225
0.026
6.42
0.211 0.023
7.98
0.221
0.029
8.61
0.305
0.032
9.22
0.171
0.031
9.06
0.247
0.034
5.51
0.285
0.021
5.29
0.267
0.020
6.10
0.396
0.026
9.16
0.237
0.032
7.81
0.173
0.026
8.13
0.331
0.033
4.62
0.147
0.018
7.64
0.203
0.028
7.90 0.208 0.028
7.04
0.191
0.026
7.60
0.169
0.026
7.07
0.305
0.027
8.52
0.325
0.030
5.99 0.129 0.022
9.38 0.301 0.034
9.65 0.360 0.036
5.68
0.225
0.021
4.34
0.163
0.017
9.25
0.228
0.033
6.93
0.229
0.026
8.31
0.149
0.029
9.20
0.165
0.033
8.42
0.082
0.030
8.08
0.099
0.027
8.16
0.137
0.029
4.68
0.207
0.018
4.70
0.137
0.017
3.35
0.044
0.012
Age
Ma
77
76
79
80
79
76
74
76
78
79
75
81
77
76
83
76
74
84
82
79
78
79
76
80
73
80
78
78
79
81
77
80
77
79
80
77
78
80
77
81
ThO2*
wt.%
11.8
7.38
11.5
11.9
6.88
7.55
8.19
7.09
8.69
9.59
9.77
9.85
6.42
6.15
7.37
9.92
8.36
9.19
5.09
8.29
8.56
7.65
8.14
8.05
9.56
6.40
10.3
10.8
6.40
4.86
9.98
7.66
8.79
9.73
8.68
8.40
8.60
5.34
5.14
3.49
Grain
No.
ThO2
wt.%
UO2
wt.%
PbO
wt.%
Age
Ma
M15-04
M15-05
M16-01
M16-02
M16-03
M16-04
M16-05
M17-01
M17-02
M17-03
M17-04
M17-05
M18-01
M18-02
M18-03
M18-04
M18-05
M19-01
M19-02
M19-03
M19-04
M19-05
M19-06
M19-07
M19-08
M19-09
M19-10
6.23
6.50
7.89
7.49
8.94
10.1
5.88
8.60
7.20
9.68
11.0
10.4
9.01
8.28
9.00
7.40
7.87
10.7
10.1
8.73
6.93
7.87
10.9
10.7
10.8
11.3
6.96
0.235
0.091
0.215
0.218
0.335
0.375
0.146
0.283
0.148
0.165
0.345
0.347
0.213
0.197
0.275
0.268
0.182
0.280
0.197
0.199
0.107
0.208
0.240
0.212
0.299
0.273
0.140
0.022
0.023
0.027
0.027
0.035
0.037
0.020
0.032
0.024
0.032
0.040
0.038
0.031
0.031
0.034
0.026
0.025
0.038
0.034
0.033
0.023
0.027
0.038
0.036
0.037
0.042
0.024
76
79
74
76
81
78
74
79
74
73
78
77
76
81
82
75
71
78
74
82
73
75
78
75
74
81
76
6.98
6.79
8.58
8.19
10.0
11.3
6.35
9.51
7.67
10.2
12.1
11.5
9.69
8.91
9.88
8.26
8.45
11.6
10.7
9.37
7.27
8.54
11.7
11.4
11.8
12.2
7.41
Sample
M01-01
M01-02
M01-03
M01-04
M01-05
M02-01
M02-02
M02-03
M02-04
M02-05
M02-06
M03-01
2905: O ta gi r i G r an i te
8.09 0.641
0.034
6.90 0.325
0.024
9.13 0.213
0.032
7.69 0.361
0.031
7.71 0.463
0.027
10.6
0.223
0.037
10.3
0.209
0.036
10.1
0.150
0.034
9.71 0.138
0.031
9.97 0.166
0.032
9.14 0.120
0.030
4.79 0.050
0.016
79
72
78
82
70
77
77
76
73
72
75
77
10.1
7.94
9.81
8.85
9.19
11.3
11.0
10.6
10.2
10.5
9.53
4.95
ThO2*
wt.%
C H I M E m o na zi t e a ge s o f t h e O t a gi r i a nd I c hi da G r an it e s
Ta b l e 1.
21
(c o nt i n ue d) .
G r ai n
No .
ThO2
wt.%
UO2
wt.%
PbO
wt.%
Age
Ma
ThO2*
wt.%
Grain
No.
ThO2
wt.%
M03-02
M03-03
M04-01
M04-02
M04-03
M04-04
M04-05
M05-01
M05-02
M05-03
M05-04
M06-01
M06-02
M06-03
M06-04
M06-05
M07-01
M07-02
M07-03
M07-04
M07-05
M07-06
M08-01
M08-02
M08-03
M08-04
M08-05
M08-06
M08-07
M08-08
M08-09
M09-01
M09-02
M09-03
M09-04
M09-05
M10-01
M10-02
M10-03
M10-04
M10-05
5.74
5.70
4.92
5.40
5.59
5.57
5.05
6.62
6.01
4.58
6.28
6.64
5.38
6.31
6.58
10.3
5.26
5.19
4.56
4.80
4.36
4.94
9.21
9.07
9.90
10.1
8.53
6.58
8.63
9.90
9.75
3.59
4.91
5.18
5.04
5.27
7.98
9.18
8.98
8.75
8.74
0.096
0.075
0.092
0.106
0.047
0.045
0.029
0.696
0.290
0.371
0.357
0.135
0.084
0.122
0.117
0.282
0.087
0.096
0.098
0.099
0.062
0.091
0.095
0.129
0.087
0.126
0.091
0.121
0.091
0.111
0.112
0.060
0.078
0.094
0.072
0.074
1.75
2.08
2.15
2.04
2.37
0.020
0.019
0.017
0.019
0.019
0.020
0.015
0.030
0.023
0.020
0.024
0.023
0.019
0.021
0.023
0.035
0.019
0.018
0.016
0.016
0.015
0.017
0.033
0.030
0.034
0.033
0.029
0.022
0.028
0.032
0.034
0.012
0.017
0.019
0.018
0.017
0.047
0.052
0.051
0.047
0.054
77
76
75
78
78
83
69
81
77
82
77
77
81
73
78
74
82
76
76
73
78
75
82
75
78
75
78
74
75
73
79
75
78
83
81
73
82
78
77
73
79
6.05
5.94
5.21
5.74
5.74
5.71
5.14
8.85
6.94
5.77
7.42
7.07
5.65
6.70
6.96
11.2
5.54
5.50
4.87
5.12
4.56
5.23
9.52
9.48
10.2
10.5
8.82
6.97
8.92
10.3
10.1
3.78
5.16
5.48
5.27
5.51
13.6
15.8
15.9
15.3
16.3
M11-01
M11-02
M11-03
M11-04
M11-05
5.63
5.62
5.71
5.73
5.35
UO2
wt.%
PbO
wt.%
0.113
0.091
0.107
0.154
0.097
0.019
0.018
0.020
0.021
0.018
Sample 2909: O t ag i r i G r a n i te
M01-01 3.44
0.918 0.020
M01-02 4.98
0.739 0.024
M01-03 3.88
1.03
0.024
M02-01 5.78
0.195 0.020
M02-02 5.90
0.249 0.019
M02-03 6.57
0.087 0.023
M03-01 4.29
0.057 0.013
M03-02 3.71
0.044 0.013
M04-01 4.65
0.077 0.018
M04-02 4.91
0.111 0.016
M04-03 5.00
0.075 0.016
M04-04 5.17
0.053 0.018
M05-01 6.99
0.121 0.024
M05-02 7.69
0.160 0.029
M05-03 7.70
0.163 0.026
M05-04 7.02
0.169 0.024
M05-05 7.26
0.186 0.025
M06-01 6.63
0.135 0.024
M06-02 6.39
0.124 0.023
M06-03 6.13
0.100 0.019
M06-04 6.03
0.069 0.022
M06-05 5.78
0.086 0.019
M07-01 7.74
0.118 0.028
M07-02 7.95
0.179 0.028
M07-03 8.91
0.165 0.031
M07-04 8.69
0.164 0.030
M07-05 7.43
0.149 0.026
M08-01 5.82
0.141 0.020
M08-02 7.21
0.172 0.026
M08-03 5.74
0.150 0.020
M08-04 6.29
0.142 0.021
M09-01 3.03
0.187 0.012
M10-01 7.87
0.220 0.027
M10-02 7.56
0.225 0.027
Age ThO2*
Ma wt.%
75
72
76
78
74
5.99
5.91
6.05
6.22
5.66
74
78
79
75
68
79
71
78
87
72
73
78
77
83
76
75
75
82
81
70
82
75
82
78
78
78
77
76
80
75
72
78
73
77
6.38
7.35
7.18
6.41
6.70
6.85
4.47
3.85
4.90
5.26
5.24
5.34
7.38
8.20
8.22
7.56
7.85
7.06
6.79
6.45
6.25
6.05
8.12
8.52
9.44
9.21
7.91
6.27
7.76
6.22
6.74
3.63
8.57
8.28
22
Ta b l e 1 .
K . Suzuki e t al.
( c o n t in u e d ) .
Grain
No.
ThO2
wt.%
UO2
wt.%
PbO
wt.%
M10-03
M10-04
M10-05
M10-06
M11-01
M11-02
M12-01
M12-02
M12-03
M13-01
M13-02
M13-03
M13-04
M13-05
M13-06
M13-07
M13-08
M13-09
M14-01
M14-02
M14-03
M15-01
M15-02
M15-03
M15-04
M15-05
M15-06
7.73
7.56
7.81
7.69
7.32
6.73
6.64
6.64
5.03
7.56
7.76
7.61
6.42
7.38
7.77
7.74
7.33
7.34
4.22
2.46
3.10
10.2
10.2
9.95
9.34
10.4
10.2
0.281
0.238
0.217
0.234
0.340
0.371
0.115
0.122
0.048
0.379
0.478
0.430
0.407
0.321
0.425
0.491
0.384
0.430
0.214
0.214
0.245
0.215
0.118
0.137
0.164
0.191
0.181
0.027
0.025
0.030
0.027
0.026
0.026
0.023
0.022
0.017
0.029
0.031
0.030
0.025
0.027
0.029
0.031
0.030
0.029
0.015
0.010
0.013
0.034
0.034
0.032
0.033
0.034
0.035
Age ThO2*
Ma wt.%
74
71
84
75
74
78
76
75
75
77
78
79
75
75
74
78
83
79
71
75
81
75
77
73
78
73
77
8.63
8.32
8.50
8.44
8.41
7.92
7.01
7.03
5.18
8.77
9.29
8.99
7.72
8.41
9.13
9.31
8.56
8.72
4.91
3.14
3.88
10.9
10.6
10.4
9.86
11.0
10.8
G r ai n
No .
ThO2
wt.%
UO2
wt.%
PbO
wt.%
Sample 2907: I c h i d a Gr a ni te
M01-01 6.07 0.122 0.022
M01-02 5.48 0.130 0.020
M02
4.48 0.124 0.017
M03
5.96 0.126 0.022
M04
3.41 0.143 0.013
M05
5.47 0.143 0.021
M06-01 7.80 0.102 0.026
M06-02 7.89 0.137 0.028
M07-02 6.97 0.129 0.024
M07-02 7.54 0.127 0.027
M08-01 5.84 0.065 0.020
M08-02 6.15 0.093 0.020
M09
2.36 0.127 0.010
M10
3.74 0.097 0.013
M11
3.30 0.032
0.010
M11-02 5.08 0.108 0.018
M12-01 5.86 0.466 0.025
M12-02 6.46 0.194 0.023
M13
3.05 0.109 0.011
M14
3.66 0.043 0.013
M15-01 6.33 0.205 0.022
M15-02 7.91 0.205 0.029
M15-03 8.01 0.166 0.028
M15-04 7.54 0.169 0.027
M15-05 8.20 0.361 0.031
M16
5.34 0.066 0.020
M17
4.10 0.070 0.014
M18-01 5.12 0.062 0.017
M18-02 6.25 0.147 0.023
M19-01 4.98 0.089 0.018
M19-02 4.93 0.076 0.018
M20-01 3.26 0.050 0.011
M20-02 3.75 0.059 0.013
M21-01 4.35 0.072 0.015
M21-02 3.49 0.061 0.013
M22
3.97 0.072 0.013
M23-01 5.17 0.130 0.019
M23-02 4.44 0.095 0.016
M24
5.04 0.078 0.017
Age ThO2*
Ma wt.%
79
80
80
80
80
84
76
79
78
80
78
74
85
78
69
78
81
76
76
81
74
79
77
79
79
84
75
74
79
81
83
73
78
75
85
75
79
80
76
6.46
5.90
4.88
6.36
3.87
5.93
8.13
8.33
7.38
7.95
6.05
6.45
2.77
4.05
3.40
5.43
7.35
7.08
3.40
3.80
6.99
8.57
8.54
8.08
9.35
5.55
4.32
5.32
6.72
5.26
5.17
3.42
3.94
4.58
3.68
4.20
5.59
4.74
5.29
C H I M E m on az i te a g es o f t he O t a gi r i a n d I ch id a G r a ni t es
23
T he O t a g ir i G r a n i te f o r m s t hr e e p lu to n s a t I n u ta g i r i , O ta g i r i a n d N a k a t a g i r i R i v e r s ( F i g . 1 ) . We t e mpo ra r i ly c a l l t he se p l u t on s a s t he no r t he r n , ce n t r a l a n d s ou t h er n pl ut o ns , r e s p e c t i v e l y. Th e no rt he r n pl u t o n c o ns i s ts e nt i r e l y
o f f i n e -g ra i n e d t wo -m i c a a d a m el l i t e . T he ce n tra l b o d y co m p r i s e s f i n e -g ra i n e d
gray-colored b i o t i te granodiorite, f i n e - g rai n e d t w o - m i c a ad a m el l i t e an d f i n e- t o
m e d i u m - g r a i n e d l eu co cr a t i c t wo -m ica g ra n i te. M ura y a m a a n d Ka t a d a ( 1 9 57 )
f o un d t h a t f in e - g r a i ne d gr ay- col o r e d b io ti t e g r a n o di o r it e i s in t r ud e d b y b o th
t w o - m ic a v ar i e t i e s w i t h a s ha r p b o u n d a ry, a n d t h a t f i n e - t o m e d i u m- g r a i n e d
l e uc o c ra ti c t wo -m i c a g ra n i t e gr a de s a br up t l y o r i n t r ud e s wi th a s ha r p bo und a r y i n t o f i n e -g r a i n e d t w o- m ica a da mel l i te ; t he c e nt ra l p l u t on fo r m e d t h r ou gh
m u l t ip l e m a g m a t i c ev en t s . T he s o ut h e rn p l u t on c o ns i s t s o f f i n e -g ra i n e d t w o m i c a a d a m e l l i t e a n d f i n e -g ra i ned t o c o a rs e- g r a i n e d l eu co cr a t i c t w o - m i c a g r a n i t e . A g a i n t h i s p l u t on p o s si b l y f o r m e d t h r o u gh m u l t i pl e m a g m a t i c e v e n t s .
We c o l l e c t e d t h r ee s a m p l e s ( s am pl e N o s . 2 9 0 3, 2 9 0 5 a n d 2 9 0 9 ) o f t he
O t a gi ri Gr a n i t e a n d a s a mpl e ( s a mpl e N o. 2 90 7) o f t he I c hi da G r a ni te (F i g. 1 ) .
M o n a z i t e wa s a n a l y z e d o n a n e l e ct r o n m i cr o p r o b e ( J C XA -7 3 3 ) b y t h e m e t h o d
d e s c r i b e d b y S u z u ki e t al . ( 1 9 91 ) , S u z u ki a n d A d a c h i (1 9 9 1) a n d Ad ac hi a n d
S u zu k i ( 1 9 9 2 ) . T h e T h , U a n d Pb a n al y t i c al r e s u l t s ar e l i s t ed i n Ta b l e 1 ; t h e
d e te cti on l im it o f P bO a t 2σ c o n f id e n c e le v e l is 0 . 0 1 w t . % , a nd t h e r e l a t iv e
er r o r i n t h e P b O d e t er m in a t i o n i s a b o u t 1 5 -2 5 % f o r 0 . 0 2 w t . % o f t h e c o n ce n t r a t i o n . E rr o r i n CH I M E ag e i s g i v e n a t 2 σ l ev e l .
CHIME MONAZITE AGES
O ta g i r i G r a n i t e
S a mp l e 2 9 0 3 f ro m t h e n or t h e r n p l u t o n i s w e ak l y f o l i at e d f i n e - g rai n e d t w o m i ca ad a m el l i t e, an d c o n s i s t s m a i n l y o f q u a r t z, K -f e ld s par, p l a g i o cl a s e, b i o t it e
a n d m u s c o v i te . T h e p ar t ia l X R F a n al y s i s o f t h i s s am p l e i s l i s t ed in Tab l e 2 .
T h e n o r m a ti v e co m p o s i t i o n i s 3 1 .8 % q u a r t z , 2 8 . 6 % o r t h o c l as e , 3 1 . 8 % p l ag i o c l as e , 2 . 2 % c o ru n d u m a n d 4 .1 % h y p e rs t hen e . T h e m o l a r A l 2 O 3 / (CaO+Na 2 O
+ K2 O ) ( A /C N K ; h e r e a f t e r ) va lue i s 1 . 15 , a nd t he m o l a r K 2 O/(Na 2 O +K 2 O ) ( K /
N K ; h e r e a f t e r ) v a l u e i s 0 . 52 . P la gi o c l a s e i s z o ne d w i th a c a l c ic c o r e a nd a
s od i c r im ; t h e c a l c ic c o re i s h i gh l y s e r i c i ti z e d . A c c e sso r i e s i nc l u d e ga r n e t ,
a p at i t e, m o n a zi t e , x en o t i m e, z i r c o n a n d i l m en i t e . Ti t an i t e i s a b s en t , b u t s e co n d a r y g r a i n s d e v el o p i n c h l o r i t i ze d b i o t i t e . M o naz it e f o r m s e u h e d r a l t o s u b h e dr a l c ry st a l s a nd i s 0 . 12 t o 0. 2 2 l on g; e x c ep t ion a l l y i t a tt a i n s t o 0. 3 0 m m .
T h e le n gt h / w i dt h r a t io is in th e r a n g e b e t w e en 1 . 1 a n d 2 . 8 a n d c o n c en tr at e s
a r o u nd 2 . 1 . M on a z i te c o n ta i n 3.34-11.3 w t . % T hO 2 and 0 . 0 9- 0 . 3 8 wt . % U O 2 . A
t o ta l o f 6 7 a na ly s e s on 1 9 mo n az i te g r a i n s a re a rr a y e d li n e ar ly o n th e Pb O T h O2 * d i ag ram ( F i g 3 A ) , an d y i el d an i s o c h r o n o f 7 7 . 4 ± 3 . 1 M a ( M S WD = 0.1 8).
S am p l e 2 9 0 5 f ro m t he ce nt r al p l ut o n i s m as s i ve fi n e -g r ai n e d g ra y-c ol o r e d
b i o ti t e g r an od i o r i te, a n d c o n s is t s o f qu ar t z , plagioclase, K - fe l ds p a r a n d b i o t it e .
T h e A /C N K an d K / N K v a l u e s a r e 1 . 1 4 and 0 . 4 1 , respectively ( Ta b l e 2) . C o mp a r ed w i t h o t h e r s a m p l e s f r o m t h e O t a g i r i Gr a n i t e , t h is i s e n r i c h e d i n Z n an d
B a b u t p oo r i n Y a n d N b . P r im a r y m us c o vi t e i s r a r e , b ut s e c o n da r y o n e
24
K . Suzuki e t al.
Ta b le 2 .
P ar t i al XR F a n al y s es o f g r a n i to i d s f r o m th e Ot ag i r i an d I c h id a
G r a n i t e s i n t h e e a s te r n Ry ok e B e l t.
S a mp l e
No .
S i O2 ( % )
Ti O 2
A l 2O 3
F e O*
M nO
M gO
CaO
N a 2O
K 2O
P 2O 5
To t a l
A / CN K
K / NK
C r ( pp m )
Co
Ni
Zn
Rb
Sr
Y
Zr
Nb
Ba
O t ag i ri G r an i te
No r t he r n Centra l
S ou t h er n
p lu t o n
p lu t o n
p l ut o n
29 0 3
29 0 5
2 90 9
72.6
71 . 1
0.23 0
0.362
14.7
15 . 2
1.74
2.36
0.04 4
0.055
0.50
0.67
1.47
2.14
2.99
3.33
4.84
3.48
0.11 8
0.177
99.232
98.874
1.15
1.14
0.52
0.41
5.8
14.6
2.5
0.6
5.1
4.6
4 1.2
70.3
157
88.6
174
362
1 5.7
11.5
132
150
1 5.5
11.6
625
1080
71 . 8
0.259
15 . 1
1.91
0.046
0.54
1.65
2.93
5.01
0.126
99.371
1.14
0.53
9.0
1.4
3.6
4 8. 6
2 00
1 91
1 6. 8
1 42
1 6. 5
7 23
I c h id a
Granite
2 90 7
72.7
0.216
14.3
1.74
0.040
0.52
1.33
2.43
5.53
0 . 11 2
98.918
1.15
0.60
5.6
1.1
5.2
38.4
128
252
9.1
115
11.4
1390
F e O * : t o t a l F e a s F e O , A / C N K : m o l a r A l 2 O 3 / ( Ca O + Na 2 O +K 2 O )
r a ti o a n d K / NK : m o l ar K 2 O / ( N a2 O + K 2 O ) r at i o
d ev e l o p s i n t h e al t er ed co re o f p l ag i o c l as e . A cc es s or i es a r e ap at i t e, m o n az i t e,
zi rco n , x e n o t i m e a n d i l m e n i te . G ar n et ca n n o t b e s ee n i n t h i s s am p l e. M o s t
m o n a z i t e g r a i n s o c cu r i n e u hed r a l f o r m o f 0 . 11 t o 0 . 1 7 m m i n l e n g th . T h e
l en g t h / w i d t h r a ti o r a r el y at t a in s 1 .8 . M o n az i t e g rai n s f r om th is s am p l e, u nli k e
t h o s e f r om s am p l e 2 9 0 3 , s h o w h i g h v ar ia t i o n i n th e U O 2 c o n t en t ; t h e U O2 co n t en t r an g e s f ro m 0 . 0 6 t o 2 . 3 6 % . T h e T h O 2 c o n t en t i s i n th e r an g e b e tw ee n 3 . 5 9
an d 1 0 . 6 % . F ig u r e 3 B s ho w s P b O v s . T HO 2 * p l o t s o f 5 8 a n al y s e s o n 11 g rai n s .
A ll d a t a p oi n t s a r e r e g r es s e d w it h a s in g l e i s o c h r o n o f 7 6 . 3 ± 2 .8 M a
( M S WD = 0 . 2 0 ).
Sa m p l e 2 9 0 9 f r o m t h e s o u t h er n p l u t o n i s w ea k l y f oli a t ed f i n e- g r a i n ed t w o m i c a a d a m e l l i te, c o n si s t i ng m a i n l y o f q u a r t z , p l a g i o c l a s e , K - f el d s par, b i ot i t e
25
C H I ME m on a zi te a ge s o f t h e O t ag i r i a n d I ch i da G r an i te s
PbO (wt.%)
(A)
0.05
S a m pl e 2 9 0 3
0 .04
6 7 a nalyses o n
1 9 grains
0.0 3
77.4±3.1 M a
0.0 2
(MSWD=0.18 )
An a l yt i c a l
uncertaint y
0.0 1
0
(B)
0.06
S am p l e 2 9 0 5
0.05
5 8 a nalyses o n
11 grains
0.04
0.03
76 .3 ± 2. 8 M a
0.02
(MSWD=0.20 )
An a l yt ic a l
unc ert a i n t y
0.01
0
(C)
0 .05
S a m pl e 2 9 09
0 .0 4
61 analyses on
1 5 gra in s
0.0 3
7 7. 1 ± 3 . 5 M a
0.0 2
(MSWD=0.23 )
An a l yt ic a l
uncertainty
0.0 1
0
Fig. 3.
0
2
4
6
8
10
T hO 2 * (wt.%)
12
14
16
P l o t s o f P b O v s . Th O 2 * o f m o n a z i t e s f r om t h e n o rt h e r n ( A ) , c e n t r a l ( B ) an d
so u t h e r n p l u t o n s o f t h e O t ag i r i G r a n i t e. Er r o r b ar s i n t h e f i g u r e r ep r e s en t 2σ
a n a l y t ic a l un c e r t ai nt y, a n d e rr o r g i v e n t o t h e a ge i s o f 2σ .
26
K . Suzuki e t al.
a n d m u s co v i t e . T h e an al y s i s , n ea r l y i d e n t i ca l w i t h t h a t o f s am p l e 2 9 0 3 , s u g g e st s s a m pl e 2 90 9 t o be pe r a l u m i n ou s wi t h t h e A / C N K v a l u e of 1. 1 4 a nd t h e
K / N K v a l u e o f 0 . 5 3 ( Tab le 2 ) . A cc es s o ri es ar e g ar n et , ap a t it e , m o n a zi t e, zi r c o n ,
x en ot i m e a nd i l me ni te . M o na z i t e f or m s e uh e dr a l t o s u bh e dr a l pr i sm s of 0 . 14
t o 0 . 25 m m i n l en g t h . T h e l e n g t h /w i d t h r ati o r a n g e s f r o m 1 .2 t o 1 .9 . T h e ThO 2
c o n t en t r an g e s f ro m 2 . 4 5 t o 1 0 . 4 % a n d t h e U O 2 c o n t en t f r o m 0 . 0 5 t o 1 . 0 3 % .
A t o t al o f 6 1 a n al y s e s o n 1 5 g r a in s ( F i g . 3 C ) y i el d s an i s o ch ro n o f 7 7 .2 ±3 .5 M a
( M S WD = 0 .2 3 ) .
Ichida Granite
S am p l e 2 9 0 7 f ro m t h e I ch i d a G r a n i t e i s m ed iu m- g r a i n ed b i o t i t e ad a m el l i t e,
a n d c o n s i s t s m a i n l y o f q u a r t z , p l ag i o c l as e , K -f el d s par an d b i o t i t e. T h i s s a mp le
s h o ws t h e A / C N K v al u e o f 1 . 1 5 a n d t h e K / NK v a l u e o f 0 .6 0 ( Ta b l e 1 ) . A cc e s s o ri es i n c l u d e a p at i t e , z i r c o n , il m e n i t e, al l a n i te a n d m o n az i t e. M o n az i t e r an g e
f r o m 0 . 1 3 t o 0 .2 8 m m i n l en g t h , an d e x ce p t i o n al l y l a rg e o n e i s u p t o 0 . 3 5 m m.
T h e T h O 2 c o n t en t r a n g e s f r o m 2 .3 6 t o 7 .9 1 % an d t h e U O2 co n te n t f r om 0 . 0 5 t o
0 . 4 7 % . T he PbO-ThO 2 * p lo t s o f 3 9 an a l y ses o n 2 4 g r a i n s ( Fi g . 4) d ef in e an i s o c h r o n of 79 . 4± 3 . 1 M a (M S W D= 0. 1 3) .
PbO (wt.%)
S a m pl e 2 9 07
0.03
3 9 a nalyses o n
2 4 g r ai n s
0.02
7 9.4± 3 . 1 Ma
( M SW D= 0 . 1 3)
0 .0 1
A na lyti c al
u ncertainty
0
0
2
4
6
8
10
T h O 2 * ( wt . % )
Fig. 4.
P l o t s o f P bO v s . T hO 2 * o f m on a z it e s f r om t h e I c h i d a G r a ni t e . E x pl a n a t i o n s f or
e r ro rs a re t h e s a me a s f o r F ig. 3.
C H IM E m on a z it e a g es o f t he O t ag i ri a n d I ch i da G ra ni te s
27
D ISCUSSION
T h e t h ree C H IME m o n a zi t e ag e s f o r th e O t ag i ri G ran i t e ar e 7 7 .4 ± 3 . 1 M a f o r
t h e n or t h er n p l u t o n , 7 6 . 3 ± 2 . 8 M a f o r t h e ce n t r a l p l u t o n a n d 7 7 . 2 ± 3 . 5 Ma f o r t h e
s o ut h er n p l u t o n . T h e t h re e C H I M E d a t es c o i n c i d e w e l l e ac h o t h e r w i t h i n
t h e l i m i t o f a n a l yt i ca l u n c e rt a i n t y. I f w e r e g r es s a l l a n a l y t ic a l d a t a f o r
t hr e e p lu t on s , w e o b ta i n a n i s o c h r o n o f 7 7 . 2 ± 1. 7 M a ( M S W D = 0 . 2 0 ) w it h a n
i nt er c ep t v a l u e o f 0 . 0 0 0 03 ± 0 . 0 0 05 7 .
As m e n t i on e d e a rl i e r, t he Ot a g i r i G r an i t e h a s b ee n d a t e d b y Yu h a r a (1 9 9 4 )
b y t h e R b - S r w h o l e - r o c k i so c h r o n m e t ho d . T h e R b - S r a g e s a r e 7 1 .6 ± 6 . 8 ,
9 9 . 0 ± 1 0 . 9 a n d 9 2 . 7 ± 1 3 . 8 M a f o r t h e n o r t h e r n , c e n t r a l a nd so u t h e r n p lu t o ns ,
r es p e ct i v e ly ( F i g . 5 ) . T h es e R b - S r ag e s , p ar t i cu l a r l y 9 9 . 0 ± 10 .9 M a f o r th e c en t r a l pl u to n , a r e in c o n si st e n t w it h th e C H I ME m on a z i te a g e s . T hi s in c o n si st e n cy m a y n o t b e a s c r i b ed t o t h e s im p l e e r ro r i n b o t h t h e m i c r o p ro b e a n d
i s o t o p i c an a l y s es . R at h e r, w e b el i e v e t h a t t h e 9 9 . 0 ±1 0 .9 M a R b - S r a g e r es u l t e d
f rom i s o to p i c d i se q ui l ib r i u m in th e Rb - S r s y s te m on th e w h ol e -r o c k s c a l e a s
..
d o c u m en t e d b y S ch ar er e t a l . ( 1 9 84 ) . I n s p e ct i o n o f t h e i s o c h r o n d i a g r a m ( Fi g .
5 ) s h o w s a m a r k e d s c at t e r i n g o f d at a p o i n t s f o r t h e ce n t r a l p l u t o n ( c i r cl e s ) .
F o r t h e a g e c alc u la ti on , Yu h a r a ( 1 9 9 4) di sc a r de d 6 d a t a ( o p e n c ir c le s ) f r o m 16
87
Sr /
86
Sr
9 9 .0 ±1 0 . 9 M a
Sr I =0 . 7 1 02 6 ± 0. 0 0 01 7
No r t h e rn p l u t o n ( t r i ang l e )
C e nt ra l p l u t o n
( c i r c l e)
0.720
7 1.1 ±6 .8 M a
Sr I = 0 . 7 1 24 3 ±0 . 0 0 03 1
0.715
9 2 . 7 ± 1 3.8 M a
S r I =0 . 7 09 1 0± 0 . 0 00 5 6
S ou t h e r n p l u t o n (s q ua re )
0.710
0
2
Fig. 5.
6
4
87
Rb /
86
8
Sr
R b-S r i s ot opi c relation s i n wh ol e -r oc k s a m pl e s of t he O t a gi r i G ra ni t e (i s o t opi c
d a t a f r o m Ta b l e 4 o f Yuh a r a, 1994). Soli d lines a r e 7 7 Ma r eference i so c h ro n s
w i t h d i ff e re n t i ni t ia l 87S r/ 86 S r ra ti os , a nd b rok e n on e s r e pr e se n t re g re s si on l i n e s
o f fi l l e d d a t a poi nt s of i ndi v i du a l pl u t on s b y Yuh a ra ( 199 4) .
28
K . Suzuki e t al.
i s o t o p i c an a l y s es o n t h e b a s ic o f p et ro g r a p h i c a n d c h e mi c al cr i t er i o n. T h e s el ec te d 1 0 d at a ( s oli d ci rcl es ) , h o w e v er, ar e s ti l l n o t s t r i c tl y al i g n e d i n t h e i s o c h r o n d ia g r a m ; th e s c a t te r i s b e y o n d a n a l y ti c al un c er t a i nt y ( m e a n sq u a re o f
w ei g hte d d e v i at e d ( M S WD ) = 3 . 3 ) . T h i s s u g g es t s t h a t t h e R b -Sr s y s t em i s n ot
i n eq u i l i b ri u m ev e n i n t h e s el ec t ed 1 0 s am p l e s . I n t h i s c o n n e ct i o n , i t i s i mp ort a n t t o n o t e t h a t t h e ce n t r al p l u t o n c o m p r i s e s at l e a s t t h r ee i n t r u s i v e u n i t s
wi t h a cl e a r-c u t i nt r us i ve r e l a t ion ( M u r a ya m a a nd K a t a da , 19 57 ) . Th e s e l i n es
of e v i de n c e s u gg e s t t ha t t he R b- S r w ho l e - r oc k i s o c hr o n a g e s o f 99 . 0± 10 . 9 M a
m i g ht b e fo r t u i t o us , an d n ot r e p re se nt t he t r u e t i m e of e m p l a c e m e n t . Th i s i s
pa r t icu l a r l y e vi de n t i f on e e xa m ine s w h ol e i s o t op i c d at a of t h e ce n t r a l p l u t on
w it h r e s p e c t t o t h e r e f er e n c e i s o c h r o n o f 77 M a ( s o li d li n e ) ; s ub s e t s of t h e
da t a p oi n t s c a n re g r e s s e d wi t h 7 7 M a i soc h r on s of di ff er e n t i n i t ia l 87S r / 86 S r
r at i o s .
Th e R b - S r da ta f or t he s ou t h e r n p l u t o n ( squ a re s) a l so a r e d i st i n c t l y n ot
a l ig n ed i n t h e i s o c h ro n d i ag r am ( F i g . 5 ) , a n d s u s e t s o f t h em a re r eg r es s e d
w i t h 7 7 M a i s oc h r on s . A g ai n t h i s i s a c o m p os it e p l u t o n ( M u ra y a m a a nd
K a t ad a , 1 9 5 7 ). We c o n s i d e r t h at t h e 9 2 . 7 ± 1 3 . 8 M a R b - S r w h o l e - ro ck i s o ch r o n
a g e i s f o r t u i t o u s . T h e R b - S r i s o c h r o n p l o t s o f d at a f o r t h e n o rth er n p l u t o n d o
n o t s h o w a ma j o r ef f e ct o f t h e i s o t o pic disequilibrium, an d t h e a g e o f 7 1 . 6 ± 6 . 8
M a i s e s se nt i a l ly i d e nt i c a l w i t h t h e C H IM E m on a z i t e a g e of 77 . 4± 3 . 1 M a . We
110
100
90
80
70 Ma
Naegi Gr a n it e
Kisokoma Granodiorite
Otagi r i Grani t e
Buset s u Grani t e
I chida Granite
Inagawa Granodiorite
Mitsu hashi Grano diorite
Ryoke m etamorphic rocks
Fig. 6.
Dia grammatic p resent ation of CHIME mona zite ages i n conte xt o f field relation s
i n t h e Komagane a n d Mikawa a r e a s. C H I M E age s a re plotted w i t h e r ro r b a rs
(2σ). B o l d lines represe n t a g e d a t a o bt ai n e d i n t h i s st u d y a n d ot h er li nes represe n t those r e p or t ed by Suzuki e t a l . (1994a, b ).
C H IM E m on a zi t e a g es o f t he O t ag i ri a n d I ch i da G ra ni te s
29
p r ef er t h e C H IM E m o n a zi t e ag e s o f c a. 7 7 M a ( 7 7 . 4 ± 3.1 , 7 6 . 3 ±2 .8 an d 7 7 . 2 ± 3 . 5
M a ) a s r e p r ese n t in g t h e t i me of i nt r u s io n a n d s o li d if ic ati o n o f t he n o r th e r n ,
c en tr al an d s o u t h e r n p l u t o n s o f t h e O t a g i r i G r a n i t e.
T h e p res e n t C H IM E m o n a zi t e ag e s o f t h e O t a g i r i a n d I ch i d a G ran i t e s a r e
p l a c e d i n t h e c o nt e xt o f f i e ld e vi d e n c e i n t h e e a s t e r n Ry ok e b e l t ( F ig . 6 ) ,
t o get h e r w i t h t h e p r e v i ou s ly r e p or t e d C HI M E m on a z i t e a ges f o r g n e i ss e s a nd
g r an it o i d s ( S u zu k i e t al . , 1 99 4 a, b ) . T h e ca . 7 7 M a C H IME ag e s o f t h e O t a g i r i
G r a ni te j u s t o ve r l a p w i t h t ho s e o f t he B u s e t su G r a ni te ( 75 . 0 ± 5. 1- 7 8. 9 ±5 . 3
M a, Su z u k i e t a l . , 1 9 9 4 b ) , a nd a p p e ar t o b e s l i g h tl y y o u n g e r t h a n t h e C H I M E
m o na zit e a g e of 79 . 4 ± 3 . 1 M a f o r t he I c h id a gr ani te ; t h e a g e s p r e s e nt e d h e r e i n
a r e i n go o d h a r m on y w i th t h e in t r u s iv e r ela ti o n t h a t th e O t a g i r i G r a n it e in t r u d e i n to t h e I c h i d a G r a n i t e . A lt h ou g h w e a r e u n c e r t a i n w h e t he r t he
O ta g ir i, K a d o s h im a a nd B u s e t su G ra n i te s a r e o f c o m a g m a t i c o r n o t, w e c an
s t a te th a t th e y w e r e e mp la c e d si mu l ta n e o u sl y a t c a . 7 7 M a .
ACKNOWLEDGEMENTS
We w o u l d l ik e to e x p r e s s o u r t h a n k s t o D r. M . A d ac h i f o r h i s c o n s t r u c t iv e
c o m m e n t s a n d M r. S. Yo g o f o r h i s t e ch n i ca l a s s i s t a n ce . T h a n k s ar e al s o ex t e n d e d t o D r. H . K ag am i a n d a n n o n y m o u s r e v i ew er f o r t h e ir t h o u gh tf u l c o m m e n ts . T h is w o r k w a s i n p a r t s up p o r te d b y G r an t - i n- A i d f o r f u n d a me n t a l
S c ie n t if i c R e s e a r c h ( N o . 0 7 64 0 6 56 ) f r o m th e M in is t r y of Ed u c a ti o n, S c i e nc e
a n d C u l t u re , J a p an .
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Adachi, M . a n d Suzuki, K. (1992) A pr e l iminary n ote o n t h e a g e of det rit a l monazite s and
z irc o n s fr o m s a n ds ton e s i n t h e U p pe r Tr i a s s ic N a b a e G r ou p , M a i zur u t e r ra ne . Me m .
Geol. Soc. J apan, 38, 111-120.
Kagami , H . ( 1 97 3 ) A Rb-Sr g eo chrono l og i cal study o f t he Ryo k e gr a ni te s i n Chubu d is t ri ct ,
central Japan. J o ur. Geol. S o c . Japan, 79, 1-10.
M err i l, R . B. , R ob e r t s o n, J . K . a nd Wyl l i e, P. J . ( 19 7 0) M e lt i n g r e a c ti o n i n t h e s y s t em
N a A l S i3 O 8 - KA lSi 3 O 8 -SiO 2 - H 2 O t o 2 0 k i lob a r s c o m pa re d w i th r e s u l t s fo r ot h e r f e l d s p ar-quar tz-H 2 O a n d ro c k-H 2 O syste ms . J. G eol., 78, 5 58-569 .
Mu ray a ma , M. an d K a t ad a , M. ( 1 9 5 7 ) E x p la n a to ry t e x t o f t h e g e o l o g i ca l m ap o f Ja p an ,
with geological s hee t m ap "Akaho" at 1:50,000 . Geol. S ur v. J apan, 4 5p .
Nakai, Y. , Takeuchi, S., Suganuma, T., Ota , S . , Sakamoto, E. , Yam a moto, N . a n d Uchida, Y.
( 19 85 ) Geology o f the Okazaki City area. Okazaki City, p.209.
Ono, A . (1969) Zoning o f th e me ta m orphic rocks in t h e Takato - Sio z ir i area, N a ga n o P r efect ur e . J ou r. G eo l. S oc . Ja pan, 75, 5 21-536 .
P a r ri s h, R . R . ( 19 90 ) U -P b d a t i n g of m o na z i te a n d i t s a pp l i ca t ion t o g e ol og i c al p ro b l e m s .
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Ya m a da , T. (1 992 ) T he m u t ua l relations of t he gr a ni t i c r oc k s o f t h e Ry oke m e t amo rp hi c be lt i n c e n t ra l J a pa n . E ar t h Sci., 26, 2 05- 21 6.
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S c h ar er, U ., H a me t, J . an d A l le g re , C. J . ( 19 8 4 ) Th e Tr an s h im al ay a ( G an g d es e ) p lu t on i s m
i n t h e Ladakh region: a U-Pb a n d Rb-Sr st u dy. Eart h Planet. S c i . Le t t ., 67, 327-33 9 .
S h i b a t a , K . a n d I sh ih a r a , S . ( 1 9 7 9 ) R b - S r wh ol e- r o c k a n d K - Ar m i n e r a l a g e s o f g r a n i t i c
ro c ks i n J a p a n. G e oc he m . J., 13 , 11 3-11 9.
S m i t h, H . A . a nd B a rre iro, B . (19 90) Mo na z i t e U - Pb da t i n g of st a uro l i t e gr a de metamorp hi s m i n p e litic schist. Contrib. Mineral . P etrol. , 105, 602-615.
S u z uk i , K . a n d A d a c h i , M. (1 9 91 ) Pr e ca m b ri an p r ov e n a n c e a n d S i l u r i a n m e ta m o rp hi s m o f
t h e Ts u b on o s a w a p ar ag n e is s i n t h e S o u t h K i ta ka mi t er r a ne , N o r th e as t J a p an , r ev ea l e d b y th e ch e m ic a l T h - U - t o ta l P b i so c h r o n ag e s o f mo n a z it e , z ir c o n an d x en o ti me . G e oc h em ic a l J. , 2 5, .357-376.
S u z uk i , K . , A da c h i , M . a nd Ta n a k a , T. ( 19 91 ) Mi dd l e P re ca m b ri a n pr ov e n a n c e o f J ur a s s i c
s an d s t o ne i n th e M i no Ter r a ne , cen t r al J a pa n : Th - U -t o ta l P b ev i d en ce f r o m a n el ec tr o n m ic ro p r o be m on a zi te s t ud y. S ed i m en ta r y G e o l., 7 5 , 14 1 - 1 47 .
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d i ff us io n i n m e ta m o r ph o s e d d e t r it a l m o n a z it e s . E a r t h P la ne t . S c i. L et t. , 1 2 8,
391- 4 05 .
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a g e s o f m o na z i te s f r om t h e Ry ok e m e t a mor p hi c r oc ks a nd s o m e gr a n i toi d s i n t h e
M i k a w a - To no a r e a , c e n t r a l J apa n . B ul l. N ag oy a U n i v. F u r uk aw a M us eum , 1 0 , 1 7- 38 .
Ts uc h i ya , T. (1 96 6) T h e st ruc ture o f t h e Ot a g i ri d om e i n t he Ryok e m e t a m or phi c b e l t , c e nt r a l J a p a n - w i t h s p e c i a l r e f e re n c e t o t h e p e t r o fa b r i c a n a l y s i s - . J o u r. F ac . S c i .
Hokk a i do U n i v., 1 9, 1 , 1 - 2 4.
Yu ha r a, M . (1 9 9 4 ) Tim in g o f i nt r u s io n o f t h e O t ag i r i gr a ni te w i th re s p ec t to t h e de f or m at i o n a n d me t am o r p h i sm i n t h e Ry o k e b e lt in t h e I n a d i st r i c t, c e n t r al Ja p an : Ex a mi n a t i on by R b-S r w h ol e ro c k i s oc h ro n a ge s . J ou r. Mi n. P e t . E c on . Geol., 89, 2 69- 28 4.