J. Earth Planet. Sci. Nagoya Univ., Vol. 42, 31 to 43, 1995

Late Permian CHIME ages of the Hida Gneiss andearly Triassic age of the Mizunashi Granite in
the Amo area of the Hida terrane, central Japan
Ibral, H. KHAN*, Kazuhiro SUZUKI**, Ken SHIBATA**
and Mamoru A DACHI***Geoscience Laboratory, Geological Survey of Pakistan, Shehzad Town,

P.O. Box 1461, Islamabad, 44,000, Pakistan** Department of Earth and Planetary Sciences, Nagoya University,

Nagoya 464-01, Japan(Received October 13, 1995 / Accepted December 15, 1995)

ABSTRACTThe CHIME (chemical Th-U-total Pb isochron method) dating of the Hida
Gneiss, the Mizunashi Granite and a pegmatite vein have been carried out by
precise microprobe analyses of monazite. The CHIME ages are 249±5 and251±7 Ma for the Hida Gneiss, 237±6 Ma for the Mizunashi Granite and180±3 Ma for the Pegmatite vein. There is no age signature older than ca.
250 Ma even in the center of monazite grains; monazite ages date the first attainment of Hida metamorphism to the grade of monazite formation, the amphibolite facies. The emplacement of the Mizunashi Granite at 237 Ma may
correspond to the peak metamorphism of the Hida Gneiss. The pegmatite veinin the Mizunashi Granite can be correlated in age with the Jurassic Funatsu
granitic rocks.

INTRODUCTION
The Hida terrane located in the northern part of central Japan consists
mainly of the Hida Gneiss, the Unazuki Schist and a series of granitoids. Although there has been extensive field, petrological and geochronologicalstudies on rocks in the Hida terrane, a controversy exists on the age of themetamorphic and granitic rocks. Up to now, the authorized facts are: (1)among the granitoids, the Funatsu Granitic Rocks are the most widespread andwere emplaced in Jurassic time (ca. 180 Ma, Shibata and Nozawa, 1984), (2)the Unazuki Schist formed in late Permian or early Triassic from upper Carboniferous to Permian sediments (Hiroi, 1981), and (3) the radiometric ages ofthe metamorphic rocks concentrate around 240 Ma (Shibata et al., 1970; Ota
and Itaya, 1989).
The main point of arguments is whether the Hida Gneiss are of true Precambrian origin or not. Minato et al. (1965) pointed out that the Hida Gneiss
bears the closest resemblance to the middle Precambrian Matenrei (Macho'llyong) System in the northeastern part of the Korean Peninsula. Sato et al.
(1967) reported preliminary Rb-Sr model ages of 680-1200 Ma for the Gray

31

32

I. H. Khan et al.

Granite that cuts the Hida Gneiss. These ages have never been confirmed, but
the age data coupled with the mode of occurrence of the Gray Granite havelong supported the idea that the Hida Gneiss is of polymetamorphosed Precambrian origin (e.g. Sato, 1968; Suwa, 1969,1990, Suzuki, 1977; Hiroi, 1981).
Subsequent Rb-Sr studies of the Gray Granite yielded whole-rock model age as
old as ca. 1100 Ma (Shibata and Nozawa, 1986) and a whole-rock isochron ageof 506 Ma (Shibata et al., 1989), whereas 230-250 Ma monazite and zircon ages
were reported by the CHIME method (Suzuki and Adachi, 1991b); the timingof metamorphism and plutonism still remains controversial. This paper presents the results of CHIME (chemical Th-U-total Pb isochron method) datingon the Hida Gneiss and related Mizunashi Granite from the Amo area in thecentral Hida terrane.
GEOLOGICAL OUTLINESouthwest Japan is divided into Inner and Outer Zones on the basis of
their position with respect to the Median Tectonic line (MTL in Fig. 1). The

Fig. 1.

Geologic framework of central Japan. MTL: the Median Tectonic Line, ISTL: the Itoigawa-Shizuoka Tectonic Line.

Late Permian CHIME ages of the Hida Gneiss

33

Inner Zone in central Japan consists of three pre-Tertiary terranes from northto south; (1) the Hida terrane of high T/P metamorphic rocks and granitoids,(2) the circum-Hida terrane of serpentinite melange with blocks of Paleozoic
sediments and high P/T type metamorphic rocks, and (3) the Mino terrane ofJurassic sedimentary complex with disorganized blocks of Paleozoic limestone
a nd gre e ns t one a nd Tri a s s i c c he rt ; t he s out he rn pa r t of t he M i no t e r ra ne i sunderlain by high T/P metamorphic rocks and granitoids, the Ryoke belt. The
Hida terrane is underlain by the Hida Gneiss on the northwestern side, theUnazuki Schist on the southeastern side, and a variety of granitoids. They arecovered with the late Jurassic to early Cretaceous Tetori Group, and Cenozoic
sediments and volcanics.
Samples of the present study were collected from the Amo area. The geological configuration of the area is given in Fig. 2. The Hida Gneiss consists
mainly of quartzo-feldspathic gneiss and crystalline limestone with subordinate amphibolite, pelitic gneiss and lime-silicate gneiss. The granite exposedin the western part of the study area is named as the Mizunashi Granite, andthat in the southeastern part is correlatable with the Jurassic Funatsu
granitic rocks (Nozawa et al., 1975).

Fig. 2.

Geologic map of the Amo area of the Hida terrane (simplified andmodified from Nozawa at al., 1975), showing sample localities.

34

I. H. Khan et al.

Two gneiss samples (Nos. 90080502A and 90080502B) were collected fromthin pelitic layers intercalated with quartzo-feldspathic gneiss in an old graphite mine near Amo. The sample of Mizunashi Granite (93080503E) was collected from an outcrop about 1 km west of the mine, and sample 93080503D
was collected from a pegmatite vein that cuts the Mizunashi Granite in the sameoutcrop as 93080503E. The gneiss samples are micaceous and consist essentially of biotite, muscovite, garnet, plagioclase and quartz with a substantial amount of graphite. Accessories include zircon, monazite, apatite and ironores. The sample of the Mizunashi Granite is medium-grained hornblendebearing biotite granite, and the pegmatite sample consists mainly of coarsegrained quartz and microcline with minor plagioclase, biotite and muscovite.
EXPERIMENTAL
Samples were crushed on a stamp mill. The size reduction was undertaken
only to the extent necessary for disaggregation of monazite through periodic
sieving to remove undersized (-60 mesh) particles before further reduction ofoversized ones. Monazite and other heavy minerals were concentrated fromeach sieved sample by a pan. The concentrates were washed with dilute HCl
a n d p u r i f i e d o n a n i s o d y n a m i c m a g n e t i c s e p a r a t o r. M o n a z i t e g r a i n s w e r ehand-picked, mounted on a glass slide with Petropoxy 153 (Fig. 3A), and driedon a hot plate at 150 oC. After covering with more Petropoxy 153 (Fig. 3B) anddrying, the samples were polished with diamond paste until the grains werethinned approximately to half in thickness (Fig. 3C).
Monazites were analyzed on a JEOL JXA-733 electron microprobe provided
with three wavelength-dispersive spectrometers (diameter of Rowland
circle=280 mm). The instrument operating conditions were 15 kV accelerating voltage, 0.15 µA probe current on Faraday cage and 5 µm probe diameter.
Intensities of ThMα, UMβ and PbMα lines only were measured using PET crystal. The background was measured at two optimum positions on both sides ofeach line peak position. Random error due to counting statistics was deducedby counting 300s at each line peak and two background. The line and background measurements were repeated twice, and the arithmetic average of thereadings was taken. The comparison standards were euxenite provided by
Smellie et al. (1978) for Th and U, and synthesized glass (56.17% PbO, 13.65%
ZnO and 30.18% SiO 2 , analyst: K. Hayashi) for Pb.
The Th, U and Pb intensities were converted into concentrations throughthe Bence-Albee correction method using an average monazite composition.
The detection limit of Pb (as PbO) at 2σ confidence level is 0.007%, and therelative error in the Pb determination is around 8% at the 0.1% PbO level andmuch better for higher concentrations. The relative errors in the Th and U
determinations are about 2% and 5%, respectively. Since the CHIME calculation has been described elsewhere (Suzuki and Adachi, 1991a,b, 1994; Adachi
and Suzuki, 1992; Suzuki et al., 1991,1992,1994), the readers, if necessary, arerecommended to refer to them.

Late Permian CHIME ages of the Hida Gneiss

Fig. 3.

35

Mounting of monazite grains on a glass slide (A), coveringwith petropoxy 154 (B), and polishing with arundum anddiamond paste (C).

RESULTSThe ThO 2 , UO 2 and PbO analytical data of monazites together with apparent ages and calculated ThO 2 * concentrations are listed in Table 1.
Hida Gneiss (Samples 90080502A and 90080502B)
A total of 95 spots on 29 monazite grains from sample 90080502A were analyzed. The ThO 2 * content varies from 3.98 to 11.4%, and the PbO content from
0.044 to 0.121%. The apparent ages for most spots concentrate around 250 Ma,
but those of spots 12-1 and 20-1 on metamict parts are about 200 Ma. The
PbO-ThO 2 * plots of 93 data are given in Fig. 4A. The regression line, having a

36

Table 1.

I. H. Khan et al.

Microprobe analyses of ThO 2 , UO 2 and PbO of monazites from samples of the
Hida Gneiss, Mizunashi Granite and pegmatite in the Amo area of the Hida
terrane.

Grain# ThO2 UO2 PbO

age ThO2*

Hida Gneiss (sample 90080502A)
M01-01 7.81 0.258 0.094 259 8.65
M01-02 7.95 0.265 0.094 254 8.81
M02-01 3.19 0.243 0.044 264 3.98
M02-02 5.38 0.264 0.064 245 6.24
M02-03 5.17 0.226 0.065 263 5.90
M02-04 5.25 0.253 0.062 241 6.07
M02-05 7.96 0.309 0.090 239 8.96
M02-06 4.58 0.231 0.054 240 5.33
M03-01 5.84 0.172 0.068 251 6.40
M03-02 5.97 0.149 0.072 263 6.45
M04-01 5.51 0.120 0.064 256 5.90
M04-02 5.61 0.150 0.067 262 6.10
M05
8.14 0.184 0.093 254 8.74
M06
6.40 0.226 0.077 257 7.13
M07
7.75 0.156 0.089 255 8.26
M08
7.73 0.154 0.088 253 8.23
M09
6.85 0.452 0.092 264 8.32
M10
7.14 0.205 0.085 257 7.80
M11
6.67 0.228 0.077 246 7.41
M12-01m 4.86 0.177 0.047 206 5.43
M12-02 5.48 0.196 0.068 263 6.12
M12-03 6.10 0.178 0.073 258 6.68
M12-04 6.33 0.159 0.070 242 6.85
M12-05 7.01 0.178 0.082 258 7.59
M12-06 7.44 0.168 0.087 260 7.99
M12-07 8.06 0.161 0.093 258 8.58
M12-08 7.27 0.182 0.081 244 7.86
M12-09 6.34 0.169 0.075 257 6.89
M12-10 8.90 0.168 0.103 260 9.45
M12-11 9.11 0.169 0.102 252 9.66
M12-12 8.82 0.169 0.099 250 9.37
M12-13 8.76 0.184 0.101 257 9.36
M12-14 8.75 0.167 0.093 239 9.29
M12-15 6.02 0.175 0.068 246 6.59
M12-16 8.62 0.163 0.097 253 9.15
M12-17 9.17 0.174 0.103 250 9.73
M12-18 8.94 0.189 0.102 252 9.55
M12-19 8.76 0.186 0.097 247 9.36
M12-20 6.80 0.198 0.081 259 7.44
M12-21 6.53 0.203 0.073 240 7.19
M12-22 7.41 0.259 0.085 244 8.25
M12-23 8.64 0.242 0.101 253 9.42
M12-24 8.76 0.250 0.107 266 9.57
M12-25 8.35 0.237 0.096 251 9.12
M12-26 8.59 0.237 0.102 260 9.36
M12-27 9.58 0.215 0.109 253 10.3
M12-28 7.13 0.267 0.082 242 8.00
M12-29 6.12 0.234 0.077 267 6.88
M12-30 6.24 0.269 0.078 259 7.11
M12-31 7.37 0.258 0.088 253 8.21
M12-32 8.56 0.219 0.098 250 9.27
M12-33 8.76 0.216 0.101 254 9.46
M12-34 8.13 0.211 0.095 257 8.81
M12-35 9.21 0.224 0.102 243 9.94
M12-36 8.78 0.188 0.104 264 9.39
M12-37 6.20 0.153 0.072 254 6.70

Grain# ThO2 UO2 PbO

age ThO2*

M12-38 6.10 0.210 0.076 265 6.78
M12-39 7.91 0.220 0.096 265 8.62
M12-40 9.86 0.183 0.110 249 10.5
M12-41 9.17 0.204 0.105 252 9.83
M12-42 8.39 0.212 0.097 255 9.08
M12-43 6.09 0.167 0.071 253 6.63
M12-44 5.80 0.180 0.065 243 6.38
M12-45 7.73 0.261 0.089 245 8.58
M13
6.85 0.219 0.083 259 7.56
M14
6.96 0.234 0.081 248 7.72
M15-01 6.34 0.201 0.075 254 6.99
M15-02 6.63 0.201 0.078 255 7.28
M15-03 5.73 0.267 0.070 253 6.60
M16-01 9.85 0.484 0.121 253 11.4
M16-02 9.28 0.442 0.113 251 10.7
M16-03 7.19 0.454 0.090 248 8.66
M17-01 5.31 0.130 0.060 247 5.73
M17-02 5.34 0.124 0.063 259 5.74
M18
6.11 0.166 0.072 258 6.65
M19
5.87 0.102 0.065 250 6.20
M20-01m 5.69 0.070 0.048 192 5.92
M20-02 6.04 0.123 0.073 270 6.44
M20-03 5.79 0.078 0.062 245 6.04
M21-01 6.06 0.133 0.071 261 6.49
M21-02 6.00 0.187 0.067 242 6.61
M22
6.05 0.180 0.068 242 6.63
M23
5.15 0.029 0.058 261 5.24
M24-01 6.90 0.392 0.085 246 8.17
M24-02 6.91 0.399 0.087 253 8.20
M24-03 5.96 0.281 0.074 257 6.87
M25
6.42 0.119 0.078 273 6.81
M26-01 6.74 0.157 0.073 240 7.25
M26-02 8.12 0.197 0.092 251 8.76
M26-03 7.04 0.196 0.083 258 7.68
M27
6.38 0.206 0.078 264 7.05
M28-01 6.39 0.155 0.073 250 6.89
M28-02 6.08 0.091 0.066 247 6.37
M29-01 8.37 0.226 0.098 257 9.10
M29-02 8.15 0.146 0.092 254 8.62
Hida Gneiss (sample 90080502B)
M01-01 8.01 0.374 0.097 249 9.22
M01-02 8.16 0.637 0.108 249 10.2
M01-03 8.77 0.442 0.109 253 10.2
M01-04m 9.62 0.462 0.101 215 11.1
M01-05 8.17 0.308 0.095 245 9.17
M02-01 8.75 0.363 0.106 252 9.93
M02-02 9.27 0.392 0.110 247 10.5
M02-03 9.60 0.415 0.117 252 10.9
M02-04 9.48 0.442 0.115 249 10.9
M03-01 8.70 0.467 0.112 259 10.2
M03-02 8.30 0.458 0.103 249 9.78
M03-03 8.83 0.459 0.111 254 10.3
M03-04 7.21 0.554 0.093 244 9.01
M03-05 7.63 0.472 0.096 248 9.16
M04-01 8.73 0.583 0.113 251 10.6

Grain# ThO2 UO2 PbO

age ThO2*

M04-02 9.32 0.333 0.108 245 10.4
M04-03 8.22 0.484 0.104 251 9.79
M04-04 8.49 0.314 0.101 251 9.51
M04-05 8.18 0.450 0.098 240 9.64
M04-06 9.74 0.323 0.114 250 10.8
M04-07 9.82 0.315 0.116 253 10.8
M04-08 9.76 0.311 0.111 244 10.8
M04-09 9.65 0.353 0.114 250 10.8
M04-10 9.37 0.321 0.110 250 10.4
M04-11 8.69 0.520 0.110 250 10.4
M04-12 9.89 0.342 0.114 245 11.0
M04-13 9.77 0.307 0.112 246 10.8
M04-14 9.13 0.314 0.106 247 10.1
M04-15 8.14 0.270 0.096 252 9.02
M04-16 8.75 0.291 0.102 249 9.69
M04-17 9.56 0.309 0.114 255 10.6
M04-18 9.42 0.364 0.109 243 10.6
M04-19 9.71 0.284 0.111 247 10.6
M04-20 9.41 0.282 0.108 247 10.3
M04-21 9.07 0.357 0.109 252 10.2
M04-22 9.54 0.335 0.110 245 10.6
M04-23 8.97 0.271 0.103 247 9.85
M04-24 9.18 0.273 0.103 242 10.1
M04-25 8.91 0.250 0.099 241 9.72
M04-26 8.91 0.268 0.108 261 9.78
M04-27 10.2 0.315 0.119 250 11.2
M04-28 9.99 0.332 0.120 256 11.1
M04-29 9.63 0.310 0.113 251 10.6
M04-30 8.79 0.353 0.101 240 9.93
M04-31 8.71 0.338 0.103 248 9.81
M04-32 9.30 0.384 0.111 249 10.5
M04-33 10.3 0.432 0.125 253 11.7
M04-34m 7.98 0.058 0.063 182 8.17
M04-35m 7.79 0.139 0.062 178 8.24
M04-36 9.79 0.313 0.112 245 10.8
M04-37 9.37 0.265 0.109 252 10.2
M04-38 9.33 0.337 0.109 247 10.4
M04-39 8.87 0.334 0.107 254 9.95
M04-40 8.76 0.346 0.102 244 9.88
M04-41 9.37 0.364 0.110 246 10.5
M04-42 9.66 0.381 0.114 247 10.9
M04-43 10.0 0.400 0.121 253 11.3
M04-44 8.54 0.227 0.099 252 9.28
M04-45 9.47 0.322 0.109 245 10.5
M04-46 9.39 0.268 0.112 258 10.3
M04-47 8.86 0.335 0.106 252 9.95
M04-18 8.62 0.320 0.103 252 9.66
M04-49 8.87 0.333 0.105 249 9.95
M04-50 9.65 0.366 0.113 246 10.8
M04-51 9.67 0.365 0.114 248 10.9
M04-52 9.26 0.207 0.102 243 9.93
M04-53 8.83 0.320 0.105 252 9.87
M04-54 8.67 0.327 0.105 255 9.73
M04-55 8.64 0.310 0.102 250 9.65
M54-56 9.32 0.343 0.111 251 10.4
M04-57 8.95 0.419 0.112 257 10.3
M04-58 8.78 0.380 0.107 252 10.0

Late Permian CHIME ages of the Hida Gneiss

Table 1.

(continued).

Grain# ThO2 UO2 PbO
M04-59M04-60
M04-61
M04-62
M05-01
M05-02
M05-03
M05-04
M05-05
M06-01
M06-02
M06-03
M06-04
M07-01
M07-02
M07-03
M07-04
M08-01
M08-02
M08-03
M08-04
M09-01
M09-02
M09-03
M09-04
M10-01
M10-02
M10-03
M10-04
M11-01
M11-02
M11-03
M11-04
M11-05
M12-01
M12-02
M12-03
M12-04
M13-01
M13-02
M13-03
M13-04
M13-05
M13-06
M14-01
M14-02
M14-03
M14-04
M14-05
M14-06
M15-01
M15-02
M15-03
M15-04
M16-01
M16-02
M17
M18
M19-0137

age ThO2*

8.69 0.338 0.101 244 9.79
8.26 0.284 0.098 252 9.18
8.93 0.325 0.106 251 9.98
7.61 0.123 0.087 257 8.01
10.6 0.474 0.128 248 12.1
9.93 0.444 0.117 243 11.4
8.98 0.381 0.110 254 10.2
9.82 0.440 0.119 250 11.2
9.85 0.475 0.119 247 11.4
8.51 0.295 0.103 257 9.47
9.43 0.294 0.111 253 10.4
9.37 0.303 0.108 246 10.4
9.12 0.291 0.107 251 10.1
7.94 0.368 0.096 248 9.13
7.89 0.359 0.094 245 9.05
10.1 0.400 0.121 251 11.4
10.6 0.418 0.124 245 12.0
11.2 0.475 0.136 252 12.7
10.1 0.528 0.124 248 11.8
8.64 0.357 0.104 251 9.80
10.7 0.432 0.126 246 12.1
10.3 0.422 0.124 252 11.7
8.63 0.386 0.107 256 9.88
8.50 0.390 0.102 247 9.76
10.4 0.421 0.125 252 11.8
6.01 0.540 0.085 259 7.76
5.53 0.500 0.075 248 7.15
5.29 0.516 0.074 251 6.96
5.72 0.477 0.078 254 7.27
9.31 0.520 0.119 256 11.0
9.96 0.537 0.123 248 11.7
7.65 0.823 0.112 256 10.3
10.8 0.531 0.134 253 12.5
11.0 0.559 0.138 255 12.8
6.24 0.639 0.089 253 8.31
5.94 0.635 0.087 257 8.00
5.45 0.675 0.081 250 7.64
6.25 0.714 0.089 246 8.56
7.83 0.395 0.097 251 9.11
8.29 0.342 0.100 251 9.40
9.29 0.356 0.109 247 10.4
9.42 0.398 0.113 249 10.7
9.57 0.416 0.115 249 10.9
8.22 0.601 0.105 244 10.2
9.92 1.37 0.149 245 14.4
9.90 1.15 0.147 255 13.6
10.9 0.725 0.139 248 13.3
8.90 1.58 0.149 251 14.0
10.3 0.495 0.126 250 11.9
10.3 0.489 0.128 255 11.9
9.34 0.363 0.113 254 10.5
7.83 0.335 0.093 246 8.92
8.43 0.311 0.101 253 9.44
8.78 0.322 0.103 248 9.82
9.53 0.295 0.111 250 10.5
10.4 0.388 0.120 244 11.7
8.93 0.352 0.108 253 10.1
8.78 0.309 0.100 242 9.78
8.35 0.368 0.101 250 9.54

Grain# ThO2 UO2 PbO

age ThO2*

M19-02 10.1 0.412 0.120 248 11.4
M20-01 7.64 0.263 0.089 248 8.49
M20-02 8.33 0.265 0.097 249 9.19
M21-01 7.12 0.280 0.085 250 8.03
M21-02 7.21 0.666 0.100 252 9.37
M22
9.31 0.447 0.115 253 10.8
M23
8.65 0.323 0.101 246 9.70
M24
9.40 0.358 0.114 255 10.6
M25
8.94 0.376 0.109 253 10.2
M26-01 7.87 0.287 0.092 247 8.80
M26-02 8.91 0.304 0.103 246 9.90
M27
9.08 0.322 0.107 250 10.1
M28
7.72 0.300 0.093 253 8.69
M29-01 8.65 0.404 0.106 251 9.96
M29-02 6.68 0.313 0.080 246 7.69
M30-01 9.04 0.258 0.105 251 9.88
M30-02 8.55 0.316 0.102 253 9.57
M30-03 7.46 0.703 0.101 245 9.74
M30-04 9.77 0.311 0.115 251 10.8
M31-01 9.62 0.351 0.117 257 10.8
M31-02 9.35 0.377 0.111 248 10.6
M31-03 9.11 0.444 0.111 250 10.5
M31-04 8.71 0.640 0.120 262 10.8
M31-05 10.0 0.399 0.118 246 11.3
M32-01 6.76 0.325 0.081 244 7.81
M32-02 9.36 0.383 0.111 247 10.6
M32-03 7.86 0.239 0.090 246 8.64
M33-01 5.05 0.346 0.066 251 6.17
M33-02 5.20 0.368 0.066 245 6.39
M33-03 5.18 0.362 0.069 255 6.35
M34-01 7.52 0.232 0.087 247 8.27
M34-02 8.21 0.228 0.096 254 8.95
M34-03 9.48 0.365 0.110 244 10.7
M34-04 8.46 0.367 0.102 249 9.65
M35-01 7.74 0.271 0.090 247 8.62
M35-02 8.15 0.217 0.093 247 8.85
M35-03 8.64 0.347 0.103 249 9.77
M36-01 6.84 0.606 0.091 245 8.80
M36-02 6.55 0.608 0.088 245 8.52
M36-03 6.37 0.621 0.091 255 8.38
M37-01 8.97 0.301 0.107 255 9.95
M37-02 9.35 0.309 0.106 242 10.4
M37-03 9.44 0.322 0.114 256 10.5
M37-04 9.24 0.309 0.109 251 10.2
M37-05 8.39 0.237 0.099 255 9.16
M37-06 9.78 0.362 0.116 250 11.0
M37-07 8.36 0.280 0.098 250 9.27
M37-08 9.47 0.356 0.110 245 10.6
M38-01 8.87 0.278 0.103 249 9.77
M38-02 8.38 0.279 0.095 243 9.28
M38-03 8.85 0.272 0.102 248 9.73
M38-04 8.59 0.271 0.100 249 9.47
M38-05 8.77 0.314 0.108 260 9.79
M38-06 8.49 0.382 0.100 242 9.73
M38-07 8.32 0.493 0.104 247 9.92
M38-08 9.02 0.255 0.102 245 9.85
M38-09 8.22 0.247 0.097 253 9.02
M39-01 7.44 0.366 0.091 248 8.63
M39-01 7.02 0.355 0.088 253 8.17

Grain# ThO2 UO2 PbO

age ThO2*

M39-03 6.78 0.338 0.083 248 7.88
M39-04 6.80 0.327 0.082 245 7.86
M39-05 6.85 0.329 0.082 245 7.92
M39-06 7.13 0.349 0.089 255 8.26
M39-07 7.15 0.349 0.085 242 8.28
M40-01 8.98 0.314 0.104 247 10.0
M40-02 8.48 0.336 0.104 257 9.57
M41-01 8.37 0.836 0.116 247 11.1
M41-02 8.34 0.833 0.117 250 11.0
M41-03 8.38 0.815 0.119 256 11.0
M41-04 8.39 0.810 0.114 245 11.0
M41-05 8.50 0.745 0.117 253 10.9
M41-06 8.48 0.745 0.116 251 10.9
M41-07 9.08 0.343 0.108 251 10.2
M41-08 8.81 0.459 0.108 248 10.3
M41-09 8.52 0.642 0.110 244 10.6
M41-10 8.53 0.717 0.114 247 10.9
M41-11 8.44 0.810 0.117 250 11.1
M41-12 8.66 0.768 0.117 248 11.2
M41-13 8.85 0.726 0.116 244 11.2
M12-14 9.97 0.475 0.121 248 11.5
M42-01 8.44 0.246 0.098 249 9.24
M42-02 8.17 0.240 0.097 256 8.95
M42-03 7.63 0.563 0.100 251 9.45
M43-01 8.27 0.361 0.099 248 9.44
M43-02 8.09 0.362 0.100 255 9.26
M43-03 8.02 0.371 0.096 246 9.22
M43-04 8.49 0.370 0.100 244 9.69
M43-05 8.71 0.259 0.101 251 9.55
M43-06 8.22 0.219 0.092 244 8.93
M43-07 8.02 0.388 0.096 244 9.28
M43-08 7.88 0.443 0.099 250 9.32
M44-01 8.79 0.658 0.114 246 10.9
M44-02 8.91 0.612 0.115 250 10.9
M44-03 8.15 0.654 0.112 257 10.3
M44-04 8.42 0.680 0.110 244 10.6
M44-05 8.38 0.640 0.111 251 10.5
M44-06 9.27 0.473 0.112 244 10.8
M44-08 9.69 0.419 0.114 245 11.0
M44-09 9.33 0.491 0.117 252 10.9
M45-01 8.49 0.269 0.103 260 9.36
M45-02 8.39 0.278 0.097 247 9.29
M45-03 8.48 0.245 0.099 252 9.27
M45-04 7.71 0.554 0.100 248 9.50
M45-05 7.67 0.520 0.099 250 9.36
M45-06 10.3 0.276 0.118 249 11.2
M45-07 11.0 0.244 0.123 247 11.8
M45-08 11.3 0.246 0.128 250 12.1
M45-09 8.35 0.852 0.115 245 11.1
M45-10 6.36 0.630 0.091 256 8.40
M45-11 6.74 0.667 0.095 252 8.90
M45-12 6.68 0.307 0.079 244 7.67
M46-01 5.43 0.399 0.070 245 6.72
M46-02 5.41 0.427 0.073 254 6.80
M46-03 5.07 0.381 0.067 252 6.31
M46-04 4.56 0.279 0.057 248 5.47
M46-05 5.27 0.389 0.069 249 6.53
M46-06 4.99 0.363 0.065 250 6.17

38

Table 1.

I. H. Khan et al.

(continued).

Grain# ThO2 UO2 PbO

age ThO2*

Mizunashi Granite (Sample 93080503E)
M01-01 10.7 0.216 0.117 242 11.4
M01-02 6.72 0.121 0.072 238 7.11
M01-03 5.90 0.074 0.064 246 6.14
M01-04 3.16 0.069 0.035 247 3.38
M01-05 8.01 0.100 0.086 244 8.33
M01-06 7.55 0.071 0.078 238 7.78
M01-07 8.96 0.055 0.096 247 9.14
M02-01 8.62 0.128 0.092 241 9.03
M02-02 8.79 0.138 0.098 251 9.24
M02-03 8.78 0.133 0.093 239 9.21
M02-04 13.3 0.153 0.139 239 13.8
M02-05 9.27 0.139 0.097 236 9.72
M02-06 13.7 0.178 0.143 236 14.3
M02-07 12.2 0.181 0.132 245 12.8
M02-08 10.7 0.181 0.115 241 11.3
M03-01 10.4 0.304 0.112 233 11.4
M03-02 11.2 0.291 0.120 233 12.1
M03-03 12.9 0.295 0.143 243 13.9
M03-04 10.9 0.285 0.118 236 11.8
M03-05 11.0 0.305 0.124 245 12.0
M03-06 10.1 0.304 0.114 243 11.1
M04-01 10.1 0.093 0.105 237 10.4
M04-02 7.23 0.108 0.077 241 7.58
M04-03 10.2 0.122 0.106 235 10.6
M04-05 10.1 0.128 0.105 236 10.5
M05-01 9.76 0.106 0.100 235 10.1
M05-02 9.35 0.103 0.097 236 9.68
M05-03 12.1 0.140 0.127 239 12.6
M05-04 12.9 0.081 0.129 233 13.2
M05-05 12.5 0.078 0.129 238 12.8
M05-06 8.95 0.070 0.092 237 9.18
M06-01 6.35 0.023 0.064 234 6.42

Grain# ThO2 UO2 PbO

age ThO2*

M06-02 8.82 0.061 0.092 240 9.02
M06-03 7.62 0.022 0.078 239 7.69
M06-04 7.30 0.030 0.074 235 7.40
M06-05 11.5 0.026 0.116 236 11.6
M06-06 4.88 0.070 0.051 236 5.11
Pegmatite (sample 93080503D)
M01-01 2.45 0.023 0.019 181 2.52
M01-02 2.07 0.016 0.018 196 2.12
M01-03 1.84 0.013 0.015 188 1.88
M02-01 2.49 0.035 0.019 176 2.60
M02-02 2.05 0.019 0.015 173 2.11
M02-03 1.84 0.017 0.015 181 1.90
M02-04 2.41 0.027 0.019 183 2.50
M02-05 2.46 0.018 0.020 189 2.52
M02-06 1.90 0.015 0.014 165 1.95
M03-01 2.84 0.030 0.022 178 2.94
M03-02 2.02 0.039 0.015 162 2.15
M03-03 1.95 0.034 0.016 188 2.06
M03-04 1.71 0.017 0.013 174 1.76
M03-05 1.86 0.020 0.014 168 1.92
M04-01 13.9 0.140 0.109 180 14.4
M04-02 15.1 0.185 0.115 174 15.7
M04-03 15.0 0.199 0.122 184 15.6
M04-04 17.5 0.160 0.136 178 18.0
M04-05 12.0 0.059 0.092 178 12.2
M04-06 31.1 0.294 0.244 180 32.0
M04-07 15.0 0.130 0.117 179 15.4
M05-01 4.56 0.026 0.034 174 4.64
M05-02 3.63 0.024 0.029 186 3.71
M05-03 3.70 0.051 0.028 172 3.86
M06-01 3.74 0.018 0.030 187 3.80

Grain# ThO2 UO2 PbO
M06-02
M06-03
M07-01
M07-02
M08-01
M08-02
M08-03
M08-04
M09-01
M10-01
M10-02
M10-03
M10-04
M10-05
M10-06
M10-07
M10-08
M11-01
M11-02
M11-03
M11-04
M11-05
M11-06
M12-01
M12-02
M12-03
M12-04
M12-05
M12-06
M13-01
M13-02
M13-03
M13-04age ThO2*

1.58 0.022 0.014 195
1.62 0.079 0.014 177
2.62 0.014 0.019 164
2.69 0.029 0.021 176
2.79 0.021 0.021 171
1.70 0.020 0.012 166
1.67 0.023 0.013 176
2.31 0.037 0.018 170
2.61 0.021 0.021 185
2.29 0.014 0.018 183
3.08 0.025 0.024 180
1.18 0.165 0.012 168
4.38 0.138 0.038 188
2.46 0.023 0.020 186
4.44 0.024 0.034 178
2.35 0.037 0.018 171
4.95 0.108 0.041 183
5.94 0.116 0.048 179
3.10 0.030 0.026 188
4.43 0.042 0.035 183
2.99 0.025 0.024 182
4.50 0.035 0.037 191
4.09 0.038 0.030 169
2.30 0.015 0.017 175
2.07 0.037 0.016 171
4.34 0.053 0.034 180
5.29 0.083 0.043 182
4.40 0.340 0.039 166
7.49 0.059 0.059 180
2.30 0.024 0.018 175
3.20 0.013 0.025 181
3.44 0.019 0.026 178
4.74 0.030 0.036 177

slope of 0.0105±0.0003 and an intercept value of 0.0007±0.0023,yields an ageof 250.5±6.8 Ma (MSWD=0.17).
A total of 248 spots on 47 monazite grains from sample 90080502B wereanalyzed. The ThO 2 * content varies from 5.47 to 14.4% and the PbO content
from 0.057 to 0.149%. The apparent ages are in the range between 240 and 261
Ma,but three spots (M01-04, M04-33 and, M04-34) on metamict parts give muchyounger apparent ages of 200-180 Ma. The PbO-ThO 2 * plots of 245 data are
given in Fig. 4B. They are regressed with an isochron of 248.9±4.5 Ma(MSWD=0.13) with a slope of 0.0105±0.0002 and an intercept value of
0.0005±0.0019.
Mizunashi Granite (Sample 93080503E)
A total of 37 spots on transparent portions of 6 monazite grains were analyzed. Monazite contains 3.38 to 14.3% ThO 2 * and 0.0353 to 0.1428% PbO. Alldata points are arrayed linearly on the PbO vs. ThO 2 * diagram (Fig. 5A),
yielding isochron (MSWD=0.12) of 237±5 Ma with a slope of 0.0100±
0.0002 and an intercept value of 0.0009±0.0020.

1.65
1.87
2.66
2.78
2.86
1.76
1.74
2.43
2.68
2.33
3.16
1.71
4.82
2.53
4.52
2.47
5.30
6.32
3.20
4.57
3.07
4.61
4.21
2.35
2.19
4.51
5.56
5.50
7.68
2.38
3.24
3.50
4.84

Late Permian CHIME ages of the Hida Gneiss

Fig. 4.

Plots of PbO vs. ThO 2 * of monazite grains from sample 90080502A (A)and from sample 90080502B (B). The error given to the age calculation isof 2σ.

39

40

I. H. Khan et al.

Fig. 5.

Plots of PbO vs. ThO 2 * of monazite grains from the Mizunashi Granite (A)and a pegmatite vein (B). The error given to the age calculation is of 2σ.

Late Permian CHIME ages of the Hida Gneiss

41

Pegmatite vein (Sample 93080503D)
A total of 58 spots on transparent portions of 13 monazite grains were analysed. The ThO 2 * content varies from 1.71 to 32.0%, and the PbO content from
0.012 to 0.244%. The apparent ages for individual spots are in the range between 169 and 196 Ma. The PbO-ThO 2 * plots of 58 data are given in Fig. 5B.
The regression line, having a slope of 0.0076±0.0001 and an intercept value of-0.0001±0.0004, yields an age of 180.1±2.5 Ma (MSWD=0.17).
DISCUSSION
The CHIME monazite ages for samples of the Hida Gneiss, 250.5±6.8 and248.9±4.5 Ma, agree well with each other. The closure temperature of Pb diffusion in m o n a z i t e i s 6 5 0 - 7 0 0 oC ( S u z u k i e t a l . , 1 9 9 4 ) . S m i t h a n d B a r r e i r o( 1 9 7 0 ) f o u n d that monazite in gneisses formed as metamorphic mineral at thelower amphibolite facies conditions records the time since the formation evenwith subsequent excursions into upper amphibolite facies. The stable association o f m u s c o v i t e a n d q u a r t z s h o w s t h a t t h e g n e i s s s a m p l e s d i d n o t e x p e r i ence the upper amphibolite facies conditions. We thus consider that the ca.
250 Ma monazite ages date the first attainment to the lower amphibolite facies
gra de duri ng t he progra de s t a ge of t he H i da m e t a m orphi s m .
If the Hida Gneiss is of polymetamorphosed Precambrian origin as has
b e e n s p e c u l a t e d p r e v i o u s l y, m o n a z i t e g r a i n s s h o u l d b e a r s i g n a t u r e o f o l d e rages. Despite our search for Precambrian monazite cores over 29 grains fromsample 90080502A and 46 grains from sample 90080502B, no parts older thanca. 250 Ma have been found. Suzuki and Adachi (1994) reported the occurrence of ca. 350 Ma detrital zircons from psammitic gneiss samples in the Tsunokawa area, east of the Amo area. This indicates that sedimentation of thegneiss protolith took place in the middle-late Paleozoic. Again, we can statethat the Hida metamorphic rocks in the Amo area formed in the late Permian,
and are not of polymetamorphosed Precambrian rocks.
The CHIME monazite age of the Mizunashi Granite is 236.8±4.7 Ma. Thisage is slightly younger than the previously reported Rb-Sr whole-rock isochron age of 296.7±25.6 Ma (Shibata and Nozawa, 1984). Since the Rb-Sr isotopic data are scattered, we here regard the 236.8±4.7 Ma age as the time of
granite emplacement. Another Rb-Sr mineral isochron age for the Mizunashi
Granite was reported as 210.9±1.8 Ma (Shibata and Nozawa, 1984). This age isevidently older than the ages for the post-tectonic Funatsu granitic rocks(188.9±4.4 Ma; Shibata and Nozawa, 1984), and is correlatable with K-Ar hornbl e nde a n d m u s c o v i t e a g e s f o r t h e H i d a G n e i s s .
Therefore, it seems much
more likely that the Mizunashi Granite was emplaced possibly at the peakmetamorphism and cooled together with the Hida Gneiss to the blocking temperatures of hornblende and muscovite at ca. 210 Ma.
The CHIME monazite age of 180.1±2.5 Ma for the pegmatite vein cutting the
Mizunashi Granite is correlative with the 188.9±4.4 Ma Rb-Sr whole-rock isochron age of the Funatsu granitic rocks (Shibata and Nozawa, 1984). Probably42

I. H. Khan et al.

the pegmatite vein had been derived from the late stage residual of the Funatsu granitic rocks.
CONCLUDING REMARKSThe CHIME ages of monazite have revealed that the Hida Gneiss in the
Amo area originated from late Paleozoic sediments and attained at the lower
amphibolite facies grade in late Permian time (ca. 250 Ma). The peakm e t a m o r p h i s m was possibly contemporaneous with the emplacement of the
Mizunashi Granite at ca. 237 Ma. Taking available K-Ar and Rb-Sr mineral ages
into consideration, we can conclude that both the gneisses and granite cooled
to blocking temperature of hornblende and muscovite at ca. 210 Ma. The Hida
Gneiss and the Mizunashi Granite subsequently underwent the intrusion of
pegmatite at ca. 180 Ma. The previous view that Hida metamorphic rocks
formed in Precambrian time is no longer valid.
ACKNOWLEDGEMENTS
We thank Mr. S. Yogo of Nagoya University for his technical assistance andanonymous reviewers for their constructive comments. One of the authors(I.H. Khan) wishes to express his profound thanks to Japan International Cooperation Agency for providing an opportunity for his research, and is inde bt e d t o J I C A a n d D e p a r t m e n t o f E a r t h a n d P l a n e t a r y S c i e n c e s , N a g o y a
University. The coordinating and arranging effort of Dr. T. Shirahase, JICA
Expert for Geoscience Laboratory and Dr. N. Fujii, Director of Applied Geology, Tamano Consultant Co. Ltd. are also thanked.
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