J. Earth Sci. Nagoya Univ., Vol. 38, 11 to 38, 1991

The Chemical Th-U-total Pb Isochron Ages of Zircon and Monazite
from the Gray Granite of the Hida Terrane, Japan
Kazuhiro SUZUKI and Mamoru ADACHI
Department of Earth Sciences, School of Science,
Nagoya University, Nagoya 464-01, Japan(Received November 6, 1991)

ABSTRACT
Zircon and monazite from thirteen "Gray Granite" samples in the central part of the
Hida terrane were analyzed precisely on an electron microprobe. The U-Th-Pb relation of
zircon was examined in terms of PbO and UO2* (UO2* means measured UO2 plus UO2equivalent of measured ThO 2), and that of monazite in terms of PbO and ThO 2*(measured ThO2 plus ThO2 equivalent of measured UO2). Most zircon grains from theGray Granite form euhedral prisms with concentric growth zones; only a few exhibit
rounded and/or overgrowth forms. Analytical data for clear portions of euhedral zircon
grains from individual samples are arrayed linearly on the PbO-UO2* diagram and form asingle isochron. The chemical Th-U-total Pb isochron ages of zircon are in the range
from 230±10 to 250±20 Ma. Several zircon grains have rounded cores of Precambrianage (as old as 1270 Ma), suggesting contributions of recycled older materials to theformation of the Gray Granite. Monazite, recognized only in one sample, gives achemical Th-U-total Pb isochron age of 230±10 Ma. These zircon and monazite ages
strikingly contrast to previously measured Rb-Sr whole-rock isochron ages of 506±37 Ma
for the Gray Granite (Shibata et al., 1989). Our microprobe observation of Sr- and Rbdistribution in granite samples discloses that the Gray Granite was open with respect tothese elements during chloritization of biotite and sericitization of plagioclase.
Consequently, the 506 Ma Rb-Sr isochron age as well as several Precambrian Rb-Sr
model ages for the Gray Granite appears to be incorrect. Rather, we believe that the Gray
Granite was emplaced into the Hida gneiss in early Triassic time (ca. 240 Ma).

INTRODUCTION
The Hida terrane occupying the northern part of Southwest Japan is one of preCretaceous terranes in the Japanese Islands (e.g. Ichikawa, 1990). It consists of a variety
of gneisses, schists, and granitoids (Suwa, 1990); which are unconformably overlain by
the Jurassic-Cretaceous Tetori Group. Although extensive field, petrological, and geochronological investigations have been done on these rocks, the history of the Hida
terrane is still unclear. The main point of arguments on the Hida terrane is whether or not
it is composed of true Precambrian metamorphic and igneous rocks. Up to now, only thefollowing four facts have received general consensus: (1) Among the granitoids, theFunatsu granitic rocks are the most widespread and were intruded in Jurassic time (ca.
180 Ma, Shibata and Nozawa, 1984), (2) Most of the metamorphic rocks are of the11Suzuki and Adachi

12

amphibolite facies and their radiometric ages concentrate around 240 Ma and 180 Ma(see the compilation of Ota and Itaya, 1989); no age data to indicate the Precambrianmetamorphism have been obtained, (3) The Unazuki schists distributed in the eastern
Hida terrane formed in Triassic time from upper Carboniferous to Permian sediments(Hiroi et al., 1978), and (4) A middle Precambrian 206Pb/207 Pb age of 1493 Ma (dated for
detrital grains of zircon in a gneiss sample from Amo, Ishizaka and Yamaguchi, 1969)shows that recycled Precambrian materials are present in some gneisses of the Hida
terrane.
In the central part of the Hida terrane the so-called Gray Granite occurs as veins anddikes cutting the Hida gneisses. These bodies are too small to be shown on the geologicalmap. The Gray Granite is characterized by bluish-tint K-feldspars which distinguish itfrom the Jurassic Funatsu granitic rocks. Sato et al. (1967) reported three preliminary
Rb-Sr model ages from the Gray Granite at Futatsuya, about 5 km north of Kamioka(Fig. 1); 680 Ma for biotite, 810 Ma for microcline and 1200 Ma for whole-rock.
Although these ages have never been confirmed, the age data coupled with the mode ofFig. 1.

Simplified geological map with sample localities of the Hida terrane in the central Japan(slightly modified from Kano, 1982).

The CHIME Ages of Zircon and Monazite from the Gray Granite

13

occurrence of the Gray Granite have supported the idea that the Hida gneisses are ofpolymetamorphosed Precambrian origin (e.g. Sato, 1968; Suwa, 1969; Suzuki, 1977).
Recently, Shibata and Nozawa (1986) reported ca. 1100 Ma and 700 Ma Rb-Sr model ages
for two varieties of Gray Granite at Kagasawa (Fig. 1).
Shibata et al. (1989) reported a
Rb-Sr whole-rock isochron age of 506±37 Ma for the Gray Granite in the Kubusu River
area (Man-nami River, Fig. 1).
They demonstrated that the isochron age indicates anupper limit for the time of the Hida metamorphism. However, since their Rb-Sr isotopic
data exhibit a significantly large scatter, we consider that the 506 Ma isochron age isinconclusive. To check the existence of the 506 Ma granitic veins and dikes as well as"Precambrian" Gray Granite by an alternative method, we have studied zircon andmonazite from thirteen Gray Granite samples on an electron microprobe.
Precisemicroprobe analyses of Th, U and Pb of zircon and monazite grains give us reliablegeochronological information (Suzuki et al., 1991; Suzuki and Adachi, 1991).

GEOLOGIC SETTING AND SAMPLE DESCRIPTION
The metamorphic rocks in the central part of the Hida terrane are composed mainlyof quartzo- feldspathic gneiss and crystalline limestone with subordinate amphibolite,
pelitic gneiss and lime-silicate gneiss. The Gray Granite occurs as well-defined veins anddikes, usually several cm to ca. 2 m thick, in the Hida gneisses (Plates I-III).
Thirteen Gray Granite samples were collected from five localities (Fig. 1), Futatsuya(36 ゜
22'18", 137゜
16'40": Sample Nos. 0701A and 0701B; 36゜
22'20", 137゜
16'41": 0702A
and 0702B), Kagasawa (36゜
26'8", 137゜
13'16" : Sample Nos. 1501A and 1502A;
36゜
26'25"N, 137 ゜
13'21"E: Sample No. 0401), Man-nami (36゜
23'46"N, 137゜
8'30"E:
Sample No. 0402), Man-nami River (a tributary of the Kubusu River, 36゜
25'44"N,137゜
9'28"E: Sample Nos. 0403A, 2804, 0403B and 0403C) and Amo (36゜
16'51"N,137゜
0'44"E: Sample No. 2801B).
Most samples are of medium-to coarse-grained granite, quartz monzonite and granodiorite, but 0702A and 0403B samples are of pegmatite and 0701A, 1501A and 0403C
samples are of pegmatitic granite to quartz monzonite. All samples consist mainly ofquartz, cryptoperthite-perthite, plagioclase (mainly oligoclase in composition) and biotite
with or without muscovite. Hornblende is rare. Plagioclase is occasionally sericitizedand biotite, except for that contained in quartz is highly chloritized. Epidote and calcite
are not uncommon especially in highly sericitized samples. Accessories include zircon,
allanite, apatite, titanite and garnet. One sample (No. 0702B) contains thorite, and 1501B
sample contains monazite but lacks titanite.

EXPERIMENTAL METHODS
Zircon and monazite for the present study were separated from samples of 2-3 kg.
The size reduction of samples was undertaken on a stamp mill only to the extent necessary for disaggregation of zircon and monazite through periodic sieving to remove undersized (minus 80 mesh) particles before further reduction of oversized ones. From each

Suzuki and Adachi

14

sieved sample, we concentrated zircon, monazite and other heavy minerals with a pan.
The concentrates were washed with dilute HCl and purified on an isodynamic magnetic
separator. Zircon and monazite grains were hand-picked, mounted on a glass slide withepoxy resin, and polished with diamond paste until the grains were thinned approximately
to half in thickness.
Zircon and monazite were analyzed on a JEOL JXA-5A electron microprobe
equipped with three wavelength dispersive-type spectrometers. The operating conditions
were 15 kV accelerating voltage, 0.02-0.15 µA probe current and 5 µm probe diameter.
Analyzing crystals were TAP, PET and LiF crystals. The comparison standards were
euxenite provided by Smellie et al. (1978) for Th and U, synthesized glass (56.17 % PbO,
13.65 % ZnO and 30.18 % SiO2, analyst: H. Haramura, University of Tokyo) for Pb, synthesized glasses of Drake and Weill (1972) for Y and rare earths, and natural minerals

Fig. 2.

X-ray emission spectra around the ThMα, UMβ and PbMα regions obtained through thepulse-height discriminator from spot 2 of Z03 zircon grain in 0403A sample. The ordinate
represents X-ray intensity (cps) and the scale of abscissa represents the X-ray detection
position (mm) of the JEOL spectrometer. The ThMα intensity is ca. 25 cps for 0.277 wt. %ThO2 on a background intensity of ca. 24 cps, the UMβ intensity is ca. 150 cps for 2.05
wt. % UO2 on a 25 cps background, the and the PbMα intensity is ca. 5 cps for 680 ppm
PbO on a 9 cps background.

The CHIME Ages of Zircon and Monazite from the Gray Granite

15

and synthesized oxides for other elements. The M-lines were used in the Th, U, Pb
analysis; they give satisfactory intensities (Fig. 2). Spectral interferences including YLγ
on PbMα were corrected through the procedure described by Amli and Griffin (1975).
X-ray intensities of ThMα, UMβ and PbMα were integrated for 200 or 400 seconds. The
measurement was repeated twice or three times, and the arithmetic average was taken.
Raw intensity data were corrected for drift, background, dead time, atomic number,absorption and fluorescence. The detection limits of PbO at 2σ confidence level is 0.0050.008 wt.%, and the relative error in the determination is about 5% for 0.1 wt.% and10-15% for 0.02 wt.% of the concentration.
Age was calculated through the chemical Th-U-total Pb isochron method (CHIME;
Suzuki et al., 1991; Suzuki and Adachi, 1991).
Details of CHIME are shown inAppendix 1 for convenience.

RESULTSThe UO2, ThO2 and PbO analytical data and apparent ages are given in Appendix 2,
and the PbO-UO2* (ThO2*) plots are shown in Figs. 3 to 11.
0701A: Futatsuya
This sample was collected from a ca. 10 cm thick vein of pegmatitic granite exposed
along the Takahara River at Futatsuya (Plate I). The vein cuts subconcordantly thesurrounding biotite gneiss. Zircon grains form euhedral prisms of 0.14-0.38 mm in lengthand 0.04-0.12 mm in width, and show concentric growth zoning (Plate IV-A). Somegrains, however, exhibit rounded forms (Plate IV-B). Most zircon grains show low UO2and PbO contents, but several grains contain measurable amounts of UO2 and PbO,
0.3-1.0 and 0.010-0.044 %, respectively. The ThO2 contents are usually smaller than
0.1 %, but attain 2.4 % at rims of U-rich grains. The data points (solid circles) for core
portions of euhedral grains are arrayed linearly in the PbO-UO2* diagram (Fig. 3). These
data points yield an isochron (MSWD=0.29) of 240±20 Ma with an intercept value of
0.0001±0.0016 (errors quoted in this paper for ages and intercept values are of 2σ ). Twodata points (open circles) for Th-rich rims do not show the same tendency as those forcores, but are plotted in an area below the 240 Ma isochron.
0701B: Futatsuya
This sample was collected from another vein that cuts nearly perpendicularly thebiotite gneiss and the above-mentioned pegmatitic vein in the same outcrop as 0701A
sample (Plate I). This vein, ca. 5 cm in width, consists of fine-grained quartz monzonite.
Zircon occurs as euhedral prisms of 0.09-0.35 mm in length. Several grains, however,show rounded to subrounded forms. The ThO2 contents rise up to 0.92 % and the UO2contents up to 1.04 %. Data points (solid triangles) for euhedral grains give an isochron of
250±20 Ma (MSWD=0.25) with an intercept value of -0.0003±0.0014 (Fig. 3). Three data
points (open triangles) for rims of euhedral grains are plotted below the 250 Ma isochron,
and the data point (square) for Z23 rounded grain is plotted above the isochron.

Suzuki and Adachi

16

Fig. 3.

Plot of PbO vs. UO2* for zircon grains in 0701A and 0701B samples from Futatsuya.
0701A: solid circles represent data points for transparent portions of euhedral grains, andopen circles represent data points for Th-rich rims. 0701B: solid triangles represent data
points for transparent portions of euhedral grains, open triangles represent for rims ofeuhedral grain, and square for the core of a rounded zircon grain. Error bars in the figure
represent 2σ analytical uncertainty, and errors referred for the age, slope (m) and intercept(b) are of 2σ .

0702A: Futatsuya
This sample was collected from a pegmatitic part of a ca. 20 cm thick vein at
Futatsuya, about 60 m downstream from the outcrop for 0701A and 0701B samples.
Zircon forms euhedral elongated crystals (0.31-0.43 mm in length and 0.08-0.16 mm inwidth), and displays concentric growth zoning. The ThO2 contents of analyzed grains
range from 0.16 to 5.77 %, the UO2 contents from 0.25 to 2.44 %, and the PbO contentsfrom 0.010 to 0.136 %. Most data points (solid circles) are arrayed linearly on the PbOUO2 diagram (Fig. 4), and give an isochron age of 240±10 Ma (MSWD=0.22) with an intercept value of -0.0001±0.0010. Three data points (open circles) for rims of Z03 and Z16
grains are plotted below the 240 Ma isochron, and one data point (square) for Z17
subrounded grain is plotted above the isochron.

The CHIME Ages of Zircon and Monazite from the Gray Granite

Fig. 4.

17

Plot of PbO vs. UO2* for zircon grains in 0702A sample from Futatsuya. Solid circles
represent data points for transparent portions of euhedral grains, open circles represent forrims of Z03 and Z16 euhedral grain, and square for the core of Z17 rounded zircon grain.
Explanation for errors is the same as Fig. 3.

0702B: Futatsuya
This sample was collected from a network vein that agmatized the hornblendebearing biotite gneiss in the same outcrop as 0702A sample (Plate II). Unlike the other
Gray Granite samples, this sample contains thorite and hornblende. Most zircon grains, 0.170.36 mm in length and 0.08-0.14 mm in width, exhibit euhedral form with concentric
growth zoning, and some grains show a distinct core-overgrowth relation. The cores ofcore-overgrowth grains are well-rounded (Plate IV-C) to subrounded (Plates V-A and
V-B), suggesting abrasion during transportation and/or metamorphic corrosion. The overgrown rims are euhedral, show concentric zoning as the result of magmatic crystallization,and have the same optical orientation as that for cores. The rounded detrital cores arehigher in the Th and U contents than the overgrown rims; the core (Z03-01) of Z03 grain, forexample, shows 31.9 % SiO2, 61.5 % ZrO2, 1.92 HfO2, 1.53 % ThO2, 1.25 % UO2, 1.51 %Y2O3, 0.031 % Yb2O3, 0.303 % PbO and 0.339 % P2O5, whereas the rim (Z03-03) shows
32.5 % SiO2, 64.6 % ZrO2, 2.04 % HfO2, 0.168 % ThO2, 0.376 % UO2, 0.192 % Y2O3,
0.009 % PbO and 0.095 % P2O5. The chemical composition of the overgrown rims does
not differ from that of the euhedral grains. Analytical data of zircon are summarized inSuzuki and Adachi

18

Fig. 5. Most data points (solid circles) for euhedral grains and mantles of heterogeneous
core-overgrowth grains give an isochron of 240±10 Ma (MSWD=0.82) and an intercept
value of -0.0002±0.0014. Analytical data for cores of c ore-overgrowth grains Z03(solid squares), Z16 (crosses) and Z14 (triangles) yield isochrons of 1270±80 Ma, 500±50
Ma and 340±20 Ma, respectively. Data points (open squares) for the cores of other coreovergrowth grains fall around the 500 and 340 Ma isochrons.

Fig. 5.

Plot of PbO vs. UO2* for zircon grains and cross sections of zircon grains Z03 and 16
showing analyzed spots in 0702B sample from Futatsuya. Solid circles represent data point
for portions with concentric growth zoning, open circles for rims of euhedral zircon grains,open squares for cores of zircon grains with distinct core-overgrowth relations. Data points
for cores of Z03 (solid squares) and Z16 (crosses) core-overgrowth grains are labeled, andthose for the core of Z14 core-overgrowth grain are marked with triangles. Explanation forerrors is the same as Fig.3.

1501A: Kagasawa
This sample was collected from the pegmatitic part of a ca. 50 cm thick network-veinin the hornblende-bearing biotite gneiss (Plate III). Shibata and Nozawa (1986) described
this vein-rock as coarse-grained granite, carried out three Rb-Sr whole-rock analyses,
and obtained 1090-1130 Ma Rb-Sr model ages taking an initial 87Sr/86Sr ratio of 0.704.
Zircon grains form euhedral prisms of 0.20-0.40 mm in length and 0.11-0.17 mm in

The CHIME Ages of Zircon and Monazite from the Gray Granite

19

width, and have concentric growth zones. They contain 0.02-4.8 % ThO2 and 0.2-5.2 %UO2. The PbO content varies from less than the detection limit to 0.19 %. Analytical
data (solid circles) for transparent portions of euhedral crystals define an isochron(MSWD= 0.32) of 240±10 Ma with an intercept value of 0.0005±0.0008 (Fig. 6). Three
data points (open circles) for metamict rims are plotted below the 240 Ma isochron.

Fig. 6.

Plot of PbO vs. UO2* for zircon grains in 1501A and 1501B samples from Kagasawa.
1501A: solid circles represent data points taken for core portions of euhedral grains, andopen circles represent data points for rims. 1501B: microprobe analyses were carried out
only for transparent portions of euhedral grains (solid triangles). Explanation for errors isthe same as Fig. 3.

1501B: Kagasawa
This sample was collected from another network vein in the same outcrop as 1501A
sample. Shibata and Nozawa (1986) described this vein-rock as medium-grained granodiorite. On the basis of six Rb-Sr whole-rock analyses, they obtained 590-810 Ma model
ages taking an initial 87 Sr/ 86Sr ratio of 0.704. Zircon grains are euhedral, 0.14-0.27 mm
in length, and 0.06-0.12 mm in width. Monazite forms euhedral to subhedral crystals of
0.09-0.18 mm in size. Most zircon grains contain less than 0.1 % UO2, but several grains
contain 0.5-1.1 % UO2. Six analytical data from three U-rich zircon grains give anisochron (MSWD= 0.01) of 230±10 Ma with an intercept value of 0.0006±0.0006 (solidSuzuki and Adachi

20

triangles, Fig. 6). Monazite grains contain 4.9-15.7 wt.% ThO2, 0.16-0.34 wt.% UO2 and0.055-0.0169 wt.% PbO, and give an isochron (MSWD= 0.82) of 230±10 Ma with an intercept value of 0.0012±0.0056 (Fig. 7).

Fig. 7.

Plot of PbO vs. ThO2* for monazite grains in 1501B sample from Kagasawa. Explanationfor errors is the same as for Fig. 3.

0401: Kagasawa
Zircon forms euhedral prisms with concentric growth zoning; the length hardly
exceeds 0.25 mm, and the length to width ratio ranges from 1.7 to 3.3. Neither overgrowth nor outgrowth can be observed. Most zircon grains show low UO2 and PbO
contents, but 6 grains contain measurable amounts of UO2 and PbO, 0.50-2.18 and0.018-0.059%, respectively. The ThO2 content ranges from 0.09 and 0.64% in cores, butrises higher than 1.1 % in rims. Data points (solid circles) for transparent portions ofeuhedral grains yield an isochron (MSWD= 0.38) of 250±30 Ma with an intercept value of-0.0004±0.0046 (Fig. 8). The data points (open circles) for Th-rich rims are plotted in anarea below the 250 Ma isochron. Regression of these data points yields an isochron(MSWD= 0.12) of 150±30 Ma with an intercept value of 0.0047±0.0064.

The CHIME Ages of Zircon and Monazite from the Gray Granite

Fig. 8.

21

Plot of PbO vs. UO2* for zircon grains in 0401 sample from Kagasawa and 0402 sample
from Man-nami.
0401: solid circles represent data points for transparent portions ofeuhedral grains, and open circles for Th-rich rims. 0402: solid triangles represent data
points for transparent core portions of 5 euhedral grains, and crosses represent those forrounded Z05 zircon grain. Explanation for errors is the same as Fig. 3.

0402: Man-nami
Zircon grains form elongated euhedral prisms of 0.24-0.45 mm in length and 0.110.17 mm in width. Their UO2 contents rise up to 0.75 % and the ThO2/UO2 ratios range
between 0.11 and 0.57. Most data points (solid triangles) define a 250±20 Ma isochron(MSWD= 0.04) with an intercept value of -0.0007±0.0012 (Fig. 8). Data points (crosses) forZ05 rounded grain, plotted above of the 250 Ma isochron, yield an isochron of 470±40
Ma (MSWD= 0.05) with an intercept value of -0.0001±0.0014.
0403A: Man-nami River
This sample was collected from the same dike as No. 3 sample of Shibata et al.
(1990). Most zircon forms euhedral prisms of 0.2-0.45 mm in length, and no roundedgrain can be seen. Overgrowth texture is seen in Z11 grain where a subrounded core ismantled by a thick rim with euhedral growth zoning. The UO2 contents range between
0.32 and 1.08 %, and the ThO2 contents range between 0.04 and 0.43 % with two
exceptionally high ThO2 values of 2.23 and 2.43% in Z10 and Z13 grains. Most

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analytical data (solid circles) are arrayed linearly on the PbO-UO2 coordinates (Fig. 9).
The best-fitting isochron (MSWD= 0.19) yields an age of 240±20 Ma with an intercept valueof 0.0007±0.0014. Data points (squares) for Z01 and Z07 grains and the core of Z11 coreovergrowth grain are plotted above the 240 Ma isochron. These five data points cannot
regress together, but give apparent ages of 410 - 600 Ma.

Fig. 9.

Plot of PbO vs. UO2* for zircon grains in 0403A and 2804 samples from Man-nami River.
0403A: solid circles represent data points for transparent portions of euhedral grains, open
circle for Z10 grain with a high ThO2 content, and squares data points for the rounded coreof Z11 grain and for rounded Z01 and Z07 grains. 2804: microprobe analyses were carriedout only for transparent portions of 6 euhedral grains (solid triangles). Explanation forerrors is the same as for Fig. 3.

2804: Man-nami River
This sample was collected from the same dike as 0403A sample. Euhedral zircon
grains have concentric growth zones, and their length hardly exceeds 0.2 mm. Only six
grains were available for the microprobe analysis. The analytical data are plotted linearlyon the PbO-UO2 diagram (solid triangles, Fig. 9), and give an isochron (MSWD= 0.92) of
240±20 Ma with an intercept value of 0.0024±0.0040.

The CHIME Ages of Zircon and Monazite from the Gray Granite

23

0403B: Man-nami River
This sample was collected from a subconcordant dike intruded into the biotite gneissat a road cut, ca. 120 m northeast of 0403A locality. It probably corresponds to No. 2
sample of Shibata et al. (1989). Zircon occurs as euhedral prisms of 0.15-0.30 mm inlength. No rounded grains can be seen in this sample, but some show abrasion at theedges. The UO2 contents are usually less than 5 %, but some grains contain as much as
13.7 % UO2. The ThO2 contents range between 0.02 and 1.35 %. Although data points forrims and metamict portions (open circles) are highly scattered, those for clear portions(solid circles) are arrayed linearly on the PbO-UO2* diagram (Fig. 10). The best-fitting
isochron (MSWD= 0.44) gives an age of 240 ± 10 Ma and an intercept value of
0.0009±0.0019. The Z10 grain with abraded edges (square) gives an apparent age of 670 Ma.

Fig. 10. Plot of PbO vs. UO2* for zircon grains in 0403B and 0403C samples from Man-nami River.
0403B: solid circles represent data points for transparent portions of euhedral grains, open
circles for rims and metamict portions, and square subrounded Z10 grain. 0403C: solid
triangles represent data points for transparent and translucent portions of euhedral grains,open triangle for a metamict rim, and crosses for the subrounded core of Z09 coreovergrowth grain. Explanation for errors is the same as for Fig. 3.

0403C: Man-nami River
This sample, collected from the central part of a ca. 1 m thick dike in the biotite
gneiss, probably corresponds to No. 10 sample of Shibata et al. (1989). Zircons form
euhedral prisms of 0.18 - 0.31 mm in length, and their length to width ratios range

24

Suzuki and Adachi

between 2.0 and 3.3.
Most grains are well faceted and show concentric growth zones.
Overgrowth is seen only in one grain (No. Z09), where a clear subrounded core ismantled by translucent rim with concentric growth zoning. Turbidity in the zircon isvariable from portion to portion within a single grain. The UO2 contents range from 0.57
to 24.6 %, and the ThO2 contents from 0.01 to 2.58 %. Analytical data (triangles) areplotted within a narrow band passing through the origin (Fig. 10). Data points for rims
and highly metamict portions are plotted outside of the area shown in Fig. 10, and tend to
shift toward the lower side within the band. Two data points plotted above the band
(crosses) are those for the rounded core of Z09 grain. All data listed in Appendix 2
define an isochron (MSWD= 2.52) of 220±10 Ma. If the data points (solid triangles on
Fig. 10) for transparent and translucent portions regress together, the MSWD decreases to1.07 and the isochron age is 230±10 Ma with an intercept value of 0.0011±0.0018.
2801B: Amo
Zircon forms well-faceted crystals of 0.2-0.4 mm in length. They have high length
to width ratios between 2.8 and 6.9. The UO2 contents rise up to 5.35 % and the ThO2
contents up to 6.90 %. The ThO2/UO2 ratios, ranging between 0.063 and 1.48, average
0.75. Most data points (solid circles, 28 of 29 analyses on 17 zircon grains) are arrayed

Fig. 11. Plot of PbO vs. UO2* for zircon grains in 2801B sample from Amo. Solid circles represent
data points for transparent and translucent portions of euhedral grains, and open circles
represent highly metamict spot 3 of Z01 and spot Z of 210 grains. Explanation as for Fig. 3.

The CHIME Ages of Zircon and Monazite from the Gray Granite

25

linearly (Fig. 11), and yield an isochron of 240±10 Ma with an intercept value of-0.0019±0.0024.

DISCUSSION
The chemical Th-U-total Pb isochron ages of euhedral zircon grains and monazite
grains concentrate at about 240 Ma. Ages as old as 1270 Ma are obtained only in therounded zircon grains and the rounded cores of core-overgrowth grains. Although the 240
Ma CHIME age for zircon and monazite coincides well with the age of main metamorphism in the Hida terrane (Ishizaka and Yamaguchi, 1969; Shibata et al., 1970), it issignificantly younger than the Rb-Sr whole-rock isochron age of 506ｱ37 Ma for the Gray
Granite (Shibata et al., 1989). This discrepancy between the CHIME age and the isotopic
age cannot be ascribed to error in the microprobe analysis. It might be thought that the
240 Ma age could result from the combined effects of partial Pb loss from restricted
portions of single grains and a heterogeneous distribution of the initial Pb. These effects,
if present, would displace the isochron significantly below or above the origin of the
PbO-UO2* (ThO2*) diagram.
Since the isochrons pass through the origin, such possibilities are excluded.
Some may consider that the Gray Granite originated 506 Ma ago and underwent total
recrystallization and/or rejuvenescence through a significant thermal event at 240 Ma.
Most 240 Ma zircons, however, show concentric growth zones attributable tocrystallization from viscous felsic melts (Speer, 1980). The intra-grain distributions of Th
and U coincide well with the growth zones. These suggest that ta otal loss of Pb fromzircon through the ca. 240 Ma thermal event is unlikely. Furthermore, the concordant
PbO-UO 2* relations (dated back to 1270 Ma) within some rounded cores of coreovergrowth crystals do not favor the possibility.
To obtain a better understanding of the cause for the discrepancy between the
CHIME and isotopic ages, we examined the distribution of Sr and Rb in the Gray Granite.
Figure 12 illustrates X-ray emission spectra around SrLα and RbLα for constituent
minerals in 0403-A sample. The figure clearly shows that Sr is concentrated in
K-feldspar, plagioclase, allanite and apatite; the Sr content of K-feldspar amounts to440-680 ppm, plagioclase 740-1050 ppm, allanite 910-1720 ppm, and apatite about
130 ppm. The Sr contents of biotite, muscovite, epidote and titanite are in less than the2σ detection limit (ca. 40 ppm). On the other hand, Rb concentrates mainly in biotite,
K-feldspar and muscovite; the Rb content of biotite is in the range between 220-890 ppm
with a maximum value of 1040 ppm on a least altered portion, K-feldspar 340-370 ppm
and muscovite about 420-470 ppm. The Rb content of plagioclase is less than 80 ppm.
Figure 13 illustrates a line profile of SrLα intensity across constituent minerals in H7
sample of Shibata et al. (1989).
The SrLα intensity rises extraordinarily on some
sericitized portions of plagioclase and some grain-boundaries. The maximum count rate
on sericitized portions of plagioclase is 2190 ppm Sr, and that on grain-boundaries 2570
ppm Sr. The sericitized portions of plagioclase showing 2190 ppm Sr contain 290 ppm
Rb, and the grain-boundaries with 2570 ppm Sr contain only 70 ppm Rb. Some large

26

Suzuki and Adachi

Fig. 12. X-ray spectra in the SrLα and RbLα regions for biotite, K-feldspar, plagioclase, muscovite, allanite, apatite, epidote and titanite in 0403A sample. The ordinates represent X-ray
intensity (cps) and the scales of abscissas represent the X-ray detection position (mm) of
the JEOL spectrometer. The rise in count toward 219 mm represents the low-energy
envelope of SiKβ band. The high background intensity in the SrLα spectra of K-feldspar
and plagioclase comes from the high concentration of SiO2.

flakes (0.02 - 0.04 mm long) of secondary sericite in plagioclase contain 110 - 180 ppm
Rb.
Since biotite has been mostly chloritized in entire samples of the Gray Granite, Rb in
biotite, probably a significant portion of the whole-rock Rb, has been lost through thealteration process.
Sericitization of plagioclase is also a common feature in the Gray
Granite. Apart from the question of what phase has concentrated Sr, these alteredportions apparently gained a certain amount of Sr. The microprobe examination showsthat the Gray Granite was open with respect to Rb and Sr during the alteration. As noted
previously, the whole-rock Rb-Sr isotopic data presented by Shibata et al. (1989) arehighly scattered. This may result, at least in part, from the heterogeneous disturbance of
Rb and Sr, as recognized here. Presumably the 506 Ma Rb-Sr whole-rock isochron age
does not represent any significant episode. Instead, it seems much more likely that theThe CHIME Ages of Zircon and Monazite from te Gray Granite

27

Gray Granite was emplaced about 240 Ma ago with new growth of zircons. Furthermore,
the zircon and monazite ages of ca. 240 Ma correspond well with the 231±11 Ma K-Ar
hornblende age determined on a Gray Granite sample from Amo (Ota and Itaya,
unpublished data appeared in Ota and Itaya, 1989).
Some of the Gray Granite contains rounded zircon both as discrete grains and coresof core-overgrowth grains. Because these rounded grains give varying CHIME ages from
340 to 1270 Ma, they should be of inherited origin. The presence of such inherited zircon
grains indicates that recycled older materials contributed to the generation and/or
evolution of the Gray Granite magma(s). The contribution of recycled materials also
accounts for the significant intra-sample scatter of whole-rock Rb-Sr data for the Gray
Granite. It seems, then, that the Precambrian Rb-Sr whole-rock model ages reported bySato et al. (1967) and Shibata and Nozawa (1986) come probably from a higher
proportion of the recycled Precambrian materials, such as the 1493 Ma rounded zircon
population (Ishizaka and Yamaguchi, 1969). Obviously these Precambrian detritus
cannot be considered indicative of the Precambrian origin of the Hida gneiss.

Fig. 13. Line profile of X-ray intensity (SrLα ) across constituent minerals (K: K-feldspar, P:
plagioclase, Q: quartz, B: biotite) in H7 sample of Shibata et al. (1989). The X-ray
intensity includes the SrLα characteristic line, interferences from SiKβ and backgrounds.
The count rate rises on the sericitized portion of plagioclase at left and the grain-boundaries
between quartz grains and between quartz and plagioclase grains at central right.

CONCLUDING REMARKSThe chemical Th-U-total Pb isochron ages of zircon and monazite have revealed
that the Gray Granite originated in Triassic time (ca. 240 Ma), but evidently not in28

Suzuki and Adachi

Precambrian time. Since the Gray Granite was open with respect to Rb and Sr duringchloritization of biotite and sericitization of plagioclase, the Rb-Sr whole-rock isochron
age of 506 Ma by Shibata et al. (1989) is incorrect. Consequently, the previous view that
the Gray Granite is of Precambrian origin and that the Hida gneisses cut by the Gray
Granite were therefore formed in Precambrian time, is no longer valid. Age determinations of zircon and monazite from the Hida gneisses are essential to a correct understanding of the origin and development of the Hida gneiss complex.
ACKNOWLEDGEMENTS
We thank Drs Y. Tsuzuki and T. Tanaka for their helpful discussion, Dr. H. Shimizu
for his careful review, and Mr. K. Ota for providing samples 2801B and 2804. This work
was supported in part by a Grant- in-Aid for Fundamental Scientific Research (Nos.
63460053 and 03452068) from the Ministry of Education, Science and Culture, Japan.
REFERENCES
AMLI, R. and W. GRIFFIN (1975) Microprobe analysis of REE minerals using empirical correction
factors. Am. Mineral., vol. 60, 599 - 606.
DRAKE, M.J. and D.F. WEILL (1972) New rare earth element standards for electron microprobe
analysis. Chem. Geol., vol. 10, 179 - 181.
HIROI, Y., N. FUJI and Y. OKIMURA (1978) New fossil discovery from the Hida metamorphic
rocks in the Unazuki area, central Japan. Proc. Japan Acad., vol. 64, B, 268 - 271.
ICHIKAWA, K (1990) Pre-Cretaceous terranes of Japan. In Pre-Cretaceous terranes of Japan,
edited by K. Ichikawa, S.Mizutani, I. Hara, S. Hada and A. Yao, Publication of IGCP Project
224, 1 - 11.
ISHIZAKA, K. and M. YAMAGUCHI (1969) U-Th-Pb ages of sphene and zircon from the Hida
metamorphic terrain, Japan. Earth Planet. Sci. Lett., vol. 6, 179 - 185.
KANO, T. (1982) Tectonic divisions and their development of the Hida metamorphic region, central
Japan. Mem. Geol. Soc. Japan, No. 21, 9 - 24.
OTA, K. and T. ITAYA (1989) Radiometric ages of granitic and metamorphic rocks in the Hida
metamorphic belt, central Japan. Bull. Hiruzen Res. Inst., Okayama University of Science,
No. 15, 1 - 25.
SATO, S. (1968) Precambrian - Variscan polymetamorphism in the Hida massif, basement of theJapanese Islands. Sci. Rept. Tokyo Univ. Educ., Sec. C, vol. 10, 15 - 129.
SATO, S., T. SHIRAHASE and H. SHIBATA (1967) Older granite based on the Rb-Sr dating in the Hida
massif. Jour. Geol. Soc. Japan, vol. 73, 72.
SHIBATA, K., T. KANO and M. ASANO (1989) Isotopic ages of the Gray granite from the upper
Kubusu River area, Hida Mountains. Jour. Min. Pet. Econ. Geol., vol. 84, 243 - 251.
SHIBATA, K. and T. NOZAWA (1984) Isotopic ages of the Funatsu Granitic Rocks. Jour. Japan Assoc.
Min. Pet. Econ. Geol., vol. 79, 289 - 298.
SHIBATA, K. and T. NOZAWA (1986) Late Precambrian ages for granitic rocks intruding the Hida
Metamorphic Rocks. Bull. Geol. Surv. Japan, vol. 36, 43 - 51.
SHIBATA, K., T. NOZAWA and R.K. WANLESS (1970) Rb-Sr geochronology of the Hida metamorphic
belt, Japan. Can. Jour. Earth Sic., vol. 7, 1383 - 1401.
SMELLIE, J.A.T., N. COGGER and J. HERRINGTON (1978) Standards for quantitative microprobe
determination of uranium and thorium with additional information on the chemical formulaeof davidite and euxenite-polycrase. Chem. Geol., vol. 22, 1 - 10.

The CHIME Ages of Zircon and Monazite from the Gray Granite

29

SPEER, J.A. (1980) Zircon. In: Orthosilicates (Review in Mineralogy 5), edited by P.H. Ribbe,
Mineralogical Society of America, 67 - 112.
..
STEIGER, R.H. and E. JAGER (1977) Subcommission on geochronology: convention on the use of
decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett., vol. 36, 359 - 362.
SUWA, K. (1969) Polymetamorphism in the Hida metamorphic complex. Mem. Geol. Soc. Japan,,No. 4 113 - 116.
SUWA, K. (1990) Hida-Oki terrane. In: Pre-Cretaceous terranes of Japan, edited by K. Ichikawa, S.
Mizutani, I. Hara, S. Hada and A. Yao, Publication of IGCP Project 224, 13 - 24.
SUZUKI, K. and M. ADACHI (1991) Precambrian provenance and Silurian metamorphism of the
Tsubonosawa paragneiss in the South Kitakami terrane, Northeast Japan, revealed by theTh-U-total Pb chemical isochron ages of monazite, zircon and xenotime. Geochemical J., vol.
25, 357 - 376.
SUZUKI, K., M. ADACHI and T. TANAKA (1991) Middle Precambrian provenance of Jurassic
sandstone in the Mino terrane, central Japan: evidence from an electron microprobe Th-Utotal Pb monazite study. Sediment. Geol., vol. 75, 141 - 147.
SUZUKI, M. (1977) Polymetamorphism in the Hida Metamorphic Belt, central Japan. Jour. Sci.
Hiroshima Univ., vol. 7, 217 - 296.
YORK, D. (1966) Least-squares fitting of a straight line. Can. J. Phy., vol. 44, 1079 - 1088.

The CHIME Ages of Zircon and Monazite from the Gray Granite

30

Appendix 1: Outline of the chemical Th-U-total Pb isochron method (CHIME).
Zircon and monazite contain both U and Th. Natural Th consists of only aradioactive isotope of 232 Th, and natural U consists of two radioactive isotopes, 235 U and238
U. These radioactive isotopes decay into Pb:
232

Th

235

U
U

238

(a)

6 alphas + 4 betas + 208Pb
7 alphas + 4 betas + 207Pb
8 alphas + 6 betas + 206Pb

(b)
(c)

The number of atoms of 208 Pb, 207 Pb and206 Pb which accumulate
terms of the present-day amounts of 232 Th, 235 U and 238 U;

in a time τ is given in

208

Pb = 232 Th {exp(λ232τ) - 1}

(d)

207

Pb = U {exp(λ235τ) - 1}
Pb = 238 U {exp(λ238τ) - 1}

(e)
(f)

206

235

-11

where λ symbolizes the decay constant of each isotope; λ232= 4.9475x10 /y,..
9.8485x10 -10 /y and λ238=1.55125x10 -10/y (Steiger and Jager, 1977).

λ235=

・・

Uranium- and Thorium-bearing zircon and monazite contain initial (common) Pb as
well as radiogenic one
Total Pb = Pbinitial + 208Pb + 207Pb + 206Pb= Pbinitial + 232 Th{exp(λ232τ) -1}+ 235 U{exp(λ235τ) - 1}
+ 238 U{exp(λ238τ) - 1}

(g)

..

Since the 235U/ 238 U ratio is 137.88 (Steiger and Jager, 1977),

Eq. (g) can be written:

Total Pb = Pbinitial+ 232Th {exp(λ232τ) - 1}τ) + exp(λ τ)- 1}{ 137.88 exp(λ138.88

+ U

238

235

(h)

The chemical Th-U-total Pb isochron method (CHIME; Suzuki et al. 1991; Suzuki
and Adachi, 1991) is as follows. We obtain, in the first step, an apparent age (t) from each
set of the UO2, ThO2 and PbO concentrations (wt.%) by solving the equation;
PbO
ThO2 {exp(l232t) - 1}
=
WTh
WPb
UO2 exp(λ235t) + 138exp(λ238t)
- 1
+
139
WU

{

}

(l)

Suzuki and Adachi

31

where W symbolizes the gram-molecular weight of each oxide. We assume WPb= 222(206+16) for U-rich zircon, WPb=224 (208+16) for Th-rich monazite, WTh=264(232+32), WU=270 (238+32) and 238 UO2/ 235 UO2=138. Taking the apparent age (t), weturn the sum of ThO2 and UO2 into ThO2* (monazite) or UO2* (zircon) by:
ThO2* = ThO2+.

UO2 WTh
WU{exp(λ232t) - 1}

UO2* = UO2+{ exp(λ t) +139138exp(λ t) - 1}
235

238

(2)

.

139 ThO2 WU {exp(λ232t) - 1}
(2‘)

WTh {exp(λ235t) + 138exp(λ238t) - 139 }If cogenetic mineral grains and/or individual parts of a single mineral grain contain thesame amounts of initial Pb but different amounts of Th and U, and have remained in aclosed system, all analytical data will be plotted on a straight line with the slope (m) andintercept (b),.
PbO = m . UO * + b

PbO = m T hO2* + b
2(monazite)(3)(zircon)(3‘)

We determine the best-fitting regression line through the procedure proposed by York
(1966), taking account of uncertainties in the microprobe analyses, and calculate the first
approximation of age (T) from the slope (m) of equations:

T=
m

1
WTh
ln 1 + m
λ232
WPb

(

WU
WPb

=

)

exp(λ235T) + 138exp(λ238T)
139

- 1

(monazite)(4)(zircon)(4‘)

Then, we can obtain the second approximation by replacing the apparent ages (t) of
equation (2 or 2') with the first approximation age (T), and so on. The intercept (b) of theline is assumed to represent the concentration (wt.%) of the initial PbO. Normally, asignificant amount of initial Pb or Pb-loss, if any, would deviate the line from the origin,or would not form an isochron.

32

The CHIME Ages of Zircon and Monazite from the Gray Granite

Appendix 2: Analytical data of zircon (Z), monazite (M), allanite (A) and thorite (T).
nd: not detected, C: core of core-overgrowth grain, M: mantle of core-overgrowth
grain, R: rim, r: rounded grain, m: metamict grain, UO2*: sum of measured UO2 andUO2 equivalent of measured ThO2 for zircon, and ThO2*: sum of measured ThO2 andThO2 equivalent of measured UO2 for others.
Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*

Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*(No. 0701A)

0.704 0.939 0.041
Z10-030.353 0.536 0.023
Z11-010.685 0.784 0.032
Z11-020.280 0.536 0.023
Z14
10.9
0.246 0.090
A15
0.248 0.680 0.024
Z16
0.197 0.313 0.013
Z17
Z18
0.143 0.196 nd
Z19
0.522 0.779 0.021
Z20
0.623 0.689 0.027
Z21
0.240 0.413 0.016
Z22
r 0.017 0.153 0.013
Z23
0.190 0.399 0.014
Z24
0.180 0.324 0.013
Z25
0.562 0.685 0.027
Z26
0.186 0.478 0.018
Z27-010.220 0.579 0.022
Z27-020.184 0.418 0.014
Z28
0.364 0.368 0.015
Z29-010.846 0.845 0.034
Z29-02 R 0.644 0.653 0.021
Z30
0.376 0.790 0.030
Z31-01 R 0.531 0.793 0.025
Z31-020.265 0.381 0.017
Z32
0.133 0.252 0.009
Z33
0.298 0.510 0.019
Z34-010.446 0.557 0.023
Z34-020.371 0.547 0.022
Z35
0.044 0.089 nd
Z36
0.215 0.347 0.013
Z37
0.024 0.041 nd
A38
10.9
0.246 0.090

Z03
Z04-01
Z04-02
Z04-03
Z04-04
Z07-01
Z07-02
Z07-03
Z07-04 R
Z08
Z09-01 R
Z09-02
Z12
Z13-01
Z13-02
Z13-03
Z14-01
Z14-02
Z14-030.011 0.043 nd
0.038 0.281 0.010
0.260 0.476 0.018
0.164 0.312 0.012
0.039 0.291 0.010
1.44 0.847 0.044
0.768 0.587 0.027
0.089 0.149 nd
1.25 0.679 0.028
0.073 0.168 nd
2.41 0.738 0.036
0.178 0.196 nd
1.17 0.984 0.044
0.341 0.339 0.014
0.345 0.305 0.014
0.280 0.283 0.013
0.169 0.177 nd
0.237 0.629 0.025
0.793 0.473 0.021

250
240
240
240
250
240
190
180
240
230
250
260
260
220

0.293
0.556
0.363
0.303
1.29
0.824
1.07
1.49
1.35
0.444
0.411
0.369
0.702
0.718

nd
0.040 nd0.723 0.761 0.032
0.255 0.445 0.017
0.251 0.419 0.017
nd0.039 nd0.924 1.04 0.045
0.280 0.459 0.017

240
240
250
250
230

0.984
0.524
0.496
1.32
0.545

(No. 0701B)
Z06
Z07-01
Z07-02
Z07-03Z08
Z10-01
Z10-02260 1.16
260 0.645
240 0.995
270 0.622
180 11.7
230 0.757
260 0.374
170 0.941
230 0.881
240 0.487
590 0.158
230 0.458
250 0.380
230 0.859
250 0.535
250 0.647
220 0.475
230 0.480
230 1.11
180 0.853
240 0.906
190 0.958
270 0.463
230 0.293
230 0.602
240 0.695
250 0.661
230 0.413
180 11.7

Suzuki and Adachi

33

Appendix 2 (cont’d)

Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*

Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*(No. 0702A)

Z05-01 M 0.643 0.825 0.035
Z05-02 M 1.75 1.29 0.053
Z05-03 C 1.03 1.55 0.091
Z05-04 C 1.08 1.13 0.100
Z05-05 C 1.87 2.35 0.136
Z05-06 C 0.511 0.537 0.040
Z05-07 C 1.05 0.825 0.073
Z05-08 C 1.59 1.71 0.127
Z05-09 C 1.46 1.49 0.086
Z05-10 C 1.64 1.38 0.177
Z05-11 C 0.669 0.816 0.075
Z05-12 C 1.50 1.69 0.160
Z08 R
1.16 1.67 0.051
Z09-01 r
1.46 1.37 0.082
Z09-02 r
2.83 1.80 0.133
Z10
0.145 0.267 0.010
Z11
0.860 1.06 0.043
T12-0168.7 7.10 0.271
T12-0268.4 9.40 0.277
Z14-01 C 1.51 1.26 0.079
Z14-02 C 0.512 0.750 0.041
Z14-03 C 0.454 0.582 0.035
Z14-04 C 1.45 1.96 0.111
Z14-05 C 0.464 0.916 0.048
Z14-06 M 0.580 1.28 0.056
Z14-07 M 0.703 0.847 0.031
Z14-08 M 0.222 0.447 0.015
Z14-09 M 0.216 0.459 0.017
Z14-10 M 0.408 0.778 0.029
Z14-11 M 0.084 0.249 0.009
Z14-12 R 0.111 0.303 0.008
Z14-13 R 0.106 0.333 0.009
Z14-14 M 0.013 0.375 0.012
Z14-15 R 0.426 0.482 0.017
Z14-16 M 0.899 2.11 0.083
Z14-17 M 0.132 0.245 0.010
Z14-18 M 0.733 1.33 0.056
Z14-19 M 2.52 1.24 0.057
Z15-01 r
0.172 0.446 0.024

Z01-01
Z01-02
Z03-01
Z03-02
Z04
Z05
Z06
Z07
Z08
Z09-01
Z09-02
Z10-01
Z10-02
Z11-01
Z11-02
Z12
Z15
Z16
Z17
Z18

5.77 2.44 0.136
1.31 1.09 0.046
0.242 0.542 0.020
2.98 0.963 0.047
0.386 0.463 0.017
0.155 0.248 0.010
0.347 0.330 0.014
0.429 0.550 0.022
0.418 0.433 0.019
0.333 0.601 0.024
0.846 1.35 0.052
0.255 0.576 0.022
0.084 0.725 0.024
2.81 1.20 0.071
1.90 1.33 0.064
2.19 1.24 0.051
0.955 0.751 0.034
2.28 1.64 0.052
1.85 1.37 0.096
0.500 1.14 0.042

240
230
240
180
220
250
240
240
250
250
240
250
240
250
250
200
240
160
360
240

4.22
1.49
0.617
1.89
0.582
0.296
0.437
0.682
0.562
0.704
1.61
0.655
0.751
2.07
1.92
1.92
1.05
2.35
1.94
1.29

0.020 0.039 nd
1.53 1.25 0.303 1190
0.669 0.709 0.157 1160
1.25 1.09 0.261 1200
0.735 0.710 0.161 1170
1.53 1.13 0.289 1230
1.81 1.28 0.312 1160
0.168 0.376 0.015 260
0.113 0.425 0.014 220
0.084 0.378 0.015 270
0.112 0.408 0.014 230
0.050 0.262 0.010 270
0.104 0.420 0.011 180

1.69
0.900
1.45
0.920
1.57
1.80
0.428
0.460
0.404
0.443
0.277
0.452

(No. 07002B)
Z01
Z03-01 C
Z03-02 C
Z03-03 C
Z03-04 C
Z03-05 C
Z03-06 C
Z03-07 M
Z04-01
Z04-02
Z04-03
Z04-04
Z04-05 R

250 1.02
210 1.83
360 1.87
500 1.46
340 2.92
420 0.693
460 1.14
420 2.19
320 1.94
670 1.87
530 1.02
530 2.14
190 2.03
330 1.82
360 2.67
240 0.312
240 1.33
70 91.4
70 98.5
330 1.72
330 0.907
350 0.721
340 2.40
330 1.06
280 1.46
220 1.06
220 0.516
240 0.526
240 0.904
240 0.275
180 0.337
180 0.366
230 0.379
200 0.614
260 2.39
260 0.286
270 1.56
210 2.02
350 0.499

The CHIME Ages of Zircon and Monazite from the Gray Granite

34

Appendix 2 (cont’d)
Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*
Z15-02 r
Z16-01 C
Z16-02 C
Z16-03 C
Z16-04 C
Z16-05 C
Z16-06 C
Z16-07 C
Z16-08 C
Z16-09 M
Z16-10 M
Z16-11 M
Z16-12 M
Z16-13 M
Z17
Z20-01
Z20-02 R
Z21-01
Z21-02
Z21-03Z22
Z23
Z24

0.173 0.268 0.015
1.34 1.84 0.152
1.46 1.62 0.138
1.50 1.68 0.140
2.49 2.95 0.268
1.85 2.73 0.214
2.08 2.30 0.205
2.17 1.89 0.159
0.484 0.819 0.064
0.173 0.462 0.018
0.092 0.251 0.010
0.177 0.483 0.018
0.171 0.471 0.017
0.269 0.588 0.023
0.998 0.773 0.034
0.035 0.433 0.015
0.933 1.62 0.045
0.370 0.335 0.015
0.179 0.424 0.016
0.186 0.366 0.013
0.193 0.362 0.013
0.136 0.315 0.011
0.426 0.770 0.026

340
490
480
470
520
470
500
450
480
260
260
250
240
250
230
250
170
250
250
230
230
230
210

0.321
2.25
2.06
2.13
3.70
3.29
2.93
2.55
0.966
0.515
0.279
0.538
0.524
0.671
1.08
0.444
1.91
0.449
0.479
0.423
0.422
0.357
0.902

230
230
270
280
260
240
240
260
260
250

0.602
0.846
0.653
0.531
0.364
0.525
1.53
1.43
0.451
1.41

(No. 15001A)
Z01
Z02-01
Z02-02
Z02-03
Z03-01
Z03-02Z04
Z05-01
Z05-02
Z05-03
Z05-040.018 0.236 nd
0.116 0.566 0.019
0.196 0.785 0.026
0.126 0.614 0.024
0.119 0.494 0.020
0.062 0.345 0.013
0.116 0.489 0.017
0.427 1.40 0.050
0.397 1.31 0.051
0.083 0.425 0.016
0.478 1.26 0.047

Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*
Z06
Z07
Z08-01
Z08-02
Z08-03
Z08-04
Z09
Z10
Z11-01
Z11-02
Z11-03 R
Z11-04
Z11-05
Z12-01
Z12-02
Z12-03 R
Z13
Z14
Z15
Z16-01
Z16-02 R
Z16-03
Z16-04
Z16-05
Z16-06
Z16-07
Z16-08
Z16-09Z17
Z18-01
Z18-02
Z18-03
Z19-01
Z19-02
Z19-03
Z19-04
Z19-05
Z19-06

0.073 0.455 0.016
0.118 0.459 0.017
0.273 0.772 0.030
0.229 0.719 0.026
0.222 0.670 0.024
0.208 0.639 0.026
0.067 0.491 0.018
0.072 0.348 0.012
1.30 1.87 0.072
0.291 0.790 0.027
4.81 2.90 0.098
1.27 1.77 0.073
0.358 0.655 0.026
0.209 0.974 0.034
0.183 0.876 0.030
0.269 1.29 0.032
0.083 0.328 0.011
0.140 0.493 0.018
0.107 0.398 0.015
1.96 5.24 0.192
0.306 1.73 0.047
0.053 0.425 0.015
0.236 1.42 0.045
0.281 1.15 0.044
1.05 2.05 0.082
0.576 1.34 0.052
0.064 0.464 0.016
0.064 0.692 0.023
0.114 0.516 0.020
0.532 1.28 0.049
0.193 0.528 0.019
0.590 1.38 0.050
0.608 0.710 0.031
0.168 0.545 0.020
0.197 0.692 0.024
0.387 1.02 0.039
1.40 2.17 0.086
0.565 1.21 0.045

250
250
260
240
240
270
260
240
230
230
170
250
250
240
240
170
230
250
260
240
190
250
220
260
250
250
240
240
270
250
240
240
250
250
240
250
240
240

0.478
0.495
0.856
0.790
0.739
0.703
0.512
0.370
2.27
0.880
4.39
2.16
0.765
1.04
0.932
1.37
0.354
0.536
0.431
5.84
1.82
0.441
1.49
1.24
2.37
1.52
0.484
0.712
0.551
1.44
0.588
1.56
0.898
0.597
0.753
1.14
2.60
1.38

Suzuki and Adachi

35

Appendix 2 (cont’d)

Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*

Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*(No. 15001B)(No. 0401)

Z01-01
Z01-02
Z02-01
Z02-02
Z02-03Z03
Z04
M01-01
M01-02
M01-03
M01-04
M01-05
M01-06
M02-01
M02-02
M02-03
M02-04M03
M04-01
M04-02
M05-01
M05-02
M05-03
M05-04
M06-01
M06-02
M06-03
M07-01
M07-02
M07-03
M07-04
M07-05
M07-06

0.109 0.507 0.018
0.220 0.962 0.033
0.070 0.406 0.014
0.323 1.12 0.039
0.138 0.573 0.020
0.023 0.491 0.016
0.029 0.081 nd
10.3
0.334 0.115
10.5
0.284 0.117
8.42 0.268 0.094
9.97 0.289 0.103
9.61 0.267 0.102
8.58 0.316 0.094
9.57 0.234 0.104
7.29 0.195 0.075
10.4 0.232 0.105
10.1
0.231 0.102
4.87 0.174 0.055
6.25 0.157 0.066
6.91 0.161 0.073
6.40 0.077 0.065
6.19 0.064 0.068
6.42 0.071 0.066
7.24 0.086 0.071
9.72 0.193 0.106
7.76 0.162 0.087
9.43 0.185 0.100
15.7 0.336 0.169
15.6 0.319 0.160
12.2 0.330 0.130
9.99 0.265 0.114
8.72 0.261 0.097
9.35 0.269 0.102

250
240
240
240
240
240
240
240
240
220
230
230
240
220
220
220
240
230
230
230
250
240
220
240
250
240
240
230
230
250
240
240

0.541
1.03
0.428
1.22
0.616
0.498
11.4
11.4
9.29
10.9
10.5
9.60
10.3
7.92
11.2
10.8
5.43
6.76
7.43
6.65
6.40
6.65
7.52
10.3
8.29
10.0
16.8
16.6
13.3
10.8
9.57
10.2

Z01
Z02-01
Z02-02
Z02-03
Z02-04 R
Z02-05 R
Z03-01
Z03-02
Z03-03
Z03-04 R
Z03-05 R
Z04
Z04
Z04

0.087 0.495 0.018
0.643 1.18 0.044
0.204 0.815 0.029
0.367 1.00 0.034
1.64 1.23 0.040
1.96 1.51 0.047
0.255 0.910 0.035
0.260 1.32 0.049
0.611 1.38 0.053
1.15 1.67 0.044
1.70 2.18 0.059
0.093 1.15 0.036
0.306 0.892 0.032
0.143 0.850 0.029

250
240
240
230
170
160
260
260
250
160
160
230
240
240

0.522
1.38
0.878
1.11
1.74
2.12
0.989
1.40
1.57
2.03
2.71
1.18
0.986
0.894

0.066 0.420 0.014
0.191 0.409 0.016
0.168 0.293 0.011
0.071 0.667 0.022
0.070 0.601 0.021
0.060 0.475 0.016
0.075 0.353 0.012
0.191 0.323 0.025
0.102 0.186 0.014
0.016 0.148 0.010
0.156 0.263 0.019
0.167 0.312 0.024
0.079 0.575 0.020
0.116 0.530 0.019
0.191 0.752 0.027

230
250
240
240
250
240
240
470
470
470
440
480
250
250
250

0.440
0.468
0.345
0.689
0.623
0.494
0.376
0.381
0.217
0.153
0.310
0.363
0.599
0.566
0.811

(No. 0402)
Z01
Z02
Z03
Z04-01
Z04-02
Z04-03
Z04-04
Z05-01 r
Z05-02 r
Z05-03 r
Z05-04 r
Z05-05 r
Z06-01
Z06-02
Z06-03

The CHIME Ages of Zircon and Monazite from the Gray Granite

36

Appendix 2 (cont’d)

Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*

Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*(No. 0403A)(No. 2804)

Z01-01 r
Z01-02 r
Z02-01
Z02-02
Z03-01
Z03-02
Z03-03
Z04-01
Z04-02
Z05-01
Z05-02
Z06
Z07 r
Z08
Z09
A10
Z11-01 C
Z11-02 C
Z11-03 M
Z11-04 M
Z12-01
Z12-02
Z13-01
Z13-02
Z13-03
Z14
Z15
Z16
Z17
Z19
Z20
Z21

0.185 0.388 0.026 420
0.112 0.324 0.020 410
0.126 0.423 0.016 260
0.091 0.351 0.013 250
0.132 0.476 0.017 240
0.176 0.428 0.016 240
0.187 0.462 0.017 240
0.144 0.593 0.019 220
0.216 0.522 0.021 260
0.294 0.807 0.030 250
0.433 0.888 0.035 250
0.252 0.648 0.024 240
0.146 0.499 0.031 410
0.130 0.556 0.021 260
0.101 0.350 0.013 250
2.43 0.567 0.031 170
0.201 0.942 0.085 600
0.163 0.964 0.074 520
0.076 0.414 0.013 220
0.083 0.468 0.015 220
0.105 0.522 0.017 230
0.177 0.566 0.021 250
2.23 0.966 0.052 230
0.214 0.952 0.036 260
0.037 0.669 0.022 240
0.110 0.558 0.020 250
0.113 0.532 0.019 250
0.359 1.08 0.038 240
0.198 0.834 0.028 230
0.134 0.589 0.021 250
0.111 0.447 0.016 250
0.199 0.538 0.021 260

0.444
0.358
0.462
0.379
0.517
0.482
0.520
0.638
0.589
0.898
1.02
0.726
0.543
0.596
0.381
4.26
1.00
1.01
0.437
0.494
0.554
0.621
1.65
1.02
0.680
0.592
0.567
1.19
0.895
0.630
0.481
0.599

Z01
Z02
Z03
Z04
Z05
Z06

0.173 0.565 0.021
0.260 1.28 0.049
0.381 3.39 0.113
0.238 0.607 0.024
0.134 1.33 0.049
0.423 1.83 0.060

250
260
240
260
260
230

0.618
1.36
3.51
0.680
1.37
1.96

(No. 0403B)
Z01
0.067 1.60 0.051
Z02-01 mR 0.148 2.50 0.063
Z02-020.119 1.29 0.043
Z02-030.139 2.11 0.059
Z02-040.095 1.29 0.043
Z03-010.395 3.98 0.138
Z03-020.277 2.05 0.068
Z04-010.078 1.21 0.039
Z04-020.190 1.87 0.058
Z04-030.507 9.06 0.294
Z04-040.812 3.07 0.103
A05-01
5.39 0.136 0.042
A05-02
17.2 0.546 0.110
A05-034.08 0.123 0.034
Z06-010.019 0.676 0.023
Z06-02
0.098 1.53 0.053
Z06-030.126 1.94 0.063
Z07-01 m 0.035 2.16 0.054
Z07-02 m 0.031 2.18 0.052
Z07-03 m 0.020 2.02 0.048
Z08-010.012 0.875 0.029
Z08-02 mR 0.196 3.51 0.091
Z08-03 mR 0.041 2.50 0.046

230 1.62
180 2.55
240 1.33
200 2.15
240 1.32
250 4.10
240 2.14
230 1.23
220 1.93
240 9.22
230 3.32
170 5.83
140 19.09
180 4.48
250 0.682
250 1.56
240 1.98
180 2.17
180 2.19
180 2.03
240 0.879
190 3.57
140 2.51

Suzuki and Adachi

37

Appendix 2 (cont’d)

Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*
Z08-04
Z09-01
Z09-02
Z09-03
Z10 r
Z11
Z12 m
Z13-01
Z13-02 R
Z14 m
Z15-01
Z15-02
Z16
Z17
Z18
Z19 R

0.014 1.18 0.038
0.148 3.20 0.098
0.022 1.12 0.039
0.040 0.664 0.022
0.092 1.49 0.143
0.144 4.29 0.121
0.554 13.7 0.387
0.025 1.71 0.053
0.051 2.64 0.066
0.030 4.90 0.063
0.039 2.19 0.075
0.058 2.17 0.072
0.013 0.587 0.020
0.172 2.46 0.084
0.689 1.91 0.067
1.35 2.58 0.069

240 1.18
220 3.25
250 1.13240 0.676
670 1.52
210 4.33
210 13.9
230 1.72
180 2.66
100 4.91
250 2.20
240 2.19
250 0.591
250 2.51
230 2.12
170 3.00(No. 0403C)
Z01-01
Z01-02
Z01-03Z01-04
Z02-01
Z02-02
Z02-03
Z03-01
Z03-02
Z03-03
Z04-01
Z04-02
Z04-03
Z04-04
Z04-05
Z05-01
Z05-02
Z05-03
Z05-040.038 1.00 0.032
0.062 1.20 0.038
0.401 7.29 0.233
0.043 1.17 0.037
0.029 0.572 0.019
0.060 0.654 0.022
0.014 0.623 0.020
0.028 0.615 0.020
0.077 1.57 0.051
0.035 0.451 0.015
0.455 6.42 0.225
0.307 6.88 0.226
0.227 5.06 0.173
0.066 0.970 0.032
0.309 5.45 0.186
0.043 1.95 0.062
0.081 2.94 0.092
0.063 2.55 0.080
0.040 1.86 0.056

230
230
230
230
240
240
240
240
240
240
250
240
250
240
250
230
230
230
220

1.01
1.22
7.41
1.18
0.581
0.673
0.627
0.624
1.59
0.462
6.56
6.97
5.13
0.990
5.55
1.96
2.97
2.57
1.87

Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*
0.010 0.860 0.027
Z05-050.101 1.26 0.040
Z06-010.150 2.74 0.087
Z06-020.078 1.79 0.059
Z06-030.946 3.49 0.124
Z07
0.050 0.796 0.027
Z08
Z09-01 C 0.035 0.509 0.027
Z09-02 C 0.099 1.69 0.086
Z09-03 M 0.153 2.24 0.073
Z09-04 M 1.62 9.44 0.291
Z09-05 M 0.127 2.08 0.066
Z10-010.174 2.09 0.072
Z10-020.170 2.06 0.064
Z10-030.162 2.08 0.069
Z10-040.349 1.12 0.040
Z10-050.173 1.64 0.056
Z10-060.070 1.38 0.043
Z11-010.411 3.01 0.097
Z11-020.998 16.1 0.489
Z12-010.055 1.63 0.057
Z12-020.041 0.895 0.030
Z13-010.100 2.73 0.088
Z13-020.203 3.55 0.109
Z13-030.070 1.44 0.046
Z14-011.47 24.6 0.764
Z14-020.414 5.17 0.153
Z14-030.106 3.15 0.096
Z14-040.305 7.29 0.216
Z14-050.015 0.821 0.027
Z15
2.58 11.2 0.369
Z16
0.059 1.73 0.054
Z17-011.34 4.66 0.141
Z17-02
0.053 1.58 0.049
Z17-030.161 2.47 0.076
Z17-04
0.493 2.43 0.086
Z18-01 mR 0.466 6.85 0.174
Z18-020.239 3.64 0.112
Z18-030.126 2.60 0.084
Z18-040.235 4.18 0.135

230 0.863
230 1.29
230 2.79
240 1.81
240 3.78
250 0.811
380 0.520
360 1.72
240 2.29
220 9.94
230 2.12
250 2.14
220 2.11
240 2.13
240 1.23
240 1.69
230 1.40
230 3.14
220 16.4
250 1.65
240 0.908
240 2.76
220 3.61
230 1.46
230 25.1
210 5.30
220 3.18
220 7.38
240 0.826
230 12.0
230 1.75
210 5.07
230 1.60
220 2.52
250 2.58
180 6.99
220 3.71
230 2.64
230 4.25

The CHIME Ages of Zircon and Monazite from the Gray Granite

38

Appendix 2 (cont’d)
Sample & ThO2 UO2 PbOt
UO2*Grain No. (wt%) (wt%) (wt%) (Ma) ThO2*(No. 28001B)
Z01-01
Z01-02
Z01-03 mR
Z01-04 m
Z01-05 m
Z01-06
Z01-07
Z01-08Z02
Z03-01
Z03-02
Z03-03
Z03-04
Z03-05
Z03-06
Z04
Z05
Z06
Z06
Z07
Z08
Z09-01
Z09-02
Z09-03
Z09-04
Z10-01
Z10-02 R
Z11
Z12
Z13

2.56 2.99 0.128
2.91 2.83 0.129
6.10 4.63 0.163
6.90 5.35 0.237
5.80 3.94 0.192
1.13 2.29 0.079
3.41 2.48 0.121
2.40 2.14 0.092
0.278 1.03 0.034
0.419 1.19 0.038
0.511 1.13 0.040
0.225 1.34 0.047
3.93 3.26 0.145
1.11 0.870 0.035
0.150 0.462 0.016
0.396 1.40 0.046
0.322 0.987 0.037
3.10 3.19 0.135
0.084 1.33 0.041
1.48 1.88 0.075
4.99 4.32 0.196
4.73 4.11 0.181
0.722 1.30 0.051
2.50 3.02 0.126
1.93 2.39 0.094
0.082 0.862 0.028
0.448 1.22 0.032
3.99 3.74 0.156
2.24 3.20 0.119
0.421 1.34 0.050

250
250
190
230
250
220
250
240
230
210
230
250
240
210
230
220
250
240
220
240
250
240
250
240
230
230
180
230
230
250

3.78
3.73
6.52
7.48
5.73
2.64
3.53
2.88
1.12
1.32
1.29
1.41
4.47
1.21
0.508
1.52
1.09
4.15
1.36
2.34
5.86
5.57
1.52
3.79
2.99
0.887
1.36
4.97
3.89
1.47

Plate I
Occurrence of the Gray Granite as veins cutting the Hida gneiss at Futatsuya. Two veins of coarse-grained topegmatitic granite (light gray in color) have intruded subconcordantly into the hornblende-biotite gneiss, and are
in turn cut by a discordant vein of fine-grained quartz monzonite (gray in color). The 0701A sample wascollected from the extension of the coarse-grained older veins, and 0701B sample from the fine-grained youngervein.

Suzuki and Adachi

PLATE I

Plate II
Occurrence of the Gray Granite at Futatsuya, about 60 m downstream from the outcrop shown in Plate I. The
Gray Granite (light gray in color) has agmatized the hornblende-biotite gneiss (dark gray in color). No reactionof the granite with the agmatized blocks is observed.

Suzuki and Adachi

PLATE II

Plate III
Occurrence of the Gray Granite as light gray network veins at Kagasawa. This outcrop is the same as that
described in Figs. 1 and 2 of Shibata and Nozawa (1986). They grouped the vein rocks into three; coarsegrained granite, medium-grained granodiorite and fine-grained granite in the order of intrusion.

Suzuki and Adachi

PLATE III

Plate IV
A: Transmitted light photomicrograph of Z08 euhedral zircon grain with concentric growth zoning from 0701A
sample at Futatsuya. Microprobe analysis shows the composition to be 32.3% SiO2, 65.3% ZrO2, 1.50% HfO2,
0.020% ThO2, 0.089% UO2, 0.168% Y2O3 and 0.066% P2O5. The PbO content is less than 0.006%.
B: Transmitted light photomicrograph of Z03 rounded zircon grain from 0701A sample at Futatsuya. This grain
contains 0.011% ThO2, 0.043% UO2 and a hardly detectable amount of PbO.
C: Transmitted light photomicrograph of Z01 zircon grain with a distinct core-overgrowth relation from 0702B
sample at Futatsuya. The well-rounded core is mantled by a rim forming a euhedral outline. both the core andrim have the same optical orientation. The core contains hardly detectable amounts of ThO2, UO2 and PbO.
D: Transmitted light photomicrograph of M07 monazite grain from 1501B sample at Kagasawa.
transparent under the microscope despite the high ThO2 content (8.72-15.7%).

It isSuzuki and Adachi

PLATE IV

Plate V
A and B: Transmitted light (A) and reflected light (B) photomicrographs of Z16 zircon grain from 0702B
sample at Futatsuya. Note a distinct core-overgrowth relation; the subrounded core is translucent owing tometamictization and is mantled by a euhedral rim with growth zoning. The CHIME age for the core is 500±50
Ma (Fig.5).
C and D: Transmitted light (C) and reflected light (D) photomicrographs of Z01 zircon grain from 2801B
sample at Amo, showing a clear concentric growth zoning. This grain contain 1.1-6.9 % ThO2 and 2.1-5.4 %UO2. Most parts are translucent owing to metamictization, but show a concordant PbO- UO2* relation.

Suzuki and Adachi

PLATE V