J. Earth Planet. Sci. Nagoya Univ., Vol.49, 1 to 13, 2002

CHIME ages of Mg-rich garnet-bearing gneiss clasts
from the Jurassic Sawando conglomerate
in the northeastern Mino terrane, central Japan

Shiro TANAKA , Kazuhiro S UZUKI and Mamoru A DACHI
Department of Earth and Planetary Sciences, Graduate School of Science,
Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan
Center for Chronological Research, Nagoya University,
Chikusa-ku, Nagoya 464-8602, JapanThe Nagoya University museum, Nagoya University,
Chikusa-ku, Nagoya, 464-8601, Japan(Received December 10, 2002 / Accepted December 25, 2002)

ABSTRACTCHIME (chemical Th-U-total Pb isochron method) has been applied to determine
the ages of two gneiss clasts which are bearing Mg-rich garnet (pyrope content up to
30-40 mol%) from the Jurassic Sawando conglomerate in the northeastern Mino terrane, central Japan. The CHIME monazite ages of gneiss clasts range from 1809 to 1494
Ma with two data concentrations of 1750-1700 Ma and 1650-1600 Ma (sample Sawa-13),
and from 1601 to 1040 Ma with a data concentration of 1300-1200 Ma (sample Sawa43). A detrital zircon in the gneiss clast (Sawa-13) gives the oldest CHIME age of 2064
±146 Ma. These CHIME age of the gneiss clasts indicate occurrence of middle Precambrian poly-phase meramorphism (between ca. 1800 and 1000 Ma) and igneous activity (ca. 2050 Ma), and suggest that the Mg-rich garnet-bearing gneisses were formedby the middle Precambrian metamorphism.
Detrital garnets, mostly Mg-rich garnet, are commonly observed in the Jurassic
sandstone of the Mino terrane. Mg-richgarnet-bearingrockslike the analyzed gneissclasts are considered as major source rocks of the Jurassic clastsintheMinoterrane.
The CHIME ages of the gneiss clasts suggest that a part of the Jurassic terrigenous
clastics, for example detrital monazite and Mg-rich garnet in the Mino terrane were
derived from the area which is underlain by middle Precambrian metamorphic andigneous rocks.

INTRODUCTION
Provenance analysis on clastic rocks is important for the reconstruction of geological history, since characteristics of clastic rocks have reflected tectonics andgeological setting. Recent development of dating method has made spot dating on
mineral grain. CHIME (chemical Th-U-total Pb isochron method: e.g. Suzuki et al.,
1991; Suzuki and Adachi, 1991a, 1991b; Suzuki et al., 1999) is one of the spot dating
method adapted for fine mineral grains including Th, U, and Pb like zircon and monazite.
This report focuses on chronological analysis with CHIME for the Mg-rich garnetbearing gneiss clasts (pyrope content up to 30-40 mol%; Tanaka and Adachi, 1999)
from the Jurassic Sawando conglomerate of northeastern Mino terrane, central

1

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S. Tanaka et al.

Japan, and discuss the relationship between sandstone grains and clasts in the conglomerate.
The Mino terrane which widely occupies central Japan is Jurassic to lower mostCretaceous acrretionary complex, mainly composed of sandstone, shale, chert, limestone and greenstone (e.g. Otsuka, 1988; Wakita, 1988). On the terrigenous detritalgrains, Suzukiet al. (1991) and Adachi and Suzuki (1994) have reported a large numberof CHIME ages of detrital monazite from Jurassic sandstone. Their data revealed thatmetamorphic and/or igneous rocks of middle Precambrian to Middle Jurassic age are
the major origins of detrital monazites, nevertheless the type of original rocks are still
unknown except for some middle Precambrian gneiss (e.g. Shibata and Adachi, 1974;
Adachi and Suzuki, 1993).
In the Mino terrane, intraformational conglomerates that contain clasts of granitoid
and gneiss are present (e.g. Kanuma and Irie, 1962; Adachi, 1971; Hattori et al., 1985;
Otsuka, 1985, 1988; Nakano et al., 1995). Though gneiss clasts are rare in the conglomelates, these are important for the provenance analysis based on petrologic features.
The Jurassic sandstones contain abundant clastic plagioclase and Mg-rich garnet derived from metamorphic rocks (Mizutani, 1959; Suzuki, 1977). Mg-rich garnet has been
formed under upper amphibolite to granulite facies conditions (e.g. Miyashiro, 1953).
Some gneiss clasts from the Jurassic Sawando conglomerate in the northeastern Mino
terrane are characterized by containing Mg-rich garnet (Tanaka and Adachi, 1999). To
clarify the chronological setting of the Mg-rich garnet-bearing gneiss clasts is also
important for recognizing the origin of detrital Mg-rich garnet.
GEOLOGICAL SETTING
The Mino terrane situated in central part of the Japanese Islands is Jurassic tolower most Cretaceous accretionary complex composed mainly of sandstone, shale,chert, limestone and greenstone (e.g. Otsuka, 1988; Wakita, 1988). Microfossil analysis
has reconstructed paleo-oceanic plate stratigraphy: Permian greenstone, limestone andchert, Triassic to Jurassic chert and siliceous shale, and Jurassic sandstone and shale(e.g. Otsuka, 1988; Wakita, 1988). The mino terrane is sucdivided into several geological complexes based on lithology, structure and age (e.g. Otsuka, 1988; Wakita, 1988;
Nakae, 2000; Yamakita and Otoh, 2000). Figure 1 shows the distribution and subdivision of the northeastern part of the Mino terrane.
In this srudy, specimens are sampled from the Sawando conglomerate (Tanaka etal., 1952) in the Sawando complex (Otsuka, 1988; Fig. 2). The Shirahone complex(Otsuka, 1988) which consists mainly of greenstone and Permian limestone and chert
is thrust over the Sawando complex. The Sawando complex consists of the repetitionof chert, siliceous shale, shale and alternating beds of sandstone and shale. Otsuka(1985) named this succession as "the chert-clastics sequence" and interpreted that thelithological transition reflects the change of sedimentary environment from pelagic tohemipelagic. Sandstone and shale occupy the majority of the Sawando complex. In
the shale-dominant beds, shale is alternated with fine-grained sandstone. In the sandstone-dominant beds, some sandstone is massive and usually has coarse to very coarse
grain size with shale patches. Sandstone and shale beds, most commonly distributed,

CHIME ages of Mg-rich garnet-bearing gneiss clasts

Fig. 1.

3

Index map showing the study area and the distribution of five geological complexes in
the northeastern part of the Mino terrane (modified from Otsuka and Watanabe, 1992).

trend northeast and dip northwest. Cross bedding and grading indicate that most of
the strata are normal. Sandstone rarely passes into granule conglomerate. Sandstone
consists of quartz, potassium feldspar (both orthoclase and microcline), minor plagioclase, and rock fragments of limestone, shale, chert, acidic igneous rock, gneiss andrarely granophyre and sillimanite (fibrolite) gneiss, with heavy minerals such as biotite, muscovite, zircon, agrnet, tourmaline, apatite, rutile and opaque minerals. The
Sawando conglomerate is associated with alternating beds of sandstone and shale, andestimated thickness of the conglomerate layer is more than 50 m.
The Sawando conglomerate is poorly sorted, matrix-supported, intraformational
and polymict. This conglomerate includes clasts of sandstone and calcareous sandstone

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S. Tanaka et al.

Fig. 2.

Geological map of the study area. Star symbol shows sampling locality.

(23%), shale (25.5%), chert (14%), marl (13%), limestone (2.5%), acidic-intermediate
volcanic to hypabyssal rocks (9%), granitoid (2.5%) and metamorphic rocks (0.5%).
Clast size is from granule to boulder. Clasts are usually subrounded to well-rouded.
Igneous rocks and gneiss clasts are usually rounded to well-rounded shape, bur pebbles
of chert, shale and acidic tuff tend to be more angular. Matrix of the conglomerate isusually sandstone but partially muddy sandstone. Imbrication and grading of clasts are
not observed. Radiolarian fossils obtained from the Sawando complex (Otsuka, 1985,
1988) provide a late Middle Jurassic age for sedimentation of coarse-grained clastic
rocks.
METHOD AND MATERIALSCHIME age analysis
Monazite and zircon grains in conventional polished thin sections were analyzed on
a JEOL JCXA-733 electron-probe microanalyzer. Accelerating voltage, probe current
and probe diameter were kept at 15 kV, 0.2 µA and 5 µm, respectively. The ThMα,UMβ, PbMα and YLα lines were measured simultaneously with PET crystals. The

CHIME ages of Mg-rich garnet-bearing gneiss clasts

Fig. 3.

5Photomicrographs and back scattered electron (BSE) images of the gneiss clasts fromthe Sawando conglomerate in the Mino terrane. A: Texture in the Sawa-13. Garnets,
graphite and biotite show weak lateral orientation. One polar. B: Analyzed roundedzircon grain in quartz from the Sawa-13. One polar. C: BSE image of analyzed zircon
shows oscillatory zoning and monazite inclusion (white part). Zoning is discordant withgrain shape (same grain in Fig. 3-B). D: Analyzed monazite grain in quartz from the
Sawa-13. One polar. E: BSE image of analyzed monazite (same grain in Fig. 3-D) fromthe Sawa-13 shows concentric zoning. Many of light spots are sites for CHIME dating.
F: Cracked texture in the Sawa-43. Cracks are filled with calcite (ordinary) and chlorite(in case of garnet). G: Analyzed cracked monazite grain in cracked quartz from the
Sawa-43. Crossed polars. H: BSE image of analyzed monazite (same grain in Fig. 3-G)
from the Sawa-43. BSE image clearly shows that monazite includes rounded zircon andsececondary apatite. I: BSE image of analyzed monazite (same grain in Figs. 3-G and H)
from the Sawa-43. shows concentric zoning which is in accord with grain shape mostpart. Ap: apatite, Bt: biotite, Chl: chlorite, Gr: graphite, Grt: garnet, Mnz: Monazite, Pl:
Plagioclase, Qtz: Quartz, Zrn: Zircon.

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S. Tanaka et al.

standards were euxenite provided by Smellie et al. (1978) for Th and U, and synthesized glass (10.8% PbO; Suzuki and Adachi, 1998) for Pb. X-ray intensities were integrated over 300 seconds by 5-times cyclic stepping of spectrometers on individual
peak and background positions. The measurements were repeated twice, and thearithmetic averages were taken. The spectral interference of YLγ on PbMα was cororected with the method described by Amli and Griffin (1975). The X-ray intensity data
were converted into concentrations with the Bence and Albee (1968) method using theanalyses of natural zircon and monazite as matrix compositions (Suzuki et al., 1999),
since small differences in the matrix between analyzed and reference minerals havelittle effect on ThO2, UO2 and PbO determinations. The detection limits at 1σ confidence level are 0.007, 0.011 and 0.003wt.% for ThO2, UO2 and PbO, respectively. The
relative errors are about 30% for 0.01 wt.% of PbO concentration, 6% for 0.3 wt.% of
UO 2 concentration, and 1% for 10 wt.% of ThO 2 concentration. The details of theanalytical procedure and the CHIME age calculation are described in Suzuki and Adachi(1991a) and Suzuki et al. (1991).
Analyzed samples[Sawa-13]
This specimen is 6 cm in diameter and well-rounded shape. it is a fine-grained
rutile (0.7%)-graphite (1.0%)-garnet (2.3%)-biotite (5.7%)-quartz (82%) gneiss. Secondary minerals are muscovite, chlorite, pyrite, ilmenite and leucoxene. MgO contents ofthe garnets up to 10 wt.% (about 40 mol% of pyrope). One monazite grain, 150 µm
across, has been found and analyzed (Fig. 3-D). Back scattered electron (BSE) imageof the monazite (Fig. 3-E) shows concentric zoning. Zircon is usually subrouded torouded shape, and one subrouded zircon was analyzed (Fig. 3-B). BSE image of thiszircon shows oscillatory zoning which does not accords with grain shape (Fig. 3-C).
The original rock is assumed to have been quartzose arenite on the basis of texture
and mineral composition (Fig. 3-A; Tanaka and Adachi, 1999). Details of this specimen
were described in Tanala and Adachi (1999).
[Sawa-43]
This specimen is about 15 cm in diameter and sub-rounded shape. it is a coarsegrained microcline (very rare)-garnet (0.2%)-biotite (0.2%)-quartz (24.1%)-plagioclase(oligoclase; 74.1%) gneiss. MgO contents of the garnets are about 8 wt.% (about 32
mol% of pyrope). This clast has undergone cataclastic deformation and has had cracks,
which are filled with calcite (Figs. 3-F and G). BSE image of the analyzed monazite(Fig. 3-H) shows that monazite includes rounded zircon and has cracks. BSE imageof the monazite (Fig. 3-I) shows zoning which partially accords with the grain shape.
This specimen was described in Tanaka and Adachi (1999).
RESULTSThe ThO2, UO2 and PbO contents of analyzed monazite and zircon grains, ThO2*
values (ThO2* values: measured ThO2 plus ThO2 equivalent of the measured UO2) andUO2* values (UO2* values: measured UO2 plus UO2 equivalent of the measured ThO2),
and apparent ages are listed in Table 1.

7

CHIME ages of Mg-rich garnet-bearing gneiss clasts

Table 1.

Representative analytical data of monazite and zircon in the gneiss clasts from the
Sawando comglomerate, in the northeastern part of the Mino terrane.

Sample.
ThO2 UO2 PbO Age# ThO2*
Grain No. (wt.%) (wt.%) (wt.%) (Ma) (wt.%)

Sample.
Grain No.

ThO2
UO2 PbO(wt.%) (wt.%) (wt.%)

Age# ThO2*(Ma) (wt.%)

Sawa-13
Mnz01-01
Mnz01-02
Mnz01-03
Mnz01-04
Mnz01-05
Mnz01-06
Mnz01-07
Mnz01-08
Mnz01-09
Mnz01-10
Mnz01-11
Mnz01-12
Mnz01-13
Mnz01-14
Mnz01-15
Mnz01-16
Mnz01-17
Mnz01-18
Mnz01-19
Mnz01-20
Mnz01-21
Mnz01-22
Mnz01-23
Mnz01-24
Mnz01-25
Mnz01-26
Mnz01-27
Mnz01-28
Mnz01-29
Mnz01-30
Mnz01-31
Mnz01-32
Mnz01-33
Mnz01-34
Mnz01-35
Mnz01-36
Mnz01-37

Mnz01-38
Mnz01-39
Mnz01-40

2.62
3.05
3.28

0.956
0.678
0.595

0.491
0.422
0.423

1809
1732
1758

Zrn01-01
0.006
Zrn01-02
0.009
Zrn01-03
0.006
Zrn01-04
0.008
Zrn01-05
0.008
Sawa-43
Mnz01-01 6.49
Mnz01-02 5.99
Mnz01-03 8.41
Mnz01-04 8.36
Mnz01-05 8.43
Mnz01-06 8.30
Mnz01-07 9.20
Mnz01-08 9.30
Mnz01-09 8.30
Mnz01-10 8.96
Mnz01-11 9.89
Mnz01-12 8.69
Mnz01-13 7.32
Mnz01-14 8.39
Mnz01-15 15.53
Mnz01-16 8.00
Mnz01-17 4.78
Mnz01-18 7.52
Mnz01-19 0.012
Mnz01-20 0.019

0.014
0.021
0.052
0.011
0.037

0.005
0.008
0.019
0.005
0.013

0.427
0.611
0.204
0.182
0.194
0.179
0.228
0.221
0.210
0.246
0.240
0.210
0.420
0.231
0.385
0.231
0.644
0.318
0.114
0.257

0.489
0.515
0.487
0.478
0.494
0.476
0.517
0.543
0.542
0.519
0.528
0.475
0.482
0.475
0.755
0.499
0.444
0.485
0.030
0.065

4.92
5.14
5.33
5.35
4.89
5.19
5.20
5.02
5.00
4.83
4.68
4.68
4.73
4.52
3.35
2.83
3.41
4.06
4.58
4.37
4.44
4.18
4.30
4.80
4.36
3.68
4.32
4.35
3.98
3.94
2.28
3.99
4.27
2.96
2.46
3.05
3.99

0.331
0.321
0.404
0.386
0.476
0.413
0.388
0.354
0.353
0.385
0.396
0.388
0.368
0.421
0.701
0.925
0.702
0.547
0.453
0.395
0.398
0.395
0.437
0.398
0.269
0.516
0.415
0.412
0.473
0.496
0.928
0.469
0.428
0.744
0.910
0.739
0.426

0.439
0.441
0.468
0.474
0.474
0.476
0.461
0.436
0.444
0.456
0.452
0.447
0.456
0.459
0.447
0.402
0.450
0.437
0.437
0.441
0.451
0.425
0.451
0.469
0.385
0.406
0.448
0.453
0.432
0.435
0.445
0.432
0.444
0.445
0.457
0.447
0.382

1636
1599
1577
1605
1633
1624
1595
1583
1614
1667
1681
1671
1709
1724
1720
1494
1711
1648
1604
1724
1741
1717
1739
1706
1644
1663
1745
1757
1720
1719
1770
1723
1733
1770
1783
1758
1577

6.13
6.31
6.80
6.76
6.63
6.70
6.62
6.31
6.29
6.25
6.14
6.11
6.09
6.07
5.93
6.17
6.00
6.06
6.23
5.83
5.91
5.64
5.92
6.27
5.35
5.58
5.86
5.88
5.73
5.77
5.72
5.72
5.85
5.73
5.84
5.79
5.54

6.18
5.56
5.49
UO2**
2019 0.016
2112 0.023
2113 0.053
2173 0.014
2036 0.039
ThO2**
1400 8.02
1447 8.18
1231 9.13
1227 9.00
1251 9.11
1231 8.93
1194 10.00
1243 10.08
1379 9.05
1221 9.83
1140 10.72
1166 9.42
1264 8.81
1192 9.20
1040 16.86
1306 8.81
1438 7.09
1293 8.64
1601 0.429
1568 0.954

MnZ: monazite
Zrn: zircon# : apparent age* : sum of the measured ThO2 and ThO2 equivalentof the measured UO2**: sum of the measured UO2 and UO2 equivalentof the measured ThO2

[Sawa-13]
A total of 5 points on a rounded zircon grain were analyzed. The ThO2 content
varies from 0.006 to 0.009 w.%, UO2 from 0.011 to 0.052 wt.%, and PbO from 0.005 to
0.019 wt.%. Apparent ages are from 2019 to 2173 Ma. On the PbO vs. ThO2* diagram,thse data define an isochron of 2064±146 Ma (Fig. 4-C).
A total of 40 points on 1 monazite grain were analyzed. The ThO2 content varies
from 2.28 to 5.35 wt.%, UO2 from 0.269 to 0.956 wt.%, and PbO from 0.382 to 0.491 wt.%.
Apparent ages of the monazite grains range from 1809 to 1494 Ma, and cluster around1750-1700 Ma and 1650-1600 Ma (Figs. 4-A and 5-A). In the moanzite grain, distributions of apparent ages are divided into two areas whose boundary is about 1700 Ma(Figs. 5-A and 6). This age distribution is not in accord with concentric zoning patternshown in BSE image (Figs. 3-E and 6). Its contrast pattern (Fig. 3-E) is in accord wellwith UO 2 and ThO 2 contents, and grain shape of monazite (Fig. 6).

S. Tanaka et al.

8[Sawa-43]
The cracked monazite was analyzed at 20 points. The ThO2 content varies from
0.012 to 15.5 wt.%, UO2 from 0.114 to 0.644 wt.% and PbO from 0.030 to 0.755 wt.%.
Apparent ages are from 1601 to 1040 Ma, and cluster around 1300-1200 Ma (Figs. 4-B
and 5-B). Two analyzed points show exceptionally low ThO2, UO2 and PbO contents
and older apparent ages (1601 and 1568 Ma), while one point shows higher thesecontents and younger apparent age (1040 Ma).

(A)
PbO(wt.%)(B)

Sawa-13 monazite
40 analyses on 1 grain

0.8

a
1 8 0 9 M1 7 0 6 M a

0.3

1809 - 1706 Ma
1705 - 1494 Ma

0.4

7.0

0.2

5.5

5.0

6.5

6.0

14

0.5

0.0

(C)

10

5

0

ThO 2*(wt.%)

0.010

2064±146 Ma

0.005

MSWD=0.04
m=0.3473±0.0319
b=0.0001±0.0006
Analytical uncertainty

0.000

0

0.01 0.02 0.03 0.04 0.05 0.06

4
3

1700

1600

1500

(Ma)

1400

1800

1850

1750

1700

0
1650

1

0
1600

2

1300

2

1200

4

5

1100

6

6

1000

number frequency

8

1500

Sawa-43
N=20

7

10

15

0.015

8

12

1550

a

0.020

Sawa-13
N=401450

number frequency

M

Sawa-13 zircon

(B)

(A)

Fig. 5.

0

PbO(wt.%) 5 analyses on 1 grain

UO 2*(wt.%)

14

4
10

0.1

Results of CHIME age analysis ofmonazite and zircon from the gneissclasts. (A) Plots of PbO vx. ThO2* formonazite from sample Sawa-13 ofgneiss clasts. (B) Plots of PbO vs.
ThO 2 * for monazite from sample
Sawa-43 of gneiss clast. Dash lines in(A) and (B) represent reference isochrons (C) Plots of PbO vs. UO2* forzircon from sample Sawa-13. Error
bars and quoted error are of 2σ.
MSDW: Mean Square Weighted Deviates. ThO2*: measured ThO2 plus ThO2
equivalent of the measured UO 2 .
UO2*: measured UO2 plus UO2 equivalent of the measured ThO 2 .

16

a

0.3

ThO 2*(wt.%)

Fig. 4.

M

011494 M

1

6
16

Ma

0.6

a

0.4

0.2

38

0.7

16

0.5

M

a

0.6

Sawa-43 monazite

PbO(wt.%) 20 analyses on 1 grain(Ma)

Histograms of apparent CHIME ages for monazite from Sawa-13 (A)and Sawa-43 (B). N: number of analysis.

9CHIME ages of Mg-rich garnet-bearing gneiss clasts

1770

2.46

ThO 2

1783

Age

1770

2.28

1719

1720

2.96
3.94

3.35

4.80

1706

3.28

1758
1724
1636

1709
1614

1599 1583
1605
1677
1633
1809 - 1706 Ma

1671
1681

1577
1720
1739
1717
1733
1757
1723
1741
1745
1724
1604

1667
1595
1624
1648

1663

1497

UO 2

.321

.353
.354

.388
.396
.385

.386

.388
.413

.404
.476

5.35
5.33
5.00 - 5.35
4.50 - 4.99

4.89

4.44
3.99
4.32
4.38
4.83
2.62
5.20
4.58
5.19
3.68
3.06
4.06
3.41
2.83

3.50 - 4.49

PbO

.744

.457
.445

.547
.702
.925

.269

.739

.445
.435

.447

.595
.428
.473
.678
.437
.395
.428
.412
.469
.398
.415
.395
.956
.516

4.38

2.28 - 3.49

.496

.368
.331

5.02

4.27
4.35

4.68
4.68

.469

.398

.421

.269 - .359

1809
1758

.910

.701

.380 - .499

5.14

1644

.928

5.00

3.04

4.30
4.18

4.73

4.92

1711

1705 - 1494 Ma.500 - .928.400 - .499

1732

.423
.382
.432

.459

.439
.441

.444
.436

.474
.468
.460 - .491
.450 - .459
.440 - .449

3.88

3.98

4.52

.474

.456

.451
.425

.447
.452

.451

.453
.441

.461
.476

.437

.450
.402

.432

.448

.458

.437

.422

.444

.491

.406

.447

.385

.385 - .439

0.05 mmFig. 6.

Distribution of apparent CHIME ages, and ThO2, UO2 and PbO contents (wt.%), andconcentric zoning (fine solid line) in BSE image of Mnz-01 from Sawa-13. Solid circle,
open circle and solid square indicate analyzed spots which correspond to light spots in
BSE image of Fig. 3-E. BSE image's contrast is in accord with ThO2 and UO2 contents.
On the other hand, distribution of CHIME ages is discordant with BSE image. Grain
shape in BSE image (dash line) is different from the shape observed in microscope(coarse solid line. See Fig. 3-E).

DISCUSSIONCHIME ages of gneiss clasts
The gneiss clasts yield middle Precambrian CHIME monazite ages. Though data
points of Sawa-13 and Sawa-43 are not arrayed linearly on the PbO vs. ThO2* diagrams(Figs. 4-A and B) these ages cluster around 1750-1700 Ma and 1650-1600 Ma, and1300-1200 Ma, respectively. Overstreet (1967) indicated that monazite in pelitic rockwas formed under lower amphibolite facies metamorphism. In addition, lower limit ofmonazite-forming temperature is estimated about 525±25 oC in pelitic schists in Appalachia (Smith and Barreiro, 1990). Since the gneiss clasts have suffered upper
amphibolite facies metamorphism, the analyzed monazites are thought to show agesof metamorphism. It, however, is not clear whether the analyzed monazites are originally detrital or not detrital, because analyzed monazite was only one grain from eachclast.

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S. Tanaka et al.

Of the Sawa-13, the analyzed monazite could be considered as either originallydetrital (1) or not detrital (2). In the case (1), 1750-1700 Ma is regarded as thermal
event before sedimentaiton of quartzose sandstone. 1650-1600 Ma shows metamorphosed age of the gneiss. In the case (2), both 1750-1700 Ma and 1650-1600 Ma areregarded as metamorphosed age of the Sawa-13. Both cases, ca. 1650-1600 Ma shows
metamorphosed age of the Sawa-13.
The zircon in the Sawa-13 is thought to be a detrital grain because of its roundedshape. In addition, zoning in BSE image is discordant with grain shape, and shows nodistinct overgrowth formed by metamorphism (Fig. 3-C). Oscillatory zoning of zircon
shows that this grain was originally formed in igneous rocks, so ca. 2064 Ma indicatesthe age of igneous activity as forming zircon before sedimentation of quartzose arenite.
Of the Sawa-43, zoning in the BSE image of the monazite seems that this grain is
fractured (detrital grain, or fractured and displaced after metamorphisms) or subhedralgrain, because lower right part of the grain looks like to be lacked (Figs. 3-G and I).
Whereas upper right corner of the grain is angular, so this grain seems not to be
detrital. Furthermore, the monazite occurs in single quartz grain; this quartz grain iscracked but all fragments of quartz around the monazite are optionally continuous andare not displaced. Therefore, we regarded the monazite as subhedral grain which wasformed during metamorphism, and cluster of apparent ages around 1300-1200 Ma anda minor data concentration around 1400 Ma could be regarded as ages of metamorphisms, The youngest age of 1040 Ma probably indicates the age of thermal event.
The oldest stages of 1601 and 1568 Ma may remnant of previous igneous activity or
metamorphism, though obtained data of these two points have exceptionally low ThO 2
and PbO contents. Since analyzed points are only at inner part of the zoning, theSawa-43 may record younger thermal events at rim part. More detailed dating isnecessary to be clear the thermal history of the clast. Consequently, the protolith of
the Sawa-43 is metamorphosed at middle Precambrian (ca. 1400, 1300-1200 and 1040
Ma), and might be metamorphosed at unidentified younger period.
Relationship between the gneiss clasts and sandstone grains
Suzuki et al. (1991) and Adachi and Suzuki (1994) reported CHIME age of detritalmonazite from the Jurassic sandstone in the Mino terrane. According to their reports,
apparent ages of detrital monazite grains are especially concentrated between 1740
and 1420 Ma, and between 274 and 161 Ma, and also lie along 1250, 800 and 400 Ma
reference isochrons. These spectrum peaks of the detrital monazite age frequency arecorrelated well with peaks of monazite age frequency from the gneiss clasts (Fig. 5):
1750-1700 Ma, 1650-1600 Ma, 1400 Ma and 1300-1200 Ma. This suggests that 18001000 Ma detrital monazites in the sandstones were derived from poly-metamorphosed
gneisses. In addition, Tanaka and Adachi (1999) indicated the importance of sample
Sawa-13 and Sawa-43 as origin of detrital Mg-rich garnet (pyrope content up to 30-40mol%) which is usually occurred in the Jurassic sandstone of the Mino terrane (e.g.
Suzuki, 1977; Adachi and Kijima, 1983; Tanaka, 2001). The CHIME ages of the gneissclasts also revealed that detrital Mg-rich garnets in the Jurassic sandstone of the Mino
terrane had been derived from the middle Precambrian metamorphic rocks.
Ca. 2064 Ma CHIME zircon age from the Sawa-13 corresponds to 2058±89 Ma

CHIME ages of Mg-rich garnet-bearing gneiss clasts

11

detrital zircon from sandstone in the Mino terrane (Adachi and Suzuki, 1994). Twosimilar ages were also reported; 2050±50 Ma CHIME zircon age of the quartzofeldspathic gneiss clast (sample 4-50; Adachi and Suzuki, 1993) and 2050±30 Ma RbSr whole-rock isochron age of gneiss clasts (including sample 4-50; Adachi et al., 1992,
recalculated from Shibata and Adachi, 1974) from the Kamiaso conglomerate in thecentral Mino terrane which is correlated with the Sawanso conglomerate (Adachi,1976). Ca. 2050 Ma is presumably emplacement time of granitoids, some ot which are
undergone ca. 1700 Ma metamorphism (Shibata and Adachi, 1974; Adachi and Suzuki,1993).
On the other age data from the Jurassic clastic rocks, Suzuki et al. (1991) reported
ca. 1740 Ma (core) and 1420 Ma (rim) Ma apparent ages of monazite from the garnet-sillimanite gneiss clast (sample 4-50) from the Kamiaso conglomerate. This also shows polymetamorphised gneiss as source rock of the detrital monazites in the Mino terrane.
Adachi et al. (1992) reported 1660-959 Ma of K-Ar and 1750-1370 Ma or Rb-Sr mineralages for biotite, muscovite, plagioclase and K-feldspar for gneiss clasts, some of which
form above mentioned ca. 2050 and 1880 Ma whole-rock isochrons. These K-Ar and
Rb-Sr mineral ages are probably ages of metamorphism and thermal event, and areroughly corresponded to cluster of monazite ages from gneiss clasts and sandstone.
Consequently, the source rocks of detrital monazite showing middle Precambrian
age in the Jurassic sandstone of the Mino terrane are igneous rocks and gneisses
which have upper amphibolite facies. According to the present data, each gneiss clast
from the Mino terrane records a few middle Precambrian thermal events, neverthelessrecords no Paleozoic and Mesozoic events. In present, souorce rocks of the detritalmonazite younger than 1000 Ma is still not clear, except for ca. 180 Ma granodiorite
clast (Tanaka et al., 2000) and Paleozoic detrital moanzite in the orthoquartzite clast(Adachi and Suzuki, 1997) from the jurassic conglomerates.
As for middle Precambrian, preliminary comparison shows that distribution ofspectrum peaks of CHIME ages from the sandstone (Suzuki et al., 1991) and gneissclasts in the Mino terrane are similar to the distribution of radiometric age data for
granitoids and metamorphic rocks in the North China region reported by Yang et al(1986), which is one of the candidate for the provenance of hte Jurassic clastics in the
Mino terrane.
SUMMARY(1) The Mg-rich garnet-bearing gneiss clasts were formed by poly-phase middle Precambrian metamorphisms. These gneisses are thought to be source rocks of the
detrital Mg-rich garnet in the Jurassic sandstone of the Mino terrane.
(2) Some Precambrian detrital monazites (ca. 1800-1000 Ma) were derived from polymetamorphosed gneisses and igneous rocks.
(3) Zircon in the gneiss clast indicates ca. 2050 Ma igneous activity. This zircon grain
is considered as detrital grain based on morphology and older age than monazite.
(4) The poly-metamorphosed gneiss clasts from the Jurassic conglomerate in the Mino
terrane have no record of Paleozoic and Mesozoic thermal events.

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ACKNOWLEDGEMENTS
We would like to thank Drs. M. Takeuchi and H. Yoshida of Nagoya University forvaluable discussion, and Mr. S. Yogo for his technical support. We are also muchindebted to journal reviewer for his helpful review. Thanks are also due to menmer
of Tectonics Group of Nagoya University for their encouragement.
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