The Island Arc (2000) 9, 55–63
Thematic Article
Zircon U–Pb sensitive high mass-resolution ion microprobe dating of
granitoids in the Ryoke metamorphic belt, Kinki District,
Southwest Japan
TERUO WATANABE,1 TREVOR IRELAND,2* YOSHIAKI TAINOSHO3 AND YUTAKA NAKAI4
1
Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido, University, Kita-ku,
Sapporo 060-0810, Japan (email: [email protected]), 2Research School of Earth Sciences,
The Australian National University, ACT 2601, Australia, 3Department of Natural Environment,
Kobe University, 3-11 Tsurukabuto, Nada-ku, Kobe 657-0011, Japan, 4Aichi University of Education,
Igaya, Hirosawa, Kariya 448-8542, Japan
Abstract Zircon U–Pb sensitive high mass-resolution ion microprobe dating was carried
out on three types of granitic rock (gneissose biotite granodiorite, biotite granite and twomica granite) from the Cretaceous Ryoke belt of the Kinki district, Southwest Japan. The
results give the ages of granitic magmatism in the Shigi-san area of between 87 and 78 Ma
and suggest extensive melting of the Cretaceous Ryoke granitic crust to form the twomica granite, probably at ca 80 Ma. Discrimination into older and younger granites based
on development of gneissosity does not appear to represent the sequence of magma generation, although there is some scope in the interpretation of the zircon U–Pb data that
would allow all three granites to form at 83 Ma. Compilation of chemical Th-U-total Pb
isochron dating method ages, whole rock Rb–Sr isotope ages and U–Pb isotope ages indicates that most Ryoke plutonism occurred from ca 70 Ma to ca 100 Ma. Younger (85 Ma–70
Ma) plutonism with the formation of two-mica granite occurred only in the eastern sector
of the Ryoke belt, including the Kinki District.
Key words: U–Pb age, Ryoke belt, Kinki district, Cretaceous, two-mica granite, zircon,
sensitive high mass-resolution ion microprobe.
INTRODUCTION
The Ryoke Belt is an 800 km-long structure that
forms part of the northwest Pacific Cretaceous
granitic province and is the inner side of the paired
metamorphic belt of Miyashiro (1961); the pair is
separated by the Median Tectonic Line. The
Ryoke belt is characterized by high-temperature–
low-pressure mineral assemblages; whereas, the
Sanbagawa Belt is the high-pressure side of the
pair; it is located to the outer (Pacific Ocean) side
of the Ryoke Belt.
The Ryoke Belt consists primarily of Cretaceous
granitoids, along with minor amounts of lowpressure metamorphic rocks. The Sanyo Belt, a
*Present address: Department of Earth and Environmental Sciences,
Stanford University, Stanford, California, USA.
Accepted for publication 23 July 1999.
© 2000 Blackwell Science Pty Ltd.
second Cretaceous granite zone, runs along the
inner side of the Ryoke Belt to the north. Kinoshita
& Ito (1986, 1988) identified apparent migration
(along-arc lateral variation) of isotope ages, is
younging toward the east. These trends have been
interpreted in relation to ridge migration. On the
basis of a comprehensive compilation of isotope
ages, Nakajima et al. (1990, 1994) refined the
model and interpreted the along-arc variation and
granitic magma production as an episodic event,
such as the collision of a mid-oceanic ridge with the
trench (Nakajima 1997). Nakajima (1994) considered that the Ryoke plutono-metamorphic belt
represented a deeper crustal section of the Cretaceous Eurasian continental margin, and that the
Sanyo zone to the north represented a shallower
crustal section.
Rb–Sr mineral ages and K–Ar ages for hornblende and biotite constrain a trend ranging from
100 Ma to 110 Ma in the western margin, is young-
56
T. Watanabe et al.
ing toward east. Ages of 55–70 Ma are seen at the
eastern margin. However, some whole rock Rb–Sr
isotope ages do not fit with the along-arc lateral
variation trend shown by the Rb–Sr and K–Ar
mineral ages. In particular, Yuhara (1994) identified three stages of granitic plutonism (ca 99 Ma,
ca 93 Ma and ca 71 Ma) in the Otagiri two-mica
granite complex in the eastern margin of the
Ryoke belt. Yuhara & Kagami (1996) subsequently
examined variation in cooling trends in the
complex. The complex cooled below the biotite
closure temperature (~350°C) in the Rb–Sr system
at 52–55 Ma. Nakajima (1996) also re-examined
the along-arc variation of isotopic ages on the basis
of sensitive high mass-resolution ion microprobe
(SHRIMP) data from two localities in the eastern
part of the Ryoke (ca 86 Ma) and Sanyo (ca 71 Ma)
zones, and proposed that the Ryoke granitoid had
a longer crustal residence time than did the Sanyo
granitoid. He also assumed separated magmatic
pulses in the Ryoke and Sanyo zones and concluded that along-arc lateral variation of isotope
ages reflected regional uplift and cooling to below
500°C. Thus, we would not accept the ridge subduction model once proposed.
Recent chemical Th-U-total Pb isochron method
(CHIME) dating in the western (Iwakuni area)
and eastern (Chubu district) parts of the Ryoke
Belt by Suzuki & Adachi (1998) sheds light on the
magmatic and denudation history of the Ryoke
Belt. They concluded the peak metamorphism
occurred ca 100 Ma in both the eastern (Chubu district) and western (Iwakuni area) parts and
younger (less than 85 Ma) igneous activity in the
Chubu district. They deciphered rapid denudation
of the western part (1.5 mm/y) compared with the
eastern part (0.8 mm/y) during the time span from
91 Ma to 85 Ma. These ages do not support older
whole-rock Rb–Sr ages and a wide range of Rb–Sr
ages (77–99 Ma) of the Otagiri granite and twomica granite (Suzuki et al. 1995; Suzuki, Adachi
1998).
Suzuki & Adachi (1998) revealed younger magmatism 1(ca 70 Ma–ca 85 Ma) in the eastern part of
the Ryoke Belt. A real distribution of the younger
granitoids including two-mica granite, however, is
not yet demonstrated in the Ryoke Belt. Further
U–Pb dating is, thus, relevant to the discussion of
granitic magma generation in Southwest Japan,
due to the high closure temperature of the U–Pb
system and the relative scarcity of data. We
discuss here SHRIMP zircon U–Pb dates for
granitic rocks in the Kinki region, Southwest
Japan.
GEOLOGIC OUTLINE
The Ryoke granitoids are often classified into two
groups (older and younger) on the basis of field
relations. In the remainder of the text, these
apparent age designations based on field relationships are shown in italics. The older granites often
exhibit gneissosity, and intrusions are concordant
with the general trend of surrounding gneisses.
The younger granites are massive or only weakly
schistose, and are never intruded by gneissose
granitoid. In the eastern Kinki district, the older
granites occur in the southern side, along the
Median Tectonic Line. However, in the western
Kinki region some younger granites also occur
near the Median Tectonic Line (Fig. 1), although
generally they are distributed widely on the northern side of the older granites. Comparatively large
amounts of mafic variants (meta-gabbro or quartz
diorite) occur in the Kinki district. Two-mica granites also occur in a comparatively narrow zone
(Fig. 2).
In the Shigi-san area, a two-mica granite is the
youngest of the granites. It intrudes both older
granites (medium-grained quartz diorite, medium
grained gneissose leucocratic biotite granite and
K-feldspar porphyritic gneisssose biotite granite)
and the younger granite (coarse-grained schistose
granite) (Fig. 2). The younger granite in turn
intrudes medium-grained quartz-diorite of the
older granite suite. For this chronological study we
have examined single samples of two-mica granite,
as well as older and younger granites from the
Shigi-san area. It should be noted that any directcontact field relation between the older and
younger granitoids selected here was not
observed.
PETROGRAPHY OF ANALYZED SAMPLES
Three rock types were selected for SHRIMP
U–Pb chronological study: (i) a medium-grained
gneissose granodiorite from the older granites; (ii)
a coarse-grained biotite granite from the younger
granites; and (iii) a fine-grained two-mica granite.
Sample sites are shown (Fig. 2).
SAMPLE J502 (MEDIUM-GRAINED
GNEISSOSE GRANODIORITE)
Sample J502 was weakly sheared, and consisted
mainly of biotite, K-feldspar, plagioclase and
quartz. Biotite clots are concentrated along and
Granitoid dating in the Ryoke belt
Fig. 1
57
Locality map showing the Shigi-san area and distribution of the Ryoke and Sanyo rocks.
Table 1 Major element analysis of the three samples
Element
J502
Sample
J302
J31
SiO2
TiO2
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
P2O5
60.34
0.83
16.93
6.95
0.14
1.67
5.39
3.83
2.00
0.22
73.43
0.23
14.67
2.03
0.05
0.72
2.62
3.50
3.11
0.11
72.23
0.36
14.80
3.53
0.05
0.53
2.41
3.32
2.64
0.10
Total
98.29
100.47
99.97
* Total iron as FeO.
Fig. 2
Geologic map of the Shigi-san area and sampling sites.
define the foliation. K-feldspar is weakly perthitic
and plagioclase is almost unzoned and displays
rounded morphology. Modal contents of the main
constituents are quartz 34%, K-feldspar 23%, and
plagioclase 28%. Major element analysis (Table 1)
shows that J502 is Na2O-rich, and MgO-poor, relative to SiO2 content.
SAMPLE J302 (COARSE-GRAINED BIOTITE GRANITE)
The lithology of sample J302 corresponded to that
of the Yagu granite which is extensively distributed throughout the Kinki region. The main constituent minerals were quartz (45 modal percent),
plagioclase (29%), K-feldspar (21%) and biotite
(4.2%). Plagioclase was weakly zoned with composition ranging from An30 to An40. Perthite texture
was predominant in K-feldspar. The biotite sometimes formed clots.
SAMPLE J31 (FINE-GRAINED TWO-MICA GRANITE)
Sample J31 was the youngest granite in the area
and was intruded along the east–west trending
boundary between the medium-grained quartz
diorite and gneissose biotite granite of the
older granite suite. The northern margin of the
gneissose biotite granite contained porphyritic
K-feldspar. A few stocks of this lithotype were
intruded also into the coarse-grained biotite
granite. The two-mica granite sampled was weakly
58
T. Watanabe et al.
schistose and characteristically contained garnet.
The main mineral constituents were quartz (36
modal percent), K-feldspar (30%), plagioclase
(30%), biotite and muscovite. Chemically, it was
SiO2-rich and poor in CaO and FeO (Table 1).
All three samples were peraluminous with
alumina saturation indices (ASI) of 1.00–1.14
(Table 1). The garnet-bearing two-mica granite
(J31) has the highest ASI, but the other two also
exceed 1.0. This is consistent with other Ryoke
Belt granitoids (Nakajima 1996), although the
samples here plot in the upper part of the range.
SHRIMP ZIRCON U–PB CHRONOLOGY
Zircon separation from the crushed samples was
performed by panning to avoid contamination. The
main objective of this work was to determine the
magmatic ages; therefore, only clear idiomorphic
zircon grains suggestive of a magmatic origin were
chosen for SHRIMP analysis.
Zircon U–Pb chronological SHRIMP study was
carried out on SHRIMP I at the Research School
of Earth Sciences of the Australian National
University. Sensitive high mass-resolution ion
microprobe zircon U–Pb analysis enabled spotage determinations at a microscale (20–30 mm)
in single zircon grains. Full details of the analytical procedure and data reduction are given by
Muir et al. (1996) as well as Compston et al. (1992).
Discussion of the interpretation of zircon U–
Pb geochronology in granitic rocks is given by
Williams (1992).
Briefly, Pb isotopic compositions were taken as
measured. Ratios of U/Pb were normalized to the
572 Ma SL13 standard zircon. Nine standard runs
throughout the days’ analyzes gave a satisfactory
weighted mean of 572 ± 1.8 Ma (1sm) with a mean
squares of weighted deviates (MSWD) of 1.04.
Common Pb contributions were assessed by
204
Pb/206Pb ratios but, in order to avoid correlated
errors from the removal of common 207Pb from a
small measured total, 206Pb/238U ages were based
on the use of 207Pb/206Pb as the common Pb estimator. Essentially, any datum was assumed to be
a mixture between a radiogenic Pb composition
and a common Pb composition. The common Pb
composition was derived from Cumming &
Richards’ (1975) model of common Pb growth
curve. The radiogenic composition was obtained by
extrapolation from the common Pb composition
through the measured datum to concordia. If the
data all reflect the same magmatic age, then all
points should lie along the mixing line on a TeraWasserburg concordia plot (Tera & Wasseburg
1972), or the ages should form a single population
on a cumulative probability diagram. Xenocrysts
will appear as being older than the main population and Pb loss will show as ages distributed on
the young side of the magmatic population. This
procedure relies on the minimum likelihood of
any mixed ages (e.g. through Pb loss, or analysis
of overlapping core-rims of differing ages) having
exactly the same degree of mixture in any
analysis.
The results (Table A1) are shown in composite
Tera-Wasserburg and cumulative probability diagrams (Fig. 3). Approximately 10 zircons from
each of the samples were chosen for analysis so
that the three samples could be run on the same
day to avoid any bias associated with normalizing
to a different standard calibration. The data were
assessed through their measurement errors alone;
the excellent behavior of the standard removes the
need for any external variability contribution as
is often seen for SL13 calibration.
The ten J502 zircons showed a range in U concentrations from 140 to 650 p.p.m., Th from 120 to
590 p.p.m. and Th/U from 0.43 to 1.25. Two U–Pb
analyses had very large errors (~ 10%) and so they
did not contribute to the interpretation of the data.
The weighted mean of the remaining eight analyses was 82.6 ± 0.4 Ma (1sm) with an MSWD of
2.44. The elevated MSWD was caused by the first
analysis which was 3.5 s higher than the mean and
was clearly displaced from the main peak on the
cumulative probability plot. Excluding this point
gave a weighted mean of 82.1 ± 0.5 Ma (1sm) with
an MSWD of 0.75. Summing in the error of the
standard gave a final result of 82 ± 2 Ma. The older
point in this case was interpreted as inheritance.
There was no indication in the chemistry of this
grain that it was different from the rest of the
population.
Eleven J302 zircons showed a large range in U
concentrations from 74 to 1050 p.p.m., Th from 40
to 460 p.p.m., and Th/U from 0.22 to 0.76. The 11
U–Pb analyses gave a weighted mean of 86.3 ±
0.4 Ma (1 sm) with an MSWD of 2.31. The cumulative probability plot showed that the cause of the
high MSWD were two analyses lying on the
younger side of the peak at ca 83 Ma. Excluding
these two analyses gave a weighted mean of 87.2 ±
0.4 Ma (1sm). Summing in the error in the standard
gave a final age estimate of 87 ± 2 Ma. In this case,
the two younger analyses could be interpreted as
Pb loss and their agreement at 83 Ma as coinci-
Granitoid dating in the Ryoke belt
59
Fig. 3 Tera–Wasserberg concordia diagrams and cumulative probability diagrams for the three granitic rocks: J502,
gneissose granodiorite; J302, biotite
granite; J31, garnet-bearing two-mica
granite. Tera–Wasserburg concordia plots
are for (common Pb) uncorrected Pb compositions. Error ellipses are 1s measurement errors. The dashed line represents
the best estimate of the line from the
common Pb composition to the mean age
of the data at the concordia. Data which lie
significantly off the best-fit line are shown
in open symbols. Cumulative probability
plots are derived by assigning each point
a unit area Gaussian and summing all data
from the sample in 0.1 Ma bins over the
range shown. Data which lie within error
of the common Pb mixing line on the TW
concordia diagrams should produce
single cumulative Gaussian curves. J502
and J302 appear to give well-constrained
ages with well-peaked distributions. Two
analyses of poor precision from J502 have
not been plotted on the TW concordia plot;
these do not contribute to the determination of the age. One analysis is significantly older than the rest and has been
removed from the age calculation. J302
has two outliers from the main population,
but these are younger than the main peak,
suggesting Pb loss. The high apparent
common Pb in the analyses of J302 is
largely due to the low U concentrations of
some of the grains. J31 is scattered and it
is not clear whether the magmatic age is
represented in the zircon ages obtained.
dental. Alternatively, the 83 Ma age could possibly
be magmatic with the remainder being inherited
from an earlier phase of Ryoke granite. In this
case there would be no age difference between the
older and younger granites. However, this interpretation is tenuously based on only two analyses.
The good agreement of the remaining analyses
would not be expected if inheritance were playing
a major role; a more diverse range in Pb/U ages
might be expected. Without any persuasive
reasons to conclude that the two 83 Ma zircons are
magmatic, we prefer the interpretation that the 87
± 2 Ma peak was reflecting the magmatic age and,
hence, the younger granite is older than the older
granite.
Of the 10 zircons analyzed from J31, one grain
was Proterozoic (ca 1700 Ma) while the rest were
Cretaceous. The nine Cretaceous grains were
spread between 92 and 78 Ma and the MSWD of 46
for the weighted mean indicated that these were
not a single population. The cumulative probability
plot showed that there were two main peaks in the
data at ca 91 Ma and ca 83 Ma (four grains contributing to each), with a single analysis at 78 Ma.
The four older grains have on average higher U
concentrations 960–3150 p.p.m., Th 400–1450
p.p.m., with Th/U from 0.19 to 0.46, while the
younger grains have 570–1300 p.p.m. U, 200–660
p.p.m. Th, with Th/U ranging from 0.34 to 0.50. The
systematics of this granite clearly indicated there
was inheritance of older protoliths contributing to
the magma. However, it is not possible from the
data to discern whether the magmatic age was represented. In detail, the four analyses in the 83 Ma
peak did not overlap well giving an MSWD of 3.5.
Furthermore, one of the analyses had a poor U–Pb
determination (zoned U concentration) and so it
did not contribute significantly to the weighted
mean. No weight was given to the 78 Ma analysis as
a magmatic age without further analyses showing
this age. More work is required to obtain a more
definitive result for this granite.
60
T. Watanabe et al.
DISCUSSION
Our SHRIMP zircon U–Pb dating indicates peraliminous granitic magma generation took place
probably from 87 to 83 Ma. The so-called younger
granite (J302; 87 ± 2 Ma) in fact yields an older age
than the gneissose granodiorite (J502; 82 ± 2 Ma)
which is classed as an older granite on the lithological criteria. Insufficient magmatic zircon was
obtained from the two-mica granite (J31) to give a
robust age estimate. However, it remains possible
that all three granites formed at 83 Ma, allowing
the lithological criteria to be consistent with the
geochronological data.
The protolith of the Ryoke igneous rocks is considered to be a Jurassic accretionary complex, and
the Proterozoic zircon age (1720 Ma) seen in J31
may be due to detrital zircon supplied from a Proterozoic provenance in the Jurassic. It is noteworthy that a cluster of ages at 92 Ma is clearly
identifiable in J31 which is homogeneous to the
naked eye. This age implies formation of granitic
magma as a precursor. The range from 92 Ma to 78
Ma recognized in the two-mica granite sheds some
light on the timing of extensive melting in the
Ryoke crust. Although we must wait further accumulation of SHRIMP data, our results suggest
that the two-mica granite, produced at a temperature below muscovite breakdown, was formed by,
at least, re-melting of older (92 Ma) granitic materials. The ages of J302 indicate a possible source
material for melting.
A previous compilation of Rb–Sr mineral ages
and K–Ar ages suggests along-arc lateral variation, as discussed by Nakajima (1990, 1994).
However, whole rock Rb–Sr ages and zircon U–Pb
ages are not necessarily concordant with migration (along-arc lateral variation) of the other isotopic ages. Ishizaka (1969) obtained an age of 97
Ma for granitoid from the eastern Kinki district
by conventional U–Th–Pb method. According to
Tainosho et al. (1985), Ryoke granitic magmatism
in the Kinki region is grouped into two stages, that
is 135–110 Ma (first stage) and 110–80 or 70 Ma
(second stage). This stage division is not yet fully
described, due to insufficient chronological study.
The first stage, ‘magmatism’, was especially
speculative. Similar staging classifications for
the Ryoke granitic rocks together with those of
the San’yo and San’in belts (Japan Sea side) in the
Chubu (eastern part) and Chugoku (western
side of the Kinki district) districts are comprehensively discussed by Harayama et al. (1985) and
Iizumi et al. (1985), respectively. Stage 1 in the
Chubu district is older than 90 Ma (Harayama et
al. 1985) and that in the Chugoku district is 70–115
Ma (Iizumi et al. 1985). According to some, the
beginning of granitic magmatism occured from 100
Ma to 115 Ma, on the basis of the chronology of volcanic activity and Rb–Sr isotope study (e.g.Seki
1981). As mentioned earlier, along-arc migration of
the Ryoke plutonic rocks mainly based on K–Ar
ages is explained by denudation history.
Suzuki & Adachi (1998) pointed out the granitic
magmatism associated with the peak metamorphism of ca 100 Ma in the western (Iwakuni
area) and the eastern part (Chubu district) by
CHIME monazite ages. Furthermore, they
revealed younger (ca 70 Ma–85 Ma) magmatism in
the Chubu district. Their compilation of the
CHIME ages also shows overestimation of whole
rock Rb–Sr ages older than ca 100 Ma.
Migration of granitic magmatism from the outer
side of Japan (along the Median Tectonic Line)
towards the Japan Sea side is, however, widely
accepted (e.g. Kagami et al. 1988). Thus, the
younger (less than 85 Ma) plutonism occurred in
the Japan Sea side and eastern side.
We would not accept the east–west trending
ridge subduction model used to interpret along-arc
lateral variation of isotope ages in the Ryoke and
Sanyo zones However, broadly speaking, in the
Ryoke and Sanyo belts the oldest magmatism is
recognized in the west and the youngest in the
east. In the western part (Higo area) of the Ryoke
zone, Rb–Sr mineral ages are older than 100 Ma
(Nakajima et al. 1995) and a whole-rock Rb–Sr age
of 121 ± 14 Ma has recently been reported by
Kamei et al. (1997). A similar age (124 ± 11 Ma) has
also been reported from the western part of the
Sanyo zone (Owada et al. 1995) but the CHIME
monazite ages are younger (ca 85 Ma) (Suzuki &
Adachi 1998). Thus, further U–Pb dating study is
now required for the rocks in the Higo area.
In the Kinki district of the Ryoke Belt the
youngest SHRIMP U–Pb zircon age from
gabbroic rock is ca 72 Ma (Watanabe et al., unpubl.
data., 1993) and in the eastern part of the Ryoke
Belt, the youngest age of granitic magmatism is
reported to be 71 Ma for two-mica granite and
Otagiri granite (Yuhara 1994), which are consistent
with the youngest age of magmatism in the eastern
Sanyo zone (Nakajima 1996). However, the ages of
Otagiri granite as revealed by Suzuki et al. (1995)
are problematic; CHIME monozite ages (ca 77 Ma)
are contradictory when compared with wholerock Rb–Sr ages (71 Ma–99 Ma) by Yuhara (1994).
This contradiction may imply the same origin of
Granitoid dating in the Ryoke belt
the two-mica granite in the Shigi-san area shown
by a wider range of SHRIMP zircon ages (78 Ma–
92 Ma). It may be worth noting that CHIME ages
of two-mica granitoids of Otagiri granite and
Busetsu granite are as young as ca 77 Ma
The age and mechanism of this melting remains
unclear. It is noted that younger (less than 85 Ma)
granitoids with two-mica granite are distributed in
the eastern part of the Ryoke Belt (eastern side
from the Kinki district). Our results suggest that
the Ryoke crust in the Shigi-san area underwent
extensive melting to produce the two-mica granite.
An increase of the geothermal gradient in the
Kinki district or an increased water supply may
have promoted the melting. Further to the east in
the Abukuma belt, we meet older ages, such as
112–122 Ma(Hiroi et al. 1994). To explain the
timing of the Cretaceous granitoids in Japan we
must await additional geochronological data.
CONCLUSIONS
1. In the Shigi-san area, Kinki district, Ryoke
granitic magmatism has been dated through two
granites at 87 ± 2 Ma and 82 ± 2 Ma. A third granite
cannot be dated because of the presence of widespread inheritance and possible Pb loss. All three
granites may be 83 Ma.
2. Granitic magmatism occurred over most of the
Ryoke and Sanyo zones from 80 Ma to 110 Ma and
younger plutonism (ca 70 Ma–ca 85 Ma) occurred
in the eastern part of the Ryoke belt, including the
Kinki district.
3. The two-mica granite in the study area was
produced from melting of older Ryoke crust, probably due to an increase in the geothermal gradient
and/or water supply. Its age is not well defined.
It is, however, worth noting that the ages of
two-mica granitoids are as old as ca 77 Ma–83 Ma
(ca 80 Ma).
The pattern of magmatism is clearly not a
simple one and further geochronological studies
are required.
ACKNOWLEDGEMENTS
We would like to express our thanks to Prof. W.
Compston, Research School of Earth Sciences,
Australian National University, for his understanding and encouragement of our SHRIMP
study. We have also benefited from the critical
comments by Prof. K. Suzuki of Nagoya Univer-
61
sity, the support and guidance of Prof. H. Kagami
of Niigata University and Prof. S. Iizumi of
Shimane University. Dr T. Nakajima (Geologic
Survey of Japan), Dr Y. Kawachi (Otago University), Prof. H. Honma (Research Institute of Earth
Interior of Okayama University), Prof. T. Itaya
(Okayama University of Science) and A. Prof. B.
Roser (Hokkaido University) are also acknowledged for their co-operative work in the study of
Circum Pacific granitic magmatism.
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Granitoid dating in the Ryoke belt
63
APPENDIX
Table A1 Analytical data of U–Pb isotopes
Labels
J502
1.1
2.1
3.1
4.1
5.1
6.1
7.1
8.1
9.1
10.1
J302
1.1
2.1
3.1
4.1
5.1
6.1
7.2
8.1
9.1
10.1
J31
1.1
2.1
3.1
3.1
5.1
6.1
7.1
8.1
9.1
10.1
207
Pb/206Pb*
238
U/ 206Pb*
f 206Pb† (%)
Age‡ (Ma)
U (p.p.m)
Th (p.p.m)
Th/U
282
415
267
360
571
650
517
141
403
513
345
444
116
450
290
549
230
121
506
592
1.22
1.07
0.43
1.25
0.51
0.84
0.45
0.85
1.25
1.15
0.07385 ± 0.00651
0.06344 ± 0.00280
0.08367 ± 0.01020
0.06928 ± 0.00634
0.05767 ± 0.00383
0.06187 ± 0.00263
0.07588 ± 0.00713
0.07826 ± 0.00793
0.06804 ± 0.00679
0.06332 ± 0.00461
71.03 ± 0.93
75.34 ± 0.83
86.41 ± 12.74
77.02 ± 0.94
69.27 ± 9.26
76.38 ± 1.19
76.49 ± 0.85
73.95 ± 1.40
75.79 ± 0.85
76.76 ± 1.27
3.29
1.99
4.58
2.73
1.24
1.79
3.57
3.86
2.57
1.98
87.2 ± 1.4
83.3 ± 1.0
70.8 ± 10.4
80.9 ± 1.2
91.3 ± 12.1
82.3 ± 1.3
80.8 ± 1.2
83.3 ± 1.8
82.3 ± 1.2
81.8 ± 1.4
817
468
1046
944
396
688
207
73
159
146
367
238
231
463
186
205
111
39
121
80
0.45
0.51
0.22
0.49
0.47
0.30
0.54
0.54
0.76
0.55
0.06447 ± 0.00500
0.08621 ± 0.01039
0.05742 ± 0.00234
0.05620 ± 0.00212
0.06792 ± 0.00498
0.07236 ± 0.00408
0.05097 ± 0.00208
0.07691 ± 0.00428
0.09393 ± 0.01162
0.17400 ± 0.01943
75.21 ± 0.58
69.41 ± 1.22
73.00 ± 0.49
72.00 ± 1.27
71.72 ± 1.42
70.92 ± 0.58
70.62 ± 1.74
69.55 ± 1.73
72.10 ± 1.29
63.32 ± 1.08
2.12
4.85
1.22
1.06
2.54
3.10
0.40
3.67
5.83
15.91
83.3 ± 0.8
87.8 ± 2.0
86.6 ± 0.6
88.0 ± 1.5
87.0 ± 1.8
87.5 ± 0.8
90.3 ± 2.2
88.7 ± 2.2
83.6 ± 2.0
85.1 ± 2.9
575
3146
956
2374
2534
994
1325
1014
696
572
107
1450
397
458
1081
121
658
394
235
195
0.19
0.46
0.42
0.19
0.43
0.12
0.50
0.39
0.34
0.34
0.11664 ± 0.00100
0.07074 ± 0.00177
0.05025 ± 0.00095
0.06152 ± 0.00273
0.05610 ± 0.00136
0.05537 ± 0.00142
0.05552 ± 0.00103
0.05568 ± 0.00285
0.08611 ± 0.00354
0.07792 ± 0.00833
3.21 ± 0.04
69.16 ± 0.71
69.79 ± 0.49
68.73 ± 0.31
68.84 ± 0.74
81.39 ± 0.80
77.15 ± 0.55
74.64 ± 0.53
72.82 ± 14.53
71.68 ± 1.26
1.12
2.89
0.30
1.72
1.04
0.99
0.99
1.00
4.85
3.81
1729.7 ± 20.0
89.9 ± 0.9
91.4 ± 0.6
91.5 ± 0.5
92.0 ± 1.0
77.9 ± 0.8
82.2 ± 0.6
84.9 ± 0.7
83.7 ± 16.6
85.9 ± 1.8
* Uncorrected for common Pb contribution.
†
Fraction of common Pb as estimated from 207Pb/206Pb assuming each point is only a mixture of radiogenic and common Pb.
‡
Concordant age after common Pb correction.