Bull. Natl. Mus. Nat. Sci., Ser. C, 44, pp. 1–11, November 26, 2018

Zircon U-Pb dating of plutonic rocks at the Tsukuba area, central Japan
Wataru Koike1 and Yukiyasu Tsutsumi2
12

Department of Geology, Ibaraki Nature Museum, 700 Osaki, Bando-city, Ibaraki 306–0622, Japan
Department of Geology and Paleontology, National Museum of Nature and Science, 4–1–1 Amakubo,
Tsukuba, Ibaraki 305–0005, Japan

Abstract Late Cretaceous to Paleogene plutonic rocks occur at the Tsukuba Mountains and the surrounding area. The rocks are mostly granitic rocks with minor amounts of pyroxene gabbro and hornblende gabbro. A total of eight plutonic rocks were analyzed by U-Pb dating of zircon using Laser Ablation Inductively Coupled Plasma Mass Spectrometry. The obtained zircon U-Pb ages cluster at 61-64
Ma and one granite sample has an age of 80 Ma. Although the age data by K-Ar methods showed that
all the granites belong to younger group, i.e. 58-63 Ma, and the gabbros belong to the older group, thepresent results showed that the gabbros belong to the younger cluster as well as most of the granite.
Older granite occurs around the Tsukuba Mountains. The present age result will contribute to extensionof the Late Cretaceous to Paleogene plutonic rocks in the Japanese Islands.
Key words: age, zircon, granite, gabbro, Tsukuba Mountains

Introduction
Since 1964, the K-Ar method has been widelyused for age analyses of granitoids in Japan (e.g.
Kawano and Ueda, 1964, 1965 and 1966). Basedmostly on the K-Ar method of the Kawano group,
radiometric age data of granitoids in the JapaneseIslands were summarized by Nozawa (1975). From
the K-Ar method, it is well known that Cretaceous
granitoids are the most ubiquitous in the islands. In
central and northern Japan, Cretaceous granitoids,
partly Paleogene, with an age of around 65 Ma
occur along the eastern part of the Asahi Mountains
and Tsukuba-Nikko area. In western Japan, granitoids with age of 60-70 Ma occur linearly along thecoastal zone of the Sea of Japan. Yokoyama et al.
(2016) obtained the thorite and uraninite ages fromabout 400 granitoids in the Japanese Islands.
Although their data are mostly similar to the K-Ar
age data, the uraninite and thorite ages are much
more concentrated in each province. The K-Ar
method involves a few problems. One is Ar loss
during later stage thermal event and alteration. The
others are an age difference among minerals used
for the age analyses. Uraninite and thorite ages also
have a closure temperature problem as shown by
Yokoyama et al. (2016). They are lower in closure
temperature than zircon and monazite, but higher
than micaceous minerals.

There are many other methods used to obtain theages of granitoids. Among them, SHRIMP and
Laser Ablation Inductively Coupled Plasma (LAICP-MS) are the most reliable and useful for granitoids due to the common presence of zircon, its highclosure temperature and strong resistance against
alteration. Watanabe et al. (2000) applied the
SHRIMP method to Cretaceous granite in the
Ryoke low-pressure type metamorphic belt at the
Kinki District. The Tsukuba area belongs to the
Ryoke Belt. Although the SHRIMP and LA-ICPMS methods have been used for the age analyses ofCretaceous and older granite in Japan (e.g., Kon etal., 2015; Ishihara and Orihashi, 2015), the age
methods have rarely applied to granites in the
Ryoke Belt.
Geological setting and samples
Figure 1 shows a geological map around the Tsukuba Mountains (cf. Takahashi, 2006). Basementrock of the plutonic rocks belongs to the Yamizo
Belt that is Jurassic to Early Cretaceous subduction
complex. The Yamizo Belt in this area is more orless metamorphosed by low-pressure type metamorphism and is thought to be eastern extension of the
Ryoke Metamorphic Belt. The plutonic rocks aremostly granite with a minor amount of gabbro. The
granite is classiﬁed geologically and geographically2

W. Koike and Y. Tsutsumi

Fig. 1. Geological map of the Tsukuba area (partly revised after Takahashi, 2006).
analyzed samples.

into three bodies called as Inada, Mt. Kaba and Tsukuba granite bodies (Takahashi, 1982; Miyazaki etal., 1996). There are two blocks of gabbro called as
the Mt. Tsukuba and Dorokushin (Miyazaki et al.,1996).
The Inada Body occurs at the northeastern part of
the Tsukuba Mountain and is subdivided into three
types based on grain size, mineral assemblage andNumbers in black squares are

texture such as equigranular and porphyritic. The
Inada Body is represented by coarse-grained biotite
granite. The Mt. Kaba Body occurs at the Kaba andAshio mountains and is subdivided into four types.
The Tsukuba Body occurs around the Tsukuba
Mountain. It is also subdivided into several types.
The Mt. Tsukuba Gabbro Body occurs at the upper
part of the Tsukuba Mountain and shows layered

Zircon U-Pb dating of plutonic rocks at the Tsukuba area

structure with locally anorthositic part (Tagiri et al.,2013). It is pyroxene gabbro with a small amount ofhornblende. Intrusive relations of the gabbros andsurrounding granites are observed on a wall rock of
the Tsukuba-tunnel (Sasada et al., 1987) and at an
outcrop shown in Fig. 2. On the other hand, theDorokushin Gabbro Body is composed mostly of
hornblende gabbro occurring at northeastern part of
the Tsukuba Mountain. It is elongated in shape withlength of 4 km. Although it apparently seems to

Fig. 2. Outcrop showing gabbro-granite boundary at
the western hillside of the Mt. Tsukuba (locality 4
and 7 in Fig. 1.). Scale: 1 meter.

3

intrude into the Mt. Kaba Granite Body (Fig. 1).
Geological relations of the granites and gabbros
were summarized by Miyazaki et al. (1996). Figure
3 shows the geological relations of granites andgabbros, referring various age data based mainly on
K-Ar age of biotite, Rb-Sr isochron of minerals andwhole rocks (e.g. Kawano and Ueda, 1966;
Miyazaki et al., 1996). Age of gabbro from the Tsukuba Mountain was obtained by K-Ar method of
hornblende. They concluded that gabbros wereformed at 75 Ma before the formation of granites of
58-63 Ma. Recently, Yokoyama et al. (2016)
obtained uraninite and thorite ages from 400 granitoids in the Japanese Islands. Eleven samples of the
granites in the Tsukuba area were analyzed. Most of
the granites were ages from 65 to 70 Ma. In two
samples from the Tsukuba and Mt. Kaba granite
bodies, they obtained age around 80 Ma. They conclude that the combination with 65-70 Ma and 80
Ma may be correlated with granitic rocks from thesouthwestern part of the Asahi Mountains where isconsidered to be northern extension of the Ryoke
Belt.

Fig. 3. Intrusive relationships of the Tsukuba plutonic bodies and radiometric ages (after Miyazaki et al., 1996). Solidarrows show the relationships between two units conﬁrmed at the outcrops. Dotted arrows show intrusive relationships estimated from the areal distributions of the rocks. Radiometric ages were from Kawano and Ueda (1966),
Shiba et al. (1979), Arakawa and Takahashi (1988), Shibata and Uchiumi (1992) and Miyazaki et al. (1996)).

4

W. Koike and Y. Tsutsumi
Table 1.

Rock- types and U-Pb zircon ages of the analyzed plutonic rocks in the Tsukuba area.

Sample No.

Complexand unit*

Rock type

U-Pbage（Ma)MSWD

The number of culculated zircon grain

1
2
3
4

Ka1
Ts3
Ts5
Ts5

Medium-grained biotite granite
Porphyritic biotite granite
Porphyritic biotite muscovite granite
Porphyritic biotite muscovite granite

63.9±1.2
79.6±1.3
61.1±1.362.5±1.9

2.9
1.5
1.04
1.5

18
16
14
6

5
6
7
8

Gb1
Gb1
Gb1
Gb2

Anorthositic pyroxene gabbro
Pyroxene gabbro
Pyroxene gabbro
Hornblende gabbro61.1±1.9—63.1±0.862.1±1.1

0.18
—
1.16
1.6

3
0
28
27

*In: Inada Granite Body, Ka: Mt. Kaba Granite Body, Ts: Tsukuba Granite Body, Gb1: Mt. Tsukuba gabbro Body,
Gb2: Dorokushin gabbro Body.
The unit numbers of Ka1, Ts3-5, Gb1-2 are based on Miyazaki et al. (1996).

Analytical methods of U-Pb dating and results
Totally eight samples were collected from the
Tsukuba area (Table 1). Both granites and gabbros

are mostly coarse-grained. All samples were apparently fresh and any alteration has not been
observed. After careful cleaning of the samples byultrasonic washing machine, around 200 g and morethan 1 kg of granites and gabbros, respectively, werecrushed by the selective fragmentation device using
high-voltage pulsed electric discharges (selFragLab) installed in the National Museum of Natureand Science to maximize the yield of zircon recovery. In granite samples, euhedral zircon grains for
LA-ICP-MS analysis were handpicked from heavy
fractions separated by heavy liquid (diiodomethane). Although zircon was not found in one gabbro
sample, sample 7, from the Tsukuba Mountain, subhedral or partly broken zircon grains were recovered from two pyroxene gabbros from the Tsukuba
body and one hornblende gabbro from the
Dorokushin body.
Zircon grains from the samples, the zircon standard FC1 (206Pb/238U ＝ 0.1859; Paces and Miller,
1993), and the glass standard NIST SRM610 were
mounted in an epoxy resin and polished till the surface was ﬂattened with the center of the embedded
grains exposed. Both the backscattered electron andcathodoluminescence images were used to select thesites for analysis (Fig. 4). U-Pb dating of these samples was carried out using an LA-ICP-MS that wasassembled by NWR213 (Electro Scientiﬁc IndusFig. 4. Back-scattered image (BEI) of the analyzed
zircon grains from the representative plutonic
rocks. A: granite from the Mt. Kaba body (sample1), B: granite from the Tsukuba body (samples 2),
C: pyroxene gabbro from the Tsukuba gabbro body(samples 7). Small circles show analytical positionin zircon grain. Scale bar: 100 μm.

Zircon U-Pb dating of plutonic rocks at the Tsukuba area

5

Fig. 5. Tera-Wasserburg concordia diagrams and age distribution plots of the analyzed granite samples. Left: TeraWasserburg concordia diagrams, right: age distribution plots. A & Aʼ: granite from the Mt. Kaba Body, sample 1. B
& Bʼ: granite from the Tsukuba Body, sample 2. C & Cʼ: granite from the Tsukuba Body, sample 3. D & Dʼ: granitefrom the Tsukuba Body, sample 4. Error ellipses represent 68.3% conﬁdence intervals. Quoted ages are weighted
means of 207Pb-corrected 206Pb/238U spot ages (n analyses, errors at 95% conﬁdence intervals). Solid ellipses, analyses used in weighted mean age calculation; dotted ellipses, excluded analyses.

tries) and Agilent 7700x (Agilent Technologies) thatis installed at the National Museum of Nature andScience, Japan. The experimental conditions and theprocedures followed for the measurements werebased on Tsutsumi et al. (2012). The spot size of thelaser was approximately 25 μm. A correction forcommon Pb was made on the basis of the measured207
Pb (207Pb correction) or 208Pb and Th/U ratio
208( Pb correction) (e.g. Williams, 1998) and themodel for common Pb compositions proposed by6

W. Koike and Y. Tsutsumi

Fig. 6. Tera-Wasserburg concordia diagrams and age distribution plots of the analyzed gabbro samples. Left: TeraWasserburg concordia diagrams, right: age distribution plots. A & Aʼ: anorthositic pyroxene gabbro from the Mt.
Tsukuba Body, sample 5. B & Bʼ: pyroxene gabbro from the Mt. Tsukuba Body, sample 7. C & Cʼ: hornblende gabbro from the Dorokushin Body, sample 8. Error ellipses represent 68.3% conﬁdence intervals. Quoted ages areweighted means of 207Pb-corrected 206Pb/238U spot ages (n analyses, errors at 95% conﬁdence intervals). Solidellipses, analyses used in weighted mean age calculation; dotted ellipses, excluded analyses.

Stacey and Kramers (1975). The pooled ages presented in this study were calculated using Isoplot/
Ex software (Ludwig, 2003). The uncertainties in
the mean 238U-206Pb* ages represent 95% conﬁdence intervals (95% conf.). 206Pb* indicates radiometric 206Pb. Most of the analyzed zircon grains in
the granite samples exhibit magmatic oscillatory
zonings (e.g. Fig. 4) and lack of textural evidence
for secondary modiﬁcation, suggesting that theobtained age for each sample represents the timingof magmatic crystallization of the zircon. On theother hand, subhedral or partly broken zircons fromthe gabbros have a weak zoned texture as shown in
Fig. 4, different from those from the granite sam-ples. We obtained zircon U-Pb mean ages from four
granite and three gabbro samples. Tera-Wasserburg
Concordia plots of granites and gabbros andweighted mean 207Pb-corrected 206Pb/238U ages areshown in Fig. 5 and 6. Age and detailed isotope data
are summarized in Table 1 and 2. Three granites
have a narrow range from 61 Ma to 64 Ma. One
granite from the Tsukuba Body has an older age: 80
Ma. The younger and older granites were recognized from the Tsukuba Granite Body. Ages of threegabbro samples from the Mt. Tsukuba andDorokushin bodies are well concentrated in a range
from 61 to 63 Ma, similar to young cluster of the
granite samples (Table 1).

Zircon U-Pb dating of plutonic rocks at the Tsukuba area
Table 2.

Detailed data for the U-Pb ages of zircons from Tsukuba plutonic rocks.

206

SampleNo.

Labels

1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1

T12_01.1
T12_01.2
T12_02.1
T12_03.1
T12_04.1
T12_04.2
T12_05.1
T12_06.1
T12_07.1
T12_08.1
T12_09.1
T12_10.1
T12_11.1
T12_11.2
T12_12.1
T12_13.1
T12_14.1
T12_15.1
T12_16.1
T12_17.1
T12_17.2
T12_18.1
T12_18.2
T12_19.1
T12_19.2
T12_20.1
T12_20.2

iz(1)

2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2

T10_01.1
T10_01.2
T10_02.1
T10_02.2
T10_03.1
T10_04.1
T10_05.1
T10_06.1
T10_07.1
T10_08.1
T10_09.1
T10_09.2
T10_10.1
T10_11.1
T10_12.1
T10_12.2
T10_13.1
T10_14.1
T10_15.1
T10_15.2
T10_16.1
T10_17.1
T10_17.2
T10_18.1
T10_19.1
T10_19.2
T10_20.1
T10_21.1

iz

3
3
3
3
3
3
3
3
3
3
3
3

K09_01.1
K09_01.2
K09_02.1
K09_02.2
K09_03.1
K09_03.2
K09_04.1
K09_05.1
K09_06.1
K09_06.2
K09_07.1
K09_08.1

iz

iz
iz
iz
iz

iz
iz
iz

iz
iz
iz
iz
iz
iz
iz
iz
iz

iz

iz

iz
iz

7

238

U/206Pb*age (2)(Ma)

238

Pb*/206Pb* (2)4.12±0.08
101.13±2.37
96.78±1.94
98.39±2.10
2.47±0.04
101.01±2.40
96.47±2.12106.58±1.80
84.71±2.23106.98±2.67
82.48±3.25
100.82±2.47
3.24±0.05
96.75±2.19
97.33±1.87
4.34±0.07
96.33±2.16
98.35±1.96
106.74±2.63
97.03±1.45103.41±2.54
3.02±0.06
91.25±1.8696.30±5.13102.27±2.22
31.22±0.63
98.81±2.08

0.1184±0.0033
0.0478±0.0035
0.0468±0.0027
0.0468±0.0031
0.1306±0.0027
0.0492±0.0038
0.0420±0.0043
0.0502±0.0027
0.0344±0.0074
0.0403±0.0051
0.0411±0.0058
0.0504±0.0039
0.1258±0.0022
0.0504±0.0041
0.0505±0.0048
0.1148±0.0023
0.0455±0.0040
0.0460±0.0031
0.0548±0.0046
0.0597±0.0026
0.0438±0.0055
0.1487±0.0030
0.0482±0.0027
0.0390±0.0120
0.0444±0.0049
0.0572±0.0076
0.0481±0.0039

1400.4±23.2
63.4± 1.566.3± 1.365.2± 1.4
2192.2±28.1
63.5± 1.566.5± 1.560.2± 1.075.7± 2.0
60.0± 1.577.7± 3.0
63.6± 1.5
1733.5±25.0
66.3± 1.565.9± 1.3
1336.8±18.6
66.6± 1.565.2± 1.3
60.1± 1.5
66.1± 1.0
62.0± 1.5
1841.9±30.7
70.3± 1.4
66.6± 3.562.7± 1.4
203.2± 4.1
64.9± 1.4

1357.4±22.8
63.4± 1.566.3± 1.365.2± 1.4
2192.2±28.1
63.4± 1.566.5± 1.560.0± 1.076.8± 1.960.0± 1.577.7± 3.0
63.4± 1.6
1699.2±24.8
66.0± 1.565.6± 1.3
1295.8±18.4
66.6± 1.565.2± 1.359.5± 1.565.1± 1.062.3± 1.5
1775.4±30.2
70.2± 1.4
66.6± 3.562.8± 1.3
201.5± 3.9
64.8± 1.4

0.68
0.47
0.49
0.49
1.88
0.31
0.36
0.20
0.41
0.35
0.37
0.49
0.41
1.32
0.08
0.36
1.07
0.43
0.91
0.52
0.62
1.37
0.39
0.41
0.56
0.45
1.25
0.52

59.48±2.4181.64±2.4528.22±0.71
70.24±1.7973.21±2.2380.32±2.0275.35±2.13
3.46±0.05
71.26±1.6582.17±1.75
3.19±0.05
67.51±1.60
79.93±2.3176.60±2.36
3.93±0.05
75.47±1.99
67.96±2.2179.69±1.70
81.41±2.8582.20±2.1426.96±0.56
81.99±2.38
83.46±1.4577.89±2.09
30.27±0.72
81.83±2.48
82.54±4.60
81.17±1.98

0.0453±0.0141
0.0397±0.0094
0.0496±0.0043
0.0414±0.0062
0.0474±0.0050
0.0467±0.0064
0.0494±0.0038
0.1157±0.0014
0.0486±0.0029
0.0471±0.0029
0.1130±0.0036
0.0524±0.0067
0.0480±0.0048
0.0464±0.0129
0.1167±0.0020
0.0525±0.0045
0.0516±0.0052
0.0371±0.0060
0.0527±0.0047
0.0501±0.0039
0.0536±0.0034
0.0451±0.0119
0.0443±0.0028
0.0459±0.0050
0.0494±0.0033
0.0504±0.0054
0.0584±0.0182
0.0483±0.0036

107.5± 4.3
78.5± 2.3
224.5± 5.6
91.1± 2.387.5± 2.679.8± 2.085.0± 2.4
1636.9±19.3
89.8± 2.1
78.0± 1.6
1757.9±25.0
94.8± 2.280.1± 2.3
83.6± 2.6
1461.6±15.7
84.9± 2.2
94.2± 3.0
80.4± 1.778.7± 2.777.9± 2.0
234.8± 4.8
78.1± 2.376.8± 1.382.2± 2.2
209.5± 4.9
78.3± 2.477.6± 4.3
78.9± 1.9

107.9± 4.2
78.8± 2.2
224.5± 5.6
91.8± 2.387.5± 2.679.9± 2.084.8± 2.4
1611.7±19.2
89.7± 2.1
78.0± 1.6
1748.8±25.2
94.3± 2.280.1± 2.3
83.7± 2.3
1423.8±15.6
84.3± 2.3
93.7± 3.1
81.0± 1.778.2± 2.877.7± 2.0
234.0± 4.8
78.4± 2.076.8± 1.382.2± 2.2
209.5± 4.9
78.0± 2.476.6± 4.678.9± 1.9

0.67
0.01
0.92
0.96
0.73
0.78
0.85
0.07
0.81
0.74
1.18
1.24

3.89±0.07
29.24±0.59109.20±6.06
104.60±4.38109.87±5.32
105.66±4.69
94.39±4.28108.83±2.57
103.27±5.62101.28±4.55
106.28±3.90102.67±5.93

0.1061±0.0050
0.0470±0.0025
0.0419±0.0239
0.0548±0.0089
0.0348±0.0133
0.0438±0.0095
0.0607±0.0131
0.0485±0.0055
0.0274±0.0235
0.0400±0.0097
0.0262±0.0061
0.0398±0.0164

1475.6±24.4216.8± 4.3
58.8± 3.2
61.3± 2.6
58.4± 2.860.7± 2.767.9± 3.1
59.0± 1.462.1± 3.4
63.3± 2.860.4± 2.262.5± 3.6

1456.8±23.9
217.0± 4.3
58.8± 3.2
60.8± 2.6
58.4± 2.860.7± 2.766.8± 3.2
58.9± 1.462.6± 3.0
63.3± 2.860.4± 2.262.5± 3.6

Pbc(2)
(%)

U
(ppm)

Th
(ppm)

Th/U

238

0.78
0.00
0.00
0.00
0.00
0.00
0.00
0.00
1.51
0.00
0.00
0.00
0.15
0.00
0.15
0.00
0.00
0.00
0.00
0.00
0.44
0.00
0.00
0.00
0.11
0.41
0.13

353
1348
1855
1350
193
1224
1211
2627
775
1030
341
1532
720
1311
1713
518
1295
1808
1098
3803
1498
293
1686
175
1309
489
1835

82
406
408
323
126
344
434
591
261
573
138
177
61
323
477
27
305
393
380
1645
544
172
310
106
321
397
430

0.24
0.31
0.23
0.25
0.67
0.29
0.37
0.23
0.35
0.57
0.41
0.12
0.09
0.25
0.29
0.05
0.24
0.22
0.36
0.44
0.37
0.60
0.19
0.62
0.25
0.83
0.24

1.43
0.37
0.00
0.74
0.00
0.25
0.00
0.00
0.00
0.00
0.35
0.59
0.00
0.29
0.45
0.00
0.00
0.78
0.00
0.00
0.00
0.48
0.00
0.00
0.00
0.00
0.00
0.00

225
512
232
953
494
763
646
2331
1145
1733
228
899
569
571
733
609
511
1179
509
839
437
812
1354
509
461
536
144
869

148
233
111
455
908
227
230
445
456
595
82
433
229
735
54
214
535
493
453
425
264
1086
519
206
254
236
176
436

2.12
0.09
0.00
0.00
0.00
0.00
0.00
1.55
0.76
0.00
0.00
0.00

168
567
119
276
202
221
204
1266
171
232
345
150

110
6
107
257
143
168
170
81
135
168
397
182

U/ Pb* (2)
206

207

U/206Pb*age (3)(Ma)

8

W. Koike and Y. Tsutsumi
Table 2. Continued.
206

238

U/206Pb*age (2)(Ma)

238

Pb*/206Pb* (2)2.38±0.04
9.00±0.23
97.47±2.86
106.21±3.33
105.01±4.06
104.83±8.34
2.45±0.06
11.74±0.18
85.66±2.5687.29±1.65

0.1657±0.0031
0.1100±0.0042
0.0513±0.0058
0.0379±0.0077
0.0615±0.0084
0.0450±0.0295
0.1533±0.0065
0.1047±0.0021
0.0566±0.0081
0.0473±0.0027

2264.6±35.0
679.4±16.3
65.8± 1.960.4± 1.961.1± 2.461.2± 4.8
2208.1±45.3
527.0± 7.7
74.8± 2.273.4± 1.4

2215.7±35.0642.7±15.6
65.5± 1.960.7± 1.860.0± 2.461.2± 4.8
2177.7±45.2
497.8± 7.4
74.0± 2.273.4± 1.4

0.40
0.61
0.53
0.38
0.49
0.26
0.45
1.01
0.34
0.29
0.42
1.05
0.52
0.18
0.65
0.59
0.11
0.40

40.88±1.03
104.92±2.12105.16±1.9793.84±2.27
99.56±2.48
78.65±1.52
98.71±3.50
98.15±3.33
103.15±2.26
6.36±0.11
77.16±2.9985.75±2.8873.45±2.02
63.76±1.94
91.54±2.4390.30±2.4854.54±1.27
90.62±1.87

0.0501±0.0034
0.0434±0.0051
0.0483±0.0040
0.0461±0.0060
0.0427±0.0064
0.0418±0.0036
0.0537±0.0091
0.0446±0.0137
0.0422±0.0041
0.1069±0.0021
0.0593±0.0074
0.0251±0.0132
0.0377±0.0068
0.0419±0.0050
0.0446±0.0035
0.0597±0.0053
0.0431±0.0033
0.0377±0.0051

155.8± 3.961.2± 1.2
61.0± 1.1
68.3± 1.664.4± 1.681.4± 1.6
65.0± 2.3
65.3± 2.2
62.2± 1.4
940.9±15.5
83.0± 3.274.7± 2.587.2± 2.4
100.3± 3.0
70.0± 1.871.0± 1.9
117.1± 2.7
70.7± 1.5

155.6± 3.9
61.4± 1.260.9± 1.1
68.4± 1.664.6± 1.582.0± 1.664.5± 2.365.5± 2.062.6± 1.4
902.8±14.9
81.8± 3.2
76.5± 2.387.4± 2.3
101.1± 3.0
70.0± 1.869.9± 2.0
117.9± 2.7
71.1± 1.4

224
227
49
184
380

0.35
0.37
0.26
0.23
0.42

91.86±2.6185.00±2.37105.49±5.20
106.09±2.61
104.03±2.57

0.0523±0.0074
0.0375±0.0072
0.0467±0.0109
0.0450±0.0036
0.0467±0.0075

69.8± 2.075.4± 2.1
60.8± 3.0
60.5± 1.561.7± 1.569.4± 2.075.5± 2.060.8± 3.0
60.5± 1.561.7± 1.5

130
102
362
423
521
514
183
76
97
136
113
129
94
377
430
47
44
53
112
90
91
162
145
131
81
415
370
487

145
71
588
696
564
560
91
72
125
149
119
152
72
378
361
58
44
47
80
113
92
174
282
145
71
542
467
292

1.14
0.72
1.66
1.69
1.11
1.12
0.51
0.97
1.32
1.12
1.09
1.21
0.78
1.03
0.86
1.27
1.04
0.90
0.73
1.28
1.03
1.10
1.99
1.14
0.90
1.34
1.30
0.62

107.97±3.7799.26±2.79
100.09±2.02
104.23±2.53102.26±2.36
106.13±2.08
104.92±3.38
106.52±5.56
104.15±5.13102.17±3.89101.56±3.7797.12±4.00
106.23±4.88102.16±2.27
97.93±2.17
96.09±4.93
98.01±5.18
100.71±5.24
99.05±3.29103.44±5.22105.76±5.25
94.63±2.93104.49±4.62
94.46±3.35105.59±4.76
93.51±1.89
94.06±2.39
100.54±2.19

0.0222±0.0155
0.0411±0.0059
0.0474±0.0031
0.0533±0.0109
0.0403±0.0086
0.0493±0.0030
0.0382±0.0090
0.0379±0.0130
0.0270±0.0232
0.0418±0.0071
0.0524±0.0075
0.0444±0.0143
0.0563±0.0208
0.0390±0.0086
0.0449±0.0030
0.0498±0.0141
0.0322±0.0126
0.0575±0.0111
0.0593±0.0070
0.0423±0.0230
0.0308±0.0192
0.0408±0.0055
0.0262±0.0267
0.0527±0.0067
0.0335±0.0180
0.0520±0.0033
0.0406±0.0104
0.0472±0.0059

59.4± 2.1
64.6± 1.864.1± 1.3
61.6± 1.562.7± 1.460.5± 1.2
61.2± 2.0
60.2± 3.1
61.6± 3.062.8± 2.4
63.2± 2.3
66.0± 2.760.4± 2.862.8± 1.465.5± 1.466.7± 3.465.4± 3.463.7± 3.364.8± 2.1
62.0± 3.1
60.7± 3.0
67.8± 2.1
61.4± 2.767.9± 2.460.8± 2.768.6± 1.468.2± 1.763.8± 1.460.7± 1.864.6± 1.864.1± 1.3
61.1± 1.3
63.3± 1.3
60.3± 1.261.5± 1.960.2± 3.1
62.6± 2.662.8± 2.462.8± 2.466.3± 2.659.7± 2.6
63.1± 1.365.5± 1.466.5± 3.665.4± 3.462.9± 3.463.8± 2.262.4± 2.861.9± 2.867.8± 2.1
63.0± 2.0
67.4± 2.4
61.6± 2.568.2± 1.468.8± 1.663.8± 1.4

73
164
97
122
339

47
89
54
57
171

0.65
0.55
0.57
0.48
0.52

104.33±5.61
109.39±4.20
100.71±5.53
110.30±5.19
103.05±3.33

0.0210±0.0192
0.0334±0.0145
0.0450±0.0109
0.0259±0.0140
0.0480±0.0125

61.5± 3.3
58.7± 2.2
63.7± 3.558.2± 2.762.3± 2.0

61.6± 3.1
59.7± 2.2
63.7± 3.559.0± 2.662.2± 2.0

Pbc(2)
(%)

U
(ppm)

Th
(ppm)

Th/U

238

0.00
5.57
0.51
0.50
0.00
0.00
0.32
0.00
0.14
0.00

190
558
648
809
291
62
72
1178
552
2050

168
139
71
327
160
54
62
28
194
722

0.90
0.26
0.11
0.41
0.56
0.90
0.88
0.02
0.36
0.36

0.00
0.65
0.02
1.12
0.29
1.55
0.09
0.29
0.86
1.02
0.00
2.30
0.27
0.74
0.00
0.00
1.16
0.56

219
811
961
427
535
832
158
204
767
419
103
211
249
416
331
347
609
631

86
479
500
160
257
211
68
201
251
119
42
217
128
75
210
199
62
246

0.28
0.14
0.00
0.00
0.82

655
633
199
810
934

T13_01.1
T13_01.2
T13_02.1
T13_02.2
T13_02.3
T13_02.4
T13_02.5
T13_03.1
T13_03.2
T13_03.3
T13_04.1
T13_04.2
T13_04.3
T13_05.1
T13_05.2
T13_06.1
T13_06.2
T13_07.1
T13_07.2
T13_07.3
T13_08.1
T13_08.2
T13_09.1
T13_10.1
T13_10.2
T13_11.1
T13_12.1
T13_12.2

2.14
0.00
0.00
0.29
1.75
0.00
0.57
0.00
1.56
0.00
0.00
0.87
2.47
0.53
0.00
0.00
0.00
0.00
0.00
1.50
1.96
0.00
2.59
0.00
1.34
0.00
1.10
1.30

T14_01.1
T14_01.2
T14_02.1
T14_02.2
T14_03.1

0.25
2.10
0.00
1.47
4.45

SampleNo.

Labels

3
3
3
3
3
3
3
3
3
3

K09_09.1
K09_09.2
K09_09.3
K09_10.1
K09_10.2
K09_11.1
K09_12.1
K09_12.2
K09_13.1
K09_13.2

iz
iz

4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4

T15_01.1
T15_02.1
T15_02.2
T15_03.1
T15_04.1
T15_05.1
T15_05.2
T15_06.1
T15_07.1
T15_08.1*(4)T15_09.1
T15_09.2
T15_10.1
T15_11.1
T15_11.2
T15_11.3*
T15_12.1
T15_12.2

iz

5
5
5
5
5

T08_01.1
T08_01.2
T08_02.1
T08_03.1
T08_04.1

7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
8
8
8
8
8

iz
iz
iz
iz

iz
iz

iz
iz
iz
iz
iz
iz
iz
iz
iz
iz
iz

U/ Pb* (2)
206

207

U/206Pb*age (3)(Ma)Zircon U-Pb dating of plutonic rocks at the Tsukuba area
Table 2.
SampleNo.

Labels

8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8

T14_04.1
T14_05.1
T14_05.2*
T14_06.1
T14_06.2*
T14_07.1
T14_08.1
T14_08.2
T14_09.1
T14_10.1
T14_11.1
T14_12.1
T14_12.2
T14_13.1
T14_14.1
T14_15.1
T14_16.1
T14_16.2
T14_17.1
T14_18.1
T14_19.1
T14_20.1

206

9Continued.
Pb*/206Pb* (2)U/206Pb*age (2)(Ma)

0.0505±0.0107
0.0413±0.0103
0.0708±0.0092
0.0283±0.0124
0.0693±0.0077
0.0391±0.0119
0.0444±0.0039
0.0232±0.0164
0.0353±0.0127
0.0283±0.0231
0.0465±0.0065
0.0399±0.0044
0.0365±0.0129
0.0416±0.0107
0.0433±0.0064
0.0495±0.0045
0.0279±0.0116
0.0432±0.0135
0.0554±0.0079
0.0435±0.0039
0.0253±0.0129
0.0404±0.0073

61.2± 1.758.2± 2.0
63.6± 2.3
59.6± 1.865.2± 2.659.9± 2.1
59.2± 1.664.6± 3.562.0± 2.1
62.6± 3.764.1± 1.761.0± 1.858.8± 2.560.6± 1.767.6± 2.262.1± 1.961.5± 2.1
61.6± 2.962.9± 2.566.6± 1.764.6± 2.2
67.9± 2.5

238

Pbc(2)
(%)

U
(ppm)

Th
(ppm)

Th/U

238

0.20
0.73
0.00
1.74
0.00
0.75
0.00
1.16
0.33
1.21
0.09
0.00
2.47
0.98
0.00
0.00
1.62
0.63
0.00
0.00
1.53
0.00

457
263
159
330
187
285
440
127
300
119
433
349
175
771
209
384
408
152
136
321
148
150

483
134
82
304
90
223
452
62
249
95
306
300
90
958
131
356
349
85
78
199
78
83

1.08
0.52
0.53
0.94
0.49
0.80
1.06
0.51
0.85
0.82
0.72
0.88
0.52
1.27
0.64
0.95
0.88
0.57
0.59
0.63
0.55
0.57

104.80±3.00
110.18±3.87
100.88±3.65
107.74±3.32
98.37±4.01
107.08±3.81108.47±2.9299.22±5.41
103.45±3.60
102.50±6.15
100.13±2.61
105.12±3.16109.14±4.62105.81±2.95
94.88±3.14103.26±3.17104.26±3.49104.16±4.88102.07±4.06
96.23±2.4399.32±3.42
94.42±3.54

U/ Pb* (2)
206

207

U/206Pb*age (3)(Ma)

238

61.0± 1.6
58.7± 2.061.7± 2.3
60.6± 1.763.4± 2.660.4± 2.059.2± 1.665.4± 3.462.2± 2.0
63.4± 3.564.1± 1.6
61.0± 1.859.6± 2.561.1± 1.6
67.6± 2.2
61.9± 1.962.5± 1.961.9± 2.862.2± 2.566.6± 1.765.6± 2.1
67.9± 2.5

Errors are 1-sigma; Pbc and Pb* indicate the common and radiogenic portions, respectively.
(1) iz: inheritad zircon(2) Common Pb corrected by assuming 206Pb/238U-208Pb/232Th age-concordance(3) Common Pb corrected by assuming 206Pb/238U-207Pb/235U age-concordance(4) *: discordant

Discussion
Since 1965, the K-Ar method has been widelyused for age analyses of granitic rocks in Japan (e.g.
Kawano and Ueda, 1964, 1966) and radiometric age
data of granitic rocks in the Japanese Islands weresummarized by Nozawa (1975). Recently many
authors have used SHRIMP and LA-ICP-MS methods for the age analyses in the Japanese Cretaceous
granites (e.g. Ishihara and Orihashi, 2015; Kon etal. 2015; Tani et al. 2016). In the Ryoke metamorphic belt, however, the reliable age method wasapplied only to the Cretaceous granites at the Kinki
District by Watanabe et al. (2000).
Basement rock of the Tsukuba area belongs to the
Ryoke Metamorphic Belt. The area is an eastern endof the belt. In this paper, reliable age analyses weredone for the plutonic rocks in the Tsukuba area. The
age results are summarized in Fig. 7. It shows that
igneous event of granite occurred at least twice inthis area, different from the K-Ar age data. The
younger cluster is roughly consistent with K-Ar
ages of 58-63 Ma. The older granites with age
around 80 Ma were found in the Tsukuba Granite
Body. Our result is rather similar to the uraninite
and thorite age data obtained by Yokoyama et al.

(2016). They concluded that the similar igneous
events, i.e. 60-65 Ma and around 80 Ma, occurred at
the western part of the Asahi Mountains where isconsidered to be northern extension of the Ryoke
Belt. On the other hand, granites with 80 Ma occur
sporadically in the Ryoke Belt from the Chubu to
Kinki provinces. The older granites with age around
80 Ma may be related to the major event of the
Ryoke Belt.
The most unexpected result was ages of gabbros
in the Tsukuba and Dorokushin bodies. Ages ofthree gabbro samples are well concentrated from 61
to 63 Ma, corresponding to younger cluster of the
granite samples with age from 61 to 64 Ma. As
shown in Fig. 2, granite is apparently intruded into
pyroxene gabbro of the Mt. Tsukuba body. The
granite at the contact zone is 62.5 ± 1.9 Ma, faintly
younger than nearby gabbro with 63.1 ± 0.8 Ma.
Unfortunately, it is hard to conclude simply thatgabbro was formed before the intrusion of the granite, considering of the standard deviation of the age.
As probable explanations, zircons collected fromgabbro were contaminated from the surrounding
granite or zircons were affected by the thermal
event of the later stage granitic activity. The zircons
in gabbros, however, have different BEI texture,

10

W. Koike and Y. Tsutsumi

Fig. 7. The radiometric ages of the Tsukuba plutonic rocks. White squares are from Kawano and Ueda (1966), Shiba etal., (1979), Arakawa and Takahashi (1988), Miyazaki et al. (1966), Shibata and Uchiumi (1992). White stars arefrom Yokoyama et al. (2016). Black circles are from this study.

weakly zoned structure (Fig. 4c), from oscillatory
zoned zircons in granites. Any zircon analyzed inthis paper shows no secondary texture affected bylater stage thermal event. Furthermore, age datafrom three gabbros have a narrow in range from 61
to 63 Ma. Hence, it is reasonable that the gabbrosand younger granites were formed almost at thesame period.
As a future work, it is important to analyze plutonic rocks in the Ryoke Belt by SHRIMP and LAICP-MS to correlate the relationships among therocks. Even if only a little, the present study willcontribute a solution to the problem of the plutonic
activity in Japan.

Acknowledgements
The authors thank Mrs. M. Shigeoka and Dr. K.
Tani of the National Museum of Nature and Sciencefor her help in sample preparations. Special thanks
go to Dr. K. Yokoyama of the Ibaraki Nature
Museum for his critical reading of this manuscript.
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