国立科博専報，(51), pp. 89–98 , 2016 年 3 月 28 日
Mem. Natl. Mus. Nat. Sci., Tokyo, (51), pp. 89–98, March 28, 2016

Chevkinite-bearing Tuffs from the Boso and Noto Peninsulas inCentral Japan and from Primorye, Far East Russia
Koichi Momma*, Yukiyasu Tsutsumi, Takashi Sano, Ritsuro Miyawaki,
Masako Shigeoka and Kazumi Yokoyama
Department of Geology and Paleontology, National Museum of Nature and Science,
4–1–1 Amakubo, Tsukuba, Ibaraki 305–0005, Japan*E-mail: k-momma@kahaku.go.jp

Abstract. Speciﬁc tuffs including chevkinite-(Ce) or monazite-(Ce) were found in Oligocene to
Miocene formations on the Boso and Noto peninsulas in central Japan. Monazite tuff was also
found in Nezhino, Primorye, Far East Russia, where chevkinite-(Ce) was found in the river sands.
The chemical composition of chevkinite-(Ce) from the Boso Peninsula is almost the same as thatfrom the Noto Peninsula. Furthermore, ages of the chevkinite-bearing tuffs are around 23 Ma in
both the tuffs. As chevkinite-(Ce) and monazite-(Ce) are very rare minerals in tuff, it is concluded
that tuffs from the Boso and Noto peninsulas were derived from the same volcanic eruptions. The
chevkinite-(Ce) collected from river sand from Nehino, Primorye, has chemical compositions similar to those mentioned above. Monazite-bearing tuff has ages similar to those from the Boso andNoto peninsulas. Hence, it is reasonable that the three tuffs were formed by the same or consanguineous volcanic eruption at around 23 Ma. These data will be important when we reconstruct theJapanese Islands before the opening of the Sea of Japan.
Key words: chevkinite, monazite, zircon, Sea of Japan

Introduction
Chevkinite-(Ce), (Ce,La,Ca)4(Ti,Fe3＋,Fe2＋,Mg)5Si4O22, is a rare mineral occurring as anaccessory mineral in granitoid, pegmatite, tuff,
and air-fall ash deposits (Mitchell, 1966; Izett etal., 1968; Sokolova et al., 2004; Platt et al.,
1987; Macdonald et al., 2012). Tsutsumi et al.
(2012) described chevkinite from the tuff in the
Boso Peninsula. It is the ﬁrst record of chevkinite-(Ce) in Asia found from tuff or ash deposits.
They also described a monazite-bearing tuff
closely associated with the chevkinite-bearing
tuff. While monazite-(Ce) is a common accessory mineral in granitoid and high-grade metamorphic rock, it is rarely found in a tuff (Das etal., 2009). The deposition age of these tuff or
tuffaceous sandstones was around 23 Ma, justbefore the opening of the Sea of Japan.
To deduce the origin of the chevkinite-(Ce) inthe Boso Peninsula, we recently collected tuffs
and tuffaceous rocks that occur along the Sea ofJapan from the Japanese Islands and Primorye,Fig. 1. Index map of the Boso and Noto peninsulas and Nejino of Primorye, Far East Russia.

90

Koichi Momma et al.

Far East Russia (Fig. 1). In this paper, we
describe the newly discovered chevkinite-(Ce)and monazite-(Ce) from tuffs and river sand in
the Noto Peninsula and Primorye. Ages of thesechevkinite-bearing or monazite-bearing tuffs provide important insight for the reconstruction of
the Japanese Islands before the opening of the
Sea of Japan.
Occurrence of chevkinite-bearing tuffs
In the Hegurinaka quarry, Boso Peninsula,more than ten tuff layers occur (Tsutsumi et al.,2012). The heavy fractions, speciﬁc gravity＞3.3, of all the tuffs were collected for moredetailed analyses under Energy Dispersive Spectrometry. As an accessory mineral, zircon was
found in all the samples. Among the tuffs, chevkinite-(Ce) is found as a common mineral in theheavy fractions of a tuff. Monazite-bearing tuff
occurs associated with the chevkinite-bearing
tuff (Tsutsumi et al., 2012). Chevkinite-(Ce) is
usually a euhedral crystal and occurs as a phenocryst ranging in size from 100 to 200 μm. The
age of the chevkinite tuff was 23 Ma and was
deposited in the deep-sea, showing that the constituent minerals were derived from the continental or island arc side.
As a possible source area of chevkinite-(Ce),we studied areas along the Sea of Japan, because
Oligocene to Miocene volcanic rocks are abundant. We collected samples from the Noto Peninsula, Sado Island, Oga Peninsula, and Nagato
areas in western Chugoku Province. Among thestudied samples, chevkinite-(Ce) was found only
in a dacitic tuff from the Goroku Formation in
the Noto Peninsula (Fig. 2). According to thegeological map presented by Yoshikawa et al.
(2002), the geological age of the Goroku Formation is at around the boundary between the Oligocene to Early Miocene, which is similar to thatfrom the Boso Peninsula. The Goroku Formationis composed mainly of dacite lava and volcaniclastic rocks. The chevkinite-bearing tuff collected is a thin layer with 15 cm thickness (Fig.
2). In addition to chevkinite-(Ce), the tuff con-Fig. 2. Outcrop of Chevkinite-bearing tuff in the
Goroku Formation from the Noto Peninsula andMonazite-bearing tuff from Nezino in Primorye, Far East Russia.

tains a phenocryst of monazite-(Ce). The size of
chevkinite-(Ce) is 100–200 μm, which is also thesize of the chevkinite-(Ce) from the Boso Peninsula (Fig. 3). From the associated dacite lava,
chevkinite-(Ce) was found only as an inclusionin zircon.
The Japanese Islands were located close to
Primorye, Far East Russia, before the opening of
the Sea of Japan at around 20 Ma. The eruption
ages of the chevkinite tuffs from the Boso andNoto peninsulas were around 23 Ma. Therefore,
as a possible occurrence of chevkinite-(Ce), wecollected samples from Primorye, Far East Russia. In Primorye, Tertiary sediments are distributed locally along the Sea of Japan where mostof the rocks are Pre-Tertiary granite and sedimentary rocks. The lava ﬂow from the Oligocene
occurs only in northern Primorye (Tatsumi et al.,Chevkinite-(Ce) from Boso, Noto and Primorye2000). Tuff and tuffaceous sandstone occur at thewestern part of Vladivostok. A tuffaceous sandstone sample was collected from a small quarry
in Nezhino (Fig. 2). It contains a phenocryst of
monazite-(Ce), but chevkinite-(Ce) is notobserved. A sand sample was collected from asmall river, the Nezhinka River, which cuts
through the Tertiary sedimentary rocks. Several
grains of chevkinite-(Ce) were found in theFig. 3. Back-scattered image of the chevkinites
from the Boso and Noto peninsulas and from
Far East Russia. Mineral abbreviation: ch＝chevkinite-(Ce), zir＝zircon.

91

heavy fraction of the sand. They are usuallyeuhedral in shape (Fig. 3), showing that they
were derived from a nearby source, i.e. Tertiary
sediments.
Chemical compositions of chevkinite-(Ce)The chemical compositions of chevkinites
from the Boso and Noto peninsulas and Primorye
were analyzed by a JEOL electron microprobe
analyzer. The accelerating voltage and current
were 15 kV and 20 nA for chevkinite-(Ce),respectively. The following synthetic and natural
materials were used as standards: wollastonite
for Si and Ca; anatase for Ti; sillimanite for Al;
Mg2SiO4 for Mg; Fe2SiO4 for Fe; rhodonite forMn; albite for Na; YP5O14 for Y; LaP5O14 for La;

Fig. 4. An empirical FeO vs. (CaO＋SrO) compositional variation of chevkinite-(Ce) from theBoso and Noto peninsulas and from Far East
Russia. (a) Comparison of the present samples
with chevkinite-group minerals from Russia
and Mongolia (Macdonald et al., 2012). (b)Enlarged plot of the present samples. The FeO
wt% is total Fe expressed as Fe2＋.

Koichi Momma et al.

92
Table 1.
Grain N.

SiO2

Chemical compositions of chevkinites from the Boso, Noto and from Primorye, Far East Russia.
TiO2

Nd2O3 Al2O3

FeO

CaO

ThO2

Y2O3

La2O3

Ce2O3

Pr2O3 Nb2O5 Sm2O3 Gd2O3 Dy2O3

Total

Chevkinite-(Ce) from the Boso Peninsula
1
2
3
4
6
7
8
9
10
11
12
13

21.15
20.87
20.69
21.02
21.31
21.23
21.06
21.24
20.98
20.88
20.92
20.93

19.12
19.16
18.45
18.53
18.47
19.02
18.80
18.09
18.93
19.06
19.24
19.29

5.78
6.17
6.81
7.26
7.01
6.82
7.12
7.33
7.32
7.08
7.13
6.95

0.44
0.28
0.41
0.36
0.28
0.32
0.37
0.43
0.40
0.51
0.47
0.38

10.76
11.06
11.08
11.18
11.05
10.72
10.90
10.49
11.02
10.70
11.00
11.12

2.72
2.86
2.71
2.73
3.00
3.07
2.65
2.88
2.66
2.95
3.13
2.71

1.72
1.13
1.09
1.53
1.14
0.97
1.42
1.48
1.80
1.74
1.53
0.71

0.92
0.58
0.70
1.07
0.95
0.58
0.93
0.85
0.95
0.90
0.93
0.67

11.36
12.70
11.21
10.29
10.91
11.68
11.65
11.26
11.13
11.16
10.44
12.21

21.38
21.09
21.73
21.13
21.57
21.77
21.15
21.39
21.68
20.97
21.05
20.88

2.36
2.52
2.32
2.63
2.47
2.42
2.33
2.39
2.26
2.34
2.37
2.26

0.47
0.29
0.44
0.52
0.49
0.30
0.46
0.56
0.51
0.43
0.50
0.29

0.90
0.69
1.01
0.92
1.02
0.66
0.64
1.06
0.78
1.09
1.03
0.61

0.61
0.45
0.38
0.62
0.46
0.37
0.34
0.62
0.60
0.54
0.55
0.17

0.33
0.19
0.27
0.43
0.22
0.19
0.26
0.27
0.33
0.36
0.26
0.20

100.02100.0399.29
100.21
100.35100.12100.10100.32101.33100.71100.55
99.37

avg.

21.02

18.85

6.90

0.39

10.92

2.84

1.36

0.84

11.33

21.32

2.39

0.44

0.87

0.48

0.27

100.20

Chevkinite-(Ce) from the Noto Peninsula
No.
1
2
3
4
5

SiO2
21.13
21.27
21.18
20.92
21.51

TiO2
18.77
19.62
19.45
18.56
18.52

Nd2O3 Al2O3
7.09 0.37
7.02 0.27
7.22 0.22
7.07 0.23
7.14 0.34

FeO
11.18
11.03
10.93
10.72
10.80

CaO
2.88
2.91
2.83
3.02
3.30

ThO2
1.31
0.61
1.07
1.04
0.70

Y2O3
0.67
0.41
0.71
0.87
0.45

La2O3
11.97
12.69
11.73
11.96
11.96

Ce2O3
21.31
21.93
22.23
21.90
21.89

Pr2O3 Nb2O5 Sm2O3 Gd2O3 Dy2O3
2.20 0.36 0.98 0.55 0.43
2.38 0.27 0.56 0.13 0.11
2.39 0.45 1.04 0.43 0.21
2.24 0.54 0.66 0.60 0.21
2.40 0.25 0.53 0.36 0.11

Total
101.19101.22102.09100.55100.25

avg.

21.20

18.98

7.11

10.93

2.99

0.94

0.62

12.06

21.85

2.32

0.21

101.06

0.29

0.37

0.75

0.41

Chevkinite-(Ce) from Nejino, Primorye, Far East Russia
No.
1
2
3
4
5
6
7
8

SiO2
21.19
20.85
21.24
21.01
21.14
20.75
21.17
19.31

TiO2
19.18
15.51
18.27
18.27
17.59
18.76
19.11
19.74

Nd2O3 Al2O3
7.55 0.38
6.85 0.07
7.70 0.27
7.42 0.30
7.50 0.30
7.46 0.31
7.19 0.43
6.38 0.37

FeO
10.25
11.97
11.03
10.67
10.56
10.57
10.31
10.42

CaO
2.82
2.53
2.62
2.27
2.45
2.24
2.98
2.99

ThO2
0.74
0.86
0.89
0.85
0.96
1.11
0.65
0.40

Y2O3
0.62
0.52
0.41
0.57
0.60
0.68
0.47
0.42

La2O3
12.18
11.37
11.53
12.42
11.19
11.92
12.43
12.96

Ce2O3
21.02
22.45
21.86
22.20
22.18
22.50
21.86
22.64

Pr2O3 Nb2O5 Sm2O3 Gd2O3 Dy2O3
2.45 0.32 0.63 0.32 0.23
2.06 2.56 1.02 0.62 0.39
2.22 0.28 0.76 0.67 0.22
2.47 0.36 0.68 0.51 0.37
2.39 0.44 0.75 0.37 0.41
2.45 0.32 0.61 0.17 0.30
2.40 0.13 0.49 0.26 0.29
2.39 0.48 0.65 0.58 0.18

Total
99.87
99.62
99.97
100.3798.84
100.15100.17
99.90

avg.

20.83

18.30

7.26

10.72

2.61

0.81

0.54

12.00

22.09

2.35

99.86

0.30

CeP5O14 for Ce; PrP5O14 for Pr; NdP5O14 for Nd;
SmP5O14 for Sm; GdP5O14 for Gd; DyP5O14 for
Dy; FeNb2O6 for Nb; MnTa2O6 for Ta; UO2 for
U; and ThO2 for Th. Chevkinite-(Ce) is mostly
homogeneous under backscattered electron
images (Fig. 3).
The chemical compositions of chevkinite-(Ce)from three localities are shown in Table 1. They
are plotted in the CaO–FeO diagram shown by
Macdonald & Bekin (2002). Although chevkinite-(Ce) shows a wide variation in CaO and
FeO, chevkinite-(Ce) from the three localities areplotted in a narrow region in Fig. 4. Even in the
more detailed diagram (Fig. 4b), chevkinites
from the Boso Peninsula are plotted in the samenarrow area as those from the Noto Peninsula.
On the other hand, chevkinites from Nezhino,
Primorye, are more variable than those from

0.61

0.70

0.44

0.30

the Boso and Noto Peninsulas in the detailed
ﬁgure. The difference is probably because thesandy grains in Nezhino were derived from somechevkinite-bearing tuffs in the drainage basin.
The empirical formulae of chevkinite-(Ce) from
Noto and Primorye, based on Si＝4 apfu, are(Ce 1.51La 0.84Ca 0.60Nd 0.48Pr 0.16Y 0.06Sm 0.05Th 0.042＋
3＋
Gd0.03Dy0.01)∑3.78(Ti2.69Fe1.00
Fe0.73Al0.06Nb0.03)∑4.51Si4.00O21.04 and (Ce1.55La0.85Ca0.54Nd0.50Y0.05Sm0.052＋
3＋
Th0.04Gd0.03Dy0.02Pr0.16)∑3.78(Ti2.64Fe1.00
Fe0.72
Al0.07Nb0.05)∑4.49Si4.00O21.03, respectively.
X-ray Crystallography
There are two structure-types in chevkinitegroup minerals: chevkinite-type (C2/m) and perrierite-type (P21/a) structures. The two structuretypes are topologically closely related and they

Chevkinite-(Ce) from Boso, Noto and Primorye
Table 2.

93

Summary of single-crystal X-ray diffraction experiments of chevkinite-(Ce) from Noto and Primorye.

Sample

Noto

Crystal Dimensions
Lattice Parameters0.080×0.070×0.040 mm
a＝13.4229(4) Å
b＝5.7444(2) Å
c＝11.0655(3) Å
β＝100.658(1)°V＝838.50(4) Å3

Space GroupZ value
Radiation
Structure Solution
Function Minimized
Dcalc
μ(Mo Kα)
Temperature2θmax
No. of Reﬂections Measured
UniqueCorrections
Weighting parameters, a, b
2θmax cutoff
No. Variables
R1 [I＞2σ(I)]R1 (all reﬂections)wR2 (all reﬂections)
Goodness of Fit Indicator
Final Δρmax (e/Å3)
Final Δρmin (e/Å3)

Primorye

0.050×0.040×0.030 mm
a＝13.3921(4) Å
b＝5.7388(2) Å
c＝11.0504(3) Å
β＝100.653(1)°V＝834.63(4) Å3
P21/a (#14)
2
MoKa (λ＝0.71075 Å, 50 kV, 24 mA)
Charge ﬂipping (Superﬂip)
Σw(Fo2−Fc2)2 where w＝1/[σ2(Fo2)＋(aP)2＋bP] and P＝(max(Fo2,0)＋2 Fc2)/3
4.592 g/cm3
4.550 g/cm3
12.160 mm−1
12.370 mm−1
25.0°C
23.0°C
59.1°
60.1°
9059
9509
2351 (Rint＝0.0460)
2442 (Rint＝0.0238)
Lorentz-polarization
Lorentz-polarization
Absorption
Absorption(trans. factors: 0.443–0.642)(trans. factors: 0.577–0.708)
0, 5.6271
0.0198, 3.6504
59.1°
60.1°
173
173
0.03290.02430.04080.02810.07790.0581
1.149
1.149
−
3
2.05 e /Å
2.58 e−/Å3−3−1.76 e /Å
−1.63 e−/Å3give similar powder X-ray diffraction patterns
that are difﬁcult to distinguish. To conﬁrm thestructure-type of the chevkinite samples from
Noto and Primorye, single-crystal X-ray diffraction studies were carried out using a microdiffractometer (Rigaku VariMax with RAPID)
equipped with a curved imaging plate withmonochromatized Mo Kα radiation. The Rigaku
RAPID AUTO software package was used forprocessing of the diffraction data, including theapplication of a numerical absorption correction.
The structure was solved by the charge ﬂipping
method using Superﬂip (Palatinus & Chapuis,2007). The SHELXL-97 software package (Sheldrick, 2008) was used for structural reﬁnement.
Atomic scattering factors for neutral atoms andthe coefﬁcients for X-ray dispersion corrections
were taken from the International Tables for
Crystallography Volume C (Wilson, 1992). Summaries of the data collection and structure reﬁnement are provided in Table 2. The single-crystal

X-ray diffraction study of chevkinite-(Ce) fromthe Boso Peninsula was reported elsewhere(Miyawaki et al., 2012).
The Noto and Primorye samples were conﬁrmed to have the chevkinite-type unit cell.
However, weak reﬂections violating extinction
conditions of the C2/m symmetry were observed
and the true symmetry was revealed to be P21/a,which is a subgroup of C2/m. Although P21/a isthe same space group as the perrierite-type structure, the unit cells of chevkinite and perrierite aredifferent, and the present samples have the chevkinite-type unit cell (Fig. 5). The reﬁned structures were basically consistent with the previously reported C2/m structures of chevkinite(Ce). The symmetry lowering from C2/m to its
subgroup P21/a is caused by small shifts of someof atoms from higher symmetry positions tolower symmetry positions. The reﬁned structure
parameters are listed in Tables 3 and 4. Based on
the number of electrons in each site, which wereKoichi Momma et al.

94
Table 3.

Final atomic coordinates and displacement parameters (Å2) of chevkinite-(Ce) from the Noto Peninsula.

Site

Occupancy

A1
A2
B
C1
C2a
C2b
Si1
Si2
O1A
O1B
O2
O3
O4
O5A
O5B
O6A
O6B
O7
O8

No. of e−

x

y

z

Ueq

Ce0.837(3)Ca0.15
Ce0.807(3)Ca0.15
Fe
Ti0.795(19)Fe0.205(19)Ti
Ti0.85(3)Fe0.15(3)
Si
Si
O
O
O
O
O
O
O
O
O
O
O

51.5
49.9
26
22.8
22
22.7
14
14
8
8
8
8
8
8
8
8
8
8
8

0.14381(2)0.06981(2)1/20.24883(6)01/20.20111(9)0.14218(10)0.0213(2)0.0225(2)0.1465(2)0.1860(3)0.1520(2)0.0707(3)0.0772(3)0.2260(3)0.2301(3)0.0848(3)0.2294(4)0.50559(5)−0.00897(6)00.24920(13)000.4994(2)0.0002(2)0.7539(6)0.2483(6)−0.0027(6)−0.0014(6)0.5014(5)0.7769(8)0.2328(7)0.2649(6)0.7340(6)0.5111(8)−0.0121(9)0.26605(3)0.74053(3)1/2
0.00009(8)000.73113(11)0.45389(12)0.1262(3)0.1261(3)0.9776(3)0.5978(3)0.0104(3)0.4059(4)0.4051(4)0.8145(3)0.8143(3)0.6704(4)0.3721(4)0.01579(11)0.01752(11)0.0132(2)0.0112(2)0.0122(2)0.0130(3)0.0107(3)0.0142(3)0.0143(7)0.0157(7)0.0135(6)0.0181(7)0.0142(6)0.0311(10)0.0290(9)0.0155(7)0.0163(7)0.0326(10)0.0391(12)

Site

U11

U22

U33

U23

U13

U12

A1
A2
B
C1
C2a
C2b
Si1
Si2
O1A
O1B
O2
O3
O4
O5A
O5B
O6A
O6B
O7
O8

0.01571(19)0.01198(19)0.0153(4)0.0115(3)0.0121(5)0.0127(5)0.0124(5)0.0149(5)0.0157(15)0.0150(15)0.0129(14)0.0217(16)0.0111(13)0.038(2)0.032(2)0.0244(18)0.0244(18)0.0164(17)0.039(2)0.01708(19)0.0288(2)0.0154(5)0.0125(4)0.0141(6)0.0135(6)0.0101(6)0.0181(7)0.0171(17)0.0162(17)0.0139(16)0.0205(18)0.0125(16)0.032(2)0.031(2)0.0101(15)0.0109(16)0.057(3)0.059(3)0.01404(17)0.01203(18)0.0081(4)0.0094(4)0.0108(5)0.0125(6)0.0095(6)0.0094(6)0.0104(16)0.0157(18)0.0147(15)0.0111(15)0.0181(16)0.021(2)0.024(2)0.0110(16)0.0131(17)0.022(2)0.026(2)0.00061(12)0.00018(14)0.0006(3)0.0001(3)0.0001(4)0.0003(4)−0.0001(4)−0.0004(5)0.0006(12)−0.0010(13)−0.0002(13)−0.0013(13)−0.0007(13)−0.0054(17)0.0054(17)−0.0013(12)−0.0026(13)0.0011(19)0.002(2)0.00135(11)0.00277(11)−0.0003(3)0.0012(3)0.0031(4)0.0010(4)0.0014(4)0.0019(4)0.0030(13)0.0022(13)0.0050(12)0.0005(13)0.0007(12)−0.0015(17)0.0028(17)0.0001(13)0.0021(14)−0.0026(15)0.0231(19)0.00011(12)0.00011(13)0.0004(4)0.0002(3)0.0002(4)0.0000(4)0.0002(4)−0.0008(5)0.0022(12)−0.0022(12)0.0006(12)−0.0020(14)−0.0006(12)−0.0145(18)0.0128(17)0.0016(13)−0.0022(13)0.0012(18)0.003(2)†Note: †Number of electrons in each site is calculated based on reﬁned site occupancy.

calculated from the reﬁned site occupancies, theB site is mainly occupied by Fe. The C2a site is
occupied by Ti, and the C1 and C2b sites areoccupied by Ti as well as a small amount of Fe.
Bond distances and polyhedral volumes of these
metal sites suggest that Fe at the B site is divalent, and Fe at other sites is largely trivalent.
Zircon age- LA-ICP-MS measurements
The zircon grains for analysis were hand-picked from heavy fractions of the chevkinite
tuff from the Noto Peninsula and of the monazite-bearing tuffaceous sandstone from theNezhino Quarry, Primorye, Far East Russia. Ageanalyses of zircons in the tuffs were carried out
using the Laser Ablation Inductively Coupled
Plasma Mass Spectrometer (LA-ICP-MS)
installed at the National Museum of Nature andScience. The method was shown in detail by Tsutsumi et al. (2012).
Almost all zircons in the samples show rhyth-Chevkinite-(Ce) from Boso, Noto and Primorye
Table 4.

95

Final atomic coordinates and displacement parameters (Å2) of chevkinite-(Ce) from Primorye.
No. of e−

x

y

z

Ueq

Ce0.853(2)Ca0.135
Ce0.819(2)Ca0.135
Fe
Ti0.799(14)Fe0.201(14)Ti
Ti0.84(2)Fe0.16(2)
Si
Si
O
O
O
O
O
O
O
O
O
O
O

52.2
50.2
26
22.8
22
22.7
14
14
8
8
8
8
8
8
8
8
8
8
8

0.143607(17)0.069259(17)1/20.24791(4)01/2
0.20050(7)0.14293(7)0.02195(19)0.02295(19)0.14622(18)0.1859(2)0.15181(18)0.0687(2)0.0799(2)0.2246(2)0.2324(2)0.0834(2)0.2319(3)0.50851(4)−0.01413(4)00.24861(9)000.49894(15)0.00049(16)0.7547(4)0.2490(4)−0.0038(4)−0.0043(4)0.5022(4)0.7815(5)0.2347(5)0.2632(4)0.7328(4)0.5195(6)−0.0192(6)0.266318(19)0.74018(2)1/20.00000(5)000.73113(8)0.45376(8)0.1266(2)0.1260(2)0.9775(2)0.5981(2)0.0105(2)0.4051(3)0.4051(3)0.8142(2)0.8144(2)0.6717(3)0.3740(3)0.01348(7)0.01460(8)0.01176(16)0.00987(16)0.01104(18)0.0118(2)0.00924(18)0.01129(19)0.0149(5)0.0136(5)0.0126(5)0.0159(5)0.0136(5)0.0242(6)0.0225(6)0.0148(5)0.0143(5)0.0284(7)0.0347(8)

Site

U11

U22

U33

U23

U13

U12

A1
A2
B
C1
C2a
C2b
Si1
Si2
O1A
O1B
O2
O3
O4
O5A
O5B
O6A
O6B
O7
O8

0.01432(14)0.01030(14)0.0157(3)0.0109(3)0.0118(4)0.0124(4)0.0108(4)0.0134(4)0.0146(12)0.0150(12)0.0110(11)0.0194(12)0.0108(11)0.0306(16)0.0249(14)0.0236(13)0.0226(13)0.0142(13)0.0334(17)0.01345(12)0.02386(14)0.0115(3)0.0107(3)0.0125(4)0.0117(4)0.0084(4)0.0130(4)0.0171(12)0.0156(12)0.0132(11)0.0187(12)0.0120(11)0.0217(14)0.0223(14)0.0105(11)0.0093(11)0.049(2)0.052(2)0.01227(12)0.00978(12)0.0072(3)0.0077(3)0.0094(4)0.0107(4)0.0083(4)0.0074(4)0.0131(12)0.0107(11)0.0148(11)0.0092(11)0.0178(12)0.0178(13)0.0191(14)0.0097(11)0.0104(11)0.0198(14)0.0250(16)0.00088(8)0.00022(9)0.0002(2)0.00008(19)0.0003(3)−0.0007(3)0.0001(3)0.0003(3)0.0024(9)−0.0015(9)−0.0003(9)0.0001(9)0.0013(9)−0.0028(11)0.0051(11)0.0017(9)−0.0009(8)−0.0013(13)0.0061(14)0.00138(8)0.00224(8)−0.0001(2)0.00084(18)0.0034(3)0.0008(3)0.0015(3)0.0019(3)0.0027(9)0.0039(9)0.0052(9)0.0011(9)0.0018(9)−0.0018(11)0.0007(11)0.0016(9)0.0013(9)−0.0033(10)0.0210(14)0.00004(8)−0.00013(9)0.0005(2)0.00015(19)0.0003(3)0.0002(3)0.0006(3)0.0001(3)0.0026(9)−0.0029(9)0.0010(9)−0.0001(10)0.0008(9)−0.0111(12)0.0088(11)0.0021(9)−0.0007(9)0.0011(13)0.0081(15)Site

Occupancy

A1
A2
B
C1
C2a
C2b
Si1
Si2
O1A
O1B
O2
O3
O4
O5A
O5B
O6A
O6B
O7
O8

†

Note: †Number of electrons in each site is calculated based on reﬁned site occupancy.

mic oscillatory and/or sector zoning on backscattered electron and/or cathodoluminescence
images, which is commonly observed in igneous
zircons. Fig. 6 shows Tera–Wasserberg concordia
diagrams for all analyzed spots from zircons
from the Noto Peninsula and Nezhino quarry
obtained by LA-ICP-MS. All zircon age data of
the sample from the Noto Peninsula (24 spots
from 13 grains) cluster around ~25 Ma, and theirweighted mean age yields 23.5±0.7 Ma (95%conf.; MSWD＝0.97) (Fig. 6a). The zircons in

the Nezhino tuffaceous sandstone indicate various ages around 25, 29, 70, 100, 180, 250, and
490 Ma. (Fig. 6b). The youngest age cluster consists of 2 data, 25.2±0.8 Ma and 25.1±0.9 Ma(1σ). Therefore the deposition age of the sample
is thought to be younger than 25 Ma.
Discussion
Even though an enormous quantity of studies
about volcanic ashes and tuffs in the JapaneseKoichi Momma et al.

96

Fig. 5. Comparison of the structures of (a) chevkinite-(Ce) (this study, the Primorye sample) and(b) perrierite-(La) (Calvo & Faggiani, 1974).
Although the two structures have the same
topology of polyhedral arrangements, unit cells
and positions of symmetry elements in the unit
cells are different.

Fig. 6. Tera–Wasserburg U–Pb concordia diagrams and the age distribution plot of samples
from the Noto Peninsula (a) and Nezhino
quarry (b). 207Pb* and 206Pb* indicate radiometric 207Pb and 206Pb, respectively.

Islands have been conducted (e.g. Machida &Arai, 1992; Takahashi et al., 2001), neither chevkinite nor monazite has previously been reported
as a phenocryst. Such a rare mineral was foundin a tuff layer from the Hegurinaka quarry, Boso
Peninsula (Tsutsumi et al., 2012). It is the ﬁrst
record from a tuff layer in East Asia. Such a raremineral was found in the tuff from the Noto Peninsula and from sands in the river cutting throughTertiary sediments in Nezhino, Far East Russia.
It is probable that the latter chevkinite-(Ce) was
also derived from tuff layers because they are
100–200 μm in size and usually euhedral, similar
to those from the Boso and Noto Peninsulas. Furthermore, chemical compositions are the same orsimilar to each other. And also, monazite as aphenocryst occurs in the tuffs from three localities. In addition to these identical natures, theages of chevkinite tuffs from the Boso and Noto
peninsulas and monazite tuff from the Nezhino
Quarry are around 23 Ma, which is the end of the
Oligocene. From these occurrences, it is concluded that the tuff was derived from the same
volcanic eruption or consanguineous volcanic
event. Tuff from the Boso Peninsula is a deep-sea
deposit, whereas the other two tuffs are terrestrial
deposits. Lava ﬂows at the end of the Oligocene
are common in the Noto Peninsula and are absent
in the Nezhino area. The most possible story isthat a volcano including chevkinite and/or monazite erupted around the Noto Peninsula and ash
fall was carried to the ocean of Boso and theChevkinite-(Ce) from Boso, Noto and Primorye

Fig. 7. Schematic illustration of the formation of
the Sea of Japan.

inland area of Primorye, Far East Russia. Reconstruction models at the opening of the Sea ofJapan have been presented by many authors. Fig.
7 shows one of the models where the Noto Peninsula is close to the Nezhino area. It is reasonable to conclude that a volcano in the Noto Peninsula provided ash to both the Boso Peninsula
and the Nezhino area. Volcanism at the end of
the Oligocene occurred along the Sea of Japan.
Further study around the Noto Peninsula willcontribute to the geotectonic reconstruction at thetime of the opening of the Sea of Japan.
Acknowledgement
We would like to thank Dr. Sergei A. Kasatkin
and Dr. Igor Chekryzhov to take us to theNezhino quarries in Primorye, Far East Russia.
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房総，能登，およびロシア極東沿海地方に分布する
チェフキン石を含む凝灰岩
門 馬 綱 一・堤之 恭・佐 野 貴 司・宮 脇 律 郎・重 岡 昌 子・横 山 一 己

房総半島，能登半島の漸新世から中新世の凝灰岩中より，チェフキン石を確認した．またロシア極東沿海地方のタフ中からモナズ石を確認し，近くの川砂からチェフキン石を確認した．房総と能登の凝灰岩のジルコン年代は日本海の拡大が始まる直前の約 23 Maで，ほぼ同年代であり，ロシア極東沿海地方のモナズ石を含む凝灰質砂岩の年代も比較的近い．モナズ石やチェフキン石を含むタフ凝灰岩は非常に稀であるにも関わらず，これら3 ヶ所の堆積物は時代的に近く，含まれるチェフキン石の化学組成もほぼ共通することか
ら，これらの凝灰岩は同一噴火による堆積物である可能性が高く，日本海拡大前のプレー
ト位置復元の鍵となる．