名古屋大学博物館報告
Bull. Nagoya Univ. Museum
No. 38, 1–21, 2023
DOI: 10.18999/bulnum.038.01

Geochemistry and K-Ar age of the mafic rocks at the Uubulan area,
Khangai-Daur belt, Central Asian Orogenic belt, central Mongolia
Bayart NADMID1）*, Kazuhiro TSUKADA2）, Munkhtsetseg OIDOV3）,
Sharav DAVAANYAM4）, Manchuk NURAMKHAAN5）, Lodoidanzan ALTANSUKH3）,
Khishigbadam DAVGADORJ3）, and Kanta UMEDA6）
1）Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8601, Japan
2）Nagoya University Museum, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan3）Department of Geology and Hydrogeology, School of Geology and Mining, MUST main building, 8th khoroo,
Baga toiruu, Sukhbaatar district, Ulaanbaatar, 14191, Mongolia
4）Institute of Geology, Mongolian Academy of Sciences, Trade Union Street, 18th khoroo, Songinokhairkan district,Ulaanbaatar, 18080, Mongolia
5）Academic Aﬀairs, Branch of Mongolian University of Science and Technology, Division of Erdenet Mining Co.,
Erdenet, Mongolia
6）Technical Planning and Coordination Department, Japan Petroleum Exploration Co., Ltd., Marunouchi, Chiyoda-ku,
Tokyo, 100-0005, Japan*Corresponding author: Bayart Nadmid (n.bayart@yahoo.com)

Abstract
This paper describes the whole-rock geochemical composition, clinopyroxene (Cpx) chemical composition, and thehornblende K-Ar age of the mafic rocks in the Carboniferous accretionary complex of the Khangai-Daur belt, Central Asian
Orogenic belt, exposed at the Uubulan area of Mongolia. The geochemical nature of the mafic rocks, enriched in large
ion lithophile elements and light rare-earth elements compared with high field-strength elements (HFSE) and heavy rareearth elements, suggest that they are alkaline basalts. The (La/Yb)cn vs. HFSE diagrams show that the samples are oceanicisland basalts. Therefore, the mafic rocks are concluded to be oceanic-island alkaline basalts. The comparatively highconcentrations of Al, Ti, Na, and Ca in the Cpx of the mafic rocks strongly support this view. The mafic rocks just besidethe Upper Silurian radiolarian chert give a hornblende K-Ar age of 412.7±8.6 Ma. The fact that Upper Silurian chert
strongly suggests that the oceanic-island alkaline basalts are conformably underlain by the radiolarian chert formed at thepelagic region as a base of oceanic plate stratigraphy.
Keywords: Mafic rocks, K-Ar age, Carboniferous accretionary complex, Khangai-Daur belt, Central Asian Orogenic
belt.

Introduction
The Central Asian Orogenic belt (CAOB: Jahn et al., 2004; Xiao et al., 2003; Windley et al., 2007; Petrov etal., 2014) is the largest Paleozoic–Mesozoic orogenic belt in the world (Fig. 1a). The development process of the
CAOB, which lies among the Siberian craton, East European craton, Tarim block, and North China block (Sengör
et al., 1993), is essential to understand the tectonic history of the Eurasian continent (Kovalenko et al., 2004). The
CAOB is generally considered to have been formed by the subduction-accreation process of the oceanic plate,magmatism of a volcanic arc magmatism, and collisions of continental fragments during the amalgamation of
―1―

a

40° E

60° E

100° E 120° E

80° E

East
EuropeancratonCe

140° E

60° N
Sea
of
Okhotsk

Siberiancratonntr
alA

Kazakhstan

Russiasian btbel

Orogenic
Mongolia

800 km Tarim block

North China
block

China

b

40° N

102° E
114° E

108° E

96° E

50°N

7

Northern superblock
Sayan-Baikal belt(Con�nental aﬃnity)

1
90° E

N

Main

2

Mongo

lian

500 km

4
3

Lin

t

en

m
ea

7
5
6

Khangai-Daur belt

(Late Paleozoic–Early Mesozoic
accre�onary complexes)

46°N

Fig. 2Southern superblock
42°N

Fig. 1a) Index map (modified from Petrov et al., 2014); b) simplified tectonic division of northern Mongolia: 1, Zag terrane;
Fig. 1 a and b
2, Tsetserleg terrane; 3, Kharhorin terrane; 4, Kharaa terrane; 5, Asralt-Khairhan terrane; 6, Ulaanbaatar terrane; 7, Onon
terrane. Modified from the study by Badarch et al. (2002), Tomurtogoo (2003), and Tsukada et al. (2018).

these cratons and blocks (Sengör et al., 1993).
The geological information of Mongolia, between the North China block and the Siberian craton, oﬀers a key tounderstanding the Paleozoic–Mesozoic tectonics of the southern margin of the “Siberian continent (Siberian craton+ accreted geologic units)” at southern CAOB. Further, the general features of the Khangai-Daur (also spelled
as Hangay-Hentey or Khangai-Hentei or Khangai-Khentei or Hangai-Daur) and Sayan-Baikal belts, a significant
constituent of northern Mongolia, has been substantially established (Badarch et al., 2002; Tomurtogoo, 2003;
Kurihara et al., 2009; Onon, 2017 MS; Onon and Tsukada, 2017: Fig. 1b).
The Khangai-Daur belt has been subdivided into the following seven geologic units: Kharaa, Zag, Tsetserleg,
Kharhorin, Asralt-Khairhan, Onon, and Ulaanbaatar terranes (Tomurtogoo, 2012; Onon and Tsukada, 2017:―2―

Fig. 1b). The Kharaa and Zag terranes are composed mostly of shelf facies rocks, and others are of accretionary
complexes (Onon and Tsukada, 2017). Onon (2017 MS) pointed out that the Kharaa and Zag terranes are similar
to the Sayan-Baikal belt rather than the other terranes of the Khangai-Daur belt in lithostratigraphy and excluded
these terranes from the Khangai-Daur belt to incorporate them into the Sayan-Baikal belt. This paper regards
the rocks of the Tsetserleg, Kharhorin, Asralt-Khairhan, Onon, and Ulaanbaatar terranes as the component of theKhangai-Daur belt, following Onon (2017 MS).
The Khangai-Daur belt is composed mainly of clastic rocks with minor amounts of radiolarian chert, siliceous
mudstone, mafic rocks, and limestone (Tomurtogoo et al., 1998, Badarch et al., 2002; Tomurtogoo, 2003; Kurihara
et al., 2009; Takeuchi et al., 2012; Tsukada et al., 2013; Ruppen et al., 2013; Onon and Tsukada, 2017). Kurihara
et al. (2009) revealed that the Ulaanbaatar terrane consists of the Late Paleozoic accretionary complex formed by
the subduction of the previous oceanic plate, and they established Late Silurian to Late Devonian oceanic-plate
stratigraphy (OPS) in the Gorkhi Formation, Ulaanbaatar terrane, based on radiolarian biostratigraphy.
The OPS can provide key information regarding the continuous evolution of oceanic plates (Matsuda andIsozaki, 1991; Wakita and Metcalfe, 2005). Kurihara et al. (2009) recovered Late Silurian conodonts from the chert
beside the mafic rocks exposed at the Uubulan area to define the basal part of the OPS in the Gorkhi Formation(Figs. 2 and 3). Although the mafic rocks have been presumed to be Upper Silurian because of the conodont fromthe chert, its precise age has not been revealed. Additionally, the mafic rocks were estimated to have an aﬃnityof oceanic alkaline basalt by X-ray ﬂuorescence (XRF) analysis (Tsukada et al., 2013); however, more accurate
geochemical data is required to clarify its detailed geochemical feature.
The geochemistry and age of the mafic rocks, which is the lowermost unit of OPS, in the accretionary complex
is an important factor in understanding the evolution of ancient oceanic-plates.
This paper describes the lithology, detailed whole-rock chemical composition, clinopyroxene (Cpx) chemicalcomposition, and hornblende K-Ar age of the mafic rocks beside the Upper Silurian chert of the Gorkhi formation
at the Uubulan area and discusses its tectonic setting and OPS position.
Geological outline
Mongolia is geologically divided into the northern and southern superblocks by the Main Mongolian Lineament.
The Khangai-Daur belt in the northern superblock, approximately 300 km wide and 1200 km long, is mostly
composed of Late Paleozoic accretionary complexes (Tomurtogoo, 2003; Onon 2017 MS) (Fig. 1b).
The accretionary complex of the Khangai-Daur belt is divided into the following five geological units:
Tsetserleg, Kharhorin, Asralt-Khairhan, Onon, and Ulaanbaatar terranes (Onon 2017 MS). The Ulaanbaatar,
Kharhorin, and Tsetserleg terranes are composed of Late Paleozoic accretionary complexes (Kurihara et al.,2009; Purevjav and Roser 2012; Tsukada et al., 2013) (Fig. 1b). The Asralt-Khairhan terrane is considered ametamorphosed part of the Ulaanbaatar terrane (Tomurtogoo 2012; Gordienko et al., 2012). The Onon terraneconsists of deformed and metamorphosed accretionary complexes, fragments of ophiolite, and minor arc-related
volcanic rocks (Bulgatov and Gordienko 1999; Zorin, 1999; Badarch, 2005; Tomurtogoo, 2012).
The rocks of the Ulaanbaatar terrane are subdivided into the following five geological units from south tonorth: Unegt, Sergelen, Dov, Gorkhi, and Altan-Ovoo & Orgioch-Uul formations (Dorjsuren et al., 2006; Minjin
et al., 2006; Kurihara et al., 2008; Takeuchi et al., 2012; Sharav et al., 2019) (Fig. 2). The Unegt, Sergelen, andDov formations are of deformed and metamorphosed clastic rocks and chert (Sharav et al., 2019). Generally, themetamorphism of the rocks is strong in the south, Sergelen Formation, and relatively weak toward the north to―3―

107 15’oo

106 45’107 00’

Selbe River

o

N

er
Tuul Riv

47 50’

Ulaanbaatar
Ulaanbaatar
city

Tuul RiveroNalaikh

r

ive

lR

u
Tu

Fig. 347 40’

Zuunmod

LegendsoTriassic to Jurassic granitoid
Altan-Ovoo&Orgioch-Uul F.
Gorkhi Formation

Sergelen

Dov Formation
Sergelen Formation

City & town

10 km

Fault

Fig. 247 30’

Unegt FormationoSimplified geologic map of the Ulaanbaatar area. The geological division follows that of Dorjsuren et al. (2006). F.:
Fig. 2Formation.

the Dov Formation (Sharav et al., 2019). The Gorkhi Formation comprises Upper Silurian to Upper Devonian
radiolarian cherts, along with siliceous mudstone and clastic rocks. The Altan-Ovoo & Orgioch-Uul Formationconsists mostly of sandstone, mudstone, and conglomerate with a minor amount of chert and felsic tuﬀ (Nakane etal., 2012; Suzuki et al., 2012; Takeuchi et al., 2012).
Tsukada et al. (2013) showed that the mafic rocks in the Gorkhi Formation are oceanic-island alkaline rocks.
Kurihara et al. (2009) reported Upper Silurian to Upper Devonian radiolarians and conodonts from the chert in the
Gorkhi Formation. Kelty et al. (2008), Bussien et al. (2011), and Hara et al. (2013) assumed that the sandstone of
the Gorkhi Formation was formed after the earliest Carboniferous based on detrital zircon chronology. Hara et al.
(2013) mentioned that the geochemical data of the sandstone and mudstone of this formation suggests a continental
margin or continental island-arc environment.
The Gorkhi Formation is probably unconformably overlain by the Carboniferous shallow marine formation,yielding bryozoans and brachiopods (Takeuchi et al., 2012), and intruded by the Triassic to Jurassic granitoid,
showing biotite K-Ar ages of 220–205 Ma (Magic Project 1998; Gerel and Lkamsuren 1999).
Geological description at the Uubulan area
At Uubulan, southeast of Ulaanbaatar city, the mafic rocks and chert of the Gorkhi Formation are exposed (Figs.
3 and 4).
―4―

N
50

60

1520 m

1500 m1480 m45

35

Legend40-Chert-Mafic rocks- Location of sampling points
45- Bedding plane- Fault- Inferred fault

100 m- Middle Devonian radiolarian locality- Late Silurian conodont locality- K-Ar age dating point

Route map showing outcrops of the mafic rocks and chert of the Gorkhi Formation at the Uubulan area with sampling points
Fig. 3for geochemical examination (blue circle) and dating (red star). Microfossil locations are indicatedby orange and sky-bluestars.

SE

NW
Fig. 5b

Middle Devonian
radiolarian locality

Fig. 5 a & c

K-Ar age
dating point
fau
lt

inf
err
ed
fau
lt

Fig. 3Late Silurian
conodont locality

Fig. 5d

15 m

Fig. 4

Aerial view of the Uubulan area from the southwest, showing outcrops of chert
Fig. 4and mafic rocks.

―5―Red chert, trending NE–SW and steeply dipping north or south, is exposed at the north and south of the mafic
rocks of dolerite, and gabbro (Fig. 5 a–d). South of the mafic rocks, the gray/reddish gray impure chert (graytuffaceous chert in Kurihara et al., 2009) is covered by gray and red bedded radiolarian chert and overlain bymassive chert. The bedded chert (approximately 5–7 cm thick in a bed) is folded and commonly cut by quartz
veins. The conodonts suggesting Upper Silurian were reported from the red bedded chert (Kurihara et al., 2009).
Red, gray/reddish gray chert is exposed near the peak of the hill, north of the mafic rocks. Middle Devonian
radiolarian assemblage was obtained from the chert (Kurihara et al., 2009). The mafic rocks are probably in fault
contact with the Middle Devonian red, gray/reddish gray chert.

(b) Gray impure

(a)

chert

Red bedded
chert
(c)

(d)

Reddish chert

Gray tufficeous
chert

(f)

(e)

Cpx

Hbl

Hbl
Pl

Hbl

Hbl
Cpx

Cpx

0.5 mmFig. 5

Hbl

Cpx

1 mm

Field photographs and photomicrographs of the rocks in the Uubulan area: (a) field occurrence of dolerite, (b) grayimpure and red bedded chert, (c) close-up of the medium-grained dolerite, (d) the gray/reddish gray impure chert, (e) the
Fig.5
photomicrograph of the Cpx-dominant dolerite, and (f) the photomicrograph of the hornblend-dominant dolerite/gabbro.
Cpx: clinopyroxene; Hbl: hornblende; Pl: plagioclase calcite.

―6―

Among the mafic rocks in this area, dolerite is dominant, followed by gabbro. The relationship among themis unclear because of poor exposure. The dolerite, showing ophitic or spinifex-like texture, consists of euhedral
plagioclase, hornblende, and anhedral Cpx (Fig. 5e). Minor amounts of opaque minerals are included. The gabbro
is hypocrystalline and consists of medium to coarse-grained (>1 mm) plagioclase, hornblende, and Cpx (Fig. 5f).
The hornblende is euhedral, dark brown. The Cpx is anhedral.
Methodology
1.1

Whole-rock chemical composition analysis

The dolerite samples were coarse-crushed using a steel bowl and pounder. After the coarse crushing, alteredparts and veins were removed from all samples under a stereomicroscope. Each sample was >100 g. The
concentrations of the major elements and Th were determined using XRF (Rigaku Primus II ZSX equipped with
Rh X-ray tube, 50 kV, 60 mA), and trace elements, except for Th, and rare earth elements (REE) were analyzed byquadrupole-type inductively coupled plasma–mass spectrometry (ICP–MS; Agilent 7700x with collision cell ofHe) at Nagoya University.
For the XRF analysis, the glass beads were prepared by fusing mixtures of 1.5 g of powdered sample with
6.0 g of lithium tetraborate for the major elements and Th analysis. Calibration was conducted using standard
rock samples issued by the Geological Survey of Japan and the composite standards prepared by Yamamoto
and Morishita (1997). The analytical precision of the major elements was estimated to be <1% for Si andapproximately 3% for other elements, except for CaO, MgO, and Na2O, whose analytical precision was >3% whenthe measured level was <0.1% (Takebe and Yamamoto, 2003), and that for trace element was estimated to be lessthan 10% (Yamamoto and Morishita, 1997).
Most trace elements and REE composition were determined using ICP–MS with a technique based on themethodology described by Yamamoto et al. (2005). Approximately 30 mg of each sample was digested with amixed solution of HF–HClO4 (2:1 by volume) at 150°C. After the acids had completely evaporated, 2 mL of 1.7
N HCl was added to dissolve the cake. Thereafter, the residue was separated by centrifugation at 12,000 rpm
using a 2-mL polypropylene tube. After centrifugation, the supernatant was transferred to another 10-mL Teﬂon
beaker, and the residue was again fused with HF–HClO4 (2:1 by volume) at 150°C. Afterward, the fused cakewas dissolved with approximately 2 mL of 1.7 N HCl by mild heating, and the resulting solution was centrifuged
at 12000 rpm. No residue was observed after centrifugation in most cases. Subsequently, the HCl solution was
evaporated to dryness, and the fused cake was redissolved in a 2% HNO3 solution and analyzed using ICP–MS.
In and Bi were used to trace ICP sensitivities, and the In and Bi concentrations were mostly identical throughout
the analysis. The oxide generation factor (LnO/Ln) was determined for each 20 ppb solution and used for REE
analytical data correction. In the ICP–MS analysis, the correlation coefficients (R values) of each element,
calculated for five standard samples, were 0.9994–1.0000, and the concentration relative standard deviation of thedata was mostly < 3%.
1.2

Mineral chemical composition analysis

The Cpx chemical composition in the samples was determined using a method described by Tsukada (2018)using energy-dispersive X-ray spectrometer (EDX, Oxford X-Max) linked with scanning electron microscope(SEM, Hitachi S-3400N) at Nagoya University. The SEM-EDX analyses were performed with an acceleration
voltage of 15 kV, analytical time of 40 seconds, and a working distance of 10 mm. Quartz (for Si) and augite (for―7―

Ti, Al, Fe, Mg, Ca, and Na) were used for analytical standards. The total values of each analysis were normalized
as 100%, following Tsukada (2018). The analytical precision of the analysis was estimated to be <5% when themeasured level was more than 1 wt%, and <10% when the measured level was less than 1 wt% (Tsukada, 2018).
Elements with a concentration of less than 0.5 wt% may not have been detected (Tsukada, 2018).
1.3

Hornblende K-Ar dating

The hornblende in the sample for the K-Ar dating was observed, and a significant alteration was not recognized(Fig. 5f). The sample was coarse-crushed using a steel bowl and pounder. Afterward, the hornblende wasconcentrated using heavy liquid (Na polytungstate solution) and ferrite and neodymium magnets.
The K-Ar dating was performed by the Hiruzen Institute for Geology and Chronology, Japan. Approximately 50
mg of hornblende, powdered in an agate mortar, was placed in a Teﬂon container and decomposed with HNO3, FH,
and H2O2 in a closed system box for more than 12 hours. After decomposition, the sample solution was evaporated
to dryness in a closed system, and thereafter, the fused cake was re-dissolved with HCl for analysis. The analysis
was performed using a ﬂame atomic absorption spectrometer (Hitachi 180-30).
Ar isotope ratio was measured using a mass spectrometer (HIRU, Itaya et al., 1991) by isotope dilution method(Nagao et al., 1984; Nagao and Itaya, 1988; Itaya et al., 1991). Approximately 50 mg of sample was used for the
analysis. The sample was evacuated at approximately 180°C to 200°C for three days to remove the adsorbed Ar.
After reducing the background in the analytical system, the sample was dissolved at approximately 1500°C. The
gas extracted from the sample was mixed with the spike (nearly 100% pure 38Ar). The gas other than inert gas wasremoved using a Ti-Zr getter, and the purified gas was introduced into the mass spectrometer.
Analytical results
1.4

Whole-rock geochemical characteristics of the mafic rocks

Major elements, trace elements, and REE compositions of the mafic rocks from the Uubulan area are listed inTable 1. The SiO2 concentration of the samples is 44.5–48.9 wt%. The loss of ignition (LOI) is <3.3 wt% for most
samples.
In variation diagrams (Fig. 6), the examined samples have nearly identical TiO2, Al2O3, Fe2O3, MnO, P2O5,
V, Cr, Co, Hf, Ni, Zn, Ta, Pb, Zr, Nb, and Y concentrations. Na2O and Sr increase and MgO and CaO decreaseagainst increasing SiO2. A mid-ocean ridge basalt (MORB)-normalized multi-element pattern (called “spidergram”
hereinafter) shows enrichment in large ion lithophile elements (LILE) compared to high-field strength elements(HFSE) (Fig. 7). In the chondrite-normalized REE patterns, it is observed that samples are enriched in light rareearth elements (LREE) and decrease toward heavy rare-earth elements (HREE) (Fig. 8).

―8―

―9―

10.3

15.8

12.6

0.17

6.24

10.4

3.24

1.46

0.75

99.9

2.97

Al2O3

Fe2O3*

MnO

MgO

CaO

Na2O

K2O

P2O5

Total

LOI

248

12.1

Th**

1.14

2.85

0.351

2.17

0.281

Ho

Er

Tm

Yb

Lu

1.15

6.57

Tb

8.61

Gd

Dy

9.27

3.09

Sm

Eu

0.289

2.18

0.314

2.84

1.06

6.21

1.16

8.14

2.99

9.13

42.7

10.3

10.6

42.6

Pr

Nd

87.9

43.2

87.8

La

43.7

3.90

5.69

5.71

4.89

539

72.8

613

23.5

116

Ce

Rare earth elements (ppm)

6.23

5.65

Hf

Ta

503

20.4

Ba

Pb

257

70.7

559

27.7

Sr

Y

Zr

23.9

Rb

Nb

26.9

114

Zn

41.1

56.8

39.0

45.9

Co

13.9

289

3.25

99.7

0.74

1.48

3.26

0.19

12.9

16.1

Ni

297

26.6

V

Cr

Trace elements (ppm)

6.18

3.34

3.34

46.0

SiO2

TiO2

45.2

0.284

2.13

0.365

2.96

1.28

7.15

1.30

9.19

3.22

9.74

46.1

11.3

93.0

44.9

5.80

5.54

6.19

3.17

472

69.6

261

29.6

538

27.5

108

36.4

33.0

60.9

265

3.12

100

0.88

1.93

3.16

10.5

6.09

0.17

11.9

15.4

3.26

46.8

0.321

2.33

0.391

3.18

1.26

7.24

1.29

10.1

3.36

10.6

50.4

12.6

105

52.1

4.80

5.90

5.85

4.19

483

76.0

257

31.6

508

32.7

116

32.3

32.5

9.45

233

3.54

99.9

0.88

2.36

2.75

10.6

5.54

0.18

12.3

16.3

3.20

45.7

0.269

2.03

0.346

2.98

1.12

6.39

1.18

8.47

3.03

8.85

42.9

10.7

90.6

44.1

9.50

5.74

5.76

3.56

491

74.1

250

27.7

549

27.0

135

40.1

36.5

9.60

269

4.05

99.6

0.77

2.20

2.97

10.2

6.25

0.18

12.6

16.1

3.27

45.1

0.254

2.11

0.365

2.99

1.22

7.03

1.26

9.47

3.26

10.0

48.9

12.1

100

48.3

11.0

5.60

5.81

4.94

552

75.0

259

29.7

514

29.3

122

34.5

35.6

11.3

255

3.37

99.6

0.90

2.24

2.94

10.1

5.69

0.18

12.3

16.2

3.29

45.8

0.245

1.94

0.316

2.81

1.09

6.57

1.16

8.73

2.97

9.18

44.8

11.0

91.1

44.3

9.30

6.03

5.06

3.48

691

79.2

208

27.6

696

39.7

123

40.6

39.5

8.51

267

3.72

100

0.78

2.78

3.42

6.40

6.77

0.20

13.0

16.5

3.37

46.7

0508218163a 0508218163b 0508218163c 0508218163d 0508218163e 0508218164f 0508218163g

element analyzed by XRF.

0.264

1.93

0.311

2.75

1.12

6.41

1.16

7.96

2.73

8.23

35.0

7.93

62.2

28.4

11.6

3.43

6.33

2.23

616

39.5

223

27.2

456

49.0

73.0

78.4

40.7

489.6

319

5.27

99.4

0.47

2.08

2.38

16.6

7.55

0.14

9.0

12.2

3.04

46.0

608197180

0.242

1.77

0.291

2.56

1.10

5.72

1.02

7.84

2.83

8.19

38.7

9.17

76.5

36.3

17.7

4.39

5.41

3.73

331

58.0

217

24.9

592

23.2

118

77.5

43.4

296

282

1.56

100

0.64

0.94

4.54

9.39

5.53

0.21

12.4

14.2

3.25

48.9

608197181

0.225

1.99

0.322

2.68

1.16

6.33

1.17

8.97

3.14

9.52

46.7

11.8

98.4

48.3

9.80

6.23

4.83

3.41

631

79.3

199

27.3

556

42.8

124

34.5

40.2

6.97

261

3.62

100

0.88

2.88

3.12

6.73

6.37

0.19

14.3

16.3

3.40

46.0

608197182

0.260

1.87

0.280

2.65

1.07

5.91

1.07

8.02

2.66

7.81

34.3

7.87

62.2

28.7

8.20

3.43

6.06

2.51

236

40.2

214

25.6

550

33.3

82.2

83.7

31.5

801

316

3.47

99.5

0.45

1.33

2.58

15.2

7.41

0.15

11.5

12.2

3.00

45.7

608197183

0.271

1.92

0.345

2.72

1.09

6.53

1.11

8.34

2.97

9.00

44.3

10.5

89.0

43.3

12.3

5.32

5.62

3.76

388

70.5

245

27.2

1180

55.2

125

43.1

32.4

76.6

280

0.87

99.9

0.74

1.75

4.28

10.0

4.50

0.18

12.2

15.2

3.29

47.9

608197184

0.265

2.06

0.386

2.96

1.22

6.72

1.24

9.19

3.17

9.59

45.7

11.2

93.7

43.3

13.9

5.31

5.67

4.02

306

69.2

241

28.6

941

24.3

120

41.7

34.7

71.6

266

1.42

98.7

0.74

1.05

4.92

8.37

4.95

0.19

12.7

14.9

3.24

47.7

608197185

0.213

1.76

0.331

2.62

1.11

6.06

1.09

7.91

2.55

7.56

34.7

8.02

62.5

29.2

8.30

3.72

6.25

2.48

353

43.0

225

25.6

438

28.7

90.1

101.1

35.6

769

342

3.18

99.6

0.48

1.11

2.33

14.0

8.16

0.16

14.0

11.9

3.03

44.5

608197186

0.292

2.04

0.334

3.09

1.18

7.15

1.26

9.24

3.20

9.66

47.1

11.3

94.3

46.3

12.1

5.50

5.66

4.18

337

69.4

236

29.2

889

29.9

114

39.0

31.2

54.8

230

2.12

100

0.74

1.42

4.70

9.65

4.36

0.20

12.5

15.4

3.23

48.0

608197187

0.317

2.36

0.430

3.08

1.24

6.92

1.26

8.87

3.18

9.16

44.5

11.1

90.4

45.7

12.8

5.39

6.07

14.65

518

67.3

263

32.7

762

37.6

194

45.9

35.5

73.3

245

1.96

100

0.77

1.44

4.23

8.83

4.50

0.28

13.4

15.3

3.29

48.1

608197188

0.285

2.36

0.441

3.38

1.32

7.66

1.35

10.1

3.46

10.7

50.9

12.4

102

50.2

9.20

5.55

5.97

4.62

298

69.4

243

33.0

1091

46.4

122

41.8

31.8

79.3

284

2.08

99.7

0.74

1.52

4.24

10.0

4.65

0.19

12.7

15.0

3.32

47.4

608197189

0.264

1.99

0.330

2.78

1.15

6.46

1.13

8.71

3.00

9.26

45.9

11.5

97.2

48.1

14.0

6.11

5.23

2.63

676

76.1

209

28.0

653

43.4

137

39.9

37.7

12.7

251

3.59

99.2

0.79

3.33

3.21

6.45

6.67

0.21

12.3

16.2

3.23

46.8

608197190

0.229

1.96

0.312

2.70

1.12

6.28

1.12

8.14

2.82

8.72

41.1

10.1

84.5

41.3

9.90

5.36

5.60

6.05

435

66.1

234

27.0

757

66.4

135

40.0

30.3

40.1

273

5.60

100

0.71

2.68

3.52

13.4

4.35

0.18

10.8

15.2

3.06

46.5

608197191

0.257

2.02

0.333

2.60

1.08

5.90

1.08

7.99

2.79

8.37

40.1

9.75

81.2

39.1

9.00

4.46

6.07

3.46

472

64.3

238

24.9

638

58.9

95.6

37.4

31.6

25.2

230

6.24

97.2

0.71

2.85

3.33

13.6

4.47

0.17

10.5

14.8

3.13

45.5

2019615B6

0.245

2.17

0.391

3.17

1.20

7.06

1.30

8.82

2.93

9.11

37.7

8.17

62.4

28.1

5.00

3.00

7.54

1.65

176

38.2

238

31.3

354

24.7

68.9

93.3

34.4

409

312

3.60

99.0

0.47

1.00

1.90

17.9

8.96

0.15

9.0

11.1

3.09

45.5

2019615B7

0.366

2.82

0.503

4.28

1.65

9.26

1.56

10.6

3.45

10.1

38.6

8.66

66.4

30.0

10.0

3.48

5.99

3.58

256

47.6

209

41.1

553

23.2

155

83.3

38.0

57.4

156

4.67

101

0.64

1.48

2.35

6.61

5.72

0.29

15.8

16.9

4.01

46.9

2019615B10

Whole-rock chemical composition of the mafic rocks from the Uubulan area. The data are displayed in three significant digits except for MnO and P2O5.* Fe2O3 total iron as Fe2O3. **trace

Major elements (wt%)

Sample No.

Table 1

(wt%)

(wt%)

5

(wt%)

20

TiO2

3

20

Al2O3

10

10

0

0

Fe2O3

1

(wt%)

(wt%)

0.4

MnO

(wt%)

MgO

8

0.2

20

10

4

0

0

0

(wt%)

(wt%)

6

Na2O

(wt%)

1.2

K2O

4

4

2

0.4

0

0

(ppm)

(ppm)

400

V

0

(ppm)

60

Cr

800

200

20
0

0

(ppm)

(ppm)

Ni

(ppm)

80

Zn

200

100

(ppm)

0

0

1600

60

(ppm)

Ba

600

Rb

40

100

800

Co

40

400

0

0

P2O5

0.8

2

200

CaO

(ppm)

Sr

Y

40
800

400

20

200

0

0

0

(ppm)

(ppm)

300

Zr

(ppm)

25

Nb

80

Pb

20

200

15
10

40

100

5

(ppm)

(ppm)

8

Hf

6

(ppm)

20

Ta

6

10

2

2

44

46

48

SiO2 (wt%)

Fig. 6

50

0
44

Th

15

4

4

0

0

0

0

5

46

48

SiO2 (wt%)

50

0

44

46

Variation diagrams for the dolerite samples from the Uubulan area.

― 10 ―

48

50

SiO2 (wt%)

Fig. 6

1000

Sample/ MORB

100

10

1

0.1
Rb Ba Th U Nb K La Ce Sr Nd Sm Zr Hf Eu Ti Gd Tb Dy Y Er Yb
Fig. 7Mid-ocean ridge basalt (MORB)-normalized multi-element patterns for the dolerite samples from the Uubulan area. The
Fig. 7order of the elements follows Nakamura et al. (2000). The normalizing values are from Sun and MacDonald (1989).

Sample/ C1-chondrite

1000

100

10

1
Fig. 81.5

La Ce

Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

8
C1-chondrite-normalized REE patterns for the dolerite samples from the Uubulan area. The Fig.
normalizing
values are from
Anders and Grevesse (1989) and Yamamoto et al. (2005).
Cpx chemical composition of the mafic rocks

Representative mineral compositions of Cpx in the mafic rocks are listed in Table 2. The Cpx is assigned
to augite and diopside in the pyroxene quadrilateral diagram (Morimoto, 1988) (Fig. 9). The Cpx chemicalcomposition of each sample is described below.
Sample 2019615B6
Mg# (Mg/(Mg + Fe)) of Cpx is 0.60–0.65, 0.62 average, and was generally higher than that of sample― 11 ―

2019615B7. TiO2 ranges from 3.09 to 5.38 wt%, Al2O3 varies from 7.62 to 11.1 wt% (10.2 wt% average), and
CaO is 22.4–23.3 wt% (22.9 wt% average). Na2O is less than 0.58 wt%.
Sample 2019615B7
The Cpx varies from 0.65 to 0.75 in Mg# (0.70 average). TiO 2 ranges from 1.53 to 3.68 wt% (2.58 wt%
average), Al2O3 is 4.16 to 10.2 wt% (7.47 wt% average), and CaO is 21.0–22.4 wt% (21.8 wt% average). Na2O isless than 0.54 wt%.
Sample 2019615B9
Mg# of Cpx is 0.51–0.62, 0.58 average. TiO2 ranges from 1.9 to 3.9 wt%, Al2O3 varies from 5.16 to 10 wt% (7.1
wt% average), and CaO is 22.6–23.3 wt% (22.9 wt% average). Na2O is less than 0.5 wt%.
Table 2

Representative mineral compositions of the clinopyroxene in the mafic rocks from the Uubulan area.

Sample No.

2019615B6

2019615B7

2019615B9
Cpx (n=3)

Cpx (n=15)

Cpx (n=30)

SiO2

41.8

45.4

44.6

TiO2

4.25

2.58

2.65

Al2O3

10.2

7.49

7.10

Cr2O3

-

-

-

FeO*

10.5

9.60

12.4

MnO

-

-

-

MgO

9.80

12.7

9.84

CaO

22.9

21.9

22.9

Na2O

0.490

0.354

0.47

Total

100.00100.00100.00

Mg#

0.624

0.701

0.585

The data are displayed to three significant digits.
FeO*is total iron as FeO.
Mg# (Mg/(Mg + Fe)) on molar basis.

MgCaSi2O6

FeCaSi2O6

hedenbergite

diopside
augiteSample number
2019615B6
2019615B7
2019615B9

pigeonite
Enstatite

Mg2Si2O6
Fig. 9Ferrosilite

MgFeSi2O6

Fe2Si2O6

Dolerite samples from the Uubulan area with their clinopyroxene chemical composition plotted on the pyroxene trapezoid.

Fig. 9― 12 ―

Table 3

Sample No

Sample2019615B9

1.6

Hornblende

Results of the K-Ar dating. STP: standard temperature and pressure.

K concentration

Radioactive 40Ar

K-Ar age

Stable 40Ar

(wt%)

(10-8cc STP/g)

(Ma)

(%)

0.517 ± 0.010

930.3 ± 11.0

412.7 ± 8.6

14.7

Hornblende K-Ar age of the mafic rocks

The result of the hornblende K-Ar dating for the sample is shown in Table 3. K concentration is 0.517 ± 0.010
wt%, and radioactive 40Ar is 930.4 ± 11.0 (10-8cc (STP)/g); then the K-Ar age is calculated to be 412.7 ± 8.6 Ma.
Analytical error under the same condition was less than 1% (Itaya et al., 1991; Yagi et al., 2015).
Discussion
1.7

Magma type and tectonic setting of the mafic rocks

The geochemistry of mafic rocks, which varies based on their origins, provides evidence for the volcanic
activity that formed in the tectonic setting, and discrimination diagrams for mafic rocks have been proposed (Pearce,1982). In this section, discrimination diagrams are used to discuss the magma type and tectonic setting of the mafic
rocks in the Uubulan area.
The samples are plotted in the alkaline field, in (Na2O + K2O) vs. SiO2, Nb/Y vs. SiO2, and TiO2 vs. Zr/(P2O5
× 10000) diagrams (MacDonald and Katsura, 1969; Winchester and Floyd, 1976, 1977) (Fig. 10a, b, and c).
The samples show geochemical features similar to those of alkaline basalts, such as enriched LILE and LREE incomparison to the HFSE and HREE (Figs. 7 and 8). As observed from the Cpx chemical composition, all data fallon the field of alkaline basalt in Al vs. Si and Ti vs. (Ca + Na) diagrams (Leterrier et al., 1982) (Fig. 11a, b). Thus,
the examined mafic rocks have an alkaline nature.

3

(a)

2.5

Alkaline

2

6

1.5

4

non-Alkaline

2

0

44

Fig. 1046

48

SiO2 (wt%)

50

1Alkaline

0.5

non-Alkaline

0

(c)

4

TiO2(wt%)

8

5

(b)

Nb/Y

Na2O + K2O (wt%)

10

44

46

48

SiO2 (wt%)

3

Alkalinebasalt

2

1

50

0

Tholeiitic
basalt00.05

Zr/ (P2O5×10000)

0.1Plot of the whole-rock chemical composition on the diagrams discriminating alkaline vs. non-alkaline mafic rocks for thedolerite samples from the Uubulan area: (a) (Na2O + K2O) vs. SiO2 diagram from MacDonald and Katsura (1969); (b) Nb/Y

10a, b andandFloydc
vs. SiO2 diagram from Winchester and Floyd (1977); (c) TiO2 vs. Zr/(P2O5 × 10000) diagram fromFig.
Winchester(1976).

― 13 ―

Al
Sample number
2019615B6
2019615B7

0.6
0.5

Ti
0.15

2019615B9

0.4

0.10

alt
as
eb
lin
ka
Al

0.3
0.2
0.1
0
Fig. 11

(a)
1.5

0.05

Th
o
ba leiit
sa ic
lt
1.6

1.7

1.8

1.9

2.0

0

Si

Alkaline basalt

Tholeiitic
and calcalkali
basalt

(b)
0.6

0.7

0.8

0.9

1.0 Ca+Na

The chemical composition of clinopyroxene in dolerite samples from the Uubulan area distinguishes between alkaline andnon-alkaline (tholeiitic and calc-alkali) basalt: (a) Al vs. Si diagram (Leterrier et al., 1982); (b) Ti vs. (Ca + Na) diagram(Leterrier et al., 1982).

Fig.11a and b

(a)

2Nb

(b)

Hf/3

N-MORB

WPA
AT
WP

IAB

E-MORB

E-MORB
OIB

VAB

Zr/4
Fig. 12N-MORB

Y

Ta

Th

Discrimination diagrams for the dolerite samples from the Uubulan area: (a) 2Nb–Zr/4–Y diagram (Meschede, 1986) and(b) Hf/3–Th–Ta diagram (Wood, 1980) show the fields for the different tectono-magmatic types. Abbreviations: WPA:
within-plate alkaline; WPAT: within-plate alkaline and tholeiites; N-MORB: normal-type MORB; E-MORB: enriched-type
MORB; VAB: volcanic arc-basalt; OIB: ocean-island basalt; IAB: island-arc basalt; BAB: back-arc basin basalts.

Fig. 12 a, b

Zr/4–2Nb–Y diagram points that the samples were formed at a within-plate setting, and Th–Hf/3–Ta and HFSE
vs. (La/Yb)cn diagrams suggest that the samples are oceanic island alkaline basalts (OIA) (Wood, 1980; Meschede,
1986; Nakamura et al., 2000) (Figs. 12 and 13). Therefore, it is quite probable that the mafic rocks from the Gorkhi
Formation at the Uubulan area are OIA. Tsukada et al. (2013) studied the major and trace elemnent compositionof the mafic rocks in the Uubulan area using XRF and suggested that these were OIA. The results of this study,
detailed examination for trace elements including REE using ICP–MS, strongly supported Tsukada’s view.
Four mantle end-members, i.e., depleted mantle (DM), high time-integrated μ (238U/204Pb) mantle (HIMU),
enriched mantle I (EM I), and enriched mantle II (EM II), have been suggested for the mafic magma-source (Zindler― 14 ―

10

ppm

10

TiO 2

1

1

0.1

0.1

1000

ppm

ppm

Hf

ppm

10

Zr

Ta

1

100

0.1
10
0.01
1000

ppm

Nb

1

10

(La/Yb) cn

100

MORB

100

Volcanic arc Basalt
Island Arc Tholeiite
Island Arc Alkaline
Calc-Alkaline
Back Arc Basin

10
1
0.1

0.1

Within-plate Basalt

0.1

1

10

100

(La/Yb) cn
Fig. 13

Oceanic Island Tholeiite
Oceanic Island Alkaline
Intra-continental Tholeiite
Intra-continental AlkalineRelationship between the concentration of high-field strength elements (HFSE) and the ratio of (La/Yb)cn in the dolerite
samples from the Uubulan area. The diagrams are drawn based on the study by Nakamura et al. (2000). Fig. 13

and Hart, 1986). In plots of Th/Yb vs. Nb/Yb and TiO2/Yb vs. Nb/Yb diagrams, the examined rocks show a strong
aﬃnity to the mafic rocks from St. Helena Island and Society Islands, which was originated from a HIMU source(Pearce, 2008) (Fig. 14a, b). The HIMU mafic rocks were known to show superplume activities (Kogiso et al.,
1997; Homrighausen et al., 2018; Tominaga and Hara, 2021), and therefore, the present data, which shows traceelements composition similar to that of the HIMU mafic rocks, may imply the Late Silurian superplume activity atpelagic region. Further studies on 238U/204Pb (μ value), 232Th/204Pb, and Pb isotopic ratios are required to confirm if
the Uubulan rocks are of HIMU-origin.
― 15 ―

10

10

(a)

(b)

(HIMU)(HIMU)

TiO2 /YbO
IBAlkaline

OIB array

OIB(deep melting)

1

ar

ra

y

1

RB

-O

IB

Th/Yb

Tholeiitic

E-MORBMORB array(shallow melting)

0.1

EM
O

RB

MO

N-MORB

10

1

Fig. 14

Nb/Yb

100

0.1

0.1

1

Nb/Yb

10

100

Discrimination diagrams of (a) Th/Yb vs. Nb/Yb (Pearce, 2008) and (b) TiO2/Yb vs. Nb/Yb (Pearce, 2008) for the dolerite
samples from the Uubulan area. For comparison, the fields for HIMU lavas are shown. The data source of the HIMU lavas
is Chaﬀey et al. (1989), Willbold and Stracke (2006), and Kawabata et al. (2011). Abbreviations: N-MORB: normal-type
204 a and b
MORB; E-MORB: enriched-type MORB; OIB: ocean-island basalt; HIMU: high time-integrated μFig.
(238U/14Pb) mantle.

1.8

Relationship between the mafic rocks and radiolarian chert at the Uubulan area

Upper Silurian (Ludlovian/Pridolian) chert is exposed beside the mafic rocks in the Uubulan area. Kurihara etal. (2009) mentioned that the mafic rocks are conformably overlain by gray tuﬀaceous chert and then overlain by
the Ludlovian/Pridolian chert at this area. However, the exact age of the mafic rocks has not been presented.
Kurihara et al. (2009) described the latest Ludlovian to Pridolian or Pridolian conodonts, such as Ozarkodina
remscheidensis eosteinhornensis, Ozarkodina sp., and Belodella sp., from the gray and red cherts beside the mafic
rocks. The hornblende K-Ar age, 412.7 ± 8.6 Ma, for the mafic rocks here coincides with the conodont age of thechert within its error range, and thus, it supports the conformable relationship between the mafic rocks and Upper
Silurian chert.
Considering the error range of the K-Ar dating, the mafic rocks might be of Early Devonian. However, as
discussed below, the time of the accretion of the oceanic plate and its cover, a component of the Gorki Formation,is generally considered as Early Carboniferous (Kurihara et al., 2009; Hara et al., 2013). That is, the radiolarianchert at the Uubulan area, likely lithified after Early Carboniferous, had probably been presented on a bed ofpelagic ocean as radiolarian ooze at the Early Devonian time. If the mafic magma intruded into or erupted on thehydrous unconsolidated/semi-consolidated Silurian radiolarian ooze, the magma quite likely brecciated because of
rapid cooling by the surrounding water, and hyaloclastite, crumble breccia, pillow/autobrecciated lava, paragonite
layers, and others should have been formed instead of the dolerite and gabbro exposed here. In reality, there is noevidence showing contact between magma and hydrous unconsolidated/semi-consolidated sediments in this area,
and therefore, it is reasonable to suppose that the mafic rocks had already been formed before the radiolarian oozeaccumulation at the Late Silurian time.
Radiolarian assemblage suggesting Pragian to Emsian, including Deflantrica sodium, Deflantrica sp.,
Pactarentinia sp., Palaeoscenidium sp., and Tlecerina horrida, was recovered from the upper chert horizon of― 16 ―

the Upper Silurian chert (Kurihara et al., 2009). Consequently, an OIA mafic rocks and Upper Silurian to Lower
Devonian radiolarian chert sequence is exposed at the Uubulan area.
1.9

Width of the Middle Paleozoic Ocean facing the “Siberian continent”

The OPS can provide key information regarding the continuous evolution of oceanic plates (Matsuda andIsozaki, 1991; Wakita and Metcalfe, 2005).
In this study, it is confirmed that the OIA mafic rocks are probably overlain by the Upper Silurian (Ludlovian/Pridolian) radiolarian chert in the Gorkhi Formation. The Lower to Upper Devonian radiolarian cherts wererecovered from several sections of the Gorkhi Formation. The Upper Devonian (Frasnian) chert is overlain bysiliceous mudstone and clastic rocks, and Kurihara et al. (2009) proposed reconstructed OPS, i.e., OIA mafic
rocks, Upper Silurian–Upper Devonian radiolarian chert, and post-Devonian clastic rocks in ascending order, for
the Gorkhi Formation.
Detrital zircons of 340–320 Ma were reported from the sandstone, which is the uppermost part of the OPS, of
the Gorkhi Formation (Kelty et al., 2008; Bussien et al., 2011; Hara et al., 2013). Additionally, Takeuchi et al.
(2012) described the Carboniferous (Visean) shallow marine formation, which probably unconformably overlaid
the mafic rocks and chert of the Gorkhi Formation. These facts suggest that an oceanic island formed at thepelagic region in the Late Silurian time (Ludrovian/Pridorian) migrated toward a continent, accumulated deep-sea
sediments on it during the movement, and reached the trench in the Early Carboniferous.
The time spent traveling the oceanic plate from ridge/oceanic island to trench can be estimated from the fossil
record in the radiolarian chert between the mafic rocks and clastic rocks of the OPS. The accretionary complexof the Gorkhi Formation is regarded as a fossilized remain of the subducted oceanic plate beneath the “Siberiancontinent” (e.g., Onon and Tsukada, 2017). According to the radiolarian fossil record in the OPS of the Gorkhi
Formation, the Middle Paleozoic oceanic plate facing the “Siberian continent” is estimated to have traveled fromthe Late Silurian to the Late Devonian for nearly 50 million years, from oceanic island to trench (Kurihara et al.,2009). The relative motion rate of the Pacific plate against the Eurasian eastern margin, which is a typical pair of
the continent and subducted oceanic plate in Quaternary time, varies from 65 to 110 mm/yr, except for Eocene(Northrup et al., 1995). Assuming that the ancient oceanic plate subducted beneath the “Siberian continent”
moved at a rate of 83 mm/yr, same as the current rate of the Pacific plate at 38°N, 144°E (DeMets et al., 1994),
the distance from the oceanic island to the trench at Silurian–Devonian can be estimated to be approximately 4200
km. This is comparable to the distance from the Midway Islands in the central Pacific to the Japan Trench. As theoceanic island is located between ridge and trench, the ridge that generated the oceanic plate at Silurian–Devonian
time had probably been located far more than the estimated distance here from the “Siberian continent.”
Conclusion(1) The mafic rocks of the Gorkhi Formation, Khangai-Daur belt at the Uubulan area show geochemical features
similar to those of oceanic island alkaline basalt.
(2) The hornblende K-Ar age, 412.7 ± 8.6 Ma, from the mafic rocks here, coincides with the conodont age of theUpper Silurian chert exposed beside the mafic rocks within its error range. Therefore, the mafic rocks are very
likely conformably overlain by the Upper Silurian chert.
(3) According to the OPS of the Gorkhi Formation (Kurihara et al., 2009), the mafic rocks at the Uubulan area hadtaken 50 million years to travel from an oceanic island to a trench along the margin of the “Siberian continent.”― 17 ―

Assuming that the relative motion rate of the ancient oceanic plate subducted beneath the “Siberian continent”
had been the same as the rate of the Pacific plate against the Eurasian eastern margin (83 mm/yr), the distancefrom the oceanic island to the trench at Silurian–Devonian time can be estimated to be approximately 4,200
km.
Acknowledgements
We would like to thank Dr. K. Yagi at the Hiruzen Institute for Geology and Chronology for his helpful advice.
We are grateful to Prof. K. Yamamoto and Prof. Y. Asahara at Nagoya University for technical supports in XRF and
ICP-MS analysis. We thank Dr. Y. Shimura at the Geological Survey of Japan and Dr. Onon. G at the Japan GoldK. K. for the helpful discussion. We would like to thank Profs. M. Takeuchi and H. Yoshida at Nagoya University
for valuable advice. Special thank goes to Prof. Zorigtkhuu, D. and Prof. Purev, N. at the Mongolian University of
Science and Technology for their help in using the university facilities. We are grateful to Dr. Oyunchimeg T. at theInstitute of Geology, Mongolian Academy of Sciences, and Dr. S. Fujiwara at Nagoya University for their valuablecomments on the manuscript. We would like to thank Enago (www.enago.jp) for the English language review. A
part of this study was supported by the Daiko Foundation, Japan.
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