名古屋大学博物館報告
Bull. Nagoya Univ. Museum
1–12, 2022
No. 37, 00–00,
published online as early view
DOI: 10.18999/bulnum.037.01

Ages from the “Mogod Formation” of the Permian–Triassic igneous rocks
in the Sayan–Baikal belt, northern Mongolia
TSUKADA Kazuhiro1*, UMEDA Kanta2, NADMID Bayart2, SODNOM Khishigsuren3,
and NURAMKHAAN Manchuk4
1

Nagoya University Museum, Nagoya 464-8601, Japan

2

Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan3

School of Geology and Mining Engineering, Mongolian University of Science and Technology, Ulaanbaatar, Mongolia

4

Academic Affairs, Branch of Mongolian University of Science and Technology, Division of Erdenet Mining Co., Erdenet,

Mongolia*Corresponding author: Tel.: +81-52-789-5768; Fax: +81-52-789-5896
E-mail address: tsukada@num.nagoya-u.ac.jp (K. Tsukada)

Abstract
Late Paleozoic–Early Mesozoic volcano-plutonic rock unit at the southern margin of the Sayan–Baikal belt, Mongolia
is a significant factor in understanding the Paleozoic–Mesozoic arc system along the Siberian continental margin. The
Permian–Triassic igneous rocks have been divided into the Lower–Middle Permian Khanui Group, Late Permian–Early
Triassic Selenge plutonic rock complex, Permian?–Middle Triassic Erdenet plutonic rock complex, and Upper Triassic–
Lower Jurassic Mogod Formation. However, a U-Pb age pointing latest Permian was recently reported from the KhanuiGroup, and stratigraphy of the Permian–Triassic igneous rocks is required to be reexamined. This paper describes zircon
U-Pb age of 257.6 ± 3.6 Ma (Late Permian) and hornblende K-Ar age of 292.9 ± 14.1 Ma (Early Permian) from the Mogod
Formation, which has previously been assigned to Upper Triassic–Lower Jurassic, and presents a possibility that thestratigraphic division of the Permian–Triassic igneous rocks is revised.
Key words: Permian–Triassic igneous rocks, Mogod Formation, U-Pb age, K-Ar age.

Introduction
The Central Asian Orogenic belt (CAOB), which lies among the Siberian craton, North China block, Tarim
block, and East European craton, is a critical geologic unit in understanding the development process of theEurasian continent (e.g., Sengör et al., 1993; Kovalenko et al., 2004: Fig. 1). In general, the CAOB has been
formed through various processes, such as subduction-accretion of the oceanic plate, volcanic arc magmatism,
and collisions of continental fragments during the amalgamation of these cratons and blocks (e.g., Sengör et al.,1993). Geological data from Mongolia, an area between the Siberian craton and North China block, is essential
in revealing the Paleozoic–Mesozoic tectonics of the southern margin of the “Siberian continent” (Siberiancraton and accreted geologic units) in the southern part of the CAOB. Here, the features of the Sayan–Baikal belt
(Precambrian–Paleozoic continental basement and intrusive rocks) and Khangai–Daur belt (Late Paleozoic–Early
Mesozoic accretionary complexes) have been established (e.g., Tomurtogoo, 1998, 2003, 2014; Badarch et al.,
2002; Kurihara et al., 2008; Onon and Tsukada, 2017: Figs. 1 and 2).
Permian–Triassic igneous rock unit (Permian–Triassic igneous rocks, hereinafter) at the southern margin of the―1―Sayan–Baikal belt is a signiﬁcant factor in understanding the previous arc system along the Siberian continental
margin (e.g., Tomurtogoo, 1998, 2003, 2014; Badarch et al., 2002; Gerel and Munkhtsengel, 2005; Munkhtsengel
et al., 2007a; Tsukada et al., 2018: Fig. 2). The Permian–Triassic igneous rocks are classiﬁed into the volcanicclastic rock sequences (Permian Khanui Group, and Triassic–Jurassic Mogod, Bugat, and Baruunburen
formations) and plutonic rock complexes (Permian–Triassic Selenge and Permian?– Triassic Erdenet complexes)

Fig. 1

Index map (modiﬁed from Kurihara et al., 2008). CAOB: Central Asia Orogenic belt, Siberia: Siberian craton, North China:
North China block, Tarim: Tarim block.

Fig. 2

Simpliﬁed tectonic division of northern Mongolia showing Jargalant and Mogod areas (modiﬁed from Badarch et al., 2002;
Tomurtogoo, 2003, and Tsukada et al., 2018).

―2―

(e.g., Tomurtogoo, 2003; Gerel and Munkhtsengel, 2005; Munkhtsengel, 2007; Munkhtsengel et al., 2007a: Figs.
3 and 4). The Khanui Group had been subdivided into four conformable successions in ascending order: maﬁc–intermediate volcanic rocks, felsic volcanic rocks, volcaniclastic rocks, and maﬁc volcanic rocks (e.g., Gabcho etal., 1967; Tovuudorj et al., 1971; Mossakovsky and Tomurtogoo, 1976; Gerel and Munkhtsengel, 2005: Fig. 3). A
ﬂora suggesting Cisuralian–Guadalupian was obtained from the felsic volcanic rock formation (e.g., Mossakovsky
and Tomurtogoo, 1976: Figs. 3 and 4). The Mogod, Bugat, and Baruunburen formations are composed mainlyof maﬁc–intermediate volcanic rocks (Kepezhinskas and Luchitsky, 1974: Figs. 3 and 4). The Bugat Formation
produces 220–195 Ma of K-Ar ages. The Selenge and Erdenet complexes consist of granitic rocks accompanied
by minor gabbro, diorite, and syenite–granosyenite, intruded by intermediate dikes (e.g., Gerel and Munkhtsengel,
2005; Munkhtsengel et al., 2007a; Tsukada et al., 2018). The Selenge plutonic rock complex gives 40Ar-39Ar agesof 247 to 258 Ma and SHRIMP ages of 243 to 250 Ma (e.g., Gerel and Munkhtsengel, 2005; Sotnikov et al., 2005;
Munkhtsengel, 2007; Munkhtsengel et al., 2007b: Figs. 3 and 4). The Erdenet plutonic rock complex shows 253± 18 Ma Ma (Rb-Sr age), ca. 235 Ma (40Ar-39Ar age), 246 to 236 Ma (U-Pb age), and 224 to 191 Ma (K-Ar age)
(Sotnikov et al., 1995, 2005; Munkhtsengel, 2007; Munkhtsengel et al., 2007b; Berzina et al., 2009: Figs. 3 and 4).
Recently, Amgalan et al. (2017) classiﬁed the Khanui Group in ascending order as the Cisuralian Khustai (maﬁcto felsic volcanic rocks), Guadalupian Khargana (volcaniclastic rocks), and Guadalupian Tulbur (maﬁc volcanic

Fig. 3Comparison of the stratigraphic divisions of the Permian–Triassic igneous rocks among previous studies and this study. Agedata were from Kepezhinskas and Luchitsky (1974), Mossakovsky and Tomurtogoo (1976), Sotnikov et al. (1995, 2005),
Izawa and Ohsawa (2004), Munkhtsengel (2007), Munkhtsengel et al. (2007b), Berzina et al. (2009), and Tsukada et al.
(2018). Amph: amphibole, F.: Formation, Hbl: hornblende, Ser: sericite, Wr: whole rock.

―3―

Fig. 4

Summary of the previously published absolute ages and fossil record from the Permian–Triassic igneous rocks. Data werefrom Kepezhinskas and Luchitsky (1974), Mossakovsky and Tomurtogoo (1976), Sotnikov et al. (1995, 2005), Izawa andOhsawa (2004), Munkhtsengel (2007), Munkhtsengel et al. (2007b), Berzina et al. (2009), and Tsukada et al. (2018). F.:
Formation.

rocks) formations. Besides, they uniﬁed the Mogod, Bugat, and Baruunburen formations as the Mogod Formation(Fig. 3). Tsukada et al. (2018) reported zircons dated as 256.9 ± 2.2 Ma from the Tulbur Formation. It shows thata part of the Khanui Group is contemporaneous to the Selenge plutonic rock complex, and the stratigraphy of thePermian–Triassic igneous rocks are required to be reexamined (Fig.3). This paper describes the zircon U-Pb age
from the Baruunburen Formation at the Jargalant Village and the hornblende K-Ar age from the Mogod Formation
at Mogod Village to reexamine the stratigraphy of the Permian–Triassic igneous rocks (Fig. 5).

Fig. 5

Index map of the study areas. Localities of Figs. 6 and 8 are also shown.

―4―

Geological Description of the Study Areas
Lithology and structure at Jargalant Village
The Jargalant Village, located 20 km east of Erdenet City, Mongolia, exposes intermediate vesicular lava andtuff breccia of the Baruunburen formation (Amgalan et al., 2017: Figs. 5 and 6). The lava has platy joints striking
NW and dipping south by 20° to 40°. The vesicles are, places, ﬁlled with calcite or chlorite to form amygdule. Thelava is ﬁne to coarse and has porphyritic, aphyric, or trachytic textures, and in places, holocrystalline (Fig. 7a, b).
104˚20’ EN

104˚30’ E

49˚7’ N
40

40

37

54
32

49

20

35

Road

25

20

49˚5’ N62
15

Jargalant

Talus deposit and alluvial sand58

Intermediate dake
Baruunburen Formation (Dejidmaa et al., 2011; Amgalan et al., 2017)River

49˚3’ NIntermediate tuff and tuff breccia with lava intercalations
Intermediate lava and minor tuff breccia
54

Strike and dip of bedding plane and platy joint

5 km

Dated point

Fig. 6

Geologic map of the Jargalant area showing the dated point. Geologic division in the map follows Dejidmaa et al. (2011) andAmgalan et al. (2017).

a

b

Pl

aphyric lava

Hbl

coarse lava

c d
Hbl
Hbl
Pl

10 cm
Fig. 71 mm(a) Intermediate lava showing trachytic texture with plagioclase and hornblende phenocrysts of the Baruunburen Formation
at the Jargalant area. (b) Aphyric–coarse lava of the Baruunburen Formation at the Jargalant area. The boundary between theaphyric lava and the coarse lava is obscure. (c) Intermediate lava showing trachytic texture with plagioclase and hornblende
phenocrysts of the Mogod Formation at the Mogod area. (d) A photomicrograph of the dated sample from the Mogod
Formation. Pl: plagioclase, Hbl: hornblende.

―5―Fig. 8

Simpliﬁed geologic map of the Mogot area (Sabidulla et al., 1978; Batbayar et al., 2014) with dated point. F.: Formation, G.:Group.

The lava was partly altered so that some minerals could be replaced by chlorite, muscovite, and opaque minerals.
In the lava with porphyritic texture, subhedral or euhedral plagioclase, clinopyroxene, and hornblende, more than
0.5 mm of the major axis lie in a groundmass. The coarse lava rarely includes subhedral/euhedral zircon. The tuff
breccia shows clear bedding striking NW and dipping south by 5° to 30°.
Lithology and structure at Mogod Village
The Mogod Village, located 70 km southwest of Bulgan City, Mongolia, exposes intermediate volcanic rocksof the Khanui Group and the Mogod Formation (Sabidulla et al., 1978; Batbayar et al., 2014: Figs. 5 and 8). The
Mogod Formation’s volcanic rocks are composed of intermediate vesicular lava, tuff breccia, and conglomerate
formed by a volcanic mudﬂow. The lava, ﬁne- to coarse-grained, has porphyritic, aphyric, or trachytic textureswith plagioclase and hornblende (Fig. 7c). The lava has platy and columnar joints and partly includes abundantfragments of intermediate lava. The vesicles of the lava, partly ﬁlled with calcite or chlorite, forms amygdule. The
tuff breccia includes abundant angular clasts of intermediate volcanic rocks, up to 10 cm in diameter, in a cognate
matrix. The tuff breccia has sub-horizontal bedding.
Absolute ages from the Baruunburen and Mogod Formations
Zircon U-Pb age of the Baruunburen Formation
U-Pb dating of zircons from the coarse lava of the Baruunburen Formation in the Jargalant area (49° 3’ 39” N,
104° 23’ 7” E) was conducted to clarify the age of the volcanism (Figs. 5 and 6). The zircons, colorless with about
200 μm in the major axis, generally show clear oscillatory zoning in the cathode luminescence image (Fig. 9a).
―6―

Fig. 9(a) Backscattered electron (upper) and cathode luminescence (lower) images of selected zircons. The circles show dated
spots. Scale bars denote 100 μ m. (b) Concordia plots (left), and individual zircon concordia ages (right). The weighted mean
age of 24 concordant data is 257.6 ± 3.6 Ma. Gr. No.: grain number, Wtd. mean: weighted mean.

The analysis was performed using Inductively Coupled Plasma Mass Spectrometry (Quadrupole type ICP-MS;
Agilent 7700x) connected with the NWR-213 LA system (Electro Scientiﬁc Industries, Inc.) at Nagoya University
in Japan, with a conventional procedure. Details of the analytical procedure are described in Kouchi et al. (2015).
The data obtained from 24 grains were plotted on a concordia diagram using Isoplot 3.70 software (Ludwig, 2008).
The ages, except for two data sets (315.3 ± 8.6 Ma and 289.1 ± 13.4 Ma), which are likely to be inherited ages,
cluster at ca. 276–246 Ma (257.9 ± 3.8 Ma in weighted mean age) (Fig. 9b, Table 1).
Hornblende K-Ar age of the Mogod Formation
K-Ar dating of hornblende extracted from a lava sample of the Mogod Formation in the Mogod area (48° 17’
42” N, 102° 56’ 5” E) was conducted (Sabidulla et al., 1978: Figs. 5 and 8). The sample contains plagioclase andhornblende with a trachytic texture. The secondary minerals are chlorite, epidote, and opaque minerals. Hornblendeis generally fresh (Fig. 6d). The sample was coarse crushed using a stainless steel bowl and pounder. Hornblende
was then concentrated using heavy liquid (sodium polytungustate solution) and Ferrite and Neodymium magnets.
Dating was performed by the Hiruzen Institute for Geology and Chronology, Japan. Quantitative analysis forpotassium was conducted using the ultra-low blank method (Itaya et al., 1996). About 50 mg of hornblende,―7―

Table 1

Results of Laser-ICP-MS analysis.

powdered in an agate mortar, was placed in a Teﬂon container and decomposed with nitric acid, hydroﬂuoric acid,and hydrogen peroxide in a closed system box for more than 12 hours. After decomposition, the sample solution
was evaporated to dryness in a closed system, and then the fused cake was re-dissolved with hydrochloric acid foranalysis. The analysis was performed using atomic absorption and ﬂame spectrometer (Hitachi 180-30).
Argon isotope ratio was measured using a mass spectrometer (HIRU, Itaya et al., 1991) with isotope dilution
method (Nagao et al., 1984; Nagao and Itaya, 1988; Itaya et al., 1991). About 50 mg of sample was used for the
analysis. The sample was evacuated at about 180 to 200°C for three days to remove the adsorbed argon. After
reducing the background in the analytical system, the sample was dissolved at about 1500°C. The gas extracted
from the sample was mixed with the spike (nearly 100% pure 38Ar). The gas other than inert gas was removed
using a Ti-Zr getter, and the puriﬁed gas was introduced into the mass spectrometer.
The results of the analysis showed that potassium concentration is 0.0931 ± 0.0047 wt%, and radioactive 40Ar is−8

114.9 ± 1.8 (10 cc (STP)/g); then K-Ar age is calculated as 292.9 ± 14.1 Ma (Table 2). Analytical error under thesame condition is less than 1% (Itaya et al., 1991; Yagi et al., 2015).

Table 2

Results of K-Ar dating.

―8―Stratigraphy of the volcanic rocks of the Permian–Triassic igneous rocks
Mossakovsky and Tumurtogoo (1976) described the upper volcanic rocks than the Khanui Group at the Bugat –Baruunburen area as Triassic–Jurassic formation based on K-Ar ages of 220 to 195 Ma (Kepezhinskas and Luchitsky,1974). Dejidmaa et al. (2011) divided the Triassic–Jurassic volcanic rocks into the Mogod Formation exposedat the Mogod area, and Bugat and Baruunburen formations at Bugat–Baruunburen area (Fig. 5). Amgalan et al.
(2017) uniﬁed these formations as Mogod Formation on the grounds that there is no difference in lithology and age
among them. The lithology of these formations is remarkably similar to each other (Fig. 7a, c). However, the Mogod
Formation and the Bugat and Baruunburen formations are exposed in different areas, and the stratigraphic correlation
between these areas’ rocks is unclear (e.g., Batbayar et al., 2014; Amgalan et al., 2017: Fig. 5). Therefore, theintermediate volcanic rocks previously regarded as “Upper Triassic–Lower Jurassic” exposed in the Mogod area arecalled Mogod Formation, and that in the Bugat–Baruunburen area is referred to as Bugat/Baruunburen Formation, in
this paper (Fig. 3).
The Bugat/Baruunburen Formation has previously been assigned to Upper Triassic–Lower Jurassic based on
the K-Ar ages (220–195 Ma) (Kepezhinskas and Luchitsky, 1974). The Bugat/Baruunburen Formation rocks,
close to the Erdenet Mine, are partly metasomatized and K-Ar age, which is easily modiﬁed by alteration, is not
the proper approach for magmatic age constraint. It is generally accepted that zircon is comparatively less affectedby weathering and hydrothermal alteration. And, clear oscillatory zoning in the cathode luminescence image
evidences less alteration for the dated zircons. We could conclude that the zircon U-Pb age, 257.9 ± 3.8 Ma, from theintermediate lava of the Bugat/Baruunburen Formation points to its magmatic age. Therefore, the Bugat/Baruunburen
Formation in the Jargalant area is not Triassic–Jurassic; however, the Upper Permian volcanic rock formation (Fig.
3). The Selenge plutonic rock complex producing 258 to 243 Ma is partly contemporaneous to the Tulbur andBugat/Baruunburen formations (e.g., Gerel and Munkhtsengel, 2005; Sotnikov et al., 2005; Munkhtsengel, 2007;
Munkhtsengel et al., 2007b; Tsukada et al., 2018: Fig. 3). The geochemical similarity among the rocks of the Selenge
plutonic rock complex, Tulbur Formation of the Khanui Group, and the Bugat/Baruunburen Formation was identiﬁed,
and these were likely attributed to single magmatism at the end of Permian (e.g., Berzina et al., 2009; Tsukada et al.,2018; Umeda, 2018 MS).
A hornblende K-Ar age of 292.9 ± 14.1 Ma (Early Permian) was obtained from the intermediate lava assigned
to the Mogod Formation in the geologic map by Sabidulla et al. (1978) (Fig. 8). Argon in a rock can be outgassed
when it undergoes alteration. Thus, it is generally said that K-Ar dating of an altered rock could give a younger age
than its formation (e.g., Kaneoka, 1998; Vural, 2017). The dated hornblende is fresh and hardly altered (Fig. 6d).
Therefore, the K-Ar dating here is supposed to show the original (magmatic) age. Although, the Mogod Formationin the Mogod area has been regarded as equivalent to Bugat/Baruunburen Formation as an upper geologic unit of thePermian–Triassic igneous rocks, the present data, if the geologic map by Sabidulla et al. (1978) is correct, indicate
the lowermost geologic unit for it (Fig. 3). The accurate stratigraphic position of the Mogod and Bugad/Baruunblen
formations remains to be confirmed when more age data from these formations become available. Besides, ageconstraint of the lower part of the Khanui Group, Khustai and Khargana formations, will give a clearer view of thestratigraphy and magmatic history of the Permian–Triassic igneous rocks.
Conclusion
1.

The intermediate lava of the Bugat/Baruunburen Formation from the Jargalant area (Amgalan et al., 2017)
gives zircon U-Pb age of 257.9 ± 3.8 Ma.
―9―

2.

The intermediate lava of the Mogod Formation from the Mogod area (Sabidulla et al., 1978) gives hornblende
K-Ar age of 292.9 ± 14.1 Ma.

3.

If previously published geologic map is correct, the volcanic rocks of the Permian–Triassic igneous rocks canbe divided into the Lower Permian Mogod Formation, the Lower –Upper Permian Khanui Group, and theUpper Permian Bugat/Baruunburen Formation.
Acknowledgements
We would like to gratitude to Dr. K. Yagi at the Hiruzen Institute for Geology and Chronology for his helpfuladvice. We thank Dr. O. Munkhtsetseg at Mongolian University of Science and Technology for useful discussion.
We would like to thank Profs. M. Takeuchi and H. Yoshida at Nagoya University for valuable comments. We are
grateful to Prof. K. Yamamoto of the Nagoya University and S. Davaanyam at Institute of Geology, Mongolian
Academy of Science for their technical support. Special thank goes to Dr. Munkhtsengel, B. at Mongolian
University of Science and Technology for her reviewing this paper. 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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