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Permian–Triassic Adakitic Igneous Activity at Northern Mongolia: Implication for Permian–Triassic Subduction System at the Siberian Continental Margin

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Abstract

The geochemistry of the Permian–Triassic large-scale igneous rock body of northern Mongolia is a key factor in understanding the subduction-related magmatism at the margin of “Siberian continent.” Several studies have been done in the Permian–Triassic igneous body; however, its detailed magmagenesis and tectonic significance remain unclear. This paper investigates the geochemistry of the Upper Permian andesites (Bugat/Baruunburen Formation) and the Late Permian–Middle Triassic plutonic rocks (Selenge plutonic rock complex) of the Permian–Triassic igneous body, and intermediate dike intruding into them, and discusses the Late Permian–Middle Triassic magmatism of the Siberian continental margin. These rocks show a linear distribution on the variation diagram. They are therefore likely to be derived from a single magmatic source. The rocks, characterized by low K2O/Na2O, high Sr/Y, high La/Yb, and high Sr/La ratios are adakitic rocks of basaltic slab-melt origin. The samples are enriched in Cr and Ni and have a high Mg# compared with the typical slab-melt. This is likely due to an interaction between the slab-melt and the overlying mantle peridotite during its ascent. The Nb/Ta variation of the samples may point crustal contamination to the magma. The paleolatitude of the Bugat/Baruunburen Formation is calculated to be 37.1° N based on thermal remanent magnetization. Therefore, the Late Permian–Middle Triassic large-scale adakitic igneous activity had taken place in the volcanic arc along the Siberian continental margin in the mid-latitudes of the Northern Hemisphere. The geochemical characteristics of the intermediate dike are almost the same as those of the Bugat/Baruunburen Formation and the Selenge plutonic rock complex, indicating that adakitic igneous activity continued after the Early Triassic.
Journal of Geodynamics 151 (2022) 101918
Available online 13 May 2022
0264-3707/© 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
PermianTriassic adakitic igneous activity at Northern Mongolia:
Implication for PermianTriassic subduction system at the Siberian
continental margin
Kanta Umeda
a
, Nemekhbayar Purevsuren
b
, Kazuhiro Tsukada
c
,
*
, Lodoidanzan Altansukh
d
,
Bayart Nadmid
a
, Khishigsuren Sodnom
e
, Manchuk Nuramkhaan
f
, Taro Kabashima
g
,
Tomoyuki Kondo
g
a
Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan
b
Natural History Museum of Mongolia, Ulaanbaatar, Mongolia
c
Nagoya University Museum, Nagoya, Japan
d
Field Research Center, Mongolian University of Science and Technology, Ulaanbaatar, Mongolia
e
School of Geology and Mining Engineering, Mongolian University of Science and Technology, Ulaanbaatar, Mongolia
f
Academic Affairs, Branch of Mongolian University of Science and Technology, Division of Erdenet Mining Co., Erdenet, Mongolia
g
Japan Oil, Gas and Metals National Co., Tokyo, Japan
ARTICLE INFO
Keywords:
Late PermianMiddle Triassic
Large-scale igneous rock body
Slab-melt origin adakitic rocks
Siberian continental margin
ABSTRACT
The geochemistry of the PermianTriassic large-scale igneous rock body of northern Mongolia is a key factor in
understanding the subduction-related magmatism at the margin of Siberian continent.Several studies have
been done in the PermianTriassic igneous body; however, its detailed magmagenesis and tectonic signicance
remain unclear. This paper investigates the geochemistry of the Upper Permian andesites (Bugat/Baruunburen
Formation) and the Late PermianMiddle Triassic plutonic rocks (Selenge plutonic rock complex) of the Per-
mianTriassic igneous body, and intermediate dike intruding into them, and discusses the Late PermianMiddle
Triassic magmatism of the Siberian continental margin. These rocks show a linear distribution on the variation
diagram. They are therefore likely to be derived from a single magmatic source. The rocks, characterized by low
K
2
O/Na
2
O, high Sr/Y, high La/Yb, and high Sr/La ratios are adakitic rocks of basaltic slab-melt origin. The
samples are enriched in Cr and Ni and have a high Mg# compared with the typical slab-melt. This is likely due to
an interaction between the slab-melt and the overlying mantle peridotite during its ascent. The Nb/Ta variation
of the samples may point crustal contamination to the magma. The paleolatitude of the Bugat/Baruunburen
Formation is calculated to be 37.1N based on thermal remanent magnetization. Therefore, the Late Per-
mianMiddle Triassic large-scale adakitic igneous activity had taken place in the volcanic arc along the Siberian
continental margin in the mid-latitudes of the Northern Hemisphere. The geochemical characteristics of the
intermediate dike are almost the same as those of the Bugat/Baruunburen Formation and the Selenge plutonic
rock complex, indicating that adakitic igneous activity continued after the Early Triassic.
1. Introduction
The Central Asian Orogenic belt (CAOB), which lies among Siberian
craton, North China block, Tarim block, and East European craton
(Sengӧr et al., 1993) is a crucial geologic unit in understanding the
development process of the Eurasian continent (Kovalenko et al., 2004)
(Fig. 1). It is generally accepted among scholars that the CAOB has been
formed due to several 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. In an
area between the Siberian craton and the North China block, the
geological setting of Mongolia is a signicant factor to reveal the Pale-
ozoicMesozoic tectonics of the CAOB (Fig. 1). The northern Mongolia,
southern margin of the Siberian continent(Siberian craton +accreted
geologic units due to Pre-Permian orogeny), widely exposes Precam-
brianPaleozoic basement rocks of the Sayan-Baikal belt and Late
* Corresponding author.
E-mail address: tsukada@num.nagoya-u.ac.jp (K. Tsukada).
Contents lists available at ScienceDirect
Journal of Geodynamics
journal homepage: www.elsevier.com/locate/jog
https://doi.org/10.1016/j.jog.2022.101918
Received 15 August 2021; Received in revised form 3 April 2022; Accepted 12 April 2022
Journal of Geodynamics 151 (2022) 101918
2
PaleozoicEarly Mesozoic accretionary complexes of the Khangai-Daur
belt and the basement rocks are covered/intruded by the Permian-
Triassic igneous rocks to form a large-scale igneous rock belt (Mineral
Resource Authority of Mongolia and Mongolian Academy of Sciences,
1998; Tomurtogoo, 2003; Badarch et al., 2002; Kurihara et al., 2008;
Onon and Tsukada, 2017) (Fig. 2).
The PermianTriassic igneous rocks are classied into the volca-
nicclastic rock sequences (i.e., Permian Mogod Formation, Bugat/
Fig. 1. Simplied tectonic division of NE Asia
(modied from Sengӧr et al., 1993; Jahn et al.,
2000; Tomurtogoo, 2003; Petrov et al., 2014;
Onon and Tsukada, 2017).Yellow stars show
Late Triassic adakitic rock localities (Donskaya
et al., 2012; Li et al., 2017; Sheldrick et al.,
2020; Tang et al., 2021). CAOB: Central Asian
Orogenic belt, SM: South Mongolian block
(including ErgunaXinganSongliao block), K:
Kizil Kum block, T-Qd: Tarim-Qaidam block, B:
Bureya block, NC: North China block, Kt:
Kataev Formation, E: Erguna massif, MG: Mid-
dle Gobi volcanic belt, UB: Ulaanbaatar, EN:
Erdenet. (For interpretation of the references to
color in this gure legend, the reader is referred
to the web version of this article.)
Fig. 2. Simplied tectonic division of Mongolia (modied from Badarch et al., 2002; Tomurtogoo, 2003).
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
3
Baruunburen Formation, and Khanui Group) and plutonic rock com-
plexes (i.e., PermianTriassic Selenge and Erdenet plutonic rock com-
plexes) (Kepezhinskas and Luchitsky, 1974; Mossakovsky and
Tomurtogoo, 1976; Sotnikov et al., 1995, 2005; Izawa and Ohsawa,
2004; Gerel and Munkhtsengel, 2005; Munkhtsengel, 2007; Munkht-
sengel et al., 2007; Berzina et al., 2009; Tsukada et al., 2018a; Tsukada
et al., 2021) (Fig. 3). The plutonic rock complexes, embedding sub-
stantial quantities of metal ores, have been well-studied mainly from the
viewpoint of metallogeny (Khasin et al., 1977; Koval et al., 1989; Gav-
rilova and Maksimyuk, 1990; Jargalsaihan et al., 1996; Lamb and Cox,
1998; Watanabe and Stein, 2000; Dejidmaa et al., 2002; Berzina et al.,
2005). However, no established theory seems to be there to explain the
magmagenesis of the PermianTriassic igneous rocks, which offers a key
to understand the ancient arc system along the Siberian continental
margin.
Morozumi (2003) suggested that the Selenge plutonic rock complex
has an adakitic nature in the Sr/Y ratio. In addition, Munkhtsengel et al.
(2007) mentioned that the Selenge plutonic rock complex corresponds
to adakitic rocks formed at an arc condition. Tsukada et al. (2018a)
found that the Upper Permian volcanic rocks of the Tulbur Formation of
Khanui Group are adakitic rocks derived from the oceanic slab-melt. As
mentioned above, several studies in PermianTriassic magmatism have
been made on the Khanui Group and the Selenge plutonic rock complex;
on the other hand, little attention has been given to the Mogod and
Bugat/Baruunburen Formations. In order to clarify the PermianTriassic
magmatism along the Siberian continental margin, geochemical
investigation of these crucial formations and comparison with the other
volcano-plutonic rocks are necessary. This paper describes the lithology
and geochemistry of the Bugat/Baruunburen Formation at the Jargalant
area, Selenge plutonic rock complex at the Erdenet north area, and in-
termediate dike intruding into these rocks around the Erdenet city,
northern Mongolia. It discusses the magmagenesis and subduction sys-
tem along the PermianTriassic SiberiaSouth MongolianBureya con-
tinental margin.
2. Geological setting of the PermianTriassic igneous rocks
Mongolia is geologically divided into the northern and southern
superblocks by the Main Mongolian lineament (Tomurtogoo, 2003)
(Fig. 2). The northern superblock, the southern margin of the Siberian
continent, is mainly composed of the Late PaleozoicEarly Mesozoic
accretionary complexes of the Khangai-Daur belt and the continental
(K-Ar, Hbl)
Early–Middle Permian Late
Permian Early
Triassic Middle Triassic
259 Ma
252 Ma
247 Ma
237 Ma
Erdenet plutonic
rock complex
?
Selenge plutonic
rock complex
Mogod
Formation
Bugat/Baruun
-buren F.
Tulbur F.
(Khanui G.)
Khargana F. (Khanui Group)
Khustai F. (Khanui Group)
258–247 Ma (40Ar-39Ar, Bt)
250–243 Ma (SHRIMP)
253 ± 18 Ma (Rb-Sr, Wr)
ca. 235 Ma (40Ar-39Ar,
Wr, Ser, Amph)
246–236 Ma (SHRIMP)
256.9 ± 2.2 Ma
(U-Pb, Zircon)
257.9 ± 3.8 Ma
(U-Pb, Zircon)
292.9 ± 14.1 Ma
Fig. 3. Stratigraphic division of the PermianTriassic igneous rocks. Age data were from Sotnikov et al. (1995, 2005), Munkhtsengel (2007), Munkhtsengel et al.
(2007), Berzina et al. (2009), and Tsukada et al. (2018a). F.: Formation, G.: Group, Wr: whole rock, Ser: sericite, Amph: amphibole, Hbl: hornblende, Bt: biotite.
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
4
Table 1
Geochronological data from the Selenge plutonic rock complex, Bugat/Baruunburen Formation, and Khanui Group.
Geologic unit Rock type Ages (Ma) Method References
Selenge plutonic rock complex
Granitoid 247 ±3.7
39
Ar-
40
Ar Sotnicov (2005)
Granitoid 258.6 ±3.3
39
Ar-
40
Ar Sotnicov (2005)
Granitoid 243.3 ±4.6 SHRIMP U-Pb
Gerel and Munkhtsengel (2005),
and Munkhtsengel (2007),
Munkhtsengel et al. (2007)
Granitoid 249.7 ±4.0 SHRIMP U-Pb
Gerel and Munkhtsengel (2005),
Munkhtsengel (2007) and
Munkhtsengel et al. (2007)
Granitoid 244.3 ±4.5 SHRIMP U-Pb
Gerel and Munkhtsengel (2005),
Munkhtsengel (2007) and
Munkhtsengel et al. (2007)
Bugat/Baruunburen Formation
Trachyandesite 206 K-Ar Kepezhinskas and Luchitsky
(1974)
Trachyandesite 205 K-Ar Kepezhinskas and Luchitsky
(1974)
Andesite-basalt 210 K-Ar Kepezhinskas and Luchitsky
(1974)
Olivine trachyandesite 195 K-Ar Kepezhinskas and Luchitsky
(1974)
Andesite-basalt 206 K-Ar Kepezhinskas and Luchitsky
(1974)
Andesite-basalt 220 K-Ar Kepezhinskas and Luchitsky
(1974)
Basalt 228 ±6 K-Ar Kepezhinskas and Luchitsky
(1974)
Andesite 257.6 ±3.6 LA-ICP-MS U-Pb Tsukada et al. (2021)
Khanui Group
(Tulbur Formation) Andesite 256.9 ±2.2 LA-ICP-MS U-Pb Tsukada et al. (2018a)
(Khustai Formation) Felsic volcanic rock CisuralianGuadalupian Flora Mossakovsky and Tomurtogoo
(1976)
Fig. 4. Simplied geologic map at the BulganErdenetJargalant area of Mongolia (modied from Makhbadar, 1990).
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
5
afnity of the Sayan-Baikal belt (Petrov et al., 2014; Onon and Tsukada,
2017) (Fig. 2). The Khangai-Daur belt is mainly composed of sandstone
and mudstone with subordinate amounts of radiolarian chert, siliceous
mudstone, basalts, and limestone (Badarch et al., 2002; Tomurtogoo,
2003; Minjin et al., 2006; Kurihara et al., 2008; Takeuchi et al., 2012;
Tsukada et al., 2013; Erdenesaihan et al., 2013; Lkhagvasuren et al.,
2020). The belt rocks are in places intruded by PermianJurassic
granitic rocks (Mineral Resource Authority of Mongolia and Mongolian
Academy of Sciences, 1998; Tomurtogoo, 2003; Takeuchi et al., 2012).
The Sayan-Baikal belt mainly consists of PrecambrianCambrian meta-
morphic rocks and limestone, PrecambrianOrdovician granitic rocks,
and SilurianCarboniferous limestone, volcanic rocks, and clastic rocks
(Onon and Tsukada, 2017). These rocks are covered/intruded by the
PermianTriassic igneous rocks, and then are intruded by Mesozoic
granitic rocks (Mineral Resource Authority of Mongolia and Mongolian
Academy of Sciences, 1998; Tomurtogoo, 2003; Onon and Tsukada,
2017) (Fig. 2).
The PermianTriassic igneous rocks are divided into the following
ve geologic units: Lower Permian Mogod Formation, LowerUpper
Permian Khanui Group, Upper Permian Bugat/Baruunburen Formation,
Late PermianMiddle Triassic Selenge plutonic rock complex, and
Permian?Late Triassic Erdenet plutonic rock complex (Mossakovsky
and Tomurtogoo, 1976; Tomurtogoo, 2003; Gerel and Munkhtsengel,
2005; Munkhtsengel et al., 2007; Dejidmaa et al., 2011; Tsukada et al.,
2018a; Tsukada et al., 2021) (Fig. 3).
The Khanui Group is classied, in ascending order, as the Lower
Permian Khustai, Middle Permian Khargana, and Upper Permian Tulbur
formations (Amgalan et al., 2017; Tsukada et al., 2018a; Tsukada et al.,
2021). The Khanui Group comprises rhyolite, diorite, andesite, basalt,
and tuffaceous clastic rocks (Amgalan et al., 2017; Tsukada et al.,
2018a). A ora suggesting CisuralianGuadalupian was obtained from
the felsic volcanic rock formation (e.g., Mossakovsky and Tomurtogoo,
1976) (Table 1). Andesite lava of the Tulbur Formation gives a zircon
U-Pb age of 256.9 ±2.2 Ma (Tsukada et al., 2018a) (Table 1). The
Mogod and Bugat/Baruunburen formations are mainly composed of
intermediate volcanic rocks. The Mogod Formation yields a hornblende
KAr age of 292.9 ±14.1 Ma, and the Bugat/Baruunburen Formation
gives zircon U-Pb age of 257.6 ±3.6 Ma (Tsukada et al., 2021)
(Table 1). Although K-Ar ages of 230195 Ma were obtained from the
Bugat Formation (Kepezhinskas and Luchitsky, 1974), it is likely the
modied ages due to later alteration (Tsukada et al., 2021) (Table 1).
While further chronological conrmation would be necessary for the
Mogod Formation which had been correlated to the Bugat/Baruunburen
Formation as TriassicJurassic formation, in this paper, the Mogod
Formation is tentatively regarded as a Lower Permian formation,
following Tsukada et al. (2021). The Selenge plutonic rock complex,
mainly composed of granitoid and diorite, gives
40
Ar
39
Ar ages of
258247 Ma and sensitive, high-resolution ion-microprobe (SHRIMP)
U-Pb ages of 250243 Ma (Gerel and Munkhtsengel, 2005; Sotnikov
et al., 2005; Munkhtsengel, 2007; Munkhtsengel et al., 2007) (Table 1).
The Erdenet plutonic rock complex shows 253 ±18 Ma (RbSr age), ca.
235 Ma (
40
Ar
39
Ar age), and 246236 Ma (SHRIMP U-Pb age) (Sotnikov
et al., 1995, 2005; Munkhtsengel, 2007; Munkhtsengel et al., 2007b;
Berzina et al., 2009).
3. Geological description of the study areas
3.1. Lithology and structure of the Bugat/Baruunburen Formation at
Jargalant area
The study area, Jargalant village, 20 km east of Erdenet city,
Mongolia, exposes intermediate volcanic rocks of the Bugat/Bar-
uunburen Formation (Amgalan et al., 2017) (Figs. 4, 5). The rocks of this
area, striking northeast and gently dipping south, are further divided
into the following two parts in ascending order: (1) lower part (inter-
mediate lava with minor tuff breccia) and (2) upper part (intermediate
tuff and tuff breccia with lava intercalations) (Fig. 5).
The lava in the lower and upper parts is ne- to coarse-grained and
vesicular, and has porphyritic or trachytoid textures and is hol-
ocrystalline in places (Fig. 6a, b). The vesicles, some of which are lled
with calcite or chlorite to form amygdules, are elongated to oval shape.
The platy joint strikes northeast and dips south by 2050, and the
Fig. 5. Geological map of the Jargalant area, northern Mongolia (modied from Tsukada et al., 2021). Sampling localities are also shown.
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
6
Fig. 6. Photographs of the Bugat/Baruunburen Formation, Selenge plutonic complex, and intermediate dike. (a) Photograph of the lava of the Bugat/Baruunburen
Formation with trachytoid texture. (b) Photograph of the porous lava of the Bugat/Baruunburen Formation. (c) Tuff breccia of the Bugat/Baruunburen Formation
including abundant angular clasts of intermediate volcanic rocks in a cognate matrix. (d) Bedding of the pyroclastic rocks of the Bugat/Baruunburen Formation. (e)
Intermediate dike intruding into the intermediate lava of the Bugat/Baruunburen Formation. (f) Granodiorite of the Selenge plutonic complex with mac enclave. Gr:
granodiorite. (g) Granodiorite of the Selenge plutonic complex intruded by intermediate dike. D: intermediate dike. (h) Close-up view of the boundary between the
granodiorite and the intermediate dike. CM: chilled margin.
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
7
elongated vesicles plunge southeast by 060. The lava is partly altered
to the extent that some minerals have been replaced by calcite, chlorite,
and opaque minerals such as hematite. In the lava showing porphyritic
or trachytoid textures, subhedral or euhedral plagioclase, clinopyroxene
(Cpx), and hornblende, more than 0.5 mm in major axis, lie in a
groundmass composed of smaller plagioclase and interstitial chloritic
material (Fig. 7a, b). Later overgrowth and entrapped inclusions are not
recognized at each mineral (Fig. 7b). The coarse lava rarely includes
subhedral/euhedral zircon. The mineral assemblage and texture of the
lava are monotonous throughout the study area. The tuff breccia in the
upper part includes abundant angular clasts of intermediate volcanic
rocks, up to 10 cm in diameter, in a cognate matrix (Fig. 6c). The tuff
breccia has clear bedding, striking northeastsouthwest, and dips south
by 15(Fig. 6d).
The rocks of the Bugat/Baruunburen Formation are intruded by in-
termediate dikes (Fig. 6e). The dikes, having clear chilled margin in
some places, generally strike northeast and are less than 100 m wide.
Numerous veins of calcite, epidote, and chlorite are, in places, devel-
oped around the dike. The dikes are composed of subhedral or euhedral
plagioclase, Cpx with porphyritic or trachytoid textures same as the lava
(Fig. 7c, d). Later overgrowth and entrapped inclusions are not recog-
nized at each mineral (Fig. 7d).
3.2. Lithology of the Selenge plutonic rock complex at Erdenet north area
The study area broadly exposes medium-grained granodiorite and
diorite with a subordinate amount of andesite in the Selenge plutonic
rock complex (Fig. 8). The andesite gradually changes into the diorite
and granodiorite to show no distinct boundary. The rocks are mainly
composed of euhedral or subhedral plagioclase, potassium feldspar,
quartz, and biotite with subordinate amounts of apatite, zircon, and
opaque minerals. The granodiorite and diorite commonly include mac
enclaves and sub-vertical joints developed in places (Fig. 6f, g).
The rocks of the Selenge plutonic rock complex are intruded by in-
termediate dikes of andesite and diorite with a clear chilled margin
(Fig. 6g, h). The intermediate dike includes the xenolith of granodiorite.
The dikes are the mostly same, in mineral assemblage and texture, to
that intrudes into the Bugat/Baruunburen Formation.
4. Whole-rock and clinopyroxene chemical compositions of the
Bugat/Baruunburen Formation, Selenge plutonic rock complex,
and intermediate dike
4.1. Methodology
4.1.1. Whole rock chemical composition
Twenty-two samples of lava from the Bugat/Baruunburen Forma-
tion, 21 samples of plutonic rocks from the Selenge plutonic rock com-
plex, and 26 samples from the intermediate dike were examined. Major
element composition was determined using X-ray uorescence (XRF;
Rigaku Primus II ZSX equipped with Rh X-ray tube, 50 kV, 60 mA)
installed at Nagoya University of Japan and Mongolian University of
Science and Technology (MUST). Some trace elements (i.e., Co, Rb, Y,
Zr, Nb, and Th) of the Selenge plutonic rock complex and intermediate
dike were determined using XRF at MUST. In the XRF analysis, glass
beads were prepared by fusing mixtures of 0.5 g of powdered sample
with 5.0 g of lithium tetraborate for the major elements and mixtures of
1.5 g of powdered sample with 6.0 g of lithium tetraborate for the trace
Fig. 7. Photomicrographs of the Bugat/Baruunburen Formation (a and b) and intermediate dike (c and d). (a) Photomicrograph of the lava with trachytoid texture.
Plane polarized light. (b) Photomicrograph of the porphyritic lava. Euhedral/subhedral clinopyroxene lies in groundmass composed of smaller plagioclase. (c)
Photomicrograph of the intermediate dike. Plane polarized light. (d) Intermediate dike including clinopyroxene phenocrysts. Plane polarized light. Pl: plagioclase,
Cpx: clinopyroxene. Ag: amygdule.
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
8
elements. Calibration was carried out using standard rock samples is-
sued by the Geological Survey of Japan. Analytical precision of major
elements was estimated to be <1% for Si and about 3% for other ele-
ments, except for CaO, MgO, and Na
2
O, whose analytical precision is
>3% when the measured level is <0.1% (Takebe and Yamamoto,
2003). Trace elements and rare earth elements (REE) were analyzed
using inductively coupled plasma mass spectrometry (quadrupole type
ICP-MS; Agilent 7700x) installed at Nagoya University, with the method
described in Yamamoto et al. (2005). About 30 mg of each sample was
digested with a mixed solution of HF-HClO
4
(2:1 by volume) at 150 .
After complete evaporation of the acids, 2 ml of 1.7 N-HCl was added to
dissolve the cake. The residue was separated by centrifugation at 12,
000 rpm with a 2 ml polypropylene tube. The supernatant after centri-
fugation was transferred to another 10 ml Teon beaker. The residue
was then fused with HF-HClO
4
(2:1 by volume) again at 150 . The
fused cake was dissolved with about 2 ml 1.7 N-HCl by mild heating,
and the solution was centrifuged at 12,000 rpm. In most cases, no res-
idue was recognized after centrifugation. The HCl solution was evapo-
rated to dryness. The fused cake was re-dissolved in 2% HNO
3
solution
and determined by ICP-MS. In and Bi were used to trace ICP sensitivities;
the In and Bi concentrations were mainly the same 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 coefcients (R-value) of each element,
calculated for ve standard samples, were >0.9994. The concentration
relative standard deviation of the data was mostly less than 3%. The
whole-rock composition of the samples is listed in Tables 2, 3, and 4. It is
displayed on variation diagrams for selected elements against SiO
2
(Fig. 9).
4.1.2. Clinopyroxene chemical composition
The Cpx chemical composition in the samples was determined with a
method described by Tsukada (2018) using energy-dispersive X-ray
spectrometer (EDX, Oxford X-Max) linked with scanning electron mi-
croscope (SEM, Hitachi S-3400 N) at Nagoya University. The total
values of each analysis were normalized as 100%, following Tsukada
(2018). The analytical precision was estimated to be <5% when the
measured level was more than 1 wt%, and <10% when the measured
level was less than 1 wt% (Tsukada, 2018). Elements with a concen-
tration of less than 0.5 wt% may not have been detected (Tsukada,
2018).
4.2. Results geochemical description of the samples
4.2.1. Whole rock chemical composition
The data of Bugat/Baruunburen Formation, Selenge plutonic rock
complex, and intermediate dike form a linear trend in the variation di-
agrams for many elements (Fig. 9). The Al
2
O
3
concentration of all
samples is nearly identical as about 17 wt% (Fig. 9, Tables 24). The
Na
2
O, K
2
O, and Rb show an increasing trend, and TiO
2
, FeO*, MnO,
CaO, MgO, CaO, P
2
O
5
, Cr, Co, Ni, Sr, Y, Ba, and Mb show a decreasing
trend against the increase of SiO
2
in the variation diagram (Fig. 9). The
data from these rocks show almost the same pattern on the mid-ocean
ridge basalt (MORB)-normalized multi-element concentration dia-
grams (called spidergramfrom now on), i.e., reducing trend with
distinctive negative Nb anomaly (Fig. 10). The chondrite-normalized
REE patterns for all data show a reducing trend, enriched in light rare
earth elements (LREE) and depleted in heavy rare earth elements
(HREE) (Fig. 11). Geochemical features of the Bugat/Baruunburen
Formation, Selenge plutonic rock complex, and intermediate dike are as
follows: (1) Bugat/Baruunburen Formation: SiO
2
concentration is
Fig. 8. Geological map of the Erdenet north area, northern Mongolia (modied from Amgalan et al., 2017). Sampling localities are also shown. SPC: Selenge plutonic
rock complex.
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
11
5057 wt% and Na
2
O +K
2
O is 5.49.6, mostly assigned to basaltic
trachy andesite (Fig. 12, Table 2). The samples have FeO* /MgO ratio
ranging from 1.8 to 4.0; Mg# (100 ×Mg / (Mg +Fe) calculated on a
molar basis using total Fe
2+
content) is 3150. The samples have high Sr
concentration ranging from 986 to 2309 ppm (avg. 1453 ppm); Sr/Y
ratio is 44190 (avg. 95); and La/Yb ratio ranges from 23 to 41. (2)
Selenge plutonic rock complex: SiO
2
concentration of the samples is
5369 wt%, Na
2
O +K
2
O is 3.79.4, and plotted near the boundary
between alkaline and subalkaline rocks in the (Na
2
O +K
2
O) vs. SiO
2
diagram (Fig. 12, Table 3); FeO* /MgO ratio is from 0.8 to 3.0; and Mg#
is 3769. The Sr concentration of the samples is 5742572 ppm (avg.
1157 ppm); Sr/Y ratio is 50212 (avg. 92); La/Yb ratio ranges from 13 to
26. (3) Intermediate dike: SiO
2
concentration of the samples is 5059 wt
% and Na
2
O+K
2
O is 4.18.4, assigned to basaltic trachy andesite or
trachy andesite (Fig. 12, Table 4); FeO* /MgO ratio ranges from 0.9 to
7.2; and Mg# is 2067. The samples show high Sr concentration
(10802531 ppm, avg. 1527 ppm), and their Sr/Y ratio is 70195 (avg.
100); La/Yb ratio is 24100.
4.2.2. Cpx chemical composition
Representative mineral compositions of Cpx in the lava of the Bugat/
Baruunburen Formation and intermediate dike are in Table 5. The Cpx is
assigned to augite and diopside in the pyroxene quadrilateral diagram
(Morimoto, 1988) (Fig. 13a). The Cpx chemical composition is described
below.
(1) Cpx in the lava of the Bugat/Baruunburen Formation
FeO concentration of the central and marginal parts are 9.2 wt%
average (standard diviasion (SD): 0.63 wt%) and 9.8 wt% average (SD:
0.66 wt%) respectively, and those of MgO are 15 wt% average (SD:
0.79 wt%) and 15 wt% average (SD: 0.66 wt%) respectively (Fig. 13b,
Table 5). Average Mg# is 0.74 (SD: 0.020) at central part of each crystal,
and is 0.74 (SD: 0.018) at marginal part (Fig. 13c, Table 5). CaO is 21 wt
% (SD: 0.73 wt%) in the cental and is 20 wt% (SD: 0.49 wt%) in the
marginal (Table 5). In all crystals, no specic structures such as zoning
and compositional contrast are not appeared within the crystals in back
scattered electron and chemical compositional mapping images
(Fig. 14a, b, c, d and 15a). TiO
2
ranges from 0.61 to 1.6 wt% (0.98 wt%
average), Al
2
O
3
varies from 1.8 to 5.3 wt% (3.4 wt% average), and.
Na
2
O is less than 0.53 wt% (Table 5).
(2) Cpx in the intermediate dike.
FeO conentration of the central and marginal parts are 7.9 wt%
average (SD: 0.28 wt%) and 8.3 wt% average (SD: 0.34 wt%) respec-
tively, and those of MgO are 15 wt% average (SD: 0.42 wt%) and 14 wt
% average (SD: 0.47 wt%) respectively (Fig. 13b, Table 5). Mg# is 0.77
(SD: 0.0090) at central part of each crystal, and is 0.75 (SD: 0.012) at
marginal part (Fig. 13c, Table 5). The mean of CaO is 22 wt% (SD:
0.53 wt%) in central, and is 22 wt% (SD: 0.34 wt%) in the marginal
(Table 5). There is no variation within each crystal for Mg, Fe, and Ca
concentrations and no specipic structures such as zoning and composi-
tional contrast are not appeared within the crystals in back scatterd
electron and chemical compositional mapping images (Fig. 14e, f, g, h
and 15b). TiO
2
ranges from 0.68 to 1.5 wt% (1.1 wt% average), Al
2
O
3
varies from 3.1 to 5.4 wt% (4.2 wt% average), and CaO is 2123 wt%.
Na
2
O is less than 0.45 wt% (Table 5).
0
0.5
1.0
1.5
2.0 wt%
TiO2
0
5
10
15
20
25 wt%
Al2O3
0
2
4
6
8
10 wt%
FeO
MnO
0
0.05
0.10
0.15
0.20
0.25wt%
MgO
0
2
6
4
8
10
12
wt%
CaO
0
2
4
6
8
10
12
14
wt%
Na2O
0
2
4
6
8
10 wt%
K2O
0
1
2
3
4
5wt%
P2O5
0
0.2
0.4
0.6
0.8
1.0
1.2wt%
Bugat/Baruunburen Fotmation
Selenge plutonic rock complex
Intermediate dike
Fig. 9. Variation diagrams for the selected elements against SiO
2
.
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
12
5. Discussion
5.1. Discrimination of the samples
The chemical composition of volcanic rocks gives evidence for the
tectonic setting of the volcanic activity, and many diagrams to
discriminate igneous rocks have been proposed (e.g., Miyashiro, 1974;
Pearce, 1982). In this section, discrimination diagrams are used to
discuss the tectonic setting of the rocks from the Bugat/Baruunburen
Formation, Selenge plutonic rock complex, and intermediate dike.
Data from the Bugat/Baruunburen Formation, Selenge plutonic rock
complex, and intermediate dike are arranged in a straight line in the
variation diagrams for many elements. It likely suggests that these three
rocks originated from a magma source through a fractionation process,
or products as a result of magma-mixing between mac and felsic
magmas (Fig. 9).
The spidergrams of the samples show geochemical features similar to
those of arc-related igneous rocks, such as enriched large ion lithophile
elements and LREE in comparison to high-eld strength elements and
HREE with a negative Nb anomaly (Gill, 1981; Pearce et al., 2005)
(Fig. 10). The majority of the samples are plotted in the calc-alkaline
eld in the SiO
2
vs. FeO* /MgO diagram (Miyashiro, 1974) (Fig. 16a).
Taking all these lines of evidence into consideration, it can be concluded
that the samples are of calc-alkaline rocks, which were formed in a
volcanic arc environment. Extremely high Sr concentration and Sr/Y
ratio of the samples correspond to adakites(Defant and Drummond,
1990). The adakites are generally characterized by SiO
2
>56 wt%,
Al
2
O
3
>15 wt%, and Sr >400 ppm, a high Sr/Y ratio, and fractionated
REE (Defant and Drummond, 1990; Drummond et al., 1996; Martin,
1999). The samples in this study, SiO
2
5069 wt% (avg. 56 wt%), Al
2
O
3
1520 wt% (avg. 17 wt%), Sr 5742572 ppm (avg. 1379 ppm), Sr/Y
44212 (avg. 96), and La/Yb 13100 (avg. 35), have adakitic nature in
this aspect. It is supported by examination using Sr/Y vs. Y and La/Yb vs.
Yb diagrams (Defant and Drummond, 1990; Defant et al., 1991; Moyen,
2009) (Fig. 16b, c). In the (K
2
O +Na
2
O) vs. SiO
2
diagram, most of data
are plotted within the eld of the adakite (Zhang et al. 2021) (Fig. 12).
Martin (1999) classied adakites into the high silica adakite (HSA) with
SiO
2
>60 wt% and the low silica adakite (LSA) with SiO
2
<60 wt%,
and described that their geochemical features are different each other.
Many samples of this study are assigned to Martins LSA in terms of SiO
2
<60 wt%, and most samples are plotted across the LSA and HSA elds
in the discrimination diagrams (Martin, 1999) (Fig. 17). In the Cr/Ni vs.
TiO
2
diagram, although the most samples are fall on the LSA eld, the
trend plots showing is similar to HSAs one (Fig. 17). Zhang et al. (2021)
mentioned that the compositional range of the HSA given by Martin
et al. (2005) falls within the initial denition of adakites established by
Defant and Drummond (1990), whereas LSA can be termed adakitic
rocks, but not strictly adakites. Following Zhang et al. (2021), the
examined samples here are called to the adakitic rocks.
5.2. Magmagenesis of the examined rocks
It is suggested that adakitic magma is generated by partial melting of
the subducted oceanic slab (Defant and Drummond, 1990; Rapp, 1995;
Sen and Dunn, 1994; Tsuchiya et al., 2007), and various processes
during its ascent led to the diversity of the adakitic rocks (Castillo, 2006;
Cr
0
100
200
300 ppm
0
50
100
150
200
250 ppm
Ni
Co
0
10
20
30
40
50 ppm
Rb
0
20
40
60
80
100
120 ppm
Sr
0
500
1000
1500
2000
2500
3000
ppm
0
5
10
15
20
25
30 ppm
Y
Ba
0
500
1000
1500
2000 ppm
45 50 55 60 65 70 75
wt%
Nb
0
5
10
15 ppm
45 50 55 60 65 70 75
wt%
Zr
45 50 55 60 65 70 75
wt%
0
50
100
150
200
250
300
350
400 ppm
SiO2SiO2SiO2
Fig. 9. (continued).
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
14
Tsuchiya, 2008; Moyen, 2009; Zhang et al., 2021). Besides, an alterna-
tive way of the adakitic high Sr/Y and La/Y magma generation is pro-
posed as magma-mixing, partial melting of the basaltic continental
lower crust, and others (Atherton and Petford, 1993; Guo et al., 2007;
Streck et al., 2007; Zhang et al., 2013; Zhang et al., 2021).
It is known that an adakitic signature in rocks is produce by some
magma-mixing processes at continental crust (Guo et al., 2007; Streck
et al., 2007; Zhang et al., 2013), and the liner arrangement of the data
from the Bugat/Baruunburen Formation, Selenge plutonic rock com-
plex, and intermediate dike in the variation diagrams might implies such
magma-mixing.
Guo et al. (2007) considered that the adakitic melts caused by
various degrees of partial melting were generated in the lower crust, and
the mixing of potassicultrapotassic magma with these melts produced a
variety of adakitic rocks in southern Tibet after the collision of Indian
and Eurasian continents. The trace elements compositions and Sr-Nd-Pb
isotope characteristics of the Tibetan adakite are in close agreement
with those of both the melt resulting from partial melting of the sub-
ducted oceanic plate and that from partial melting of the lower crustal
macintermediate rocks. And, Guo et al. (2007) considered the latter as
a candidate for the source of the Tibetan adakite on the ground that the
subducted oceanic plate was perhaps too cold to cause a melt at the end
stage of continentcontinent collision. They also mentioned that the
heat required to melt the rocks of the lower continental crust could have
been supplied by the contemporaneous potassicultrapotassic magmas
combined with crustal heating induced by lithospheric extension. In the
case of Mongolia, unlike the case of Tibet, the Mongol-Okhotsk oceanic
plate is considered to have subducted beneath the Siberian continental
marginin the Late Paleozoic (Kurihara et al., 2008; Bussien et al., 2011;
Gordienko et al., 2012; Takeuchi et al., 2012; Onon and Tsukada, 2017).
Although there is no concrete data on the stress eld of the northern
Mongolia around the PermianTriassic, a compressional eld, rather
than extensional one, would be expected from the tectonic situation.
Indeed, there is no evidence here for lithospheric extension, for example
regional normal fault system, graben deposits, back-arc volcanism etc.,
that would have allowed thermal intrusion into the lower continental
crust. Thus, it might not be possible to apply the model proposed by Guo
et al. (2007) to the Mongolian case.
Streck et al. (2007) concluded that the adakite-like high-Mg andesite
at the Mount Shasta, western coast of US, was formed by mixing of
dacitic and basaltic magmas and entrainment of ultramac crystal ma-
terial, along with the evidences such as orthopyroxene with high-Mg#
rim and entrapped glass inclusions in olivine. And, Zhang et al. (2013)
suggested that the adakite-like high-Mg diorites in eastern Dabie orogen,
East China is a product resulted as a magma-mixing between the
crust-derived granodioritic magma and the differentiation products of
mantle-derived gabbronoritic magma with the following evidences. (1)
the gabbronorite is usually occurred as enclaves in the high-Mg diorite
host, and these are gradually changed each other without sharp
boundary. (2) plagioclase phenocrysts in the high-Mg diorite show a
strong compositional zoning, i.e. Ca-rich core and Ca-poor rim. (3) The
amphibole and biotite in the high-Mg diorite have distinctively higher F
concentrations than those in the gabbronorite implying that the high-Mg
diorite is a result of mixing of F-rich granodioritic magma and F-poor
gabbronoritic magma. (4) In the high-Mg diorites, Cpx is included in
amphibole to suggest a reaction between pre-existing Cpx and sur-
rounding hydrous melts causing the amphibole formation. (5) Zircons
from the gabbronorite and the diorite are different each other in their
morphology, internal texture, Hf isotope, and trace element
composition.
In the case of this study, evidences positively suggesting the magma-
mixing, such as compositional zoning and entrapped glass inclusions in
Fig. 10. MORB-normalized multi-element concentration diagrams (spidergram) of the examined samples. Normalizing MORB composition by Sun and Mcdonough
(1989) is used. The data of the Tulbur Formation are from Tsukada et al. (2018a). The range of the Kitakami adakitic granites is from Tsuchiya et al. (2005).
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
15
crystals, are not recognized at least in microscopic and SEM observations
for 46 samples (Figs. 7, 13, and 14, Table 5). Besides, monotonous
chemical composition within Cpx crystals also support this view
(Figs. 14 and 15). The intermediate dikes always intrude into the host
lava and plutonic rocks with clear boundary, and not appear to have
been reacted each other (Fig. 6e, g, and h). Therefore, it is considered
Fig. 11. Chondrite-normalized REE patterns of the examined samples. Normalizing chondrite values are after Sun and McDonough (1989) and Yamamoto et al.
(2005). The data of the Tulbur Formation are from Tsukada et al. (2018a).
Fig. 12. (Na
2
O +K
2
O) vs. SiO
2
diagram (Irvine and Baragar, 1971). The data of the Tulbur Formation are from Tsukada et al. (2018a). The range of adakite was from
Zhang et al. (2021).
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
16
unlikely, here, that the samples in this study are the result of magma-
mixing.
Zhang et al. (2005) classied the adakitic rocks according to their
origins into Type 1 (derived from the partial melting of the subducted
oceanic slab) and Type 2 (derived from the partial melting of the basaltic
lower continental crust). Type 2 adakitic rocks have lower Al
2
O
3
con-
centration (less than about 16 wt%), higher K
2
O/Na
2
O ratio (about 0.5
or more) than Type 1 reecting their source composition: the basaltic
rocks of the lower continental crust generally have a higher K
2
O/Na
2
O
ratio than the oceanic basalt. An examination using K
2
O/Na
2
O vs. Al
2
O
3
diagram highly suggests that the present samples, low K
2
O/Na
2
O ratio
(mostly less than 0.6) and high Al
2
O
3
concentration (1520 wt%, avg.
17 wt%), correspond to the Type 1 adakitic rocks (Kamei et al., 2009;
Liu et al., 2010) (Fig. 16d).
When an adakite magma are produced by various levels of partial
melting of an eclogite or garnet amphibolite, a positive correlation
would be expected for Sr/Y and La/Yb because Sr and La are incom-
patible whereas Y and Yb are compatible in garnet-bearing and
plagioclase-free residues (e.g., Defant and Drummond, 1990; Rapp and
Watson, 1995; Moyen, 2009; Liu et al., 2010) (Fig. 16e). Liu et al.
(2010), based on geochemical studies of Cretaceous adakitic rocks in
central-eastern China, demonstrated that adakitic rocks of oceanic
slab-origin and those of lower continental crust-origin show quite
different arrays on the Sr/Y vs. (La/Yb)
N
diagram, i.e. the former ex-
hibits much higher Sr/Y value than the latter for a given (La/Yb)
N
value.
Such a sharp trend of increasing Sr/Y ratio of oceanic slab-derived
adakitic rocks is unlikely to be caused by plagioclase and garnet frac-
tionation, and it probably reects the low-temperature alteration of the
oceanic slab (Liu et al., 2010). The most data here are plotted in the eld
of the slab-derived adakitic rocks in the Sr/Y vs. (La/Yb)
N
diagram, and
therefore the examined rocks are quite likely of slab-melt origin
(Fig. 16e).
The continental crust generally has a lower Ce/Pb than oceanic crust
(Sun and McDonough, 1989; Taylor and McLennan, 1985; Rudnick and
Gao, 2003). And the altered oceanic crust has a much higher Sr/La, than
the lower continental crust, due to the LREE-depleted N-MORB
composition and enrichment of Sr by seawater alteration (Liu et al.,
2010). Therefore, the Sr/La of oceanic slab-derived adakitic rocks tends
to be higher than that of lower continental crust-derived adakitic rocks.
The present data, characterized by a comparatively wide range of Ce/Pb
ratio and high Sr/La ratio, imply that partial melting of an altered
oceanic slab and contamination of sediments on oceanic crust or con-
tinental crustal materials have played a role in the magma-formation
(Liu et al., 2010) (Fig. 16f).
Some magma of adakitic rocks is considered to have been formed
accompanying fractional crystallization (Macpherson et al., 2006; Jia
et al., 2017). Macpherson et al. (2006) explained that the Pleistocene
typical adakitic volcanic rock at the Mindanao Island, Philippines, was
not attributed to the subducted slab-melting, but either was produced by
fractional crystallization of garnet-bearing assemblage from basaltic arc
magma, or was produced by melting of garnet-bearing solidied basalt.
They pointed that an important feature of the Mindanao rock is the
positive correlation between Dy/Yb and SiO
2
concentration, and
chondrite-normalized REE pattern shows an increasing trend from me-
dium to heavy REE in the rocks of which SiO
2
concentration is less than
60 wt%. The examined samples, unlike the Mindanao rocks, show
negative correlation in Dy/Yb vs. SiO
2
diagram, and show a monotonous
decreasing trend from light to heavy REE even in rocks with SiO
2
<60 wt% at chondrite-normalized REE pattern (Figs. 11 and 18a). As
the data of Mindanao and this study are quite different in behavior of
REE, at present, it is more reasonable to suppose that the Mongolian
adakitic initial melt was produced by slab-melting as described before,
rather than by the fractional crystallization of basaltic arc magma/-
melting of solidied basalt as seen in Mindanao.
Incidentally, it is generally known that adakitic rocks are closely
associated with porphyry copper deposits (Thiéblemont et al., (1997);
Sajona and Maury, 1998; Oyarzun et al. 2001; Ballard et al. 2002; Ish-
ihara and Chappell, 2010; Liu et al., 2010; Sun et al. 2011, 2013).
Thiéblemont et al., (1997) noted that 38 of 43 large Au, Ag, Cu, and Mo
deposits in the world are associated with adakitic rocks, and Leng et al.
(2007) showed that most porphyry copper deposits in China are
adakite-related. Ishihara and Chappell (2010) summarized the
geochemical characteristics of copper-mineralized granitoids from
Chile, Highland Valley (Canada), Erdenet (Mongolia), Dexing (China),
Medet (Bulgaria), and Ani (Japan), and suggested that many of porphyry
copper deposits are closely related to adakitic rocks originating from
melt of oceanic slab which has undergone a hydrothermal alteration at
oceanic ridge. They remarked that alteration by S-rich hydrothermal
uids in the ridge increases the S concentration in the comparatively
copper-rich MORB, and the partial melting of such altered basalt favors
Table 5
Representative chemical composition of clinopyroxene from the Bugat/Baruunburen Formation and intermediate dike.
Lava of the Bugat/Baruunburen
Formation (n =11) Intermediate dike (n =8)
(wt%) central part marginal part central part marginal part
SiO2 49.2 50.5 49.9 49.3
TiO2 1.03 1.07 0.88 1.18
Al2O3 3.90 2.60 3.95 4.87
FeO* 9.36 9.97 7.97 8.44
MgO 14.9 15.4 14.7 14.3
CaO 21.2 20.0 22.2 21.5
Na2O 0.408 0.475 0.434 0.412
Total 100 100 100 100
Oxygen 6.00 6.00 6.00 6.00
Si 1.84 1.89 1.89 1.82
Ti 0.0290 0.0301 0.0258 0.0367
Al 0.172 0.115 0.156 0.216
Fe* 0.294 0.312 0.238 0.257
Mg 0.836 0.857 0.783 0.788
Ca 0.852 0.801 0.903 0.904
Na 0.0297 0.0345 0.0312 0.0289
Total cation 4.06 4.04 4.02 4.05
Mg# 0.740 0.733 0.767 0.754
difference of central and marginal parts of Mg# 0.007 0.012
FeO* : total iron as FeO, Fe* : total iron as Fe
2+
, Mg#: Mg / (Mg +Fe*).
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
17
the formation of large-scale porphyry copper deposits including copper
sulde minerals such as chalcocite, chalcopyrite, covellite. Furthermore,
Liu et al. (2010) demonstrated, in comparison the Cretaceous
ore-bearing and ore-barren high-Mg adakitic rocks in central-eastern
China, that the deposits are characteristically occurred in adakitic
rocks of slab-melt origin. In terms of whether mineralization is associ-
ated, the PermianTriassic igneous rocks of Mongolia host one of the
major porphyry copper deposits in the world, and this fact likely sup-
ports the oceanic slab-melt origin for the present samples.
It is known that the Mg# in the melt generated by partial melting of
MORB is up to 45, and interaction with peridotite increases the Mg# in
the melt (Rapp, 1997; Rapp et al., 1999). The Mg# of the present
samples is generally higher than that of the experimental slab-melt, and
many of them are more than 45 (Rapp et al., 1991; Rapp and Watson,
1995; Sen and Dunn, 1994; Winther and Newton, 1991) (Fig. 18b). This
fact points that the initial magma of the examined rocks interacted with
mantle peridotite. That is, the magma of present samples has not been
derived from basaltic lower crust-melt, but is highly possible to have
been derived from the oceanic slab-melt interacted with mantle peri-
dotite during its rising. The rocks in this study have a higher concen-
tration in Fe, Mg, Cr, Co, and Ni than the adakitic granites of the
Kitakami Mountains in Japan, which represent the primitive slab-melt
composition (Tsuchiya et al., 1999) (Fig. 10). Such rocks have been
reported from some regions, e.g. Aleutian-arc and Kitakami Mountains.
For example, Drummond et al. (1996) described Ni-enriched adakitic
rocks from the Aleutian-arc, and they attributed it to the interaction
between slab-melt and mantle peridotite. Furthermore, adakitic rocks
with high Mg, Cr, and Ni concentration, similar to the rocks of this study,
exposed at the Kitakami Mountains result from the interaction between
the slab-melt and mantle peridotite (Tsuchiya et al., 1999, 2005).
The examined samples may correspond to the LSA of Martin (1999).
Martin (1999) considered that the LSA was resulted from the melting of
mantle peridotite that had been metasomatized by reaction with basaltic
slab-melt. While, Tsuchiya et al. (2005) showed that the Ni/Cr ratio of
the modied mantle-melt is smaller than that of the melt produced by
slab-melt and mantle interaction. In the Ni-Cr correlation, the present
data are not plotted in the eld of modied mantle-melt, but appear at or
near the mixing line of the mantle and slab melt (Fig. 18c). This suggests
that the initial magma of the Bugat/Baruunburen Formation, Selenge
plutonic rock complex, and the intermediate dike were formed by
slab-melting and then interacted with mantle peridotite during its
ascent.
Nb and Ta are known to have same incompatibility each other, and
are not fractioned during mantle magmatism, but fractioned during
crustal processes (Sun and McDonough, 1989; Foley et al., 2002; Xiao
et al., 2006). Therefore, Nb/Ta is likely a good indicator to know the
magmatic origin and its contamination at the continental crust. Nb/Ta
of the MORB and primitive mantle is almost constant in 17 (Sun and
Fig. 13. (a) Data plot of the clinopyroxene chemical composition on the pyroxene trapezoid (Morimoto, 1988). (b) FeO vs. MgO diagram for clinopyroxene chemical
composition. (c) Mg# ((Mg/(Mg +Fe) calculated on a molar basis using total Fe content) vs. Si diagram. F.: Formation, Di: diopside, Hd: hedenbergite, En: enstatite,
Fe: ferrosilite.
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
18
Fig. 14. Back scatter electron (BSE) and chemical compositional mapping images of the representative clinopyroxene crystals in the Bugat/Baruunburen Formation
and intermediate dike. (a) BSE image of a clinopyroxene crystal in the Bugat/Baruunburen Formation. (bd) Compositional mapping images of (a) for Ca, Ma, and
Mg. (e) BSE image of a clinopyroxene crystal in the intermediate dike. (fh) Compositional mapping images of (e) for Ca, Ma, and Mg. Cpx: clinopyroxene, Pl:
plagioclase, Ab: albite.
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
19
McDonough, 1989). The Nb/Ta ratios of the upper and middle conti-
nental crusts are 13.4 and 16.5 respectively, and that of the lower crust
is 8.3 (Nb/Ta: 12.4 at total crust in average) (Rudnick and Gao, 2003).
Nb/Ta of the adakite and trondhjemite-tonalite-granodiorite (TTG) melt
ranges from 1.5 to 20, and basically have lower Nb/Ta ratio than the
N-MORB and primitive mantle (Sun and McDonough, 1989; Foley et al.,
2002). The Zr/Hf ratios in the N-MORB, primitive mantle, and conti-
nental crust are nearly identical, ranging from 33 to 37 (Sun and
McDonough, 1989; Foley et al., 2002). In the Nb/Ta vs. Zr/Hf plot, the
present data are plotted along an array including N-MORB, primitive
mantle, and continental crust within the Nb/Ta range of adakite and
TTG (Fig. 18d). This may indicate that upwelling adakitic magma un-
derwent various degrees of continental contamination.
5.3. Tectonic implications of the adakitic magmatism
The Bugat/Baruunburen Formation and Selenge plutonic rock com-
plex rocks have a signature of adakitic rocks. Source magma was formed
by an interaction between slab-melt and mantle peridotite. The Bugat/
Baruunburen Formation gives zircon U-Pb age of 257.9 ±3.8 Ma and
the Selenge plutonic rock complex gives
40
Ar-
39
Ar ages of 247258 Ma
and SHRIMP ages of 243250 Ma (Gerel and Munkhtsengel, 2005;
Sotnikov et al., 2005; Munkhtsengel, 2007; Munkhtsengel et al., 2007;
Tsukada et al., 2021) (Table 1). Besides, Tsukada et al. (2018a) reported
that the intermediate lava of the Tulbur Formation of the Khanui Group,
dated as 256.9 ±2.2 Ma by zircon U-Pb method, has almost the same
geochemical characteristics as the examined samples (Figs. 10, 11, 12,
16, 18). Furthermore, the rocks of the Khanui Group and Selenge
plutonic rock complex exposed around Darkhan city, 250 km northeast
of Bulgan city, show a signature of slab-melt-origin adakitic rocks
(Tsukada et al., 2018b). Therefore, it is evident that adakitic magmatism
caused by subducted slab-melting occurred at the Siberian continental
marginaround 250 Ma.
The partial melting of MORB requires a particular condition at
2.0 GPa and 900950 , and hot and young (<5 Ma) slab subduction
model is considered to be the best explanation for the generation of
Type 1 adakitic magma (Defant and Drummond, 1990; Sen and Dunn,
1994; Rapp and Watson, 1995; Tsuchiya et al., 2007).
Onon and Tsukada (2017) deduced that the Khangai-Daur belt, a
Late PaleozoicEarly Mesozoic accretionary complex, is the fossilized
remains of the subducted Mongol-Okhotsk oceanic plate and its cover
beneath the Siberian continent. The migration time of oceanic plates
from ridge/oceanic islands to trench can be estimated from the fossil
record of radiolarian chert in the accretionary complex. The movement
of the Mongol-Okhotsk oceanic plate from the oceanic island to the
trench is inferred to have had taken more than 50 million years based on
the radiolarian biostratigraphy of the chert in the Late Paleozoic
accretionary complex of the Khangai-Daur belt (Kurihara et al., 2008;
Tsukada et al., 2013; Bayart et al., 2022). The subducted oceanic plate
underneath the Siberian continentat the Late Paleozoic time was old
and cold when it reached the trench. Therefore, the generation of initial
magma of the PermianTriassic adakitic rocks was probably the result of
melting of cold and old subducted slab, and the partial melting of hot
and young (<5 Ma) subducted slabmodel is incompatible with this
case.
Several specic settings, such as at subduction and oblique sub-
duction, have been proposed to account for the melting of the cold and
old slab (Gutcher et al., 2000; Yogodzinski et al., 2001). Gutcher et al.
(2000) demonstrated that the at subduction which causes isobaric
heating produces partial melting of the 50 million years old subducted
slab, causing a broad adakitic rock belt of 100 km width or more. Per-
mianTriassic adakitic rocks have been reported from a wide area,
covering 200 km ×50 km, including Burgan, Erdenet, and Darkhan in
northern Mongolia (Morozumi, 2003; Munkhtsengel et al., 2007; Tsu-
kada et al., 2018a, b) (Figs. 2 and 4). Although it might be possible that
the PermianTriassic igneous rocks correspond to a broad adakitic rock
belt attributed to the at subduction of the Mongol-Okhotsk oceanic
plate, precise age and geochemical feature of the PermianTriassic
igneous rocks are, in many areas, still unclear. Further study on age and
geochemistry at the western part of the PermianTriassic igneous rocks
is needed for clarication (Fig. 2).
Yogodzinski et al. (2001) proposed that oblique subduction and slab
tearing along an L-shaped trench result in the melting of cold and old
slab according to studies in the AleutianKamchatka region. According
to them, oblique subduction and tearing of the slab lead to heat intrusion
and slab-melting at the torn edge of the slab. The paleolatitude of the
Upper Permian Bugat/Baruunburen Formation is calculated to be
37.1N based on thermal remanent magnetization data presented in
Pruner (1987). According to the reconstructed Late Permian continental
distribution, the adakitic volcanism had occurred along the L-shaped
trench between the eastern margin of the Siberian continent and the
northern margin of the South MongolianBureya continent (Ueno, 2006;
Metcalfe, 2011; Cleal, 2018) (Fig. 19a). The shape of the subduction
zone along the SiberiaSouth MongolianBureya continental margin is
similar to that of the AleutianKamchatka region. The magma genera-
tion of the PermianTriassic adakitic rocks may also have been related to
oblique subduction, like the AleutianKamchatka case (Fig. 19). How-
ever, this remains to be conrmed when further data on relative plate
motion between the continents and the Mongol-Okhotsk oceanic plate,
e.g., moving path of the Mongol-Okhotsk oceanic plate based on
paleomagnetic data, become available.
It is known that the superplume activity that formed the Siberian
Fig. 15. Line proles of signal intensity for Ca, Mg, and Fe of clinopyroxene crystals. (a) Line prole along the AB at Fig. 14a (Bugat/Baruunburen Formation). (b)
Line prole along the CD at the Fig. 14e (intermediate dike). Cpx: clinopyroxene, Pl: plagioclase, Ab: albite, gm: groundmass, kcps: 1000 counts per second.
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
20
traps had taken place just behind the volcanic arc at Late Permian
(Sobolev et al., 2011) (Fig. 19). Another possible scenario is that the heat
supply from the superplume that formed the Siberian traps caused
partial melting of the oceanic slab subducted beneath the continental
margin (Fig. 19). At present, there is no clear evidence that the heat
supply from the superplume played a role in the slab-melting, but it is
unlikely that the mantle wedge was completely free from the thermal
effects of superplume activity. It might be necessary to examine the
cause of the adakitic igneous activity in northern Mongolia concerning
the superplume in future studies.
The intermediate dike has remarkably similar geochemical charac-
teristics to the Bugat/Baruunburen Formation and Selenge plutonic
complex (Figs. 10, 11, 12, 16, 18). This indicates that the adakitic
magmatism had lasted after the Early Triassic. The Erdenet plutonic rock
complex which is MiddleLate Triassic, the younger igneous rock unit
than the Selenge plutonic rock complex, also shows adakitic nature
(Munkhtsengel et al., 2007). In addition, Donskaya et al. (2012) pre-
sented the Late Triassic adakitic volcanic rocks from the Kataev For-
mation at the western Transbaikalia, northeastern extension of the
PermianTriassic igneous rocks. The adakitic igneous activity caused the
dike, therefore, likely corresponds to the post-Middle Triassic adakitic
igneous activity such as the Erdenet plutonic rock complex and the
Kataev Formation. The Late Triassic adakitic volcanic-plutonic rocks
were also reported from the Erguna massif at northeast China and
Middle Gobi volcanic belt of south Mongolia on the South Mongolian
block (Li et al., 2017; Sheldrick et al., 2020; Tang et al., 2021) (Figs. 1,
19). Li et al. (2017) mentioned that the Middle Triassic volcanic rocks at
the Erguna massif have an afnity to normal arc-type volcanic rocks, but
Fig. 16. (a) FeO*/MgO vs. SiO
2
diagram
(Miyashiro, 1974). FeO* : total iron as FeO. The
data of the Tulbur Formation are from Tsukada
et al. (2018a). (b) Sr/Y vs. Y diagram (Defant
and Drummond, 1990). ADR:
andesite-dacite-rhyolite. Adakite and Island arc
ADR elds are after Defant et al. (1991); Defant
and Drummond (1990), and Martin et al.
(2005). The data of the Tulbur Formation are
from Tsukada et al. (2018a). (c) La/Yb vs. Yb
diagram (Moyen, 2009). Data of the Tulbur
Formation (Tsukada et al., 2018a) are also
plotted. (d) K
2
O/Na
2
O vs. Al
2
O
3
diagram
(Kamei et al., 2009, 2013; Liu et al., 2010). (d)
Sr/Y vs. La/Yb diagram (Liu et al., 2010). The
data of the Tulbur Formation are after Tsukada
et al. (2018a). OSA: oceanic slab-derived ada-
kitic rocks in the central-eastern China. LCCA:
lower continental crust-derived adakitic rocks
at Dabie region, China. The OSA and LCCA
elds are referred from Liu et al. (2010). (e)
Ce/Pb vs. Sr/La diagram (Liu et al., 2010). The
data of the Tulbur Formation are after Tsukada
et al. (2018a). OSA: oceanic slab-derived ada-
kitic rocks in the central-eastern China, LCCA:
lower continental crust-derived adakitic rocks
in the central-eastern China. The OSA and LCCA
elds are after Liu et al. (2010).
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
21
the Late Triassic one is slab-melt-origin adakitic rocks. This might imply
that the place of the melting of subducted oceanic plate spread from the
Siberian continental margin to the South MongolianBureya continental
margin during the Middle to Late Triassic (Figs. 1, 19).
6. Conclusions
Geochemical investigations of the Bugat/Baruunburen Formation,
the Selenge plutonic rock complex, and the intermediate dike were
carried out in order to clarify the subduction system along the Siberian
continental marginat PermianTriassic. As a result, the following
points were established.
i. Data from the Bugat/Baruunburen Formation, Selenge plutonic
rock complex, and intermediate dike forming a linear trend in the
variation diagrams for many elements imply a single magmatic
source for these three rocks.
ii. The spidergrams of the examined samples show geochemical
characteristics similar to the arc-related igneous rocks, and the
majority are plotted in the calc-alkaline eld in the SiO vs. FeO* /
MgO diagrams. The conclusion is that the Bugat/Baruunburen
Formation, Selenge plutonic rock complex, and intermediate dike
are calc-alkaline igneous rocks formed at a volcanic arc
environment.
iii. The samples have an extremely high Sr concentration, Sr/Y ratio,
and fractionated REE, which gives evidence of adakitic rocks.
iv. Low K
2
O/Na
2
O, high Sr/Y, high La/Yb, and high Sr/La ratios
point that the magma of the Bugat/Baruunburen Formation,
Selenge plutonic rock complex, and the intermediate dike was
derived from oceanic slab-melt.
v. Higher concentration of Cr and Ni in the samples than the
primitive slab-melt suggests that the magma of the Bugat/Bar-
uunburen Formation, Selenge plutonic rock complex, and inter-
mediate dike resulted from an interaction slab-melt and mantle
peridotite. The high Mg# value in many samples supports this
view.
vi. The fact that Nb/Ta ratio of the present samples are plotted along
an array including N-MORB mantle and continental crust likely
suggests that the upwelling adakitic magma had undergone
various degrees of contamination within the continental crust.
vii. The paleolatitude of the Upper Permian Bugat/Baruunburen
Formation is calculated to 37.1N. Therefore, the Late Per-
mianMiddle Triassic adakitic igneous activity had taken place in
the volcanic arc along the Siberian continental margin in the mid-
latitude region of the Northern Hemisphere.
Fig. 17. Discrimination diagrams of high-silica and low-silica adakites (Martin, 1999). HAS: high-silica adakite, LSA: low-silica adakite.
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
22
viii. The intermediate dike having almost the same geochemical
feature as the Bugat/Baruunburen Formation and Selenge
plutonic complex is evidence that the adakitic magmatism lasted
till after the Early Triassic.
It is generally considered that slab-melting occurs when a young/hot
slab subducts beneath a continent. However, the oceanic plate sub-
ducted beneath the Siberian continent in the late Paleozoic is
considered to be old and cold. Possible explanations for the old/cold
slab-melting include at subduction, slab tearing due to oblique sub-
duction, and heat supply from the superplume that caused the Siberian
traps. In order to clarify this issue, it is necessary to do further research
on the movement direction of the Mongol-Okhotsk oceanic plate and the
PermianTriassic thermal structure beneath the Siberian continent.
CRediT authorship contribution statement
Kanta Umeda: Field research, ICP-MS and XRF analysis, Original-
draft preparation. Nemekhbayar Purevsuren: Field research, ICP-MS
and XRF analysis. Kazuhiro Tsukada: Supervision, Conceptualization,
Field research, Final-draft preparation, Visualization. Lodoidanzan
Altansukh: Rock-processing for ICP-MS and XRF analysis. Bayart
Nadmid: ICP-MS and XRF analysis. Khishigsuren Sodnom: Field
research. Manchuk Nuramkhaan: Field research. Taro Kabashima:
Field research. Tomoyuki Kondo: Validation of draft.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
Fig. 18. (a) Dy/Yb vs. SiO
2
diagram. The trend of adakitic rocks in the Mindanao Island is after Macpherson et al. (2006). (b) Mg#-SiO
2
relationship for the
examined samples. The Field of the experimental slab-melt is after Rapp et al. (1991), Rapp and Watson (1995), Sen and Dunn (1994), and Winther and Newton
(1991). The data of the Tulbur Formation are from Tsukada et al. (2018a). (c) Ni-Cr relationship for the examined samples. Data of the Tulbur Formation (Tsukada
et al., 2018a) are also plotted. Ni and Cr compositions of the primitive mantle and inferred slab-melt are from Sun (1982) and Tsuchiya et al. (2005) respectively.
Range of melt derived from modied mantle and inferred slab-melt are after Tsuchiya et al. (2005). (d) Nb/Ta vs. Zr/Hf diagram. LCC: lower continental crust, MCC:
middle continental crust, UCC: upper continental crust, TTG: trondhjemite-tonalite-granodiorite. The data on N-MORB, primitive mantle, LCC, MCC, and UCC were
from Sun and McDonough (1989) and Rudnick and Gao (2003). Nb/Ta upper and lower limits of adakite and TTG were from Foley et al. (2002).
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
23
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgements
This work, for the most part, is based on the bachelors study of the
rst author at Nagoya University, Japan. We wish to thank to Prof. K.
Yamamoto and Associate Prof. Y. Asahara at Nagoya University for their
technical support. We would like to thank Dr. H. Morozumi and Dr. Y.
Sasaki at the Japan Oil, Gas and Metals National Co. and Dr. Munkht-
sengel, B. at Mongolian University of Science and Technology for joining
eld research. We greatly gratitude to an anonymous reviewer for
valuable comments. Special thank goes to Profs. Zorigtkhuu, D. and
Prev, N. of Mongolian University of Science and Technology for
Fig. 19. (a) Paleogeographic reconstruction for the Late Permian time (modied from Ueno, 2006; Xiao et al., 2009; Metcalfe, 2011; Bazhenov et al., 2016; and
Cleal, 2018). SMB: South MongolianBureya continent, T: Tarim continent, NC: North China continent, SC: South China continent, Cimmeria: Cimmeria continent,
Bg/Bb F.: Bugat/Baruunburen Formation. (b) Schematic cross-section for Permian Siberian continental margin corresponding to the xy line of the (a). No scale.
K. Umeda et al.
Journal of Geodynamics 151 (2022) 101918
24
facilitating the use of the facilities in the university. We would like to
thank Enago (www.enago.jp) for the English language review.
Part of the data was obtained through the Regional survey on the
infrastructure development project for promoting rare metal resource
exploration in 2013 (Mongolia) by the Ministry of Economy, Trade and
Industry of Japan (METI). A part of this study was supported by the
Daiko Foundation (Grant number: 9219), Japan.The second author, Mr.
Nemekhbayar, P. passed away 1 July 2021 at the age of 32 years. This
study was inspired by his masters study at Nagoya University on the
geochemistry of the Tulbur Formation, Khanui Group, Mongolia.
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