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Metamorphic evolution and tectonic implications of the metamorphic rock series in theXilinhot-Linxi area, Inner MongoliaLi, Y.
2011
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citation for published version (APA)Li, Y. (2011). Metamorphic evolution and tectonic implications of the metamorphic rock series in the Xilinhot-Linxi area, Inner Mongolia.
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111
Chapter 6 Petrology and geochemistry of the Shuangjing Schist
In the Linxi-Xilinhot area along the northern margin of the North China Craton
(GS-CUG, 2008), Devonian-Carboniferous strata and Middle-Late Triassic strata are
absent, reflecting two episodes of crustal uplift in the area that may be related to the
suturing of the Solonker suture zone. Permian radiolarian fossils were found in the
Linxi-Xilinhot area (Wang and Fan, 1997; Shang, 2004), suggesting the presence of a
deep oceanic basin between the two stages of uplift. The Shuangjing Schist outcrops in
the Linxi area along the Xar Moron fault belt, which marks the southern boundary of the
eastern section of the Solonker suture zone (see Fig. 1-2). The unit is composed of various
schists and local lenticular limestones. It was identified as a Late Archaean tectonic sheet
in a Precambrian metamorphic terrane called the Shuangjing microcontinent
(SRGST-IMAR, 1997). However, our research on a carbonaceous biotite-plagioclase
schist in Shuangjing Schist indicated that its protolith is a proximal sedimentary rock that
mainly sourced from intermediate magmatic materials with a source age of 298 ± 2 Ma
and was intruded by granite at 272 ± 2 Ma by LA-ICPMS U-Pb zircon dating (Chapter 7).
Therefore, the Shuangjing Schist is a Late Carboniferous-Early Permian unit that formed
between the two stages of crustal uplift in the area. This chapter presents detailed and
systematic petrology and geochemistry of the Shuangjing Schist, to provide more
information on the Permian oceanic basin in the Linxi area and to resolve the dispute
regarding the timing of final suturing of the Solonker suture zone.
§6.1 Geological setting
Xiao et al. (2003) present an overview of the eastern section of the CAOB, including its
geological framework and a tectonic model for its evolution. The area is divided into
three parts (see Fig. 1-2): the southern accretionary zone between North China Craton and
the Solonker suture zone, the Solonker suture zone itself and the northern accretionary
zone between the suture zone and the South Mongolia microcontinent. The southern
accretionary zone is characterized by the Middle Ordovician to Early Silurian Ondor Sum
subduction-accretion complex and the contemporaneous Bainaimiao arc. The northern
accretionary zone extends southward from a continental margin that was active during
Petrology and geochemistry of the Shuangjing Schist
112
Devonian to Carboniferous times, through the Hegenshan ophiolite-arc accretionary
complex into the Late Carboniferous Baolidao arc. Complete subduction of the
Paleo-Asian Ocean caused the two opposing active continental margins to collide, leading
to formation of the Solonker suture (Xiao et al., 2003). The southern accretionary zone is
separated from the Solonker suture zone by the Xar Moron fault belt, which is a ductile
dextral strike-slip zone of dozens of kilometers wide and hundreds of kilometers long
(Fang et al., 1997).
The study area is located in central Inner Mongolia around the Xilinhot City and the
Linxi County. It is divided into two Paleozoic tectonic units separated by the Xar Moron
fault: the Tuchengzi Early Paleozoic tectonic belt, which is the eastwards extension of the
Ondor Sum subduction-accretion complex, and the Linxi-Xilinhot Late Paleozoic-Early
Triassic tectonic belt (Fig. 6-1). The latter can in turn be divided into 3 parts: the Xar
Moron fault belt, the Shangde Ardg anticlinorium that is equivalent to the Baolidao
arc-accretion complex, and the Linxi synclinorium that is equivalent to the Solonker
suture zone.
Fig. 6-1 Tectonic frame of the research area in the Xilinhot-Linxi area (GS-CUG, 2008)
Chapter 6
113
The Shuangjing Schist developed in the Xar Moron fault belt and outcrops in the
Lianhuashan-Fangkuangzi-Nadaga area in the southeast of Linxi County (Fig. 6-2). It is
in fault contact with the northern Late Silurian Xibiehe sedimentary formation and in the
south it is intruded by Middle-Late Permian granitic gneiss and Mesozoic granite (Li et al.,
in review). A NEE striking penetrative schistosity developed in the Shuangjing Schist.
Many blocks of Permian ophiolite are located around Nadaga-Xingshuwa area, whose
relation with the Shuangjing Schist is difficult to assess because of the Quatenary cover.
Fig. 6-2 Geological map of southern Linxi County in Inner Mongolia showing the
distribution of the Shuangjing Schist.
The Shuangjing Schist is well exposed at Lianhuashan, Fangkuangzi and Nadaga in
the southeast of Linxi County. In the three exposures the rocks show consistent
metamorphism and deformation, but different lithologies. In Lianhuashan, the rock
assemblage is composed of various schists; in Fangkuangzi, sericite-quartz schist and
Petrology and geochemistry of the Shuangjing Schist
114
carbonaceous biotite-plagioclase schist are interbedded; and in Nadaga,
garnet-sericite-quartz schist is the only lithology. Lenticular limestones occur in
Fangkuangzi and Nadaga. We collected typical Shuangjing Schist samples from the three
localities for petrological and geochemical study.
§6.2 Petrology
6.2.1 Petrography
The Shuangjing Schist in Lianhuashan is mainly composed of plagioclase-amphibole
schist, plagioclase-biotite schist, amphibole-plagioclase-epidote schist and felsic leptynite.
With the exception of some unfoliated thick-bedded samples, most rocks show a clear
schistosity (Fig. 6-3a). Columnar plagioclase or subhedral quartz are present in some
samples (Fig. 6-4a), suggesting the presence of volcanic components in protolith. Felsic
leptynites occur as thin layers with a fine-granular blastic texture intercalated between the
schists, and are mainly composed of quartz and plagioclase with minor chlorite, biotite,
epidote and actinolite. The contact zone between the leptynite and schist has a transitional
structure with a higher content of actinolite and biotite than in leptynites.
The Shuangjing schist in Fangkuangzi and Nadaga has an obvious bedded structure
with locally developed fine bedding-parallel lamination. Sericite-quartz schist and
biotite-plagioclase schist alternate as 20-60 cm layers in Fangkuangzi. Carbonaceous
components are usually present in the biotite-plagioclase schist, which locally grades into
carbonaceous schist (Fig. 6-3b). The presence of subhedral plagioclase with lamellar
twinning (Fig. 6-4b) indicates that volcanic components were present in the protolith of
the biotite-plagioclase schist. The sericite-quartz schist has well sorted and moderately
rounded grains with very fine-grained texture (Fig. 6-4c), reflecting a sedimentary
protolith. Alternating layers of sericite-quartz schist and biotite-plagioclase schist reflect a
long-lived alternating depositional environment. The single lithology of
garnet-sericite-quartz schist in Nadaga is thick-bedded (Fig. 6-3c). Garnet porphyroblasts
show spiral inclusion trails and pressure shadows in the matrix of sericite due to
metamorphism and deformation during and after garnet growth (Fig. 6-4d). The protolith
of the schist at Nadaga may be allitic argillaceous rock formed in a stable deep-water
facies judged by its mineral assemblages, small grainsize and thick bedding. Lenticular
limestone bodies of several meters long that are commonly enclosed in the schists at
Nadaga and occasionally at Fangkuangzi (detailed shown in Fig. 6-3d) also suggest that
the protolith of the schists in Nadaga-Fangkuangzi was sedimentary. The lenticular
Chapter 6
115
limestones are brecciated with moderate sorting and poor rounding, suggesting a
turbiditic depositional environment.
Fig. 6-3 Field aspect of the Shuangjing Schist. (a) Strongly developed ENE striking
schistosity in the schists from Lianhuashan and weakly developed schistosity in some
thick bedded rocks. (b) Biotite-plagioclase schist with carbonaceous components from
Fangkuangzi, locally grading into carbonaceous schist. (c) Thick-bedded
garnet-sericite-quartz schist from Nadaga with leucocratic veins. (d) Lenticular brecciated
limestone of different sizes enclosed in the schists in the Nadaga-Fangkuangzi area.
Petrology and geochemistry of the Shuangjing Schist
116
Fig. 6-4 Photomicrographs in cross-polarized light of the Shuangjing Schist; L is image
width. (a) Fresh columnar plagioclase with lamellar twinning from Ep-Pl-Bi schist in
Lianhuashan. (b) Fresh columnar plagioclase with lamellar twinning from carbonaceous
Bi-Pl schist in Fangkuangzi. (c) Fine-grained, well sorted and moderately rounded Se-Q
schist in Fangkuangzi. (d) Garnet with spiral inclusion trail and pressure shadows in
Gt-Se-Q schist from Nadaga.
The mineral assemblages of collected samples from the Shuangjing Schist are listed
in table 6-1. Samples 805-1 to 805-13 from Lianhuashan reveal alternation of
intermediate (805-1 to 805-5) rocks at the base, acidic (805-6 to 805-9) in the middle and
again intermediate (805-10 to 805-13) lithologies at the top. Considering the volcanic
components in the schists from Lianhuashan, the rhythmic alteration may be related to
volcanic eruption cycles. Sample 8876-1 and 8876-2 are from Fangkuangzi, sample
8886-1 is from Nadaga. Localized layer-parallel veining is suggestive of fluid-rock
interaction in the Shuangjing Schist (Fig. 6-3c).
Chapter 6
117
Table 6-1 Mineral assemblage of typical rocks from the Shuangjing Schist
Mineral assemblage (%) Position Sample Name
Pl/Kfs Q Ac Bi Ser Chl Ep Gt Sph accessory Species
805-1 Ep-Pl-Ac
schist 25 15 37 1 0 0 15 0 5 Ap+Zr
805-2 Ep-Ac-Pl
schist 35 23 25 1 0 0 10 0 5 Ap+Zr
805-3 Pl-Ep-Ac
schist 35 9 30 0 0 0 25 0 0 Mt+ Zr
805-4 Pl-Ac-Ep
schist 20 9 20 0 0 0 45 0 5 Mt
805-5 Bi-Ac-Pl
schist 35 25 20 10 0 0 5 0 3 Mt+Ap+Zr
intermediate
805-6 Felsic
leptynite 45 45 0 5 2 0 2 0 0 Mt+Ap+Zr
805-7 Felsic
leptynite 45 40 0 5 0 2 7 0 3 Mt+Ap+Zr
805-8 Felsic
leptynite 40 40 0 3 0 10 5 0 0 Mt+Ap+Zr
805-9 Felsic
leptynite 30 40 0 5 5 19 0 0 0 Mt+Ap+Zr
acidic
805-10 Ac-Pl-Bi
schist 25 15 25 25 0 0 8 0 1 Mt+Zr
805-11 Ac-Pl-Ep
schist 25 15 20 8 0 0 30 0 1 Mt+Zr
805-12 Ep-Pl-Bi
schist 20 15 5 40 0 3 15 0 0 Mt+Ap+Zr
Lianhuashan
805-13 Pl-Bi schist
29 20 0 50 0 0 0 0 0 Mt
intermediate
8876-1 Ser-Q shcist
20 40 0 5 35 0 0 0 0 Mt pelitic
Fangkuangzi
8876-2 Bi-Pl schist
40 20 0 30 10 0 0 0 0 Mt+Zr+C intermediate
Nadaga 8886-1 Gt-Ser-Q
schist 20 40 0 5 30 0 0 4 0 Mt+Zr+C pelitic
Abbreviations: Ac = actinolite, Ap = apatite, Bi = biotite, C = carbonaceous components,
Chl = chlorite, Ep = epidote, Gt = garnet, Kfs = K-feldspar, Mt = magnetite, Q = quartz,
Ser = sericite, Sph = sphene, Zr = zircon.
Petrology and geochemistry of the Shuangjing Schist
118
6.2.2 Mineral chemistry
Feldspar and amphibole from the schist in Lianhuashan and garnet from the schist in
Nadaga were analyzed with the electron microprobe. Both plagioclase and alkali feldspar
occur in the schist from Lianhuashan (Table 6-2). Plagioclase is albite-oligoclase
(An=0-27), consistent with low-grade metamorphism. Alkali feldspar is orthoclase
(Ab=3-10).
Amphibole is present in the intermediate schists of samples 805-1 to 805-5 from
Lianhuashan (Table 6-3) and is classified as calcic ((Ca＋Na)B≥1.34 and NaB＜0.67;
Leake et al., 2004). Plotting in the Mg/(Mg+Fe2+)-Si diagram for calcic amphibole (Fig.
6-5a) indicates all amphibole is actinolite. The low content of Ti and Al� in the standard
formula of amphibole suggests metamorphism in the greenschist facies (Jin, 1991).
Almandine garnet only occurs in the schist from Nadaga (Table 6-3). It is weakly
zoned with a Mn and Ca-enriched core and a Fe and Mg-enriched rim (Fig. 6-5b). Such
componential zoning is thought to be formed during prograde metamorphism (Banno et
al., 1986), so the Shuangjing Schist is formed during upper greenschist facies prograde
metamorphism.
Fig. 6-5 (a) Classification for calcic amphiboles from schist in Lianhuashan (cf. Leake et
al., 2004). (b) Garnet zoning from the schist in Nadaga.
Tab
le 6
-2 E
lect
roni
c m
icro
prob
e an
alys
es (
%)
of f
elds
par
from
the
Shu
angj
ing
Sch
ist f
rom
Lia
nhua
shan
Sam
ple
805-
1 80
5-1
805-
1 80
5-2
805-
3 80
5-3
805-
5 80
5-5
805-
6 80
5-6
805-
6 80
5-7
805-
7 80
5-8
805-
10
805-
11
805-
12
Spec
ies
inte
rmed
iate
ac
idic
in
term
edia
te
Min
eral
P
l P
l K
fs
Pl
Pl
Pl
Pl
Kfs
P
l P
l P
l K
fs
Kfs
K
fs
Kfs
K
fs
Pl
SiO
2 63
.33
65.2
2 65
.02
65.0
8 68
.15
68.3
7 69
.44
66.5
0 67
.01
66.1
3 65
.23
66.9
5 66
.48
65.3
3 65
.69
65.8
9 64
.28
TiO
2 0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
01
0.00
0.
00
0.00
0.
00
Al 2
O3
23.6
1 23
.10
18.4
9 23
.68
20.1
4 20
.28
19.4
8 19
.14
22.3
1 22
.82
21.6
5 19
.54
18.9
8 19
.55
19.3
9 18
.90
24.8
7
FeO
0.
00
0.02
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
02
0.00
0.
00
MnO
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
MgO
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
CaO
3.
86
3.84
0.
00
4.15
0.
30
0.03
0.
45
0.00
2.
47
2.63
2.
04
0.00
0.
00
0.02
0.
00
0.00
5.
30
Na 2
O
8.82
9.
17
0.35
8.
97
10.7
1 9.
61
11.7
3 0.
96
8.82
9.
40
11.2
0 1.
09
1.01
0.
84
0.76
0.
90
7.87
K2O
0.
27
0.22
15
.67
0.10
0.
07
0.00
0.
15
15.7
0 0.
28
0.21
0.
06
15.3
1 14
.84
14.5
3 15
.82
14.9
5 0.
19
Cr 2
O3
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
0.
00
0.00
Tota
l 99
.89
101.
57
99.5
3 10
1.98
99
.37
98.2
9 10
1.25
10
2.30
10
0.89
10
1.19
10
0.18
10
2.89
10
1.32
10
0.27
10
1.68
10
0.64
10
2.51
Si
2.80
2.
83
3.01
2.
81
2.98
3.
01
3.00
2.
99
2.90
2.
86
2.87
2.
99
3.01
2.
98
2.98
3.
00
2.77
Al
1.23
1.
18
1.01
1.
21
1.04
1.
05
0.99
1.
02
1.14
1.
17
1.12
1.
03
1.01
1.
05
1.04
1.
01
1.26
Ca
0.18
0.
18
0.00
0.
19
0.01
0.
00
0.02
0.
00
0.11
0.
12
0.10
0.
00
0.00
0.
00
0.00
0.
00
0.24
Na
0.75
0.
77
0.03
0.
75
0.91
0.
82
0.98
0.
08
0.74
0.
79
0.96
0.
09
0.09
0.
07
0.07
0.
08
0.66
K
0.02
0.
01
0.92
0.
01
0.00
0.
00
0.01
0.
90
0.02
0.
01
0.00
0.
87
0.86
0.
85
0.91
0.
87
0.01
Tot
al
4.98
4.
97
4.97
4.
96
4.95
4.
88
5.00
4.
99
4.91
4.
95
5.05
4.
98
4.96
4.
95
5.00
4.
96
4.94
An
19
19
0 20
2
0 2
0 13
13
9
0 0
0 0
0 27
Ab
79
80
3 79
98
10
0 97
9
85
86
91
10
9 8
7 8
72
Or
2 1
97
1 0
0 1
91
2 1
0 90
91
92
93
92
1
Num
ber
of io
ns f
or f
elds
par
on b
asis
of
8 ox
ygen
ato
ms.
T
able
6-
3 E
lect
roni
c m
icro
prob
e an
alys
es
(wt%
ox
ide)
of
am
phib
ole
and
garn
et
from
th
e S
huan
gjin
g S
chis
t fr
om
Lia
nhua
shan
and
Nad
aga
Sam
ple
805-
1 80
5-2
805-
2 80
5-2
805-
3 80
5-3
Sam
ple
8886
-1
8886
-1
8886
-1
8886
-1
8886
-1
Spec
ies
inte
rmed
iate
Sp
ecie
s pe
litic
Min
eral
A
c A
c A
c A
c A
c A
c M
iner
al
Gt r
im
Gt m
id
Gt c
ore
Gt m
id
Gt r
im
SiO
2 52
.83
52.0
3 53
.27
51.9
5 52
.95
51.7
6 S
iO2
38.4
9 37
.45
38.0
8 38
.48
38.3
6
TiO
2 0.
00
0.16
0.
26
0.20
0.
26
0.19
T
iO2
0.00
0.
00
0.00
0.
00
0.00
Al 2
O3
3.00
2.
32
2.80
4.
01
3.05
4.
03
Al 2
O3
21.8
9 21
.00
21.7
5 22
.86
22.1
5
FeO
* 13
.08
12.9
0 13
.06
13.5
8 14
.35
13.7
3 F
eO
33.5
7 33
.98
33.4
5 33
.50
32.8
5
MnO
0.
54
0.45
0.
33
0.42
0.
45
0.43
M
nO
0.11
0.
44
0.78
0.
35
0.02
MgO
15
.15
15.2
0 15
.56
14.1
6 14
.27
14.1
9 M
gO
3.99
2.
68
2.47
2.
94
3.90
CaO
12
.64
12.9
4 12
.53
12.6
8 11
.85
12.4
8 C
aO
3.86
4.
63
4.91
4.
48
4.14
Na 2
O
0.40
0.
24
0.41
0.
46
0.44
0.
51
Na 2
O
0.00
0.
03
0.00
0.
00
0.00
K2O
0.
00
0.02
0.
06
0.20
0.
09
0.17
K
2O
0.00
0.
00
0.00
0.
00
0.00
Cr 2
O3
0.00
0.
00
0.00
0.
00
0.00
0.
00
Cr 2
O3
0.00
0.
00
0.00
0.
00
0.00
Tot
al
97.6
4 96
.26
98.2
8 97
.66
97.7
1 97
.49
Tot
al
101.
91
100.
21
101.
44
102.
61
101.
42
Si
7.60
7.
62
7.60
7.
53
7.61
7.
51
Si
3.00
3.
00
3.00
2.
98
3.00
Ti
0.00
0.
02
0.03
0.
02
0.03
0.
02
Ti
0.00
0.
00
0.00
0.
00
0.00
Al
0.51
0.
40
0.47
0.
69
0.52
0.
69
Al
2.01
1.
98
2.02
2.
09
2.04
Fe3＋
0.
28
0.18
0.
33
0.10
0.
41
0.21
Fe
3＋
0.00
0.
03
0.00
0.
00
0.00
Fe2+
1.
30
1.40
1.
23
1.55
1.
32
1.46
F
e2+
2.19
2.
24
2.21
2.
17
2.15
Mn
0.07
0.
06
0.04
0.
05
0.05
0.
05
Mn
0.01
0.
03
0.05
0.
02
0.00
Mg
3.25
3.
32
3.31
3.
06
3.06
3.
07
Mg
0.46
0.
32
0.29
0.
34
0.45
Ca
1.95
2.
03
1.91
1.
97
1.83
1.
94
Ca
0.32
0.
40
0.41
0.
37
0.35
Na
0.11
0.
07
0.11
0.
13
0.12
0.
14
Tot
al
7.99
8.
01
7.99
7.
97
7.98
K
0.00
0.
00
0.01
0.
04
0.02
0.
03
XA
lm
0.73
0.
75
0.74
0.
72
0.72
Cr
0.00
0.
00
0.00
0.
00
0.00
0.
00
XA
dr
0.00
0.
02
0.00
0.
00
0.00
Tot
al
15.0
6 15
.10
15.0
4 15
.14
14.9
7 15
.11
XG
rs
0.11
0.
12
0.14
0.
12
0.12
AlⅣ
0.
40
0.36
0.
38
0.44
0.
36
0.47
X
Pyp
0.
15
0.11
0.
10
0.11
0.
15
AlⅥ
0.
11
0.04
0.
09
0.24
0.
16
0.21
X
Sps
0.
00
0.01
0.
02
0.01
0.
00
Num
ber
of io
ns f
or a
mph
ibol
e an
d ga
rnet
on
basi
s of
6 a
nd 1
2 ox
ygen
ato
ms
resp
ectiv
ely.
Petrology and geochemistry of the Shuangjing Schist
122
6.3 Geochemistry Major and trace element compositions of 13 schist samples from Lianhuashan are
listed in Table 6-4.
6.3.1 Major elements
In the TiO2-SiO2 protolith recovery diagram (Fig. 6-6a), all samples except one plot
in volcanic rock area or just across the boundary between the volcanic and sedimentary
rock areas, indicating that the protolith of the schist in Lianhuashan is volcanic rocks. The
volcanic rocks belong to the sub-alkaline series of the (Na2O+K2O)-SiO2 diagram (Fig.
6-6b), while plotting in the AFM diagram indicates they belong to a calc-alkaline series
(Fig. 6-6c). Sample 805-2 plots in the sedimentary protolith area of the TiO2-SiO2
diagram because of its high TiO2 content (Fig. 6-6a).
Fig. 6-6 Protolith recovery and classification of the Shuangjing Schist in Lianhuashan. (a)
Chapter 6
123
TiO2-SiO2 diagram (Tarney, 1976). (b) (Na2O+K2O)-SiO2 diagram (Irvine and Baragar,
1971). (c) AFM diagram (Irvine and Baragar, 1971), A = Na2O+K2O, F = FeOt, M = MgO.
(d) Nb/Y-Zr/TiO2 diagram (Winchester and Floyd, 1977).
Samples 805-1 to 805-5 are amphibole-plagioclase schist that range widely in
composition (Table 6-4). Their protoliths are intermediate volcanic rocks and most plot in
the andesite/basalt area (Fig. 6-6d). Samples 805-6 to 805-9 and samples 805-10 to
805-13 are less variable in composition. Samples 805-6 to 805-9 are felsic leptynite. Their
protoliths are acidic rocks and most plot in or near rhyodacite/dacite area (Fig. 6-6d).
Samples 805-10 to 805-13 mainly are biotite schist. Their protoliths are intermediate
volcanic rocks and all plot in andesite area (Fig. 6-6d). These samples have higher Al2O3
and lower MgO and Mg# than samples 805-1 to 805-5. Enrichment of K in samples
805-11 and 805-13 causes their ratio of Na2O/K2O less than 1.
6.3.2 Trace elements
Chondrite-normalized REE distribution patterns of all the samples show weak LREE
enrichment without a Ce anomaly in most samples (Fig. 6-7a). Intermediate rocks,
samples 805-1 to 805-5, have a wide range of total REE abundance (75-171 ppm) and
some LREE enrichment, LaN/YbN = 1.92-3.39. There is weak fractionation of HREE,
GdN/YbN = 1.28-1.46. The negative Eu anomalies (Eu/Eu*) vary from 0.27 to 0.94.
Acidic rocks, samples 805-6 to 805-9, show a narrow range of total REE content (94-124
ppm) and obvious enrichment of LREE, LaN/YbN = 2.78-14.09. The negative Eu
anomalies are limited, Eu/Eu* = 0.90-0.95 (805-6 is 0.20). Sample 805-6 has an obvious
Eu anomaly, similar to sample 805-5, implying transitional characteristics between
intermediate volcanic rocks 805-1 to 805-5 and acidic rocks 805-6 to 805-9. The total
abundances of REE in intermediate samples 805-10 to 805-13 cluster even more tightly
(109-116 ppm) with slightly negative Eu anomalies (Eu/Eu* = 0.93-0.98, 805-13 is 0.65).
Their enrichment of LREE is between that in samples 805-1 to 805-5 and 805-6 to 805-9,
LaN/YbN = 4.98-6.11. GdN/YbN = 1.29-1.41, revealing weak fractionation of HREE.
The samples show consistent trace element distribution patterns with enrichment in
large ion lithophile elements (e.g. K, Rb, Th, Pb) and depletion in high field strength
elements (e.g. Nb, Ta, Ti, P; Fig. 6-7b), implying a similar magma source, but their low
Cr and Ni concentrations and Mg numbers indicate they do not represent primary magmas.
Low HFSE/LREE ratios (Nb/La = 0.25-0.44), enrichment of incompatible trace elements
(e.g., Th, U, Pb), and Nb, Ta, Ti negative anomalies are consistent with the patterns of
Petrology and geochemistry of the Shuangjing Schist
124
subducting sediment (GLOSS) or back-arc basin basalt (BABB), pointing towards
subduction-related magmatism resulting from mantle metasomatism (Woodhead et al.,
2001).
Fig. 6-7 (a) Chondrite-normalized REE distribution patterns and (b) primitive
mantle-normalized trace element distribution patterns of the Shuangjing Schist in
Lianhuashan. Normalizing values are after Sun and McDonough (1989). Values for global
subducting sediment (GLOSS) and back arc basin basalt (BABB) values are from Stern
(2002).
6.4 Discussion
6.4.1 Implications of results for the Shuangjing Schist
The volcanic rocks in Lianhuashan belong to the calc-alkaline series and contain
large volumes (~90%) of intermediate members and subordinate (~10%) acidic members
as is common in Andean-type calc-alkaline series. The tectonic discrimination plots of
La/Yb-Sc/Ni for andesite (Fig. 6-8a) and Rb-(Nb+Y) for granite (Fig. 6-8b), show that
most intermediate samples plot in the area of continental island arc magmas and all acidic
samples plot in volcanic arc area. The trace element compositions therefore suggest that
the Lianhuashan volcanics formed in the arc/forearc setting of a continental marginal arc.
Tab
le 6
-4 M
ajor
ele
men
ts (
wt%
oxi
de)
and
Tra
ce e
lem
ents
(pp
m)
of th
e Sh
uang
jing
Sch
ist f
rom
Lia
nhua
shan
Sam
ple
805-
1 80
5-2
805-
3 80
5-4
805-
5 80
5-6
805-
7 80
5-8
805-
9 80
5-10
80
5-11
80
5-12
80
5-13
Spec
ies
basi
c qu
artz
-fel
dspa
thic
ba
sic
SiO
2 60
.11
65.5
2 58
.21
53.0
3 67
.74
76.6
9 70
.19
70.1
1 73
.02
59.0
0 58
.76
59.4
8 59
.54
TiO
2 0.
85
1.15
0.
97
1.18
0.
31
0.17
0.
37
0.38
0.
44
0.85
0.
79
0.88
0.
86
Al 2
O3
15.4
8 13
.02
15.4
9 15
.58
11.3
9 13
.26
14.8
9 15
.01
13.8
3 16
.54
15.6
6 17
.62
16.6
2
Fe2O
3 1.
91
1.82
2.
19
3.20
1.
84
0.37
1.
81
1.46
1.
25
3.25
3.
15
2.10
1.
11
FeO
4.
12
3.32
5.
08
4.30
3.
98
0.62
0.
52
0.92
1.
12
3.32
3.
32
4.18
5.
27
MnO
0.
16
0.11
0.
16
0.16
0.
18
0.01
0.
04
0.05
0.
05
0.15
0.
16
0.11
0.
11
MgO
3.
71
2.86
3.
96
5.60
3.
60
0.21
0.
49
0.59
0.
51
2.55
2.
70
2.29
3.
10
CaO
6.
32
5.78
5.
37
11.1
5 4.
89
1.34
1.
73
1.53
2.
36
5.01
6.
51
4.05
0.
96
Na 2
O
4.31
3.
70
5.34
2.
98
4.09
5.
26
4.99
4.
71
3.93
3.
65
2.23
4.
02
1.25
K2O
1.
43
1.08
1.
22
1.28
0.
58
1.27
3.
50
3.93
2.
32
3.64
4.
57
3.02
8.
84
P2O
5 0.
11
0.21
0.
11
0.18
0.
06
0.01
0.
11
0.12
0.
10
0.28
0.
25
0.31
0.
26
LO
I 1.
32
1.26
1.
72
1.13
1.
21
0.67
1.
11
0.95
0.
87
1.46
1.
63
1.62
1.
81
Tot
al
99.8
3 99
.83
99.8
2 99
.77
99.8
7 99
.88
99.7
5 99
.76
99.8
0 99
.70
99.7
3 99
.68
99.7
3
Be
1.20
0.
98
1.74
1.
28
1.43
1.
45
1.64
1.
95
1.16
1.
69
1.67
1.
98
3.62
Sc
23.1
17
.8
23.6
26
.8
35.3
3.
11
4.26
4.
52
6.29
15
.3
19.3
14
.7
19.3
V
91.4
91
.7
149
179
59.6
7.
28
28.0
28
.8
38.9
15
0 17
6 14
1 14
6
Cr
40.5
81
.6
88.7
11
9 40
.5
3.74
5.
66
5.06
9.
18
14.2
37
.6
16.0
38
.0
Co
18.0
17
.5
25.6
25
.9
16.0
1.
53
3.07
3.
73
2.65
15
.5
16.3
12
.6
16.4
Ni
20.5
18
.5
23.9
30
.1
13.5
1.
56
2.98
2.
91
3.31
5.
79
10.1
6.
92
10.7
Cu
21.2
10
5 42
.7
84.8
8.
02
37.6
8.
47
7.29
26
.0
11.0
11
.2
40.6
22
.2
Zn
78.9
50
.0
101
64. 6
78
.5
7.11
32
.5
42.2
26
.4
101
106
95.9
17
0
Ga
20.0
17
.1
18.6
19
.1
15.0
16
.4
16.3
17
.1
14.8
20
.2
18.8
19
.1
23.0
Rb
23.2
16
.5
19.0
25
.4
7.91
13
.6
85.7
87
.4
44.1
94
.0
97.2
10
0 27
6
Sr
278
251
168
650
88.8
10
6 28
7 38
4 30
2 53
8 53
3 53
4 11
3
Y
41.4
41
.6
34.4
25
.0
71.6
35
.8
20.3
14
.9
16.3
25
.3
26.3
25
.7
29.3
Zr
65.8
10
0 13
4 12
1 30
2 23
7 17
5 17
2 10
3 13
8 13
0 15
4 15
1
Nb
6.31
7.
18
3.75
4.
06
4.89
6.
80
7.33
7.
57
6.20
7.
02
6.04
5.
88
8.12
Cs
2.00
1.
23
0.63
1.
14
0.33
0.
36
2.44
2.
74
1.67
12
.58
6.91
18
.42
39.6
4
Ba
386
282
429
235
132
177
1216
11
22
590
910
538
1048
13
66
La
19.2
18
.4
12.4
10
.9
19.4
15
.4
29.7
29
.1
19.1
20
.6
20.2
21
.9
19.3
Ce
43.1
44
.4
29.1
27
.5
49.5
34
.4
49.9
49
.1
47.0
40
.5
40.8
43
.4
44.3
Pr
6.21
6.
40
4.32
3.
25
7.67
4.
22
5.97
5.
69
4.34
5.
20
5.09
5.
44
4.90
Nd
28.0
28
.8
19.9
14
.9
36.4
16
.2
21.1
20
.0
16.9
21
.0
20.9
22
.3
21.1
Sm
7.
21
7.24
5.
46
3.61
10
.8
3.72
3.
75
3.57
3.
19
4.73
4.
76
5.00
4.
63
Eu
1.58
1.
14
1.48
1.
12
0.98
0.
25
1.07
1.
01
0.89
1.
43
1.39
1.
54
0.97
Gd
7.00
7.
02
5.43
3.
64
11.2
3.
86
3.42
2.
77
2.68
4.
25
4.27
4.
37
4.33
Tb
1.22
1.
20
0.97
0.
65
2.03
0.
77
0.54
0.
40
0.44
0.
68
0.69
0.
71
0.75
Dy
7.94
7.
51
6.33
4.
19
13.3
3 5.
46
3.29
2.
39
2.80
4.
30
4.33
4.
46
4.76
Ho
1.58
1.
56
1.27
0.
89
2.72
1.
19
0.68
0.
52
0.60
0.
93
0.94
0.
94
1.04
Er
4.25
4.
22
3.57
2.
31
7.50
3.
61
1.95
1.
42
1.60
2.
55
2.59
2.
64
2.63
Tm
0.
60
0.59
0.
52
0.34
1.
06
0.57
0.
28
0.21
0.
27
0.36
0.
38
0.39
0.
40
Yb
4.07
3.
98
3.40
2.
32
7.25
3.
97
2.03
1.
49
1.92
2.
64
2.66
2.
57
2.78
Lu
0.63
0.
60
0.53
0.
30
1.14
0.
65
0.30
0.
24
0.26
0.
41
0.42
0.
42
0.38
Hf
2.26
3.
75
3.72
2.
98
8.42
8.
25
4.72
4.
75
2.96
3.
97
3.74
4.
30
3.91
Ta
0.38
0.
50
0.27
0.
30
0.44
0.
74
0.55
0.
57
0.57
0.
48
0.42
0.
39
0.46
Tl
0.13
0.
10
0.14
0.
16
0.05
0.
09
0.40
0.
40
0.18
0.
44
0.46
0.
52
0.99
Pb
7.74
12
.30
9.33
13
.58
5.39
5.
45
15.6
17
.4
30.1
20
.7
17.0
19
.9
8.98
Th
2.71
5.
78
2.73
2.
56
6.86
12
.20
7.56
7.
85
7.28
5.
32
5.22
5.
81
5.30
U
0.64
1.
46
0.77
0.
68
1.58
2.
19
1.43
1.
71
1.48
1.
38
1.38
1.
79
4.69
Ti
5094
68
92
5814
70
72
1858
10
19
2218
22
77
2637
50
94
4735
52
74
5154
K
1187
1 89
66
1012
8 10
626
4815
10
543
2905
5 32
625
1925
9 30
217
3793
7 25
070
7338
5
TR
EE
13
3 13
3 94
.7
75.9
17
1 94
.3
123
118
102
110
109
116
112
(La/
Sm
) N
1.72
1.
64
1.46
1.
95
1.16
2.
67
5.11
5.
27
3.87
2.
82
2.74
2.
82
2.69
(La/
Yb)
N
3.39
3.
32
2.61
3.
38
1.92
2.
78
10.4
8 14
.03
7.12
5.
61
5.46
6.
11
4.98
(Ce/
Yb)
N
2.95
3.
10
2.38
3.
30
1.90
2.
41
6.81
9.
16
6.79
4.
26
4.26
4.
68
4.43
(Gd/
Yb)
N
1.42
1.
46
1.32
1.
30
1.28
0.
80
1.39
1.
54
1.15
1.
33
1.33
1.
41
1.29
Sm
/Nd
0.26
0.
25
0.27
0.
24
0.30
0.
23
0.18
0.
18
0.19
0.
22
0.23
0.
22
0.22
Th/
Ce
0.06
0.
13
0.09
0.
09
0.14
0.
35
0.15
0.
16
0.15
0.
13
0.13
0.
13
0.12
Ba/
Th
142
48. 8
15
7 91
.7
19.3
14
.4
161
143
81.1
17
1 10
3 18
0 25
8
Eu/
Eu*
0.
67
0.48
0.
82
0.94
0.
27
0.20
0.
90
0.95
0.
90
0.95
0.
93
0.98
0.
65
Ce/
Ce*
0.
96
1.00
0.
97
1.12
0.
99
1.03
0.
87
0.88
1.
22
0.93
0.
96
0.95
1.
09
K/K
* 2.
55
1.84
3.
28
3.66
0.
99
2.09
4.
10
4.64
3.
70
5.79
7.
66
4.84
14
.90
Nb/
Nb*
0.
23
0.32
0.
18
0.20
0.
29
0.30
0.
13
0.12
0.
17
0.13
0.
09
0.12
0.
07
Ti/
Ti*
0.
29
0.39
0.
42
0.77
0.
07
0.10
0.
25
0.30
0.
36
0.46
0.
43
0.46
0.
46
Petrology and geochemistry of the Shuangjing Schist
128
Fig. 6-8 Tectonic background discrimination diagrams for the intermediate-acidic
volcanic rocks in the Shuangjing Schist from Lianhuashan. (a) La/Yb-Sc/Ni diagram for
andesite after Bailey (1981), (b) Rb-(Nb+Y) diagram for granite after Pearce et al. (1984).
Middle Permian radiolarian fossils were found in the ophiolitic sequence of the
Nadaga-Xingshuwa area that is in fault contact with the Shuangjing schist (Wang and Fan,
1997), reflecting the initiation of an oceanic spreading center and the formation of new
oceanic crust. The widespread lenticular limestones between Fangkuangzi and Nadaga
could be the result of the long-term transgressive beach sedimentation and/or reefs.
Together, the continental marginal arc in Lianhuashan, the alternating sedimentary facies
in Fangkuangzi and the deep-water facies in Nadaga outline a continental shelf and slope
environment. Along the line Lianhuashan- Fangkuangzi- Nadaga- Xingshuwa, the
characteristics of the Shuangjing Schist reflect a smooth transition of depositional regimes
from a continental facies through shallow water facies to deep water facies. In other
words, we see various types of sediments deposited at a continental margin, representing
continental shelf, slope and deep ocean floor environments (Fig. 6-9b).
Our work on Cathode Luminescence images and LA-ICPMS U-Pb dating of zircons
from a carbonaceous biotite-plagioclase schist in Fangkuangzi indicates its protolith was a
proximal sedimentary rock that mainly sourced from intermediate magmatic materials
with a source age of 298 ± 2 Ma and was intruded by granite at 272 ± 2 Ma (Li et al., in
review). The protolith of the Shuangjing Schist was therefore formed during the Late
Carboniferous to Early Permian at the margin of an arc/forarc-related basin (Fig. 6-9b).
The hiatus of Devonian-Carboniferous strata in Linxi-Xilinhot area (GS-CUG, 2008)
suggests that the arc/forearc basin contained newly formed oceanic crust of Late
Carboniferous to Permian age (Fig. 6-9b,c). In the Linxi area, sedimentary sequences
from the Early Permian to Early Triassic age include flysch of the Shoushangou formation
(P1s), intermediate to basic volcanic rocks and flysch of Dashizhai formation (P1ds),
Chapter 6
129
marine flysch and carbonate rocks of Zhesi formation (P2z) and inland river-lake
sediments with some turbidite and marine sediment of Linxi formation (P3-T1l; GS-CUG,
2008). This sequence reflects pediplanation of an orogenic belt (P1s), initial development
of a continental rift (P1ds), oceanization and deep water sedimentation (P2z) and closure
of an oceanic basin (P3-T1l), which summarizes the evolution of an oceanic basin from
continental extension to closure from the Late Carboniferous to Early Triassic (Fig.
6-9b,c).
6.4.2 Broader tectonic context and significance
Two contrasting intrusive suites were identified at the northern margin of the North
China Craton: a Late Carboniferous (324-300 Ma) suite of diorite-granodiorite and a Late
Permian-Middle Triassic (254-237 Ma) suite of granitoid intrusions (Zhang et al., 2009a).
The Late Carboniferous rocks are subduction-related intrusions that were emplaced at an
Andean-style continental margin. Their formation resulted from anatectic melting of the
underplated ancient lower crust with variable involvement of enriched mantle materials
during southward subduction of the Paleo-Asian Oceanic plate beneath the North China
Craton. The Late Permian-Middle Triassic postcollisional granitoids were produced by
extreme fractional crystallization of hybrid magmas resulting from mixing of coeval
mantle- and crust-derived melts, linked to postcollisional lithospheric extension and
asthenospheric upwelling after final collision and suturing of the Mongolian arc terranes
with the North China Craton (Zhang et al., 2009a). Tang et al. (2010) suggested that the
Benbatu bimodal volcanic rocks (313-308 Ma) in the Solonker suture zone formed in a
post-collisional extensional setting (Fig. 9b), implying the suture zone closed completely
before the Late Carboniferous (Fig. 6-9a). A TIMS U-Pb zircon age of 284.8 ± 1.1 Ma of
alkali granite in Dong Ujimq in the suture zone reflects establishment of a post-orogenic
regime (Zhang et al., 2009b). Shi et al. (2004) reported a SHRIMP U-Pb zircon age of
276 ± 2 Ma for an A-type granite in the Xilinhot area, implying extension lasted into the
Early Permian. Thus, the Late Carboniferous diorite-granodiorite belt is thought to result
from post-orogenic extension at the northern margin of the North China Craton, including
the research area (Zhang et al., 2009a). This extension is thought to have lasted until at
least the Early Permian, as indicated by a 282 ± 5 Ma LA-ICPMS U-Pb zircon age of
quarzdiorite in the Wuchan area (Yuan and Wang, 2006), SHRIMP U-Pb zircon ages of
288 ± 5 and 280 ± 6 Ma of quartz diorite in northern Hebei province (Wang et al., 2007b),
and the LA-ICPMS U-Pb zircon age 285.6 ± 1.3 Ma of a diorite in the Linxi area (Li et al.,
in press).
Petrology and geochemistry of the Shuangjing Schist
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131
As outlined above, a stage of collision occurred in the Solonker suture zone before
the Late Carboniferous (Fig. 6-9a) and turned into post-orogenic extension during the
Late Carboniferous to Early Permian (Fig. 6-9b). The formation age (298 ± 2 Ma) and
intermediate magmatic nature of the source rock of the carbonaceous biotite-plagioclase
schist in the Shuangjing Schist (Li et al., in review) are consistent with the Late
Carboniferous (324-300 Ma) diorite-granodiorite suite of Zhang et al. (2009a). The
intermediate volcanic rocks in Lianhuashan lie in the continental marginal arc area,
implying the intrusion may have formed in a continental marginal arc background.
However, considering the extensional regime in the Solonker suture zone during the Late
Carboniferous to Early Permian when the magma intruded, the continental marginal arc
background was probably fed from slab-derived melts contributing to the magma source
region (Fig. 6-9b).
Two closures of oceanic basins are recorded in the Linxi area, related to the early
Paleo-Asian Ocean (Fig. 6-9a) and the late arc/forearc basin (Fig. 6-9c), respectively. This
resulted in the two contrasting interpretations of the timing of suturing in the Solonker
suture zone. The hiatus of Devonian-Carboniferous strata in the Xilinhot-Linxi area
suggests the final closure of Paleo-Asian Ocean happened before the Late Carboniferous
(Zhang et al., 2008). The arc/forearc basin developed from the Early Permian, so the
subducting slabs that caused metasomatism of the source of the volcanic rocks in
Lianhuashan investigated in this study must be related to earlier subduction, probably
induced by the closure of the Paleo-Asian Ocean during Caledonian Orogeny (Liu et al.,
2003). Considering the substantial volume of the Solonker suture zone, other arc/forearc
basins may have existed along the suture zone during Late Carboniferous to Early Triassic
times. Permian radiolarian fossils in the Xilinhot area (Shang, 2004) may represent one
such oceanic basin. The hiatus of Mid-Late Triassic strata in Linxi-Xilinhot area
(GS-CUG, 2008) is related to the final closure of the arc/forearc basin at this time. Final
closure of the marginal basin and suturing of the Solonker suture zone finished in the Late
Permian to Early Triassic (Fig. 6-9c).
Fig. 6-9 Tectonic model showing the formation environment for the protolith of the
Shuangjing Schist. (a) The Solonker suture zone closed before the Late Carboniferous,
resulting in closure of the Paleo-Asian Ocean. (b) Late Carboniferous to Early Permian
extension in the Solonker suture zone led to the opening of an arc/forearc basin with newly
formed oceanic crust. The continental marginal arc background of intermediate volcanic
samples in Lianhuashan was obtained from slab-derived melts contributing to the magma
source region. (c) Late Permian to Early Triassic closure of marginal basin and suturing of
Solonker Suture zone.
Petrology and geochemistry of the Shuangjing Schist
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6.5 Conclusion
(1) The Shuangjing Schist was formed during the greenschist facies prograde
metamorphism. Its protolith is a volcanic-sedimentary rock series, whose formation is
related to an arc/forearc basin during the Late Carboniferous-Early Triassic in the Linxi
area, central Inner Mongolia. The volcanic parts of the Shuangjing Schist belong to a
calc-alkaline series with large volumes of intermediate members and subordinate acidic
members. The volcanism was induced by subduction-related magmatism resulting from
mantle metasomatism and erupted in a continental marginal arc. The sedimentary parts of
the Shuangjing Schist reveal characteristics of various depositional sequences including
shelf, slope and deep sea sediments.
(2) In the Linxi area, final closure of the Paleo-Asian Ocean in the Late
Carboniferous was followed by closure of the arc/forearc basin, which induced
subduction of oceanic crust and the leading continental margin in the Late Permian-Early
Triassic. The closure of multiple oceanic basins led to two contrasting hypotheses
regarding the timing of final suturing in the Solonker suture zone. Our new data,
considered in the context of published work, indicate that final suturing of the Solonker
suture zone finished in the Late Permian to Early Triassic.
