Early Paleozoic tectonic evolution of the northern West Junggar (NW China): Constraints from Early Cambrian–Middle Silurian felsic plutons of the Chagantaolegai ophiolitic mélange

Journal Pre-proof Early Paleozoic tectonic evolution of the northern West Junggar (NW China): Constraints from Early Cambrian–Middle Silurian felsic plutons of the Chagantaolegai ophiolitic mélange Yaqi Yang, Lei Zhao, Qinqin Xu, Rongguo Zheng, Jianhua Liu, Jin Zhang PII:

S0024-4937(19)30384-6

DOI:

https://doi.org/10.1016/j.lithos.2019.105225

Reference:

LITHOS 105225

To appear in:

LITHOS

Received Date: 16 July 2019 Revised Date:

23 September 2019

Accepted Date: 23 September 2019

Please cite this article as: Yang, Y., Zhao, L., Xu, Q., Zheng, R., Liu, J., Zhang, J., Early Paleozoic tectonic evolution of the northern West Junggar (NW China): Constraints from Early Cambrian–Middle Silurian felsic plutons of the Chagantaolegai ophiolitic mélange, LITHOS, https://doi.org/10.1016/ j.lithos.2019.105225. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.

Abstract: The early Paleozoic tectonic evolution of the Junggar Ocean (a major branch of the southern Paleo-Asian Ocean (PAO)) remains a topic of debate. This study mapped the Chagantaolegai ophiolitic mélange (COM) in the northern West Junggar (NWJ) on a large scale and identified several tectonically juxtaposed lithotectonic units, including the ophiolite suite and felsic plutons. The ophiolite suite mainly comprises serpentinized ultramafic rock, pyroxenolite, gabbro, dolerite, plagiogranite, basalt, volcanic rock, and radiolarian chert. Zircon U–Pb ages for two plagiogranite samples yielded ages of 515 ± 4 Ma and 513 ± 6 Ma, constraining the existence of the Junggar Ocean to the Early Cambrian at least. The felsic plutons can be subdivided into two groups. Group I (503–481 Ma), which intruded into the COM, shows low-K, calc-alkaline features with remarkable depletion of Nb, Ta, and Ti, resembling rocks formed in an intra-oceanic arc. Group II (435–428 Ma) was found on both sides of the COM and displays high-K, calc-alkaline series characteristics, similar to the Silurian to Early Devonian A-type granites in the NWJ, implying a post-collisional environment. Based on a combination of existing observations and our new observations and data with regional geological evidence, our new data indicate that the NWJ underwent a transition from the tectonic processes of seafloor spreading and subduction to arc–arc collision and post-collisional extension during the Early Cambrian–Early Devonian.

1

Early Paleozoic tectonic evolution of the northern West Junggar

2

(NW China): Constraints from Early Cambrian–Middle Silurian

3

felsic plutons of the Chagantaolegai ophiolitic mélange

4

Yaqi Yang a, Lei Zhao a,∗, Qinqin Xu a, Rongguo Zheng a, b, Jianhua Liu a, Jin Zhang a

5

a

6

Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037,

7

China

8

b

9

Geography, Chinese Academy of Sciences, Urumqi 830011, China

10

Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources,

Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and

Abstract:

11

The early Paleozoic tectonic evolution of the Junggar Ocean (a major branch of

12

the southern Paleo-Asian Ocean (PAO)) remains a topic of debate. This study

13

mapped the Chagantaolegai ophiolitic mélange (COM) in the northern West Junggar

14

(NWJ) on a large scale and identified several tectonically juxtaposed lithotectonic

15

units, including the ophiolite suite and felsic plutons. The ophiolite suite mainly

16

comprises

17

plagiogranite, basalt, volcanic rock, and radiolarian chert. Zircon U–Pb ages for two

18

plagiogranite samples yielded ages of 515 ± 4 Ma and 513 ± 6 Ma, constraining the

∗

serpentinized

ultramafic

Corresponding author. Email: [email protected]. (L. Zhao)

rock,

pyroxenolite,

gabbro,

dolerite,

19

existence of the Junggar Ocean to the Early Cambrian at least. The felsic plutons can

20

be subdivided into two groups. Group I (503–481 Ma), which intruded into the COM,

21

shows low-K, calc-alkaline features with remarkable depletion of Nb, Ta, and Ti,

22

resembling rocks formed in an intra-oceanic arc. Group II (435–428 Ma) was found

23

on both sides of the COM and displays high-K, calc-alkaline series characteristics,

24

similar to the Silurian to Early Devonian A-type granites in the NWJ, implying a

25

post-collisional environment. Based on a combination of existing observations and

26

our new observations and data with regional geological evidence, our new data

27

indicate that the NWJ underwent a transition from the tectonic processes of seafloor

28

spreading and subduction to arc–arc collision and post-collisional extension during

29

the Early Cambrian–Early Devonian.

30

Keywords: Ophiolite suite; Intra-oceanic subduction; Post-collisional extension;

31

West Junggar; Paleo-Asian Ocean

32

1. Introduction

33

The southwest segment of the Central Asian Orogenic Belt (CAOB) is

34

dominated by accretionary complexes and Paleozoic arcs (Xiao et al., 2010). As the

35

core part, the Kazakhstan and West Junggar (WJ) terranes are mainly composed of

36

several curved and coaxial ribbon-like Paleozoic geologic units (Fig. 1b). The

37

Paleozoic arc units are also major components of the Kazakhstan Orocline (Xiao et

38

al., 2010, 2015). However, the Paleozoic tectonic evolution process in the

39

Kazakhstan and WJ has not been clearly established. Increasing evidence suggests

40

that the subduction–accretion processes of the Kazakhstan and WJ strongly resemble

41

those of present-day archipelago systems in the southwestern Pacific, which are

42

broadly regarded as responsible for the mutual collision of multiple micro-continents

43

and/or island arcs (e.g., Chen et al., 2015, 2019; Degtyarev and Ryazantsev, 2007;

44

Ren et al., 2014; Xiao et al., 2015).

45

The WJ, among Altay, Junggar basin, Tianshan and Kazakhstan, is an important

46

area for examining the lateral correlation of Paleozoic tectonic units (Zhao and He,

47

2013) and reconstructing the tectonic evolution of the southwestern CAOB (e.g.,

48

Chen et al., 2019; Fig. 1b). Recently, various tectonic models have been used to

49

explain the Paleozoic evolution process in the WJ, including intra-oceanic

50

subduction (e.g., Ren et al., 2014), oceanic-ridge subduction (e.g., Zhang et al., 2018;

51

Windley and Xiao, 2018, and references therein), arc–arc collisions (e.g., Yang et al.,

52

2019), and subduction and accretion of seamounts (Yang et al., 2015a). The diversity

53

of these models further indicates that the Paleozoic evolution of the WJ was complex.

54

A similar situation exists in the NWJ. Large amounts of Silurian–Early Devonian

55

igneous rocks occur in the NWJ, but these models proposed to explain their setting

56

have been controversial (Chen et al., 2019, and references therein), such as

57

oceanic-ridge subduction (Shen et al., 2014; Zhang et al., 2018), slab roll-back

58

(Chen et al., 2019; Yin et al., 2017a), and post-collisional extension (Chen et al.,

59

2015; Yang et al., 2019). The Chingiz–Tarbagatai orogenic belt (CTOB), also known

60

as the Boshchekul–Chingiz (BC) arc, is one of the major orogenic belts extending

61

from Kazakhstan to NWJ. The CTOB is thought to have been formed by

62

intra-oceanic subduction, giving rise to the Cambrian–Silurian volcanic-sedimentary

63

rock series, arc-like granitoids, and Silurian alkaline igneous rocks (Degtyarev and

64

Ryazantsev, 2007). However, compared with in the Kazakhstan area, the extension

65

of the CTOB to China is not well constrained regarding the Cambrian–Ordovician

66

igneous rocks (Shen et al., 2015). Therefore, our understanding of the formation of

67

the CTOB in the NWJ is inadequate. To better constrain the early Paleozoic tectonic

68

evolution in the NWJ, this study investigated the early Paleozoic ophiolite and felsic

69

plutons in the southern Xiemisitai Mountains.

70

2. Geological setting

71

The WJ region, as a typical subduction–accretion area, provides an ideal

72

laboratory for studying the orogenic processes in the southern CAOB (Choulet et al.,

73

2012a, b). A growing number of studies have revealed that this region contains

74

considerable amounts of Paleozoic igneous rocks, ophiolite and seamount relics (Xu

75

et al., 2012; Yang GX et al., 2015a; Yang YQ et al., 2019). Generally, the WJ is

76

considered to be the region between the Irtysh–Zaysan suture zone and the North

77

Tianshan suture zone (Liu et al., 2017; 2019). Based on the distinct differences of

78

fault strike, strata and isotope-dated intrusions in the WJ (e.g., Zhao and He, 2013,

79

2014), the region can be divided into two parts along the Baiyanghe–Heshituoluogai

80

valley (Yang et al., 2019; Zhao and He, 2014). Considering the significant

81

differences in the strike of faults as well as the ages of ophiolite and strata in the

82

southern WJ, it can be further subdivided into two parts, namely, Central West

83

Junggar (CWJ) and Southern West Junggar (SWJ) (Chen et al., 2015; Du et al., 2019;

84

Liu et al., 2017; Fig. 2).

85

The NWJ, lying north of the Baiyanghe–Heshituoluogai valley and south of the

86

Irtysh–Zaisan suture zone, is composed of the Zharma–Sawur orogenic belt (ZSOB)

87

and the CTOB (Fig. 2). The ZSOB is generally regarded as a late Paleozoic

88

magmatic arc formed by the south-dipping subduction of the Irtysh–Zaisan Ocean

89

(Zhao and He, 2013, 2014). The CTOB, south of the South Saur Fault, is generally

90

regarded as an E–W-striking early Paleozoic tectonic belt, consisting of two arcs (the

91

Taer-E’min arc and Xiemisitai arc) separated by one ophiolitic belt (Fig. 2). The

92

ophiolitic belt consists of numerous ophiolitic mélanges, including the Hongguleleng,

93

Hebukesaier, Chagantaolegai, E’min and Kujibai ophiolitic mélanges (Fig. 2).

94

Prolonged magmatic episodes occurred in the NWJ, ranging from the Late

95

Ordovician to Early Permian (Chen et al., 2019, and references therein; Yang et al.,

96

2019; Figs. 2–3). In addition, Ordovician–Carboniferous sedimentary rocks are

97

widely scattered in the region (Fig. 2).

98

The CWJ consists of Early Devonian ophiolites (Gu et al., 2009; Xu et al., 2006)

99

and Devonian–Carboniferous igneous rocks and strata; early Paleozoic strata are

100

sparse in this area (Fig. 2). Notably, Ordovician conodonts and radiolarians were

101

identified from the deep-sea chert in the Karamay and Darbut ophiolitic mélanges,

102

implying an Ordovician residual oceanic crust (He et al., 2007; Samygin et al., 1997;

103

Shu et al., 2001). However, there is still controversy regarding the tectonic setting of

104

the widely-distributed Devonian and Carboniferous igneous rocks in the CWJ, which

105

are an important factor in constraining the timing of the closure of the Junggar

106

Ocean (Choulet et al., 2012b; Liu et al., 2017).

107

The SWJ is dominated by Ordovician to Devonian volcanic-sedimentary rocks

108

and several ophiolites, the Tangbale, Barleik, and Mayile ophiolites (Fig. 2).

109

Radiometric dating on the ophiolitic blocks ranges from 572 Ma (Yang et al., 2012)

110

to 508 Ma (Xiao et al., 1992); the ophiolites show characteristics of SSZ-type

111

ophiolites (Liu et al., 2019; Xu et al., 2012; Table. 1). Moreover, the

112

newly-recognized Cambrian to Ordovician (509–485 Ma) plutons in the Barleik and

113

Mayile Mountains have been interpreted as the products of intra-oceanic subduction

114

of the Junggar Ocean and called the Southwest Junggar arc (Chen et al., 2019; Ren et

115

al., 2014; Xu et al., 2013), consistent with the presence of 504–492 Ma blueschist

116

within the mélange in this area (Liu et al., 2016). The Ordovician strata and mélange

117

are unconformably overlain by Early Silurian pyroclastic rock and Devonian

118

siltstone (Chen et al., 2019; Ren et al., 2014).

119

3. Chagantaolegai ophiolitic mélange

120

The COM outcrops along two sub-E–W-trending faults in the southern part

121

ofthe Xiemisitai Mountains (Figs. 2–3) and is surrounded by Silurian intermediate to

122

acidic volcanic rocks and Devonian–Carboniferous sedimentary rocks (IGSCUG,

123

2013; Fig. 3). To document the constituents and structural relationships of the COM,

124

detailed field investigations were conducted on the southern slope of the Xiemisitai

125

Mountains (Fig. 4). Several tectonically juxtaposed lithotectonic units (the ophiolite

126

suite and felsic plutons) were identified in the mélange (Fig. 4).

127

3.1. Ophiolite suite

128

The Chagantaolegai ophiolite suite exhibits a typical block-in-matrix fabric

129

with variably-sized blocks, including serpentinized ultramafic rock, pyroxenolite,

130

gabbro, dolerite, plagiogranite, basalt, volcanic rock, and radiolarian chert (Figs. 4,

131

5f). Gabbroic blocks occur in the foliated, fine-grained serpentinite matrix of the

132

COM with reported zircon U–Pb ages of 519–517Ma (Zhao and He, 2014; Fig. 4;

133

Table 1). However, the basalt blocks only outcrop in the center of the map (Fig. 4).

134

Some plagiogranites 1 × 3 m in size are present as blocks in the ultramafic matrix

135

(Figs. 4, 5a). The plagiogranite consists mainly of plagioclase, quartz and accessory

136

minerals with a poikilitic texture (Fig. 5i). The cherts have red jasper rocks and

137

jade-green siliceous rocks. Some Early–Middle Ordovician radiolarian fossils were

138

found in the red chert of the northern part of the region (Zong et al., 2014; Fig. 4).

139

3.2. Felsic plutons

140

Felsic plutons have been identified from the COM. Diorite (XMST18-42),

141

granite (XMST18-47) and K-feldspar granite (XMST18-49) were identified from the

142

mapping region (Fig. 4) and intruded locally into the chert, matrix, or volcanic rock

143

(Figs. 3, 4, 5e). However, the granodiorite (XMST18-48) was collected in the north

144

of the eastern segment of the COM (or also called the Yinisala ophiolitic mélange)

145

(Fig. 3). The grayish-black diorite displays massive structure with some cleavage

146

(Figs. 5d, g) and is composed of plagioclase, hornblende, and quartz with minor

147

zircon, apatite, and iron oxides, with a medium-grained texture. Some hornblendes

148

have been altered to chlorite (Fig. 5j). The granitic pluton exhibits E-dipping

149

cleavage and a coarse-grained texture (Figs. 5b, c), consisting mainly of variable

150

amounts of quartz, plagioclase, and K-feldspar (Fig. 5k).The K-feldspar granite lies

151

south of the COM and intrudes into the volcanic rocks with obvious chilled margins

152

(Fig. 5e). The flesh-red granite mainly contains K-feldspar (40%–50%), quartz

153

(30%–40%), and minor hornblende (<5%). The granodiorite pluton shows

154

fine-grained structure and consists of quartz, plagioclase, and K-feldspar (Figs. 5l,

155

m). In addition, the plagioclases show partial alteration to sericite (Fig. 5m).

156

4. Analytical methods

157

The processes of zircon selection and mounting zircon target used the common

158

techniques, similar to these of Yang et al. (2019). U–Pb isotopic data of some zircons

159

from samples XMST18-42, 47–49 were tested by Laser Ablation Inductively

160

Coupled Plasma Mass Spectrometry (LA-ICP-MS) at Beijing Createch Test

161

Technology Co. Ltd., China (BCTT), and zircons from samples XMST13-11 and

162

XMST18-45 were measured by the Sensitive high-resolution ion microprobe

163

(SHRIMP) II at the Beijing SHRIMP Center, Institute of Geology, Chinese Academy

164

of Geological Sciences, Beijing, China. For the method of LA-ICP-MS, laser

165

sampling was performed using an ESI NWR 193 nm laser ablation system and an

166

AnlyitikJena PQMS Elite ICP–MS instrument was used to acquire ion-signal

167

intensities. The analytical procedures are consistent with Du et al. (2019). The

168

analytical procedures of SHRIMP II follow those of Yang et al. (2019). Common Pb

169

correction was made using the measured

170

Supplementary Table A. Uncertainties for each analysis are at 1σ, whereas the

171

weighted mean age is quoted at 2σ.

204

Pb. Analytical results are listed in

172

Zircon Hf-isotopic analyses were conducted on the dated sites of those zircons

173

from the felsic plutons (XMST18-42, 47–49) at BCTT. A Neptune MC-ICP-MS,

174

equipped with a 193-nm laser, was used to analyse the zircon Lu–Hf isotopic ratios.

175

During the analysis, the

176

operation parameters have been integrally described in Du et al. (2019).

176

Hf/177Hf ratio of zircon GJ1 (standard zircon) and other

177

Whole-rock analyses of all samples were completed at the Wuhan Sample

178

Solution Analytical Technology Co., Ltd., China. Operational processes and

179

parameters follow Liu et al. (2008). Besides, the international reference materials

180

and 10% of the samples were selected to retest and estimate the instrument stability.

181

The precision during major and trace element analysis was generally better than 5%. Rb–Sr and Sm–Nd isotope of samples XMST18-42, 47–49 analyses were also

182

87

Sr/86Sr and

183

undertaken on a Neptune plus MC-ICP-MS at BCTT. The measured

184

143

185

0.7219, respectively. The 87Sr/86Sr and 143Nd/144Nd values were adjusted to the NBS

186

987 standard with

187

0.512185. Procedural details are described by Yang et al. (2010).

188

5. Results

189

5.1 Zircon U–Pb geochronology

Nd/144Nd isotope ratios were normalized to

87

86

Sr/88Sr = 0.1194 and

Sr/86Sr = 0.710248 and GSB Nd standard with

146

143

Nd/144Nd =

Nd/144Nd =

190

Cathodoluminescence (CL) images of representative zircon grains are shown in

191

Fig. 6. All the zircon grains show well-preserved crystals of different sizes and

192

exhibit clear oscillatory zoning without overgrowths or inherited cores. The

193

SHRIMP and LA-ICP-MS U–Pb zircon data are shown in Supplementary Table A

194

and Table B and are depicted in concordia diagrams (Fig. 6).

195

5.1.1 Ophiolitic plagiogranites

196

Zircons from two plagiogranite samples (XMST13-11 and XMST18-45) exhibit

197

columnar and concentric oscillatory zoning (Fig. 6). The Th/U ratios are greater than

198

0.1 (0.2–0.74), within the range of magmatic zircon. These samples were analyzed

199

by SHRIMP II and yielded ages of 515 ± 4 Ma (XMST13-11) and 513 ± 6 Ma

200

(XMST18-45) (Figs. 6e–f).

201

5.1.2 Felsic plutons

202

All zircon grains from the felsic plutons show well-developed oscillatory

203

growth zoning (Fig. 6) and have a narrow range of Th/U ratios of 0.29–1.16

204

(Supplementary Table B). The weighted mean age of the diorite (XMST18-42) is

205

503 ± 2 Ma (Fig. 6a). Twenty-six valid spots from sample XMST18-47 yielded a

206

concordant age of 428 ± 2 Ma (Fig. 6b). The pluton of the coarse-grained granite

207

(XMST18-48) yielded a weighted mean 206Pb/238U age of 481 ± 3 Ma (Fig. 6c). The

208

K-feldspar granite (XMST18-49) gave a weighted mean age of 435 ± 2 Ma (Fig. 6d).

209

These data indicate the emplacement ages of these felsic plutons.

210

5.2 Geochemical data

211 212

The major- and trace-element data are given in Supplementary Table C. The major oxides were LOI-free normalized.

213

5.2.1 Ophiolitic plagiogranites

214

The ophiolitic plagiogranite samples from the COM had distinctly different

215

SiO2 contents: Sample XMST13-11 had relatively low SiO2 contents of 63.62–64.64

216

wt%, whereas XMST18-45 showed relatively high SiO2 contents of 72.18–75.67

217

wt% (Fig. 7a). In addition, the two ophiolitic plagiogranites exhibited low TiO2

218

(0.12–0.22 wt%), K2O (0.08–1.23 wt%), and P2O5 (0.03–0.007 wt%) contents. They

219

also exhibited similar rare earth element (REE) patterns with no obvious

220

fractionation ((La/Yb)N = 0.71–1.68) and moderately-weak Eu negative anomalies

221

(Eu/Eu⁎ = 0.58–0.97) (Fig. 8a). Furthermore, they were depleted in Nb, Ta, P, and

222

Ti and enriched in K and Pb (Fig. 8b).

223

5.2.2 Felsic plutons

224

The diorite was low-K, calc-alkaline and metaluminous (A/CNK = 0.84–0.93;

225

Fig. 8) with relatively low SiO2 (58.48–59.32 wt%) and high MgO (3.58–4.31 wt%)

226

contents and Mg# values (57–58). The coarse-grained granite also had low levels of

227

K, and was calc-alkaline and metaluminous (A/CNK = 1–1.08, Fig. 7), but had

228

higher SiO2 (72.06–74.99 wt%) and lower Al2O3 (12.02–12.26 wt%) and MgO

229

(1.13–1.38 wt%) contents. Both the diorite and granite showed approximately flat

230

REE patterns (Fig. 8c), negative anomalies of Th, Nb, Ta, P, Ti, and Zr, and positive

231

anomalies of Cs, Pb, and Sr (Fig. 8d). In addition, the granodiorite and K-feldspar

232

granite samples were both medium- or high-K, calc-alkaline, and metaluminous (Fig.

233

7). The granodiorite contained lower SiO2 (67.85–68.70 wt%), K2O (1.07–1.85 wt%)

234

and CaO (1.53–4.27 wt%) contents where the K-feldspar granite displays higher

235

SiO2 (69.87–72.04 wt%) and K2O (2.6–3.79 wt%) contents. All the rocks had similar

236

REE patterns with notable enrichment in LREE ((La/Yb)N = 4.73–9.57) (Fig. 8e).

237

Furthermore, the granodiorite and K-feldspar granite samples showed similar

238

features in the multi-element variation diagram with relative depletion in Nb, Ta, P,

239

and Ti and enrichment in Cs, Pb, and Sr (Fig. 8f).

240

5.3 Sr–Nd–Hf isotopes

241 242 243

Whole-rock Sr–Nd isotopic and zircon Hf isotopic data of four felsic plutons are given in Supplementary Table D and Table E. All the samples from the felsic plutons displayed similar 176

87

Sr/86Sr (0.70458 to

Hf/177Hf (0.282754 to 0.282942) and εHf (t) values (+9.52 to +15.03).

244

0.70595),

245

Specifically, the 503 Ma diorite and 481 Ma coarse-grained granite exhibit relatively

246

higher εNd (t) values (+3.72 to +4.64) than the 428 Ma granodiorite and 435 Ma

247

K-feldspar granite (+2.21 to +2.96) (Supplementary Table D). The two-stage zircon

248

Hf model ages of the diorite and coarse-grained granite (average = 671 Ma, 701 Ma,

249

respectively) are slightly older than those of the Silurian felsic plutons (average =

250

640 Ma, 644 Ma, respectively) (Supplementary Table E); however, the average

251

two-stage depleted mantle Nd model ages of the diorite (831 Ma) and coarse-grained

252

granite (871 Ma) are considerably younger than that of the granodiorite (935 Ma)

253

and K-feldspar granite (988 Ma) (Supplementary Table D).

254

6. Discussion

255

6.1 Correlation of early Paleozoic ophiolites in the WJ

256

The Chagantaolegai ophiolite consists of serpentinized ultramafic rock,

257

pyroxenolite, gabbro, dolerite, plagiogranite, basalt, volcanic rock, and radiolarian

258

chert. The two plagiogranite samples have the Early Cambrian crystallization age of

259

515–513 Ma, coeval with the age of the gabbroic blocks (519–517 Ma) in the COM

260

(Zhao and He, 2014). This result indicates a formation age of 519–513 Ma for the

261

Chagantaolegai ophiolite. Other ophiolites in the WJ show some regular

262

spatio-temporal characteristics: (1) The Chagantaolegai ophiolite (519–513 Ma)

263

represents the oldest oceanic crust (Anonymous, 1972) in the NWJ, coeval with the

264

gabbroic blocks in the SWJ ophiolites (572–508 Ma). (2) The ophiolites distributed

265

in the NWJ show similar ages ranging from 488 Ma to 472 Ma and form an

266

E–W-trending ophiolitic belt, implying an early Paleozoic fossil ocean (called the

267

Hongguleleng–Balkybey Ocean (HBO), Yang et al., 2019). (3) The radioisotopic

268

ages of the CWJ ophiolites are mostly in the Late Devonian, but these ophiolites also

269

revealed some Ordovician radiolarians and conodont fossils in the deep-sea chert

270

(He et al., 2007; Samygin et al., 1997; Shu et al., 2001).

271

Most of the SWJ ophiolites are Early Cambrian in age (531–512 Ma),

272

representing an Early Cambrian Junggar Ocean (Liu et al., 2019). However, some

273

researchers are still debating whether the Chagantaolegai ophiolite is associated with

274

the Junggar Ocean or the Hongguleleng–Balkybey Ocean (HBO) (Du et al., 2019;

275

Zhao and He, 2014). A recent detailed geochronological study of the Hebukesaier

276

ophiolite suggested that the HBO opened during 512–502 Ma (Yang et al., 2019),

277

close to the ages of the Chagantaolegai ophiolite (519–513 Ma; Zhao and He, 2014;

278

this study). These two ophiolites bear similar MOR-type geochemical characteristics,

279

(Zhao and He, 2014) which are notably different from those of the SSZ-type SWJ

280

ophiolites (Table 1). Thus, the Chagantaolegai ophiolite may represent a

281

continuation of the northern ophiolitic belt. In addition, no terrane has been found

282

between the Junggar Ocean and the HBO during the Cambrian. Therefore, the

283

Junggar Ocean is likely to have been a large ocean basin that merged with the HBO

284

during the Cambrian; thus, the Chagantaolegai ophiolite is a crucial connection point

285

between the two fossil oceans.

286

6.2 Petrogenesis

287

6.2.1 Ophiolitic plagiogranites

288

The two plagiogranite samples form the COM have relatively high SiO2

289

contents. They are characterized by low K2O contents (mostly <1 wt%), high

290

Na2O/K2O ratios (mostly >4), relatively low (La/Sm)N (0.63–1.37) and (Ce/Yb)N

291

ratios (1.22–1.86), enrichment in LILEs (Cs, Rb, and K), and depletion in HFSEs

292

(Nb, Ta and Ti). These characteristics are typical of ocean plagiogranites (Coleman

293

and Peterman, 1975; Pearce et al., 1984; Rollinson, 2009; Fig. 8b).

294

Plagiogranites are felsic rocks that contain diorite, quartz diorite, tonalite,

295

trondhjemite, and albitite/anorthosite, and generally occur in ophiolites and modern

296

oceanic crust (Coleman and Donato, 1979); thus, they are important for

297

understanding evolution of the oceanic crust. Two petrogenesis models for oceanic

298

plagiogranites have been widely accepted: (1) differentiation of basaltic magmas

299

(Coleman and Donato, 1979), and (2) partial melting of metasomatized gabbros or

300

amphibolites (Grimes et al., 2013). The incompatible elements Zr and Y have a

301

strong effect on the partial melting process (Hanson, 1978). The plagiogranite

302

samples show low Zr and Y contents, compatible with anatectic-type plagiogranite

303

(Fig. 10a), which implies that the plagiogranites may have been generated by gabbro

304

or amphibolite anatexis rather than by basaltic magma fractionation. Furthermore,

305

the plagiogranites in the COM exhibit low La contents and relatively high La/Sm

306

ratios (Fig. 10b) and do not show signs of fractional crystallization in the Harker

307

diagrams (Fig. 9), similar to the partial melting process. Therefore, the

308

Chagantaolegai plagiogranites are most likely to have originated from partial melting

309

of metasomatized gabbros or amphibolites.

310

6.2.2 Felsic plutons

311

Based on analysis of the field relationships, formation ages, and geochemical

312

characteristics of the felsic plutons, we divided them into two groups: Group I which

313

formed during 503–481 Ma and intruded into the COM, and Group II which formed

314

in 435–428 Ma and is distributed on both sides of the COM. The diorite and

315

coarse-grained granite of Group I (503–481 Ma) show significantly older

316

crystallization ages than those of the granodiorite and K-feldspar granite of Group II

317

(435–428 Ma) (Fig. 3). They also show differences in the magma evolution trends as

318

well as trace-element features (Fig. 9). All of these results indicate their different

319

origins.

320

6.2.2.1 Felsic plutons – Group I

321

The felsic plutons of Group I show similar petrographic features and

322

trace-element and isotope compositions. Their major and trace elements display

323

remarkably linear trends in the Harker diagrams (Fig. 9), which reveal a relatively

324

homogeneous source. Both samples show characteristics of low K, calc-alkalinity,

325

high Na2O, and low Ga/Al values (<2.6) without Al-rich minerals (Fig. 9), consistent

326

with I-type granites.

327

In general, the intermediate to felsic I-type granites may originate from three

328

sources: (1) crustal origin (Chappell and White, 2001), (2) mantle-derived origin

329

(Chiaradia, 2009), or (3) a mixed origin of both crustal and mantle-derived

330

components (Barbarin, 1999). The felsic plutons of Group I show moderate to high

331

SiO2 content and relatively low MgO, Mg#, Cr, and Ni contents (Supplementary

332

Table C), incompatible with magmas derived from direct partial melting of the

333

mantle. The crystallization ages (503–481 Ma) of the Group I felsic plutons show

334

obviously older zircon Hf model ages (701–617 Ma), implying the same conclusion

335

as above. Moreover, there are no mafic enclaves or disequilibrium petrographic

336

characteristics within the rocks, which excludes a mixed origin. The Hf-isotope

337

compositions of the diorite and granite samples show a narrow range, which is

338

atypical of magmas that incorporate both mantle and crustal components (Yin et al.,

339

2017b). However, depletion in Th and low levels of Rb (4.88–13.54 ppm), (La/Yb)N

340

(0.65–0.84), and Sr/Y (5.3–13.44) also rule out the possibility of partial melting of

341

oceanic crust (Fowler et al., 2008). Additionally, the positive εNd (t) (+3.72 to

342

+5.93), high εHf (t) (+10.16 to +14.6) values, and low initial

87

Sr/86Sr values

343

(0.70505–0.70595) (Figs. 10g–h) suggest that the felsic plutons of Group I may have

344

been generated by partial melting of juvenile crust (Rapp and Watson, 1995).

345

6.2.2.2 Felsic plutons – Group II

346

The felsic plutons of Group II show high-K and calc-alkaline series (Fig. 9).

347

Similarly, they are magnesian and metaluminous to weakly peraluminous, and have

348

low Ga/Al values (<2.6), typical of I-type granite affinity (Chappell and White,

349

1974). Moreover, the Group II felsic plutons display similar co-variation trends of

350

decreasing TiO2, Fe2O3T, MgO, CaO, Al2O3, P2O5, and V abundances with increasing

351

SiO2 (Fig. 9), indicating significant fractional crystallization of plagioclase,

352

K-feldspar, hornblende or biotite during magma evolution. Furthermore, the La vs.

353

La/Sm and Sr vs. Ba diagrams suggest that these rocks may have been derived from

354

fractional crystallization of K-feldspar and plagioclase (Figs. 10b–c). Considering

355

the crustal origins of Group I proposed above, the felsic plutons of Group II show

356

similar characteristics of petrography, geochemistry, and Sr–Nd–Hf isotopes (Figs.

357

9–10). Group II was also likely derived from juvenile lower crust. Notably, Group II

358

felsic plutons have consistent two-stage whole-rock Nd model ages (994–929 Ma),

359

older than those of plutons of Group I (TDM2(Nd) = 914–783 Ma), indicating that the

360

felsic plutons of Group II are likely to have originated from lower crust formed

361

earlier.

362

6.3 Tectonic implications

363

6.3.1 Intra-oceanic subduction prior to the Early Ordovician

364

It has been widely agreed that the BC arc, as the northern limb of the

365

Kazakhstan Orocline (Fig. 1b), was formed by intra-oceanic subduction of the

366

Junggar Ocean during the Cambrian to Ordovician (e.g., Shen et al., 2014, 2015, and

367

references therein). Geographically, the BC arc may extend westward to eastern

368

Kazakhstan and possibly eastward to East Junggar (Zhang et al., 2013; Fig. 1b). In

369

the NWJ, the BC arc is composed of the Taer-E’min arc and the Xiemisitai arc (Yang

370

et al., 2019), exposed in the E–W-striking Tarbagtay–Xiemisitai–Sharbuti Mountains

371

(Fig. 2). However, the NWJ is mainly dominated by Silurian–Devonian igneous

372

rocks (Chen et al., 2010, 2015; Yang et al., 2015b; Fig. 11a); thus, the extension of

373

the tectonic belt in the NWJ cannot be demonstrated as there is no evidence of

374

Cambrian–Ordovician arc igneous rocks.

375

In this study, the Cambrian to Ordovician subduction-related plutons were first

376

recognized from the COM in the NWJ. The diorite and coarse-grained granite

377

yielded zircon U–Pb (LA-ICP-MS) ages of 503 ± 2 Ma and 481 ± 3 Ma, respectively.

378

Petrologically, the diorite samples contain abundant hornblende (Fig. 5j), implying

379

an H2O-rich magmatic origin (Naney, 1983). The hydrated magma probably

380

originated from the subducted slab. Geochemically, these samples show low-K,

381

calc-alkaline characteristics, enrichment in LILEs and pronounced depletion in

382

HFSEs, compatible with arc-related origins. Furthermore, the trace-element features

383

of these rocks are almost identical to those of the Mariana intra-oceanic arc (Tamura

384

et al., 2014; Fig. 8d). Based on their high silicon contents, the Middle

385

Cambrian–Early Ordovician plutons formed in a highly mature intra-oceanic arc

386

environment where the oceanic subduction began before the Middle Cambrian in the

387

NWJ.

388

6.3.2 Collision and post-collision during the Silurian to Early Devonian

389

Silurian–Early Devonian igneous rocks are widely distributed in the NWJ (Figs.

390

2, 11a), but their tectonic settings are still under debate (Du et al., 2019; Yin et al.,

391

2017a). These igneous rocks display subduction-related fingerprinting (i.e. relatively

392

enrichment in LILEs and depletion in HFSEs) (Chen et al., 2015, 2019; Yang G et al.,

393

2015b; Yang YQ et al., 2019; Yin et al., 2017a). However, the arc-related setting is

394

hard to interpret for such large-area coeval peralkaline A-type granites in the

395

Xiemisitai Mountains (Chen et al., 2015; Yang et al., 2015b; Fig. 11a). Some studies

396

have indicated that the depletion of Nb and Ta in magmatic rocks can be inherited

397

from an earlier subduction episode (Turner et al., 1996; Zhang et al., 2013). In

398

addition, the peak age of granitic plutons in the region is ca. 428 Ma (Fig. 11b),

399

consistent with the age of the biggest granitic batholith exposed in the central

400

Xiemisitai Mountains (Fig. 11a). This batholith has been proved to be formed in a

401

high-T and low-P extensional regime (Chen et al., 2015; Yang et al., 2015b; Yin et al.,

402

2017a), comprising mainly A2- and I-type granites.

403

The Group II felsic plutons (granodiorite and K-feldspar granite), located in the

404

two sides of the COM (Fig. 3), gave zircon LA-ICP-MS U–Pb ages of 428 ± 2 Ma

405

and 435 ± 2 Ma, respectively. They also exhibited spatio-temporal characteristics

406

similar to those of the 445–418 Ma magmatic rocks in the Xiemisitai–Saier

407

Mountains (Fig. 11a). Geochemically, the major- and trace-element contents of the

408

Group II felsic plutons and the 445–418 Ma magmatic rocks show clear linear trends

409

in the Harker diagrams (Fig. 9), and similar normalized incompatible element

410

patterns (Fig. 8f), εNd (t) and εHf (t) values (Figs. 10g–h). All these features suggest

411

that the Group II felsic plutons and the 445–418 Ma magmatic rocks of the

412

Xiemisitai–Saier Mountains originated from a similar source and tectonic setting.

413

Compared with the felsic plutons of Group I, the Silurian–Early Devonian magmatic

414

rocks show higher K and less depletion in HFSEs (Nb, Ta, and Ti) (Fig. 8f), as well

415

as different magmatic evolution trends (Fig. 9). Additionally, in the tectonic

416

discrimination diagrams, most of the Silurian–Early Devonian magmatic rocks plot

417

in the post-collisional, syn-collisional, or intraplate regions (Figs. 10c, d).

418

Geologically, (1) the Lower Silurian conglomerate that contains ophiolitic blocks

419

(Yang et al., 2019) unconformably overlies the upper Ordovician strata in the Saier

420

Mountains; (2) the Silurian limestone olistostrome and Devonian terrigenous clastic

421

rocks and molasse overlie Ordovician strata in the Shaerbuerti Mountains (Wei et al.,

422

2009); (3) Early Silurian igneous rocks intrude into the Heukesaier ophiolitic

423

mélange (Du et al., 2019; Yang et al., 2018, 2019). Consequently, we propose that

424

the Silurian–Early Devonian magmatic rocks formed in a post-collisional

425

environment.

426

In

addition,

the

occurrence

of

regional

unconformity

between

the

427

Silurian–Devonian and lower strata in the WJ is interpreted as the result of a

428

pre-Silurian soft collision between the Xiemisitai arc and Southwest Junggar arc

429

(Choulet et al., 2012a). The detrital zircons from the Devonian strata in the Mayile

430

region and Sharburti region (Fig. 2) show similar age spectra, indicating a common

431

source (Choulet et al., 2012b). Despite the lack of valuable paleomagnetic data, the

432

pre-Silurian collision model between the Xiemisitai arc and Southwest Junggar arc

433

provides a reasonable explanation of the above-mentioned data and geological

434

evidences. In addition, it is generally accepted that the joint Kazakhstan–West

435

Junggar Block had formed in the Silurian (Chen et al., 2015; Windley et al., 2007),

436

which also provides a potential setting for the pre-Silurian arc–arc collision in the

437

WJ.

438

6.3.3 Early Paleozoic tectonic evolution

439

The early Paleozoic is an important period for determining the geodynamic

440

processes of the Junggar Ocean. In our study, we identified Cambrian to Silurian

441

igneous rocks from the COM. The 515–513 Ma plagiogranites indicate that the

442

paleo-ocean basin existed during the Early Cambrian, joining the Junggar Ocean and

443

the HBO. Moreover, the 503–481 Ma Group I arc plutons may represent

444

north-dipping intra-oceanic subduction in the NWJ. A large number of Cambrian to

445

Ordovician subduction-related igneous rocks were identified in the SWJ and can be

446

attributed to south-dipping subduction (in today’s direction) of the Junggar Ocean

447

(Ren et al., 2014; Zheng et al., 2019b; Figs. 1b, 8d). Notably, no larger terranes dated

448

Cambrian to the Early Ordovician or older were identified between the Xiemisitai

449

arc and Southwest Junggar arc except for some seamounts (Yang et al., 2015a). Thus,

450

it is very likely that a double subduction system involving these two intra-oceanic

451

arcs was active at that time (Fig. 12a).

452

The oldest ophiolite age implies that the Junggar Ocean may have opened

453

during the Ediacaran (Yang et al., 2012) and continuously subducted until the Early

454

Silurian (Fig. 12b). Notably, the Early Silurian alkaline magmatism is developed and

455

the Late Ordovician strata is overlain uncomfortably by the Early Silurian red

456

molasse in Kazakhstan (Degtyarev and Ryazantsev, 2007; Shen et al., 2015). In the

457

SWJ, Silurian magmatism is uncommon; however, it appears unconformably

458

overlapping outcrops between the Lower Silurian Qiaergaye group and Middle

459

Ordovician Keshayi group (IGSCUG, 2013). The NWJ is dominated by

460

Silurian–Early Devonian alkaline magmatic rock; a clearly angular unconformity

461

occurs between the Early Silurian and Late Ordovician strata in the Saier Mountains

462

(Yang et al., 2019). Moreover, Early Silurian stitching plutons intrude into the

463

Hebukesaier ophiolitic mélange (Du et al., 2019; Yang et al., 2018, 2019). All of

464

these evidences indicate that arc–arc collision occurred between the Southwest

465

Junggar arc and BC arc, and that the Junggar Ocean and HBO may have closed

466

before the Early Silurian (Fig. 12c). However, the model cannot give a reasonable

467

explanation for the 414–391 Ma age of the ophiolites in the CWJ (Gu et al., 2009).

468

Moreover, the Ordovician microfossils in the Karamay and Darbut ophiolitic

469

mélanges cannot be ignored (He et al., 2007; Samygin et al., 1997; Shu et al., 2001).

470

Recently, various models have been proposal to interpret the evolution of the CWJ,

471

such as oceanic-ridge subduction (Yin et al., 2010), and subduction and accretion of

472

seamounts (Yang et al., 2015a). These tectonic models cannot account for the

473

two-stage ophiolitic ages in the region. Rather, the “remnant ocean basin model” (Li

474

et al., 2009) seems to interpret the geodynamic processes in the region more

475

accurately. The two-stage ophiolitic ages in the CWJ demonstrate that the remnant

476

ocean was inactive until the Early Devonian. Even though the Junggar Ocean may

477

not have died out completely in the Early Silurian, the model also provides a

478

reasonable explanation for the arc–arc collision (soft collision, Choulet et al., 2012a).

479

Finally, the remnant ocean may have locally reopened in the Early Devonian (Fig.

480

12d).

481

7. Conclusions

482

(1) The COM consists of serpentinized ultramafic rock, pyroxenolite, gabbro,

483

dolerite, plagiogranite, basalt, volcanic rock, and radiolarian chert. Two

484

plagiogranite samples have zircon U–Pb (SHRIMP) ages of 515 ± 4 Ma and 513 ± 6

485

Ma, constraining the formation date of the oceanic crust.

486

(2) The arc plutons in the COM yielded ages ranging from 503 Ma to 481 Ma,

487

confirming the existence of Middle Cambrian to Early Ordovician intra-oceanic

488

subduction in the NWJ. The BC arc may extend eastward to the NWJ, China and

489

form a double subduction system with the Southwest Junggar arc.

490

(3) The middle Paleozoic Group II felsic plutons (435–428 Ma) in the COM

491

show similar spatio-temporal and geochemical characteristics to those of the

492

Silurian–Early Devonian magmatic rocks in the NWJ, having probably formed in a

493

post-collisional setting.

494

(4) Our results indicate that the WJ underwent a complex process of tectonic

495

evolution from the Cambrian to the Early Devonian, including seafloor spreading,

496

intra-oceanic subduction, arc–arc collision, and post-collisional extension.

497

Acknowledgments

498

We acknowledge the Editors-in-Chief (Prof. Andrew Kerr), and two anonymous

499

reviewers for their detailed and constructive comments that have resulted

500

insignificant improvements in the paper. We are grateful to Su-Mei Zhang for

501

mineral identification, to Shi-wen Xie, Lu Yang and Hong-fang Chen for their

502

assistance in the laboratory analyses. This study was cosponsored by the Fund from

503

the National Natural Science Foundation of China (41572206), the Key Laboratory

504

of Deep-Earth Dynamics of Ministry of Natural Resources (J1901-14) and the China

505

Geological Survey (DD20190358).

506

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English abstract).

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Ocean: evidence from zircon U–Pb geochronology and geochemistry of the

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Mayile ophiolitic mélange in West Junggar, NW China. Lithos 140–141, 53–65.

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tectonic implication. Acta Geologica Sinica 92(2), 298–312 (in Chinese with

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from geochronological, geochemical, and Sr–Nd–Hf isotopic data on alkali

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northern Junggar and comparative study on their connection. Acta Petrologica

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741

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742

Ma) Chagantaolegai ophiolite in northern West Junggar (NW China):

743

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744

megazone. International Geology Review 56, 1181–1196.

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Barleik ophiolite in southern part of western Junggar, Xinjiang. In: International

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748

Karamay, Xinjiang, China, pp. 32–39 (in Chinese).

749

Zheng, R.G., Zhao, L., Yang, Y.Q., 2019a. Geochronology, geochemistry and

750

tectonic implications of a new ophiolitic mélange in the northern West Junggar,

751

NW China. Gondwana Research 74, 237–250.

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Zheng, B., Han, B.F., Liu, B., Wang, Z.Z., 2019b. Ediacaran to Paleozoic

753

magmatism in West Junggar Orogenic Belt, NW China, and implications for

754

evolution of Central Asian Orogenic Belt. Lithos 338–339, 111–127.

755

Zhu, Y.F., Xu, X., 2006. The discovery of Early Ordovician ophiolite mélange in

756

Taerbahatai Mts., Xinjiang, NW China. Acta Petrologica Sinica 23, 1739–1748

757

(in Chinese with English abstract).

758

Zong, R.W., Wang, Z.Z., Gong, Y.M., Wang, G.C., Xiao, L., Wang, Z.H., Fan, R.Y.,

759

2014. Ordovician radiolarians from the Yinisala ophiolitic mélange and their

760

significance in western Junggar, Xinjiang, NW China. Science China: Earth

761

Sciences 58, 776–783.

762

Figure captions

763

Fig. 1. (a) Tectonic framework map of the Central Asian Orogenic Belt (CAOB)

764

(modified after Jahn et al., 2004); (b) Paleozoic geotectonic map of southwest

765

CAOB (modified after Windley et al., 2007).

766

Fig. 2. Regional geology map of the WJ area (modified after Yang et al., 2019);

767

Ophiolitic ages refer to: 1–Zhu and Xu, 2006; 2–Zheng et al., 2019a; 3–Du et al.,

768

2019; 4–Yang YQ et al., 2019; 5–Zhang and Guo, 2010; 6–Zhao and He, 2014;

769

7–Zhao, 2011; 8–Yang GX et al., 2012; 9–Ren et al., 2014; 10–Jian et al., 2005;

770

11–Xu et al., 2006; 12–He et al., 2007; 13–Gu et al., 2009; 14–Samygin et al., 1997;

771

15–Shu et al., 2001.

772

Fig. 3. 1:250,000 geologic map of the southern Xiemisitai Mountains (modified after

773

IGSCUG, 2013). Felsic plutons age data are from: 1–Yang et al., 2015b; 2–Chen et

774

al., 2019; 3–Chen et al., 2010; 4–Wang et al., 2017; 5–this study.

775

Fig. 4. 1:2,000 geological map of the COM.

776

Fig. 5. Outcrop photographs and photomicrographs from ophiolitic rocks and felsic

777

plutons in the COM.

778

(a) Plagiogranite block; (b) granite intruding into chert; (c) coarse-grained granite; (d)

779

diorite pluton in outcrop; (e) K-feldspar granite intruding into volcanic rock; (f)

780

panorama of the COM; (g) diorite specimens; (h) granodiorite pluton into outcrop;

781

(i)–(m) photomicrographs showing the textures and mineral assemblages of felsic

782

rocks corresponding to (a), (c), (g), (e), and (h), respectively. Abbreviations: Hb =

783

hornblende; Kf = K-feldspar; Pl = plagioclase; Q = quartz.

784

Fig. 6. U–Pb concordia diagrams of the dating samples and cathodoluminescence

785

(CL) images of representative zircons in this study.

786

Fig. 7. Geochemical classification of all the rock samples. (a) TAS diagram (after

787

Wilson, 1989); (b) (Na2O+K2O) – CaO vs. SiO2 diagram (after Frost and Frost,

788

2008); (c) FeOT/(FeOT +MgO) vs. SiO2 diagram (after Frost and Frost, 2008); (d)

789

A/CNK vs. A/NK diagram (after Maniar and Piccoli, 1989).

790

Fig. 8. REE and multi-element variation diagrams of all the felsic rocks normalized

791

to chondrite and the primitive mantle, respectively (Sun and McDonough, 1989).

792

Data for island arc magmatic rocks in the Mariana arc are from Tamura et al. (2014);

793

data for 445–418 Ma magmatic rocks in the Xiemisitai–Saier Mountains are from

794

Chen et al. (2019) and references therein.

795

Fig. 9. Harker diagrams for the rock samples

796

Fig. 10. (a) Plot of Y vs. Zr for the oceanic plagiogranites in the COM (after

797

Pedersen and Malpas, 1984); (b) plot of La vs. La/Sm; (c) Ba vs. Sr (after Janoušek

798

et al., 2004); (d) Nb/Yb vs. Th/Yb diagram (Pearce, 2014); (e) Rb vs. (Y + Nb) and

799

(f) Nb vs. Y diagrams for discriminating the tectonic settings (after Pearce, 1996); (g)

800

I(Sr) vs. εNd(t) correlation plot for the rock samples; (h) εHf(t) vs. age plot. Data

801

from: Chen et al. (2015, 2019), Du et al. (2019), Shen et al. (2014, 2015) Yang G et

802

al. (2015b), Yang YQ et al. (2019), Yin et al. (2017a), Zhang et al. (2018).

803

Fig. 11. (a) 1:200,000 geological map of the NWJ; (b) age histogram of early

804

Paleozoic ophiolites and magmatic rocks in the NWJ. Age data of plutons are listed

805

in the Supplementary Table F; Ophiolitic ages are listed in the Table 1.

806

Fig. 12. Tectonic model of the WJ from the Cambrian to the Early Devonian.

807

Abbreviations: BK–Barleik ophiolite; MY–Mayile ophiolite; EM–E'min ophiolite;

808

HB–Hebukesaier ophiolite.

809

Supplementary data

810

Supplementary Table A. SHRIMP U–Pb analytical data for zircons from the

811

Chagantaolegai ophiolite.

812

Supplementary Table B. LA-ICP-MS U–Pb isotopic analysis for zircons from the

813

felsic plutons in the COM.

814

Supplementary Table C. Major (wt.%) and trace element (ppm) contents of the

815

rocks in the COM.

816

Supplementary Table D. Sr–Nd isotope compositions of the felsic plutons from

817

south of the Xiemisitai Mountains.

818

Supplementary Table E. Lu–Hf isotopic data of zircons extracted from felsic

819

plutons from south of the Xiemisitai Mountains.

820

Supplementary Table F. Summary of zircon U–Pb ages of plutons in the NWJ.

Table. 1 The ages of ophiolites in the WJ. Locality

Ophiolite

Rocks

Age

SHRIMP

Anorthosite

475 Ma

Jian et al., 2005

SHRIMP

Cumulate gabbro

472 ± 8 Ma

Zhang and Guo, 2010

LA-ICP-MS

Cumulate gabbro

488 ± 3 Ma

Choulet et al., 2012a

LA-ICP-MS

Metagabbro

484 ± 3 Ma

SHRIMP

Gabbro

512 ± 9 Ma

SHRIMP

Gabbro

505 ± 5 Ma

SHRIMP

Plagiogranite

502 ± 5 Ma

E'min

SHRIMP

gabbro

Kujibai

SHRIMP

Hongguleleng

Du et al., 2019

MOR

Yang YQ et al., 2019

476 ± 2 Ma

SSZ

Zheng et al., 2019a

Altered gabbro

478 ± 3 Ma

MOR

Zhu and Xu, 2006

LA-ICP-MS

Metagabbro

517 ± 3 Ma

LA-ICP-MS

Metagabbro

519 ± 3 Ma

MOR

Zhao and He, 2014

SHRIMP

Plagiogranite

515 ± 4 Ma

SHRIMP

Plagiogranite

513 ± 6 Ma

Darbut Karamay

LA-ICP-MS SHRIMP

Gabbro Metagabbro

391 ± 7 Ma 414 ± 9 Ma

Barleik

LA-ICP-MS LA-ICP-MS

Gabbro Gabbro

512 ± 7 Ma 521 ± 3 Ma

LA-ICP-MS

Gabbro

572 ± 9 Ma

MOR

Yang GX et al., 2012

SIMS

Gabbro

516 ± 5 Ma

SSZ

Ren et al., 2014

LA-ICP-MS

Gabbro

512 ± 7 Ma

LA-ICP-MS

Gabbro

531 ± 12 Ma

SSZ

Weng et al., 2016

Sphene Pb-Pb

Plagiogranite

508 ± 20 Ma

SHRIMP

Gabbro

531 ± 15 Ma

NWJ

Chagantaolegai

SWJ

References

MOR

Hebukesaier

CWJ

Types of ophiolite

Methods

Mayile

Tangbale

This study SSZ SSZ

Gu et al., 2009 Xu et al., 2006 Wen et al., 2016

Xiao et al.,1992 SSZ

Jian et al., 2005

E 20°

N 60°

72°

78°

E 100° E 140° E 60°

N 60°

a

b Other tectonic units

100Km Lat

Russian Craton

Siberian Craton

e

Pro

Fig.1b

ia

40° N Other tectonic units 0

e

Be

lt

N Belt 45°

Orogenic

China

Mongolia

North China-Tarim Craton

E 85°

E 86° Russia

800Km

Mongolia

oz sh ak

48°

ol -C hi ng

Ce

iz

ntr

ar c

al K aza

Kazakhstan O roc lin

og

c ni

B

e

Karaganda

Or

As

Astana

l

Kazakhstan 50 2~ 490Ma Shen et al., 2015

ic

ra

zo

nt

Russia

ter

Ce

West Siberian basin

kha

50 3~ 481Ma This study

Tacheng

Kazakhstan

tan

China

fau

karamay

lt

Ti a

ns

Junggar basin

ha

n

509 ~ 485Ma Xu et al., 2012 Ren et al., 2014

44° Urumqi

Almaty Microcontinent

78°

84° Middle CambrianEarly Silurian

Active continental margin arc Early-Middle Devonian

90° Sedimentary basin

Middle-Late Ordovician

Volcanic arc Cambrian

Late Devonian-Permian

Middle Cambrian- Accretionary wedge and Early Silurian suture zone Devonian-Permian

Late CambrianEarly Ordovician

Middle DevonianMiddle Permian

Early Paleozoic

Late DevonianMiddle Carboniferous

Ordovician-Late Devonian

Middle CarboniferousMiddle Permian

Late Devonian-Carboniferous

Mesozoic-Cenozoic

Accretionary and collisional belt Late Paleozoic

Inferred direction of subduction

Fault National boundary

800

15

a

b

a Fr

200

0 10

0

c

n tio

io

50

100

g tin el

0

200

10

P l fr a c ti o n 0. 3

a ti o n v e c 0. 5

ct

i

r

0. 7

0. 7

15

20

La(ppm)

25

30

d

10 - 1

E-MORB Subduction zone

0. 5

n ve

0. 3

0. 3

10 - 2

ctor

K

fs

fra

n

10

10 0

natio

10 1

on

io at

to e c5 v0.

5

0

1

Oceanic arcs

0. 7

actio

2

Fractional crystallisation

to r

Bt fr

Ba(ppm)

150

Y(ppm)

4

c

m

5

10 3

10

Differentiated-type plagiogranite

al

n

nd

10

rti

at

tre

e

Pa

Anatectic-type plagiogranite

La/Sm

400

lin

Th/Yb

Zr(ppm)

600

MO

RB

ar

ra

y

N-MORB

10 - 3

10 0 0 10

10 1

10 2

10 4

10 3

10 - 2

10 - 1

10 0

Sr(ppm)

10 1

Nb/Yb 10 3

10

3

e

f WPG

syn-COLG WPG

10

Rb(ppm)

Nb(ppm)

10 2

10 1

2

VAG+ syn-COLG 10 1

post-COLG

VAG

ORG 10 0

ORG

10 0 0 10

10 1

10 2

10 - 1 - 1 10

10 3

10 0

10 1

(Y+Nb)ppm 20

g

Silurian-Early Devonian magmatic rocks in the northern West Junggar

eA rra y

0

10

5

CHUR 0

-5 0.702

0.704 87

0.706 86

( Sr/ Sr) i

0.708

h DM

Zircon εHf(t)

ntl

5

10 3

15

Ma

Whole-rock εNd(t)

10

10 2

Y(ppm)

-5 400

Silurian-Early Devonian magmatic rocks in the northern West Junggar




Middle-Late Cambrian magmatic rocks in the central Kazakhstan

CHUR

450

500

U-Pb age(Ma)

550

47°00′N

46°40′N

46°20′N

478

tay

O2

bag

460

480

500

520

C-O Intrusions

Ma

411

S-D 1 Intrusions

440

Ophiolitic age in the northern West Junggar

N=40

Early Paleozoic magmatic rocks in the northern West Junggar

455

E’min

84°E

411 419

434

(14) 429

20km

Faults

10km

Ophiolitic melange

0

440

428

418 429 445

425

436

419

85°E

423 428

429

413

420

436 413 422

402

ts .

414

414

405

410 407

Xiemisitai Fault

420

u r F a u lt

S au r M ts .

C 3 -P 1 Intrusions National border

Tectonic boundary

86°E

Ordovician

Junggar Basin

475 472 488

China

Hebukesaier ophiolitic melange

S a ie r M

435 435

512 502 484

S o u th S a

420 422

428


Chagantaolegai ophiolitic melange

428 452 503 519 517 428 435 515 513 481

Silurian

kas

hier

. Mts

Xi em is ita i M ts .

Devonian

er Wu

429

476

429

ZSOB

Xie mis itai Arc

416

c

Mts.

min Ar

E’min ophiolitic melange

C TO B

.

Ta e r - E

Mts

Kazakhstan

Carboniferous

420

428Ma

Kujibai ophiolitic melange

455

Ta r

N

PermianCenozoic

83°E

400

b

Qoqek

b

0 380

2

4

6

8

10

a

Number

aer

b

Mts

Hongguleleng ophiolitic melange

427

Sh

C 1 Intrusions

420 S-D age of magmatic rock 1

503 C-O age of magmatic rock

512 Ophiolitic
age

412

ti uer

.

(a)
Cambrian

North

Southwest Junggar Arc seamount
fragments

Xiemisitai Arc Junggar Ocean

Future E’min ophiolite (SSZ-type)

BK CT

MY

503Ma diorite

509 ~ 493Ma

Ta

(b)
Early
-Middle
Ordovician

Xiemisitai Arc HBO

Junggar Ocean

EM

( c)
Silurian-Early Devonian

HB

435 Ma rhyolite porphyry




Silurian-Early Devonian (~428Ma) A 2＆I-types granitoides




Early Silurian volcansediments







Early Silurian volcansediments

435~428Ma granitoides

(d)
Early-Middle Devonian





Lower Silurian conglomerate





Hebukesaier ophiolitic melange

Remnant ocean



392Ma Darbut ophiolite





414Ma Keramay ophiolite

O2






Remnant ocean


Early Silurian alkaline magmatism

seamount

481Ma coarse-grained granite

488 ~ 485Ma

Taer-E’min Arc





Chagantaolegai ophiolitic melange

Zaisan ocean

E 83°

E 84° Chin g P a l e o i z - Ta r b a g a zoic Orog tai Early enic Belt Ta e r

N 47°

-Emi

ta

n

Kujibai ophiolite 478Ma(1)

hs ak

E’min

N 46°

Central West

Ba

rl

k ei

Fa

u

lt

Chagantaolegai ophiolite 519-517Ma(6) lt au tF u Junggar erb Da

Mayile ophiolite 572Ma(8);516Ma(9)

N 45°

Aib

N 47°

Hongguleleng ophiolite 473Ma(5)

Xiemisit ai Fault

Fig.3

lt a u Darbut ophiolite eF l i 392Ma(13) y M a















O (14, 15)



S Ju out ng hw ga es rA t rc

ult Hebukesaier ophiolite 484Ma(3);512-502Ma(4)

i Arc

Fig.13 Barleik ophiolite 512Ma(7)

r Fa

E 86°

Northern West Junggar

X ie m is it a

az K

n Arc

E’min ophiolite 476Ma(2)

Qoqek

E 85° Zhar m Pale a ozoi -Sawur c Or L ogen ate Sou ic Be th S lt au

China Karamay ophiolite 414Ma(11)





O 2 - 3 (12)

N 46°

Junggar Basin

Karamay 0





Southern West Junggar i L ak e

Tangbale ophiolite 531Ma(10)

E 83°

E 84°

50

100Km

Cenozoic and Mesozoic

Ordovician

Ultramafic rocks

Carboniferous

C 3 -P 1 Intrusions

Faults

Devonian

C 1 Intrusions

Tectonic boundary

S-D 1 Intrusions

National border

Silurian

E 85°

E 86°

429Ma (1)

428Ma (1) 46°36′N

440Ma (2) 445Ma (2)

XMST18-47

46°32′N

XMST18-42 503Ma (5) Fig.4

XMST18-48

0

2

4Km 85°03′E

85°09′E

435Ma (5)

428Ma (5)

420Ma (3) 452Ma (4)

XMST18-49 481Ma (5)

Chagantaolegai ophiolitic melange 85°15′E

MesozoicCenozoic

Junggar Basin 85°21′E

Granite

D-C

Diorite

S1-4

Ultramafic rock

Carbonate rocks

Fault

Chert

Tectonic boundary

85°27′E

85°11′45″E

85°12′25″E

S1-4 O 1 - 2 radiolarian Zong et al., 2014

*

N

503±2Ma XMST18-42 513±6Ma XMST18-45

46°34′00″N

Fig.5f

515±4Ma XMST13-11

* *

* *

C-D

CarboniferousDevonian

Basalt

Diorite/ Granite

S1-4

Early-Late Silurian

Plagiogranite

K-feldspar granite

Carbonate rocks

Dolerite/ Gabbro

Fault

Chert

Ultramafic block

Tectonic boundary

Volcanic rock

Melange matrix

*

435±2Ma XMST18-49

Sample/ radiolarian

C-D

46°33′40″N

481±3Ma XMST18-48

517～519Ma Zhao and He, 2014

* 200m

a

c

b Plagiogranite

Coarse-grained granite

Chert

d Diorite

Granite

e

f

g

300°

Volcanic rock Metaperidotite Diorite K-feldspar granite

h

Gabbro Chert

Q

Silurian granite Ultramafic matrix

Granodiorite

Pl

Pl

Gabbro Pl

j

k Hb

Hb

Q

Pl

Kf Kf

Pl

Pl

Q Hb

Pl Q

Hb 2mm

Q Q 2mm

0.5mm

m

l

Pl

Q

Q

Q 2mm

Kf

2mm

0.087

a

0.074

530

Diorite Mean=503±2
Ma n=28, MSWD=1.04

0.085

b

XMST18-42

0.072

520

XMST18-47 Granodiorite Mean=428±2 Ma n=26, MSWD=0.98

450

0.083 0.070

Pb/238U

507±6Ma

500

206

0.081

206

Pb/238U

510

+14.37

490

0.079

430 0.068

430±5Ma

520

442

+10.98 510

480

100μm

0.077

434

0.066

500

100μm410

426

490

470 0.075 0.50

0.54

0.58

0.62 207

0.083

c

0.66

0.70

410

0.064 0.46

0.74

0.48

0.50

0.52

Pb/235U

207

0.075

XMST18-48

0.56

0.58

0.60

Pb/235U

470

d

500 0.073

460

K-feldspar granite Mean=435±2 Ma n=26, MSWD=1.2

450

0.079

Pb/238U

480 0.077

0.071

440 434±4Ma

206

479±5Ma

206

Pb/238U

0.54

XMST18-49

Coarse-grained granite Mean=481±3
Ma n=22, MSWD=1.4

0.081

418

480

455

430

0.069

500

+13.68

0.075

+12.33

445

490

460 100μm

480

100μm

0.067

0.073

420

435

470

425

410

460

0.071 0.52

0.56

0.60 207

0.64

0.68

0.065 0.46

0.72

0.50

415

0.54

Pb/235U

207

0.58

0.62

0.66

Pb/235U

0.092

e XMST18-45 0.090

f

560

Plagiogranite Mean=513±6
Ma n=12, MSWD=1.2

560

XMST13-11 Plagiogranite Mean=515±4
Ma n=10, MSWD=0.51

0.088

540

540

Pb/238U

520

514±10Ma

0.082

520

0.084

519±6Ma

206

206

Pb/238U

0.086

500

0.080

500

540

524

480

520

0.078

100μm

0.076

480

508

100μm

500

516

460

500

480

0.074

0.072 0.5

0.6

0.7 207

235

Pb/ U

0.8

0.35

0.45

0.55 207

Pb/235U

0.65

0.75

0.85

Ultrabasic

Basic

a

b

nepheline syenite

12

Na 2 O+K 2 O(wt%)

12

Acid

8

(Na 2 O+K 2 O)-CaO(wt%)

14

Intermediate

syenite syenite

10

alkali granite 8 ijolite 6 4

syeno-diorites alic Alk gabbro lic ka -al b gabbro Su diorite

quartz diorite

granite

(granodiorite)

4

alkalic

0

alk

ca

-4

alc

ic

al lc-

ka

-c ali

calcic

gabbro

2 0 40

50

lic

60

-8 50

70

60

SiO 2 (wt%) 1.0

3

c

I-type

S-type

Ferroan

Peraluminous

2

Metaluminous

A/NK

0.8

0.7

0.6

80

d

0.9

FeO T /(FeO T +MgO)

70

SiO 2 (wt%)

XMST13-11 XMST18-45 XMST18-42 XMST18-48 XMST18-47 XMST18-49

1

Magnesian

Peralkaline

0.5

0.4 50

60

70

SiO 2 (wt%)

80

0 0.5

445-418Ma granitoids in northern West Junggar (data from references)

0.7

0.9

1.1

1.3

A/CNK

1.5

1.7

1.9

10 2

10 3

Rock/Primitive
Mantle

Rock/Chondrite

a

10 1

b Plagiogranite of Hebukesaier ophiolite

10 2

10 1

10 0 Yang et al. 2019

10 0

Pr Nd

10 - 1

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

CsRbBa Th U Nb Ta K LaCe PbPr Sr P Nd Zr SmEu Ti Dy Y Yb Lu 10 3

c Rock/Primitive
Mantle

Rock/Chondrite

10

La Ce

2

10 1

d

10 2

10 1

10 0 Shen et al. 2014 Ren et al. 2014

Rock/Chondrite

10 2

La Ce

Pr Nd

CsRbBa Th U Nb Ta K LaCe PbPr Sr P Nd Zr SmEu Ti Dy Y Yb Lu 10 3

e

10 1

10 0

10 - 1

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Rock/Primitive
Mantle

10 0

La Ce

Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Mariana Arc IAB

f

10 2

10 1

10 0 445-418Ma magmas in the Xemisitai Mts. and Saier Mts. from other studies 10 - 1 CsRbBa Th U Nb Ta K LaCe PbPr Sr P Nd Zr SmEu Ti Dy Y Yb Lu

1.0

25

a

12

b

c

10

0.8

y=-0.394x+33.352 R 2 =0.99

Al 2 O 3 (wt%)

TiO 2 (wt%)

y=-0.0439x+3.4434 R 2 =0.96

0.6

0.4

Fe 2 O 3 T (wt%)

20

15

6 y=-0.3981x+31.089 R 2 =0.98 4

y=-0.0077x+0.8395 R 2 =0.88

10

0.2

2

0 50

60

70

5 50

80

60

SiO 2 (wt%) 6

8

70

80

50

60

SiO 2 (wt%) 8

d

70

80

SiO 2 (wt%) 0.5

e

f

yp S-t

0.4

e tr

end

6

y=-0.1889x+15.386 R 2 =0.99

y=-0.2532x+20.824 R 2 =0.97

4 y=-0.2834x+22.203 R 2 =0.84

y=-0.2527x+19.064 R 2 =0.95

2

P 2 O 5 (wt%)

CaO(wt%)

MgO(wt%)

4 0.3

y=-0.0167x+1.2921 R 2 =0.97

0.2

I-ty

pe

tre

nd

2 0.1 y=-0.002x+0.2078 R 2 =0.92 0 50

60

70

0

0 50

80

60

SiO 2 (wt%) 6

70

80

50

60

SiO 2 (wt%) 30

g

5

70

h

25

250

20

200

La(ppm)

K 2 O(wt%)

High-K 3

Medium-K

2

1

V(ppm)

Shoshonitic 4

80

SiO 2 (wt%) 300

15

y=-13.723x+1081 R 2 =0.99

150

10

100

5

50

y=-10.344x+780.68 R 2 =0.95

Low-K 0 50

60

70

SiO 2 (wt%)

80

0 50

0 60

SiO 2 (wt%)

70

80

50

60

70

SiO 2 (wt%)

80

Highlights
Three rock units of different settings were identified from the COM of the NWJ.
New ages for Chagantaolegai ophiolitic plagiogranites are 515 ± 4 Ma and 513 ± 6 Ma.
Discovery of 503–481 Ma arc plutons constrains the timing of intra-oceanic subduction in the NWJ.
Arc–arc collision may have occurred before the Early Silurian in the NWJ.