Timing of two separate granulite-facies metamorphic events in the Helanshan complex, North China Craton: Constraints from monazite and zircon U–Pb dating of pelitic granulites

Journal Pre-proof Timing of two separate granulite-facies metamorphic events in the Helanshan complex, North China Craton: constraints from monazite and zircon U–Pb dating of pelitic granulites Longlong Gou, Jian-Wei Zi, Yunpeng Dong, Xiaoming Liu, Zhenghui Li, Xiaofei Xu, Chengli Zhang, Liang Liu, Xiaoping Long, Yuhang Zhao PII:

S0024-4937(19)30375-5

DOI:

https://doi.org/10.1016/j.lithos.2019.105216

Reference:

LITHOS 105216

To appear in:

LITHOS

Received Date: 7 June 2019 Revised Date:

17 September 2019

Accepted Date: 19 September 2019

Please cite this article as: Gou, L., Zi, J.-W., Dong, Y., Liu, X., Li, Z., Xu, X., Zhang, C., Liu, L., Long, X., Zhao, Y., Timing of two separate granulite-facies metamorphic events in the Helanshan complex, North China Craton: constraints from monazite and zircon U–Pb dating of pelitic granulites, LITHOS, https:// doi.org/10.1016/j.lithos.2019.105216. 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.

This study presents the results of monazite and zircon U–Pb dating of garnet–biotite gneiss and recently discovered spinel-bearing ultrahigh-temperature (UHT) pelitic granulites from the Paleoproterozoic Helanshan complex of the North China Craton, and considers implications for the formation of the complex and the tectonic history of this region. SHRIMP monazite U–Pb dating of the garnet–biotite gneiss yielded a weighted mean 207Pb/206Pb age of 1944.4 ± 4.2 Ma, which is the same within error as a weighted mean laser ablation–inductively coupled plasma–mass spectrometry (LA– ICP–MS) 207Pb/206Pb age of 1959 ± 18 Ma obtained from metamorphic zircons within the same unit. The age of 1959 ± 18 Ma is similar to the published metamorphic ages of ca. 1.96–1.95 Ga from high-temperature (Tmax < 900 °C) pelitic granulites in the Helanshan, which represent the timing of continental collision between the Yinshan and Ordos terranes as suggested by previous published works. Metamorphic monazites from the spinel-bearing UHT pelitic granulite unit in this area yielded weighted mean 207Pb/206Pb ages of 1930.8 ± 2.6 and 1933.6 ± 3.1 Ma, which are consistent with the ages of metamorphic zircons. These ages record the timing of UHT metamorphism within the Helanshan complex and are similar to 1.93–1.92 Ga ages previously reported from typical UHT pelitic granulites within the Jining and Daqingshan complexes of the Khondalite Belt, where ca. 1.92 Ga metamorphic zircons are thought to record the cooling of the UHT rocks to the solidus. The new data presented here indicate that the ca. 1.93 Ga UHT metamorphic event occurred across a wider area than previously thought and extended into the Helanshan complex. Combining these new data with the results of previous research suggests that the entire Khondalite Belt was influenced by a ca. 1.93 Ga UHT metamorphic event that was preceded by metamorphism associated with continental collision between the Yinshan and Ordos terranes at ca. 1.95 Ga. This suggests that the Khondalite Belt underwent a tectonic transition from compression to extension between 1.95 and 1.93 Ga, a process likely controlled by a late-stage shallow slab-breakoff event.

1 2 3

Timing of two separate granulite-facies metamorphic events in

4

the Helanshan complex, North China Craton: constraints from

5

monazite and zircon U–Pb dating of pelitic granulites

6 7

Longlong Goua*, Jian-Wei Zib,c, Yunpeng Donga, Xiaoming Liua, Zhenghui Lia,

8

Xiaofei Xua, Chengli Zhanga, Liang Liua, Xiaoping Longa, Yuhang Zhaoa

9 10 11

a

12

University, Xi’an 710069, China

13

b

14

Geosciences, Wuhan 430074, China

15

c

State Key Laboratory of Continental Dynamics, Department of Geology, Northwest

State Key Laboratory of Geological Processes and Mineral Resources, University of

John de Laeter Centre, Curtin University, Bentley, WA 6102, Australia

16 17

(*corresponding author: [email protected])

18 19

1 / 42

20

Abstract:

21

This study presents the results of monazite and zircon U–Pb dating of garnet–biotite

22

gneiss and recently discovered spinel-bearing ultrahigh-temperature (UHT) pelitic

23

granulites from the Paleoproterozoic Helanshan complex of the North China Craton,

24

and considers implications for the formation of the complex and the tectonic history

25

of this region. SHRIMP monazite U–Pb dating of the garnet–biotite gneiss yielded a

26

weighted mean 207Pb/206Pb age of 1944.4 ± 4.2 Ma, which is the same within error as

27

a weighted mean laser ablation–inductively coupled plasma–mass spectrometry (LA–

28

ICP–MS) 207Pb/206Pb age of 1959 ± 18 Ma obtained from metamorphic zircons within

29

the same unit. The age of 1959 ± 18 Ma is similar to the published metamorphic ages

30

of ca. 1.96–1.95 Ga from high-temperature (Tmax < 900 °C) pelitic granulites in the

31

Helanshan, which represent the timing of continental collision between the Yinshan

32

and Ordos terranes as suggested by previous published works. Metamorphic

33

monazites from the spinel-bearing UHT pelitic granulite unit in this area yielded

34

weighted mean 207Pb/206Pb ages of 1930.8 ± 2.6 and 1933.6 ± 3.1 Ma, which are

35

consistent with the ages of metamorphic zircons. These ages record the timing of

36

UHT metamorphism within the Helanshan complex and are similar to 1.93–1.92 Ga

37

ages previously reported from typical UHT pelitic granulites within the Jining and

38

Daqingshan complexes of the Khondalite Belt, where ca. 1.92 Ga metamorphic

39

zircons are thought to record the cooling of the UHT rocks to the solidus. The new

40

data presented here indicate that the ca. 1.93 Ga UHT metamorphic event occurred

41

across a wider area than previously thought and extended into the Helanshan complex. 2 / 42

42

Combining these new data with the results of previous research suggests that the

43

entire Khondalite Belt was influenced by a ca. 1.93 Ga UHT metamorphic event that

44

was preceded by metamorphism associated with continental collision between the

45

Yinshan and Ordos terranes at ca. 1.95 Ga. This suggests that the Khondalite Belt

46

underwent a tectonic transition from compression to extension between 1.95 and 1.93

47

Ga, a process likely controlled by a late-stage shallow slab-breakoff event.

48

Keywords: SHRIMP monazite U–Pb dating, Zircon geochronology, UHT

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metamorphism, Khondalite Belt, North China Craton

50

1. Introduction

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The North China Craton (NCC) is composed of multiple terranes or micro-continental

52

blocks that were amalgamated during the Paleoproterozoic (Zhao et al., 2005, 2012;

53

Zhai and Santosh, 2011; Zhao and Zhai, 2013; Peng et al., 2014; Wei et al., 2014;

54

Kusky et al., 2016). The oldest crustal remnants within the NCC formed at around 3.8

55

Ga (Liu et al., 1992), indicating that the craton records a long history of early

56

Precambrian continental crustal evolution (Guo et al., 2002; Zhai, 2009, 2014; Peng et

57

al., 2010; Wan et al., 2011; Zhai and Santosh, 2011; Zhang et al., 2015; Wu et al.,

58

2016, 2017; Zhou et al., 2017), as well as the processes involved in Paleoproterozoic

59

continental collisional events (Liu et al., 2006; Zhao et al., 2008, 2012, 2017; Santosh

60

et al., 2009a; Qian et al., 2013; Zhang et al., 2018; Wang et al., 2018).

61 62

The Khondalite Belt is one of several Paleoproterozoic collisional belts within the

63

NCC, and the eastern end of this belt contains sapphirine-bearing 3 / 42

64

ultrahigh-temperature (UHT) granulites (Guo et al., 2006, 2012; Santosh et al., 2007a).

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Research over the past decade has furthered our understanding of these types of rocks,

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and quantifying the pressure–temperature–time (P–T–t) evolution of these units has

67

provided unique insights into the geodynamic implications of UHT metamorphism

68

during the Paleoproterozoic (Guo et al., 2006, 2012; Santosh et al., 2007a, b, 2009a, b,

69

2013; Liu et al., 2010, 2011, 2012; Jiao and Guo, 2011; Jiao et al., 2011, 2015, 2017;

70

Tsunogae et al., 2011; Shimizu et al., 2013; Gou et al., 2014, 2015; Li and Wei, 2016,

71

2017; Zhou et al., 2017; Lobjoie et al., 2018). Two periods of metamorphism at 1.93–

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1.92 Ga (Santosh et al., 2007b, 2009b, 2013; Li and Wei, 2017; Lobjoie et al., 2018)

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and 1.88–1.85 Ga (Yang et al., 2014; Jiao et al., 2015, 2017) have been reported for

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the UHT granulites within the Daqingshan and Jining complexes. However, it remains

75

controversial regarding interpretation of these metamorphic ages. For example, Yang

76

et al. (2014) suggested that these two periods represent separate UHT metamorphic

77

events, whereas Li and Wei (2017) argued that these dates potentially represent a

78

prolonged single-stage UHT event at ca. 1.93 Ga with the younger ages from these

79

units recording post-peak cooling. This unclarity means that additional

80

geochronological data for UHT pelitic granulites from other localities within the

81

Khondalite Belt are needed to clarify the geological implications of these previously

82

determined metamorphic ages.

83 84

Recently, Gou et al., (2018) identified spinel-bearing pelitic granulites that record

85

UHT metamorphic conditions within the Helanshan complex at the western end of the 4 / 42

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Khondalite Belt. The present study reports the results of a combined monazite and

87

zircon U–Pb dating study that focuses on garnet–biotite gneiss and UHT pelitic

88

granulite units within this complex and uses the resulting data to constrain the timing

89

and tectonic implications of UHT metamorphism within this area, which is distal from

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the Daqingshan and Jining complexes elsewhere within the Khondalite Belt. These

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offer insights into whether the UHT metamorphism recorded by granulites in the

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Helanshan complex was simultaneous with the UHT metamorphism within the

93

eastern Khondalite Belt, on which basis we draw the geodynamic implications of this

94

metamorphism for Paleoproterozoic continental collisional orogenesis.

95

2. Geological setting

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The NCC is divided into the Western and Eastern Blocks along the Trans-North China

97

Orogen (TNCO; Fig. 1a; Zhao et al., 2005, 2012), with the Western Block consisting

98

of the Yinshan and Ordos terranes and the Khondalite Belt (Fig. 1a; Kusky and Li,

99

2003; Zhao et al., 2005, 2012). Zircon U–Pb dating has enabled the identification of

100

Neoarchean rocks that underwent metamorphism at ca. 2.5 Ga within the Yinshan

101

Terrane (Jian et al., 2012; Zhang et al., 2013; Chen et al., 2017a). In addition, the

102

Ordos Terrane is thought to be an Archean micro-terrane, as evidenced by zircons

103

within ∼2.5 Ga granitoids intercepted by drilling in this area, which show positive

104

εHf(t) values of 0.1–4.1 with a peak mantle extraction model age (TDM) of ∼2.7 Ga

105

(Zhang et al., 2015). However, this view is disputed by Wan et al. (2013a), who

106

questioned the prevailing view that the Ordos basement is an Archean cratonic block

107

on the basis of a lack of Archean components during sensitive high-resolution ion 5 / 42

108

microprobe (SHRIMP) zircon U–Pb dating of samples obtained by drilling in this

109

area.

110 111

The Khondalite Belt is a collisional belt that was formed by southward subduction of

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oceanic crust under the Ordos Terrane and the final collision between the Yinshan and

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Ordos terranes (Zhao et al., 2005; Gou et al., 2016, 2018). The collision between the

114

Yinshan and Ordos terranes occurred at ca. 1.96–1.95 Ga, as indicated by the ages of

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metamorphic zircons within pelitic granulites in the Khondalite Belt (Zhao et al., 2005,

116

2012; Yin et al., 2009, 2011; Dong et al., 2013). Previous research has identified both

117

high-pressure (HP) and medium-pressure (MP) granulite-facies rocks in this area

118

(Zhao et al., 2005, 2012; Li et al., 2011; Cai et al., 2014; Yin et al., 2014), as well as

119

sapphirine-bearing UHT metamorphic rocks (Guo et al., 2006, 2012; Santosh et al.,

120

2007a). The MP granulite-facies rocks are dominated in the belt (Zhai, 2009; Zhao

121

and Zhai, 2013), whereas the HP granulite-facies rocks have been identified only

122

within the Helanshan and Qianlishan complexes (Fig. 1a; Zhou et al., 2010; Yin et al.,

123

2014, 2015). The sapphirine-bearing UHT rocks have been identified only within the

124

Jining and Daqingshan complexes, in the eastern Khondalite Belt (Guo et al., 2006,

125

2012; Santosh et al., 2007a). Detrital zircon U–Pb ages and Hf–O isotopic data

126

suggest that the protoliths for khondalite series may have been deposited in either an

127

active continental margin or a back-arc tectonic setting (Wan et al., 2009; Dan et al.,

128

2012). Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS)

129

and SHRIMP zircon U–Pb dating showed that the age of metamorphism was broadly 6 / 42

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1.96–1.83 Ga (Wan et al., 2006; Santosh et al., 2007b, 2009b, 2013; Yin et al., 2009,

131

2011; Dan et al., 2012; Jiao et al., 2013, 2015, 2017; Xu et al., 2018). The

132

metamorphic ages and pressure–temperature (P–T) evolution of pelitic granulites

133

within northwestern Ordos basin are identical to the ages and P–T paths of pelitic

134

granulites within the Khondalite Belt, suggesting that the Khondalite Belt extended

135

further south than previously thought and is present within the Ordos Terrane (Fig. 1a;

136

Wang et al., 2014, 2017, 2019; Gou et al., 2016).

137 138

The Helanshan complex is located within the western Khondalite Belt (Fig. 1a) and

139

contains khondalites and garnet-bearing S-type granites (Yin et al., 2011; Dan et al.,

140

2012, 2014). The khondalites within the complex have previously referred to as the

141

Helanshan Group, which was further split into the Liushugou, Alenghuduge and

142

Tulugen formations (Fig. 1b; Yin et al., 2011). LA–ICP–MS zircon U–Pb dating has

143

yielded two metamorphic age groups for the pelitic granulites within the complex, one

144

at ca. 1.95 Ga and the other at ca. 1.87 Ga, with the former being considered to reflect

145

the timing of collision between the Yinshan and Ordos terranes (Yin et al., 2011). The

146

HP pelitic granulites from the Helanshan complex are composed of garnet, sillimanite,

147

cordierite, perthite and quartz, with minor amounts of biotite, plagioclase, kyanite and

148

muscovite (Zhou et al., 2010; Yin et al., 2015). The four metamorphic mineral

149

assemblages identified within these granulites define a clockwise P–T path with an

150

isothermal decompression (ITD) segment based on the phase equilibrium modelling

151

(Zhou et al., 2010; Yin et al., 2015). The MP pelitic granulites also record a similar 7 / 42

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clockwise P–T path with the style similar to that of HP pelitic granulites, although the

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former underwent lower-pressure conditions during Pmax (Xu et al., 2018), suggesting

154

that all of the granulites within this region formed as a result of a single geological

155

event that caused their protoliths to be buried at different depths.

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3. Sample descriptions

157

A garnet–biotite gneiss (sample HL1527; Fig. 2a, b) and two UHT spinel-bearing

158

pelitic granulite (samples HL1501-3 and HL1502-3; Fig. 2c) were selected for

159

monazite and zircon U–Pb dating in this study. The garnet–biotite gneiss in the study

160

area has a clear boundary with the adjacent garnet–sillimanite–cordierite gneiss (Fig.

161

2a) and consists of garnet (5%), biotite (10%), plagioclase (15%), K-feldspar (40%)

162

and quartz (30%) (Figs. 2a, b, 3a, b), with accessory zircon and monazite. The garnet–

163

biotite gneiss is texturally similar to the garnet–sillimanite–cordierite gneiss in the

164

study area (Fig. 2a), suggesting that they record similar metamorphic conditions but

165

have different whole-rock compositions. The UHT spinel-bearing pelitic granulites

166

were collected from large granulite lens (0.5–2 m in diameter) within a garnet-bearing

167

S-type granite (Fig. 2c), and is composed of garnet, cordierite, sillimanite, plagioclase,

168

K-feldspar, and quartz with minor amounts of biotite, spinel, and diaspore (Fig. 3c, d)

169

and accessory rutile, ilmenite, zircon and monazite. Biotite is a minor phase in sample

170

HL1501-3 but is relatively abundant in sample HL1502-3. Most of the garnet within

171

these samples has been replaced by cordierite, and the relict garnet remaining contains

172

sillimanite, spinel and quartz inclusions. Sillimanite is present in the matrix or as

173

inclusions within garnet, cordierite, and quartz (Fig. 3c, d), whereas spinel is present 8 / 42

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in the matrix or as inclusions in garnet, sillimanite, plagioclase and cordierite (Fig. 3c),

175

or as rims around diaspore and chlorite. Petrological and phase equilibrium modelling

176

indicates that the peak metamorphic of garnet + K-feldspar + sillimanite + spinel +

177

ilmenite + quartz + melt phase assemblage within these samples formed at a T of

178

∼960–1030 ºC and a P of 6.3–7.3 kbar, with a very steep geothermal gradient that

179

extends well into the UHT field (Gou et al., 2018).

180

4. Analytical results

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4.1. SHRIMP monazite U–Pb ages

182

4.1.1. Sample HL1527

183

Monazite grains from this sample are nearly rounded in shape with a size from 70 to

184

120 µm, and are structurally homogeneous structure (Fig. 4). A total of twenty spot

185

analyses were conducted on twenty monazites during this study, three of which were

186

significantly (≥15%) discordant and hence were disregarded in age determinations

187

(Table S1). The remaining seventeen analyses are all concordant (disc. ≤5%; Table S1)

188

and yielded relatively limited variation of U and Th concentrations, ranging from

189

1060 to 2600 ppm and 29000 to 36000 ppm, respectively, with Th/U ratios from 13–

190

33 (Table S1; Fig. 5). Spot 4 is a statistical outlier, if it is excluded, the remaining 16

191

analyses yielded radiogenic 207Pb/206Pb ratios ranging from 0.1185 to 0.1207, with a

192

weighted mean of 0.11921 ± 0.00028, equivalent to a 207Pb/206Pb age of 1944.4 ± 4.2

193

Ma (MSWD = 1.6) (Fig. 6a, b).

9 / 42

194

4.1.2. Sample HL1501-3

195

This sample contains equant monazites that range in size from 80 to 120 µm (Fig. 4).

196

The majority of these monazites show a core–rim structure characterized by

197

homogeneous cores surrounded by less luminescent rims (Fig. 4). A total of

198

twenty-four spot analyses were obtained from this sample, among them six are from

199

the darker rims of composite monazites. Monazite cores and rims have overlapping U

200

(4300–9900 ppm) and Th (33000–94000 ppm) concentrations that yield Th/U ratios

201

of 4–14 (Table S1; Fig. 5). All of the monazites from this sample contain <0.1%

202

common 206Pb, but two of these analyses are significantly (>15%) discordant and

203

were therefore not used in age determinations. The remaining twenty-two analyses are

204

all concordant (disc. <5%; Table S1; Fig. 6c). Sixteen analyses of homogeneous

205

monazites or the cores of composite monazites yielded radiogenic 207Pb/206Pb ratios

206

ranging from 0.1165 to 0.1189, with a weighted mean value of 0.11828 ± 0.00024,

207

which is equivalent to a 207Pb/206Pb age of 1927.4 ± 3.7 Ma (MSWD = 1.9). Spot 16.1

208

is a statistical outlier that if excluded changes the weighted mean 207Pb/206Pb age to

209

1930.3 ± 3.6 Ma (Fig. 6c) and improves the MSWD to 1.2. The six analyses of

210

monazite rims yielded a weighted mean 207Pb/206Pb age of 1932.5 ± 5.2 Ma (MSWD

211

= 0.28; Fig. 6c). Since the results from both cores and rims of the monazite from this

212

sample are indistinguishable within analytical uncertainties, they are combined to give

213

a weighted mean 207Pb/206Pb age of 1930.8 ± 2.6 Ma (MSWD = 0.97; Fig. 6d), which

214

is interpreted as the best estimate of the timing of monazite growth.

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4.1.3. Sample HL1502-3

216

Monazites within sample HL1502-3 have a similar crystal habit to that of the

217

monazites within sample HL1501-3, with most have core–rim textures (Fig. 4). All of

218

the analyses of monazite from this sample yielded low common 206Pb values (f206

219

<0.1%) and are concordant (disc. <5%). A total of fifteen spot analyses of the

220

homogeneous monazites and the cores of composite monazites yielded variable U

221

(3100–23800 ppm) and Th (27900–76200 ppm) contents with Th/U ratios of 1–25

222

(Table S1; Fig. 5) and a weighted mean radiogenic 207Pb/206Pb age of 1934.5 ± 3.4 Ma

223

(n = 15, MSWD = 1.5; Fig. 6e).

224 225

A further five spot analyses of monazite rims within this sample yielded similar U and

226

Th concentrations to those of the cores and homogeneous monazites within this

227

sample (Table S1; Fig. 5) and a weighted mean 207Pb/206Pb age of 1928.9 ± 6.0 Ma (n

228

= 5, MSWD = 1.1; Fig. 6e). This age is indistinguishable within errors from that of

229

the homogeneous monazites and the cores of composite monazites within this sample.

230

Thus, the two data groups can be pooled together to give a weighted mean 207Pb/206Pb

231

age of 1933.6 ± 3.1 Ma (n = 20, MSWD = 1.5; Fig. 6f), interpreted as age of the

232

monazite growth event.

233

4.2. LA–ICP–MS zircon U–Pb age for sample HL1527

234

The zircons within this sample are rounded and range in size from 80 to 140 µm (Fig.

235

7). The majority of zircons within this sample appear to have patchy zoning or are

236

structureless under cathodoluminescence (CL) imaging (Fig. 7), indicating they are 11 / 42

237

metamorphic, although a few have core–rim structures, with igneous zircon cores and

238

metamorphic rims. All of the zircon analyses undertaken during this study were

239

performed on homogeneous metamorphic zircon grains. A total of twenty-two spot

240

analyses on twenty-two zircon grains yielded variable but high concentrations of U

241

(290–590 ppm), but low concentrations of Th (10–710 ppm), and Th/U ratios of 0.04–

242

1.87 (Table S2). Removing four discordant analyses left eighteen spots that yielded a

243

weighted mean 207Pb/206Pb age of 1959 ± 18 Ma (n = 18, MSWD = 0.43; Fig. 8),

244

which are considered as the timing of metamorphism of the hosting garnet–biotite

245

gneiss.

246

4.3. SHRIMP zircon U–Pb ages for samples HL1501-3 and HL1502-3

247

4.3.1. Sample HL1501-3

248

Zircons within HL1501-3 are rounded in shape and range in size from 50 to 70 µm.

249

Two types of zircon are present within the sample, as evidenced by variations in

250

internal textures visible during CL imaging (Fig. 7). The first type of zircon is

251

characterized by core–rim textures where the cores show blurred zoning, and the rims

252

are patchy or structureless (Fig. 7). The second type of zircon is homogeneous and is

253

either structureless or shows a patchy structure (Fig. 7), both of which are indicative

254

of a metamorphic origin. A total of sixteen spot analyses on sixteen zircons yielded

255

high concentrations of U (410–3540 ppm) and low concentrations of Th (20–110

256

ppm), with Th/U ratios in the range of 0.01–0.07 (Table S3). All analyses have f206

257

values of <1%. Post-SHRIMP imaging indicates that spots 9.1, 10.1 and 15.1

258

analyzed more than one age domain and were therefore disregarded. The remaining 12 / 42

259

thirteen analyses yielded generally discordant (disc. >5%, mostly >10%) data except a

260

single analysis that yielded a concordant 207Pb/206Pb age of 1932 ± 6 Ma (1σ). Plotting

261

these analyses on a Tera–Wasserburg concordia diagram yielded a discordia line that

262

intercepts the concordia curve at 1937 ± 30 Ma (Fig. 9a), suggesting that these zircons

263

formed at ca. 1.93 Ga.

264

4.3.2. Sample HL1502-3

265

Zircon grains from sample HL1502-3 range from 70 to 110 µm in size (Fig. 7), and

266

CL imaging indicates that the majority have core–rim structures (Fig. 7). These

267

structures consist of cores that are clear and have blurred oscillatory zoning (Fig. 7),

268

indicative of a magmatic origin and which are surrounded by structureless

269

metamorphic rims (Fig. 7). A few of zircons within the sample also appear entirely

270

homogeneous during CL imaging (Fig. 7), suggesting that they have a metamorphic

271

origin. Analyses 3.1, 6.1, 12.1 and 13.1 were conducted on metamorphic overgrowth

272

rims and homogeneous metamorphic zircons, yielding apparently discordant ages (Fig.

273

9b). The remaining eleven analyses were carried out on magmatic zircon cores and

274

yielded variable U (99–2225 ppm) and Th (1–647 ppm) concentrations with Th/U

275

ratios of <0.001–1.33 (Table S3). The four most concordant analyses yielded a

276

weighted mean 207Pb/206Pb age of 1997 ± 43 Ma (MSWD = 1.9), whereas the

277

discordant analyses define a discordia with an intercept age of 2064 ± 96 Ma (Fig. 9b),

278

which is considered as the timing of formation of inherited detrital magmatic zircons

279

within the pelitic protolith of the granulite.

13 / 42

280

5. Discussion

281

5.1. Timing of the granulite-facies metamorphism in the Helanshan complex

282

SHRIMP monazite U–Pb dating of a garnet–biotite gneiss (sample HL1527) during

283

this study yielded a weighted mean 207Pb/206Pb age of 1944.4 ± 4.2 Ma (Fig. 6a, b),

284

which is the same within uncertainty as the weighted mean LA–ICP–MS

285

metamorphic zircon 207Pb/206Pb age of 1959 ± 18 Ma. Previously published

286

metamorphic zircon U–Pb LA–ICP–MS ages from nearby high-temperature (HT, Tmax

287

< 900 °C) granulite-facies garnet–biotite and garnet–sillimanite–cordierite gneisses

288

range from ca. 1960 to 1950 Ma (Fig. 1b; Yin et al., 2011, 2015). This is thought to

289

represent the timing of collision between the Yinshan and Ordos terranes (Zhao et al.,

290

2005, 2012; Yin et al., 2009, 2011) and encompasses the 1959 ± 18 Ma age obtained

291

from zircons from sample HL1527.

292 293

The garnet–biotite gneisses in the study area do not contain HP or UHT mineral

294

assemblages but are in contact with the garnet–sillimanite–cordierite gneisses in this

295

region and have identical gneissic textures (Fig. 2a), suggesting that both of these

296

units underwent MP granulite-facies-type metamorphism. Pelitic granulites with HP

297

mineral assemblages are rare within the Helanshan complex (Zhou et al., 2010; Yin et

298

al., 2015), whereas MP granulite-facies rocks are the dominated rocks. It is possible

299

that some of the MP granulite-facies rocks in this area formed as a result of isothermal

300

decompression of the HP pelitic granulites (Zhou et al., 2010), leading to the total

301

replacement of the HP mineral assemblage by a MP mineral assemblage. Nevertheless, 14 / 42

302

HT granulite-facies rocks with metamorphic age of ca. 1.95 Ga certainly formed

303

during continental collision between the Yinshan and Ordos terrans (Yin et al., 2011,

304

2015). This means that the metamorphic zircon of 1959 ± 18 Ma age obtained during

305

this study is similar (if not identical) to the timing of the HT granulite-facies

306

metamorphism that occurred within the Helanshan complex.

307 308

One of the most important discoveries made in the Precambrian NCC in the past

309

decade is the identification of UHT pelitic granulites in the Tuguiwula section of the

310

Jining complex and within the Daqingshan complex (Guo et al., 2006, 2012; Santosh

311

et al., 2007a). A significant amount of previous research has been undertaken to try

312

and constrain the timing of formation and geodynamic significance of these UHT

313

pelitic granulites (Table 1; Santosh et al., 2007b, 2009b, 2013; Yang et al., 2014; Jiao

314

et al., 2015, 2017; Li and Wei, 2016, 2017; Lobjoie et al., 2018). Zircon U–Pb dating

315

of the sapphirine-bearing UHT pelitic granulites from the Tuguiwula area of the

316

Jining complex have yielded metamorphic ages of 1934–1919 Ma (Santosh et al.,

317

2007b, 2009b), and spinel-bearing UHT pelitic granulites within the Heling’er area of

318

the same complex have given metamorphic ages of 1913–1910 Ma (Santosh et al.,

319

2013), both of which are considered to reflect the timing of UHT metamorphism.

320

More recent research by Lobjoie et al. (2018) has yielded a zircon U–Pb age of 1925

321

Ma for an olivine-bearing UHT migmatite that crops out within the Liangcheng area

322

of the Jining complex, which was also interpreted as the timing of the UHT

323

metamorphism in this region. However, Li and Wei (2017) obtained a metamorphic 15 / 42

324

age of 1919 Ma for the sapphirine-bearing UHT pelitic granulites mentioned above

325

and suggested that this age represented the cooling of these UHT rocks to the solidus

326

in pseudosections, a model that is consistent with the mean Ti-in-zircon temperatures

327

of these rocks (876 °C and 842 °C; Li and Wei, 2017).

328 329

Monazite is superior to zircon in dating metamorphism, because zircon is more

330

susceptible to metamictization and, as shown in the present study, most metamorphic

331

zircons have experienced significant Pb loss (Fig. 9). The SHRIMP monazite U–Pb

332

dating undertaken during this study yielded ages of 1931 and 1934 Ma for the UHT

333

pelitic granulites in the Helanshan complex, consistent with an upper-intercept age of

334

1937 Ma and an apparent age of 1932 Ma obtained by SHRIMP dating of

335

metamorphic zircons within the same sample. All of the monazite analyzed during

336

this study appears relatively homogeneous during the CL imaging, in contrast to

337

igneous monazites, which show clear oscillatory zoning (Li et al., 2017), providing

338

more evidence of a metamorphic origin for the monazite in the study area. This in turn

339

implies that the ca. 1.93 Ga ages obtained during this study represent the timing of a

340

metamorphic event in this region, the first ages of this type to be obtained from the

341

Helanshan complex.

342 343

Examination of representative monazite within thin-sections of samples HL1501-3

344

and HL1502-3 using backscattered electron (BSE) imaging was undertaken to better

345

understand the petrographic context of the monazite within these samples and the 16 / 42

346

geological meaning of their U–Pb ages in terms of the formation and evolution of the

347

spinel-bearing UHT pelitic granulites from the Helanshan complex. The majority of

348

monazite is either matrix-hosted (Fig. 10a–d) or present as mineral inclusions within

349

K-feldspar, cordierite, and quartz (Fig. 10e–j), with only a single monazite inclusion

350

identified within garnet in these samples (Fig. 10k, l). These in situ monazites have

351

similar crystal habits and internal structures to those of monazites separated from

352

these samples for age dating during this study (Figs. 4 and 10). The fact that garnet

353

and K-feldspar are stable during UHT metamorphism (Gou et al., 2018) and similar

354

apparent ages were obtained from different monazites (Table S1; Fig. 6c–f) means

355

that the monazite is likely to record the timing of UHT metamorphism within the

356

Helanshan complex. This in turn indicates that the ca. 1.93 Ga ages obtained from the

357

metamorphic monazite analyzed during this study provide evidence of the timing of

358

peak UHT metamorphism within the complex. These ages are similar to the older

359

metamorphic ages (ca. 1.94–1.90 Ga) obtained for sapphirine-bearing UHT pelitic

360

granulites within the Tuguiwula area of the Jining complex (Santosh et al., 2007b,

361

2009b) and from the Daqingshan complex (Jiao et al., 2017). The data presented by Li

362

and Wei (2017) also suggest that these UHT metamorphic rocks may have cooled to

363

the solidus by ca. 1.92 Ga.

364 365

The ca. 1.93 Ga UHT metamorphic rocks within the Daqingshan and Jining

366

complexes of the eastern Khondalite Belt formed as a result of the intrusion of mafic

367

magmas. This is supported by the presence of coeval ca. 1.93–1.92 Ga gabbronorite 17 / 42

368

intrusions in this region (Peng et al., 2010; Guo et al., 2012), which contrasts with the

369

absence of contemporaneous mafic magmatism within the Helanshan complex. The

370

spinel-bearing UHT metamorphic rocks within the Helanshan complex are present as

371

large lenses (0.5–2 m in diameter) within garnet-bearing S-type granites, suggesting

372

that these metamorphic rocks may also have been generated as a result of the intrusion

373

of mafic magma within the lower crust and were captured and uplifted from lower- to

374

middle-crustal levels by S-type granitic magmas (Gou et al., 2018). The presence of

375

the ca. 1.93 Ga UHT metamorphic rocks within both the eastern and western

376

Khondalite Belt indicates that this metamorphic event was regional in extent. The HP

377

and MP pelitic granulites in this region did not undergo UHT metamorphism, which

378

may be due to rapid exhumation during the HT granulite-facies metamorphism at ca.

379

1.96–1.95 Ga as indicated by their ITD P–T path (Zhou et al., 2010; Yin et al., 2015;

380

Xu et al., 2018). Further works are needed to clarify this question.

381 382

UHT metamorphism involves extreme temperature conditions that are much higher

383

than those involved in HT granulite-facies metamorphism. This means that HT

384

granulite-facies metamorphic assemblages will be replaced by UHT mineral

385

assemblages during UHT granulite-facies metamorphism. It also means that any

386

monazite and/or zircon formed during HT granulite-facies metamorphism most likely

387

dissolved during the late UHT event (Kelsey et al., 2008; Wei, 2016), explaining why

388

the spinel-bearing UHT garnet-cordierite gneiss in the study area does not preserve

389

any evidence of the ca. 1.95 Ga metamorphic event in this region. 18 / 42

390

5.2. Two separate UHT metamorphic events within the Khondalite Belt?

391

In addition to the metamorphic ages of ca. 1.94–1.90 Ga obtained from UHT

392

metamorphic rocks within the Jining and Daqingshan complexes, secondary ion mass

393

spectrometry (SIMS) zircon and monazite and LA–ICP–MS zircon U–Pb dating have

394

also yielded metamorphic ages of 1881–1849 Ma for these UHT metamorphic rocks

395

(Table 1; Yang et al., 2014; Jiao et al., 2015, 2017). This led Yang et al. (2014) to

396

suggest that the Khondalite Belt records two separate UHT metamorphic events that

397

occurred at ca. 1.93–1.92 Ga and ca. 1.88–1.85 Ga. This was supported by the

398

construction of a clockwise P–T path for the UHT metamorphism characterized by

399

isobaric heating followed by cooling and decompression at ca. 1.88 Ga, in contrasting

400

with the anti-clockwise P–T path for the ca. 1.92 Ga UHT rocks at Tuguiwula that

401

was defined by Santosh et al. (2007b, 2009b). In fact, these ca. 1.92 Ga UHT rocks

402

also underwent a clockwise P–T evolution, as discussed by Li and Wei (2017), and the

403

ca. 1.93 Ga UHT rocks within the Helanshan area also record a clockwise P–T path

404

(Gou et al., 2018). All of these data suggest that a younger (i.e., 1.88–1.85 Ga) UHT

405

event in the Khondalite Belt cannot be substantiated solely on the basis of the P–T

406

paths of these UHT rocks.

407 408

Gou et al. (2016) summarized available metamorphic ages from metasedimentary and

409

meta-igneous rocks in the Khondalite Belt, which range almost continuously from ca.

410

1.97 to 1.80 Ga, with four age peaks at ca. 1.95, 1.92, 1.89 and 1.85 Ga. Wan et al.

411

(2013b) suggested that the metamorphic ages in the Khondalite Belt cannot be divided 19 / 42

412

into different stages, arguing that the metamorphic ages of ca. 1.95–1.83 Ga instead

413

recorded long-term extension and exhumation of metamorphic basement material

414

from lower- to upper-crustal levels. Yin et al. (2009, 2011) also proposed that the

415

metamorphic age of ca. 1.95 Ga in the Khondalite Belt represented the timing of

416

collision between the Yinshan and Ordos terranes, whereas the ages of ca. 1.92–1.87

417

Ga within the belt reflect post-collisional extension and exhumation processes.

418

Moreover, Li and Wei (2016, 2017) argued that the metamorphic ages of ca. 1.94–

419

1.83 Ga within the Khondalite Belt recorded the slow cooling of the UHT rocks to the

420

solidus based on LA–ICP–MS zircon U–Pb dating of the spinel- and

421

sapphirine-bearing pelitic granulites. Nevertheless, it is worth investigating the

422

significance of ca. 1.88–1.85 Ga metamorphic ages from the belt, which overlap with

423

ca. 1.87–1.84 Ga mafic magmatism within the Daqingshan complex (Wan et al.,

424

2013b; Liu et al., 2014). LA–ICP–MS zircon U–Pb analysis has also yielded ages of

425

ca. 1.88–1.84 Ga for S-type granites and some pelitic granulites within the Helanshan

426

complex (Fig. 1b; Yin et al., 2011), and S-type granites in the Qianlishan (Yin et al.,

427

2009). All of these data suggest that the Khondalite belt may record ca. 1.88–1.84 Ga

428

HT or even UHT metamorphic event.

429

5.3. Tectonic implications

430

Spinel-bearing pelitic granulites within the Helanshan complex underwent UHT

431

metamorphism along a clockwise P–T path that involved pre-Tmax decompression

432

followed by nearly isobaric cooling (Gou et al., 2018). Combining this with

433

previously reported metamorphic ages for HT and UHT pelitic granulites in the 20 / 42

434

Khondalite Belt suggests that ongoing southward subduction in this area (cf. Gou et

435

al., 2016) led to continent–continent collision between the Yinshan and Ordos terranes

436

at ca. 1.95 Ga (e.g., Yin et al., 2009, 2011; Zhao et al., 2012; Zhao and Zhai, 2013).

437

This is consistent with the LA–ICP–MS metamorphic zircon age of 1959 ± 18 Ma

438

obtained for the garnet–biotite gneiss analyzed during this study. In addition, the

439

detrital igneous zircons from the pelitic granulites analyzed during this study yielded

440

U–Pb ages of ca. 2.0 Ga (Fig. 8b), indicating that the protoliths for these units were

441

deposited after ca. 2.0 Ga. This is in agreement with the findings by Wan et al. (2009)

442

and Dan et al. (2012), who also used detrital zircon U–Pb ages and Hf–O isotopic

443

compositions to determine that the sedimentary protoliths of these granulites were

444

deposited along an active continental margin or in a back-arc setting. Slab breakoff

445

occurred shortly after the ca. 1.95 Ga continent–continent collisional event, which

446

terminated subduction and induced the emplacement of mafic magmas. This caused

447

the ca. 1.93 Ga UHT metamorphic event within the Khondalite Belt, as recorded in

448

the Daqingshan and Jining complexes (Peng et al., 2010; Guo et al., 2012; Lobjoie et

449

al., 2018). This slab breakoff model can also be proposed to explain the formation of

450

S-type granites at ca. 1.95 Ga (Dan et al., 2014). The new data presented here indicate

451

that the UHT pelitic granulites in the Helanshan complex formed at ca. 1.93 Ga,

452

contemporaneous with the formation of the UHT pelitic granulites within the Jining

453

complex (Santosh et al., 2007b, 2009b; Lobjoie et al., 2018). This suggests that the

454

Khondalite Belt was an ultra-hot orogen that underwent regional UHT metamorphism

455

at ca. 1.93 Ga. The Khondalite Belt also most likely experienced a transition of 21 / 42

456

tectonic regime from compression to extension during the period of ca. 1.95–1.93 Ga,

457

with this tectonic transition having most likely been controlled by a later-stage

458

shallow slab-breakoff event.

459

6. Conclusion

460

Our conclusions are summarized as follows.

461

(1) SHRIMP monazite U–Pb dating of a garnet–biotite gneiss yielded a weighted

462

mean 207Pb/206Pb age of 1944.4 ± 4.2 Ma, which is the same within uncertainty as the

463

weighted mean 207Pb/206Pb age of 1959 ± 18 Ma obtained by LA–ICP–MS U–Pb

464

analysis of metamorphic zircon within the same sample.

465

(2) SHRIMP metamorphic monazite and zircon U–Pb dating of spinel-bearing UHT

466

pelitic granulites within the Helanshan complex yielded ages that cluster around ca.

467

1.93 Ga, representing the timing of UHT metamorphism.

468

(3) The ca. 1.93 Ga UHT metamorphism within the Helanshan complex is similar to

469

that recorded by the Tuguiwula UHT pelitic granulites of the Jining and Daqingshan

470

complexes, suggesting that this UHT metamorphism at ca. 1.93 Ga was a prevalent,

471

regional-scale event that influenced both the western and eastern segments of the

472

Khondalite Belt.

473

(4) The Khondalite Belt most likely underwent a tectonic transition from collision to

474

extension at 1.95–1.93 Ga, with this change possibly being associated with shallow

475

slab-breakoff after the continental collisional event in this region.

476

22 / 42

477

Appendix: Analytical methods

478

(1) SHRIMP monazite and zircon U–Pb isotopic analysis

479

Monazite and zircon were separated from crushed samples HL1527, HL1501-3 and

480

HL1502-3 using a combination of standard density and magnetic separation

481

techniques before representative monazite and zircon separates were hand-picked

482

under a binocular microscope. The resulting monazite and zircon separates were

483

mounted in epoxy resin together with a Qinghu standard zircon (Li et al., 2013). Then,

484

these mounts were polished to expose zircon and monazite centers. The monazite

485

reference materials were set into a separate mount. All of the monazites and zircons

486

were photographed under transmitted and reflected lights, as well as

487

cathodoluminescence (CL) in order to identify internal textures and to guide spot

488

selection for U–Pb isotopic analyses. The CL imaging of monazite and zircon was

489

performed at the Institute of Geology and Geophysics, Chinese Academy of Sciences,

490

Beijing, China.

491 492

SHRIMP monazite and zircon U–Pb analysis were undertaken at the John de Laeter

493

Centre, Curtin University, Perth, Australia. Instrument setup for monazite followed

494

procedures similar to that described by Fletcher et al. (2010), where a primary beam

495

of O2− ions was focused through a 50 µm Kohler aperture to produce an oval spot of

496

~12 µm in diameter, with a current intensity of ~0.6 nA. Mass peaks with flat tops and

497

mass resolutions of >5000 (at 1% peak heights) were achieved throughout the

498

analytical session. Data were collected in sets of 8 scans. Monazite reference 23 / 42

499

materials FRENCH, Z2234 and Z2908 were analyzed concurrently and used to

500

correct for instrumental mass fractionation in 207Pb/206Pb and matrix effects in

501

206

502

Raw data were reduced using a custom task file for the SQUID2 (v2.50) software

503

(Ludwig, 2009) and were plotted using ISOPLOT3 (Ludwig, 2012). More details

504

about the instrument setup parameters and data processing procedures can be found in

505

Fletcher et al. (2010) and Zi et al. (2017).

Pb/238U and 208Pb/232Th, following protocols established by Fletcher et al. (2010).

506 507

SHRIMP zircon U–Pb analysis used an analytical spot size of ~15 µm in diameter.

508

The chips of Plešovice zircon (206Pb/238U age of 337.1 Ma, Sláma et al., 2008)

509

analyzed during the session were fairly homogeneous in terms of U and Th

510

compositions (~750 ppm U and ~70 ppm Th), and were used as a primary reference

511

standard for calibration of Pb/U ratios and for approximation of U concentrations of

512

the samples. A Qinghu zircon standard (Li et al., 2013) was used as a secondary

513

reference material to monitor U–Pb ages. Data were reduced using SQUID2 (v2.50;

514

Ludwig, 2009) and were visualized using ISOPLOT3 (Ludwig, 2012). Weighted

515

mean values are quoted at the 95% confidence interval and individual analyses are

516

presented with 1σ errors.

517

(2) LA–ICP–MS zircon U–Pb isotopic analysis

518

The LA–ICP–MS analysis of zircon was performed at the State Key Laboratory of

519

Continental Dynamics, Department of Geology, Northwest University, Xi’an, China.

520

This analysis used a 30 µm laser spot size and a laser frequency of 6 Hz were used. 24 / 42

521

Helium was used as the carrier gas to transport the ablated material. Details of

522

analytical procedures can be found in Yuan et al. (2004). The standard silicate glass

523

NIST SRM 610 was used for determination of U, Th, and Pb concentrations, and

524

207

525

being corrected using zircon 91500 as an external standard. The resulting data were

526

processed using ISOPLOT3 (Ludwig, 2012). Weighted mean values are quoted at the

527

95% confidence interval and individual analyses are presented with 1σ errors.

Pb/206Pb and 206Pb/238U ratios were calculated using the GLITTER program before

528

529

Acknowledgements

530

We thank Prof. Yi Chen and Dr. Saihong Yang for their assistance during the CL

531

imaging of monazite and zircon, and editor Prof. Xianhua Li and two anonymous

532

reviewers for their comments that significantly improve the manuscript. This work

533

was jointly supported by the National Natural Science Foundation of China (Grants

534

No. 41772051, 41421002 and 41430209) and a research grant from the State Key

535

Laboratory of Continental Dynamics (SKLCD-04).

536 537

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629

Multiple mafic magmatic and high-grade metamorphic events revealed by

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zircons from meta-mafic rocks in the Daqingshan–Wulashan Complex of the 29 / 42

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Khondalite Belt, North China Craton. Precambrian Research, 246, 334–357.

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Liu, S.J., Li, J.H., Santosh, M., 2010. First application of the revised Ti-in-zircon

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Tuguiwula, Inner Mongolia, North China Craton. Contributions to Mineralogy

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Liu, S.J., Bai, X., Li, J.H., Santosh, M., 2011. Retrograde metamorphism of

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640

Liu, S.J., Tsunogae, T., Li, W.S., Shimizu, H., Santosh, M., Wan, Y.S., Li, J.H., 2012.

641

Paleoproterozoic granulites from Heling'er: Implications for regional

642

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constraints on the tectonothermal evolution of the Trans-North China Orogen.

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olivine-bearing migmatite from the Khondalite Belt, North China Craton. Journal

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evidence of crust-mantle interaction. Precambrian Research 183, 635–659.

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Peng, P., Wang, X.P., Windley, B.F., Guo, J.H., Zhai, M.G., Li, Y., 2014. Spatial

661

distribution of ~1950–1800 Ma metamorphic events in the North China Craton:

662

Implications for tectonic subdivision of the craton. Lithos 202–203, 250–266.

663

Qian, J.H., Wei, C.J., Zhou, X.W., Chu, H., 2013. Metamorphic P–T paths and New

664

Zircon U–Pb age data for garnetemica schist from the Wutai Group, North China

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Mg–Al granulites in the North China Craton: implications for Paleoproterozoic

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669

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SHRIMP U–Pb zircon geochronology. Precambrian Research 159, 178–196.

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Santosh, M., Sajeev, K., Li, J.H., Liu, S.J., Itaya, T., 2009a. Counterclockwise

673

exhumation of a hot orogen: the Paleoproterozoic ultrahigh-temperature

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granulites in the North China Craton. Lithos 110, 140–152. 31 / 42

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from an ultrahot Orogen: the amalgamation of North China Craton within the

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supercontinent Columbia. Journal of Geology 117, 429–443.

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Santosh, M., Liu, D., Shi, Y., Liu, S.J., 2013. Paleoproterozoic accretionary

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Shimizu, H., Tsunogae, T., Santosh, M., Liu, S.J., Li, J.H., 2013. Phase equilibrium

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modelling of Palaeoproterozoic ultrahigh-temperature sapphirine granulite from

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the Inner Mongolia Suture Zone, North China Craton: Implications for

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counterclockwise P–T path. Geological Journal 48, 456–466.

685

Sláma, J., Košler, J., Condon, D.J., Crowley, J.L., Gerdes, A., Hanchar, J.M.,

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Horstwood, M.S.A., Morris, G.A., Nasdala, L., Norbeg, N., Schaltegger, U.,

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Schoene, B., Tubrett, M.N., Whitehouse, M.J., 2008. Plešovice zircon — a new

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natural reference material for U–Pb and Hf isotopic microanalysis. Chemical

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Geology 249, 1–35.

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Tsunogae, T., Liu, S.J., Santosh, M., Shimizu, H., Li, J.H., 2011.

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Ultrahigh-temperature metamorphism in Daqingshan, Inner Mongolia Suture

692

Zone, North China Craton. Gondwana Research 20, 36–47.

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Wan, Y.S., Song, B., Liu, D.Y., Wilde, S.A., Wu, J.S., Shi, Y.R., Yin, X.Y., Zhou,

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H.Y., 2006. SHRIMP U–Pb zircon geochronology of Palaeoproterozoic

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metasedimentary rocks in the North China Craton: evidence for a major Late

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Palaeoproterozoic tectonothermal event. Precambrian Research 149, 249–271. 32 / 42

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Wan, Y.S., Liu, D.Y., Dong, C.Y., Xu, Z.Y., Wang, Z.J., Wilde, S., Yang, Y.H., Liu,

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Z.H., Zhou, H.Y., 2009. The Precambrian Khondalite Belt in the Daqingshan

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area, North China Craton: evidence for multiple metamorphic events in the

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Paleoproterozoic era. Geological Society of London, Special Publications 323,

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73–97.

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Wan, Y.S., Liu, D.Y., Wang, S.J., Yang, E.X., Wang, W., Dong, C.Y., Zhou, H.Y.,

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Du, L.L., Yang, Y.H., Diwu, C.R., 2011. ∼2.7 Ga juvenile crust formation in the

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North China Craton (Taishan-Xintai area, western Shandong Province): Further

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evidence of an understated event from U–Pb dating and Hf isotopic composition

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Wan, Y.S., Xie, H.Q., Yang, H., Wang, Z.J., Liu, D.L., Kröner, A., Wilde, S.A., Geng,

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709

Block Archeanor Paleoproterozoic in age? Implications for the Precambrian

710

evolution of theNorth China Craton. American Journal of Science 313, 683–711.

711

Wan, Y.S., Xu, Z.Y., Dong, C.Y., Nutman, A., Ma, M.Z., Xie, H.Q., Liu, S.J., Liu,

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715

whole-rock geochemistry. Precambrian Research 224, 71–93.

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Wang, W., Liu, X.H., Hu, J.M., Li, Z.H., Zhao, Y., Zhai, M.G., Liu, X.C., Clarke, G.,

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719

Basin, NW China: petrology, phase equilibrium modelling and U–Pb

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721

Wang, W., Gao, S.L., Liu, X.C., Hu, J.M., Zhao, Y., Wei, C.J., Xiao, W.J., Guo, H.,

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Gong, W.B., 2017. Prolonged anatexis of Paleoproterozoic metasedimentary

723

basement: First evidence from the Yinchuan Basin and new constraints on the

724

evolution of the Khondalite Belt, North China Craton. Precambrian Research 302,

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74–93.

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Wang, W., Zhao, Y., Liu, X.C., Hu, J.M., Wei, C.J., Xiao, W.J., Du, J.X., Wang, S.L.,

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Zhan, L.Q., 2019. Metamorphism of diverse basement gneisses of the Ordos

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Basin: Record of multistage Paleoproterozoic orogenesis and constraints on the

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evolution of the western North China Craton. Precambrian Research 328, 48–63.

730

Wang, X., Li, X.P., Han, Z.Z., 2018. Zircon ages and geochemistry of amphibolitic

731

rocks from the Paleoproterozoic Erdaowa Group in the Khondalite Belt, North

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China Craton and their tectonic implications. Precambrian Research 317, 253–

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Wei, C.J., 2016. Granulite facies metamorphism and petrogenesis of granite (II):

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Quantitative modeling of the HT-UHT phase equilibria for metapelites and the

736

petrogenesis of S-type granite. Acta Petrologica Sinica 32, 1625–1643 (in

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Chinese with English abstract).

738

Wei, C.J., Qian, J.H., Zhou, X.W., 2014. Paleoproterozoic crustal evolution of the

739

Hengshan–Wutai–Fuping region, North China Craton. Geoscience Frontiers 5,

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Wu, J.L., Zhang, H.F., Zhai, M.G., Guo, J.H., Liu, L., Yang, W.Q., Wang, H.Z., Zhao,

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L., Jia, X.L.,Wang, W., 2016. Discovery of pelitic high-pressure granulite from

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Manjinggou of the Huai’an Complex, North China Craton: Metamorphic P–T

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evolution and geological implications. Precambrian Research 278, 323–336.

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Wu, J.L., Zhang, H.F., Zhai, M.G., Guo, J.H., Li, R.X., Wang, H.Z., Zhao, L.,

Jia,

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747

high-pressure-high-temperature pelitic granulites from Datong in the North

748

China Craton and their geological implications: Constraints from geology,

749

petrology and phase equilibrium modeling. Precambrian Research 303, 727–748.

750

Xu, X.F., Gou, L.L., Long, X.P., Dong, Y.P., Liu, X.M., Zi, J.W., Li, Z.H., Zhang,

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U–Pb dating of medium-pressure pelitic granulites in the Helanshan complex of

753

the Khondalite Belt, North China Craton, and their tectonic implications.

754

Precambrian Research 314, 62–75.

755

Yang, Q.Y., Santosh, M., Tsunogae, T., 2014. Ultrahigh-temperature metamorphism

756

under isobaric heating: New evidence from the North China Craton. Journal of

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Asian Earth Sciences 95, 2–16.

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Yin, C.Q., Zhao, G.C., Sun, M., Xia, X.P., Wei, C.J., Leung, W.H., 2009.

759

LA-ICP-MS U–Pb zircon ages of the Qianlishan Complex: constrains on the

760

evolution of the Khondalite Belt in theWestern Block of the North China Craton.

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Precambrian Research 174, 78–94.

762

Yin, C.Q., Zhao, G.C., Guo, J.H., Sun, M., Xia, X.P., Zhou, X.W., Liu, C.H., 2011. 35 / 42

763

U–Pb and Hf isotopic study of zircons of the Helanshan Complex: constrains on

764

the evolution of the Khondalite Belt in the Western Block of the North China

765

Craton. Lithos 122, 25–38.

766

Yin, C.Q., Zhao, G.C., Wei, C.J., Sun, M., Guo, J.H., Zhou, X.W., 2014.

767

Metamorphism and partial melting of high-pressure pelitic granulites from the

768

Qianlishan Complex: Constraints on the tectonic evolution of the Khondalite

769

Belt in the North China Craton. Precambrian Research 242, 172–186.

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Yin, C.Q., Zhao, G.C., Sun, M., 2015. High-pressure pelitic granulites from the

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Helanshan Complex in the Khondalite Belt, North China Craton: metamorphic

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P–T path and tectonic implications. American Journal of Science 315, 846–879.

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age and trace element determinations of zircon by laser ablation-inductively

775

coupled plasma mass spectrometry. Geostandards and Geoanalytical Research 28,

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777

Zhai, M.G., 2009. Two kinds of granulites (HT-HP and HT-UHT) in the North Chian

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780 781 782 783 784

Zhai, M. and Santosh, M., 2011. The early Precambrian odyssey of the North China Craton: A synoptic overview. Gondwana Research 20, 6–25. Zhai, M., 2014. Multi-stage crustal growth and cratonization of the North China Craton. Geoscience Frontiers 5, 457–469. Zhang, J.X., Gong, J.H., Yu, S.Y., Li, H.K., Hou, K.J., 2013. Neoarchean– 36 / 42

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786

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787

and Hf isotopic composition. Precambrian Research 235, 36–57.

788

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789

H.S., Zhao, J., 2015. Archean-Paleoproterozoic crustal evolution of the Ordos

790

Block in the North China Craton: Constraints from zircon U–Pb geochronology

791

and Hf isotopes for gneissic granitoids of the basement. Precambrian Research

792

267, 121–136.

793

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794

high-pressure mafic granulites from northern Hengshan, North China Craton:

795

Insights from phase equilibria and geochronology. Precambrian Research 312,

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1–15.

797

Zhao, G.C., Wilde, S.A., Sun, M., Li, S.Z., Li, X.P., Zhang, J., 2008. SHRIMP U–Pb

798

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accretion and evolution of the Trans-North China Orogen. Precambrian Research

800

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801

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802

evolution of the North China Craton: key issues revisited. Precambrian Research

803

136, 177–202.

804

Zhao, G.C., Cawood, P.A., Li, S.Z., Wilde, S.A., Sun, M., Zhang, J., He, Y.H., Yin,

805

C.Q., 2012. Amalgamation of the North China Craton: Key issues and discussion.

806

Precambrian Research 222–223, 55–76. 37 / 42

807

Zhao, G.C., Zhai, M.G., 2013. Lithotectonic elements of Precambrian basement in the

808

North China Craton: Review and tectonic implications. Gondwana Research 23,

809

1207–1240.

810

Zhao, J., Gou, L.L., Zhang, C.L., Guo, A.L., Guo, X.J., Liu, X.Y., 2017. P–T–t path

811

and tectonic significance of pelitic migmatites from the Lüliang Complex in

812

Xiyupi area of Trans-North China Orogen, North China Craton. Precambrian

813

Research 303, 573–589.

814

Zhou, L.G., Zhai, M.G., Lu, J.S., Zhao, L., Wang, H.Z., Wu, J.L., Liu, B., Shan, H.X.,

815

Cui, X.H., 2017. Paleoproterozoic metamorphism of high-grade granulite facies

816

rocks in the North China Craton: Study advances, questions and new issues.

817

Precambrian Research 303, 520–547.

818

Zhou, X.W., Zhao, G.C., Geng, Y.S., 2010. Helanshan high-pressure pelitic granulites:

819

Petrological evidence for collision event in the Western Block of the North

820

China Craton. Acta Petrologica Sinica 26, 2113–2121 (in Chinese with English

821

abstract).

822

Zi, J.W., Gregory, C.J., Rasmussen, B., Sheppard, S., Muhling, J.R., 2017. Using

823

monazite geochronology to test the plume model for carbonatites: The example

824

of Gifford Creek Carbonatite Complex, Australia. Chemical Geology 463, 50–

825

60.

826

827

Figure captions

828

Fig. 1. (a) Tectonic subdivisions of the North China Craton (modified after Zhao et al., 38 / 42

829

2005). Abbreviations of metamorphic complexes are as follows: JN, Jining; DU,

830

Daqingshan–Ulashan; QL, Qianlishan; HL, Helashan; HS, Hengshan; LL, Lüliang;

831

TNCO, Trans-North China Orogen. (b) Schematic geological map of the Helanshan

832

complex (modified after Yin et al., 2011) showing the distribution of lithologies

833

within the complex and sample locations. Published metamorphic ages for pelitic

834

granulites are from Yin et al. (2011), whereas published crystallization ages for S-type

835

granites are from Yin et al. (2011) and Dan et al. (2012).

836 837

Fig. 2. Field photographs showing representative examples of garnet–biotite gneiss (a,

838

b), and spinel-bearing UHT pelitic granulite lens enclosed by garnet-bearing S-type

839

granite (c) within the Helanshan complex. Mineral abbreviations: g, garnet; sill,

840

sillimanite; cd, cordierite; bt, biotite.

841 842

Fig. 3. Photomicrographs (all taken under cross-polarized light) of spinel-bearing

843

UHT pelitic granulites in the Helanshan complex. (a, b) Biotite, K-feldspar,

844

plagioclase and quartz within a garnet–biotite gneiss (sample HL1527). (c) Relict

845

garnet, cordierite, and spinel inclusion-bearing sillimanite assemblage (sample

846

HL1501-3). (d) K-feldspar, quartz, and sillimanite inclusion-bearing cordierite

847

assemblage (sample HL1501-3). Mineral abbreviations are as follows: g, garnet; cd,

848

cordierite; sill, sillimanite; bt, biotite; kfs, K-feldspar; q, quartz; sp, spinel.

849 850

Fig. 4. Representative CL images of monazites within garnet–biotite gneiss (sample 39 / 42

851

HL1527) and spinel-bearing UHT pelitic granulite (samples HL1501-3 and HL1502-3)

852

of the Helanshan complex. The ages and circles in red color correspond to the

853

monazite cores and the homogeneous monazites, whereas those in green color

854

correspond to the monazite rims.

855 856

Fig. 5. Diagram showing variations in Th and U concentrations within monazite from

857

garnet–biotite gneiss (sample HL1527) and spinel-bearing UHT pelitic granulites

858

(samples HL1501-3 and HL1502-3) in the Helanshan complex. Hom. means

859

homogeneous monazites without internal texture.

860 861

Fig. 6. Diagrams showing SHRIMP monazite U–Pb data for garnet–biotite gneiss and

862

spinel-bearing UHT pelitic granulite samples from the Helanshan complex. (a) Tera–

863

Wasserburg concordia diagram for sample HL1527. (b) Bar chart showing variation in

864

age data for sample HL1527. (c) Tera–Wasserburg concordia diagram for sample

865

HL1501-3. (d) Bar chart showing variation in age data for sample HL1501-3. (e)

866

Tera–Wasserburg concordia diagram for sample HL1502-3. (f) Bar chart showing

867

variation in age data for sample HL1502-3. Hom. means homogeneous monazites

868

without internal texture.

869 870

Fig. 7. Representative CL images of zircons from garnet–biotite gneiss (sample

871

HL1527) and spinel-bearing UHT pelitic granulite (samples HL1501-3 and HL1502-3)

872

within the Helanshan complex. 40 / 42

873 874

Fig. 8. Tera–Wasserburg concordia diagram showing distribution of the LA–ICP–MS

875

zircon U–Pb data obtained from garnet–biotite gneiss sample HL1527. The ellipses

876

marked by dashed line represent discordant data that were not used in weighted mean

877

age calculation.

878 879

Fig. 9. Tera–Wasserburg concordia diagrams showing the distribution of SHRIMP

880

zircon U–Pb data obtained for samples HL1501-3 (a) and HL1502-3 (b). Green

881

ellipses (i.e., 9.1, 10.1 and 15.1) in (a) represent the data obtained by analysis on more

882

than one age domain and were therefore disregarded, whereas green ellipses (i.e., 3.1,

883

6.1, 12.1 and 13.1) in (b) represent the data from metamorphic zircons.

884 885

Fig. 10. BSE images showing the location of monazite (mnz) within spinel-bearing

886

UHT pelitic granulite (samples HL1501-3 and HL1502-3) from the study area. (a, b)

887

A matrix-hosted monazite in sample HL1501-3. (c, d) A matrix-hosted monazite in

888

sample HL1502-3. (e, f) A monazite inclusion within K-feldspar in sample HL1501-3.

889

(g, h) A monazite inclusion within cordierite in sample HL1501-3. (i, j) A monazite

890

inclusion within quartz in sample HL1502-3. (k, l) A monazite inclusion within garnet

891

in sample HL1501-3. Mineral abbreviations are the same as those in Fig. 3.

892

893

Tables

894

Table 1. Summary of metamorphic ages for UHT pelitic granulites within the 41 / 42

895

Khondalite Belt of the NCC.

896 897

Supplementary Table S1. SHRIMP monazite U–Th–Pb isotopic data for garnet–

898

biotite gneiss (sample HL1527) and spinel-bearing UHT pelitic granulite (samples

899

HL1501-3 and HL1502-3) within the Helanshan complex of the Khondalite Belt,

900

NCC.

901 902

Supplementary Table S2. LA–ICP–MS zircon U–Pb isotopic data for garnet–biotite

903

gneiss sample HL1527 from the Helanshan complex of the Khondalite Belt, NCC.

904 905

Supplementary Table S3. SHRIMP zircon U–Th–Pb isotopic data for spinel-bearing

906

UHT pelitic granulite within the Helanshan complex of the Khondalite Belt, NCC.

907 908

42 / 42

Table 1 Summary of metamorphic ages from UHT pelitic granulites in the Khondalite belt, NCC. Sample

Rock

Area

HL1501-3

Spl-bearing UHT granulite

HL1501-3

Location

Metamorphic Ages (Ma)

Method

Reference

Helanshan

1930±4 (15)

SIMS (monazite core or homgeneours grain)

This study

Spl-bearing UHT granulite

Helanshan

1933±5 (6)

SIMS (monazite rim)

This study

HL1501-3

Spl-bearing UHT granulite

Helanshan

1931±5 (21)

SIMS (core and rim)

This study

HL1502-3

Spl-bearing UHT granulite

Helanshan

1935±3 (15)

SIMS (monazite core or homgeneours grain)

This study

HL1502-3

Spl-bearing UHT granulite

Helanshan

1934±3 (20)

SIMS (core and rim)

This study

HL1502-3

Spl-bearing UHT granulite

Helanshan

1929±6 (5)

SIMS (monazite rim)

This study

SIMS (zircon)

This study

1932±6 (1)

HL1501-3

Spl-bearing UHT granulite

Helanshan

6662

Spr-bearing UHT granulite

Jining

Tuguiwula

1919±10 (26)

SIMS (zircon)

Santosh et al. (2007)

70601

Spr-bearing UHT granulite

Jining

Tuguiwula

1926±13 (15)

SIMS (zircon)

Santosh et al. (2009)

70601

Spr-bearing UHT granulite

Jining

Tuguiwula

1934±16 (15)

SIMS (zircon)

Santosh et al. (2009)

TGWL-6

Spr-bearing UHT granulite

Jining

Tuguiwula

1927±11 (10)

SIMS (zircon)

Santosh et al. (2009)

TGWL-9

Spr-bearing UHT granulite

Jining

Tuguiwula

1930±11 (12)

SIMS (zircon)

Santosh et al. (2009)

90927

Spl-bearing UHT granulite

Jining

Heling’er

1913±17 (17)

SIMS (zircon)

Santosh et al. (2013)

90928

Spl-bearing UHT granulite

Jining

Heling’er

1910±18 (15)

SIMS (zircon)

Santosh et al. (2013)

1937±30 (Upper intercept)

OY-XH-13a

Spl-bearing UHT granulite

Jining

Hongsigou

1881±7 (27)

LA-ICP-MS (zircon)

Yang et al. (2014)

TMT04

Spr-bearing UHT granulite

Daqingshan

Dongpo

1854±4 (9)

SIMS (monazite)

Jiao et al. (2015)

TMT10

Spr-bearing UHT granulite

Daqingshan

Dongpo

1853±3 (12)

SIMS (monazite)

Jiao et al. (2015)

J1432

Spr-bearing UHT granulite

Jining

Tuguiwula

1919±4 (39)

LA-ICP-MS (zircon)

Li and Wei (2017)

J1429

Spl-bearing UHT granulite

Jining

Tuguiwula

1919±5 (37)

LA-ICP-MS (zircon)

Li and Wei (2017)

Jdp06

Spr-bearing UHT granulite

Daqingshan

Dongpo

1850±9 (12)

SIMS (zircon)

Jiao et al. (2017)

Jpd01

Spr-bearing UHT granulite

Daqingshan

Dongpo

1849±6 (18)

SIMS (zircon)

Jiao et al. (2017)

Jpd01

Spr-bearing UHT granulite

Daqingshan

Dongpo

1940–1900

SIMS (zircon)

Jiao et al. (2017)

Jpd05

Spr-bearing UHT granulite

Daqingshan

Dongpo

1850±30 (7)

SIMS (zircon)

Jiao et al. (2017)

FZ49

Ol-bearing migmatite (UHT)

Jining

Liangcheng

1925±11 (11)

SIMS (zircon)

Lobjoie et al. (2018)

100°E

(a)

110°E

105°E

115°E

125°E

120°E

130°E

Changchun

Duolun

Fig. 1b

Yinshan domain

40°N

e DU JN rran TeQL n Alxa domain Khondalite Belt a Jiayuguan sh Yi n Shizuishan HS HL Qitan 1

Pyeonrang

Beijing

Ordos Basin Ch

ina

Or

og

en

lt Be

Xi’an

Songpan

Ce nt (Q ral C inl h i n g i n a Xinyang –D Oro ab ie) gen Wuhan

Archean–Paleoproterozoic basement beneath the Ordos Terrane Archean–Paleoproterozoic basement in the Yinshan Terrane Exposed basement in the Khondalite Belt Archean–Paleoproterozoic basement in the Khondalite Belt Exposed basement in the Trans-North China Orogen Archean–Paleoproterozoic basement in the Trans-North China Orogen Archean–Paleoproterozoic basement in the Eastern Block

i gg on na

30°N

400 km

Exposed basement in the Jiao–Liao–Ji Belt Archean–Paleoproterozoic basement in the Jiao–Liao–Ji Belt Major fault

Seoul ye

Lia

TNCO

ng

ng

al

o–

ntr

Jia

35°N

Taiyuan

tral C h in (Su- a Orog en Lu)

Ce

ga

G

Ordos Terrane

Eastern Block

jin

Ye o

LL

Ji

Western Block

Im

o–

Yinchuan

Cen

BY

W

LS

F

40°N

Jianping

m

Bayan Obo

Boundary between the Khondalite Belt and the Ordos Basin

125°E

BYWLSF, Bayanwulashan fault

(b)

106°10'E

106°20'E

HL3-3 1962 ± 14 Ma

N 4 km

HL3-1 1946 ± 13 Ma

HL1501-3 HL1502-3

39°20'N

HL2-5 1955 ± 15 Ma 1865 ± 12 Ma

09AL233 1947 ± 6 Ma

HL2-1 1858 ± 23 Ma

HL1-2 1958 ± 7 Ma

HL1-5 1840 ± 15 Ma HL3-2 1963 ± 15 Ma HL2-3 Zongbieli 1953 ± 7 Ma 1869 ± 29 Ma HL1527

Shitanjing

Liushugou Formation Alenghuduge Formation Tulugen Formation

HL2-4 1952 ± 9 Ma 1865 ± 26 Ma

Migmatite Sandstone K-feldspar granite S-type granite TTG

Fault Sample location

39°10'N

Pelitic granulite with published metamorphic age S-type granite with published crystallization age

(a)

(b)

(c)

(d)

kfs

bt mnz

mnz kfs

q

q

100 µm

(e)

100 µm

10 µm

(f)

kfs

pl

(g)

mnz

(h)

cd

10 µm

40 µm

mnz

10 µm

20 µm

(l)

(k)

(j)

(i)

15 µm

g

q

mnz g

mnz 200 µm

8 µm

20 µm

10 µm

(a) g – bt gneiss

garnet

g – sill – cd gneiss

(b) g – bt gneiss

garnet

(c) S-type granite

Spinel-bearing pelitic granulite

(a)

kfs

kfs

(b)

kfs pl bt

q

kfs

bt

q q

kfs

400 µm

(c)

400 µm

(d)

sill cd g

cd

sill

sill

kfs sp sill 500 µm

q

500 µm

HL1527 12.1

10.1

2.1

1940 ± 4 Ma

1944 ± 6 Ma

1953 ± 12 Ma 40 µm

30 µm

30 µm

HL1501-3 4.1/4.2

9.1

6.1

1929 ± 8 Ma

1933 ± 3 Ma 1929 ± 7 Ma

1928 ± 5 Ma 1931 ± 6 Ma

40 µm

1937 ± 6 Ma 11.1

30 µm

40 µm

15.1

13.1/13.2

1929 ± 19 Ma

1928 ± 11 Ma 1940 ± 8 Ma 20 µm

1910 ± 19 Ma

30 µm

1930 ± 6 Ma

20 µm

HL1502-3 1.1

3.1

2.1

40 µm

8.1

1890 ± 35 Ma

1942 ± 3 Ma

1937 ± 5 Ma

30 µm

12.1 1942 ± 7 Ma

1925 ± 7 Ma

20 µm

14.1 1932 ± 3 Ma 1940 ± 15 Ma

1925 ± 6 Ma

30 µm

1941 ± 7 Ma

30 µm

1927 ± 6 Ma

20 µm

Th (ppm)

9.0x10

4

6.0x10

4

3.0x10

4

HL1501-3 (core & Hom.) HL1501-3 (rim) HL1502-3 (core & Hom.) HL1502-3 (rim) Hl15027

0

1x10

4

2x10

U (ppm)

4

3x10

4

(a) HL1527

2010 Mean = 1944.4 ± 4.2 Ma n = 16, MSWD = 1.6 95% confidence

207

Pb/ 2 0 6 Pb

0.126

0.122 6 1

0.118 3

1970

1950

1930

1910

0.110 2.2

2.6

3.0

3.4 238

U/

3.8 206

4.2

1890

4.6

Pb

(c) HL1501-3

(d) HL1501-3 1950

2020

1980

2.1

1940

0.118 16.1

0.116 1880

0.114

Cores/Hom. Mean = 1930.3 ± 3.6 Ma n = 15, MSWD = 1.2 95% confidence

1920 1910

1890

Cores and rims Mean = 1930.8 ± 2.6 Ma n = 21, MSWD = 0.97 95% confidence

1880 2.8

3.2 238

U/

3.6 206

4.0

Pb

(e) HL1502-3

1980

2040

Rims Mean = 1928.9 ± 6 Ma n = 5, MSWD = 1.1 95% confidence

0.124 2000

Pb 206

1920

0.116

207

1880

2.75

1900

1840

1860 1800

2.85 238

U/

2.95 206

Pb

3.05

3.15

Cores/Hom.

1920

1840

2.65

(f) HL1502-3

1940

1880

Cores/Hom. Mean = 1934.5 ± 3.4 Ma n = 15, MSWD = 1.5 95% confidence

0.108 2.55

1960

Age (Ma)

1960

0.120

Pb/

Rims

1930

1900

1860

0.112 2.4

Cores/Hom.

1940

0.120

207

Pb/ 2 0 6 Pb

0.122

Rims Mean = 1932.5 ± 5.2 Ma n = 6, MSWD = 0.28 95% confidence 1.1

Age (Ma)

0.124

0.112

Mean = 1944.4 ± 4.2 Ma n = 16, MSWD = 1.6 95% confidence

4

0.114

0.126

(b) HL1527

1990

Age (Ma)

0.130

Cores and rims Mean = 1933.6 ± 3.1 Ma n = 20, MSWD = 1.5 95% confidence

Rims

HL1527 10

4

20

19

1950 ± 39 Ma

1937 ± 37 Ma 1983 ± 38 Ma

1941 ± 36 Ma 25 µm

25 µm

25 µm

25 µm

HL1501-3 3.1

1.1 1921 ± 8 Ma

8.1 1932 ± 6 Ma

10 µm

10 µm

10 µm

13.1 1834 ± 13 Ma

1901 ± 6 Ma 10 µm

HL1502-3 6.1

4.1

13.1

12.1 1809 ± 69 Ma

1869 ± 11 Ma

1537 ± 16 Ma

2026 ± 24 Ma 20 µm

20 µm

20 µm

20 µm

0.14

HL1527

Mean = 1959 ± 18 Ma n = 18, MSWD = 0.43 95% confidence

18 1

0.12

12

207

Pb/ 2 0 6 Pb

0.13

17 0.11

0.10 2.2

2.6

3.0 238

3.4

U/

206

Pb

3.8

4.2

(a) HL1501-3 0.13

20 7 Pb/ 2 0 6 Pb

10.1

Intercepts at 531 ± 38 Ma & 1937 ± 30 Ma MSWD = 6.0

15.1

0.11

0.09

9.1

0.07

0.05 0

2

4

6 238

U/

8 206

10

Pb

(b) HL1502-3 Mean = 1997 ± 43 Ma n = 4, MSWD = 2.6

20 7 Pb/ 2 0 6 Pb

0.13

12.1 6.1

0.11

3.1 13.1 0.09

Intercepts at 831 ± 180 Ma & 2064 ± 96 Ma MSWD = 2.6 0.07 1

3

5 238

U/

206

Pb

7

12

Highlights:
A combined of monazite and zircon U–Pb dating is firstly conducted in the Helanshan
Two separate granulite-facies metamorphic events at ca. 1.96 Ga and 1.93 Ga
A tectonic transition from collision to extension during the period of 1.95–1.93 Ga