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Precambrian Research 235 (2013) 36– 57

Contents lists available at ScienceDirect

Precambrian Research

journa l h om epa ge : www.elsev ier .com/ locate /precamres

eoarchean–Paleoproterozoic multiple tectonothermal events in the westernlxa block, North China Craton and their geological implication: Evidence fromircon U–Pb ages and Hf isotopic composition

ianxin Zhanga,∗, Jianghua Gonga, Shengyao Yua, Huaikun Lib, Kejun Houc

State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang, Beijing 100037, ChinaTianjin Institute of Geology and Mineral Resources, CGS, 8th Road, Dazhigu, 300170 Tianjin, ChinaInstitute of Mineral Resources, Chinese Academy of Geological Sciences, 26 Baiwanzhuang, Beijing 100037, China

r t i c l e i n f o

rticle history:eceived 25 January 2013eceived in revised form 19 April 2013ccepted 10 May 2013vailable online 31 May 2013

eywords:lxa blockorth China Craton–Pb datingf isotopeeoarchean–Paleoproterozoic

a b s t r a c t

The Alxa block was traditionally considered to be part of the North China Craton, but its metamor-phic basement has been poorly studied. Here we present a systematic zircon U–Pb and Hf isotopicinvestigation on four orthogneiss samples in the Beidashan area of the western Alxa block. The pet-rographic and geochemical data show that these rocks are granodioritic and trondhjemitic gneisses withTTG (tonalite–trondhjemite–granodiorite) characteristics. Zircons from the TTG gneisses display typicalcore–rim or core–mantle–rim structures. U–Pb datings and Hf isotopic analyses reveal two distinct agepopulations: the Latest Neoarchean (∼2.5 Ga) and the Late Palaeoproterozoic (∼1.85 Ga). The magmaticzircon cores and metamorphic mantles (rims) of the TTG gneisses were dated at similar ages around 2.5 Ga,supporting the existence of Archean rocks in the western Alxa block. The short time interval between theLatest Neoarchean magmatism and the subsequent metamorphism suggests that they were related tothe same Latest Neoarchean tectonothermal event. The ∼2.5 Ga zircons have εHf(t) mainly between 0.8and 5.0, TDM (Hf) model ages mainly between 2.6 and 2.8 Ga (with a peak at ∼2.7 Ga) and TDMC (Hf) modelages mainly between 2.7 and 3.0 Ga (with a peak at ∼2.8 Ga). The age of ∼1.85 Ga obtained from two

trondhjemitic gneisses is interpreted as the age of the Late Paleoproterzoic high-grade metamorphism.Our combined datasets show that the TTG gneisses in the Baidashan area of the western Alxa blockexperienced a main 2.7–2.8 Ga crust growth, a ∼2.5 Ga magmatic–metamorphic event and a ∼1.85 Gahigh-grade metamorphic event. The sequence of events is very similar to that of the other North ChinaCraton. A Combination of the data of Paleoproterozoic metamorphic rocks in the Alxa block suggests thatthe Alxa block is the western extension of the Khondalite Belt rather than the Yinshan block.

. Introduction

In the last decade, extensive structural, metamorphic, geochem-cal, geochronological and geophysical investigations on the Northhina Craton (NCC) have been carried out. These investigationsroduced an abundant amount of new data and competing inter-retations (e.g. Zhao and Zhai, 2012; Zhao et al., 2012 and referencesherein). Although controversies still remain about the timing andectonic processes of the NCC, a long and complex history ofrecambrian tectonic events has been recognized. These include:1) several ancient nuclei in the eastern NCC indicate the pres-

nce of Paleoarchean–Eoarchean continental crust (Liu et al., 1992;ong et al., 1996; Wan et al., 2001, 2005); (2) a major phase ofontinental growth at ca. 2.7–2.8 Ga (e.g. Zhai and Santosh, 2011;

∗ Corresponding author. Tel.: +86 68999723; fax: +86 68994781.E-mail address: zjx66@yeah.net (J. Zhang).

301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.precamres.2013.05.002

© 2013 Elsevier B.V. All rights reserved.

Geng et al., 2012; Jiang et al., 2010; Wan et al., 2011); (3) cra-tonization at ca. 2.5 Ga (Zhai and Santosh, 2011; Jian et al., 2012; Lüet al., 2012; Wan et al., 2012; Zhang et al., 2012a); and (4) Pale-oproterozoic subduction–accretion–collision tectonics during ca.1.95–1.82 Ga (Zhao et al., 2001, 2004, 2005, 2008, 2012; Zhai andSantosh, 2011; Zhao and Guo, 2012). However, these conclusionsare mainly based on studies on the eastern-middle part of the NCC.The westernmost NCC, termed the Alxa block, remains the leaststudied area. This region is traditionally considered as part of theNCC (Ren et al.,1980; Wu et al., 1998; Zhai and Bian, 2000). It hasbeen considered to constitute the western extension of the Yinshanblock (Zhao et al., 2005; Zhao, 2009), or to be part of the Khon-dalite Belt (Dong et al., 2007; Geng et al., 2010). However, based onthe absence of Archean rocks and different Paleoproterozoic geol-

ogy of the eastern Alxa area from the other part of the NCC, theAlxa block has been suggested recently as a separated Paleopro-terozoic terrane from the Western Block of the NCC (e.g. Dan et al.,2012a). Thus, the Archean–Paleoproterozoic evolution is crucial for

J. Zhang et al. / Precambrian Research 235 (2013) 36– 57 37

ng theM

uf

ztcp

2

ttQcZa

bwamwgDa3DttaLiNsDdttr

Fig. 1. Geological sketch map showiodified from Gong et al. (2012).

nderstanding the tectonic affinity of the Alxa block as well as theormation and evolution of the NCC.

In this study, we present U–Pb dating and Hf isotopic data ofircons for four TTG gneiss samples from the Beidashan complex inhe western Alxa block. The aims of this work are to place robusthronological and isotopic constraints on magmatic and metamor-hic events and the crust growth history of the TTG gneisses.

. Geological setting

The westernmost part of the North China Craton (NCC), termedhe Alxa block, is bounded by the Central Asian Orogenic Belt andhe Tarim Craton to the north and west, respectively, and the Qilian-inling Orogen to the south (Fig. 1). The Alxa block is traditionallyonsidered to be part of the NCC (Ren et al.,1980; Wu et al., 1998;hai and Bian, 2000). Zhao et al. (2005, 2010) further considered its the western extension of the Yinshan block.

The Alxa block is largely covered by desert, with Precam-rian basement rocks sporadically exposed in the eastern andestern parts. In eastern Alxa block, Geng et al. (2006, 2007) dis-

ssembled the traditional Alxa Group into medium to high-gradeetamorphic complexes and low-grade sedimentary sequences,ith the latter being redefined as the new Alxa Group. Three high-

rade metamorphic complexes were recognized: the Neoarcheaniebusige Group, the Paleoproterozoic Bayanwulashan Groupnd the Boluositanmiao Complex. Li et al. (2006) obtained a018 ± 49 Ma Nd model age of the amphibolite in the loweriebusige Group and proposed that a Mesoarchean crust exists in

he Alxa block. Other scholars obtained Paleoproterozoic ages fromhe metamorphic basement of the Bayanwulashan and Qinggeletureas (Li et al., 2006; Zhou et al., 2007; Dong et al., 2007).ately, zircon U–Pb dating of detrital zircons from paragneissesn the Diebusige complexes, which were previously considered aseoarchean basement, yielded ages ranging from 2.0 to 2.45 Ga,

uggesting a Paleoproterozoic depositional age for protoliths of theiebusige paragneisses (Dan et al., 2012a). These U–Pb zircon age

ata do not support the existence of exposed Archean rocks inhe eastern Alxa block, although zircon Hf isotopic data imply thathe possibility of Archean rocks at deeper crustal levels cannot beuled out.

geological setting of the Alxa block.

The Precambrian basement of the western Alxa block is mainlyexposed in the Longshoushan and Beidashan areas, which lie tothe north of the Hexi Corridor and are separated from the easternAlxa block by the Tenggeli Desert (Fig. 1). The Longshoushan areais located at the north of the Hexi Corridor in Gansu Province, ofwhich crystalline basement is composed predominantly of Pale-oproterozoic amphibolite-facies metamorphosed igneous rocksand metasedimentary rocks (Xiu et al., 2002, 2004; Lu et al.,2006; Tung et al., 2007). Our latest geochronology results alsoindicated that the primary magmatic ages of the orthogneissesin the Longshoushan area range between 2.04 and 2.17 Ga andthat the depositional ages of the protoliths of the metasedi-mentary rocks are between 1.93 Ga and 2.01 Ga (Gong et al.,2011). Both types of rocks were overprinted by metamorphicevents at ca. 1.85–1.93 Ga. These data indicate that the Long-shoushan Complex formed in the Paleoproterozoic rather than inthe Archean.

The Beidashan, which is the focus of this contribution, extendsin a NWW-SEE trend and is bounded by the Yabulaishan andthe Badanjilin Desert to the north, by the Tengeli Desert to theeast, and is separated from the parallel Longshoushan by theChaoshui basin to the south. As shown in Fig. 2, the high-grademetamorphic rocks in the Beidashan area are intruded by abun-dant granitic rocks. These rocks, called previously as “BeidashanGroup”, although it is not a simple lithostratigraphic successionin terms of international stratigraphic nomenclature, were dividedinto three parts (BGMG, 1989): Part A is mainly exposed in thewestern part of the Beidashan area and consists of amphibolite-facies metamorphic rocks, including biotite–plagioclase gneiss,biotite–amphibole–plagioclase gneiss, micaschist, amphibolite,quartzite and marble; Part B is exposed in the central part of the Bei-dashan area and consists of amphibolite-facies metamorphic rocks,including marble, amphibole-bearing gneiss, biotite-plagioclasegneiss and garnet- and sillimanite-bearing micaschist with minoramounts of quartzite; Part C is exposed in the Eastern part of theBeidashan area and consists of amphibolite facies metamorphicrocks, including micaschists, amphibolites and biotite-plagioclase

gneiss. Given that the exposed basement in the Beidashan area con-tains both metamorphosed pluton and supracrustal sequences thatwere strongly deformed, we renamed “the Beidashan Group” as“the Beidashan Complex”.

38 J. Zhang et al. / Precambrian Research 235 (2013) 36– 57

ent ro

3

3

(tIali

masc(iaLa(Tp(fi8ra

3

eYs(wscaNTpSaY

Fig. 2. Distributions of Precambrian basem

. Field geology and sample descriptions

.1. Granodioritic gneisses (samples LS10-8-1.1 and LS11-4-2.1)

Samples LS10-8-1.1 (39◦ 9.613′, 101◦ 47.613′) and LS11-4-2.1N 39◦ 10.12′, E 101◦ 47.72′) were collected from the central part ofhe Beidashan Complex 10 km southeast of Alxa Youqi, in westernnner Mongolia (Fig. 2). The Beidashan Complexes at this locationre mainly composed of banded gneisses intercalated with a fewayered amphibolites with minor graphite-bearing marble and arentruded by pegmatites and some mafic dykes (Gong et al., 2012).

The both samples show similar petrographic characteristics andineral assemblages. They are gray, medium- to coarse-grained

nd display a homogenous granoblastic texture with gneissictructure. Granodioritic gneisses are mainly composed of plagio-lase (30–35%), quartz (25–30%), hornblende (10–15%), K-feldspar10–15%) with minor epidote, chlorite and accessory minerals,ncluding apatite, sphene and zircon (Fig. 3). Whole-rock majornd trace element analysis indicates that gneisses LS10-8-1.1 andS11-4-2.1 share similar geochemical characteristics with the aver-ge Archean TTG with relatively high SiO2 (∼70%), Al2O3 (>15%), Sr>500 ppm) and low Y (<20 ppm), Yb (<1 ppm), Nb (<1 ppm), anda (<0.05 ppm), corresponding to high La/Yb and Sr/Y ratios (Sup-lementary Table 1A). Based on the normative An–Ab–Or diagramsee Supplementary Fig. 1A), both sample fall in the granodioriteeld. The two samples are characterized by high Sr (686 ppm and19 ppm) and low Y (8.99 and 6.11 ppm) contents with high Sr/Yatios (76 and 134) (see Supplementary Table 1A), analogous todakite and Archean TTG (Martin et al., 2005; Condie, 2005).

.2. Trondhjemitic gneisses (samples LS11-1-1.1 and LS11-1-5.1)

Samples LS11-1-1.1 and LS11-1-5.1 were collected from theastern part of the Beidashan Complex, about 100 km east of Alxaouqi (Fig. 2). They are gray, are medium-grained and show gneis-ic structures. Two gneisses are mainly composed of plagioclase∼35–40%), quartz (25–30%), biotite (10%) and epidote (3–5%),ith minor K-feldspar (Fig. 3c and d) and accessory zircon and

phene. Amphibole has not been observed. Two samples have SiO2ontents of 64.35 wt% and 70.15 wt%, Na2O contents of 5.22 wt%nd 4.66 wt%, Al2O3 contents of 15.72 wt% and 16.23 wt%, and higha2O/K2O ratios (4.5 and 2.1, respectively) (see Supplementaryable 1A). Based on the normative An–Ab–Or diagram (see Sup-

lementary Fig. 1A), two samples fall in the Trondhjemitic field.imilar to samples LS10-8-1.1 and LS11-4-2.1, The two samplesre also characterized by high Sr (591 ppm and 969 ppm) and low

(8.92 ppm and 13.9 ppm) contents with high Sr/Y ratios (66 and

cks sample locations in the Beidashan area.

70), analogous to adakite and Archean TTG (Martin et al., 2005;Condie, 2005).

4. Sample preparation and analytical procedures

4.1. Zircon U–Pb dating

Zircon grains from four felsic gneiss samples were mechan-ically separated from approximately 5 to 10 kg of samples bycrushing and sieving, followed by standard magnetic, heavy liquidand hand-picking methods. The zircons were mounted in epoxyresin and then ground to approximately half their original thick-nesses. Cathodoluminescence (CL) imaging of the zircon crystalswas performed using a FEI PHILIPS XL30 SFEG instrument with a2-min scanning time operating at 15 kV and 120 nA at the BeijingSHRIMP Center, Chinese Academy of Geological Sciences (CAGS).

The dating of sample LS10-8-1.1 and LS11-1-5.1 was performedon a SHRIMP-II instrument at the Beijing SHRIMP Center, CAGS.The SHRIMP analytical procedure for zircon was similar to thatdescribed by Williams (1998) and Wan et al. (2005). The mass res-olution during the analytical sessions was approximately 5000 (1%definition), and the intensity of the primary O2− ion beam was4–6 nA. The primary beam size was ∼30 �m, and each analyticalsite was rastered for 2–3 min prior to analysis. Three to four scanswere performed through the relevant mass stations (196ZrO, 204Pb,background, 206Pb, 207Pb, 208Pb, 238U, 248ThO, and 254UO) for eachanalysis of the detrital zircons, and five scans were performed forthe standards. The standards used were SL13 (U = 238 ppm) andTEM (206Pb/238U age = 417 Ma) (Williams, 1998; Black et al., 2003).The decay constants used for the age calculation are those recom-mended by the Subcommission on Geochronology of IUGS (Steigerand Jager, 1977). The measured 204 Pb was applied as the commonlead correction, and the data processing was performed using theSQUID and ISOPLOT programs (Ludwig, 2001). Ages of 207Pb/206Pbare used for all the data. The uncertainties for the individual anal-yses are quoted at a confidence level of 1, while the errors for theweighted mean ages are quoted at a 95% confidence level.

The zircons from samples LS11-4-2.1, LS11-1-1.1 and LS11-1-5.2were analyzed for U, Th, and Pb using the MC-LA-ICP-MS facil-ity at the Tianjin Institute of Geology and Mineral Resources, CGS.Laser sampling was performed using a Newwave UP 213 laser abla-tion system. All analyses were conducted with a beam diameter of25 �m, a 10 Hz repetition rate, and an energy of 2.5 J/cm. A ThermoFinnigan Neptune MC-ICP-MS instrument was used to acquire the

ion-signal intensities. Standards GJ1 and M127 were used duringour analyses, and they were assessed in two out of every 5–10 anal-yses. The data were evaluated using ICPMSDataCal 3.4 (Liu et al.,2010). Concordia diagrams and weighted mean calculations were

J. Zhang et al. / Precambrian Research 235 (2013) 36– 57 39

al asse

mtr(

4

toUwaMjhWyogtttdr1mmtdur

Fig. 3. Photomicrographs showing texture and miner

ade using Isoplot/Ex ver. 3. The detailed operating conditions ofhe laser ablation system and the MC-ICP-MS instrument and dataeduction are identical to the conditions described by Hou et al.2009).

.2. Hf isotope

Zircon grains from all four samples were separated for Lu–Hf iso-opic analyses using MC-LA-ICPMS. The analyses were conductedn the same or nearly the same zircon domains in which the–Pb dating had been conducted. The zircon Hf isotope analysisas conducted using a Newwave UP213 laser-ablation microprobe

ttached to a Neptune multi-collector ICP-MS at the Institute ofineral Resources, Chinese Academy of Geological Sciences, Bei-

ing. The instrumental conditions and data acquisition methodsave been comprehensively described by Hou et al. (2007) andu et al. (2006). A stationary spot was used for the present anal-

ses with a beam diameter of either 40 �m or 55 �m, dependingn the sizes of the ablated domains. He was used as the carrieras to transport the ablated sample from the laser-ablation cell tohe ICP-MS torch via a mixing chamber containing Ar. To correcthe isobaric interferences of 176Lu and 176Yb with that of 176Hf,he 176Lu/175Lu = 0.02658 and 176Yb/173Yb = 0.796218 ratios wereetermined (Chu et al., 2002). For the instrumental mass bias cor-ection, the Yb isotope ratios were normalized to 172Yb/173Yb of.35274 (Chu et al., 2002) and the Hf isotope ratios were nor-alized to 179Hf/177Hf of 0.7325 using an exponential law. Theass bias behavior of Lu was assumed to follow that of Yb, and

he mass bias correction protocol details were identical to thoseescribed by Wu et al. (2006) and Hou et al. (2007). Zircon GJ1 wassed as the reference standard with a weighted mean 176Hf/177Hfatio of 0.282008 ± 27 (2�) during our routine analyses. This ratio

mblages of granodioritic and trondhjemitic gneisses.

is not distinguishable from a weighted mean 176Hf/177Hf ratio of0.282013 ± 19 (2�), according to an in situ analysis by Elhlou et al.(2006).

The calculation of the Hf model age (single-stage modelage) (TDM) is based on a depleted-mantle source with present-day 176Hf/177Hf at 0.28325, using the 176Lu decay constant of1.865 × 10−11 year−1 (Scherer et al., 2001). The calculation of the“crust” (two-stage) Hf model age (TDMC) is based on the assumptionof a mean 176Hf/177Hf value of 0.015 for the average conti-nental crust (Griffin et al., 2002). The calculation of the εHf(t)values was based on the zircon U–Pb ages and the chondritic val-ues (176Hf/177Hf = 0.282772, 176Lu/177Hf = 0.0332; Blichert-Toft andAlbarede, 1997).

5. Results

5.1. U–Pb dating results

5.1.1. LS10-8-1.1The zircon grains in this sample have previously been analyzed

using the LA-MC-ICPMS method, but most of these analyses are dis-cordant, possibly resulting from Pb loss (Gong et al., 2012). Zircongrains selected for SHRIMP analyses are predominantly euhedraland oval crystals, ranging from 200 to 400 �m in length withlength-to-width ratios between 2:1 and 3:1. CL images reveal thatzircon grains commonly have core–rim structures (Fig. 4a–f). Mostcores appear weakly luminescent (Fig. 4a–c). Some zircon coresappear gray with oscillatory zoning (Fig. 5a and c). The rims have

relatively bright CL intensity and show no discernible internalstructure or blurred zoning, suggesting metamorphic origin.

Twenty spots were analyzed on 18 grains using SHRIMP II(Table 1), including 11 analyses on the oscillatory zoned cores

40 J. Zhang et al. / Precambrian Research 235 (2013) 36– 57

Fig. 4. Representative CL images of zircons from sample LS10-8-1.1. The smaller circles show locations for U–Pb analyses, and the bigger circles for Hf isotopic analyses. Eachspot is labeled with its individual 207Pb/206Pb ages and 176Hf/177Hf ratios.

Fig. 5. Concordia diagrams for zircon U–Pb analyses of granodioritic gneisses LS10-8-1.1.

J. Zhang et al. / Precambrian Re

Tab

le

1SH

RIM

P

U–P

b

anal

ytic

al

dat

a

for

zirc

on

from

sam

ple

LS10

-8-1

.1.

Spot

s

U

(pp

m)

Th

(pp

m)

Th/U

Pb*

Com

. Pb

(%)

207Pb

*/20

6Pb

*

%1

�20

7Pb

*/23

5U

1�

(%)

206Pb

*/23

8U

1�

(%)

Age

(Ma)

Text

ura

l loc

atio

ns

206Pb

*/23

8U

1�20

7Pb

*/20

6Pb

*

1�

LS10

-8-1

.1-1

75

56

0.77

31.5

0.39

0.16

38

1.3

11.0

2

2.5

0.48

8

2.1

2563

44

2495

22

Rim

LS10

-8-1

.1-2

22

19

0.91

8.94

0.86

0.17

08

5.1

11.2

5

5.8

0.47

8

2.7

2517

56

2566

86

Rim

LS10

-8-1

.1-3

94

75

0.82

40.3

0.18

0.16

43

0.98

11.2

6

1.5

0.49

73

1.1

2602

24

2500

17

Rim

LS10

-8-1

.1-4

14

10

0.73

4.69

0.85

0.15

61

3.7

8.51

5.3

0.39

5

3.7

2147

68

2414

63

Rim

LS10

-8-1

.1-5

267

216

0.84

107

0.07

0.16

34

0.77

10.5

6

2.3

0.46

9

2.2

2477

45

2491

13

Cor

eLS

10-8

-1.1

-6

69

69

1.02

26.3

0.13

0.16

04

1.3

9.75

4.0

0.44

1

3.8

2354

74

2460

21

Rim

LS10

-8-1

.1-7

146

139

0.99

59.1

0.12

0.16

51

0.77

10.7

0

2.4

0.47

0

2.3

2485

48

2508

13

Cor

eLS

10-8

-1.1

-8

199

173

0.90

77.5

0.13

0.16

49

0.67

10.2

9

2.3

0.45

3

2.2

2408

45

2507

11

Cor

eLS

10-8

-1.1

-9

398

398

1.03

148

0.04

0.15

80

0.46

9.43

2.2

0.43

27

2.2

2318

42

2434

8

Cor

eLS

10-8

-1.1

-10

38

19

0.50

15.5

0.53

0.15

81

1.8

10.1

7

3.3

0.46

6

2.7

2468

56

2436

31

Rim

LS10

-8-1

.1-1

1

52

26

0.51

20.7

0.18

0.16

22

1.5

10.4

0

2.9

0.46

5

2.5

2461

52

2479

25

Rim

LS10

-8-1

.1-1

2

187

182

1.01

72.2

0.06

0.15

92

0.65

9.88

2.4

0.45

0

2.3

2395

46

2447

11

Cor

eLS

10-8

-1.1

-13

277

268

1.00

105

0.02

0.16

05

0.50

9.74

2.2

0.44

02

2.2

2352

43

2461

8

Cor

eLS

10-8

-1.1

-14

47

42

0.93

16.5

0.41

0.15

55

1.7

8.74

3.8

0.40

8

3.3

2204

62

2407

30

Rim

LS10

-8-1

.1-1

5

230

197

0.88

88.3

0.05

0.16

07

0.60

9.92

2.3

0.44

75

2.2

2384

44

2463

10

Cor

eLS

10-8

-1.1

-16

153

107

0.72

63.9

0.11

0.16

62

0.73

11.1

1

2.4

0.48

5

2.3

2547

47

2520

12

Cor

eLS

10-8

-1.1

-17

16

13

0.81

6.20

1.37

0.16

07

3.6

9.76

5.1

0.44

1

3.6

2353

71

2463

61

Rim

LS10

-8-1

.1-1

8

104

49

0.49

44.4

0.21

0.16

32

0.93

11.1

7

2.5

0.49

6

2.4

2599

50

2489

16

Cor

eLS

10-8

-1.1

-19

268

219

0.84

104

0.07

0.16

15

0.54

10.0

9

2.2

0.45

31

2.2

2409

44

2471

9

Cor

eLS

10-8

-1.1

-20

21

18

0.86

8.51

0.40

0.15

89

2.2

10.2

1

3.8

0.46

6

3.0

2466

62

2444

38

Rim

search 235 (2013) 36– 57 41

and 9 analyses on the CL-bright and structureless rims. Themagmatic cores yield U contents ranging from 104 to 398 ppmwith Th/U ratios of 0.49–1.03, yielding 207Pb/206Pb ages rangingfrom 2434 ± 8 Ma to 2520 ± 12 Ma., and combining a concordiaupper intercept age of 2505 ± 26 Ma and a lower intercept age of1241 ± 420 Ma (MSWD = 0.91) (Fig. 5a). Of the 11 core analyses, 4concordant analyses yield 207Pb/206Pb ages between 2491 ± 13 Maand 2520 ± 12 Ma, combining to a weighted mean 207Pb/206Pb ageof 2507 ± 12 Ma (MSWD = 0.9). The rims yield lower U contentsranging from 16 to 94 ppm with Th/U ratios of 0.50–1.02. Ten rimanalyses produced 207Pb/206Pb ages ranging from 2407 ± 30 Ma to2566 ± 86 Ma and a concordia upper intercept age of 2474 ± 25 Maand lower intercept age of 949 ± 460 Ma (MSWD = 0.4) (Fig. 5b).The analyses from the zircon cores and rims are almost indis-tinguishable, implying a short time interval between the lateNeoarchean magmatism and the high-grade regional metamor-phism. Combining all the data yields a concordia upper interceptage of 2493 ± 14 Ma and a lower intercept age of 1041 ± 270 Ma(Fig. 5c).

5.1.2. Sample LS11-4-2.1The zircon grains from sample LS11-4-2.1 are primarily pris-

matic and oval crystals with pyramidal terminations. Similar tosample LS10-8-1.1, most of the grains exhibit core–rim structures(Fig. 6). The zircon cores exhibit dark and gray CL intensities withdiscernible oscillatory zoning. Some zircon grains show more com-plex structure with CL-gray mantle between CL-dark core andCL-bright rim (Fig. 6d). These CL-gray domains show blurred andwide oscillatory zoning, representing the incompletely recrystal-lized portion of the core. The CL-bright rims show no discernibleinternal structure, suggesting metamorphic origin. In addition,most zircon grains have a thin outer rim with variable CL inten-sity (Fig. 6), forming a discontinuous edge, which was too narrowto analyze.

Forty-four laser ablation analyses were obtained from 38grains (Table 2 and Fig. 7). Thirty-four analyses were con-ducted on the magmatic zircon core. Except for three coreanalyses with 207Pb/206Pb ages of 2640 ± 13 Ma, 2742 ± 4 Ma and2787 ± 5 Ma, which are probably inherited zircon grains, the other31 core analyses show various radiogenic and define a upperintercept 207Pb/206Pb age of 2550 ± 34 Ma and a lower inter-cept age of 1019 ± 32 Ma (MSWD = 1.5) (Fig. 7c). Ten analysesfrom the zircon rims produced 207Pb/206Pb ages ranging from2409 ± 4 Ma to 2588 ± 55 Ma, yielding a concordia upper interceptage of 2527 ± 49 Ma and a lower intercept age of 859 ± 390 Ma(MSWD = 0.98). Similar to sample L011-8-1.1, these data imply thatthe analyses from the zircon cores and rims are indistinguishable.Combining all the data yields a concordia upper intercept age of2554 ± 30 Ma and a lower intercept age of 1096 ± 250 Ma (Fig. 7b).

5.1.3. Sample LS11-1-5.1Zircon grains from sample LS11-1-5.1 are 100–250 �m in diam-

eter and show oval, nearly spherical and prismatic morphologies.CL images reveal that most zircons contain CL-dark and oscillatoryzoned cores, CL-bright and structureless mantle, and CL-gray rims(Fig. 8c–e and g–i). We intercept the zircon core to have a mag-matic origin, with a mantle and rim resulting from metamorphicrecrystallization and overgrowth. Some zircons have CL-dark coressurrounded by only CL-bright or CL-gray domains (Fig. 8b and m). Afew zircons do not have cores but consist entirely of CL-gray grainswith patchy or rare fir-tree patterns (Fig. 8f and k), similar to thoseof the rims surrounding the cores and mantle, suggesting that they

are entirely of metamorphic origin.

Seventy-nine analyses in total were performed on 51 grains:25 spots using SHRIMP II and 54 spots using LA-MC-ICPMS.The twenty-five SHRIMP analyses include 9 analyses from

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235 (2013) 36– 57

Table 2LA-MC-ICPMS U–Pb analytical data for zircon from sample LS11-4-2.1.

Spots Pb (ppm) Th (ppm) U (ppm) Th/U 207Pb/235U 1� 206Pb/238U 1� 207Pb/206Pb 1� Age (Ma) Textural locations

206Pb/238U 1� 207Pb/235U 1� 207Pb/206Pb 1�

LS11-4-2.1-1 399 44 89 0.4931 0.1786 0.0014 11.2184 0.2082 0.4564 0.0086 2423 38 2541 17 2640 13 In-coreLS11-4-2.1-2 211 30 26 1.1186 0.1606 0.0018 9.7605 0.3998 0.4405 0.0182 2353 81 2412 38 2463 20 CoreLS11-4-2.1-3 57 8 18 0.4445 0.1591 0.0013 9.5960 0.2643 0.4373 0.0113 2338 51 2397 25 2446 15 RimLS11-4-2.1-4 327 40 50 0.8078 0.1718 0.0009 10.7659 0.1656 0.4540 0.0059 2413 26 2503 14 2576 9 CoreLS11-4-2.1-5 147 16 13 1.2403 0.1953 0.0006 14.7755 0.1888 0.5479 0.0061 2817 25 2801 12 2787 5 In-coreLS11-4-2.1-6 102 14 11 1.2850 0.1587 0.0007 8.5215 0.0862 0.3894 0.0036 2120 17 2288 9 2442 7 RimLS11-4-2.1-7 205 22 15 1.4323 0.1900 0.0005 13.8734 0.1167 0.5295 0.0041 2739 17 2741 8 2742 4 In-coreLS11-4-2.1-8 226 23 39 0.6022 0.1619 0.0006 9.9815 0.1157 0.4473 0.0051 2383 23 2433 11 2476 7 CoreLS11-4-2.1-9 194 23 14 1.6427 0.1621 0.0008 8.8729 0.0902 0.3969 0.0036 2155 17 2325 9 2480 8 CoreLS11-4-2.1-10 1185 116 49 2.3643 0.1709 0.0016 10.9020 0.2041 0.4625 0.0069 2451 31 2515 17 2566 16 CoreLS11-4-2.1-11 289 25 57 0.4446 0.1656 0.0008 10.5512 0.1260 0.4621 0.0052 2449 23 2484 11 2514 9 CoreLS11-4-2.1-12 319 27 32 0.8337 0.1685 0.0007 10.7270 0.1367 0.4619 0.0058 2448 26 2500 12 2542 7 CoreLS11-4-2.1-13 88 7 25 0.2894 0.1594 0.0032 9.7435 0.3675 0.4444 0.0190 2370 85 2411 35 2450 33 CoreLS11-4-2.1-14 53 5 4 1.0487 0.1580 0.0020 9.7545 0.2867 0.4480 0.0119 2386 53 2412 27 2435 22 RimLS11-4-2.1-15 90 8 7 1.1324 0.1644 0.0010 10.0579 0.1221 0.4440 0.0050 2369 22 2440 11 2502 16 CoreLS11-4-2.1-16 239 17 11 1.6506 0.1664 0.0013 10.3290 0.1622 0.4504 0.0069 2397 31 2465 15 2522 13 RimLS11-4-2.1-17 1051 73 50 1.4530 0.1665 0.0008 10.5291 0.1297 0.4586 0.0052 2434 23 2482 11 2524 7 CoreLS11-4-2.1-18 292 20 13 1.5634 0.1701 0.0008 10.7336 0.1188 0.4579 0.0051 2430 22 2500 10 2559 9 CoreLS11-4-2.1-19 100 8 8 1.0098 0.1598 0.0022 9.6307 0.1884 0.4374 0.0068 2339 30 2400 18 2454 23 CoreLS11-4-2.1-20 263 21 10 2.1819 0.1617 0.0017 9.8861 0.1321 0.4439 0.0054 2368 24 2424 12 2474 17 CoreLS11-4-2.1-21 313 26 61 0.4231 0.1572 0.0010 7.9915 0.0897 0.3683 0.0028 2021 13 2230 10 2426 10 CoreLS11-4-2.1-22 91 7 6 1.1238 0.1719 0.0013 10.9551 0.1632 0.4631 0.0067 2453 30 2519 14 2576 13 RimLS11-4-2.1-23 59 20 12 1.70 0.1579 0.0013 9.0443 0.2753 0.4152 0.0124 2238 56 2342 28 2433 13 RimLS11-4-2.1-24 45 13 33 0.41 0.1556 0.0012 8.7245 0.1509 0.4070 0.0069 2201 31 2310 16 2409 13 RimLS11-4-2.1-25 139 59 31 1.93 0.1612 0.0012 9.3827 0.1466 0.4217 0.0054 2268 24 2376 14 2468 13 CoreLS11-4-2.1-26 108 48 35 1.37 0.1562 0.0008 8.6145 0.1083 0.4003 0.0048 2170 22 2298 11 2415 9 CoreLS11-4-2.1-27 50 26 17 1.51 0.1644 0.0013 9.4642 0.1915 0.4174 0.0078 2249 35 2384 19 2502 14 CoreLS11-4-2.1-28 27 13 13 1.02 0.1665 0.0015 9.7014 0.2350 0.4240 0.0103 2279 47 2407 22 2524 14 RimLS11-4-2.1-29 37 6 68 0.09 0.1648 0.0009 9.9621 0.1136 0.4385 0.0046 2344 21 2431 11 2506 8 CoreLS11-4-2.1-30 111 52 39 1.35 0.1671 0.0013 10.1038 0.1838 0.4389 0.0075 2346 34 2444 17 2529 13 CoreLS11-4-2.1-31 57 19 14 1.33 0.1644 0.0013 9.9311 0.2717 0.4371 0.0109 2338 49 2428 25 2502 14 CoreLS11-4-2.1-32 424 102 60 1.70 0.1669 0.0007 10.4606 0.1225 0.4545 0.0049 2415 22 2476 11 2527 7 CoreLS11-4-2.1-33 173 28 18 1.57 0.1600 0.0015 9.4210 0.2153 0.4278 0.0096 2296 43 2380 21 2457 16 CoreLS11-4-2.1-34 103 21 30 0.72 0.1619 0.0008 9.7508 0.1293 0.4369 0.0054 2337 24 2412 12 2476 3 CoreLS11-4-2.1-35 59 2 62 0.03 0.1714 0.0011 10.6905 0.1728 0.4528 0.0073 2408 32 2497 15 2572 10 RimLS11-4-2.1-36 236 20 75 0.27 0.1621 0.0006 9.8951 0.1014 0.4427 0.0041 2362 18 2425 9 2477 6 CoreLS11-4-2.1-37 309 31 55 0.56 0.1652 0.0007 10.2970 0.1240 0.4520 0.0051 2404 23 2462 11 2510 8 CoreLS11-4-2.1-38 138 6 4 1.46 0.1731 0.0057 11.6498 0.7549 0.4870 0.0307 2558 133 2577 61 2588 55 RimLS11-4-2.1-39 549 31 22 1.39 0.1643 0.0010 10.3802 0.1707 0.4580 0.0068 2431 30 2469 15 2502 10 CoreLS11-4-2.1-40 153 13 72 0.18 0.1649 0.0007 10.1310 0.1117 0.4456 0.0045 2375 20 2447 10 2506 7 CoreLS11-4-2.1-41 54 10 12 0.83 0.1669 0.0011 11.0456 0.2943 0.4797 0.0123 2526 53 2527 25 2528 11 CoreLS11-4-2.1-42 118 9 12 0.78 0.1647 0.0019 10.4565 0.3358 0.4610 0.0143 2444 63 2476 30 2506 19 RimLS11-4-2.1-43 240 21 18 1.18 0.1591 0.0013 9.8725 0.2102 0.4505 0.0096 2397 43 2423 20 2447 14 CoreLS11-4-2.1-44 150 18 23 0.82 0.1569 0.0009 9.0417 0.1712 0.4177 0.0075 2250 34 2342 17 2433 10 Core

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43

Table 3SHRIMP U–Pb analytical data for zircon from sample LS11-1-5.1.

Spots U (ppm) Th (ppm) Th/U Pb* Com. Pb (%) 207Pb*/206Pb* 1� (%) 207Pb*/235U 1� (%) 206Pb */238U 1� (%) Age (Ma) Textural locations

206Pb*/238U 1� 207Pb*/206Pb* 1�

LS11-1-5.1-1 122 73 0.62 50.1 0.05 0.1664 0.76 10.99 2.6 0.479 2.5 2522 53 2522 13 CoreLS11-1-5.1-2 37 4 0.11 10.5 0.66 0.1125 3.0 5.15 4.0 0.3317 2.7 1846 43 1841 54 RimLS11-1-5.1-3 461 148 0.33 181 0.06 0.1623 0.39 10.20 2.2 0.4557 2.1 2421 43 2479 7 CoreLS11-1-5.1-4 39 24 0.62 10.9 0.25 0.1331 1.9 5.97 3.2 0.3251 2.7 1814 42 2140 32 MantleLS11-1-5.1-5 38 9 0.24 11.0 0.82 0.1113 3.0 5.10 4.0 0.3324 2.7 1850 44 1822 54 RimLS11-1-5.1-6 246 143 0.60 99.3 0.04 0.1659 0.54 10.73 2.2 0.469 2.2 2479 45 2517 9 CoreLS11-1-5.1-7 42 41 1.00 17.0 0.24 0.1593 1.4 10.24 3.0 0.466 2.6 2467 53 2448 24 MantleLS11-1-5.1-8 71 34 0.49 20.8 0.11 0.1120 1.4 5.27 3.1 0.3414 2.7 1894 45 1832 25 RimLS11-1-5.1-9 133 60 0.47 53.7 0.04 0.1615 0.79 10.45 2.4 0.469 2.3 2481 47 2472 13 CoreLS11-1-5.1-10 34 33 0.99 14.1 0.62 0.1614 2.0 10.54 3.4 0.474 2.8 2501 58 2470 33 MantleLS11-1-5.1-11 44 6 0.15 12.5 0.21 0.1160 2.3 5.34 3.5 0.3337 2.7 1856 43 1895 41 RimLS11-1-5.1-12 40 6 0.14 11.6 0.56 0.1131 3.7 5.28 4.6 0.3388 2.8 1881 45 1850 66 RimLS11-1-5.1-13 60 123 2.12 16.6 0.71 0.1106 2.5 4.87 4.5 0.320 3.8 1788 59 1809 45 RimLS11-1-5.1-14 287 232 0.84 112 0.10 0.1605 0.77 10.01 2.8 0.452 2.7 2405 55 2461 13 CoreLS11-1-5.1-15 31 8 0.26 8.84 0.81 0.1077 4.5 4.85 5.4 0.3267 3.0 1823 47 1761 83 RimLS11-1-5.1-16 626 290 0.48 194 0.20 0.1465 0.49 7.25 2.2 0.3591 2.2 1978 37 2305 8 CoreLS11-1-5.1-17 26 40 1.61 8.88 0.30 0.1549 2.3 8.56 4.0 0.401 3.3 2173 61 2400 39 MantleLS11-1-5.1-18 29 8 0.30 6.99 0.77 0.1135 4.5 4.43 5.4 0.2830 3.0 1606 42 1856 82 RimLS11-1-5.1-19 125 61 0.50 43.7 0.06 0.1572 0.93 8.85 3.3 0.408 3.1 2207 58 2426 16 MantleLS11-1-5.1-20 41 137 3.49 11.6 0.00 0.1133 2.1 5.20 3.4 0.3329 2.7 1853 43 1853 37 RimLS11-1-5.1-21 132 83 0.65 51.9 0.15 0.1620 0.85 10.21 2.4 0.457 2.3 2426 46 2477 14 CoreLS11-1-5.1-22 77 53 0.71 30.2 0.26 0.1567 1.1 9.88 2.6 0.458 2.4 2429 47.7 2420 19 MantleLS11-1-5.1-23 263 161 0.63 105 0.05 0.1612 0.53 10.31 2.2 0.464 2.2 2458 44 2468 9 CoreLS11-1-5.1-24 119 74 0.65 38.1 0.15 0.1488 0.84 7.64 2.4 0.3725 2.2 2041 39 2332 14 CoreLS11-1-5.1-25 46 18 0.40 13.1 0.89 0.1103 3.5 5.00 4.0 0.3285 2.1 1831 33 1805 63 Rim

44 J. Zhang et al. / Precambrian Research 235 (2013) 36– 57

F cles shs �m)

CtztAa(d

ig. 6. Representative CL images of zircons from sample LS11-4-2.1. The smaller cirpot is labeled with its individual 207Pb/206Pb ages and 176Hf/177Hf ratios (scale is 50

L-dark oscillatory-zoned cores, 6 analyses from CL-bright man-les and 10 analyses from the CL-gray rims. The data from theircon cores produce 207Pb/206Pb ages ranging from 2305 ± 8 Mao 2520 ± 13 Ma (Table 3), indicating a variable radiogenic Pb loss.ll 9 magmatic zircon cores yield a concordia upper intercept

ge of 2503 ± 20 Ma and a lower intercept age of 1124 ± 150 MaMSWD = 0.77). Six analyses from the CL-bright mantles pro-uced 207Pb/206Pb ages ranging from 2140 ± 32 Ma to 2470 ± 33 Ma

Fig. 7. Concordia diagrams for zircon U–Pb anal

ow locations for U–Pb analyses, and the bigger circles for Hf isotopic analyses. Each.

and a concordia upper intercept age of 2490 ± 20 Ma and lowerintercept age of 1176 ± 260 Ma (MSWD = 1.5) (Fig. 9a). Ten anal-yses from the CL-gray rims produce 207Pb/206Pb ages between1761 ± 83 Ma and 1895 ± 41 Ma, yielding a concordia upper inter-cept age of 1837 ± 30 Ma and lower intercept age of 144 ± 1400 Ma

(MSWD = 0.51). Nine concordant analyses give a weighted meanage of 1838 ± 28 Ma (n = 10, MSWD = 0.43) (Fig. 9b). Most metamor-phic zircon mantles and rims are characterized by very low U, Th

yses of granodioritic gneisses LS11-4-2.1.

J. Zhang et al. / Precambrian Research 235 (2013) 36– 57 45

Fig. 8. Representative CL images of zircons from sample LS11-1-5.1. The smaller circles show locations for U–Pb analyses, and the bigger circles for Hf isotopic analyses. Eachs are fo

aac

byCadayaac(b

5

tsAs

pot is labeled with its individual 207Pb/206Pb ages and 176Hf/177Hf ratios. (e) and (f)

nd radiogenic Pb. Relatively large uncertainties on the individualnalyses are due to very low abundance and the high proportion ofommon lead.

Fifty-four laser ablation analyses yield similar results to thosey SHRIMP analyses. These laser ablation analyses include 25 anal-ses from CL-dark and oscillatory-zoned cores, 15 analyses fromL-bright mantles and 14 analyses from the CL-gray rims (Table 4nd Fig. 10). Twenty-five magmatic zircon cores yield a concor-ia upper intercept age of 2552 ± 41 Ma and a lower interceptge of 1084 ± 20 Ma (MSWD = 0.72); fifteen CL-bright mantle anal-ses produce a concordia upper intercept age of 2534 ± 49 Mand lower intercept age of 1046 ± 30 Ma (MSWD = 0.26) (Fig. 10c);nd fourteen CL-gray rim analyses yield a concordia upper inter-ept age of 1882 ± 58 Ma and lower intercept age of 618 ± 630 MaMSWD = 1.02) (Fig. 10d). These results are consistent with analysesy SHRIMP within error.

.1.4. Sample LS11-1-1.1The zircon grains from sample LS11-1-1.1 are generally similar

o those of sample LS11-1-5.1. Most of the zircons are near-pherical and prismatic grains and 100–300 �m in size (Fig. 11).

few zircon grains contain CL-dark and oscillatory zoned coresurrounded by CL-bright mantles and CL-gray rims (Fig. 11d and

Fig. 9. Concordia diagrams for zircon SHRIMP U–Pb

r SHRIMP analyze, and (g)–(m) for LA-ICPMS analyses (scale is 50 �m).

e). However, most grains contain only CL-dark cores surroundedby CL-bright mantle with variable width. Two zircon grains includeCL-bright inner core surrounded by CL-dark and oscillatory zoneddomains (Table 5 and Fig. 11a). We interpret the inner core asinherited or xenocrystic origin.

Thirty-six laser ablation analyses were obtained from 30 grains(Table 5 and Fig. 12). Two analyses were conducted on the inher-ited zircon cores, yielding 207Pb/206Pb age of 2817 ± 28 Ma and2836 ± 29 Ma (Fig. 12a). Twenty-two analyses from the CL-darkand oscillatory zoned cores yield 207Pb/206Pb ages ranging from2440 ± 28 Ma and 25 13 ± 28 Ma, indicating a variable radiogenicPb loss. Five analyses from CL-bright and structureless mantles give207Pb/206Pb ages between 2399 ± 29 Ma and 2529 ± 29 Ma. Similarto the other samples described above, the analyses from the CL-darkzircon cores and CL-bright mantles are almost indistinguishable.Combining all data from CL-dark cores and mantles yields a con-cordia upper intercept age of 2492 ± 18 Ma and a lower interceptage of 849 ± 460 Ma (Fig. 12b).

Only two analyses from CL-gray metamorphic rims obtained.

The two analyses are concordant and give 207Pb/206Pb ages of1826 ± 32 Ma and 1870 ± 34 Ma, respectively. This is consistentwith ca. 1850 Ma metamorphic age obtained in sample LS11-1-5.1(Fig. 12a).

analyses of trondhjemitic gneisses LS11-1-5.1.

46J.

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Research

235 (2013) 36– 57Table 4LA-MC-ICPMS U–Pb analytical data for zircon from sample LS11-1-5.1.

Spots Pb (ppm) Th (ppm) U (ppm) Th/U 207Pb/235U 1� 206Pb/238U 1� 207Pb/206Pb 1� Age (Ma) Textural locations

206Pb/238U 1� 207Pb/235U 1� 207Pb/206Pb 1�

LS11-1-5.1-1 398 70 99 0.71 9.7597 0.4016 0.4369 0.0208 0.1624 0.0023 2337 94 2412 38 2481 24 CoreLS11-1-5.1-2 43 12 5 2.50 8.3929 0.3297 0.3938 0.0143 0.1545 0.0024 2140 66 2274 36 2398 32 MantleLS11-1-5.1-3 146 22 18 1.20 8.7041 0.5189 0.4271 0.0283 0.1635 0.0116 2293 128 2308 54 2492 119 CoreLS11-1-5.1-4 80 14 7 2.16 9.9656 0.3393 0.4328 0.0138 0.1667 0.0017 2318 62 2432 31 2525 16 MantleLS11-1-5.1-5 296 41 63 0.65 9.7468 0.1927 0.4452 0.0090 0.1588 0.0009 2374 40 2411 18 2443 9 CoreLS11-1-5.1-6 94 12 7 1.67 9.6788 0.3978 0.4324 0.0179 0.1624 0.0023 2317 80 2405 38 2481 24 MantleLS11-1-5.1-7 296 28 30 0.90 9.0028 0.3763 0.3981 0.0184 0.1639 0.0013 2160 85 2338 38 2496 13 CoreLS11-1-5.1-8 449 48 53 0.89 9.3141 0.2502 0.4047 0.0124 0.1668 0.0024 2191 57 2369 25 2526 24 CoreLS11-1-5.1-9 12 2 13 0.15 4.5367 0.0999 0.2959 0.0068 0.1113 0.0012 1671 34 1738 18 1821 15 RimLS11-1-5.1-10 229 27 37 0.72 9.0009 0.1821 0.4140 0.0082 0.1574 0.0010 2233 38 2338 18 2429 11 CoreLS11-1-5.1-11 701 110 103 1.06 7.0972 0.3838 0.3313 0.0184 0.1551 0.0007 1845 89 2124 48 2403 7 CoreLS11-1-5.1-12 1431 260 154 1.70 9.7265 0.1905 0.4337 0.0084 0.1625 0.0012 2322 38 2409 18 2481 13 CoreLS11-1-5.1-13 11 2 8 0.31 4.3588 0.1232 0.2893 0.0076 0.1090 0.0012 1638 38 1705 23 1784 19 RimLS11-1-5.1-14 28 7 6 1.06 5.0416 0.2702 0.2762 0.0123 0.1323 0.0062 1572 62 1826 45 2129 77 MantleLS11-1-5.1-15 207 33 44 0.76 8.5695 0.2848 0.4053 0.0109 0.1530 0.0015 2193 50 2293 30 2380 18 CoreLS11-1-5.1-16 140 14 7 1.88 11.2269 0.2519 0.4779 0.0106 0.1701 0.0014 2518 46 2542 21 2559 13 MantleLS11-1-5.1-17 681 68 48 1.42 9.7624 0.4337 0.4409 0.0194 0.1605 0.0013 2355 87 2413 41 2461 14 CoreLS11-1-5.1-18 189 17 14 1.28 8.6794 0.6272 0.4059 0.0262 0.1546 0.0032 2196 120 2305 66 2398 35 CoreLS11-1-5.1-19 175 16 16 0.98 10.2446 0.4828 0.4558 0.0193 0.1626 0.0023 2421 86 2457 44 2483 24 CoreLS11-1-5.1-20 446 37 24 1.55 10.0315 0.4122 0.4347 0.0179 0.1672 0.0011 2327 81 2438 38 2529 10 CoreLS11-1-5.1-21 462 65 18 3.55 4.6152 0.0930 0.2959 0.0064 0.1131 0.0009 1671 32 1752 17 1850 15 RimLS11-1-5.1-22 3 9 9 1.01 8.7360 0.1034 0.4212 0.0149 0.1507 0.0059 2266 67 2311 11 2353 68 MantleLS11-1-5.1-23 450 36 30 1.20 9.5248 0.3667 0.4198 0.0105 0.1644 0.0032 2260 48 2390 35 2502 61 CoreLS11-1-5.1-24 161 13 11 1.23 9.7937 0.2346 0.4336 0.0092 0.1636 0.0018 2322 41 2416 22 2494 17 CoreLS11-1-5.1-25 41 13 6 2.32 10.3387 0.6636 0.4514 0.0242 0.1658 0.0023 2401 107 2466 59 2517 24 MantleLS11-1-5.1-26 259 35 43 0.82 6.6950 0.0978 0.3361 0.0046 0.1445 0.0006 1868 22 2072 13 2283 12 CoreLS11-1-5.1-27 256 30 25 1.20 10.1860 0.1809 0.4470 0.0073 0.1653 0.0010 2382 33 2452 16 2511 10 CoreLS11-1-5.1-28 57 11 4 2.58 10.7698 0.5463 0.4593 0.0207 0.1692 0.0025 2437 91 2503 47 2549 24 MantleLS11-1-5.1-29 440 31 30 1.03 10.3732 0.2133 0.4479 0.0085 0.1678 0.0014 2386 38 2469 19 2535 14 CoreLS11-1-5.1-30 198 16 16 0.96 10.5310 0.4036 0.4482 0.0152 0.1696 0.0020 2387 68 2483 36 2554 19 CoreLS11-1-5.1-31 91 7 4 1.68 9.5723 0.5677 0.4422 0.0295 0.1589 0.0034 2360 132 2395 55 2444 35 MantleLS11-1-5.1-32 49 10 4 2.25 10.1973 0.6899 0.4562 0.0272 0.1611 0.0037 2423 121 2453 63 2478 39 MantleLS11-1-5.1-33 290 24 14 1.73 9.8765 0.2211 0.4362 0.0099 0.1646 0.0016 2334 44 2423 21 2503 17 CoreLS11-1-5.1-34 1697 156 141 1.10 10.1233 0.1346 0.4463 0.0061 0.1646 0.0007 2379 27 2446 12 2506 6 CoreLS11-1-5.1-35 180 17 12 1.41 9.6633 0.2538 0.4419 0.0097 0.1581 0.0017 2359 44 2403 24 2435 19 CoreLS11-1-5.1-36 242 23 25 0.94 9.7676 0.5385 0.4408 0.0188 0.1599 0.0028 2354 84 2413 51 2455 29 CoreLS11-1-5.1-37 533 83 27 3.10 4.6403 0.0646 0.2976 0.0041 0.1131 0.0006 1679 20 1757 12 1850 9 RimLS11-1-5.1-38 14 5 15 0.30 5.0831 0.1174 0.3206 0.0067 0.1150 0.0011 1793 33 1833 20 1879 18 RimLS11-1-5.1-39 124 14 12 1.16 10.8440 0.3463 0.4687 0.0145 0.1681 0.0015 2478 64 2510 30 2539 15 MantleLS11-1-5.1-40 131 11 18 0.64 10.0193 0.1887 0.4449 0.0080 0.1634 0.0011 2373 36 2437 17 2491 11 MantleLS11-1-5.1-41 837 72 43 1.67 4.7774 0.0488 0.3085 0.0030 0.1123 0.0006 1733 15 1781 9 1839 10 RimLS11-1-5.1-42 22 <1 15 <0.01 5.0408 0.1162 0.3209 0.0070 0.1141 0.0011 1794 34 1826 20 1865 18 RimLS11-1-5.1-43 259 14 8 1.71 9.9203 0.4540 0.4519 0.0190 0.1583 0.0018 2404 84 2427 42 2439 18 MantleLS11-1-5.1-44 57 16 12 1.39 5.1057 0.1348 0.3273 0.0091 0.1136 0.0013 1825 44 1837 22 1858 21 RimLS11-1-5.1-45 996 62 24 2.61 4.8205 0.0727 0.3102 0.0045 0.1128 0.0008 1741 22 1788 13 1856 13 RimLS11-1-5.1-46 166 6 14 0.44 5.0427 0.1147 0.3200 0.0063 0.1141 0.0011 1790 31 1827 19 1865 18 RimLS11-1-5.1-47 19 4 10 0.43 5.2323 0.1669 0.3322 0.0097 0.1141 0.0013 1849 47 1858 27 1866 16 RimLS11-1-5.1-48 49 5 16 0.29 4.9234 0.1248 0.3171 0.0074 0.1126 0.0011 1776 36 1806 21 1843 13 RimLS11-1-5.1-49 397 26 16 1.62 10.6289 0.2666 0.4647 0.0105 0.1657 0.0014 2460 46 2491 23 2515 15 CoreLS11-1-5.1-50 321 19 13 1.46 8.7549 0.1981 0.4091 0.0089 0.1553 0.0012 2211 41 2313 21 2405 13 MantleLS11-1-5.1-51 174 23 12 1.87 9.9943 0.3167 0.4513 0.0131 0.1605 0.0016 2401 58 2434 29 2461 18 MantleLS11-1-5.1-52 40 4 15 0.26 5.1571 0.1139 0.3272 0.0070 0.1144 0.0011 1825 34 1846 19 1872 17 RimLS11-1-5.1-53 2719 633 296 2.14 10.3896 0.9991 0.4627 0.0397 0.1657 0.0038 2452 175 2470 89 2515 39 CoreLS11-1-5.1-54 15 7 12 0.61 5.1645 0.1328 0.3272 0.0087 0.1152 0.0014 1825 42 1847 22 1883 22 Rim

J. Zhang

et al.

/ Precam

brian R

esearch 235 (2013) 36– 57

47

Table 5LA-MC-ICPMS U–Pb analytical data for zircon from sample LS11-1-1.1.

Spots Pb (ppm) U (ppm) Th/U 207Pb/235U 1� 206Pb/238U 1� 207Pb/206Pb 1� Age (Ma) Textural locations

206Pb/238U 1� 207Pb/235U 1� 207Pb/206Pb 1�

LS11.1.1.1.1 22 35 0.55 15.0745 0.2930 0.5432 0.0042 0.2013 0.0036 2797 22 2820 55 2836 29 In-coreLS11.1.1.1.2 241 542 0.13 9.3322 0.1812 0.4283 0.0041 0.1580 0.0026 2298 22 2371 46 2434 28 CoreLS11.1.1.1.3 374 741 0.35 10.4198 0.1955 0.4637 0.0039 0.1630 0.0027 2456 21 2473 46 2487 28 CoreLS11.1.1.1.4 258 569 0.25 9.4039 0.2067 0.4247 0.0057 0.1605 0.0027 2282 31 2378 52 2461 28 CoreLS11.1.1.1.5 89 176 0.22 10.9337 0.2164 0.4745 0.0038 0.1671 0.0029 2503 20 2518 50 2529 29 MantleLS11.1.1.1.6 133 274 0.32 9.9182 0.1739 0.4475 0.0024 0.1608 0.0027 2384 13 2427 43 2464 28 CoreLS11.1.1.1.7 305 568 0.62 10.4815 0.1992 0.4625 0.0043 0.1644 0.0027 2451 23 2478 47 2501 28 CoreLS11.1.1.1.8 382 806 0.49 9.5290 0.1858 0.4246 0.0040 0.1628 0.0027 2281 21 2390 47 2485 28 CoreLS11.1.1.1.9 241 511 0.31 9.7094 0.1732 0.4345 0.0024 0.1621 0.0027 2326 13 2408 43 2477 28 CoreLS11.1.1.1.10 529 1228 0.53 8.4770 0.1713 0.3865 0.0044 0.1591 0.0027 2106 24 2283 46 2446 28 CoreLS11.1.1.1.11 264 610 0.22 8.8998 0.1541 0.4073 0.0017 0.1585 0.0026 2202 9 2328 40 2440 28 CoreLS11.1.1.1.12 209 344 0.39 14.6562 0.2580 0.5346 0.0029 0.1989 0.0033 2761 15 2793 49 2817 27 In-coreLS11.1.1.1.13 82 183 0.26 9.2090 0.1617 0.4216 0.0019 0.1584 0.0027 2268 10 2359 41 2439 28 MantleLS11.1.1.1.14 288 602 0.47 9.6205 0.1652 0.4307 0.0016 0.1620 0.0027 2309 8 2399 41 2476 28 CoreLS11.1.1.1.15 339 716 0.48 9.3549 0.1674 0.4243 0.0026 0.1599 0.0027 2280 14 2373 42 2454 28 CoreLS11.1.1.1.16 123 267 0.23 9.6768 0.1944 0.4372 0.0047 0.1605 0.0027 2338 25 2405 48 2461 28 CoreLS11.1.1.1.17 342 719 0.55 9.6252 0.1714 0.4244 0.0024 0.1645 0.0027 2281 13 2400 43 2502 28 CoreLS11.1.1.1.18 324 705 0.32 9.5850 0.1997 0.4298 0.0050 0.1617 0.0027 2305 27 2396 50 2473 28 CoreLS11.1.1.1.19 232 503 0.17 9.8675 0.1700 0.4411 0.0018 0.1622 0.0027 2355 10 2422 42 2479 28 CoreLS11.1.1.1.20 78 160 0.06 10.7401 0.2162 0.4739 0.0049 0.1643 0.0028 2501 26 2501 50 2501 28 MantleLS11.1.1.1.21 462 1088 0.62 8.4979 0.1575 0.3762 0.0030 0.1638 0.0027 2058 17 2286 42 2496 28 CoreLS11.1.1.1.22 246 618 0.22 8.2458 0.2305 0.3755 0.0092 0.1597 0.0027 2055 50 2258 63 2452 29 CoreLS11.1.1.1.23 90 202 0.24 9.0266 0.1823 0.4232 0.0047 0.1547 0.0026 2275 25 2341 47 2399 29 MantleLS11.1.1.1.24 283 641 0.64 8.8317 0.1589 0.4086 0.0029 0.1568 0.0026 2208 15 2321 42 2422 28 CoreLS11.1.1.1.25 293 619 0.41 9.4833 0.1704 0.4303 0.0020 0.1598 0.0027 2307 11 2386 43 2454 29 CoreLS11.1.1.1.26 284 604 0.49 9.4883 0.1676 0.4259 0.0022 0.1616 0.0027 2287 12 2386 42 2472 28 CoreLS11.1.1.1.27 210 411 0.38 10.6660 0.1862 0.4674 0.0022 0.1655 0.0028 2472 12 2494 44 2513 28 CoreLS11.1.1.1.28 46 132 0.59 4.9752 0.0924 0.3233 0.0020 0.1116 0.0020 1806 11 1815 34 1826 32 RimLS11.1.1.1.29 235 480 0.55 9.8667 0.1761 0.4445 0.0025 0.1610 0.0027 2371 14 2422 43 2466 28 CoreLS11.1.1.1.30 243 565 0.50 8.6268 0.1812 0.3948 0.0050 0.1585 0.0027 2145 27 2299 48 2440 28 CoreLS11.1.1.1.31 23 45 0.38 10.3907 0.2211 0.4617 0.0044 0.1632 0.0030 2447 23 2470 53 2489 31 MantleLS11.1.1.1.32 225 538 0.49 8.3171 0.1798 0.3823 0.0050 0.1578 0.0026 2087 27 2266 49 2432 28 CoreLS11.1.1.1.33 161 345 0.36 9.4902 0.1614 0.4298 0.0012 0.1602 0.0027 2305 7 2387 41 2458 28 CoreLS11.1.1.1.34 72 141 0.41 10.6007 0.1896 0.4669 0.0028 0.1647 0.0028 2470 15 2489 45 2505 28 CoreLS11.1.1.1.35 203 489 0.63 7.9594 0.1391 0.3670 0.0020 0.1574 0.0026 2015 11 2226 39 2427 28 CoreLS11.1.1.1.36 28 88 0.15 5.1002 0.1004 0.3235 0.0015 0.1143 0.0021 1807 8 1836 36 1870 34 Rim

Note: In-core: inherited zircon cores.

48 J. Zhang et al. / Precambrian Research 235 (2013) 36– 57

S U–Pb

5

sde2abvbzioF

vri0os(2

hc612

Fig. 10. Concordia diagrams for zircon LA-ICPM

.2. Hf isotopic result

The Lu–Hf isotopes were analyzed on the same or nearly theame zircon spots on which the U–Pb dating had been con-ucted (Figs. 4, 6, 8 and 11). For two granodioritic gneisses,xcept for two inherited zircon cores with 207Pb/206Pb ages of640 Ma and 2742 Ma in sample LS11-4-2.1, the εHf(t) values ofll the other Hf isotopic spots under analysis were calculatedased on 2500 Ma. For two trondhjemitic gneisses, the εHf(t)alues of the magmatic zircon cores and CL-bright mantles areased on 2500 Ma, and the εHf(t) values of the metamorphicircon rims are based on 1850 Ma. The εHf(t) values of two inher-ted zircon cores from sample LS11-1-1.1 were calculated basedn their 207Pb/206Pb ages of 2836 Ma and 2817 Ma (Table 6 andig. 13).

The zircons from the sample LS10-8-1.1 have 176Hf/177Hf ratiosarying from 0.281211 to 0.281360, mainly concentrating on aange from 0.281211 to 0.281312. Their age-corrected εHf(t) values between −0.08 and 5.77 and is concentrated on a range of.08–3.95, except for one relatively low εHf(t) values (−0.08) andne relatively high εHf(t) values (5.77) (Table 6). The correspondingingle-stage zircon Hf model age TDM is between 2.59 and 2.81 GaTable 6), and the two-stage zircon Hf model age TDMC ranges from.65 to 2.98 Ga.

Except for two inherited cores, zircons from LS11-4-2.1ave 176Hf/177Hf ratios vary from 0.281251 to 0.281377, which

orrespond to age-corrected εHf(t) values between 1.71 and.26 at 2.5 Ga, slightly higher than those of sample LS10-8-.1. The single-stage zircon Hf model age TDM is between.57 Ga and 2.73 Ga (Table 6), and the two-stage zircon Hf

analyses of trondhjemitic gneisses LS11-1-5.1.

model age TDMC ranges from 2.62 Ga to 2.94 Ga. Two inher-ited zircons have 176Hf/177Hf ratios of 0.281300 to 0.281301and age-corrected εHf(t) value of 6.05 and 8.98, correspond-ing to single-stage zircon Hf model age (TDM) of 2.70 Ga and2.68 Ga (Table 6), respectively, closing to their 207Pb/206Pbages.

CL-dark magmatic zircon cores and CL-bright zircon man-tle (∼2.5 Ga group) in sample LS11-1-5.1 have indistinguishable176Hf/177Hf ratios varying from 0.281213 to 0.281394. Their age-corrected εHf(t) value is between −0.46 and 3.7 at 2.5 Ga with TDMranging from 2.56 Ga to 2.80 Ga and TDMC ranging from 2.60 Gato 2.98 Ga (Table 6). Metamorphic zircon rims with an age of∼1.85 Ga exhibited a relatively wide range of Hf isotopic compo-sition with 176Hf/177Hf ratios varying from 0.281319 to 0.281686.The εHf(t) values range from −10.54 to 2.84 at 1.85 Ga, and theTDM values range from 2.14 Ga to 2.65 Ga and TDMC from 2.33 Gato 3.15 Ga.

Except for two inherited zircon core and two ∼1.85 Ga meta-morphic rims, the remaining 32 Hf isotopic analyses from sampleLS11-1-1.1 gave 176Hf/177Hf ratios ranging between 0.281142 and0.281346, corresponding to εHf(t) value of −2.82 to 4.32 at 2.5 Ga,TDM values ranging from 2.65 Ga to 2.92 Ga and TDMC from 2.74 Gato 3.17 Ga (Table 6). Hf isotopic analyses from two inherited zirconcores yield 176Hf/177Hf ratios of 0.280826 to 0.2811119, corre-sponding to age-corrected εHf(t) value of −5.54 and 3.67, TDMof 3.30 Ga and 2.94 Ga, and TDMC of 3.59–3.02 Ga, respectively

(Table 6). Two ∼1.85 Ga metamorphic zircons have 176Hf/177Hfratios of 0.281278 and 0.281301, εHf(t) values of −11.94 and −11.20at 1.85 Ga, TDM values of 2.70–2.67 Ga and TDMC of 3.24–3.19 Ga,respectively.

J. Zhang et al. / Precambrian Research 235 (2013) 36– 57 49

Table 6Lu–Hf isotopic compositions of zircons from four samples in the Beidashan, western Alxa block.

Spots Age (Ma) 176Yb/177Hf 2� 176Lu/177Hf 2� 176Hf/177Hf 2� 176Hf/177Hf (t) εHf(t) TDM (Ma) TDMC (Ma) fLu/Hf

Granodioritic gneiss LS10-8-1.1LS10-8-1.1-1 2500 0.010420 0.000019 0.000305 0.000001 0.281297 0.000021 0.281226 3.46 2677 2788 −0.99LS10-8-1.1-2 2500 0.029623 0.000136 0.000812 0.000008 0.281272 0.000022 0.281283 1.69 2747 2897 −0.99LS10-8-1.1-3 2500 0.027735 0.000247 0.000770 0.000003 0.281272 0.000029 0.281233 1.75 2744 2893 −0.98LS10-8-1.1-4 2500 0.012026 0.000062 0.000351 0.000001 0.281298 0.000022 0.281282 3.42 2679 2791 −0.98LS10-8-1.1-5 2500 0.026775 0.000209 0.000745 0.000004 0.281223 0.000030 0.281188 −0.08 2808 2995 −0.98LS10-8-1.1-6 2500 0.010609 0.000072 0.000302 0.000001 0.281282 0.000032 0.281268 2.93 2697 2821 −0.99LS10-8-1.1-7 2500 0.011166 0.000036 0.000327 0.000001 0.281312 0.000029 0.281297 3.95 2657 2759 −0.99LS10-8-1.1-8 2500 0.011135 0.000041 0.000375 0.000002 0.281237 0.000027 0.281219 1.17 2764 2928 −0.99LS10-8-1.1-9 2500 0.026618 0.000143 0.000840 0.000005 0.281310 0.000026 0.281270 3.00 2697 2817 −0.97LS10-8-1.1-10 2500 0.041795 0.000297 0.001237 0.000009 0.281265 0.000027 0.281206 0.71 2788 2957 −0.96LS10-8-1.1-11 2500 0.009740 0.000051 0.000284 0.000001 0.281229 0.000026 0.281215 1.05 2768 2936 −0.99LS10-8-1.1-12 2500 0.008623 0.000143 0.000282 0.000003 0.281275 0.000026 0.281261 2.69 2706 2835 −0.99LS10-8-1.1-13 2500 0.021953 0.000404 0.000645 0.000008 0.281219 0.000029 0.281188 0.08 2807 2995 −0.98LS10-8-1.1-14 2500 0.024588 0.000084 0.000672 0.000002 0.281290 0.000028 0.281258 2.56 2713 2844 −0.98LS10-8-1.1-15 2500 0.025000 0.000301 0.000687 0.000003 0.281256 0.000037 0.281224 1.35 2759 2918 −0.98LS10-8-1.1-16 2500 0.011768 0.000043 0.000346 0.000001 0.281211 0.000030 0.281194 0.30 2796 2982 −0.99LS10-8-1.1-17 2500 0.032233 0.000235 0.000857 0.000003 0.281246 0.000031 0.281205 0.70 2785 2957 −0.97LS10-8-1.1-18 2500 0.024883 0.000071 0.000633 0.000003 0.281256 0.000033 0.281226 1.44 2755 2912 −0.98LS10-8-1.1-19 2500 0.023104 0.000080 0.000589 0.000004 0.281239 0.000031 0.281210 0.88 2776 2946 −0.98LS10-8-1.1-20 2500 0.009413 0.000071 0.000253 0.000001 0.281360 0.000032 0.281348 5.77 2590 2647 −0.99

Granodioritic gneiss LS11-4-2.1LS11-4-2.1-1 2640 0.030274 0.000036 0.000677 0.000003 0.281300 0.000025 0.281266 6.08 2700 2737 −0.98LS11-4-2.1-2 2500 0.019343 0.000089 0.000411 0.000003 0.281254 0.000023 0.281234 1.73 2743 2894 −0.99LS11-4-2.1-3 2500 0.016449 0.000088 0.000372 0.000001 0.281251 0.000025 0.281234 1.71 2743 2896 −0.99LS11-4-2.1-4 2500 0.022709 0.000045 0.000500 0.000003 0.281349 0.000022 0.281325 4.97 2621 2696 −0.98LS11-4-2.1-5 2500 0.019616 0.000042 0.000419 0.000002 0.281371 0.000023 0.281351 5.89 2586 2640 −0.99LS11-4-2.1-6 2742 0.015715 0.000030 0.000315 0.000002 0.281301 0.000021 0.281281 8.98 2678 2639 −0.99LS11-4-2.1-7 2500 0.016917 0.000046 0.000356 0.000001 0.281371 0.000027 0.281354 5.98 2582 2634 −0.99LS11-4-2.1-8 2500 0.030270 0.000108 0.000636 0.000003 0.281314 0.000025 0.281284 3.48 2678 2787 −0.98LS11-4-2.1-9 2500 0.018033 0.000104 0.000392 0.000003 0.281377 0.000026 0.281362 6.26 2572 2617 −0.99LS11-4-2.1-10 2500 0.014670 0.000048 0.000297 0.000001 0.281314 0.000023 0.281300 4.06 2655 2752 −0.99LS11-4-2.1-11 2500 0.017238 0.000065 0.000342 0.000001 0.281292 0.000020 0.281275 3.19 2688 2805 −0.99LS11-4-2.1-12 2500 0.027321 0.000096 0.000573 0.000003 0.281349 0.000022 0.281322 4.85 2626 2703 −0.98LS11-4-2.1-13 2500 0.014121 0.000144 0.000331 0.000004 0.281344 0.000022 0.281328 5.07 2617 2690 −0.99LS11-4-2.1-14 2500 0.024944 0.000384 0.000521 0.000005 0.281255 0.000023 0.281230 1.57 2750 2904 −0.98LS11-4-2.1-15 2500 0.024268 0.000056 0.000491 0.000001 0.281312 0.000023 0.281288 3.64 2671 2777 −0.99LS11-4-2.1-16 2500 0.020085 0.000324 0.000415 0.000006 0.281328 0.000023 0.281308 4.36 2644 2733 −0.99LS11-4-2.1-17 2500 0.017834 0.000099 0.000383 0.000003 0.281305 0.000024 0.281286 3.58 2673 2781 −0.99LS11-4-2.1-18 2500 0.018913 0.000193 0.000411 0.000003 0.281310 0.000023 0.281290 3.71 2668 2773 −0.99LS11-4-2.1-19 2500 0.039774 0.000114 0.000853 0.000004 0.281352 0.000025 0.281311 4.47 2641 2726 −0.97LS11-4-2.1-20 2500 0.007389 0.000198 0.000132 0.000003 0.281306 0.000022 0.281300 4.07 2654 2751 −1.00LS11-4-2.1-21 2500 0.022181 0.000192 0.000477 0.000005 0.281322 0.000021 0.281300 4.05 2656 2752 −0.99LS11-4-2.1-22 2500 0.027011 0.000075 0.000632 0.000003 0.281309 0.000020 0.281278 3.30 2685 2798 −0.98LS11-4-2.1-23 2500 0.021876 0.000077 0.000479 0.000002 0.281287 0.000022 0.281264 2.80 2703 2829 −0.99LS11-4-2.1-24 2500 0.015712 0.000028 0.000346 0.000000 0.281328 0.000022 0.281312 4.48 2639 2726 −0.99LS11-4-2.1-25 2500 0.014174 0.000086 0.000325 0.000002 0.281352 0.000023 0.281336 5.35 2606 2673 −0.99LS11-4-2.1-26 2500 0.010868 0.000032 0.000252 0.000001 0.281284 0.000023 0.281272 3.08 2691 2812 −0.99LS11-4-2.1-27 2500 0.020553 0.000157 0.000474 0.000003 0.281329 0.000029 0.281307 4.31 2646 2737 −0.99LS11-4-2.1-28 2500 0.015607 0.000034 0.000365 0.000000 0.281273 0.000018 0.281256 2.49 2714 2848 −0.99LS11-4-2.1-29 2500 0.015236 0.000084 0.000356 0.000001 0.281351 0.000019 0.281334 5.27 2609 2678 −0.99LS11-4-2.1-30 2500 0.017809 0.000220 0.000416 0.000003 0.281320 0.000019 0.281300 4.06 2655 2751 −0.99LS11-4-2.1-31 2500 0.020497 0.000090 0.000457 0.000002 0.281312 0.000019 0.281290 3.70 2669 2774 −0.99LS11-4-2.1-32 2500 0.022931 0.000126 0.000480 0.000001 0.281286 0.000021 0.281263 2.75 2705 2832 −0.99

Trondhjemitic gneiss LS11-1-5.1 (same zircons as SHRIMP datings)LS11-1-5.1-1 2500 0.012129 0.000174 0.000383 0.000006 0.281213 0.000023 0.281194 0.31 2796 2981 −0.99LS11-1-5.1-2 1850 0.016877 0.000743 0.000489 0.000022 0.281375 0.000024 0.281355 −2.27 2586 2863 −0.99LS11-1-5.1-3 2500 0.058250 0.000399 0.001646 0.000015 0.281310 0.000026 0.281231 1.61 2756 2902 −0.95LS11-1-5.1-4 2500 0.003519 0.000051 0.000113 0.000001 0.281344 0.000020 0.281338 5.42 2603 2668 −1.00LS11-1-5.1-5 1850 0.010135 0.000057 0.000287 0.000001 0.281319 0.000024 0.281308 −10.54 2648 3150 −0.99LS11-1-5.1-6 2500 0.028859 0.000384 0.000805 0.000006 0.281346 0.000032 0.281307 4.33 2646 2735 −0.98LS11-1-5.1-7 2500 0.010678 0.000035 0.000335 0.000001 0.281269 0.000021 0.281253 2.40 2717 2853 −0.99LS11-1-5.1-8 1850 0.008397 0.000083 0.000243 0.000004 0.281350 0.000025 0.281341 −9.37 2603 3078 −0.99LS11-1-5.1-9 2500 0.035310 0.000531 0.000961 0.000019 0.281279 0.000028 0.281233 1.70 2748 2896 −0.97LS11-1-5.1-10 2500 0.013365 0.000079 0.000397 0.000001 0.281223 0.000028 0.281204 0.64 2784 2961 −0.99LS11-1-5.1-11 1850 0.001207 0.000057 0.000032 0.000002 0.281438 0.000029 0.281437 −5.97 2471 2869 −1.00LS11-1-5.1-12 1850 0.001693 0.000054 0.000039 0.000002 0.281454 0.000032 0.281453 −5.41 2451 2834 −1.00LS11-1-5.1-13 1850 0.010715 0.000083 0.000301 0.000002 0.281465 0.000028 0.281455 −5.34 2452 2830 −0.99LS11-1-5.1-14 2500 0.029066 0.000523 0.000798 0.000011 0.281283 0.000028 0.281245 2.10 2731 2871 −0.98LS11-1-5.1-15 1850 0.001532 0.000047 0.000034 0.000001 0.281474 0.000020 0.281473 −4.70 2424 2790 −1.00LS11-1-5.1-16 2500 0.032449 0.000302 0.000798 0.000006 0.281279 0.000022 0.281241 1.96 2737 2880 −0.98LS11-1-5.1-17 2500 0.015992 0.000038 0.000406 0.000000 0.281327 0.000022 0.281308 4.35 2644 2734 −0.99LS11-1-5.1-18 1850 0.014053 0.000063 0.000381 0.000003 0.281442 0.000023 0.281428 −6.29 2489 2888 −0.99LS11-1-5.1-20 1850 0.014427 0.000189 0.000362 0.000004 0.281380 0.000026 0.281367 −8.46 2571 3022 −0.99LS11-1-5.1-21 2500 0.036853 0.000123 0.000807 0.000002 0.281340 0.000023 0.281302 4.12 2654 2748 −0.98

50 J. Zhang et al. / Precambrian Research 235 (2013) 36– 57

Table 6 (Continued)

Spots Age (Ma) 176Yb/177Hf 2� 176Lu/177Hf 2� 176Hf/177Hf 2� 176Hf/177Hf (t) εHf(t) TDM (Ma) TDMC (Ma) fLu/Hf

LS11-1-5.1-22 2500 0.010834 0.000130 0.000322 0.000002 0.281218 0.000023 0.281203 0.61 2784 2963 −0.99LS11-1-5.1-23 2500 0.044375 0.000432 0.000958 0.000013 0.281346 0.000027 0.281300 4.08 2656 2750 −0.97LS11-1-5.1-24 2500 0.028313 0.000231 0.000813 0.000005 0.281287 0.000029 0.281248 2.23 2726 2864 −0.98LS11-1-5.1-25 1850 0.000930 0.000011 0.000016 0.000000 0.281686 0.000022 0.2816852 2.84 2140 2325 −1.00

Trondhjemitic gneiss LS11-1-5.1 (same zircons as LA-ICPMS datings)LS11-1-5.1-1 2500 0.011674 0.000073 0.000279 0.000001 0.281314 0.000019 0.2813006 4.09 2653 2750 −0.99LS11-1-5.1-3 2500 0.049066 0.001156 0.001109 0.000027 0.281264 0.000024 0.2812109 0.90 2779 2945 −0.97LS11-1-5.1-4 2500 0.015824 0.000139 0.000373 0.000002 0.281311 0.000018 0.2812930 3.82 2664 2767 −0.99LS11-1-5.1-6 2500 0.043382 0.000296 0.000930 0.000003 0.281301 0.000026 0.2812565 2.52 2716 2846 −0.97LS11-1-5.1-7 2500 0.044314 0.001786 0.000918 0.000034 0.281267 0.000022 0.2812229 1.33 2762 2919 −0.97LS11-1-5.1-8 2500 0.039191 0.000085 0.000873 0.000004 0.281338 0.000024 0.2812961 3.93 2662 2760 −0.97LS11-1-5.1-9 1850 0.086719 0.000665 0.001773 0.000006 0.281425 0.000029 0.281362 −8.63 2605 3032 −0.95LS11-1-5.1-10 2500 0.018312 0.000126 0.000396 0.000001 0.281298 0.000018 0.2812793 3.33 2683 2796 −0.99LS11-1-5.1-12 2500 0.021737 0.000057 0.000463 0.000003 0.281262 0.000026 0.2812401 1.94 2735 2882 −0.99LS11-1-5.1-13 1850 0.003489 0.000388 0.000076 0.000009 0.281571 0.000023 0.2815685 −1.30 2296 2581 −1.00LS11-1-5.1-15 2500 0.035876 0.000140 0.000797 0.000002 0.281338 0.000029 0.2812996 4.05 2657 2752 −0.98LS11-1-5.1-16 2500 0.039377 0.000216 0.000866 0.000005 0.281270 0.000028 0.2812288 1.53 2753 2906 −0.97LS11-1-5.1-17 2500 0.028099 0.000306 0.000657 0.000006 0.281345 0.000027 0.2813139 4.56 2637 2721 −0.98LS11-1-5.1-18 2500 0.040288 0.000295 0.000918 0.000004 0.281253 0.000025 0.2812087 0.82 2781 2950 −0.97LS11-1-5.1-19 2500 0.034767 0.000166 0.000869 0.000004 0.281273 0.000023 0.2812315 1.63 2750 2900 −0.97LS11-1-5.1-20 2500 0.012392 0.000051 0.000325 0.000001 0.281341 0.000021 0.2813252 4.96 2621 2696 −0.99LS11-1-5.1-21 1850 0.007794 0.000026 0.000171 0.000001 0.281400 0.000028 0.2813939 −7.51 2531 2963 −0.99LS11-1-5.1-22 2500 0.033956 0.000158 0.000849 0.000002 0.281304 0.000026 0.2812638 2.78 2706 2830 −0.97LS11-1-5.1-23 2500 0.025359 0.000231 0.000616 0.000003 0.281314 0.000023 0.2812844 3.51 2677 2785 −0.98LS11-1-5.1-24 2500 0.074802 0.000550 0.001800 0.000006 0.281273 0.000030 0.2811874 0.06 2817 2996 −0.95LS11-1-5.1-25 2500 0.034216 0.001182 0.000881 0.000032 0.281305 0.000022 0.2812625 2.73 2708 2833 −0.97LS11-1-5.1-27 2500 0.033629 0.000205 0.000854 0.000001 0.281293 0.000021 0.2812519 2.36 2722 2856 −0.97LS11-1-5.1-30 2500 0.036401 0.000819 0.000835 0.000019 0.281295 0.000025 0.2812556 2.49 2717 2848 −0.97LS11-1-5.1-33 2500 0.012775 0.000034 0.000299 0.000001 0.281271 0.000021 0.2812562 2.51 2713 2847 −0.99LS11-1-5.1-34 2500 0.017797 0.000689 0.000475 0.000017 0.281394 0.000022 0.2813714 6.61 2559 2596 −0.99LS11-1-5.1-35 2500 0.016463 0.000283 0.000377 0.000008 0.281366 0.000024 0.2813478 5.76 2591 2647 −0.99LS11-1-5.1-37 1850 0.000956 0.000011 0.000027 0.000000 0.281498 0.000020 0.2814975 −3.83 2391 2737 −1.00LS11-1-5.1-38 1850 0.000706 0.000014 0.000014 0.000000 0.281460 0.000020 0.2814596 −5.17 2441 2820 −1.00LS11-1-5.1-40 2500 0.011333 0.000057 0.000276 0.000001 0.281315 0.000020 0.2813017 4.13 2652 2748 −0.99LS11-1-5.1-41 1850 0.001063 0.000015 0.000019 0.000000 0.281505 0.000019 0.2815048 −3.57 2381 2721 −1.00LS11-1-5.1-42 1850 0.001230 0.000007 0.000022 0.000000 0.281520 0.000021 0.2815197 −3.04 2361 2688 −1.00LS11-1-5.1-44 1850 0.015733 0.000471 0.000360 0.000012 0.281422 0.000022 0.2814090 −6.97 2514 2930 −0.99LS11-1-5.1-45 1850 0.013927 0.000216 0.000362 0.000004 0.281408 0.000020 0.2813953 −7.46 2533 2960 −0.99LS11-1-5.1-47 1850 0.009930 0.000042 0.000251 0.000001 0.281412 0.000023 0.2814031 −7.18 2520 2943 −0.99LS11-1-5.1-52 1850 0.011949 0.000060 0.000333 0.000001 0.281397 0.000024 0.2813856 −7.80 2545 2981 −0.99

Trondhjemitic gneiss LS11-1-1.1LS11-1-1.1-1 2836 0.011893 0.000035 0.000279 0.000002 0.280826 0.000021 0.2808111 −5.54 3302 3594 −0.99LS11-1-1.1-2 2500 0.023361 0.000083 0.000555 0.000002 0.281271 0.000023 0.2812442 2.08 2730 2873 −0.98LS11-1-1.1-3 2500 0.039236 0.000198 0.000927 0.000009 0.281304 0.000026 0.2812601 2.65 2711 2838 −0.97LS11-1-1.1-4 2500 0.020937 0.000225 0.000491 0.000003 0.281261 0.000021 0.2812372 1.83 2739 2888 −0.99LS11-1-1.1-5 2500 0.016586 0.000094 0.000387 0.000001 0.281318 0.000024 0.2812996 4.05 2655 2752 −0.99LS11-1-1.1-6 2500 0.060991 0.000330 0.001377 0.000005 0.281237 0.000029 0.2811714 −0.51 2836 3031 −0.96LS11-1-1.1-7 2500 0.026071 0.000228 0.000555 0.000006 0.281246 0.000025 0.2812197 1.21 2763 2926 −0.98LS11-1-1.1-8 2500 0.012890 0.000201 0.000328 0.000001 0.281235 0.000021 0.2812195 1.20 2762 2926 −0.99LS11-1-1.1-9 2500 0.027909 0.000448 0.000638 0.000007 0.281246 0.000022 0.2812160 1.08 2769 2934 −0.98LS11-1-1.1-10 2500 0.068103 0.000852 0.001345 0.000008 0.281327 0.000029 0.2812623 2.72 2711 2833 −0.96LS11-1-1.1-11 2500 0.029711 0.000522 0.000692 0.000013 0.281312 0.000021 0.2812789 3.32 2684 2797 −0.98LS11-1-1.1-12 2817 0.034320 0.000268 0.000686 0.000005 0.281119 0.000024 0.2810824 3.67 2943 3021 −0.98LS11-1-1.1-13 2500 0.010734 0.000110 0.000233 0.000002 0.281219 0.000024 0.2812083 0.80 2776 2951 −0.99LS11-1-1.1-14 2500 0.037434 0.000331 0.000822 0.000009 0.281346 0.000020 0.2813071 4.32 2647 2736 −0.98LS11-1-1.1-15 2500 0.059756 0.000252 0.001227 0.000007 0.281321 0.000028 0.2812625 2.73 2710 2833 −0.96LS11-1-1.1-16 2500 0.040686 0.000596 0.000882 0.000017 0.281322 0.000024 0.2812796 3.34 2684 2796 −0.97LS11-1-1.1-17 2500 0.035111 0.000202 0.000755 0.000007 0.281142 0.000030 0.2811063 −2.82 2918 3172 −0.98LS11-1-1.1-18 2500 0.022410 0.000059 0.000509 0.000001 0.281242 0.000024 0.2812173 1.12 2766 2931 −0.98LS11-1-1.1-19 2500 0.052661 0.000217 0.001137 0.000005 0.281192 0.000029 0.2811373 −1.72 2880 3105 −0.97LS11-1-1.1-20 2500 0.041624 0.000362 0.000869 0.000004 0.281311 0.000031 0.2812696 2.99 2698 2817 −0.97LS11-1-1.1-21 2500 0.044403 0.000586 0.000975 0.000015 0.281272 0.000025 0.2812258 1.43 2758 2913 −0.97LS11-1-1.1-22 2500 0.024557 0.000580 0.000510 0.000010 0.281250 0.000024 0.2812255 1.42 2755 2913 −0.98LS11-1-1.1-23 2500 0.038166 0.000366 0.000896 0.000010 0.281297 0.000024 0.2812539 2.43 2719 2852 −0.97LS11-1-1.1-24 2500 0.037463 0.000745 0.000875 0.000017 0.281328 0.000025 0.2812858 3.56 2676 2782 −0.97LS11-1-1.1-25 2500 0.030918 0.000643 0.000730 0.000014 0.281287 0.000024 0.2812526 2.38 2720 2855 −0.98LS11-1-1.1-26 2500 0.038637 0.000794 0.000961 0.000020 0.281318 0.000023 0.2812717 3.06 2696 2813 −0.97LS11-1-1.1-27 2500 0.018961 0.000070 0.000504 0.000003 0.281241 0.000022 0.2812164 1.09 2767 2933 −0.98LS11-1-1.1-28 1850 0.008830 0.000077 0.000316 0.000003 0.281301 0.000022 0.2812899 −11.20 2673 3190 −0.99LS11-1-1.1-29 2500 0.009908 0.000048 0.000324 0.000001 0.281200 0.000018 0.2811844 −0.04 2809 3003 −0.99LS11-1-1.1-30 2500 0.049050 0.000392 0.001183 0.000005 0.281293 0.000021 0.2812364 1.80 2745 2890 −0.96LS11-1-1.1-32 2500 0.033567 0.000242 0.000773 0.000007 0.281153 0.000021 0.2811162 −2.47 2905 3151 −0.98LS11-1-1.1-33 2500 0.045676 0.000131 0.001087 0.000002 0.281293 0.000022 0.2812415 1.98 2737 2879 −0.97LS11-1-1.1-34 2500 0.034257 0.000910 0.000686 0.000015 0.281166 0.000023 0.2811328 −1.88 2881 3115 −0.98LS11-1-1.1-35 2500 0.048874 0.000438 0.001014 0.000006 0.281322 0.000021 0.2812738 3.14 2693 2808 −0.97LS11-1-1.1-36 1850 0.008790 0.000097 0.000261 0.000006 0.281278 0.000018 0.2812690 −11.94 2700 3235 −0.99

Notes: The number of analyzed spots is the same as U–Pb datings.

J. Zhang et al. / Precambrian Research 235 (2013) 36– 57 51

F circleE

6

6

mglacpc

ig. 11. Representative CL images of zircons from sample LS11-1-1.1. The smaller

ach spot is labeled with its individual 207Pb/206Pb ages and 176Hf/177Hf ratios.

. Discussion

.1. Explanation of U–Pb dating and Lu–Hf analysis results

The interpretation of zircon U–Pb age data from high-gradeetamorphic rocks, particularly those of Archean to Proterozoic

neiss terranes, is difficult. They experienced more than one geo-ogical event and their zircon may have been affected by multiple

lteration processes (e.g. Gerdes and Zeh, 2009). Zircon grainsollected from two granodioritic gneisses in the Beidashan Com-lex show a typical core–rim structure under CL. Although theores and rims can be easily distinguished by their luminescence

Fig. 12. Concordia diagrams for zircon U–Pb analyses of Trondhj

s show locations for U–Pb analyses, and the bigger circles for Hf isotopic analyses.

intensity and different U and Pb concentrations, which indicatedifferent origins, i.e. magmatic origin for CL-dark and oscillatoryzoned zircon cores and metamorphic origin for CL-bright and struc-tureless zircon rims, the U–Pb dating results show that the coresand rims yield a nearly indistinguishable 207Pb/206Pb age rangeand a similar upper intercept age (∼2.5 Ga, Figs. 5 and 7). Thisresult implies a short time interval between the crystallizationof the magmatism and high-grade regional metamorphism. It is

noteworthy that a narrow CL-structureless outer rim was alsoobserved for some zircon grains in the granodioritic gneisses, prob-ably implying a later metamorphic overprint (e.g. ∼1.85 Ga). Thisis also supported by a ∼1.85 Ga metamorphic age recorded in

emitic gneisses LS11-1-1.1. In-core: inherited zircon core.

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he metamorphic zircons of the micaschists near the granodioriticneisses (Gong et al., unpublished data). In contrast, most zirconrains from sample LS11-1-5.1 and a few zircon grains from sampleS11-1-1.1 show a more complex structure under CL, character-zed by a core–mantle–rim structure. Similar to the zircon coresnd rims in granodioritic gneisses, although the magmatic coresnd metamorphic mantle in trondhjemitic gneisses can be recog-ized in the CL investigation, U–Pb dating results show that theores and mantle yield an indistinguishable 207Pb/206Pb age rangend an upper intercept age of ∼2500 Ma (Figs. 9, 10 and 12). All zir-on rims from two trondhjemitic gneisses yield an age of ∼1.85 GaFigs. 9, 10 and 12). CL images show that these zircon rims haveypical metamorphic origin.

For all four samples, the Lu–Hf isotopic dataset reveal that the2.5 Ga zircon cores have similar measured 176Hf/177Hf ratios to

hose of nearly coeval metamorphic zircon rims (mantle). Thisesult suggests that these ∼2.5 Ga metamorphic zircon rims (man-le) were resulted from pseudomorphic alteration of primarymagmatic) zircon grains (metamorphic recrystallization) ratherhan from new zircon growth (metamorphic overgrowth) (e.g. Zeht al., 2010a). This is also support by blurred oscillatory zone inome CL-bright zircon mantle (rim) which has transitional bound-ries to the primary zircon core (Figs. 4b). These observationsuggest that these ∼2.5 Ga metamorphic zircons were formed in

Lu–Hf closed system, i.e. the U–Pb system is reset, but the Lu–Hfystem remains closed because the initial 176Hf/177Hf in the zir-on lattice is largely unaffected by the alteration (Zeh et al., 2009,010b; Gerdes and Zeh, 2009).

In contrast to the zircon cores and mantles, the zirconims of ∼1.85 Ga in two trondhjemitic gneisses show moreariable 176Hf/177Hf. The 176Hf/177Hf ratios of some ∼1.85 Gaetamorphic zircons overlap those of the magmatic cores

nd metamorphic mantles, implies a pseudomorphic (in situ)issolution/reprecipitation process (or metamorphic recrystal-

ization process) for the formation of some ∼1.85 Ga meta-orphic zircons. However, most metamorphic zircon rims

rom sample LS11-1-5.1 possess higher measured 176Hf/177Hfatios. These characteristics suggest a possible process ofissolution/transportation/reprecipitation (or metamorphic over-rowth) for formation of these metamorphic zircon rimsGerdes and Zeh, 2009; Zeh et al., 2010a,b). The higher76Hf/177Hf values are attributed to the incorporation of addi-ional radiogenic 176Hf formed by 176Lu decay (Gerdes and Zeh,009).

.2. Geological implications

.2.1. Neoarchean–Paleoproterozoic crustal evolution of theestern Alxa block

The U–Pb ages and Lu–Hf isotopic data of the zircons presentedn this study provide significant insights into the crustal growth andeworking of the western Alxa block. These dataset indicate that zir-ons in the Baidashan gneiss of the western Alxa block were formeduring at least two events: during TTG magmatic crystallizationnd subsequent metamorphic recrystallization at almost coeval2.5 Ga, and during metamorphic recrystallization or overgrowtht ∼1.85 Ga. Thus, the presented data provide unambiguous evi-ence that the Beidashan area of the western Alxa block underwenteoarchean magmatism–metamorphism, i.e. the emplacement of

he TTG magmatism and a subsequent high-grade metamorphicvent at the close of the Archean. The short time interval betweenhe initial magmatism and the high-grade regional metamorphism

ignifies the two were related to the same tectonothermal event.imilar cases have been widely reported in NCC (Grant et al., 2009;an et al., 2001; Yang et al., 2008; Nutman et al., 2011, see discus-

ion below).

search 235 (2013) 36– 57

Hf isotopic analyses reveal that the ∼2.5 Ga zircons possessεHf(t) values between −2.82 and 6.61, with depleted mantle Hfmodel ages (TDM) mainly ranging between 2.65 Ga and 2.8 Ga andtwo-stage Hf model ages (TDMC) mainly between 2.7 Ga and 3.0 Ga(Table 6, Figs. 13 and 14). Their Hf isotopic composition is verysimilar to that of the contemporaneous zircons in the NCC base-ment (as discussed below). With the exception of a few slightlynegative εHf(t) values, the Neoarchean zircons have positive εHf(t)values. In the formation age vs. εHf(t) diagram (Fig. 13), these zir-cons are mainly situated between the evolutionary trends of thedepleted mantle and the chondrite. A few ∼2.5 Ga zircons have val-ues of εHf(t) close to the depleted mantle evolutionary line (Table 6and Fig. 13), indicating the presence of juvenile materials in themagma source. This large range of εHf values may indicate mix-ing between the old crust and juvenile materials at the end ofthe Neoarchean. However, all the ∼2.5 Ga zircons show predom-inant peaks of depleted mantle model ages (TDM) at ∼2.7 Ga andtwo-stage Hf model ages (TDMC) at ∼2.8 Ga (Fig. 14). We suggestthat 2.7–2.8 Ga may be an important episode of crustal growthin the western Alxa block. The ∼2.7 Ga crustal growth has beenextensively recorded in the NCC and most ancient cratons world-wide, which is usually considered to be the peak period of globalcrustal growth (Condie et al., 2005, 2009; Jiang et al., 2010; Zhaiand Santosh, 2011 and references therein). This also implies thatthe Late Neoarchean granodioritic and trondhjemitic gneisses werederive from the 2.7 to 2.8 Ga juvenile crustal materials, which werereworked in a very short period at the end of Neoarchean.

In addition, two inherited zircons with 207Pb/206Pb ages of2640 ± 13 Ma, 2742 ± 4 Ma from granodioritic gneiss LS11-4-2.1have high εHf(t) values (6.05 and 8.98) close to the initial Hf isotoperatios of the contemporaneous depleted mantle, correspondingto single-stage zircon Hf model age (TDM) of 2.70 Ga and 2.68 Ga(Table 6). This provides direct evidence that ∼2.7 Ga was a periodof juvenile crust growth. In contrast, two inherited zircon cores(xenocrysts) with similar 207Pb/206Pb age of ∼2.8 Ga from trond-hjemitic gneisses LS11-1-1.1 show significantly lower εHf(t) valueand older Hf model age (TDM of 3.30 Ga and 2.94 Ga, and TDMC of3.59–3.02 Ga), implying the existence of a Paleo-Mesoarchean crustmaterial in the western Alxa block.

The finding of metamorphic zircon recrystallization and over-growth with an age of ∼1.85 Ga in trondhjemitic gneisses suggestsNeoarchean TTG gneisses experienced Paleoproterzoic metamor-phic overprint. This also implies that the Neoarchean crust in thewestern Alxa block was intensively reworked in the middle-latePaleoproterozoic tectonothermal event. However, the metamor-phic condition and evolution around 1.85 Ga have not been wellconstrained. In addition, a 1.93–1.85 Ga amphibolite facies meta-morphic event was also recognized in the Longshoushan area,50 km south of the Baidashan, suggesting the Precambrian meta-morphic basement of the western Alxa block gneiss commonlyexperienced middle-late Paleoproterozoic tectonothermal event.This event coincided with global orogenic events recording frommany other continental fragments, including North China Craton,Laurentia, Baltica, Amazonia and India, which has been correlatedwith the time of assembly of the Palaeoproterozoic superconti-nent Columbia (Rogers and Santosh, 2002, 2009; Zhao et al., 2002,2003, 2004, 2005; Zhao and Cawood, 2012; Condie and Aster,2010; Santosh, 2010). Recent models suggest that the assembly ofColumbia at ca. 1.85 Ga coincided with major geological events thataffected the entire globe (Rogers and Santosh, 2002, 2009; Zhanget al., 2012a,b and references therein).

6.2.2. Implications for comparison to the other part of NCCThe North China Craton (NCC) is one of the oldest cratons

in the world (Liu et al., 1992; Song et al., 1996; Rogers andSantosh, 2003). It consists of Early Archean to Paleoproterozoic

J. Zhang et al. / Precambrian Re

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ig. 13. Diagram of εHf(t) values vs. ages for zircons from the granodioritic gneissesLS10-8-1.1 and LS11-4-2.1) and trondhjemitic gneisses (LS11-1-1.1 and LS11-1-5.1)n the Baidashan area, eastern Alxa block.

asement overlain by Mesoproterozoic to Cenozoic unmetamor-hosed cover. Neoarchean basement rocks are widespreadver the whole of the NCC and constitute approximately5% of the area of the exposed Archean basement in theCC (Lu et al., 2008; Diwu et al., 2011 and references

here). These rocks consist of a 2.7–2.5 Gaonalite–trondhjemite–granodiorite (TTG) suite, ca. 2.5 Gayntectonic granites and a variety of supracrustal rocks thatnderwent greenschist to granulite facies regional metamorphismnd polyphase deformation at approximately 2.5 Ga (Zhao et al.,001 and references therein). Among these rock units, the Lateeoarchean TTG gneisses dominate in the preserved Archean crustf the NCC. Numerous geochronological and isotopic studies onhe Late Neoarchean TTG gneisses suggest that the ca. 2.5 Ga TTG

agmatic event represents the reworking of the 2.7–3.0 Ga crust,ndicating that the major crustal growth in the NCC took place at.7–3.0 Ga (Wu et al., 2005; Jiang et al., 2010; Zhai and Santosh,011 and references therein), similar to most other cratons in theorld, although some authors still consider ca. 2.5 Ga as a time of

ubdominant crustal growth in the NCC (Diwu et al., 2011).A large number of geochronological data of TTG gneisses from

he eastern and central NCC suggest that the time of magmatic crys-allization and subsequent metamorphism is almost identical at thend of the Neoarchean. A ca. 2.5 Ga magmatic event soon followed

y high-grade metamorphism with a short time span (10–50 Ma)ave become a significant feature for many areas of the NCC and areelated to the same Neoarchean major tectonothermal event (Want al., 2012; Grant et al., 2009).

Fig. 14. Probability plots of depleted mantle Hf model ages (TDM

search 235 (2013) 36– 57 53

Paleoproterozoic tectonothermal events in the NCC were com-monly considered to be mainly restricted to three Paleoproterozoiclinear tectonic belts situated in the western, central and easternparts of the NCC, which were named the “Khondalite Belt” (or “InnerMongolia Suture Zone”), “Trans-North China Orogen” (or “Centralorogenic belt”) and “Jiao-Liao-Ji Belt”, respectively (e.g. Zhao et al.,2002, 2005; Santosh, 2010; Kusky and Li, 2003; Li et al., 2005,2012; Li and Zhao, 2007). In these Paleoproterozoic tectonic belts,two main events have been recognized. One occurred at ∼1.95 Ga,which is related to the collision between the Yinshan and Ordosblocks along the Khondalite Belt to form the Western Block (Xiaet al., 2006a,b, 2008; Zhou et al., 2010). Another one occurred at∼1.85 Ga, which is related to the final assembly between the west-ern block and the eastern block along the Trans-North China Orogen(Zhao et al., 2001, 2005, 2012 and references therein).

However, the high-grade metamorphism at approximately1.85 Ga is not restricted to the Trans-North China Orogen (e.g. Kuskyet al., 2007; Kusky, 2011). It was also recognized in the other twoPaleoproterozoic mobile tectonic belts (Li and Zhao, 2007; Yin et al.,2011; Ma et al., 2012). Thus, some authors suggest that the distri-bution of 1.85 Ga metamorphic rocks in the NCC are more complexthan the model originally proposed (e.g. Wan et al., 2006; Genget al., 2010; Dan et al., 2012a).

The Alxa block is considered the westernmost component ofthe NCC. However, its nature and affinity are still controversial.Zhao et al. (2005, 2010) considered it as the western extension ofthe Yinshan block. Based on studies on the late Paleoproterozoictectonothermal events of the eastern Alxa block, some authors pro-posed that the Khondalite Belt probably extends westward to theeastern Alxa block (Dong et al., 2007; Geng et al., 2010). In con-trast, Dan et al. (2012a) believed that the Alxa block was likelyan independent Paleoproterozoic terrane rather than the westernextension of the Yinshan block or part of the Khondalite Belt. How-ever, zircon Hf isotopic data of the Bayanwulashan Paleoproterzoicorthogneisses in the eastern Alxa block gave Hf model ages around2.8–2.9 Ga, imply that the possibility of the existence of unexposedArchean rocks at deeper crustal levels in the eastern Alxa block (Danet al., 2012a).

Our combined datasets show that the TTG gneisses in theBaidashan area of the western Alxa block experienced a main2.7–2.8 Ga crust growth, a ∼2.5 Ga magmatic–metamorphic eventand a ∼1.85 Ga high-grade metamorphic event. The sequence ofevents is very similar to those of the other part of the NCC. However,

some relevant questions arise: e.g. what is about the relationshipbetween the western Alxa block and eastern Alxa block? Is the Alxablock the western extension of the Yinshan block (Zhao et al., 2005),or part of the Khondalite Belt (Inner Mongolia Suture Zone) (Geng

) and two-stage Hf model ages (TDMC) for ∼2.5 Ga zircons.

54 J. Zhang et al. / Precambrian Research 235 (2013) 36– 57

visionM

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AATbim(taLmiGham(a1tAve

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Fig. 15. Tectonic subdiodified from Zhao et al. (2005).

t al., 2010), or the western extension of the North Hebei Orogenicelt (Kusky and Li, 2003; Kusky et al., 2007)?

The western Alxa block is separated by desert from the easternlxa block (Fig. 2). As mentioned above, up to now, the exposedrchean rocks have not recognized in the eastern Alxa block.he oldest exposed metamorphic basements in the eastern Alxalock (Diebusige and Bayanwulashan complexes) were formed

n Early-middle Paleoproterozoic, and underwent two events ofetamorphism at ∼1.90 Ga and ∼1.80 Ga, and thus Dan et al.

2012a) considered the Alxa block as an independent Paleopro-erozoic terrane. However, Paleoproterozoic metamorphic rockslso exposed in the in the western Alxa block, named as theongshoushan Complex, which is composed of amphibolite-faciesetamorphosed igneous rocks and metasedimentary rocks. The

gneous protoliths were formed at 2.0–2.40 Ga (Xiu et al., 2004;ong et al., 2011). The detrital zircons of metasedimentary rocksave a main age population at 2.0–2.17 Ga and a minor populationt Early Paleoproterozoic–Neoarchean, suggesting the protoliths ofetasedimentary rocks were deposited at some time after 2.0 Ga

Tung et al., 2007; Gong et al., 2011). Both metamorphosed igneousnd metasedimentary rocks underwent metamorphic events at ca..85–1.93 Ga (Gong et al., 2011 and unpublished data). Therefore,he western Alxa block can be generally comparable to the easternlxa block although further detailed studies are needed in order toerify this (e.g. the recognization of the Neoarchean rocks in theastern Alxa block).

The Yinshan block was considered as a Neoarchean Archeanlock, separated by the Paleoproterozoic Khondalite Belt from therdos block in the south (e.g. Zhao et al., 2005). It consists of theuyang, Wuchuan and Alxa metamorphic complexes, of which the

uyang and Wuchuan complexes are dominated by NeoarcheanTG gneisses and minor supracrustal rocks, metamorphosed fromreenschist facies (low-grade granite-greenstone belts) to granuliteacies (high-grade terrains) at ∼2.5 Ga (Zhao et al., 2005; Jian et al.,

of North China Craton

2012). In the subdivision of the NCC by Kusky and Li (2003), the Yin-shan block was assigned to be part of their Paleoproterozoic InnerMongolia-North Hebei Orogen (IMNHO), resulted from the colli-sion of the northern margin of the NCC collide with an exotic arcterrane at 2.3 Ga, and then collide with the Columbia (Nuna) super-continent at 1.92–1.85 Ga (Kusky and Li, 2003; Kusky et al., 2007).However, up to now, no evidence shows a ca. 2.3 Ga collision eventin the IMNHO. This is also not supported by the absence of the Pale-oproterozoic (1.92–1.85 Ga) tectonothermal events in the Guyuanand Wuchuan complexes (Jian et al., 2012; Zhao and Zhai, 2012;Zhao et al., 2012).

The exposures of different rock types within the KhondaliteBelt are mainly distributed in the Helanshan-Qianlishan, Daqing-shan and Jining areas. The belt is dominated by the ‘khondaliteseries’ (including high-grade Al-rich pelitic granulite, garnet-bearing quartzite, felsic paragneiss, calc-silicate rock and marble)and TTG gneisses, mafic granulites, syntectonic charnockites andS-type granites (Jin et al., 1991; Lu and Jin, 1993; Zhao et al., 1999).The timing of the deposition of the khondalite protoliths was con-strained between ca. 2.00 and ca. 1.95 Ga (Yin et al., 2009, 2011; Danet al., 2012b). TTG gneisses and granitic gneisses have magmatic zir-cons record magmatic crystallization ages of ca. 2.5 Ga (e.g. Ma et al.,2012). Both the ‘khondalite series’ and meta-igneous rocks com-monly experienced Paleoproterozoic metamorphism at ca. 1.95 Ga,and some rocks also recorded metamorphism at ∼1.85 Ga (Donget al., 2007, 2011; Yin et al., 2009, 2011; Li et al., 2011; Ma et al.,2012; Wan et al., 2009, 2013).

Although the Alxa block has been considered as part of theYinshan block or Inner Mongolia-North Hebei Orogen, the avail-able data is absent. Considering that a common overprint of late

Paleoproterozoic metamorphism in various rocks of the westernAlxa block, the present dataset seems to support that the Alxablock is part of the Paleoproterozoic IMNHO. However, as men-tioned above, the existence of the Paleoproterozoic IMNHO has

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ot been supported by available data from the Inner Mongolianortion (e.g. Guyuan and Wuchuan complexes). Therefore, we spec-late that the western Alxa block is the most possibly westernxtension of the Paleoproterozoic Khondalite Belt. Our reasonsre: the present data of TTG gneisses in the Baidashan are veryimilar to the multiple tectonothermal events recorded in meta-gneous rocks from the Daqingshan within the Khondalite BeltMa et al., 2012; Wan et al., 2013). Although the metamorphicondition and PT evolution have not been well constrained, thealeoproterozoic metamorphosed igneous rocks, metasedimen-ary associations (pelitic gneiss, quartzite, calc-silicate rock and

arble) and tectonothermal sequence (igneous protolith age at2.0–2.4 Ga, sedimentary protolith age at 1.95–2.0 Ga and meta-orphic age at 1.85–1.93 Ga) in the Longshoushan of the westernlxa block are generally comparable to those of metamorphicupracrustal rocks in the Khondalite Belt (Yin et al., 2009, 2011;an et al., 2012b).

If so, the trending of the Khondalite Belt in the westernnd should be redefined. It would extend from the Qianlishan-elanshan (QL-HL), through the Bayanwulashan (BW) in theastern Alxa block, to the Beidashan (BD) and Longshoushan (LS)n the western Alxa block (Fig. 15). This is different from previ-us model in which the Khondalite Belt extends to the southwestway from the Helanshan (e.g. Zhao et al., 2005). This also implieshat the Yinshan block possibly does not extend to the Alxalock and thus it is much smaller than that previously recognizedFig. 15).

Moreover, the Paleoproterozoic Khondalite Belt possiblyxtends through the Altyn Tagh fault to the Dunhuang block inhe west, where the Neoarchean TTG magmatic–metaorphism and.85 Ga HP granulite have been recently identified in the Dunhuangomplex (Zhang et al., 2012b, Zhang et al., 2013). However, this will

nvolve the relationship between the Tarim Craton and NCC, whichs out of this discussion.

cknowledgements

This study was financially supported by grants from the Nationalatural Science Foundation of China (Nos. 41272210, 41072151nd 41772138), the Ministry of Land and Resources of China (Nos.01011034 and 201011058), and the Geological Survey Project ofhina (No. 1212011120157). The paper was substantially improvedy the constructive reviews of two anonymous journal reviewersnd Editor G.C. Zhao.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.precamres.013.05.002.

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