Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Paleoproterozoic Granulite-Facies Metamorphism and Archean Crustal Remnant in the Dabie Orogen

Journal of China University of Geosciences, Vol. 19, No. 2, p.110–134, April 2008 Printed in China

ISSN 1002-0705

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Paleoproterozoic Granulite-Facies Metamorphism and Archean Crustal Remnant in the Dabie Orogen Lei Nengzhong (䳋㛑ᖴ) State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China; Department of Urban Construction and Environment Science, West Anhui University, Lu’an 237012, China Wu Yuanbao* (ਈ‫)ֱܗ‬ State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China ABSTRACT: Zircon U-Pb age, trace elements, and Hf isotopes were determined for granulite and gneiss at Huangtuling (咘ೳኁ), which is hosted by ultrahigh-pressure metamorphic rocks in the Dabie (໻߿) orogen, east-central China. Cathodoluminescence (CL) images reveal core-rim structure for most zircons in the granulite. The cores show oscillatory zoning, relatively high Th/U and

176

Lu/177Hf

ratios, and high rare earth element (HREE)-enriched pattern, consistent with magmatic origin. They gave a weighted mean

207

Pb/206Pb age of (2 766±9) Ma, dating magma emplacement of protolith. The

rims are characterized by sector or planar zoning, low Th/U and

176

Lu/177Hf ratios, negative Eu

anomalies and flat HREE patterns, consistent with their formation under granulite-facies metamorphic conditions. Zircon U-Pb dating yields an age of (2 029±13) Ma, which is interpreted as a record of metamorphic event during the assembly of the supercontinent Columbia. The gneiss has a protolith age of (1 982±14) Ma, which is similar to the zircon U-Pb age for the granulite-facies metamorphism, suggesting complementary processes to granulite-facies metamorphism and partial melting. A few inherited cores with igneous characteristics have

207

Pb/206Pb ages of approximately 3.53, 3.24, and 2.90

Ga, respectively, suggesting the presence of Mesoarchean to Paleoarchean crustal remnants. A few Triassic and Cretaceous metamorphic ages were obtained, suggesting the influences by the Triassic continental collision and postcollisional collapse in response to the Cretaceous extension. Comparing with abundant occurrence of Triassic metamorphic zircons in ultrahigh-pressure eclogite and granitc gneiss from the Dabie-Sulu (㢣剕) orogenic belt, however, very limited availability of aqueous fluid or 

hydrous melt is evident for zircon growth in the

This article is supported by the National Natural Science

Huangtuling granulite and gneiss during the

Foundation of China (Nos. 40303003, 40521001, and

continental collision. The magmatic protolith

40673028).

zircons from the granulite show a large variation

*Corresponding author: [email protected]

in

Manuscript received December 9, 2007.

Hf model ages of 2.74 to 3.34 Ga. The 2.90 Ga

Manuscript accepted January 31, 2008.

inherited zircons show the similar Hf isotope

176

Hf/177Hf ratios from 0.280 809 to 0.281 289,

corresponding to HHf(t) values of -7.3 to 6.3 and

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Granulite-Facies Metamorphism and Crustal Remnant

111

features. These indicate that both growth of juvenile crust and reworking of ancient crust took place at the time of zircon formation. It is inferred that the Archean basement of the Yangtze block occurs in the north as the Dabie orogen, with ca. 2.90–2.95 Ga and 2.75–2.80 Ga as two major episodes of crustal formation. KEY WORDS: zircon, granulite facies, metamorphism, Paleoproterozoic, fluid availability.

INTRODUCTION The Yangtze and North China blocks are the two largest Precambrian blocks in China, colliding along the Qinling-Dabie orogenic belt during the Triassic (Zheng et al., 2003; Li et al., 1999; Cong, 1996). In the North China block, Archean rocks occur widely, with sporadic outcrops of Paleoarchean relicts (Zheng J P et al., 2004; Zhao et al., 2002; Song et al., 1996; Liu et al., 1992; Jahn et al., 1987). A widespread of 1 900–1 800 Ma metamorphism and subsequent 1 800–1 650 Ma rifting are considered as representing, respectively, a continental collision event and a subsequent extensional event within the craton, corresponding to the assembly and break-up of the supercontinent Columbia (Zhao G C et al., 2004). In contrast, Archean rocks are only recognized from the Kongling area in the Yangtze block in South China (Zhang et al., 2006a; Qiu et al., 2000; Gao et al., 1999), with poor constraints on their early evolutionary history. Paleoproterozoic metamorphic and magmatic events are also identified in South China (Yang et al., 2007; Zhang et al., 2006b). A few gneissic and granulitic rocks in the Dabie orogen gave very old Nd and Hf model ages up to 3.3 Ga (Xie et al., 2006; Bryant et al., 2004; Ma et al., 2000; Chen and Jahn, 1998), suggesting the possible presence of Paleoarchean rocks below the collisional orogen. Some inherited zircons with U-Pb ages up to 2.7 Ga were also reported in the Dabie orogen, confirming the existence of Neoarchean crustal relict (Jian et al., 1999; Chen et al., 1996). Some authors inferred occurrence of Paleoproterozoic metamorphic event in the Late Archean rocks (Chen et al., 2006), but no isotopic ages are obtained for them. In this article, we present a combined study of U-Pb ages, trace elements, and Hf isotopes for zircons from high-grade rocks at Huangtuling in the Dabie orogen. The results not only provide unambiguous evidences for the presence of Archean crust and their Paleoproterozoic granulite-facies metamorphism in

this area but also shed some light on fluid availability during continental collision between the Yangtze and North China blocks. GEOLOGICAL SETTING AND SAMPLES The Dabie orogen is the eastward extension of the Qinling orogen that separates the Yangtze block from the North China block (Zheng et al., 2005a; Ratschbacher et al., 2003; Hacker et al., 1998; Cong, 1996). It can be divided, from north to south, into five tectonic units (Fig. 1): (1) the Beihuaiyang low-grade metamorphic zone; (2) the North Dabie granulitefacies high temperature (HT)/ultrahigh pressure (UHP) zone; (3) the Central Dabie middle temperature (MT)/UHP eclogite-facies zone; (4) the South Dabie low temperature (LT)/UHP eclogite-facies zone; and (5) the Susong LT/HP blueschist-facies zone. Postcollisional mafic-ultramafic and granitic plutons are widespread in the Beihuaiyang and North Dabie zones (Zhao Z F et al., 2005, 2004; Chen et al., 2002; Zhang et al., 2002; Jahn et al., 1999). The North Dabie zone is bounded by four faults or shear zones (Fig. 1): the steeply dipping Xiaotian-Mozitan fault in the north, the Wuhe-Shuihou fault in the southeast, the Mamiao-Taihu fault in the southwest, and the Shangcheng-Macheng fault in the west (Ratschbacher et al., 2000). It experienced intensive magmatism and metamorphism during the Early Cretaceous, possibly controlled by transtension between Jurassic and Cretaceous (Hacker et al., 2000; Ratschbacher et al., 2000). Cretaceous granites make up almost 50% of the area in North Dabie and are associated with several generations of tonalitic to granitic orthogneisses (Xie et al., 2006; Byant et al., 2004; Cong, 1996). Two types of orthogneiss have been recognized, respectively, with Neoproterozoic and Cretaceous protolith ages (Xie et al., 2006, 2004a; Bryant et al., 2004; Hacker et al., 2000, 1998). The presence of eclogite containing microdiamonds in the North Dabie

Lei Nengzhong and Wu Yuanbao

112

zone as well as relevant studies of petrology and geochronology, demonstrates that this zone experienced ultrahigh-pressure (UHP) metamorphism due to deep subduction of continental crust (Liu et al., 114o E

2005; Xu et al., 2005, 2003; Xie et al., 2004b; Tsai and Liou, 2000). Granulites occur as lenses within the regional orthogneiss.

115o E

116o E

117o E

0

50 km

32o N

Xinyang

TBC

Hong'an

Shang cheng -Mac heng fault

III

IV

II

Shucheng

Tongcheng Yuexi

Wuha n I

Beihuaiyang unit

II

North Dabie HT/UHP unit

III

Central Dabie MT/UHP unit

IV

South Dabie LT/UHP unit

V

Susong LT/HP unit

TBC Tongbai complex Cretaceous granitoid

lt

fault

fau

ou Wuhe-Shuih

Ta i hu

31o N

Macheng

Luotian Yingshan Huangtuling

30o N

Xia Houshan I otia n-M ozit an f ault

III fault aihu iao-T m a M Taihu

IV

V Susong

Fault Sample location

Figure 1. Sketch geological map of the Dabie orogen in east-central China and sample location. Two granulite samples (05HTL01 and 05HTL02) were collected from the Huangtuling Primary School in the North Dabie zone (Fig. 1). They are typical felsic granulites with plagioclase (~25%), quartz (~20%), K-feldspar (~10%), garnet (~15%), hypersthene (~15%), biotite (~10%), minor cordierite and hornblende, and some accessory minerals such as zircon, magnetite, and ilmenite (Chen et al., 1998). Hornblende and plagioclase occur as symplectite around garnet and hypersthene, indicating amphibolite-facies retrograde metamorphism. Chen Y et al. (2006) and Chen N S et al. (1998) estimated the P-T conditions of the peak and amphibolite-facies retrograde metamorphism, (1.3±0.2) GPa/(850±50) ć and 0.6 GPa/700 ć, respectively. Zheng et al. (2001) obtained similar temperatures for the granulitefacies metamorphism from mineral-pair oxygen

isotope thermometry. A felsic gneiss sample (05HTL03) was also collected just tens of meters away from the granulite outcrop. The contact between the gneiss and the granulite is not exposed. The gneiss displays granoblastic texture and weakly gneissic structure. It contains plagioclase, K-feldspar, quartz, hornblende, biotite, and accessory zircon. ANALYTICAL METHODS Zircons were separated using the standard techniques (Wilfley table, Frantz magnetic separator, heavy liquid). The transparent crystals were selected by binocular microscope, mounted in epoxy resin, and then polished down to expose the grain centers. Zircons were imaged using a JEOL JXA-8900RL electron microprobe at the Institute of Mineral Resources in Chinese Academy of Geological

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Granulite-Facies Metamorphism and Crustal Remnant

Sciences, Beijing. The work conditions during the CL imaging were at 15 kV and 20 nA. Zircon U-Pb dating for granulite 05HTL02 and gneiss 05HTL03 was carried out using the sensitive high resolution ion microprobe (SHRIMP II) at Beijing SHRIMP Center in the Chinese Academy of Geological Sciences. The standard operating conditions and data acquisition methods follow Williams (1998). TEMORA standard zircon was used to calibrate U/Pb isotopic discrimination during analysis. U and Th abundances were calibrated against standard zircon SL13. The measured 204Pb was used for common Pb correction. The data were reduced using the ISOPLOT program (Ludwig, 2001). Individual analyses were reported with 2ı uncertainties; weighted average of ages was also reported at the 2ı level. Zircon U-Pb dating for granulite samples (05HLT01 and 05HTL02) was accomplished by means of LA-ICPMS at China University of Geosciences in Wuhan. The detailed analytical procedure follows Yuan et al. (2004). The GeoLas 200M laser-ablation system equipped with a 193 nm ArF-excimer laser was used in connection with an ELAN6100 DRC ICP-MS. Helium was used as the carrier gas to enhance the transport efficiency of the ablated material. The helium carrier gas inside the ablation cell is mixed with argon as a makeup gas before entering the ICP to maintain stable and optimum excitation conditions. The spot diameter was 32 µm. Each analysis includes a background acquisition interval of approximately 30 s and a signal acquisition of approximately 60 s. Raw data were processed using the Glitter program. All measurements were normalized relative to zircon 91500, with a recommended 206Pb/238U age of (1 065.4±0.6) Ma (Wiedenbeck et al., 1995). Standard silicate glass NIST SRM610 was used to calculate U and Th concentrations. The common Pb correction was carried out by using the EXCEL program of ComPbCorr#_151 (Andersen, 2002). Zircon trace element analyses for 05HTL01 and 05HTL02 were also carried out by LA-ICPMS (Excimer 193-nm ArF Laser, coupled with an ELAN6100 quadrupole MS) at China University of Geosciences in Wuhan. Repetition rate was 10 Hz and

113

the sizes of the spots were 32 µm in diameter. Oxide production rate was tuned to <0.5% ThO. A detailed compilation of instrument and data acquisition parameters for the LA-ICPMS follow Yuan et al. (2004). For samples 05HTL02 and 05HTL03, zircon Hf isotope analysis was carried out in-situ using a Geolas-193 laser-ablation microprobe, attached to a Neptune multi-collector ICPMS, at the Institute of Geology and Geophysics in the Chinese Academy of Sciences, Beijing. Instrumental conditions and data acquisition were comprehensively described by Xu et al. (2004). Spot sizes of 32 or 63 Pm, with a laser repetition rate of 10 Hz at 100 mJ, were used, depending on size of target domains. Isobaric interferences of 176Lu and 176Yb on 176Hf were corrected precisely (Wu F Y et al., 2006). In doing so, 175 Lu was calibrated using 176Lu on 176Hf; a 176 Yb/172Yb value of 0.588 7 and a mean ȕYb value obtained during Hf analysis on the same spot were applied for the interference correction of 176Yb on 176 Hf (Wu F Y et al., 2006; Iizuka and Hirata, 2005). During our analyses, 176Hf/177Hf ratios for the standard zircon 91500 were 0.282 294±15 (2ın, n = 20), identical to the commonly accepted 176Hf/177Hf ratio of 0.282 293±28 (1ı) measured using the laser method (Woodhead et al., 2004). We have adopted a decay constant for 176Lu of 1.865×10-11 a-1 (Scherer et al., 2001). Initial 176Hf/177Hf ratio, denoted as HHf(t), is calculated relative to the chondritic reservoir with a 176 Hf/177Hf ratio of 0.282 772 and 176Lu/177Hf of 0.033 2 (Blichert-Toft and Albarede, 1997). Singlestage Hf model ages (TDM) are calculated relative to the depleted mantle with a present-day 176Hf/177Hf ratio of 0.283 25 and 176Lu/177Hf of 0.038 4 (Griffin et al., 2000). RESULTS Zircon Morphology Zircons from granulite 05HTL01 are light yellow and transparent. They range in grain size from 100 to 300 µm, with length/width ratios of 1:1 to 2.5:1. In CL images (Fig. 2a), most zircon grains reveal distinct core-rim structure. Most of the rims show no zoning or weak zoning that is typical for metamorphic zircon (Wu and Zheng, 2004; Corfu et al., 2003). Most of the

114

Lei Nengzhong and Wu Yuanbao

Figure 2. Typical CL images for U-Pb dated zircons. Ellipses indicate SIMS analysis spots and corresponding 207Pb/206Pb ages, the smaller cycles show LA-ICPMS dating spots and corresponding U-Pb ages and LA-ICPMS trace element analysis spots (marked with TE), and the bigger cycles show locations of Lu-Hf isotope analysis. (a) Granulite 05HTL01; (b) granulite 05HTL02; (c) two oldest inherited zircons in granulite 05HTL02; (d) gneiss 05HTL03.

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Granulite-Facies Metamorphism and Crustal Remnant

cores exhibit oscillatory zoning, which may represent the protolith magmatic domain. A few cores illustrate anhedral shape and blurred zoning, and are probably older inherited grains (Fig. 2a), as proven by their old age. Very thin discontinued bright rims can also be discerned around most zircon grains and may have formed during subsequent retrograde metamorphism. Zircons from granulite 05HTL02 are light yellow to colorless. They are rounded to short prismatic. The lengths scatter between 100 and 400 µm, with ratios of length to width ranging from 1:1 to 2.5:1. CL imaging reveals that most of them have core-rim structure (Fig. 2b). The cores exhibit oscillatory zoning that is typical for igneous zircon and represents the protolith domain. The rims show planar, sector or weak zoning and may represent metamorphic overgrowth. Inherited cores can also be discerned by their blurred zonation and unconformable shape (Fig. 2b). Thin homogeneous outer rim occurs in a few grains. Zircons in gneiss 05HTL03 are short to long prismatic, light brown, and translucent. Their lengths range from 200 to 400 µm, with aspect ratios of 2:1 to 4:1. CL images show that most of the zircons have core-mantle-rim structure (Fig. 2d). The cores exhibit weak oscillatory or patched zoning with relatively weak luminescence. The mantles are lamellar or weakly zoned and are commonly embayed by the rims (Fig. 2c). They are all interpreted to represent magmatic zircon that underwent different degrees of metamorphic recrystallization (Zheng Y F et al., 2004; Hoskin and Black, 2000). The rims are homogeneous with very weak luminescence and may represent metamorphic overgrowths. A few zircons also contain inherited cores. Trace Elements Trace elements analyzed by LA-ICPMS for different zircon domains in granulites 05HTL01 and 05HTL02 are listed in Table 1. The protolith and inherited zircon domains have high Th/U ratios (>0.30), and high trace element concentrations compared to metamorphic zircons (Table 1). The REE patterns of the protolith and inherited zircons are characterized by HREE enrichment, a positive Ce anomaly, and a negative Eu anomaly (Fig. 3), and are

115

typical for magmatic zircons. Whereas the metamorphic domains have low Th/U ratios (<0.1), and low trace element concentrations (Table 1). In the REE patterns, they show clear negative Eu anomalies and nearly flat HREE profiles (Fig. 3). These features are best explained by assuming that the metamorphic zircons are formed in the presence of fledspar and garnet (Bingen et al., 2004; Wu and Zheng, 2004; Rubatto and Hermann, 2003; Whitehouse and Platt 2003; Rubatto, 2002). Zircon U-Pb Ages Twenty-five LA-ICPMS U-Pb analyses of zircons in sample 05HTL01 are reported in Table 2 and Fig. 4a. Five analyses were located on the inherited cores, nine analyses were obtained from the protolith magmatic domains, and the other eleven analyses were obtained from the metamorphic domains. The metamorphic domains have moderate U abundances (174 to 705 ppm) and very low Th abundances (3 to 8 ppm), resulting in very low Th/U ratios (0.01 to 0.05), which are typical for metamorphic zircon (Zheng Y F et al., 2004; Rubatto and Gebauer, 2000). While the remaining fourteen analyses on protolith and inherited zircons are characterized by high Th abundances (34 to 175 ppm) and high Th/U ratios (0.25 to 1.13). The eleven analyses on the metamorphic domain yield a discordia chord that intersects the concordia curve at (2 029±21) Ma and (39±180) Ma with an MSWD value of 1.7 (Fig. 207 Pb/206Pb ages for these analyses range 4a). The from 1 996 to 2 047 Ma, with a weighted mean of (2 025±9) Ma (MSWD=1.8). This age is more precise than the upper intercepted age and is interpreted as the formation age of the metamorphic zircons. The nine analyses on the protolith domains yield 207Pb/206Pb ages of 2 745 to 2 778 Ma, with a weighted mean of (2 761±13) Ma (MSWD=0.30) which is interpreted as the protolith of the granulite. Among the five analyses on inherited cores, three analyses yielded a weighted average 207Pb/206Pb age of (2 937±43) Ma (MSWD= 2.1), while the other two analyses gave a weighted mean 207Pb/206Pb age of (3 277±16) Ma (MSWD=0.33). These indicate two generations of inherited zircon in this sample.

Lei Nengzhong and Wu Yuanbao

116 Table 1 Spot

Th

U

P

LA-ICPMS trace element analyses data for zircon (ppm) in granulite samples 05HTL01 and 05HTL02 at Huangtuling Nb

Ta

Hf

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Eu/Eu*

05HTL01 1

48.21 119.93 370.58

1.04

0.41

9 825

0.24

7.95

0.12

1.11

1.81

0.59

9.26

3.43

47.41

20.98 108.17 26.04 285.21 60.24

0.35

2

45.92 749.82 246.03

0.69

0.60

11 943

0.03

4.78

0.07

2.06

4.17

1.46

21.65

5.62

52.85

17.44

14.89 143.46 27.62

0.38

3

116.99 284.08 253.26

3.79

0.85

11 265

0.02

22.18

0.04

1.14

3.31

1.50

22.56

8.58

105.93 42.13 195.43 42.86 421.89 78.60

0.39

559.64 94.62

0.16

0.12

13 991 0.016 4 0.501 0.008 7 0.255

1.74

0.871

8.19

2.366

19.92

4.41

8.75

1.33

1.518

0.58

156.24 355.74 81.23

1.12

0.38

11 426

0.87

1.83

0.70

10.10

3.18

37.38

15.11

74.66

17.93 208.06 47.97

0.39

0.023 1 0.622

2.92

1.204

16.53

5.54

28.44

5.45

10.2

1.072

6.66

0.904

0.42

13.62

1.737

12.48

4 5

4.71

0.04

28.23 0.71

0.06

73.25

9.53

6

4.69

309.86 156.63

0.32

0.32

12 135 0.030 9

7

3.57

211.29 134.66

0.32

0.33

13 115 0.025 9 0.676

0.034

0.524

2.41

0.884

12.17

3.73

24.78

6.09

1.706

0.41

8

122.55 256.41 314.65

1.25

0.75

10 647

0.21

14.65

0.29

2.75

3.38

0.59

12.42

4.08

51.55

20.93 104.54 25.42 275.83 58.45

0.25

9

60.48 149.26 272.29

1.35

0.46

9 384

0.01

6.71

0.07

1.28

3.68

0.76

19.40

7.51

93.57

37.60 173.20 38.05 377.91 73.32

0.22

10

385.90 492.20 177.79

2.68

0.91

9 837

0.29

52.60

0.24

3.04

3.41

1.19

17.72

5.76

67.89

26.66 129.50 30.22 334.57 71.58

0.38

11

147.88 322.58 338.05

3.60

0.94

9 625

0.28

18.10

0.21

1.36

3.11

1.01

16.39

5.87

68.08

26.44 123.55 26.46 263.74 48.87

0.34

12

96.89 172.60 471.33

1.87

0.84

8 997

6.51

36.60

4.27

21.48

9.98

1.31

26.35

9.19

110.85 45.78 217.71 49.55 516.45 105.83

0.23

13

29.14

43.73 220.46

0.45

0.10

8 486

0.42

15.85

0.34

2.88

3.40

1.82

16.07

4.57

47.27

16.07

13.59 134.28 27.04

0.62

14

88.32 143.44 217.91

0.95

0.41

9 116

0.20

28.20

0.24

2.86

3.41

1.51

18.85

5.92

70.44

26.21 119.44 28.27 291.85 59.44

0.45

15

21.04 311.14 71.84

0.18

0.16

10 692

1.48

6.7

1.258

6.06

5.55

1.822

17.54

1.91

14.28

2.22

4.64

0.605

0.526

0.52

16

44.77

73.08 217.86

0.71

0.24

7 457

0.85

17.81

0.37

2.82

4.48

0.89

18.41

5.55

57.79

21.28

90.16

18.29 179.62 33.35

0.26

67.31

3.89

05HTL02 1

3.87

727.22 74.31

0.17

0.10

11 098

0.02

0.35

0.01

0.19

0.85

0.46

6.61

1.51

9.34

1.46

3.49

0.51

4.06

0.57

0.42

2

3.91

226.05 87.15

0.27

0.18

13 678

0.04

0.66

0.05

0.76

2.87

1.14

14.50

3.58

21.10

3.61

7.86

1.06

8.90

1.15

0.44

3

131.10 428.18 224.35

4.58

1.85

15 003

1.38

19.32

0.69

3.69

3.81

0.41

18.59

6.61

85.39

35.13 169.47 38.87 400.95 80.15

0.12

4

833.32 721.15 187.56

6.68

1.51

13 373

0.18

123.72

0.28

3.83

7.47

2.48

34.42

10.66 115.59 43.48 193.37 43.50 447.48 88.05

0.39

5

368.61 615.57 156.26

3.48

0.67

13 065

0.13

72.75

0.16

2.69

5.68

2.48

26.07

8.47

90.13

31.90 139.05 30.92 315.52 63.26

0.52

6

28.04 504.08 80.77

0.48

0.16

9 387

0.06

3.92

0.05

0.34

0.86

0.30

4.49

1.22

8.02

2.01

6.10

1.21

9.71

1.52

0.38

7

25.93 637.06 61.43

0.76

0.28

13 195

0.03

5.40

0.01

0.12

2.36

0.91

10.62

3.03

21.66

5.72

19.51

2.84

20.83

2.67

0.47

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Granulite-Facies Metamorphism and Crustal Remnant

117

Continued Spot

Th

U

P

Nb

Ta

Hf

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Eu/Eu*

05HTL02 8

328.39

842.64

124.20 3.13

0.85

13 013

0.02

35.29

0.03

0.94

2.55

1.17

17.58

5.88

73.18

31.58

163.89

42.22 494.71 115.44

0.39

9

61.71

202.83

162.55 1.22

0.41

10 689

0.03

15.29

0.08

1.33

4.74

2.52

25.15

9.88

122.43 54.70

275.54

68.87 770.79 173.36

0.56

10

501.86

687.48

102.25 3.04

1.04

8 907

0.04

74.92

0.13

2.55

4.97

3.72

26.46

8.81

96.18

35.29

166.96

39.48 439.49

82.21

0.79

11

8.75

918.77

44.17

0.28

0.19

10 375

0.06

1.73

0.03

0.35

1.91

1.61

10.33

3.68

23.97

6.42

19.51

4.41

37.55

5.55

0.88

12

7.40

456.19

68.70

0.68

0.80

13 255

0.05

1.31

0.09

2.01

8.95

2.51

43.52

8.15

40.20

6.76

15.84

2.12

12.75

1.92

0.32

13

178.16

392.77

155.53 6.98

3.45

14 773

0.11

10.95

0.80

9.93

24.08

5.97

176.57 67.32 749.45 293.46 1 193.94 236.51 2 014.70 366.81

0.20

14

18.01

787.80

45.03

0.53

0.51

13 371

0.03

1.45

0.05

1.72

8.05

3.73

39.58

9.20

0.52

15

502.40 1 031.53 340.35 19.27

7.56

11 654

0.08

57.96

0.52

3.76

16.34

4.25

95.09

41.74 502.30 235.38 1 136.70 277.52 2 873.12 599.27

0.26

16

730.60 1 082.98 101.71 6.55

3.72

11 018

0.09

50.61

0.38

4.23

10.17

3.14

55.57

22.65 243.82 110.42

517.82 128.37 1 296.86 272.19

0.32

17

35.36

2.61

10 568

0.11

2.93

0.25

2.79

16.36

8.23

65.20

14.11

31.87

0.67

1 266.83 67.79

1.65

56.16

70.62

10.91

15.06

27.11

4.03

5.41

32.46

29.56

4.43

4.44

Lei Nengzhong and Wu Yuanbao

118 Table 2 Spot

Th (ppm)

U (ppm) Th/U

207

Zircon U-Pb isotopic data obtained by LA-ICPMS for granulites 05HTL01 and 05HTL02

Pb*/206Pb*

1ı

207

Pb*/235U

1ı

206

Pb*/238U

1ı

207

Pb/206Pb

1ı

207

Pb/235U

1ı

206

Pb/238U

1ı

%Conc.a

05HTL01 1

5

705

0.007

0.123 26

0.000 70

0.057 37

0.359 69

0.003 34

2 004

10

2 000

18

1 981

21

99

2

150

245

0.613

0.212 41

0.001 29 16.998 91 0.062 35

6.170 44

0.580 35

0.005 42

2 924

12

2 935

22

2 958

29

101

3

3

245

0.012

0.122 39

0.000 83

5.250 06

0.051 02

0.306 82

0.002 86

2 013

14

1 861

20

1 725

18

86

4

13

268

0.048

0.125 55

0.000 93

5.924 75

0.061 86

0.349 35

0.003 27

2 037

12

1 965

23

1 932

21

95

5

4

458

0.008

0.126 32

0.001 21

5.449 59

0.060 38

0.315 64

0.003 00

2 047

19

1 893

23

1 768

18

86

6

161

286

0.563

0.192 23

0.003 09 14.021 77 0.150 91

0.529 38

0.008 30

2 761

28

2 751

47

2 739

35

99

7

4

174

0.023

0.126 09

0.000 62

0.126 43

0.322 28

0.005 24

2 044

9

1 927

38

1 801

26

88

8

42

136

0.306

0.266 28

0.002 57 23.386 42 0.128 47

0.642 28

0.004 85

3 284

15

3 243

39

3 198

27

97

9

3

175

0.015

0.124 06

0.000 91

0.075 12

0.331 24

0.003 16

2 015

13

1 937

25

1 844

17

92

10

57

135

0.422

0.264 54

0.001 47 23.453 43 0.116 34

0.642 49

0.006 43

3 274

9

3 246

38

3 199

31

98

11

5

595

0.009

0.122 70

0.001 53

0.104 95

0.292 15

0.004 26

1 996

22

1 816

34

1 652

25

83

12

43

167

0.257

0.191 04

0.001 86 11.516 86 0.135 33

0.431 42

0.006 53

2 751

16

2 566

43

2 312

32

84

13

74

50

1.466

0.191 36

0.004 24 10.341 18 0.064 51

0.371 49

0.004 08

2 754

36

2 466

23

2 036

22

74

14

8

242

0.034

0.124 32

0.002 59

0.055 98

0.284 45

0.002 85

2 019

37

1 802

19

1 614

15

80

15

15

34

0.443

0.190 37

0.003 16 12.495 08 0.127 71

0.464 70

0.006 29

2 745

27

2 642

41

2 460

28

90

16

4

576

0.006

0.125 82

0.001 14

0.078 57

0.314 09

0.004 43

2 040

16

1 882

24

1 761

22

86

5.669 03 5.735 20 4.983 03

4.897 27 5.385 04

17

63

75

0.831

0.216 44

0.001 48 17.066 74 0.116 15

0.582 21

0.006 01

2 954

11

2 939

35

2 958

24

100

18

166

276

0.603

0.191 62

0.002 44 13.822 87 0.098 32

0.522 76

0.005 49

2 756

21

2 738

31

2 711

23

98

19

60

238

0.253

0.190 72

0.002 64 11.357 20 0.099 16

0.426 63

0.005 31

2 748

23

2 553

33

2 291

24

83

20

34

138

0.244

0.194 23

0.001 91 13.089 21 0.115 69

0.487 04

0.006 31

2 778

16

2 686

35

2 558

27

92

21

8

227

0.036

0.124 10

0.001 06

0.050 24

0.202 68

0.002 16

2 016

15

1 513

19

1 190

12

59

22

109

363

0.299

0.192 81

0.002 14 13.678 38 0.115 83

0.514 18

0.005 73

2 766

19

2 728

35

2 675

24

97

23

84

122

0.686

0.192 64

0.001 56 14.120 41 0.126 80

0.531 39

0.006 12

2 765

13

2 758

40

2 747

26

99

24

175

155

1.128

0.212 89

0.001 64 16.573 07 0.127 95

0.562 23

0.006 62

2 928

13

2 910

41

2 876

27

98

25

5

399

0.012

0.124 93

0.001 99

4.826 66

0.072 46

0.278 46

0.003 36

2 028

28

1 790

24

1 584

17

78

1

4

549

0.01

0.126 54

0.000 66

6.500 60

0.06656

0.37241

0.003 44

2 050

9

2 046

18

2 041

21

100

2

4

312

0.01

0.123 94

0.000 67

6.073 74

0.08864

0.35134

0.003 27

2 014

10

1 986

28

1 941

21

96

3.437 75

05HTL02

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Granulite-Facies Metamorphism and Crustal Remnant

119

Continued U (ppm) Th/U

207

Pb*/206Pb*

1ı

207

Pb*/235U

1ı

206

Pb*/238U

1ı

207

Pb/206Pb 1ı

207

Pb/235U

1ı

206

Pb/238U

1ı %Conca.

Spot

Th (ppm)

05HTL02 3

613

310

1.98

0.191 32

0.001 27

4

50

151

0.33

0.192 62

0.000 78

5

643

338

1.90

0.192 36

0.000 99

6

128

449

0.29

0.184 78

0.001 26

7

213

440

0.48

0.190 68

0.001 35

8

356

593

0.60

0.192 39

0.000 66

13.514 69

0.088 37

0.507 04

0.005 25

2 762

6

2 716

27

2 644

33

96

9

6

325

0.02

0.125 83

0.001 83

5.591 01

0.087 26

0.326 45

0.003 66

2 041

26

1 915

26

1 821

28

89

10

8

327

0.02

0.127 45

0.001 88

3.978 00

0.062 11

0.232 14

0.002 58

2 063

26

1 630

17

1 346

13

65

11

32

85

0.37

0.216 45

0.001 50

16.519 09

0.065 09

0.551 29

0.006 33

2 955

11

2 907

19

2 831

40

96

12

18

237

0.08

0.124 99

0.001 96

5.306 14

0.081 82

0.304 42

0.003 41

2 029

28

1 870

24

1 713

20

84

13

386

353

1.09

0.194 22

0.001 25

14.109 23

0.062 58

0.528 29

0.003 29

2 778

11

2 757

17

2 734

23

98

14

30

387

0.08

0.125 82

0.001 24

5.476 89

0.104 25

0.315 99

0.003 79

2 041

17

1 897

28

1 770

23

87

15

9

257

0.03

0.124 54

0.001 35

5.431 18

0.101 76

0.310 64

0.003 76

2 022

19

1 890

27

1 744

24

86

16

52

113

0.46

0.192 47

0.001 36

13.943 36

0.067 20

0.523 87

0.003 71

2 763

12

2 746

19

2 716

22

98

17

8

341

0.02

0.125 41

0.001 24

4.592 59

0.089 58

0.270 03

0.003 42

2 035

17

1 748

24

1 541

24

76

18

206

268

0.77

0.310 92

0.001 50

30.904 58

0.115 57

0.720 72

0.003 94

3 525

7

3 516

27

3 499

25

99

19

89

164

0.54

0.194 76

0.001 37

14.211 14

0.077 03

0.526 56

0.003 72

2 782

11

2 764

19

2 727

26

98

13.071 96

0.103 71

0.494 98

0.004 65

2 753

11

2 685

29

2 592

25

94

13.296 82

0.119 38

0.502 20

0.004 78

2 765

7

2 701

31

2 623

31

95

13.988 94

0.087 18

0.526 10

0.005 27

2 762

8

2 749

26

2 725

33

99

10.281 72

0.082 73

0.407 40

0.003 94

2 696

11

2 460

21

2 203

27

82

12.003 62

0.129 70

0.457 82

0.004 66

2 748

12

2 605

35

2 430

30

88

20

17

702

0.02

0.126 44

0.001 25

5.034 52

0.140 50

0.292 79

0.003 42

2 049

17

1 825

37

1 655

21

81

21

119

188

0.63

0.214 93

0.000 89

16.917 89

0.075 37

0.570 92

0.003 81

2 928

7

2 930

20

2 912

24

99

22

10

245

0.04

0.125 79

0.001 25

5.576 36

0.099 03

0.323 08

0.003 49

2 040

18

1 912

28

1 805

24

88

23

83

180

0.46

0.193 69

0.000 85

13.255 82

0.140 75

0.496 37

0.002 78

2 773

7

2 698

37

2 598

20

94

24

69

143

0.49

0.214 99

0.000 99

16.916 12

0.090 32

0.573 13

0.002 93

2 943

7

2 930

27

2 921

18

99

25

86

127

0.68

0.194 97

0.000 93

14.013 06

0.106 22

0.522 05

0.002 93

2 784

8

2 751

28

2 708

18

97

26

108

123

0.88

0.193 72

0.000 99

14.054 91

0.118 18

0.530 27

0.002 89

2 774

8

2 753

30

2 743

18

99

27

4

328

0.01

0.123 47

0.001 27

6.103 47

0.101 41

0.355 10

0.003 61

2 007

18

1 991

27

1 959

25

98

a. %Conc. =100 (206Pb/238U age)/(207Pb/206Pb age).

Lei Nengzhong and Wu Yuanbao

120 100 000

100 000 (b)

(a) 1 000 Sample/chondrite

Sample/chondrite

1 000

10

10

0.1

0.1

0.001

0.001 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 3. Chondrite-normalized REE patterns for different zircon domains in granulites 05HTL01 and 05HTL02. (a) 05HTL01; (b) 05HTL02. Solid triangles denote metamorphic domain, solid dots represent protolith domain, and hollow dots represent inherited domain. 0.8

(a)

Mean=(3 277f16) Ma MSWD =0.33

0.7

Pb/ U

238

Mean=(2 937f43) Ma MSWD =2.1

0.3

1 400

Mean=(2 033f10) Ma MSWD =1.4

0.2 Upper intercept:(2 043f13)Ma Lower intercept:(96f100)Ma MSWD =0.81

Upper intercept: (2 029f21) Ma Lower intercept: (39f180) Ma MSWD =1.7

0

Mean=(2 938f31) Ma MSWD =2.4

1 800

Mean=(2 025f9) Ma MSWD =1.8

1 400

0.1

(3 525f7)Ma

3 000

2 200

0.4

206

206

238

Pb/ U

0.6

2 200 1 800

Mean=(2 767f5) Ma MSWD =0.92

2 600

2 600

0.5

3 400

3 000

Mean=(2 761f13)Ma MSWD =0.30

(b)

3 400

4

8

12 207

16

20

24

28

235

Pb/ U

0.0

0

10

20 207

30

Pb/ 235U

Figure 4. Concordia diagrams of LA-ICPMS zircon U-Pb dating for granulites 05HTL01 and 05HTL02. (a) 05HTL01; (b) 05HTL02. A total of 27 analyses were carried out by LA-ICPMS for zircons in sample 05HTL02 (Table 2 and Fig. 4b). Eleven analyses on the metamorphic domains have low Th contents (4 to 30 ppm) and low Th/U ratios (0.01 to 0.08), and are typical metamorphic zircon (Zheng Y F et al., 2004; Rubatto and Gebauer, 2000). They yield a discordia chord intersecting the concordia curve at (2 043±13) Ma and (96±100) Ma (MSWD=0.81), respectively. The 207 Pb/206Pb ages of them range from (2 007±18) to (2 063±26) Ma, with a weighted mean of (2 033±10) Ma (MSWD=1.4), which is taken as the formation age of the metamorphic zircons. Whereas the twelve analyses on the protolith magmatic domain show high Th contents (50 to 643 ppm) and high Th/U ratios of 0.33 to 1.90. Except for two discordant analyses, the

other ten nearly concordant analyses (concordance >90%) have consistent 207Pb/206Pb age of (2 753±11) to (2 784±8) Ma, with a weighted mean of (2 767±5) Ma (MSWD=0.92), which is considered as the protolith age of the granulite. Three analyses on inherited cores have 207Pb/206Pb age of (2 928±7) to (2 955±11) Ma with a mean of (2 938±31) Ma (MSWD=2.4). A very old inherited core with age of (3 525±7) Ma has also been dated (Fig. 4b). Seventeen SHRIMP U-Pb isotope analyses for eleven zircon grains from granulite 05HTL02 are reported in Table 3 and Fig. 5a. Six analyses on protolith zircon domains show moderate U and Th concentrations of 66 to 1 022 ppm and 37 to 387 ppm, respectively, and variable Th/U ratios of 0.07 to 1.41. They are discordant to nearly concordant, defining a

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Granulite-Facies Metamorphism and Crustal Remnant Table 3

Spot

U (ppm)

121

SHRIMP zircon U-Pb data for granulite 05HTL02 and granitic gneiss 05HTL03 at Huangtuling Th Th/U %206Pbc* (ppm)

Isotope ratio 206

207

Pb/ U

235

Pb/ U

±%

Age (Ma) 207

±%

238

Pb/ Pb

206

206

±%

207

Pb/ U

1ı

206

Pb/ Pb

1ı

238

05HTL02 1.1

295

150

0.53

0.06

5.240 36

3.5

0.302 0

3.4

0.125 9

0.71

1 701

51

2 041

12

1.2

304

6

0.02

0.04

6.139 24

4.7

0.362 0

4.2

0.123 0

2.2

1 992

72

2 000

39

2.1

346

293

0.88

0.01

17.562 3

3.6

0.598 0

3.4

0.213 0

1.2

3 022

83

2 929

19

3.1

242

76

0.32

0.02

13.576 7

3.5

0.506 0

3.4

0.194 6

0.46

2 640

75

2 781

7.5

3.2

215

4

0.02

0.06

5.680 21

8.9

0.328 0

8.8

0.125 6

1.6

1 829

140

2 037

28

4.1

371

212

0.59

0.02

23.062 1

3.5

0.647 8

3.4

0.258 2

0.48

3 220

85

3 236

7.6

4.2

572

37

0.07

0.00

8.731 29

3.5

0.389 0

3.4

0.162 8

0.44

2 118

62

2 485

7.5

5.1

359

264

0.76

0.02

16.461 8

4.8

0.574 0

4.6

0.208 0

1.4

2 924

110

2 890

22

5.2

437

7

0.02

0.05

6.025 29

4.8

0.351 0

3.4

0.124 5

3.3

1 939

58

2 022

59

6.1

66

90

1.41

0.07

13.247 9

3.7

0.502 0

3.6

0.191 4

1.0

2 622

77

2 754

16

7.1

1 022

281

0.28

0.02

9.429 38

3.4

0.392 0

3.4

0.174 5

0.29

2 132

62

2 601

4.9

8.1

431

60

0.14

0.01

7.151 2

3.5

0.346 0

3.4

0.149 9

0.51

1 915

57

2 345

8.7

8.2

1 634

24

0.02

0.07

0.220 08

3.9

0.031 7

3.4

0.050 3

1.9

201

4.4

208

45

9.1

182

147

0.84

0.15

14.061 4

1.6

0.500 9

1.5

0.203 6

0.65

2 618

33

2 855

11

10.1

643

625

1.00

0.06

14.915

1.5

0.535 7

1.4

0.201 9

0.34

2 765

32

2 842

5.5

10.2

548

387

0.73

0.06

14.441 9

1.8

0.544 4

1.6

0.192 4

0.73

2 802

38

2 763

12

11.1

186

133

0.74

0.15

16.793 1

2.0

0.578 6

1.5

0.210 5

1.3

2 943

37

2 909

20

1.1

667

72

0.11

1.84

2.030 38

4.6

0.124 9

3.5

0.117 9

3.0

759

14

1 925

54

1.2

475

63

0.14

0.08

5.272 35

3.5

0.312 0

3.4

0.122 6

0.62

1 751

53

1 994

11

2.1

1 648

138

0.09

0.01

2.802 26

4.0

0.164 3

3.4

0.123 7

2.0

981

25

2 010

36

2.2

820

62

0.08

0.01

4.823 87

3.9

0.294 0

3.5

0.119 0

1.8

1 661

51

1 941

33

3.1

1 620

867

0.55

0.10

10.118

3.4

0.353 6

3.4

0.207 5

0.42

1 952

31

2 886

6.8

3.2

462

111

0.25

0.08

4.638 28

4.0

0.280 1

3.5

0.120 1

2.0

1 592

49

1 958

34

3.3

1 312

131

0.10

0.72

4.111 38

3.5

0.245 4

3.4

0.121 5

0.63

1 415

41

1 979

11

4.1

1 079

64

0.06

0.11

2.078 03

3.6

0.135 9

3.4

0.110 9

0.98

821

27

1 814

18

4.2

755

219

0.30

0.47

0.131 68

6.3

0.018 8

3.6

0.050 8

5.2

1 20

4.3

232

120

5.1

1 128

141

0.13

0.25

3.672 7

5.7

0.217 8

3.4

0.122 3

4.5

1 270

39

1 990

79

6.1

994

154

0.16

0.10

3.684 29

4.0

0.221 2

3.4

0.120 8

2.2

1 288

40

1 968

37

6.2

4 076

238

0.06

0.26

0.142 06

7.1

0.021 2

6.6

0.048 6

2.5

135

8.8

129

60

05HTL03

%206Pbc* is the percentage of common Pb in the total measured 206Pb.

discordia chord that intersects the concordia curve at (2 774±50) Ma and (1 239±200) Ma, respectively (MSWD=1.15). The upper intercept age, which is similar to the LA-ICPMS dating result, is interpreted to represent the protolith age of the granulite.

Analyses of four metamorphic rims yield U and Th contents of 215 to 437 ppm and 4 to 150 ppm, respectively. Except for analysis #1.1, the other three analyses have very low Th/U ratios, consistent with metamorphic origin. They are concordant to slightly

Lei Nengzhong and Wu Yuanbao

122

discordant, and yield a weighted mean 207Pb/206 age of (2 037±20) Ma (MSWD=0.35, n=4), which is considered as the metamorphic age for the granulite. Six analyses made on the inherited cores show moderate U and Th contents of 182 to 643 ppm and 133 to 625 ppm, with relatively high Th/U ratios of 0.59 to 1.00, implying that all of them are of igneous origin. Analysis #4.1 is concordant, and yields a 207 Pb/206Pb age of (3 236±8) Ma (Fig. 2c). The other five analyses define a discordia chord that intersects the concordia curve at (2 892±32) Ma and (957±570)

Ma, respectively (MSWD=1.17) (not shown). Among them the three concordant analyses gave a weighted mean 207Pb/206Pb age of (2 911±23) Ma (MSWD=0.88), which may represent the age of the inherited component. Only one analysis can be obtained from the outer rim domains. It has a relatively high U content of 1 634 ppm and a low Th content of 24 ppm, resulting in a very low Th/U ratio of 0.02, suggesting its metamorphic genesis. It is concordant with a 206 Pb/238U age of (201±4.4) Ma.

0.4 3 400

(a)

(3 236f8) Ma 3 000 (2 911f23) Ma

1 800 1 400

Lower intercept: (1 239f200) Ma Upper intercept: (2 774f50) Ma MSWD =1.15 (2 037f20) Ma

0.2

0.0

0.3

238

2 200

0.4

206

238

Pb/ U

2 600

Pb/ U

Granulite 05HTL02

2 000

(b) Gneiss 05HTL03 1 600

(2 886f7) Ma

Upper intercept: (1 994f33) Ma Lower intercept: (111f31) Ma MSWD =3.3

1 200

0.2

206

0.6

800 0.1

0

4

8

12 207

16

20

24

28

235

Pb/ U

0.0

0

2

4

6 207

8

10

12

235

Pb/ U

Figure 5. Concordia diagrams of SHRIMP zircon U-Pb dating for granulite 05HTL02 and gneiss 05HTL03. (a) 05HTL02; (b) 05HTL03. Six zircon grains from gneiss 03HTL03 were dated by twelve SHRIMP U-Pb spot analyses (Table 3 and Fig. 4c). Nine analyses were obtained from magmatic zircon domains. They have variable U contents of 462 to 1 648 ppm, relatively low Th contents of 63 to 154 ppm, and Th/U ratios of 0.06 to 0.25. All of them are variably discordant (Fig. 5b) and have 207Pb/206Pb ages scattering between (1 814±18) and (2 010±36) Ma. As shown in Fig. 5b, they are coherent and define a discordia chord that intersects the concordia curve at (1 994±33) Ma and (111±31) Ma (MSWD=3.3). Omitting analysis #4.1 that gave an apparent young 207Pb/206Pb age, the other eight analyses yield a weighted mean 207Pb/206Pb age of (1 982±14) Ma (MSWD=0.74), which is identical to the upper intercept age and is interpreted as the crystallization age of the protolith. Two analyses on metamorphic rims show U contents of 755 and 4 076 ppm and Th contents of 219 and 238 ppm,

respectively, with Th/U ratios of 0.06 and 0.30. They are nearly concordant and give 206Pb/238U ages of (120±4.3) and (135±8.8) Ma, with a weighted mean of 123 Ma (MSWD=2.4). An inherited core gives a 207 Pb/206Pb age of (2 886±7) Ma, which is a minimum age for the inherited domain. Zircon Hf Isotopes Fifty-three spot analyses for Lu-Hf isotope composition were accomplished on 45 zircons from granulite 05HTL02 (Table 4). Twenty-two analyses on the metamorphic domain show very low 176Lu/177Hf ratios of 0.000 008 to 0.000 059. Their 176Hf/177Hf ratios vary from 0.281 068 to 0.281 326, corresponding to HHf(t) values of -15.8 to -6.6 at t=2 000 Ma with a weighted mean of -12.2±0.9 (MSWD=4.2) (Fig. 6a). Hf model ages (TDM) range from 2.62 to 2.97 Ga (Fig. 7a). Twenty-one analyses for the protolith domains show relatively high

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Granulite-Facies Metamorphism and Crustal Remnant Table 4 No.

176

Yb/177Hf

123

Zircon Lu-Hf isotope compositions for granulite and granitic gneiss at Huangtuling 176

Lu/177Hf

176

Hf/177Hf

±2ı

t (Ma)a HHf(t)b

±2ı

TDM1(Ma)

±2ı

U-Pb spot

05HTL02 1mc

0.000 826

0.000 028

0.281 104

0.000 022

-14.5

0.8

2 918

58

2o

0.019 594

0.000 920

0.280 980

0.000 021

-2.2

0.7

3 152

56

3o

0.013 830

0.000 577

0.280 988

0.000 025

-1.3

0.9

3 114

67

4o

0.051 911

0.002 412

0.281 175

0.000 032

1.9

1.1

3 005

91

2 763

5o

0.015 728

0.000 736

0.280 989

0.000 023

0.8

0.8

3 125

61

6m

0.000 714

0.000 020

0.281 180

0.000 018

-11.8

0.6

2 817

48

7m

0.001 504

0.000 043

0.281 190

0.000 025

-11.4

0.9

2 805

66

8i

0.022 389

0.000 998

0.281 047

0.000 023

9m

0.001 325

0.000 042

0.281 153

0.000 022

10i

0.019 305

0.000 893

0.280 967

0.000 031

2 842 2 855

2.3

0.8

3 069

62

-12.7

0.8

2 854

57

-0.4

1.1

3 169

83

11m

0.000 876

0.000 027

0.281 147

0.000 026

-13.0

0.9

2 861

68

12o

0.019 452

0.000 815

0.281 034

0.000 027

-0.1

1.0

3 072

72

13m

0.001 153

0.000 037

0.281 182

0.000 033

-11.7

1.2

2 816

89

14m

0.000 357

0.000 011

0.281 187

0.000 022

-11.5

0.8

2 807

57

15m

0.000 204

0.000 008

0.281 257

0.000 024

-9.0

0.9

2 713

64

16o

0.020 234

0.000 929

0.280 974

0.000 035

-0.2

1.2

3 162

94

2 909

17m

0.000 720

0.000 023

0.281 151

0.000 021

-12.8

0.7

2 856

54

18o

0.015 670

0.000 673

0.280 997

0.000 030

-1.1

1.1

3 110

80

19m

0.001 163

0.000 038

0.281 176

0.000 024

-11.9

0.9

2 823

64

20o

0.044 283

0.001 987

0.281 116

0.000 025

0.6

0.9

3 054

69

21o

0.013 332

0.000 610

0.281 096

0.000 027

2.5

1.0

2 972

73

22m

0.010 366

0.000 487

0.281 101

0.000 023

3.0

0.8

2 956

63

23m

0.001 206

0.000 040

0.281 187

0.000 017

-11.6

0.6

2 809

46

24m

0.000 911

0.000 028

0.281 234

0.000 028

-9.9

1.0

2 746

73

25o

0.006 118

0.000 279

0.281 006

0.000 022

0.0

0.8

3 066

59

26m

0.000 781

0.000 027

0.281 248

0.000 025

-9.4

0.9

2 727

66

27o

0.013 472

0.000 605

0.281 038

0.000 024

0.5

0.9

3 050

66

28m

0.000 798

0.000 022

0.281 073

0.000 022

-15.6

0.8

2 958

59

29o

0.024 135

0.001 004

0.281 058

0.000 023

0.4

0.8

3 054

63

30o

0.012 331

0.000 523

0.281 040

0.000 023

0.7

0.8

3 041

63

31o

0.014 602

0.000 589

0.281 117

0.000 027

3.3

1.0

2 942

73

10.2

10.1 9.1

11.1

32i

0.021 245

0.000 940

0.281 128

0.000 020

2 929

5.3

0.7

2 955

53

2.1

33o

0.025 837

0.001 106

0.281 017

0.000 021

2 781

-1.2

0.7

3 118

57

3.1

34m

0.001 302

0.000 033

0.281 267

0.000 068

-8.7

2.4

2 702

181

35m

0.000 631

0.000 017

0.281 326

0.000 046

36i

0.019 573

0.000 782

0.280 718

0.000 018

37m

0.000 792

0.000 031

0.281 118

0.000 019

3 236

-6.6

1.6

2 623

123

-2.2

0.7

3 493

49

-14.0

0.7

2 899

49

4.1

38m

0.001 203

0.000 047

0.281 265

0.000 026

-8.8

0.9

2 706

70

39i

0.027 241

0.001 124

0.281 054

0.000 022

2 890

2.3

0.8

3 069

60

5.1

40o

0.021 518

0.000 816

0.281 206

0.000 022

2 754

6.1

0.8

2 839

60

6.1

41o

0.019 534

0.000 834

0.281 078

0.000 018

1.5

0.6

3 014

48

42m

0.000 588

0.000 023

0.281 156

0.000 018

-12.6

0.6

2 849

47

Lei Nengzhong and Wu Yuanbao

124 Continued No. 43o

176

Yb/177Hf

0.010 311

176

Lu/177Hf

176

Hf/177Hf

0.000 414

0.280 809

t (Ma)a HHf(t)b

±2ı

TDM1(Ma)

±2ı

U-Pb spot

0.000 027

2 345

-7.3

1.0

3 340

71

8.1

201

8.2

±2ı

44mr

0.018 466

0.000 875

0.282 141

0.000 019

-12.6

0.7

1 561

53

45o

0.021 970

0.001 014

0.281 289

0.000 047

6.3

1.7

2 831

127

46i

0.031 362

0.001 410

0.280 767

0.000 025

-1.8

0.9

3 484

69

47m

0.001 601

0.000 054

0.281 128

0.000 020

-13.7

0.7

2 889

53

48m

0.001 657

0.000 059

0.281 068

0.000 028

-15.8

1.0

2 967

73

49i

0.021 030

0.000 951

0.280 833

0.000 030

-5.2

1.1

3 354

80

50o

0.014 716

0.000 541

0.280 998

0.000 026

-0.8

0.9

3 099

69

51o

0.041 277

0.001 541

0.281 115

0.000 021

1.4

0.8

3 020

58

52o

0.028 675

0.001 086

0.281 046

0.000 021

-0.2

0.8

3 077

58

53m

0.001 130

0.000 036

0.281 142

0.000 019

-13.1

0.7

2 868

51

1i

0.018 152

0.000 777

0.280 722

0.000 038

2 886

-8.4

1.3

3 473

101

3.1

05HTL03 2i

0.012 635

0.000 432

0.280 778

0.000 018

1 958

-26.1

0.7

3 366

49

3.2

3o

0.019 145

0.000 722

0.280 979

0.000 021

1 979

-19.4

0.7

3 123

56

3.3

4o

0.027 861

0.001 141

0.281 011

0.000 018

1 925

-18.9

0.6

3 118

48

1.1

1 994

1.2

5o

0.007 222

0.000 262

0.280 937

0.000 016

-20.2

0.6

3 139

43

6o

0.016 432

0.000 661

0.280 994

0.000 015

-18.8

0.5

3 098

41

7o

0.005 620

0.000 212

0.281 014

0.000 019

-17.4

0.7

3 032

49

8o

0.006 407

0.000 271

0.280 950

0.000 014

2 010

-19.8

0.5

3 123

38

2.1

9o

0.003 783

0.000 164

0.280 991

0.000 015

1 941

-18.1

0.5

3 058

40

2.2

10o

0.022 373

0.000 926

0.281 079

0.000 017

1 968

-16.2

0.6

3 006

47

6.1

11o

0.018 592

0.000 727

0.280 987

0.000 021

-19.1

0.7

3 113

55

12o

0.013 834

0.000 588

0.281 055

0.000 021

1 990

-16.5

0.7

3 009

56

5.1

13mr

0.028 876

0.001 236

0.281 407

0.000 028

120

-45.0

1.0

2 583

78

4.2

14o

0.025 830

0.000 942

0.281 265

0.000 023

1 814

-9.6

0.8

2 755

64

15o

0.018 101

0.000 689

0.281 093

0.000 025

-15.3

0.9

2 967

68

16o

0.015 276

0.000 589

0.280 944

0.000 015

-20.4

0.5

3 158

40

17o

0.021 040

0.000 717

0.281 188

0.000 020

-12.0

0.7

2 841

54

18o

0.027 188

0.001 015

0.280 994

0.000 015

-19.3

0.5

3 129

40

19o

0.002 030

0.000 075

0.280 928

0.000 014

-20.2

0.5

3 134

37

20o

0.020 216

0.000 784

0.280 930

0.000 015

-21.2

0.5

3 194

41

21o

0.029 873

0.001 179

0.281 220

0.000 017

-11.5

0.6

2 836

48

22o

0.024 895

0.000 907

0.281 140

0.000 016

-14.0

0.6

2 922

44

23o

0.003 620

0.000 135

0.280 977

0.000 015

-18.6

0.5

3 074

40

24o

0.032 448

0.001 246

0.281 003

0.000 014

-19.3

0.5

3 139

39

25o

0.009 919

0.000 402

0.280 942

0.000 015

-20.2

0.5

3 145

41

26o

0.020 603

0.000 901

0.280 988

0.000 016

-19.4

0.6

3 128

44

27o

0.003 831

0.000 149

0.280 998

0.000 020

-17.9

0.7

3 048

54

28o

0.022 028

0.000 897

0.281 077

0.000 019

-16.2

0.7

3 006

53

29o

0.006 875

0.000 248

0.280 970

0.000 014

-19.0

0.5

3 094

38

30o

0.021 397

0.000 705

0.281 105

0.000 018

-14.9

0.6

2 953

49

a. The ages are given by the measured

207

Pb/

206

Pb ages, except the two Triassic and Cretaceous ages given by their 206Pb/238U ages. b.

Initial Hf isotope ratios are calculated nearly by their formation age (see detailed discussion in the text). c. m. metamorphic; o. oscillatory magmatic; i. inherited; mr. metamorphic outer rim.

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Granulite-Facies Metamorphism and Crustal Remnant 8

(a) Mean=(-11.84f0.90) Ma

Mean=(0.44f0.92) Ma MSWD =3.1

MSWD =3.2

6

Mean=(-18.2f1.0) Ma MSWD =17.0

8 Number

Number

(b)

10

125

4

6 4

2 2 0

-25

-20

-15

-10

-5

0

5

10

15

0

20

-35

-30

-25

H Hf ( t )

-20 -15 H Hf ( t )

-10

-5

0

Figure 6. Histograms of zircon HHf(t) values for granulite 05HTL02 and gneiss 05HTL03. (a) 05HTL02; (b) 05HTL03. 12

8 (a) Mean=(2 816f33 ) Ma Mean=(3 078f41 ) Ma MSWD=3.3 MSWD=3.9

10 Number

6

4

Mean=(3 075f36 ) Ma MSWD=4.1

(b)

8 6 4

2 2 0 1 400

1 900

2 400 2 900 T DM (Ma)

3 400

3 900

0 2 300

2 700

3 100 T DM (Ma)

3 500

3 900

Figure 7. Histograms of Hf model ages for granulite 05HTL02 and gneiss 05HTL03. (a) 05HTL02; (b) 05HTL03. 176

Lu/177Hf ratios ranging from 0.000 279 to 0.001 987. They have 176Hf/177Hf ratios of 0.280 809 to 0.281 289; if the forming age of ca. 2 800 Ma is used, they yield HHf(t) values of -7.3 to 6.3 (Fig. 6a). When calculated for derivation from a depleted mantle reservoir, their Hf model ages scatter between 2.74 and 3.34 Ga (Fig. 7a). Nine analyses were obtained from inherited domains. They have relatively high 176Lu/177Hf ratios of 0.000 736 to 0.001 410, and low 176Hf/177Hf ratios of 0.280 718 to 0.281 128. Analysis #36, which was dated to have a U-Pb age of ca. 3.24 Ga, has the lowest 176Hf/177Hf ratio of 0.280 718. Another analysis (#46) shows a similar 176Hf/177Hf ratio to that of analysis #36 and thus, is assumed to have the similar formation age. These two analyses yield HHf(t) values of -2.2 and -1.8 at t=3 200 Ma, respectively, and have similar Hf model ages of (3.49r0.05) and (3.48r0.07) Ga (Fig. 7a). The other seven analyses have HHf(t) values of -5.2 to 5.3 at t=2 900 Ma, corresponding to Hf model ages of 2.96 to 3.35 Ga. Only one analysis

(#20) was obtained from the outer rim with an age of ca. 200 Ma. It has 176Lu/177Hf and 176Hf/177Hf ratios of 0.000 875 and 0.282 141, respectively. Its HHf(t) value is -12.6, with an Hf model age of 1.6 Ga. Thirty analyses for Lu-Hf isotope composition were carried out for 22 zircon grains from gneiss 05HTL03, including 10 analyses that have been dated with the SHRIMP U-Pb method (Table 4). All of them show similar 176Lu/177Hf ratios of 0.000 149 to 0.001 374. Except for analyses #1, #2, and #13, the other twenty-seven analyses yield 176Hf/177Hf ratios ranging from 0.280 943 to 0.281 280, corresponding to HHf(t) values of -9.6 to -21.2 at t=2 000 Ma. Their Hf model ages scatter between 2.75 to 3.19 Ga. Analyses #1 was obviously acquired from the inherited core. It shows relatively low 176Hf/177Hf ratio of 0.280 738, corresponding to HHf(t) value of -8.4 at t=2 900 Ma. Its Hf model age is 3.47 Ga. Although analysis #2 was dated at ca. 2 000 Ma, it has obviously low 176Hf/177Hf ratio of 0.280 793,

Lei Nengzhong and Wu Yuanbao

126

corresponding to a HHf (t) value of -26.1 at t=2 000 Ma. This indicates that it is also inherited domain, but its U-Pb age has been completely reset by the ca. 2 000 Ma event. This can also be revealed by its significantly old Hf model age of 3.37 Ga. Analysis #13 obtained from the metamorphic rim gave a 176 Hf/177Hf ratio of 0.281 422 and an Hf model age of 2.58 Ga. At t=130 Ma, the calculated HHf(t) value is -45.0. DISCUSSION Paleoproterozoic Metamorphism Since Chen et al. (1996) firstly dated the felsic granulite at Huangtuling in the North Dabie zone, a great deal of effort has been made to constrain the timing of granulite-facies metamorphism by means of zircon U-Pb, mineral Sm-Nd, and Ar-Ar dating. Chen et al. (1996) obtained ID-TIMS zircon U-Pb discordia ages of (2 663±56) Ma and (1 690±82) Ma, respectively, which were interpreted as ages of protolith crystallization and regional granulite-facies metamorphism. Jian et al. (1999) used TIMS evaporation technique to date the granulite and obtained zircon 207Pb/206Pb ages of 1 992 to 2 814 Ma. The authors regarded the minimum age as the granulite metamorphic age and the maximum age as the protolith age. According to our CL images (Fig. 2), however, a multi-stage growth is evident for the zircon domains in the granulite at Huangtuling, including protolith, inherited, granulite-facies, and retrograde metamorphic domains. These have resulted in potential difficulties in obtaining metamorphic or protolith age for the granulite by the traditional TIMS techniques. Zhou et al. (1999) reported a Pb-Pb isochron age of (1 998±35) Ma by the stepwise dissolution of garnet. A less precise pyroxene + feldspar + whole rock Sm-Nd isochron age of (2 238±300) Ma was obtained by Ma et al. (2000). Wang et al. (2002) reported a ca. 195 Ma biotite Ar-Ar age and considered it close to the age of granulitefacies metamorphism. However, biotite Ar-Ar age cannot be used to date H-T granulite-facies metamorphic event because of its low closure temperature for Ar diffusion. In our granulite samples 05HTL01 and 05HTL02, the zircon rims are characterized by sector or planar

zoning internal structures (Fig. 2a) and low Th/U ratios (0.01 to 0.08), which are typical features for metamorphic zircons in high-grade metamorphic rocks (Wu and Zheng, 2004; Zheng Y F et al., 2004; Whitehouse and Platt, 2003; Rubatto et al., 1999; Vavra et al., 1999, 1996). All the metamorphic zircon domains have an obviously negative Eu anomaly and relatively flat HREE pattern. These indicate that the metamorphic zircons formed along with garnet and feldspar under granulite-facies conditions (Rubatto and Hermann, 2003; Whitehouse and Platt, 2003; Rubatto, 2002; Schaltegger et al., 1999). The weighted mean age of LA-ICPMS and SHRIMP dating results for the metamorphic domains is (2 029±13) Ma (MSWD=0.26) (Fig. 8) and is the best estimate age of the granulite-facies metamorphism. 2 060

2 040

2 020

2 000

Figure 8. Weighted mean age for the metamorphic zircons from granulites 05HTL01 and 05HTL02. Based on petrographic studies, Chen Y et al. (2006), and Chen N S et al. (1998) suggested that the granulite-facies metamorphism at Huangtuling is characterized by a clockwise P-T path, which appears to be associated with a collision process (England and Thompson, 1984). Our zircon U-Pb data demonstrate that this event occurred during the Middle Paleoproterozoic (2 029±13 Ma). To our knowledge, this is the first report for a mid-Paleoproterozoic orogenic event in the Yangtze block. Lee et al. (2000) obtained a similar U-Pb age (ca. 1.9 Ga) for Paleoproterozoic granulite-facies rocks from the Gyeonggi massif in Central Korea, suggesting a link to the Yangtze block. An SHRIMP discordia upper-intercept U-Pb age of (1.82r0.10) Ga was reported for residual zircon from LT/UHP eclogite at

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Granulite-Facies Metamorphism and Crustal Remnant

Huangzhen in the Dabie orogen (Li et al., 2004). In combination with Nd and Sr isotope studies for the eclogite, Li et al. (2004) interpreted that the eclogite protolith would be derived from melting of a paleoceanic basalt with geochemical affinity to the depleted mantle at approximately 1.8 to 1.9 Ga. Paleoproterozoic U-Pb ages of 1.8 to 2.0 Ga are also dated for inherited zircon from other metamorphic rocks in the Dabie orogen (Wu Y B et al., 2006; Grimmer et al., 2003; Ayers et al., 2002; Chavagnac et al., 2001) and at Kongling in the Yangtze block (Zhang et al., 2006a, b; Zheng J P et al., 2006). Zhang et al. (2006b) suggested a likely connection between this block and the North China block with respect to assembly and break-up of the supercontinent Columbia in this period (Zhao Z F et al., 2004, and references therein). In this context, the occurrence of granulite-facies metamorphism, migmatitization, and subsequent depleted mantle-like magmatism during the Middle Paleoproterozoic along the northern edge of the Yangtze block may be a manifestation of arc-continent collision in response to the global-scale 2.1 to 1.9 Ga collision orogeny resulting in the Columbia assembly (Zhang et al., 2006b). On the other hand, the protolith of gneiss 05HTL03 is dated to form at (1 982±14) Ma. This age is only slightly younger than that for the granulite-facies metamorphism and suggests that the granulite-facies metamorphism and the intrusion of felsic magma belong to the same tectonothermal event. In fact, granulite-facies metamorphism is characterized by dehydration, and thus linked to the formation of granitic magmas (e.g., Clemens, 1990; Vielzeuf et al., 1990). This near synchronicity probably indicates that granulite-facies metamorphism and partial melting were complementary processes (Dostal et al., 2006). The magmatic zircons in our sample have Hf model ages of 2.75 to 3.19 Ga, which are significantly older than their formation age. This may imply that they were formed by reworking of ancient crust that was extracted from the depleted mantle during the Mesoarchean to Neoarchean. Archean Crustal Growth in the Yangtze Block Zircon is a very refractory mineral and can survive erosion and metamorphism that may modify

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or destroy its host rock. It has very low Lu/Hf ratio and high Hf concentration and thus, records near initial 176Hf/177Hf at the time of its formation. U-Pb dating can provide a precise age of formation on the same sample used for Hf isotope measurement. Therefore, zircon U-Pb dating combined with Hf isotope analyses are well suited for resolving the early evolution history of ancient crust (Zhang et al., 2006a, c; Zheng J P et al., 2006, 2004; Zheng Y F et al., 2006; Harrison et al., 2005; Amelin et al., 1999). The trace element compositions of zircon depend on the concurrent growth of other minerals, which can be relevant for the identification of metamorphic conditions (Bingen et al., 2004; Hoskin and Schaltegger, 2003; Rubatto and Hermann, 2003; Whitehouse and Platt 2003; Rubatto, 2002). Plagioclase is the main sink for Eu. Therefore, its simultaneous growth with zircon results in Eu depletion of the latter mineral. As a consequence, a negative Eu anomaly appears in the chondritenormalized REE pattern of zircon formed simultaneously with plagioclase. Regarding HREE, zircon generally displays HREE enriched patterns (Hoskin and Schaltegger, 2003). Garnet which is much more abundant incorporates also HREE and its crystallization in a metamorphic environment results in a depletion of HREE. As a result, simultaneously grown zircon would display a nearly flat or even negative HREE profile (Bingen et al., 2004; Hoskin and Schaltegger, 2003; Rubatto and Hermann, 2003; Whitehouse and Platt, 2003; Rubatto, 2002). The protolith zircon domains in granulites 05HTL01 and 05HTL02 are characterized by oscillatory zoning (Fig. 2b), HREE enrichment, high Th/U and high 176Lu/177Hf ratios, which are typical for igneous zircon. The nine analyses on the protolith magmatic zircon domains in 05HTL01 yield a weighted mean 207Pb/206Pb age of (2 761±13) Ma. The protolith zircon domains in sample 05HTL02 were dated at (2 767±5) Ma by LA-ICPMS and (2 773±33) Ma by SHRIMP, respectively. They yield a weighted mean of (2 766±9) Ma (MSWD=0.11). These indicate that the protolith ages of the Huangtuling granulites are approximately 2.77 Ga. Qiu et al. (2000) also obtained ca. 2.75 Ga zircons from the Kongling area in the northern part of the Yangtze block, southwest to

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the Dabie orogen. Therefore, the ca. 2.77 Ga tectonothermal event may be widespread around the Yangtze block, and Neoarchean basement in the Yangtze block occurs as far north as the North Dabie zone. The protolith magmatic zircons from granulite 05HTL02 exhibit very large variations in 176Hf/177Hf ratios from 0.280 809 to 0.281 206, corresponding to HHf(t) values of -7.3 to 6.3 (Table 4). Because the zircons have undergone different degrees of recrystallization as shown by the U-Pb data, it is crucial to elucidate whether their Hf isotope compositions are original or just modified by granulite-facies metamorphism. As demonstrated by Zheng et al. (2005b), metamorphic newly grown zircon can elevate its HHf(t) values by 3.1 to 13.5, whereas solid-state recrystallization zircon can preserve its Lu/Hf and 176Hf/177Hf ratios resembling its protolith zircon. The similar observations of elevated HHf(t) values by metamorphism were also made for migmatite at Kongling (Zhang et al., 2006a, b) and in the Antarctic peninsula (Flowerdew et al., 2006). For our sample, however, three zircons with nearly concordant U-Pb ages (analyses #4, #33 and #40) also give HHf(t) values in the range of -1.2 to 6.1 (Table 4). These variations could be attributed to metamorphic modification by anatectic recrystallization that involves dissolution of high 176 Hf/177Hf accessory minerals. However, the Th/U and Lu/Hf ratios for the protolith zircons are inconsistent with metamorphic resetting. Thus, the large variation in HHf(t) values of -7.3 to 6.3 for the protolith zircons is original. The total range of HHf(t) values can be explained by different proportions of mantle-crust mixing in magma source or via crustal contamination during magma emplacement (Hawkesworth and Kemp, 2006; Zheng Y F et al., 2006; Griffin et al., 2002; Amelin et al., 1999). The positive HHf(t) values up to 6.3 provide unambiguous evidence for growth of juvenile crust from the depleted mantle at the time of zircon formation. On the other hand, the negative HHf(t) values indicate that the magma sources of zircon contained older crustal components, which is in agreement with the occurrence of old inherited zircons. The Hf model ages of 2.74 and 3.34 Ga imply that the oldest crust

Lei Nengzhong and Wu Yuanbao

was extracted from the depleted mantle at least as early as 3.34 Ga ago. Most inherited zircons in granulite 05HTL02 were dated at (2 938±31) Ma. They have 176Hf/177Hf ratios of 0.280 833 to 0.281 128, corresponding to HHf(t) values of -5.2 to 5.3 at t=2 900 Ma. The variations can also be ascribed to the mantle-crust mixing. Half of them show positive HHf(t) values indicating formation from juvenile crust, whereas the others with negative HHf(t) values suggesting recycling of ancient crust. Their Hf model ages fall into the range of 2.96 to 3.35 Ga, similar to, or distinctly older than the formation ages. This also reflects their hybrid gneiss, with contributions from both juvenile and ancient crustal materials. An inherited zircon in gneiss 05HTL03 has a 207Pb/206Pb age of (2 886±7) Ma. Its 176 Hf/177Hf ratio is 0.280 738 and HHf(t) value is -8.4 at t=2 900 Ma, which also favors melting of old crust at that time. Two trondhjemite gneisses from the Kongling area have been reported to have zircon U-Pb ages of 2.90 to 2.95 Ga and negative HNd(t) values of -1.27 to -1.10 (Qiu et al., 2000). The similar U-Pb ages are also acquired for zircon from five migmatite and gneiss in the Kongling area (Zhang et al., 2006a; Zheng J P et al., 2006), with negative HHf(t) values of -8.7 to -0.1. These results reinforce that the 2.90–2.95 Ga event signifies recycling of older crustal components in the Yangtze block. An inherited zircon from granulite 05HTL02 gives a 207Pb/206Pb age of 3.24 Ga. It has 176Hf/177Hf ratio of 0.280 718 and HHf(t) value of -2.2. Another inherited zircon from the same sample with the similar 176 Hf/177Hf ratio is assumed to form at the same time and thus, gives similar HHf(t) value of 1.8. Their Hf model ages are ca. 3.50 Ga. These suggest that the host rocks of these zircons were extracted from the mantle at least 3.50 Ga ago and then were remelted at 3.2 to 3.3 Ga. The inherited zircon from granulite 05HTL02 yielded an LA-ICPMS 207Pb/206Pb age of 3.53 Ga, which represents the minimum formation age. A very old Nd model age of ca. 3.1 Ga relative to the depleted mantle was obtained from the Huangtuling granulite (Ma et al., 2000), which is compatible with the U-Pb ages for the inherited zircon. It appears that the zircon U-Pb age of ca. 3.53 Ga indicates the presence of Paleoarchean crustal relict in the Yangtze

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Granulite-Facies Metamorphism and Crustal Remnant

block. The ca. 3.28 Ga xenocrystal zircons were found in metasedimentry rocks from the Kongling high-grade metamorphic unit (Qiu et al., 2000). Liu X M et al. (2006) found detrital zircons with Hf model ages of 3.3 to 3.5 Ga from Neoproterozoic sedimentary rocks. Zhang et al. (2006c) acquired zircon U-Pb ages up to 3.8 Ga for Neoproterozoic sedimentary rocks near Kongling. Xenocrystic zircons from lamproite diatremes were reported to have Hf model ages of 2.6 to 3.5 Ga (Zheng J P et al., 2006). Zhang et al. (1990) also reported a ca. 3.3 Ga inherited Pb component in a quartz-syenite from Dalongshan in Anqing of Anhui Province, south to the Dabie orogen. All these suggest that some basement rocks of the Yangtze block are as old as parts of the North China block (Zheng J P et al., 2004; Zhao et al., 2002; Song et al., 1996; Liu et al., 1992; Jahn et al., 1987), and the Paleoarchean basements with ages of 3.2 to 3.5 Ga in the Yangtze block were widespread as previously recognized. In the Dabie orogen, Paleoproterozoic to Archean components occur only as lenses or even inherited zircon cores, whereas metamorphic rocks with Neoproterozoic protolith ages are widely outcroped (Zheng Y F et al., 2006, 2004, 2003; Grimmer et al., 2003; Ayers et al., 2002; Chavagnac et al., 2001; Hacker et al., 1998). These indicate that the old components represented the wall rocks and were destructed by the strong Neoproterozoic bimodal rifting magmatisms along the northern margin of the Yangtze block. Exhumation of High-Grade Metamorphic Rocks A well-preserved growth zoning in garnet from the Huangtuling granulite was obtained by Chen Y et al. (2006) and Chen N S et al. (1998), reflecting that the Paleoproterozoic granulite-facies metamorphism was terminated very soon and then experienced amphibolite-facies metamorphism at mid-crustal levels. It seems that the rock would not be significantly reheated afterward and thus not entrapped into the Triassic subduction of the Yangtze block beneath the North China block. However, a metamorphic rim of zircon from sample 05HTL02 has a U-Pb age of 201 Ma (Table 3), indicating its growth due to a Triassic tectonothermal event. A ca. 195 Ma

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Ar-Ar plateau age for biotite in a granulite at Huangtuling was also reported by Wang et al. (2002), suggesting that it is the effect of Triassic event rather than the Plaeoproterozoic orogeny. Although the Triassic UHP metamorphic overprint was not found in the granulites and gneiss, it cannot exclude a tectonothermal effect on these rocks by this period of collision orogeny. Fluid-absent metamorphic environment and very fast exhumation rate, as revealed by the comprehensive studies of mineral O isotopes and U-Pb ages (Zheng Y F et al., 2004, 2003, 2001), may be two crucial factors for the preservation of growth zoning in the garnet and for few zircon growths from the granulites during the Triassic continental collision between the North China and Yangtze blocks. On the contrary, abundant metamorphic zircons of Triassic ages occur in Dabie-Sulu UHP metamorphic rocks with igneous protoliths of Neoproterozoic (Zheng Y F et al., 2006, 2005, 2004; Liu et al., 2004; Hacker et al., 2000, 1998) or Paleoproterozoic to Archean (Li et al., 2004; Zheng Y F et al., 2004; Grimmer et al., 2003; Ayers et al., 2002; Chavagnac et al., 2001). Even large sizes and volumes of Triassic metamorphic zircons occurred in the UHP metasedimentary rocks (Liu F L et al., 2006; Wu Y B et al., 2006). These indicate that fluid availability is a decisive factor for growth and/or recrystallization of metamorphic zircon (Wu Y B et al., 2006; Zheng Y F et al., 2004; Corfu et al., 2003). As illustrated by Söderlund et al. (2002) for polymetamorphic gneisses from the Sveconorwegian orogen in SW Sweden, Zr hosted in igneous phases was liberated during the first phase of granulite-facies metamorphic event when primary minerals were recrystallized or took part in metamorphic reactions to form metamorphically grown zircon, resulting in less mobilized Zr left for a new generation of metamorphic zircon to form in the absence of aqueous fluid or hydrous melt during a later metamorphic event. The similar situation can also be used to account for the rare occurrence of Triassic zircon growths in the Huangtuling granulite. It is consistent with the petrological results that the granulite at Huangtuling experienced the Triassic UHP metamophism under fluid-absent conditions (Chen Y et al., 2006). As for granitic gneiss 05HTL03, the lack of Triassic zircon growth may be also because of the

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fluid-absent metamorphism during the continental collision (Fig. 2c). The North Dabie zone experienced intensive magmatism and metamorphism during the Early Cretaceous, possibly associated with extensional collapse of collision-thickened orogen (Xie et al., 2006). The exhumation was suggested to have been accomplished by an asymmetric Cordilleran-type extensional complex formed between 140 and 120 Ma (Hacker et al., 2000). High-grade metamorphic rocks in the North Dabie zone were brought to rather shallow depth during these processes (Ratschbacher et al., 2000). Two zircon rims in gneiss 05HTL03 have U-Pb ages of 120 and 135 Ma (Table 3), with a weighted mean of 123 Ma (MSWD=2.4). These ages are in agreement with the widespread Cretaceous ages for the North Dabie igneous rocks (Xie et al., 2006; Zhao Z F et al., 2005, 2004; Bryant et al., 2004; Jahn et al., 1999), reflecting that the final exhumation of the high-grade metamorphic rocks at Huangtuling may be related to the Early Cretaceous extension.

Ga, respectively. The large variations in HHf(t) values for the ca. 2.75–2.80 Ga protolith magmatic and the ca. 2.90 Ga inherited zircons depict two major episodes of crust formation, including both juvenile crust growth and older crust recycling. The occurrence of the ca. 3.53 Ga inherited zircon and the ca. 3.24 Ga inherited zircon with an Hf model age of ca. 3.50 Ga provides further evidence for the presence of Paleoarchean crust in the Yangtze block. Thus, Paleoarchean basement of the Yangtze block may be sizable but destroyed by later metamorphic and magmatic events. The relative lack of Triassic and Cretaceous metamorphic ages in the zircons from the Huangtuling granulite and gneiss suggests very limited availability of either aqueous fluid or hydrous melt during the continental collision and postcollisional exhumation. ACKNOWLEDGMENTS We are grateful to Prof. Wan Yusheng for his assistance with SHRIMP U-Pb dating. REFERENCES CITED

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