Geochemistry and zircon U–Pb geochronology of Paleoproterozoic arc related granitoid in the Northwestern Yangtze Block and its geological implications

Precambrian Research 200–203 (2012) 26–37

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Geochemistry and zircon U–Pb geochronology of Paleoproterozoic arc related granitoid in the Northwestern Yangtze Block and its geological implications Yuanbao Wu a,∗ , Shan Gao a , Hongfei Zhang a , Jianping Zheng a , Xiaochi Liu a , Hao Wang a , Hujun Gong b , Lian Zhou a , Honglin Yuan b a b

State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China

a r t i c l e

i n f o

Article history: Received 6 October 2011 Received in revised form 21 December 2011 Accepted 28 December 2011 Available online 5 January 2012 Keywords: Yangtze Block Zircon Paleoproterozoic Calc-alkaline granitoid Houhe complex Arc-continental collision

a b s t r a c t The early history of the Yangtze Block has not been well constrained yet, due to the scarce outcrops of Archean to Paleoproterozoic rocks. In this study, we report an integrated study of zircon U–Pb age, major and trace element data and Sr–Nd–Hf isotope compositions for gray gneisses from the Houhe complex in the northwestern part of the Yangtze Block. Zircon U–Pb dating yields a weighted mean 207 Pb/206 Pb age of 2081 ± 9 Ma for a gneiss. This age is interpreted as the formation age of the Houhe complex and thus the Houhe complex represents the oldest rocks found in the western part of the Yangtze Block. The gray gneisses from the Houhe complex range in SiO2 contents from 58.63% to 68.59% and Na2 O from 3.88% to 5.28%, and have relatively high Fe2 O3 contents of 2.86–6.69%, Al2 O3 of 16.01–18.88%, and MgO of 0.97–2.65%. These rocks show low Sr (149–390 ppm), Cr (9.07–45.1 ppm) and Ni (4.97–21.3 ppm) contents, but high Y (12.9–32.7 ppm) and Yb (0.95–2.25 ppm). They are characterized by a relative enrichment in LILEs and LREEs, but a depletion in HFSEs. These features are similar to those of calc-alkaline granitoids, suggesting that their formation might be related to a subduction-related process or remelting of preexisted arc rocks. Compiled age spectra of Archean to Paleoproterozoic zircon grains reveal that the western and eastern parts of the Yangtze Block have similar Archean and early Paleoproterozoic age ranges, indicative of the occurrence of an old continental nucleus in both parts. On the other hand, there are large amounts of ca. 2050–2400 Ma zircon grains revealed in the western part of the Yangtze Block, implying that there might be a microcontinent with an active-type continental margin during the Paleoproterozoic times in the western part of the Yangtze Block. Because of the poor outcrops of Archean to Paleoproterozoic rocks in the Yangtze Block, it is no possible to establish the exact nature of such an active margin yet. The occurrence of ca. 2.0 Ga khondalitic and metasedimentary rocks has been documented in the eastern part of the Yangtze Block, implying that there might be a passive-type continental margin in the eastern part of the Yangtze Block. At ca. 2.0 Ga, the collision between the western microcontinent and the eastern part may have resulted in the final arc-continental collision and the amalgamation of the Yangtze Block. © 2012 Elsevier B.V. All rights reserved.

1. Introduction It has been widely recognized that significant chemical changes in compositions of juvenile continental crust occurred in the late Archean to the Paleoproterozoic (Engel et al., 1974; Taylor and McLennan, 1985; Condie, 1993, 2008). In the late Archean, mafic crust was greatly thickened and underwent partial melting to produce the tonalite-trondhjemite-granodiorite (TTG) magma suites, whereas in the Early Proterozoic, large ion lithophile element (LILE) enriched calc-alkaline granitoids widely occurred (Champion and Sheraton, 1997; Moyen et al., 2003; Condie, 2008). Such a diversity

∗ Corresponding author. Tel.: +86 2767883001; fax: +86 2767883002. E-mail addresses: [email protected], [email protected] (Y. Wu). 0301-9268/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2011.12.015

suggests that these rocks were produced by contrasting petrogenetic processes in various geodynamic settings (Moyen et al., 2003) and the modern style mantle wedges may have come into existence during the early Proterozoic times (Condie, 2008). The Yangtze and North China blocks are the two largest Precambrian blocks in China, which collided along the Qinling-Dabie-Sulu orogenic belt in the Triassic (Zheng et al., 2003; Wu et al., 2006, 2009a). The North China Block is one of the oldest blocks in the world, with widespread Archean rocks and crustal remnants as old as 3800 Ma (Liu et al., 1992; Song et al., 1996; Zheng et al., 2004), and its early evolution has been well constrained (Liu et al., 2002; Zhao et al., 2002a, 2005, 2008; Yin et al., 2009, 2011; Wang et al., 2010). The first occurrence of calc-alkaline granitoids was constrained at the late Archean to the early Paleoproterozoic, arguing for the initial subduction process in the North China Block (Liu et al., 2002; Zhao

Y. Wu et al. / Precambrian Research 200–203 (2012) 26–37

27

Fig. 1. (a) Sketch geological map showing the Houhe complex in the Yangtze and the Cathaysia blocks, and the Houhe complex in the northwestern part of Yangtze Block. Outcrops of Archean TTG and Paleoproterozoic rocks are also shown. References—*this study, 1: Qiu et al. (2000); 2: Zhang et al. (2006a); 3: Zhang et al. (2006b); 4: Zheng et al. (2006); 5: Jiao et al. (2009); 6: Wu et al. (2009b); 7: Gao et al. (2011); 8: Wu et al. (2008); 9: Sun et al. (2008); 10: Greentree and Li (2008); 11: Zhao et al. (2010); 12: Li (1997). (b) Geological map of the Houhe complex (modified after Ling et al., 2003). (1) Houhe gneiss complex; (2) Meso-Neoproterozoic Mawozi and Shangliang formations, lower and middle Huodiya Group; (3) Tiechuanshan volcanic-sedimentary succession, upper Huodiya Group; (4) Sinian and Phanerozoic sedimentary rocks; (5) Neoproterozoic gabbro; (6) granite; (7) diorite; (8) alkaline granite; (9) mafic dike; (10) fault; (11) unconformity; and (12) sample location.

28

Fig. 2. (A) Typical CL images of zircons from the gray gneiss sample 07QL14. The smaller circles show LA-ICP-MS dating spots and corresponding U–Pb ages, and the larger circles show the locations of Lu–Hf isotope analysis and corresponding epsilon Hf value. (B) Concordia diagrams for U–Pb ages of zircon grains in the gray gneiss sample 07QL14.

Spot

Th (ppm)

U (ppm)

Th/U

207

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

155 174 250 100 165 216 208 60 145 119 180 100 106 227 210 155 387 172

292 245 458 184 331 453 364 91 271 211 281 184 188 393 375 211 487 229

0.53 0.71 0.55 0.54 0.50 0.48 0.57 0.65 0.53 0.56 0.64 0.55 0.57 0.58 0.56 0.74 0.79 0.75

0.12957 0.12602 0.12856 0.12866 0.12825 0.12836 0.12982 0.12903 0.1313 0.12872 0.12867 0.12951 0.12595 0.12718 0.13048 0.12901 0.12849 0.12975

a

Pb* representing radiogenic Pb.

Pb*/206 Pb*a

1

207

Pb*/235 U

0.00139 0.00138 0.00132 0.00142 0.00133 0.00135 0.00142 0.00186 0.0014 0.00138 0.00141 0.00139 0.00148 0.00134 0.0014 0.00138 0.00138 0.00138

6.82986 6.35457 6.75198 6.76212 6.74875 6.70309 6.85122 6.71847 6.97449 6.79034 6.6644 6.91417 6.32462 6.3038 7.19145 6.7962 6.68341 6.84704

1

206

0.07622 0.07216 0.07287 0.07738 0.07338 0.07348 0.07793 0.09735 0.07772 0.07575 0.07609 0.07731 0.07668 0.06921 0.08035 0.07557 0.07484 0.07588

0.38228 0.36571 0.3809 0.38117 0.38163 0.37871 0.38273 0.37761 0.38523 0.38259 0.37564 0.38719 0.36418 0.35947 0.39972 0.38206 0.37723 0.38272

Pb*/238 U

1

207

Pb/206 Pb (Ma)

0.00408 0.00392 0.00404 0.00409 0.00406 0.00403 0.0041 0.00428 0.00411 0.00408 0.00403 0.00414 0.00395 0.00383 0.00427 0.00408 0.00403 0.00408

2092 2043 2078 2080 2074 2076 2096 2085 2115 2081 2080 2091 2042 2059 2104 2085 2077 2095

1

207

Pb/235 U (Ma)

19 19 18 19 18 18 19 25 19 19 19 19 21 18 19 19 19 19

2087 2009 2081 2082 2084 2070 2089 2065 2101 2088 2056 2110 2002 1980 2168 2086 2063 2089

1

206

Pb/238 U (Ma)

19 18 19 19 19 19 19 20 19 19 19 19 19 18 20 19 19 19

2090 2026 2080 2081 2079 2073 2092 2075 2108 2085 2068 2101 2022 2019 2135 2085 2070 2092

1 10 10 10 10 10 10 10 13 10 10 10 10 11 10 10 10 10 10

Y. Wu et al. / Precambrian Research 200–203 (2012) 26–37

et al., 2007). In contrast, the Yangtze Block consists mainly of Neoproterozic rocks (Li et al., 2003), with only sporadic Archean TTG rocks in the Kongling terrane (Gao et al., 1999, 2011; Qiu et al., 2000; Zhang et al., 2006a; Jiao et al., 2009), and Paleoproterozoic rocks in the southwestern and northwestern parts (Fig. 1a) (Greentree and

Table 1 Zircon U–Pb isotopic data obtained by LA-ICP-MS for sample 07QL14 from the Houhe complex.

Y. Wu et al. / Precambrian Research 200–203 (2012) 26–37

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Table 2 Major and trace element contents of gray gneisses in the Houhe complex. Sample SiO2 TiO2 Al2 O3 MgO Fe2 O3 CaO MnO P2 O5 Na2 O K2 O LOI Total Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Tl Pb Th U

07QL13

07QL14

07QL15

07QL19

07QL20

07QL21

68.59 0.35 16.01 1.26 3.19 2.51 1.26 0.11 3.88 2.83 1.35 100.13

64.92 0.75 16.06 1.9 4.59 2.24 1.9 0.24 4.27 4.19 0.93 100.15

58.65 0.84 18.88 1.95 6.69 2.92 1.95 0.33 5.61 2.57 1.1 99.60

65.41 0.4 16.98 1.8 4.16 2.13 1.8 0.16 4.1 3.26 1.38 99.85

69.33 0.35 16.01 0.97 2.86 2.35 0.97 0.1 4.34 1.84 1.43 99.64

62.85 0.67 16.3 2.65 5.37 4.42 2.65 0.21 5.28 0.68 1.76 100.27

63.87 0.33 17.55 1.8 4.5 3.28 1.8 0.18 4.88 2.08 1.48 100.01

7.69 44.1 12.1 8.67 6.97 3.81 34.0 19.3 57.9 331 14.3 145 6.61 0.22 1101 44.6 80.9 8.52 28.2 4.53 1.17 3.44 0.46 2.48 0.46 1.18 0.17 1.00 0.14 3.82 0.37 6.62 7.37 0.70 7.69 44.1

12.8 54.8 9.07 12.5 6.47 31.4 32.0 19.2 123 149 21.5 273 9.27 0.98 1487 101 192 21.4 73.8 11.7 1.87 7.78 0.96 4.76 0.84 2.15 0.29 1.54 0.22 6.54 0.42 3.81 12.9 0.48 12.8 54.8

12.3 95.1 14.7 13.6 9.38 2.44 25.9 27.5 100 278 22.0 337 8.87 0.82 969 56.9 102 11.7 42.1 7.38 1.67 5.57 0.72 3.82 0.74 2.05 0.29 1.73 0.26 7.84 0.36 1.68 5.93 0.65 12.3 95.1

7.15 49.2 45.1 9.51 13.5 5.93 45.1 20.6 115 281 13.8 167 9.55 0.97 1124 28.4 48.6 5.39 19.1 3.59 1.14 2.67 0.38 2.23 0.43 1.23 0.17 1.16 0.17 3.99 0.62 4.37 5.58 0.62 7.15 49.2

5.81 41.8 10.7 8.61 4.97 23.6 27.8 18.0 52.8 371 12.9 164 5.71 0.27 803 45.8 81.1 8.43 28.1 4.35 1.14 2.97 0.38 2.16 0.40 1.07 0.15 0.95 0.13 4.31 0.36 4.90 10.1 0.62 5.81 41.8

13.9 77.3 22.8 14.7 21.3 2.93 53.2 16.1 19.5 284 20.8 260 8.63 0.31 177 35.8 78.2 9.47 34.8 6.39 1.23 4.94 0.69 3.85 0.75 1.99 0.29 1.64 0.26 6.42 0.55 6.01 9.67 0.69 13.9 77.3

28.6 67.5 19.5 13.1 8.09 5.77 45.4 20.1 73.2 390 32.7 157 6.13 0.63 783 81.9 153 16.9 59.0 10.9 1.68 8.70 1.21 6.47 1.17 3.08 0.42 2.25 0.33 4.05 0.47 8.66 19.6 0.92 28.6 67.5

Li, 2008; Zhao et al., 2010). It was suggested that the Yangtze Block experienced a Paleoproterozoic high-grade metamorphic event at ca. 2.0 Ga during the assembly of the supercontinent Columbia (Wu et al., 2002, 2008, 2009b; Zhang et al., 2006a,b; Zheng et al., 2006; Sun et al., 2008), probably resulting from an arc-continent collisional orogenesis (Zhang et al., 2006a,b). If this model is correct, it might be the earliest subduction process documented in the Yangtze Block, which has crucial implications for the geochemical and tectonic evolution of this block. However, the details about this process have not been well constrained and no arc-related rocks formed in this process have been reported yet. The Houhe complex is located in the northwestern part of the Yangtze Block (Fig. 1b). The complex was taken as an Archean TTG gneiss terrane in the Yangtze Block, for it has petrological features analogous to the TTG gneisses in the Kongling terrane (Gao and Zhang, 1990), and thus it provides important information about the early evolution of the western part of the Yangtze Block. In this contribution, we provide an integrated study of zircon U–Pb age, major and trace element geochemistry, and Sr–Nd–Hf isotope compositions for the Houhe complex. The results will provide important

07QL17

insights into understanding the Paleoproterozoic tectonic evolution of the western part of the Yangtze Block. 2. Geological setting and sampling The Yangtze Block, bounded to the west by the Tibetan Plateau and to the north by the Qinling-Dabie-Sulu orogen, is composed mainly of late Paleoproterozoic and Neoproterozoic rocks with only sporadic outcrops of Archean rocks (Zhang et al., 1990; Chen and Jahn, 1998; Gao et al., 1999; Qiu et al., 2000; Jiao et al., 2009). The Kongling terrane, located in the northern part of the Yangtze Block, represents the only known Archean microcontinent in the Yangtze Block (Gao et al., 1999, 2011; Qiu et al., 2000; Zhang et al., 2006a,b; Zheng et al., 2006; Jiao et al., 2009). A population of Paleoproterozoic zircon grains (ca. 2400–2100 Ma) was reported in the western part of the Yangtze Block, which is quite absent in the eastern part, implying the two parts might have different evolution during this period (Fig. 1a) (Greentree and Li, 2008; Zhao et al., 2010). The Houhe complex is poorly exposed in the northwestern part of the Yangtze Block (Fig. 1a). It mainly consists of tonalitic

30

Y. Wu et al. / Precambrian Research 200–203 (2012) 26–37

14

06QL13 06QL14 06QL15 07QL17 06QL19 06QL20 06QL21

N a 2O + K 2O ( w t % )

12 10

8

Quartz Monzonite

Monzonite Monzodiorite

6 Granite

4 Gabbro

Gabbroic Diorite

Diorite

Granodiorite

2

0 35

45

55

65

75

SiO 2(wt% ) Fig. 3. TAS diagram of the gray gneisses from the Houhe complex.

gneisses, with minor amphibolites and marbles, and has experienced upper amphibolite facies metamorphism and migmatization (Gao and Zhang, 1990; Ling et al., 2003). The Houhe complex is unconformably overlain by the greenschist facies metamorphosed Meso-Neoproterozoic Huodiya Group, which consists of, from bottom to top, the Mawozi Formation of marbles, the Shangliang Formation of aleuvitic slates with marbles and the Tiechuanshan Formation of volcanic lavas and clastic rocks (Fig. 1b) (Ling et al., 2003). Zircon U–Pb dating for felsic gneiss and migmatite in the Houhe complex yielded 207 Pb/206 Pb ages of ca. 2100–2600 Ma by the zircon evaporation method (Ling et al., 1996). A whole rock Sm–Nd isochron age of ca. 2400 Ma has also been obtained for gneisses (Ling et al., 1996). The Archean ages were interpreted as the formation ages of the Houhe complex, while the youngest age of ca. 2100 Ma as the timing of the migmatization. However, the ages are too variable to precisely constrain the formation ages of the Houhe complex and the meaning of the ages needs to be further explored. A few tonalitic gneiss samples were assumed to be similar to the Kongling TTG rocks (Gao and Zhang, 1990), even though they show relatively low La/Yb ratios and apparent more negative Eu anomalies compared to those of the Kongling TTG rocks. In this paper, seven fresh gneiss samples were taken near the Mayuan Village, which is located in the southeastern part of the Houhe complex (Fig. 1b). They are gray in color and contain 30–60% plagioclase feldspar, 5–20% quartz, 10–20% K-feldspar, 5–10% biotite, 1–5% hornblende, and accessory opaque minerals, apatite, sphene, zircon and epidote. Some of them contain variable concordant to nearly concordant veins that are generally folded,

indicating different migmatization. Sample 07QL14 was selected as a representative sample for zircon U–Pb dating and Hf isotope analysis because of its low degree of migmatization. Major and trace element and Sr–Nd isotope analyses were carried out for all the samples. 3. Analytical methods 3.1. Zircon U–Pb dating Zircon crystals from sample 07QL14 were separated by standard techniques (heavy liquid and magnetic techniques). Transparent zircon grains without cracks were selected using a binocular microscope, mounted in epoxy resin and then polished down to expose the centers of zircon grains. Cathodoluminescence (CL) imaging was done using a Quanta 400FEG environmental scanning electron microscope equipped with an Oxford energy dispersive spectroscopy system and a Gatan CL3+ detector at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an. The operating conditions for the CL imaging were at 15 kV and 20 nA. Typical CL images were obtained to characterize each grain in terms of size, growth morphology, and internal structure, and were used to guide analytical spot selection for U–Pb dating and Hf isotope analysis. Zircon U–Pb isotope analyses were done using LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR), China University of Geosciences. A pulsed (GeoLas) 193 nm ArF Excimer (Lambda Physik, Göttingen Germany) laser with power

Table 3 Sr and Nd isotopic compositions of the Paleoproterozoic gray gneisses from the Houhe complex. Sample

Rb

Sr

Sm

Nd

87

Rb/86 Sr

07QL14 07QL15 07QL17 07QL19 07QL20 07QL21

123 100 115 52.8 19.5 73.2

149 278 281 371 284 390

11.7 7.38 3.59 4.35 6.39 10.9

73.8 42.1 19.1 28.1 34.8 59.0

2.3971 1.0433 1.1930 0.4123 0.1989 0.5442

(87 Sr/86 Sr)i and εNd (t) are calculated by assuming t = 2100 Ma.

87

Sr/86 Sr

0.744256 0.724172 0.724637 0.717071 0.713229 0.71511

±2

147

0.000002 0.000003 0.000002 0.000003 0.000003 0.000005

0.0962 0.1059 0.1138 0.0938 0.1111 0.1118

Sm/144 Nd

143

Nd/144 Nd

0.511187 0.511222 0.511344 0.511093 0.511442 0.511338

±2

(87 Sr/86 Sr)i

εNd (t)

TDM (Ga)

0.000002 0.000002 0.000002 0.000002 0.000001 0.000004

0.671699 0.692593 0.688526 0.704591 0.707210 0.698639

−1.20 −3.14 −2.89 −2.39 −0.23 −2.46

2.53 2.71 2.74 2.60 2.53 2.70

Y. Wu et al. / Precambrian Research 200–203 (2012) 26–37

Sample/Chondrite

a

10

31

3

10 2

10 1 06QL13 06QL14 06QL15 07QL17 06QL19 06QL20 06QL21 Averag e TT G

10 0

10 -1 La

b

Pr

Nd Sm Eu

Gd

Tb

Dy

Ho

Er

Tm Yb

Lu

10 3

10

Sample/Primitive Mantle

Ce

06QL13 06QL14 06QL15 07QL17 06QL19 06QL20 06QL21 Averag e TT G

2

10 1

10 0

10 -1 Rb

Ba

Th

U

Nb

Ta

La

Ce

Pb

Pr

Sr

Nd

Zr

Hf

Eu Tb Y Er Yb Sm Gd Dy Ho Tm Lu

Fig. 4. (a) Chondrite normalized rare-earth element diagram and (b) primitive mantle-normalized spidergram for the Paleoproterozoic gray gneisses from the Houhe complex in the western part of the Yangtze Block. The gray area represents the TTG rocks from the Kongling terrane (Gao et al., 1999; Zhang et al., 2006b); the average TTG is from Kemp and Hawkesworth (2003). Chondrite- and primitive mantle-normalize values are from Sun and McDonough (1989).

of 50 mJ/pulse energy at a repetition ratio of 6 Hz, coupled to an Agilent 7500a quadrupole ICP-MS, was used for ablation. Helium was used as a carrier gas to transport the ablated material from the laser ablation cell to the ICP-MS. Signal was enhanced in the LA-ICP-MS by addition of nitrogen in the central channel gas (Hu et al., 2008). The diameter of the laser ablation craters was 32 ␮m. Zircon 91500 was used as an external standard to normalize isotopic discrimination during analysis. NIST610 glass was used as an external standard to normalize U, Th and Pb concentrations of unknowns. The detailed analytical procedures follow Liu et al. (2010). Common Pb correction was applied using the method of Andersen (2002), by assuming Pb loss occurred at t = 0 Ma, which has little effect on the age results. Uncertainties of individual analyses are reported with 1; weighted average ages are calculated at

the 2 level. The data were treated with the ISOPLOT program of Ludwig (2003). 3.2. Whole-rock major and trace element and Sr–Nd isotope analyses The samples for whole-rock analyses were crushed and powdered to 200-mesh in an agate mill. Major elements were analyzed by X-ray fluorescence (XRF) using glass disks at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an. Analysis of international rock standards (BCR-2, GSR-1 and GSR3) indicates analytical precision and accuracy both better than 5%. Whole-rock trace elements were analyzed at the GPMR, China University of Geosciences. About 50 mg of sample powders were

32

Y. Wu et al. / Precambrian Research 200–203 (2012) 26–37

Table 4 Lu–Hf isotope compositions of zircons from sample 07QL14 in the Houhe complex. No.

176

1 2 3 4 5 6 7 8 9 10 11 12 13 14

0.281352 0.281339 0.281356 0.281384 0.281386 0.281452 0.281385 0.281384 0.281367 0.281487 0.281420 0.281386 0.281333 0.281376

a

Hf/177 Hf

±(2)

176

0.000017 0.000015 0.000012 0.000016 0.000014 0.000016 0.000014 0.000014 0.000016 0.000018 0.000017 0.000018 0.000021 0.000014

0.000954 0.000875 0.000952 0.000571 0.000842 0.000639 0.000656 0.000680 0.000890 0.000543 0.000819 0.000967 0.001357 0.001385

Lu/177 Hf

176

Yb/177 Hf

0.000006 0.000008 0.000002 0.000003 0.000014 0.000007 0.000003 0.000008 0.000010 0.000011 0.000008 0.000009 0.000032 0.000003

εHf (0)

εHf (t)a

±(2)

TDM1 (Ma)

±(2)

TDM2 (Ma)

±(2)

−50.2 −50.7 −50.1 −49.1 −49.0 −46.7 −49.0 −49.1 −49.7 −45.5 −47.8 −49.0 −50.9 −49.4

−4.7 −5.0 −4.5 −3.0 −3.3 −0.7 −3.1 −3.2 −4.1 0.7 −2.1 −3.5 −5.9 −4.4

0.6 0.5 0.4 0.6 0.5 0.6 0.5 0.5 0.6 0.7 0.6 0.6 0.7 0.5

2651 2663 2645 2581 2597 2494 2586 2589 2627 2441 2549 2606 2706 2648

47 42 34 42 39 44 37 38 43 50 45 48 57 39

2978 2999 2968 2874 2894 2733 2880 2885 2941 2650 2818 2906 3054 2963

74 67 54 68 62 71 60 61 69 80 73 77 89 62

Initial Hf isotope ratios are calculated by assuming t = 2100 Ma.

digested by HF + HNO3 in Teflon bombs and analyzed with an Agilent 7500a ICP-MS. The analytical precision is better than 5% for elements with concentrations >10 ppm, and less than 10% for those <10 ppm. The detailed analytical procedures are described in Liu et al. (2008a). Whole rock Sr–Nd isotope ratios were determined on a Finnigan Trition thermal ionization mass spectrometer at the GPMR Isotope Laboratory. Sample powders were digested in Teflon bombs with mixed agents of double distilled HNO3 and HF acids at 190 ◦ C for 48 h. The measured 143 Nd/144 Nd and 87 Sr/86 Sr ratios were normalized to 146 Nd/144 Nd = 0.7219 and 86 Sr/88 Sr = 0.1194, respectively. During the period of analysis, the NBS987 standard yielded an average 87 Sr/86 Sr value of 0.710215 ± 10 (2) and the La Jolla standard gave an average 143 Nd/144 Nd value of 0.511837 ± 1 (2). Total procedural Sr and Nd blanks were <4 ng and <1 ng, respectively. 3.3. Zircon Lu–Hf isotope analysis In situ zircon Lu–Hf isotope measurements were done using the Nu Plasma HR Multi-Collector (MC)-ICP-MS, equipped with a 193-nm ArF Laser, with a spot size of 44 ␮m, a 10 Hz repetition rate, and a laser power of 100 mJ/pulse at the State Key Laboratory of Continental Dynamics, Northwest University, in Xi’an. The analytical protocol was similar to that outlined in Yuan et al. (2008). The interference of 176 Lu on 176 Hf was corrected by measuring the intensity of the interference-free 175 Lu, using the recommended 176 Lu/175 Lu ratio of 0.02669 (Debievre and Taylor, 1993) to calculate 176 Lu/177 Hf. Similarly, the isobaric interference of 176 Yb on 176 Hf was corrected by using a recommended 176 Yb/172 Yb ratio of 0.5886 (Chu et al., 2002). Zircon 91500 was used as the reference standard. A decay constant value of 1.865 × 10−11 a−1 for 176 Lu (Scherer et al., 2001), and the present day chondritic ratios of 176 Hf/177 Hf = 0.282772 and 176 Lu/177 Hf = 0.0332 (Blichert-Toft and Albarede, 1997) were adopted to calculate εHf values. Single-stage Hf model ages (TDM1 ) are calculated relative to the depleted mantle with a present day 176 Hf/177 Hf ratio of 0.28325 and 176 Lu/177 Hf of 0.0384 (Vervoort and Blichert-Toft, 1999), and two-stage Hf model ages (TDM2 ) are calculated by assuming a mean 176 Lu/177 Hf value of 0.015 for the average continental crust (Griffin et al., 2002).

zircon crystals generally have oscillatory zoning (Fig. 2a), implying their magmatic genesis (Wu and Zheng, 2004). Discontinuous bright rims can also be observed around some grains (Fig. 2a), which may have resulted from subsequent metamorphism. Eighteen LA-ICP-MS U–Pb spot analyses were obtained on 18 zircon grains, which are listed in Table 1 and shown in Fig. 2b. All of the analyses have moderate Th (60–387 ppm) and U (91–487 ppm) contents, with relatively high Th/U ratios of 0.48–0.79, consistent with their magmatic origin. They have coherent 207 Pb/206 Pb ages of 2042 ± 21 to 2115 ± 19 Ma, yielding a weighted average of 2081 ± 9 Ma (MSWD = 0.94) (Fig. 2b). 4.2. Major and trace element compositions A total of 7 samples from the Houhe complex were selected for whole-rock major and trace element composition analyses and the results are listed in Table 2. They range in SiO2 contents from 58.65% to 69.33%, and Na2 O from 3.88% to 5.61%, which are relatively lower than those of the Kongling TTG rocks (Gao et al., 1999; Zhang et al., 2006b). On the other hand, they are characterized by relatively high Fe2 O3 contents of 2.86–6.69%, Al2 O3 of 16.01–18.88%, and MgO of 0.97–2.65% (Table 2). Their K2 O contents vary from 0.68% to 4.19%, and K2 O + Na2 O from 5.96% to 8.46%. In the TAS diagram (Fig. 3), most of them plot into the field of the sub-alkaline series and belong to granodiorite. All the samples show variable LREE enrichment ((La/Yb)CN = 15.6–46.8) (CN represents Chondrite-normalized),

4. Results 4.1. Zircon U–Pb age Zircon crystals recovered from the gneiss sample 07QL14 are euhedral to subhedral, transparent and light yellow. The lengths of these grains range from 120 to 300 ␮m with aspect ratios of 1.5:1–2.5:1. Cathodoluminescence imaging reveals that these

Fig. 5. Histograms of εHf (t) values for zircons in gray gneiss 07QL14.

Y. Wu et al. / Precambrian Research 200–203 (2012) 26–37

33

10

a

Eu/Eu*

Global TT G 1

06QL13 06QL14 06QL15 07QL17 06QL19 06QL20 06QL21

0.1 0.1

Kongling TT G

1

(Gd/Yb) N

10

100

b 10

Global TT G Rb/Sr

1

0.1

Kongling TT G

0.01

0.001 0.1

1

10

Eu/Eu* Fig. 6. The Eu/Eu* vs. (Gd/Yb)CN (a) and Rb/Sr vs. Eu/Eu* (b) diagrams for gray gneisses from the Houhe complex. The Kongling TTG field is from Gao et al. (1999) and Zhang et al. (2006b), while the Global TTG field is from Kemp and Hawkesworth (2003).

and insignificant HREE depletion ((Gd/Yb)CN = 1.90–4.17) profiles (Fig. 4a). They have Eu anomalies varying from 0.53 to 1.13, implying different degrees of fractionation of plagioclase. On the primitive mantle-normalized spidergram (Fig. 4b), they show enrichments of almost all the trace elements relative to the TTG rocks in the Kongling terrane (Gao et al., 1999; Zhang et al., 2006b) and the average TTG in the world (Kemp and Hawkesworth, 2003). All the samples are characterized by a relative enrichment in LILEs (Rb, Ba, and Th) and LREEs (La, Ce, and Pr), but a depletion in Nb, Ta, Sr, and Eu, which are similar to those of arc granitoids (Zhou et al., 2002). 4.3. Sr, Nd and Hf isotope compositions Whole rock Sr–Nd isotopic data for the gneiss samples from the Houhe complex are presented in Table 3. Initial Sr isotope ratios

and εNd (t) values are calculated at t = 2100 Ma. The initial Sr isotope ratios of the gneiss samples show a wide range from 0.671699 to 0.707210 (Table 3). The large variation of the initial Sr isotope ratios might result from disturbances of the Rb–Sr isotope system by subsequent metamorphism. The REEs, including Sm and Nd, are considered to be much less mobile than Rb and Sr during metamorphism and hydrothermal processes. The Sm–Nd isotopic system is usually robust, and is thus considered to closely reflect the primary magmatic signatures of the rocks (Huang et al., 2006). The gneiss samples have relatively consistent 143 Nd/144 Nd ratios ranging from 0.511093 to 0.511442, corresponding to ␧Nd (t) values varying from −3.14 to −0.23 with an average of −2.23. They yield single-stage depleted mantle Nd model ages (TDM ) of 2.53 to 2.74 Ga (Table 3). Fourteen Lu–Hf isotope spot analyses were done on 14 dated zircon grains from the gneiss sample 07QL14. They have relatively low 176 Hf/177 Hf ratios of 0.281333–0.281487 (Table 4). Assuming

34

Y. Wu et al. / Precambrian Research 200–203 (2012) 26–37

500

Kongling TT G

06QL13 06QL14 06QL15 07QL17 06QL19 06QL20 06QL21

(Sr / Y)CN

400

Adakite & TTG

300

200 Averag e TT G

Typica l AR C Rocks

100

0 0

10

20

30

40

50

Y Fig. 7. (Sr/Y)CN vs. Y diagram discriminating adakite and TTG from classical arc calc-alkaline rocks (Drummond and Defant, 1990).

t = 2100 Ma, the calculated εHf (t) values range from −5.9 to +0.7. All the analyses yield a weighted average of −3.51 ± 0.88 (MSWD = 7.7) (Fig. 5). Their two-stage depleted mantle Hf model ages range from 2650 ± 80 to 3054 ± 89 Ma (Table 4).

5. Discussion 5.1. The oldest rocks in the western part of the Yangtze Block Up to now, the oldest basement rocks of the Yangtze Block are the ca. 3.2 Ga TTG gneisses from the Kongling terrane (Jiao et al., 2009; Gao et al., 2011). In the western part of the Yangtze Block, detrital zircons were reported to have ages of ∼1.85, 2.04–2.35, ∼2.5, and ∼2.7–2.9 Ga, with the oldest up to ca. 3.7 Ga (Greentree and Li, 2008; Zhao et al., 2010). The oldest reported rocks are, however, the 1675 ± 8 Ma volcanic rocks from the Dahongshan Group (Greentree and Li, 2008; Zhao et al., 2010). In this study, the zircons from the gray gneiss 07QL14 show oscillatory zoning, and high Th/U ratios, which are typical for magmatic zircon. The zircons yield a weighted average 207 Pb/206 Pb age of 2081 ± 9 Ma (MSWD = 0.94), which is interpreted as the formation age of the gray gneiss in the Houhe complex. To our knowledge, this is the oldest rock that has been reported in the western part of the Yangtze Block. Some detrital zircons with such ages have also been reported in the Dahongshan Group in the southwestern part of the Yangtze Block. However, these grains were assumed to be an exotic source for the Yangtze Block, because of the lack of rock outcrops and age spectra of such detrital zircons in the eastern part of the Yangtze Block (Greentree and Li, 2008; Zhao et al., 2010). According to the results of this study, we suggest that the Paleoproterozoic zircons might be derived from the Houhe complex in the northwestern part of the Yangtze Block, and the absence of early Paleoproterozoic zircon ages in the eastern part of the Yangtze Block hints that the western and the eastern parts of the Yangtze Block might have different evolution during this period.

5.2. Petrogenesis of the gray gneisses The gray gneisses of the Houhe complex were speculated to have petrological features similar to the TTG gneisses in the Kongling terrane and were thus taken as Archean TTG gneisses in the Yangtze Block (Gao and Zhang, 1990). However, petrological features alone cannot give direct indication for the discrimination of different kinds of rock. A similar kind of situation has also been reported for other Archean gray gneiss complexes (Moyen, 2011). Therefore, a critical examination of the geochemical characteristics should be done. The gray gneisses of the Houhe complex have relatively lower SiO2 and Na2 O, and higher K2 O contents than those of the Kongling TTG rocks (Gao et al., 1999; Zhang et al., 2006b). They also show low Sr (149–390 ppm), Cr (9.07–45.1 ppm) and Ni (4.97–21.3 ppm) contents, but high Y (12.9–32.7 ppm) and Yb (0.95–2.25 ppm), and variable Eu anomalies (0.53–1.13) (Table 2), relative to the Kongling and global TTG gneisses. These features are analogous to typical calc-alkaline granitoids (Martin, 1999; Martin et al., 2005; Zhou et al., 2002). In the Eu/Eu* vs. (Gd/Yb) CN and Rb/Sr vs. Eu/Eu* diagrams (Fig. 6), the gray gneisses have higher Rb/Sr and lower Eu/Eu* ratios not only than those of the Kongling TTG rocks, but also than those of the global TTG. Moreover, in the discriminating diagram of (Sr/Y)CN vs. Y (Drummond and Defant, 1990), all the samples plot into the field of typical arc rocks, due to their low Sr, but high Y contents (Fig. 7). All these indicate that the gray gneisses are typical calcalkaline granitoids rather than TTGs, and their formation might be related to a subduction-related process or remelting of preexisted arc rocks. The gneisses have εNd (t) values of −3.14 to −0.23 and εHf (t) values of −5.9 to +0.7, implying that they were derived from preexisting crustal rocks. The formation of the gneisses with calc-alkaline feature in the Houhe complex might also be responsible for the secular geochemical changes of the Yangtze Block from the Archean to the Paleoproterozoic with an increase in the Chemical Index of Alteration and Eu depletion, and decreasing Nd model ages in sedimentary rocks (Gao et al., 1999; Zhang et al., 2006b).

Y. Wu et al. / Precambrian Research 200–203 (2012) 26–37

a

35

Relat ive probabilit y

West ern Yangtze Block n = 269

b Relativ e pr oba bil it y

Eastern Yangtze Block n = 659

1200

1600

2000

2400

2800

3200

3600

4000

Age Fig. 8. Age spectra of Archean to Paleoproterozoic zircon grains for the western and eastern parts of the Yangtze Block. The data of the western part are from Greentree and Li (2008), Zhao et al. (2010), and this study; The data of the eastern part are from Qiu et al. (2000); Zhang et al. (2006a,b,c), Zheng et al. (2006), Gao et al. (2011), Sun et al. (2008), and Wu et al. (2008, 2009b).

5.3. Implications for the early evolution of the Yangtze Block The Yangtze Block is one of the largest Precambrian blocks in China. The oldest rocks are about ca. 3.2 Ga (Jiao et al., 2009; Gao et al., 2011) and detrital zircons with ages of 3.3–3.8 Ga have also been found (Zhang et al., 2006c; Greentree and Li, 2008; Liu et al., 2008b; Sun et al., 2008; Wu et al., 2008; Zhao et al., 2010), indicating the widespread presence of Paleoarchean basement. Compiled age spectra of Archean to Paleoproterozoic zircon grains reveal that the Archean and early Paleoproterozoic zircon U–Pb ages are similar for the western and eastern parts of the Yangtze Block, suggesting the occurrence of old continental nuclei in both parts (Fig. 8). There are, however, numerous zircon grains with ages of ca. 2050–2400 Ma in the western part of the Yangtze Block (Fig. 8a), whereas they are absent in the eastern part (Fig. 8b). This indicates that the eastern and western parts of the Yangtze Block had a different history

during this period and the western part of the Yangtze Block might be an independent microcontinent as represented by the Houhe complex. It was suggested that the Yangtze Block experienced a Paleoproterozoic high-grade metamorphic event at ca. 2.0 Ga during the assembly of the supercontinent Columbia (Wu et al., 2002, 2008, 2009b; Zhang et al., 2006a,b; Zheng et al., 2006; Sun et al., 2008). This process was considered to result from an arc-continent collisional orogenesis, and presumably to be responsible for the cratonization of the Yangtze Block (Zhang et al., 2006a,b). The protoliths of the Paleoproterozoic metamorphic rocks include khondalitic rocks in the Kongling terrane (Zhang et al., 2006b; Wu et al., 2009b) and metasedimentary rocks in the Northern Dabie complex (Sun et al., 2008; Wu et al., 2008), along the western side of the eastern part of the Yangtze Block (Fig. 1a). This implies that the western side of the eastern part of the Yangtze Block might have

36

Y. Wu et al. / Precambrian Research 200–203 (2012) 26–37

a passive-type continental margin along which stable continental margin sediments were deposited, forming the protoliths of the khondalitic and the metasedimentary rocks (Ling et al., 2003). On the other hand, the gneisses from the Houhe complex in the western part of the Yangtze Block have a formation age of 2081 ± 9 Ma, suggesting that it formed shortly before ca. 2.0 Ga high-grade metamorphism. There are also zircon age spectra of 2050–2400 Ma for the western part of the Yangtze Block (Fig. 8a), which imply a widespread occurrence of Paleoproterozoic magmatic rocks in the western part of the Yangtze Block that have been eroded by subsequent metamorphic and magmatic events. Accordingly, it is inferred that there might be a microcontinent with an active-type continental margin during the Paleoproterozoic times in the western part of the Yangtze Block. At ca. 2.0 Ga, the collision between the western microcontinent and the eastern part of the Yangtze Block may have resulted in an arc-continent collisional orogenesis, which was coincident with global collisional events that led to the assembly of the Paleo-Mesoproterozoic Columbia (Nuna) supercontinent (Zhao, 2001; Zhao et al., 2001, 2002b, 2003, 2004). 6. Conclusions An integrated study of zircon U–Pb age, major and trace element and Sr–Nd–Hf isotope compositions for gray gneisses from the Houhe complex reveals that the gneisses formed at 2081 ± 9 Ma, which represent the oldest rocks exposed in the western part of the Yangtze Block. The gneisses are characterized by low Sr, Cr and Ni contents, but high Y and Yb, and variable Eu anomalies, analogous to typical arc rocks. These suggest that their formation might have resulted from a subduction process or remelting of preexisting arc rocks. Both the western and eastern parts of the Yangtze Block have similar Archean to early Paleoproterozoic zircon U–Pb age spectra, implying the presence of old continental nuclei in both parts. Whereas great amounts of ca. 2050–2400 Ma zircon grains occur in the western part of the Yangtze Block, implying that there might be a microcontinent with an active-type continental margin during the Paleoproterozoic times in the western part of the Yangtze Block. The occurrence of ca. 2.0 Ga khondalitic and metasedimentary rocks implies that a passive-type continental margin probably developed in the eastern part of the Yangtze Block. At ca. 2.0 Ga, the collision between the western microcontinent and the eastern part may have resulted in the final arc-continental collision. Acknowledgements We thank Z.C. Hu and Y.S Liu for their assistance with LA-ICPMS zircon U–Pb dating and whole-rock trace elements analyses, M.N. Dai for her assistance with LA-MC–ICP-MS zircon Hf isotope analyses and J.Q. Wang for his assistance with whole-rock major elements analyses. The authors would also like to thank two anonymous reviewers for their critical reviews and the editor Prof. Guochun Zhao for his constructive suggestions and detailed revision, which substantially improve the manuscript. This study was supported by funds from the National Natural Science Foundation of China (41173017, 40772042, 90714010, 40873043 and 40521001), Chinese ‘973 Project (2009CB825006), the Ministry of Education of China (IRT0441, B07039 and NCET-060659), SinoProbe Project 05-04 and China Geological Survey Project 1212011120162, and the Special Fund for Basic Scientific Research of Central Colleges, China University of Geosciences (Wuhan). References Andersen, T., 2002. Correction of common lead in U–Pb analyses that do not report 204 Pb. Chemical Geology 192, 59–79.

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