Episodic crustal growth of North China as revealed by U–Pb age and Hf isotopes of detrital zircons from modern rivers

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Geochimica et Cosmochimica Acta 73 (2009) 2660–2673 www.elsevier.com/locate/gca

Episodic crustal growth of North China as revealed by U–Pb age and Hf isotopes of detrital zircons from modern rivers Jie Yang a,b,c, Shan Gao b,c,*, Chen Chen b, Yongyong Tang b, Honglin Yuan c, Hujun Gong c, Siwen Xie a, Jianqi Wang c b

a Graduate School, China University of Geosciences, Wuhan 430074, China State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China c State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China

Received 29 September 2008; accepted in revised form 2 February 2009; available online 20 February 2009

Abstract Clastic sedimentary rocks are samples of the exposed continental crust. In order to characterize the crustal growth history of North China and its possible regional variations, 479 concordant detrital zircons in three sand samples from the lower reach of the Yellow River (which drains the Tibet–Qinghai Plateau, the Western Qinling Orogen, the Qilian Orogen and the North China Craton) and two sand samples from the Luan River and the Yongding River (which run entirely within the North China Craton) were measured for U–Pb age and Lu–Hf isotopic compositions by excimer laser-ablation ICPMS and MC-ICP-MS. Although regional variations exist, concordant detrital zircons from the Yellow River reveal three major age groups of 2.1–2.5 Ga, 1.6–2.0 Ga, and 150–500 Ma. Detrital zircons from the smaller Luan and Yongding Rivers show three broadly similar major age groups at 2.3–2.6 Ga, 1.6–2.0 Ga, and 140–350 Ma, but with narrower age ranges. Compared to the Luan and Yongding River zircons, the Yellow River zircons are characterized by a significant number of Neoproterozoic grains. Although Hf isotopic compositions show both juvenile crustal growth and crustal reworking for all age groups, much of the crustal growth of North China occurred in the Neoarchean and Mesoproterozoic. All three rivers are characterized by a common prominent group of Hf crust formation model ages at 2.4–2.9 Ga with a peak at 2.7–2.8 Ga. A less significant age group lies between 1.4 and 1.8 Ga for the Yellow River, and between 1.6 and 1.9 Ga for the Yongding River and Luan River. Crustal growth rates based on Hf continental crust formation model ages suggest 45% and 90% of the present crustal volume was formed by 2.5 Ga and 1.0 Ga, respectively, for the drainage area of the Yellow River. In comparison, 60% and 98% of the present crustal volume of the North China Craton was generated by 2.5 Ga and 1.0 Ga, respectively, for the Luan and Yongding Rivers. The 2.7–2.8 Ga age peak observed in all river samples agrees well with the coeval major peak for global crustal growth. However, the other suggested global peaks of crustal growth at 3.4 and 3.8 Ga are insignificant in North China. Taken together with our previous studies of the Yangtze Craton, which show insignificant crustal growth at 2.7–2.8 Ga, we suggest that these advocated worldwide crust formation peaks be re-examined and treated carefully. Our results also show that Phanerozoic zircons may have been derived from crustal sources separated from the mantle up to 2.0 Ga ago before the zircons crystallized, suggesting long-term preservation, reworking and recycling of the continental crust. Ó 2009 Published by Elsevier Ltd.

1. INTRODUCTION

*

Corresponding author. Fax: +86 27 67885096. E-mail address: [email protected] (S. Gao).

0016-7037/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.gca.2009.02.007

Clastic sediments and sedimentary rocks, particularly their fine-grained members (e.g., tillite, loess, shale, mudstone, and siltstone), are representative samples of the continental crust derived from large areas, and are ideal for

Episodic crustal growth of North China

studies of formation, evolution, and chemical composition of the continental crust (e.g., Goldschmidt, 1933; Taylor et al., 1983; Taylor and McLennan, 1985, 1995; Jahn et al., 2001; McLennan, 2001; Rudnick and Gao, 2003; Hu and Gao, 2008; Liu et al., 2008). Zircon, one of the robust accessory phases in sedimentary rocks, is providing the most valuable information. Different growth zones within zircon crystals can be dated using U–Pb isotopes, and their trace elements and Lu–Hf and O isotopic compositions provide an exceptional record of magmatic, and thus continental crustal evolution (Maas et al., 1992; Vervoort and Patchett, 1996; Bruguier et al., 1997; Amelin et al., 1999; DeCelles et al., 2000; Mojzsis et al., 2001; Nutman, 2001; Wilde et al., 2001; Kosˇler et al., 2002; Griffin et al., 2004; Condie et al., 2005, 2009; Iizuka et al., 2005; Veevers et al., 2005; Coogan and Hinton, 2006; Hawkesworth and Kemp, 2006a,b; Kemp et al., 2006; Weislogel et al., 2006; Zhang et al., 2006a,b; Campbell and Allen, 2008; Pietranik et al., 2008). The zircon thermometer may add additional important information on the formation and evolution of the continental crust (Watson and Harrison, 2005; Watson et al., 2006; Farry and Watson, 2007). Detrital zircons from younger sedimentary, or in modern river sediments, may record crustal material that has not been preserved or is no longer exposed. For these reasons, detrital zircons are a powerful tool to study crustal growth (e.g., Griffin et al., 2004; Condie et al., 2005; Iizuka et al., 2005; Cawood et al., 2007; Liu et al., 2008; Pietranik et al., 2008). It has been estimated that >50% volume of the present continental crust formed by the end of the Archean and >90% by the end of Precambrian (Taylor and McLennan, 1995; Hawkesworth and Kemp, 2006a). Worldwide compilations of zircon ages from juvenile crust indicate striking age peaks around 2.7 Ga, 1.9 Ga, and 1.2 Ga, which suggest episodic rapid continental growth during thermal pulses associated with emplacement of mantle plumes (Condie, 1998, 2000). Condie et al. (2005) studied U–Pb ages and Lu–Hf isotopic compositions of detrital zircons from the lower Cambrian Atan Group, northern British Columbia, and the Paleoproterozoic Lake Harbour Group in southern Baffin Island, northern Canada, as well as modern rivers from Australia, NW India, the Ukrainian shield and northern Brazil. The Hf isotopes in detrital zircons from the modern river deposits show evidence for production of juvenile crust at about 2.5 Ga, but this is not apparent for the Atan Group and Lake Harbour Group. If their measured detrital zircon populations in the 2.2–2.4 Ga time window are representative of production of juvenile crust, they yield little evidence for generation of significant volumes of juvenile crust for this age. Hf isotopic compositions of detrital zircons from the Ukraine and eastern Australia record production of juvenile continental crust between 1.65 and 1.40 Ga. Zircons from granitoids in south-central Laurentia and in western Brazil have eHf(t) values that fall near the depleted mantle growth curve, recording production of juvenile continental crust in these regions between 1.5 and 1.3 Ga. U–Pb ages and Lu–Hf isotopic compositions of detrital zircons from the Mississippi River sand show juvenile crus-

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tal formation dominated between 2.0 and 1.6 Ga (Iizuka et al., 2005). U–Pb age and Lu–Hf isotopic studies of detrital zircons from Neoproterozoic clastic sedimentary rocks and zircons from Archean basements in the Yangtze Craton, South China suggest major periods of crust formation at 3.2– 3.8 Ga and 720 and 910 Ma (Liu et al., 2008). Condie et al. (2009) compiled U–Pb ages of zircons in granitoids and of detrital zircons worldwide. Zircons of granitoids reveal seven igneous age peaks (3300, 2700, 2680, 2500, 2100, 1900, and 1100 Ma), while detrital zircons also show seven peaks at 2785, 2700, 2600, 2500, 1900, 1650, and 1200 Ma. However, only the 2700, 2500, and 1900 Ma peaks are common to the two groups. These differences in age spectra may be caused by removal of crustal sources by erosion, inadequate sampling of granitoids because of cover by younger rocks, or small age peaks hidden by large age peaks in detrital spectra. Nd isotope distributions of granitoids suggest important additions of juvenile continental crust at 2700, 2550, 2120, 1900, 1700, 1650, 800, 570, and 450 Ma (Condie et al., 2009), of which the 2700 Ma is most striking. U–Pb ages of 5246 concordant detrital zircons from sands collected near the mouths of 40 major rivers from Asia, Europe, and North and South America supplemented by 1136 Australian dune zircons and 583 zircons from Antarctic Paleozoic sediments show distinct peaks at 2.75–2.6 Ga, 1.95–1.6 Ga, 1250–950 Ma, 650–400 Ma, and 350–225 Ma, which correlate with the assembly of the Superia/Sclavia, Nuna, Rodinia, Gondwana, and Pangaea supercontinents, respectively (Campbell and Allen, 2008). However, these zircon age peaks do not necessarily correspond to crustal growth, as zircon frequently crystallizes during crustal reworking and from crustal sources. Identifying net mantle additions, and how they have changed with time, is the key to the study of crustal growth. Although Hf model ages can be used to identify periods of crust generation, significantly these need to be screened for hybrid ages derived from sedimentary source rocks. This can now be done using oxygen isotopes. Combination of Hf and O isotopes provides a robust way to reveal net mantle additions. In this way, detrital zircons from eastern Australia suggest major pulses of juvenile crust production in part of Gondwana at 3300 and 1900 Ma (Kemp et al., 2006), and worldwide Precambrian zircons show four episodes of generation of new continental crust at ca. 4.4– 4.5 Ga, ca. 3.8 Ga, ca. 3.4 Ga and ca. 2.7–2.8 Ga (Pietranik et al., 2008). The above observed peaks in zircon age and crustal growth often coincide with the periods of supercontinent formation. This coincidence might be attributed to crustal growth associated with supercontinent formation. However, it should be noted that the above models are largely based on the records of igneous and sedimentary rocks selectively preserved in ‘‘stable” areas and are therefore biased by the formation of supercontinents and underestimate the geological records from tectonic (e.g., extensional) stages with poor preservation potential (Hawkesworth et al., 2009).

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On the other hand, there are cases of classic orogenic belts (Caledonides, Hercynides, Canadian Cordillera, Lachlan and New England Foldbelts, and Central Asia orogenic belts) where Phanerozoic crustal growth is considered to be significant based on positive eNd(t) of granitoids (see Jahn et al., 2000; Wu et al., 2000; and references therein). However, in situ determinations of U–Pb age, Hf and oxygen isotopes of detrital zircons from the Lachland Fold Belt of southeastern Australian reveal two striking features (Kemp et al., 2006; Hawkesworth and Kemp, 2006a,b). Firstly, no Phanerozoic zircons have a Hf isotopic composition that approaches that of the depleted mantle at time of crystallization. This means Phanerozoic zircons were derived by melting pre-existing, rather than juvenile, crustal rocks. Secondly, zircons with mantle (5.3 ± 0.3&)-like d18O (<6.5&) define two prominent linear arrays that intersect the depleted mantle growth curve near 1.9 and 3.3 Ga ago. The data indicates that crustal generation in part of Gondwana was limited to major pulses at 1.9 and 3.3 Ga, and the Phanerozoic zircons crystallized during repeated reworking of crust formed at these times. These results highlight the importance of in situ zircon isotopic studies compared to whole-rock analysis, which may be compromised by mixing processes. The potential bias in preservation and significant regional variations of crustal growth records require that construction of the global crustal growth history has to be based upon samples that collect zircon grains from areas as large as possible and from tectonic settings as diverse as possible. Here we report U–Pb age and Hf isotopic compositions of detrital zircons from three sand samples of the Yellow River, which drains much of North China, including the

Tibet–Qinghai Plateau and the North China Craton, and two sand samples from the Luan River and Yongding River which drain the North China Craton. The isotopic data is used to constrain the history of crustal growth and reworking in North China. 2. GEOLOGICAL SETTING AND SAMPLES The Yellow River is the second-longest river in China and the sixth-longest in the world at 5464 km. The Yellow River drainage basin extend 1900 km east–west and 1100 km north–south, with a total basin area of 752,443 km2. Originating in the Bayan Har Mountains, the Yellow River extends westward to the Kunlun Mountains in Qinghai Province (Fig. 1). It flows through the western Qinling Orogen, the Qilian Orogen, and the North China Craton and empties into the Bohai Sea. Three sand samples were taken in the lower reach: (1) HHHC02 at Hanchen, Shanxi Province (GPS: 35°24.5000 N, 110°27.3250 E), (2) HHC02 at Zhengzhou, Henan Province (GPS: 34°54.3720 N, 113°39.8260 E) and (3) HH01SD at the exit into the Bohai Sea in Shandong Province (GPS: 37°36.5790 N, 118°23.5070 E), respectively. The Luan River originates in Fengning county of Inner Mongolia Province and flows southward into the Bohai Sea. This river has a total length of 833 km and the total basin area is 44,900 km2. It drains entirely within the North China craton. One sand sample LH01 was taken at Luanxian county, 60 km east of the exit into Bohai Sea (GPS: 39°430 47.800 N, 118°4533.200 E (Fig. 1). The Yongding River originates in Niwu county, Shanxi Province and flows southward into the Hai River in Tianjin. This river has a total length of 747 km with the total

Fig. 1. Simplified map of major tectonic units in China (A) and the drainage area of the Yellow River, Luan River and Yongding River under this investigation and sample location (B). The two dot–dash lines divide the North China Craton into the Eastern Block (EB), the TransNorth China Orogen (TNCO) and the Western Block (WB) (Zhao et al., 2005). YZ and SC denote the Yangtze Craton and the South China Orogen, respectively.

Episodic crustal growth of North China

basin area of 47,016 km2 (Fig. 1). It also runs entirely within the North China Craton. One sand sample YDH01 was collected at the Yongding bridge in Beijing (GPS: 39°450 50.800 , E116°140 02.300 E) (Fig. 1). The Bayan Har Fold Belt of Indosinian (late Triassic) age is composed of flysch sandstone, dolomitic limestone, and minor volcanic rocks (Wu et al., 2005c). The western Qinling Mountains is the western part of the Qinling orogenic belt and situated at the northeastern part of the Tibet–Qinghai Plateau. It is a conjunction region of the North China Craton, the Yangtze Craton and the Tibet–Qinghai Plateau. Its tectonic framework resulted from a three-direction compression of the ancient Asian Ocean from the north, the Marginal-Pacific Ocean from the southeast and the Paleo-Tethyan Ocean from the southwest. Late Paleozoic flysches, especially Devonian strata, and Triassic–Jurassic granites outcrop widespread in this region. Paleo-Tethyan ophiolites were discovered in the northern margin of the western Qinling (Su et al., 2008). The Qilian Fold Belt in the drainage area is Caledonian (Cambrian to Silurian) in age, which has been reactivated by the collision between India and Asia (Tapponnier et al., 1986). Proterozoic to Lower Paleozoic metamorphic rocks and granite, locally Triassic sandy slate with limestone, and Tertiary sandstone and mudstone are exposed (Wu et al., 2005c; Xiao et al., 2009). Zircon U–Pb dating of Proterozoic basements of the Qilian Fold Belt yielded ages of 785–891 Ma (Xu et al., 2007), which were related to the Yangtze Craton in the overall context of formation of the Rodinia Supercontinent. Cambrian–Ordovician (430–525 Ma) ophiolites and arc magmatic rocks characterize the Kunlun, Qilian and Qingling belts (Zhang et al., 1997, 2008; Xiao et al., 2009). The North China Craton (NCC) is one of the world’s oldest Archean cratons, preserving crustal remnants as old as 3800 Ma (Liu et al., 1992). The craton is divided into the Eastern Block, the Western Block and the intervening Trans-North China Orogen/Central Orogenic Belt based on age, lithological assemblage, tectonic evolution and P–T–t paths (Zhao et al., 2005) (Fig. 1). The three blocks consist of Archean and Paleoproterozoic basements and Mesoproterozoic to Cenozoic sedimentary rocks with lack of late Ordovician, Silurian and Devonian sedimentation. The Eastern Block shows two main Nd model age peaks, one between 3.6 and 3.2 Ga and the other between 3.0 and 2.6 Ga. Limited Nd isotopic data from the Western Block show a large range of model ages between 3.2 and 2.4 Ga (Wu et al., 2005b). Reactivation of the craton began in the Early Mesozoic, with uplift and the onset of magmatism, followed by basin development. The magmatism is peaked volumetrically in the late Cretaceous (120–132 Ma) (Wu et al., 2005a) and dominated by granitoid, andesite, dacite and rhyolite with minor basalts (Gao et al., 2004, 2008). Compilation of basement zircon U–Pb dates shows a striking peak at 2.5 Ga, which represents the most prominent time of magmatism. A less significant peak occurs at 1.85 Ga (Gao et al., 2004). The Mesozoic compositionally diverse magmatism was followed by Cenozoic intraplate basaltic volcanism.

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3. ANALYTICAL METHODS Zircons from >5 kg samples were separated by heavy-liquid and magnetic methods and then purified by hand picking under a binocular microscope. >500 zircon grains were picked out from all the samples. 3.1. U–Pb dating Zircons were dated in situ on an excimer (193 nm wave length) laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an. The ICP-MS used is an Agilent 7500a (with shield torch) from Agilent (Japan). The unique shield torch increase analytical sensitivity by a factor of >10 (e.g., 4500 cps/ppm 238U at a spot size of 40 lm and laser frequency of 10 Hz), which is important for LA-ICP-MS. The GeoLas 2005 laser-ablation system (MicroLasTM Beam Delivery Systems, Lambda Physik AG, Germany) was used for the laser ablation experiments. It consists of a COMPexPro 102 ArF excimer laser (with a wavelength of 193 nm, maximum energy of 200 mJ and maximum pulse rate of 20 Hz) and a GeoLas 2005 PLUS package (including a laser beam homogenizing system, a motorized sampling stage and a viewing system coupled with an Olympus microscope and color CCD). Helium was used as carrier gas to provide efficient aerosol transport to the ICP and minimize aerosol deposition around the ablation site and within the transport tube (Eggins et al., 1998; Jackson et al., 2004). The used spot size was 30 lm for HHHC02 and HH02 and 44 lm for the other three samples where age, Lu–Hf isotopic and trace element data were collected simultaneously (see below). The used laser frequency was 10 Hz. The data acquisition mode was peak jumping (20 ms for 1 point per peak). Raw count rates were measured for 29Si, 204Pb, 206Pb, 207Pb, 208Pb, 232Th and 238 U. U, Th and Pb concentrations were calibrated by using 29 Si as an internal standard and NIST SRM 610 as the reference standard. Each analysis consists of 30 s gas blank and 40 s signal acquisition. High-purity argon was used together with a home-made helium filtrating column, which resulted in 204Pb and 202Hg being less than 100 cps in the gas blank. Therefore, the contribution of 204Hg to 204Pb was negligible and no correction was made. 207Pb/206Pb, 206 Pb/238U, 207Pb/235U and 208Pb/232Th ratios, calculated using GLITTER 4.0 (Macquarie University), were corrected for both instrumental mass bias and depth-dependent elemental and isotopic fractionation using Harvard zircon 91500 as external standard. The ages were calculated using ISOPLOT 3 (Ludwig, 2003). Our measurements of GJ-01 as an unknown yielded weighted 206Pb/238U ages of 602.2 ± 2.4 Ma (2r, MSWD = 1.15, n = 19), which is in good agreement with the apparent ID-TIMS 206Pb/238U ages of 598.5–602.7 Ma (Jackson et al., 2004). Analytical details for age and trace element determinations of zircons were reported in Yuan et al. (2004). Common Pb corrections were made following the method of Andersen (2002). Because of measured 204Pb usually account for <0.3% of the total Pb, the correction is insignificant in most cases.

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3.2. Lu–Hf isotopes Lu–Hf isotope analysis was done on a Nu Plasma HR MC-ICP-MS (Nu Instruments Ltd., UK), coupled to a GeoLas 2005 excimer ArF laser-ablation system hosted at the State Key Laboratory of Continental Dynamics, Northwest University. The Nu Plasma HR MC-ICP-MS is a second-generation double-focusing MC-ICP-MS with three ion counters and 12 faraday cups. A unique feature of this instrument is a specially designed zoom lens composed of a pair of quadruple lenses. An Edwards E2M80 source rotary pump was applied in the interface region to improve sensitivity. The energy density applied is 15–20 J/cm2 and a spot size of 44 lm is used. Helium was also used as carrier gas. We used high-purity argon (99.9995%) and high-purity helium (99.9995%), purified by an in-house filtration column, which is composed of 15L 13X molecular sieve and can reduce the gas backgrounds of 208Pb and 202Hg <100 and 400 counts per second (cps) (Yuan et al., 2008). This column has similar filtering performance as charcoal filter (Hirata et al., 2005) and gold traps (Storey et al., 2006). These backgrounds were measured by ion counters (MC-ICP-MS) and correspond to 0.05 and 0.1 ppt, respectively. The sensitivity in laser ablation mode is 7–8 V per 1% of hafnium at 44 lm. Interference correction for Yb and Lu is of paramount importance for precise in situ measurements of Hf isotopes in zircon (Woodhead et al., 2004). Interference of 176Lu on 176 Hf was corrected by measuring the intensity of the interference-free 175Lu isotope and using the recommended 176 Lu/175Lu ratio of 0.02669 (DeBievre and Taylor, 1993) to calculate 176Lu/177Hf ratios. Similarly, the interference of 176Yb on 176Hf was corrected by measuring the interference-free 172Yb isotope and using the recommended 176 Yb/172Yb ratio of 0.5886 (Chu et al., 2002) to calculate 176 Hf/177Hf ratios. In doing so, a mean 173Yb/171Yb ratio for the analyzed spot itself was automatically used in the same run to calculate a mean bYb value (Iizuka and Hirata, 2005), and then the 176Yb signal intensity was calculated from the 173Yb signal intensity and the mean bYb value. The Hf isotopes were measured in two modes. For the HHHC02 and HH02, the analysis was done on the same spots or the same age domains for age determinations of the concordant grains, as guided by CL images. For the other three samples, we used our developed technique of simultaneous determinations of U–Pb age, Hf isotopes and trace element compositions of zircon by combining excimer laser ablation quadruple and multiple collector ICP-MS (Yuan et al., 2008). This allows simultaneous collections of data on U–Pb age, Hf isotopes and trace element compositions of the same aerosol from the same spot of zircon. Our measured values of well-characterized zircon standards (91500, Temora-2, GJ-1, Mud Tank, BR266 and Monastery) agree with the recommended values to within 2r. Detailed description of the technique and analyses of the standard zircons were reported in Yuan et al. (2008). The initial 176Hf/177Hf ratios were calculated with reference to the chondritic reservoir (CHUR) at the time of zircon growth from magmas. eHf(t) values are defined to denote a 0.1& difference between the sample and the chon-

dritic reservoir at the time of magma crystallization. The decay constant for 176Lu and the chondritic ratios of 176 Hf/177Hf and 176Lu/177Hf used in calculations are 1.865  1011 yr1(Scherer et al., 2001) and 0.282772 and 0.0332 (Bichert-Toft and Albare`de, 1997), respectively. The single-stage model age (TDM1) was calculated relative to the depleted mantle with a present-day 176 Hf/177Hf = 0.28325 and 176Lu/177Hf = 0.0384 (Griffin et al., 2000). A two-stage continental model age (TDM2) was also calculated by projecting the initial 176Hf/177Hf of zircon back to the depleted mantle growth curve using 176 Lu/177Hf = 0.0093 for the upper continental crust (Vervoort and Patchett, 1996). 4. RESULTS As shown by Vermeesch (2004), for provenance studies, a minimum of 117 detrital zircon grains have to be dated for a single sample in order to yield statistically significant results. 120–184 zircon grains were dated for the five samples, which yield concordant age (with age concordance in the range from 90% to 110%) dates of 84, 99, 102, 86, and 108 for HHHC02, HH02, HH01SD, YDH01, and LH01, respectively (Fig. 2). Online Table S1 presents the U–Pb age and Lu–Hf isotopic data for these concordant zircons. The following discussion is confined to the concordant zircons. We use 207Pb/206Pb ages for zircons of age P1.0 Ga and 206Pb/238U ages for zircons of age <1.0 Ga. 4.1. U–Pb ages The concordant zircons from the three sand samples of the Yellow River show broadly similar age patterns with three major age populations of 2.1–2.5 Ga, 1.6–2.0 Ga, and 150–500 Ma. There are also a significant number of zircons in the age range of 700–1500 Ma. Zircons of this age range are more abundant in HH01SD than in HHHC02 and HH02. They count for 22%, 14%, and 7% of the total populations in the three samples, respectively. At expense, the >1.5 Ga zircons are considerably less abundant in HH01SD than in the other two Yellow River samples. One 3.5-Ga old zircon is present in HHC02, while one 2.8 Ga zircon occurs in HH02. Zircons from the Yongding River and the Luan River show clear three populations of 2.6–2.3 Ga, 1.6–2.0 Ga, and 350–140 Ma. There are few zircons of ages between 2.0 and 2.3 Ga. 4.2. Lu–Hf isotopes As shown in Fig. 3a, the eHf(t) values exhibit a wide range from negative to positive for three major age groups of the Yellow River. This indicates recycled crustal materials. Of importance, several zircons at 2.5 Ga, 2.1 Ga, 1.4– 1.5 Ga and 900 Ma and 450 Ma show values identical to the depleted mantle indicating juvenile crustal addition. All zircons lie above the evolution line for an upper crust of 3.5 Ga. The Yongding River and Luan River also show negative to positive eHf(t) values for zircons of the age groups of 2.6–2.3 Ga, 1.6–2.0 Ga, and 350–140 Ma

Episodic crustal growth of North China

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0.6 14

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Fig. 2. Left panels show U–Pb concordia plots of concordant detrital zircons from the Yellow River (a–c), Yongding River (d) and Luan River (e). Right panels show corresponding relative probability plots of U–Pb ages for concordant detrital zircons.

(Fig. 3b). Zircons from the minor group at 2.0–2.3 Ga all have negative values. Few zircons of 2.4–2.5 Ga and 325 Ma have eHf(t) values close to the depleted mantle.

Fig. 4 shows distributions of the Hf continental model ages (TDM2). It can be seen that the crustal model age show a prominent group at 2.4–2.9 Ga with a peak at 2.7–2.8 Ga

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a

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Zircon age (Ma) Fig. 3. U–Pb age versus eHf(t) value plots of concordant detrital zircons from the Yellow River (a) and Yongding River and Luan River (b).

for all the three rivers under investigation. A less significant group lies between 1.4 and 1.8 Ga for the Yellow River and between 1.6 and 1.9 Ga for the Yongding River and Luan River. Although the Yellow River sands show a significant number of zircons with TDM2 between 0.5 and 1.4 Ga, zircons of TDM2 within this range are rare for the Yongding River and Luan River. 5. DISCUSSION As shown in Fig. 5, the majority of the zircons have Th/U > 0.30 indicating an igneous origin (Hanchar and Hoskin, 2003). Typical metamorphic Th/U ratios (<0.10) are few. The results are consistent with their CL images pre-

dominated by oscillatory zoning typical of igneous origin (not shown). 5.1. Control of regional provenance The Yongding River and Luan River drain entirely within the North China Craton. The lower reach of the Yellow River also run in the North China Craton. All these rivers show prominent U–Pb age peaks at 2.4–2.5 Ga and 1.8– 1.9 Ga, which are characteristic of the North China Craton (Gao et al., 2004; Liu et al., 2008). The 2.4–2.5 Ga ages are widespread in the North China Craton and recorded by voluminous granitoids and felsic volcanic rocks from the Precambrian basements (Zhao et al., 2000, 2001, 2005).

Episodic crustal growth of North China

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TDM2 (Ma) Fig. 4. Relative probability plots of two-stage Hf crust formation model ages for zircons from the Yellow River (a) and Yongding River and Luan River (b).

Inherited zircons of similar ages are also common in the Mesozoic granites and felsic volcanic rocks. The 1.9 Ga event is largely metamorphic and represented by thin zircon overgrowths around the 2.5 Ga magmatic cores (Zhao et al., 2000, 2001, 2005). The Precambrian basements of the North China Craton had been stabilized after the 1.9 Ga until Ordovician when kimberlite erupted (Gao et al., 2002). Therefore post-1.7 Ga Precambrian zircons are absent from the Yongding River and Luan River. The Yangtze Craton and Qilian Orogenic Belt are characterized by predominant Neoproterozoic magmatic event around 830 Ma (Li et al., 2002, 2003; Liu et al., 2008). Mesoprote-

rozoic zircons around 1.5 Ga are known from the Hanjian River that drains the South Qinling Belt, which belongs to the northern margin of the Yangtze Craton (Yang et al., 2007). It is thus clear that the Yongding River and Luan River are entirely contributed by sources from the North China Craton. In contrast, all the three Yellow River sands have a significant number of Neoproterozoic (700– 1000 Ma) zircons. Mesoproterozoic zircons are less abundant but still significant in the three Yellow River samples (Fig. 2). It is interesting to note that the proportion of the Neoproterozoic zircons is higher in sample HH01SD (14%) than in the other two Yellow River samples

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a

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Zircon age (Ma) Fig. 5. Plots of Th/U ratios versus U–Pb ages of concordant detrital zircons from the Yellow River (a) and Yongding River and Luan River (b).

(5–6%). This sample was taken close to the exit of the Yellow River to the Bohai Sea and is located close to the Sulu ultrahigh pressure metamorphic belt, where eclogites and host gneisses have Yangtze protoliths of Neoproterozoic ages (Zheng et al., 2003). The Neoproterozoic zircons must have not been dominantly sourced from the coeval Qilian basements (Xu et al., 2007). Otherwise, more Neoproterozoic zircons would be expected in the other two Yellow River sands than in HH01SD, as they were taken in localities closer to the Qilian belt (Fig. 1). It is suggested that small local rivers originating from the Sulu belt may contribute to the Yellow River at the lowest reach. Absence of Neoproterozoic ages in the detrital zircons from the Yongding and Luan Rivers within the North China Craton contrasts with the significant 830 Ma peak of detrital and igneous zircons from the Yangtze craton, and reinforces that the

two cratons have distinct evolutionary histories in the Precambrian (Gao et al., 2004; Liu et al., 2008). 5.2. Crustal growth of North China and global comparison As discussed above, very few zircons have eHf(t) identical to depleted mantle values. This indicates that the studied zircons contain variable amounts of recycled crustal materials. For such zircons the two-stage Hf model age TDM2 calculated by using the upper crustal Lu/Hf value (0.0093) are better approximation of crust formation ages (Iizuka et al., 2005; Kemp et al., 2006; Hawkesworth and Kemp, 2006a; Liu et al., 2008). Wu et al. (2005b) compiled available Nd isotopic compositions of Precambrian basement rocks from the North China Craton in order to study the crustal growth. Their re-

Episodic crustal growth of North China

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Zircon age (Ma) Fig. 6. Histogram of concordant U–Pb ages and crust growth curves based on U–Pb ages and two-stage Hf crust formation ages for detrital zircons from the Yellow River.

sults reveal common dominant Nd crust model ages in the range between 2.6 and 3.0 Ga for all the three subunits of the North China Craton (Fig. 1) (Wu et al., 2005b). Therefore, Hf and Nd model ages for crust formation are consistent. However their results show few, if any, Precambrian rocks of Nd model age younger than 1.8 Ga (Wu et al., 2005b). This is in contrast to the Hf model ages, which

show a significant portion of post-Mesoproterozoic model ages. We attribute this difference to the fact that samples used for Nd isotopic studies were from >1.8 Ga old basement rocks of the North China Craton. Thus, results from the modern river zircons are better indicative of crustal growth history of the North China Craton. Phanerozoic magmatism is widespread in the drainage areas of the three

100 LH01

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U-Pb age (Ma) Fig. 7. Histogram of concordant U–Pb ages and crust growth curves based on U–Pb ages and two-stage Hf crust formation ages for detrital zircons from the Luan River and Yongding River.

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rivers (Wu et al., 2005a). However, only two zircons each from the Yellow River and the Luan and Yongding Rivers show Phanerozoic model ages of 250–500 Ma (Fig. 4). It is suggested that the Phanerozoic magmatism had been predominantly products of crustal reworking with insignificant formation of juvenile crust. Figs. 6 and 7 illustrate distributions of concordant U–Pb ages and crust growth curves based on the U–Pb ages and two-stage Hf crust formation model ages for the Yellow River and Luan and Yongding Rivers, respectively. As pointed by previous studies (e.g., Iizuka et al., 2005), large differences exist between crust growth curves based on these

a

two types of ages indicates that crustal reworking is an important process in continental crust formation. Erosion and recycling add additional complications. It may take a long time, perhaps up to 1 Ga, for new crustal material to dominate the sedimentary record (Hawkesworth and Kemp, 2006a). Fig. 8 illustrates zircon crystallization age versus crustal residence time defined by zircon crust formation model age TDM2 minus zircon crystallization age. It can be seen from this figure that both the range and the maximum of the crustal residence time clearly increase with decreasing age. Phanerozoic zircons may have been derived from crustal sources separated from the mantle up to

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Zircon age (Ma) Fig. 8. Variation of crustal residence time with crystallization age of concordant zircons from the Yellow River (a) and Yongding River and Luan River (b). Crustal residence time is defined by crust formation age of zircon (TDM2) minus zircon crystallization age. Lines show trend of the maximum residence time with age.

Episodic crustal growth of North China

2.0 Ga ago before the zircons crystallized, suggesting longterm preservation, reworking and recycling of the continental crust. Bearing these complications in mind, Figs. 6 and 7 can be used as the first-order approximation to study crustal growth rates. Crustal growth rates based on TDM2 suggest that 45% and 90% of the present crustal volume were formed by 2.5 Ga and 1.0 Ga, respectively, for the drainage area of the Yellow River (Fig. 6). In comparison, 60% and 98% of the present crustal volume were formed by 2.5 Ga and 1.0 Ga, respectively, for the Luan and Yongding Rivers, which drain entirely within the North China Craton (Fig. 7). It should be noted that these calculations do not account for those zircons which may come from sources outside of the present drainage areas of the modern Yellow River and its tributaries because of recycling of detrital zircons into younger sediments. The upper reach of the Yellow River is adjacent to the Tibetan Plateau where Cenozoic granites are widespread (Yin and Harrison, 2000). However, none of our dated Yellow River zircons has ages younger than 150 Ma. This suggests that sources outside of the present drainage areas of the modern Yellow River and its tributaries had contributed insignificantly to our samples. As stated above, combination of zircon Hf and O isotopes provides a robust way to reveal net mantle additions. Four episodes of generation of new continental crust thus obtained from worldwide Precambrian zircons are ca. 4.4–4.5 Ga, ca. 3.8 Ga, ca. 3.4 Ga and ca. 2.7–2.8 Ga (Pietranik et al., 2008). The 2.7–2.8 Ga peak of Hf model ages for North China agree well with the worldwide compilations. However, other three peaks are insignificant in North China. In comparison, studies of U–Pb age and Hf isotopic compositions of detrital zircons from the Neoproterozoic strata and igneous zircons from the Archean basement in the Yangtze Craton suggest two major crust growth peaks between 3.2 and 3.8 Ga and 720 and 910 Ma (Liu et al., 2008). For this craton, the advocated worldwide 2.7–2.8 Ga crust formation peak is not observed. It is suggested that these worldwide peaks have to be re-examined and treated carefully. 6. CONCLUSIONS U–Pb ages of detrital zircons from the Yellow River reveal three major age groups of 2.1–2.5 Ga, 1.6–2.0 Ga, and 150–500 Ma. Those from the smaller Luan and Yongding Rivers show broadly similar three age groups of 2.3– 2.6 Ga, 1.6–2.0 Ga, and 350–140 Ma, but with narrower ranges. Although Hf isotopic compositions show both juvenile crustal growth and crustal reworking for all the age groups, much of the crustal growth of North China occurred in the Neoarchean and Mesoproterozoic. The three rivers are characterized by a common prominent group of Hf crust formation model ages in the range of 2.4–2.9 Ga with a peak at 2.7–2.8 Ga. A less significant group lies between 1.4 and 1.8 Ga for the Yellow River and between 1.6 and 1.9 Ga for the Yongding River and Luan River. Crustal growth rates based on Hf crust formation model ages suggest that 45% and 90% of the present crustal

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volume were formed by 2.5 Ga and 1.0 Ga, respectively, for the drainage area of the Yellow River. In comparison, 60% and 98% of the present crustal volume were generated by 2.5 Ga and 1.0 Ga, respectively, for the Luan and Yongding Rivers, which run entirely within the North China Craton. Although the 2.7–2.8 Ga peak of Hf crust formation model ages for North China agree well with the coeval major peak for global crustal growth, the other two major worldwide peaks at 3.4 and 3.8 Ga are insignificant in North China. In contrast, the 2.7–2.8 Ga peak is not observed for the Yangtze Craton. We suggest that these advocated worldwide crust formation peaks had to be reexamined with data (particularly combined Hf and O isotopes) from other major rivers worldwide and treated carefully. Our results also show that Phanerozoic zircons may have been derived from crustal sources separated from the mantle up to 2.0 Ga ago before the zircons crystallized, suggesting long-term preservation, reworking and recycling of the continental crust. ACKNOWLEDGMENTS This study is jointly supported by the MOST special funds from the State Key Laboratory of Continental Dynamics and the State Key Laboratory of Geological Processes and Mineral Resources, the National Nature Science Foundation of China (Grants 40821061, 90714010, and 40673019), and the Ministry of Education of China (B07039). We thank Chris Hawkesworth, Tim Peters and an anonymous reviewer for their constructive comments and Martin A. Menzies for editorial handling.

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