Gondwana Research 20 (2011) 194–204

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Crustal evolution of the North Qinling terrain of the Qinling Orogen, China: Evidencefrom detrital zircon U–Pb ages and Hf isotopic composition

Xi-Yan Zhu a,b, Fukun Chen b,⁎, Shuang-Qing Li b, Yi-Zeng Yang b, Hu Nie b,Wolfgang Siebel c, Ming-Guo Zhai a

a Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, Chinab CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, Chinac Institut für Geowissenschaften, Universität Tübingen, 72074 Tübingen, Germany

⁎ Corresponding author. Tel.: +86 551 3603723.E-mail address: fkchen@ustc.edu.cn (F. Chen).

1342-937X/$ – see front matter © 2011 International Adoi:10.1016/j.gr.2010.12.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 September 2010Received in revised form 25 December 2010Accepted 30 December 2010Available online 6 January 2011

Keywords:North China blockNorth QinlingPrecambrianZircon ageHf isotopes

The North Qinling terrain reveals a different geological history from the North China block to the north and theSouth Qinling micro-continental block to the south. The timing of events and geochemical features of theNorth Qinling terrain are essential for understanding the evolution of the Qinling orogenic belt that joinsthe North and South China blocks. This study presents detrital zircon ages and Hf isotopic compositions fromthe Guandaokou Group, a cover sequence of the North China block, and from the Kuanping Group, an upperbasement sequence of the North Qinling terrain. The youngest detrital zircon U–Pb ages constrain theinitiation of sedimentation in the Guandaokou Group to 1.85 Ga and the maximum depositional age of theKuanping Group to ~640 Ma. Zircon U–Pb ages and Hf data testify to amajor phase of magmatism in the NorthQinling terrain at ~1000–900 Ma. The magmatic rocks were largely produced from reworking of earlyPaleoproterozoic (~2.5–2.0 Ga) crust with minor contribution of late Paleoproterozoic (~1.7 Ga) juvenilematerial. This age record differs in time from the two important thermal events of ~1.85 Ga and ~2.5 Ga,widely manifested in the North China block. The North Qinling terrain can thus be regarded as a separatemicro-continent with a pre-Neoproterozoic evolution different from that of the North China block. The finalassembly of the North Qinling terrain to the North China block took place after about 640 Ma.

© 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

The Qinling orogenic belt, being part of the central orogenic belt inChina, was amalgamatedwith theNorth China block (NCB) to the northalong the Lo-Luan tectonic zone (e.g., R.G.S. Henan, 1989; Zhao et al.,2009). This belt was sutured to the South China block (SCB) during themiddle to late Triassic along theMian-Lue tectonic zone (~240–225 Ma;Li et al., 1993, 2000; Chavagnac and Jahn, 1996; Rowley et al., 1997;Hacker et al., 1998; Webb et al., 1999; Chavagnac et al., 2001; Liu et al.,2008; Li et al., 2009, 2010). The Shang-Dan tectonic zone is an innersuture, which separates the Qinling orogenic belt into the North Qinlingterrain (NQ) and the South Qinling terrain (SQ). Over the last decade, acharacteristic Neoproterozoic magmatic event was recorded by zirconsin magmatic and low-grade metamorphic rocks of the South Qinlingterrain and theSouthChinablock (e.g., Linget al., 2003, 2007;Chenet al.,2006; Zhu et al., 2008), indicating that these two blocks were closelyconnected in a uniform tectonic setting during the Neoproterozoic (e.g.,Zhang et al., 2001; Ratschbacher et al., 2003; Chen et al., 2006; Zhu et al.,2008, 2009; Ling et al., 2007, 2010). However, whether the North

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Qinling terrain was connected with the North China block or the SouthChina block (Yangtze Block) remains amatter of controversy (Xue et al.,1996a,b; Zhang et al., 2001). Recently, Yang et al. (2010) argued that theNorth Qinling terrain was dissimilar to the North China block and theSouth China block because it displays a unique late Mesoproterozoic toearly Neoproterozoicmagmatic event. In a number of studies, the NorthQinling terrain is regarded as a micro-continent with a distinctgeological evolution prior to the final amalgamation with the NorthChina block (e.g., O'Yang and Zhang, 1996; Dong et al., 2003).

Zircons contain high concentrations of the element Hf (generally1–2%) resulting in low Lu/Hf ratios of typically b0.001,much lower thanthe bulk Lu/Hf ratio of the continental crust. Hf is more incompatiblethan Lu during mantle melting and hence, the melt acquires a lowerLu–Hf ratio compared to the residual mantle. Therefore, crust andmantle show different Hf isotopic evolution trends with time and the176Hf/177Hf ratio of a zircon is an excellent tracer of the source materialand the time of crust/mantle differentiation (Kinny and Maas, 2003).The Hf isotopic composition of zircons has been successfully used as anindicator for sedimentary provenance and interaction between crustand mantle (e.g., Amelin et al., 1999, 2000; Griffin et al., 2000, 2002;Kemp et al., 2005; Zheng et al., 2006b, 2007).

Many previous studies have demonstrated that the Proterozoic wasan important episode for the crustal evolution of the Chinese blocks,

Published by Elsevier B.V. All rights reserved.

195X.-Y. Zhu et al. / Gondwana Research 20 (2011) 194–204

especially for the South China block (Yangtze and Cathaysia blocks;Chen and Jahn, 1998; Ling et al., 2002). U–Pb analysis of zircon grainsfrom sediments and basement rocks suggest four stages of crustalevolution of the Yangtze block, namely at N3.0 Ga, ~2.95 Ga, ~1.95 Gaand 820–750 Ma (Zhang et al., 2006; Zheng et al., 2006a). But, previousstudies have also shown that the Sm–Ndmodel ages (TDM) of differentrock types cluster around three peaks at 2.05 Ga, 1.40 Ga, and 1.05 Gathat were interpreted as representing three crustal formation stages(Wei et al., 1999).

This study presents new detrital zircon U–Pb and Hf isotopic datafrom the low-grade metamorphic rocks of the northern part of theQinling orogenic belt, and provides a discussion on the sourcecharacteristics, the crustal evolution and the tectonic relationshipbetween the North China block and the North Qinling terrain duringthe Precambrian.

2. Geological background

TheQinlingorogenconstitutes thewesternpart of theQinling–Dabieorogenic belt in central China. It was formed in a two-stage collisionprocess between the North China and South China blocks during theearly–middle Paleozoic (e.g., Meng and Zhang, 1999; Zhang et al., 2001;Sun et al., 2002; Faure et al., 2008) and early Mesozoic (e.g., Li et al.,1993, 2000; Ames et al., 1993; Meng and Zhang, 2000; Li et al., 2007b;Faure et al., 2008; Jiang et al., 2010). The Shang-Dan and the Mian-Luesuture zones formedduring these collisions. The Shang-Dan suture zoneseparates the Qinling orogen into a northern and a southern terrain,which are referred to as the North Qinling and the South Qinling,respectively (Fig. 1). Geologically, theNorth Qinling terrain is composedof Meso- to Paleoproterozoic gneiss, e.g., the Qinling and KuanpingGroups, Neoproterozoic meta-sedimentary and meta-volcanic rocks,e.g., theDanfengandErlangpingGroups, and Paleozoic low-grademeta-sedimentary rocks,whereas the South Qinling terrain consistsmainly ofNeoproterozoic meta-sedimentary and meta-volcanic rocks, e.g., theWudang and Yaolinghe Groups.

The Qinling Group is considered to be the oldest basement unitexposed in the North Qinling terrain. It is composed of gneisses,amphibolites and marbles (e.g., Ratschbacher et al., 2003; Dong et al.,2008). U–Pb zircon ages from gneisses range from 2.2 to 2.3 Ga (e.g.,Zhang et al., 1994c), whereas amphiboles yielded a Sm–Nd isochron ageof ~2.0 Ga (Zhang et al., 1994b). New U–Pb zircon analyses fromgneisses located in southeastern Shaanxi and western Henan indicatethat the Qinling Group formed mainly during the late Mesoproterozoicto early Neoproterozoic (Yang et al., 2010). The Erlangping andDanfengGroups are composed of meta-sedimentary and meta-volcanic rocksthat formed during the Neoproterozoic to early Paleozoic (Zhang et al.,1994a; Lerch et al., 1995; Xue et al., 1996a,b). Geochemical compositionindicates that the volcanic rocks range in composition from basaltthrough andesite, to dacite and that these rocks erupted in an intra-oceanic arc (Zhang et al., 1994b; Xue et al., 1996b). The upper sequenceof the Kuanping Group, consisting chiefly of low amphibolite- togreenschist-facies marbles and two-mica quartz schists (Li et al., 1995),is interpreted as part of the metamorphosed south-facing passivemargin of the North China block (Wang et al., 2000) or, as anaccretionary wedge (Ratschbacher et al., 2003). Element geochemistrysuggests that the protoliths of greenschist- to amphibolite-facies rocksof the lower sequence of the Kuangping Group were tholeiitic basaltswith N-MORB or T-MORB geochemical characteristics (Liu and Zhou,1995; Zhang et al., 1997). Sm–Nd isochron ages of these meta-basaltsrange from 0.94 to 1.2 Ga (Zhang et al., 1994c; Dong et al., 2003, 2008).The meta-volcanic rocks of the Kuanping Group are associated withmeta-gabbro and serpentinized peridotite that are considered torepresent a Grenvillian-age ophiolite. Such interpretation would implyan initial rift along the southern margin of the North China block (Weiet al., 1999; Meng and Zhang, 1999). A pre-Mesoproterozoic age has

been suggested for the formation of theKuanpingGroup (e.g.,Wei et al.,1999; Zhang et al., 2001 and references therein).

3. Analytical technique

Zircon grains were isolated from crushed rocks by standardmineral separation techniques and were handpicked for analysisunder a binocular microscope. Zircon U–Pb age dating was accom-plished by the mean of LA-ICP-MS (laser-ablation inductively coupledplasma mass spectrometer) at the CAS Key Laboratory of Crust–Mantle Materials and Environments, University of Science andTechnology of China in Hefei. The laser-ablation system was a GeoLas200 M equipped with a 193 nm ArF excimer laser. The diameter of thelaser-ablation pit was approximately 40 μm and the average poweroutput was about 4 W. Signal and background measuring durationswere about 70 s and 40 s, respectively. Zircon 91500 was used as anexternal calibration standard for age calculation, and NIST610 wasanalyzed twice for every 10 analyses for concentration calculations ofU, Th, and Pb. Further analytical details are given in Liu et al. (2007).U–Pb isotopic ratios were calculated using the Glitter Ver4.0 softwareand U–Pb age calculation was performed using the software programIsoplot (Ludwig, 2003). All errors are quoted as 2σ.

Zircon Hf analytical procedure has been described in Wu et al.(2006) and Hou et al. (2007). All analyses were performed using aNeptune MC-ICP-MS located at Chinese Academy of Geological Science(CAGS). This mass spectrometer is equipped with double-focusing andmulti-collectors. A GeoLas 200 M laser-ablation system (MicroLas,Germany) was used equipped with an ArF excimer 193 nm laser. Inthis study, a 40 μm laser spot size was selected during the ablation witha repetition rate of 8 Hz.Measured 176Hf/177Hf ratioswere not correctedfor isobaric interference of 176Lu on 176Hf due to the extremely low176Lu/177Hf in zircon (normally b0.002). 175Lu/176Lu=0.02655 wasused for elemental fractionation correction. Isobaric interference of176Yb on 176Hf was corrected using the mean fractionation indexproposed by Iizuka et al. (2005). The applied value of 176Yb/172Yb is0.5886 (Chu et al., 2002; Vervoort et al., 2004). A 176Hf/177Hf ratio of0.282007±7 (2σ, n=36) was obtained for zircon GJ1, which isidentical with values obtained by the solution method (Morel et al.,2008; Hou et al., 2007; Sláma et al., 2008). Analytical data of this studywere corrected using 176Hf/177Hf ratio of 0.281994±15 of zircon GJ1(MSWD=2.2, n=12).

4. Analytical results

Mica quartz-schist samples belonging to the middle-upperKuanping Group were collected from western Henan Province,where the sedimentary sequences are well exposed and fresh samplescan be easily obtained. In order to allow a comparison with crystallinerocks of the North China block, sedimentary rocks from theGuandaokou Group exposed along the southern margin of the NorthChina block were also collected. Sample localities and informationabout the rock types is given in Table 1. Analytical results of zirconU–Pb dating and Hf isotopic composition are given as the supple-mentary data in the journal website (Table S1 and Table S2).

Zircon grains from sandstones of the lower part of the GuandaokouGroup are well rounded, implying a long transport of the grainsbefore deposition or multiple sedimentary recycling. As seen fromcathodoluminescence (CL) images (Fig. 2), some zircons show oscillatorygrowth zoning consistent with a primary magmatic origin, while othersdisplay typical fir-tree-zoned, sector and planar internal texture. Gener-ally, the inhomogeneous CL intensity of zircons is interpreted as the resultof a complex origin. Forty-seven detrital zircons from sample GSH0806and GSH0808 were selected for U–Pb and Hf isotope analyses. The twomain peak age values of these zircons are ~1.85 Ga and ~2.50 Ga (Fig. 3).

Zircons from schist samples of the middle-upper Kuanping Groupare euhedral to subhedral, implying a rapid weathering cycle and

Fig. 1. Geological map of the North Qinling terrain. Abbreviations in the inset map: SD, Shang-Dan tectonic zone; ML, Mian-Lue tectonic zone; LN, Lo-Nan fault.

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short transportation of materials. The CL images show that mostgrains have preserved magmatic oscillatory zonation. Some grainsdisplay characteristics of a metamorphic origin without oscillatoryzoning (Fig. 2), indicating a metamorphic event. A total of onehundred and twenty-seven detrital zircons from samples 08HN46,08HN49 and 08HN60 were analyzed for U–Pb and Hf isotopiccomposition. Concordia diagrams for the detrital zircons from theserocks are shown in Fig. 4. Zircons with U–Pb ages ranging fromNeoarchean (~2.5–2.4 Ga) to early Neoproterozoic (~1.1–1.0 Ga and~950–810 Ma) can be distinguished. Minor zircons of Mesoproterozoic(~1.7–1.4 Ga) and Paleoproterozoic (~2.5–2.0 Ga) age are also identi-fied in these diagrams.

Forty-eight detrital zircons from the Guandaokou Group give εHf(t)values ranging from−13.4 to 7.0 (Fig. 5). More than half of the grainshave εHf(t) values N0, indicating that the detrital zircons crystallizedfrommagmas that originated from juvenile crustal material or directlyfrom a depletedmantle source and reworked ancient crustal material.Two-stage Hf model age values (TDM2) of the detrital zircons range

Table 1Lithology and mineralogy of the analyzed samples.

Sample Rock group Lithology Texture

GSH0806 Guandaokou Sandstone MediumGSH0808 Guandaokou Sandstone Fine-gra08HN46 Kuanping Quartz schist Laminate08HN49 Kuanping Quartz schist Laminate08HN60 Kuanping Mica quartz schist Laminate

Mineral: Qz = quartz, Sc = sericite, Cc = calcite, Ms = muscovite, Bi = biotite.

from ca. 3.3–2.2 Ga, some are even older than 4.0 Ga, implying pre-existing basement material with a prolonged crustal evolution.

εHf(t) values of seventy-nine detrital zircon grains from theKuanping Group range from –26.5 to 10.9 (Fig. 5). About 80% ofthese grains have negative εHf(t) values, meaning that most detritalzircons crystallized from the magmas derived by melting of crustalmaterial. The corresponding Hf model ages (TDM2) range from 3.8 Gato 1.5 Ga. Similar to the sandstone samples from the GuandaokouGroup, some detrital zircons from the Kuanping Group also yield Hfmodel ages older than 4.0 Ga.

5. Discussion

5.1. Deposition age of the sedimentary sequences

The Guandaokou Group is usually considered as aMesoproterozoicsequence (R.G.S. Henan, 1989; Zhao et al., 1998; Zhai et al., 2007;Kusky et al., 2007a,b) and the formation time probably corresponds to

Major mineralogy Locality

-grained Qz N34°22′00″E111°02′00″ined Qz+Sc N34°22′00″E111°02′00″d and oriented Qz+Ms+Bi N33°44′04″E111°42′59″d and oriented Qz+Ms+Bi N33°44′45″E111°43′28″d and oriented Ms+Bi+Qz N33°51′11″E111°12′51″

Fig. 2. Cathodoluminescence (CL) images of typical zircon populations. (a) rounded zircons from sandstones of the Guandaokou Group (samples GSH0806 and GSH0808). Lateoverprint zones characterized by high CL intensity can be observed in some grains; (b) euhedral or subhedral grains from the schists of the Kuanping Group (samples 08HN46,08HN49, and 08HN60). Most grains display CL oscillatory zoning typical for magmatic origin.

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that of the Changcheng Group in the North China block (e.g., Zhao et al.,2001b; Fig. 6). Sandstones of the lower part of the sequence overlie thevolcanic-sedimentary sequence of the Xiong'er Group distributed in

Fig. 3. Zircon U–Pb concordia diagrams for san

northwestern Henan Province (e.g., R.G.S. Henan, 1989; Zhao et al.,2001a, 2009). These rocks are covered by marine-facies sandstone,stromatolitic dolomite and limestone of the upper Guandaokou Group,

dstone samples of the Guandaokou Group.

Fig. 4. Zircon U–Pb concordia diagrams for quartz-schist samples of the Kuanping Group.

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which, in turn, is overlain by the Sinian to Triassic section (R.G.S. Henan,1989). Most of the detrital zircons from the lower part of theGuandaokou Group yield concordant 207Pb/206Pb ages between1.85 Ga and 2.5 Ga, except for a few grains, which are older than3.0 Ga. These grains are highly rounded (Fig. 2), with various internaltexture. Most grains with oscillatory zoning have high Th/U ratios(N0.4), characteristic of magmatic zircon. Because sedimentary rocks ofthe Guandaokou Group have only undergone a merely low-grademetamorphic overprint, the U–Pb system of the most zircon grainsshould remain closed during late metamorphic event(s). Hence, theyoungest detrital zircon ages of 1.85 Ga (MSWD=0.91, n=9) canconstrain the maximum depositional age of the Guandaokou Group.However, according to field observations, volcanic rocks and theirintrusive counterparts in the underlying Xiong'er Group were emplacedat ~1.78 Ga (Zhao et al., 2001a, 2002b, 2009; Peng et al., 2007, 2008; Hou

et al., 2008a,b; He et al., 2009), hence, the deposition time of the lowerGuandaokouGroupcanbebetter constrained to the lateMesoproterozoic.

Earlier studies have suggested that the Kuanping Group, especiallyamphibolites and other meta-mafic rocks of the lower part, formedduring the Paleo- to Mesoproterozoic (e.g., Zhang et al., 1994a,b,c).Recently, He et al. (2007) reported a U–Pb zircon age of 1.75 Ga for theamphibolites of themiddle part of the KuanpingGroup, Gansu Province,by using the LA-ICP-MS zircon dating technique. However, in thepresent study, most of the investigated detrital zircons are euhedral orsubhedral with magmatic zoning (Fig. 2) and Th/U ratios N0.3 and theU–Pb systematics of these grains constrain the maximum depositionalage of the quartz schists from themiddle part of the Kuanping Group at~640 Ma (n=7, Fig. 7). Two detrital zircon grains even yield younger206Pb/238U concordant ages of ca. 510 Ma and ca. 540 Ma. Meanwhile,Yan et al. (2008) presented a SHRIMP U–Pb age of 611±13Ma for

Fig. 5. Temporal evolution of εHf(t) values of detrital zircons younger than 3.0 Ga. The major magmatic activity in the North Qinling terrain occurred around ~1.0–0.9 Ga and thismagmatism was largely produced by reworking of early Paleoproterozoic crust with minor contribution of late Paleoproterozoic juvenile material.

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meta-mafic volcanic rocks of the lower part of the Kuanping Group,Shaanxi Province. Paleontological evidence also supports the conclusionthat the Kuanping Group is much younger than previously suggested(Wang et al., 2009).

5.2. Provenance of the sedimentary sequences

Material sources of sedimentary rocks are mainly derived fromthree major sources, that are high-grademetamorphic rocks of deeplyeroded cratonic basement, young volcanic or plutonic rocks fromaccretionary basins along active continental margins, and recycledrock complexes of old crustal sections (Chen et al., 2009). Detritalminerals in sedimentary rocks (e.g., zircon, garnet, apatite andmuscovite) have been used to distinguish their sources and thehistory of ancient continental margins in regions adjacent to the

Fig. 6. Precambrian stratigraphic sequences in the southern margin of the North China Block a

Qinling orogen, e.g., in the North China block, Yangtze Block and theDabie orogen (e.g., Chen et al., 2009; Sun et al., 2009; Hu et al., 2009;Wang et al., 2010a,b; Yan et al., 2010). Furthermore, whole-rock Nd(e.g., Barovich and Foden, 2000; Krogstad et al., 2003; Wade et al.,2005) and zircon Hf isotope analyses (e.g., Iizuka et al., 2005; Kempet al., 2006; Hawkesworth and Kemp, 2006a,b) have been broadlyapplied in such studies.

Sandstones of the Guandaokou Group are exposed along thesouthern margin of the North China block. The U–Pb age spectrumoutlined for detrital zircons (Fig. 7) shows that the sedimentsobtained material mainly from Neoarchean (~2.5 Ga) and middlePaleoproterozoic (~1.85 Ga) sources. Evidence for significant mag-matic– tectonic events at about 2.5 Ga can be found throughout theNorth China block. This tectonothermal episode was characterized bythe formation of granulite belts, magmatism andmetamorphism (e.g.,

nd the North Qinling terrain (modified by Gao et al., 2008 and 1:200,000 geologic maps).

Fig. 7. Age spectra outlined for detrital zircons of sedimentary samples from theGuandaokou and Kuanping Groups. Age peaks are at ~2.5 Ga, ~1.85 Ga for theGuandaokou Group and at ~2.5 Ga and ~1.0–0.9 Ga for the Kuanping Group.

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Zhao et al., 2002a; Rogers and Santosh, 2009; Santosh et al., 2006,2007a,b, 2009a,b, 2010), indicating that various micro-blocks amal-gamated to form a coherent craton (e.g., Zhai et al., 2000; Geng et al.,2006; Zhai, 2010). Shortly after ~1.85 Ga the North China blockunderwent an extensional tectonic regime, characterized by anoro-genic magmatism, rift-magmatism or volcanism, and retrogrademetamorphism (e.g., Zhao et al., 2001b, 2005; Zhao, 2009; Guo et al.,2002). Volcanic rocks of the Xiong'er Group and dyke swarms thatformed at ~1.8 Gamark the endof this tectonic phase in theNorth Chinablock (e.g., Zhai, 2010; Zhao et al., 2002b, 2004, 2009; Peng et al., 2007,2008). As an important finding, detrital zircons from sedimentary rocksof the Zhaertai and Huade Groups along the northern margin of theNorth China block (Li et al., 2007a; Hu et al., 2009), the Penglai Group inthe eastern part (Li et al., 2007c; Zhou et al., 2008), the Changcheng(Wan et al., 2003), Gaofan, Hutuo, Dongjiao Groups in the central part(Wan et al., 2010), provide the same age peaks as the detrital zirconsfrom the Guandaokou Group, all corresponding with the two majorgeological events of the North China block. The sedimentary sources ofthis group evidently originated from the basement rocks of the NorthChina block.

The detrital zircon populations from schists of the middle-upperKuanping Group exposed in the northernmost Qinling orogen, havedominant age clusters at ~2.5 Ga and ~1000–900 Ma (Fig. 7),indicating that the material was derived from late Neoarchean andearly Neoproterozoic sources. As both the North China and SouthChina blocks contain late Archean basement rocks or detritus, itremains unclear from which the Neoarchean detrital zircons in theKuanping Group were derived. But, with the exception of somediabase dikes (~1000 Ma; Liu et al., 2006), Neoproterozoic rocks arescarce along the southern margin of the North China block, indicatingthat the North China block was tectonically inactive during this timeperiod (Zhai, 2010). In contrast, in the South China block, includingthe South Qinlingmicro-continental block, large-scale Neoproterozoictectono-magmatic activity is evident (e.g., Li et al., 2002, 2003a,b;Zheng et al., 2004; Chen et al., 2006; Ling et al., 2003, 2007; Yu et al.,2008; Sun et al., 2009; Zhu et al., 2008, 2009; Wang et al., 2010a,b).Besides the outcrops of the Grenvillian rocks (~1000 Ma) exposedalong the western margin of the South China block, e.g., theHuiqinggou granitic gneiss and contemporaneous deposits (Li et al.,2002) and the Laowushan Formation and the Kunyan Formation(Greentree et al., 2006), products associated with the Neoproterozoicevents (~720–850 Ma) occur in the southern Qinling orogen andalong the northern margin of the South China block (e.g., Li et al.,2003a,b; Yu et al., 2008; Sun et al., 2009; Wang et al., 2010a). TheGrenville-aged formations display a 50 Myr time difference from theearly Neoproterozoic depositional source of detritus in the KuanpingGroup.

Recent studies indicate a dominant phase of Neoproterozoicmagmatism in the Qinling Group of the North Qinling terrain.Abundant granitoid rocks formed between about 980 Ma and930 Ma, e.g., from west to east, the Lianghekou granite (Chen et al.,2004a, 2007), the Laoyu pluton (Lu et al., 2005), the Cai'ao granite(Zhang et al., 2004), the Niujiaoshan granite (Wang et al., 2002, 2005),the Dehe monzogranite (Chen et al., 2004b) and the Zhaigen pluton(Chen et al., 2006). These plutons are regarded as syn-collisional orpost-collisional intrusives following the about 1.3–1.0 Ga Grenvillianorogenic period (e.g., Zhang et al., 2004; Chen et al., 2004b, 2006,2007). In addition, a Sm–Nd whole-rock isochron age of 1.03 Ga,obtained from the Songshugou ophiolite, also represents theGrenvillian-aged event in the North Qinling terrain (Dong et al.,2008). Thus, it is reasonable to suggest that the early Neoproterozoicsource for the sedimentary rocks from the middle-upper KuanpingGroup has close affinity with the North Qinling terrain, rather than theNorth China block and the South China block.

5.3. Crustal evolution of the North Qinling terrain

Hf isotope composition of detrital zircons is a very powerful tool todecipher reworking and growth of continental crust (Wu et al., 2007and references therein). As shown below, together with detrital zirconU–Pb ages, they help us to constrain the crustal evolution of the NorthChina block and the North Qinling terrain during the Precambrian.

Analyses of Hf isotopic composition in Neoarchean detrital zircons(ca. 2.5 Ga) in sedimentary samples from the Guandaokou andKuanping Groups reveal that a sub- group of the grains have negativeεHf(t) values of −6.3 to −0.2 and Hf model ages of about 3.4 Ga to3.0 Ga, and the Paleoproterozoic detrital zircon grains (ca. 2.2 Ga to1.85 Ga) from the Guandaokou Group samples also show negative εHf(t) values of−8.9 to−1.9 and Hf model ages of about 3.0 Ga to 2.7 Ga,indicating the reworking of Archean crust (Wan et al., 2005a; Zhaiet al., 2005). This conclusion is consistent with the evidence providedby Nd isotopic analysis inWu et al. (2005). The oldest zircons found inthe two groups in this study have U–Pb ages of about 3.3 Ga to 3.0 Ga,εHf(t) values of −13.4 to −3.2, and Hf model ages of about 4.5 Ga to3.8 Ga and these isotopic features are similar to those of the oldestArchean rocks presently exposed in the North China block (Liu et al.,1992; Song et al., 1996; Wan et al., 2005b; Wu et al., 2008).

Half of Neoarchean and Paleoproterozoic zircon grains of theGuandaokou Group is characterized by positive εHf(t) values of up toabout +7, corresponding to Hf model ages (TDM2) of 2.8 Ga to 2.5 Ga.Several Paleoproterozoic detrital zircon grains of about 2.5 Ga to2.0 Ga from the Kuanping Group also yield positive εHf(t) values of upto +2.3 and Hf model ages (TDM2) of ~2.8–2.6 Ga. These values areconsistent with derivation from variously depleted mantle sources.

TheU–PbandHf zircondata fromtheKuanpingsedimentary sequencealso help to unravel the late Precambrian crustal evolution of the NorthQinling terrain. Detrital zirconU–Pb ages and in-situ zircon Lu–Hf isotopedata show that the major magmatic activity in the North Qinling terrainoccurred around ~1000–900Ma (Fig. 5) and this magmatismwas largelyproduced by reworking of early Paleoproterozoic crust with minorcontribution of late Paleoproterozoic juvenile material.

The oldest basement rocks exposed in the North Qinling terrainrange from 2.2 to 2.3 Ga (U–Pb zircon ages, Zhang et al., 1994c),whereas amphibolites have yielded a Sm–Nd isochron age of ~2.0 Ga(Zhang et al., 1994b). Previous studies have demonstrated that theNorth Qinling terrain is distinguished from the South Qinling terrainand the South China block by the absence of Archean basement rocks(e.g., O'Yang and Zhang, 1996; Zhang et al., 2001; Lu et al., 2006).But, the oldest zircon ages from the Kuanping Group presented hereshow the existence of recycled Archean material in the North Qinlingterrain.

Early Neoproterozoic to Mesoproterozic detrital zircons of theKuanping Group from the North Qinling terrain yield negative εHf(t)

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values (mostb−10), corresponding with Hf model ages from 2.5 Gato 2.8 Ga (Fig. 8). Noticeably, two magmatic zircons with U–Pb ages of615 Ma and 1083 Ma have Hf isotopic compositions that suggest theparental magmas were derived from old sources of 3.2 Ga to 3.4 Ga.Therefore, it could be concluded that Archean basement rocks beneaththe North Qinling terrain were reworked during Neoproterozoic andMesoproterozoic times to contribute to the detrital zircon population.

The majority of zircons grains from the Kuanping Group define anarrow age cluster at ca. 1000–900 Ma (Figs. 4 and 7). These grainshave negative εHf(t) values ranging from −10.8 to −1.6 (Fig. 5),corresponding to Hf model ages (TDM2) of 2.5–2.0 Ga. The large agedifference between Hf model ages and zircon crystallization ages canbe readily explained by reworking of early Paleoproterozoic crust.Some detrital zircons of primary magmatic origin have relatively highor positive εHf(t) values (−0.3 to +5.3), with Hf model ages rangingfrom ca. 1.85 to 1.55 Ga. A remarkable positive εHf(t) value of +10.9was obtained for a zircon grain from sample 08HN60. This zircon hasan indistinct magmatic structure and a Th/U ratio of 0.68 (Fig. 2). Thecorresponding Hf model age is 1.73 Ga, which is equal to itscrystallization ages. Accordingly, when the Hf model ages closelyapproaches the U–Pb age of the detrital zircon, geological processesthat led to the accretion of new juvenile crust are likely to haveoccurred (e.g., Wu et al., 2007). Hence, these grains reflect derivationfrom magma that contains juvenile material derived from the mantleduring the late Paleoproterozoic.

The majority of Neoproterozoic detrital zircons of the KuanpingGroup have negative εHf(t) values in the range −12.5 to −4.8,corresponding to TDM2 Hf model ages of 2.5–2.0 Ga, and the minorityhave relatively high εHf(t) values ranging from −1.7 to −0.3,corresponding to TDM2 Hf model ages of ca. 1.7 Ga, indicating thatboth ancient crust and juvenile igneous material had contributed tothe magma sources.

Summarizing, it can be concluded that themajormagmatic activity inthe North Qinling terrain occurred around 1000–900Ma (Fig. 7). Thismagmatismwas largely produced by reworking of early Paleoproterozoiccrust with minor contributions of late Paleoproterozoic juvenile material.The U–Pb ages and Hf data of detrital zircons presented in this studydemonstrate that no juvenile material contributed to the magmasources of the about 1000–900Ma magmatic rocks for the KuanpingGroup.

Fig. 8. Plot of TDM2(Hf) values vs. U–Pb ages of zircons, showing different materialsources between the Kuanping Group and the Guandaokou Group.

5.4. Implication for Precambrian evolution of the southern North Chinablock

The detrital zircons of igneous origin in meta-sediments of theGuandaokouand theKuanpingGroupsprovide twoU–Pbagepeaks, onearound ~2.5–2.4 Ga and another around ~1.85 Ga (Fig. 5). This agedistribution is reminiscent to that of theNorthChinablock,withpeaks at2.6–2.45 Ga and 1.95–1.75 Ga. These age peaks constrain the mostimportant tectonothermal events in the evolution of the craton and theyare considered to be related to Precambrian supercontinental cycles(Zhao et al., 2001b, 2002a; Zhai et al., 2000,2005; Rogers and Santosh,2009; Santosh et al., 2006, 2007a,b, 2009a,b, 2010).

However, U–Pb ages of about 1000–900 Ma obtained from detritalzircons of the Kuanping Group indicate a 100-million year period ofmagmatic activity along the northern margin of the North Qinlingterrain during the early Neoproterozoic. There is abundant evidencefor Neoproterozoic (~980–930 Ma) graniticmagmatism in the QinlingGroup (e.g., Lianghekou granite, Laoyu pluton, Cai'ao granite,Niujiaoshan granite, Dehe monzogranite, Zhaigen pluton; Wang et al.,2001, 2002, 2005; Zhang et al., 2004; Chenet al., 2004a,b, 2006, 2007; Luet al., 2005, 2006; Yang et al., 2010). The remaining zircon data from theKuanping Group is best explained in terms of reworking of earlyPaleoproterozoic crustal material (~2.5–2.0 Ga), different from that ofthe North China block (Fig. 8).

Accordingly, it can be concluded that the North Qinling terrain andthe North China block are characterized by different geologicalhistories. It seems that during the Precambrian, the North Qinlingterrain was indeed a separate micro-continent (O'Yang and Zhang,1996; Dong et al., 2003). The conclusion that the North Qinling andthe North China block collided after 640 Ma can be drawn from thedating results of the youngest detrital zircons from the middleKuanping Group. The Sinian sequences, composed of shallow-marinecarbonates and siliciclastic deposits, are widely distributed along thesouthern margin of the North China block (Zhai et al., 2003), and canbe compared with coeval strata (b640 Ma) from the sedimentaryfacies of the upper Kuanping Group and the Taowan Group (Fig. 6).This sedimentary record testifies that the North Qinling terrain andthe North China block were assembled no early than 640 Ma.

Meta-gabbros and serpentinized peridotites associated with theKuanping Group have been interpreted as a Grenvillian-age ophiolite(Wei et al., 1999; Meng and Zhang, 1999). This would be in line withthe subduction of oceanic lithosphere beneath the North Qinlingterrain, and a period of major magmatism that actually has beenestablished between 1000 Ma and 900 Ma indicated by zircon Hfanalysis. Sedimentation of the middle Kuanping Group started after640 Ma, demonstrating a temporal hiatus of ~260 Myr betweenterrain assembly and the onset of sedimentation.

6. Conclusions

The youngest detrital zircon U–Pb ages constrain a maximumdeposition age of ~1.85 Ga for the Guandaokou Group (lowest coversequence along the southern margin of the North China block), and~640 Ma for the Kuanping Group, the basement sequence along thenorthern margin of the North Qinling terrain. Peak U–Pb age values ofdetrital zircons around ~2.5–2.4 Ga and ~1000–900 Ma for theKuanping Group suggest a mixed provenance with material derivedfrom the North China block and the North Qinling terrain.

Major magmatic activity in the North Qinling terrain occurredaround 1000–900 Ma and the magmatic rocks constitute a majorcomponent of reworking of the early Paleoproterozoic crust withminor contribution of late Paleoproterozoic juvenile material.

Detrital zircon U–Pb and Hf data from the sediments of theGuandaokou and Kuanping Groups accentuates the disparate mag-matic and geochemical character of the North Qinling terrain and the

202 X.-Y. Zhu et al. / Gondwana Research 20 (2011) 194–204

North China block. The final accretion and assembly of these blockstook place after about 640 Ma.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.gr.2010.12.009.

Acknowledgements

This studywas financially supported by theMinistry of Science andTechnology of China (Project No. 2009CB219305) and the NationalNature Science Foundation of China (NSFC project nos. 40525007 and40973042). Sincere thanks are due to M. Santosh, L.-C. Zhang and tworeviewers for assistance during the elaboration of this work. K.-J. Hou,Y.-B. Zhang, L.-L. Du, and S. Zhang are thanked for help during theanalyses.

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