Gondwana Research 19 (2011) 881–893

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U-Pb and 40Ar/39Ar geochronological constraints on the exhumation history of theNorth Qinling terrane, China

Yunpeng Dong a,b,⁎, Johann Genser b, Franz Neubauer b, Guowei Zhang a, Xiaoming Liu a,Zhao Yang a,c, Bianca Heberer b

a State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, Chinab Department of Geography and Geology, University of Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austriac Geowissenschaften, Technische Universitat Bergakademie Freiberg, D-09599 Freiberg, Germany

⁎ Corresponding author. State Key Laboratory of ContinGeology, Northwest University, Xi'an 710069, China. Tel.:+88303531.

E-mail address: dongyp@nwu.edu.cn (Y. Dong).

1342-937X/$ – see front matter © 2010 International Adoi:10.1016/j.gr.2010.09.007

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 July 2010Received in revised form 16 September 2010Accepted 17 September 2010Available online 1 October 2010

Handling Editor: W.J. Xiao

Keywords:U-Pb and 40Ar/39Ar geochronologyCooling ageExhumationNorth Qinling terrane

The amphibolite facies grade North Qinling metamorphic unit forms the centre of the Qinling orogenic belt.Results of LA-ICP-MS U-Pb zircon, 40Ar/39Ar amphibole and biotite dating reveal its Palaeozoic tectonic history.U-Pb zircon dating of migmatitic orthogneiss and granite dykes constrains the age of two possible stages ofmigmatization at 517±14Ma and 445±4.6 Ma. A subsequent granite intrusion occurred at 417±1.6 Ma. The40Ar/39Ar plateau ages of amphibole ranging from 397±33Ma to 432±3.4 Ma constrain the cooling of theQinling complex below ca. 540 °C and biotite 40Ar/39Ar ages at about 330–368 Ma below ca. 300 °C. The ages areused to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling ata rate b2.5 °C/Maduring the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from397 Ma to368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay inexhumation after peakmetamorphic conditions and a long period of tectonic quiescence after the suturing of theNorthChina and South Chinablocks along the Shangdan suture. These relationships exclude classical exhumationmodels of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peakconditions of metamorphism.

© 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

The Qinling orogen is a part of the Qinling–Dabie mountain range(Fig. 1), which was formed by the collision of the North China and SouthChina blocks along the Shangdan suture (e.g. Mattauer et al., 1985; Hsuet al., 1987; Zhao and Coe, 1987; Xu et al., 1988; Zhang et al., 1991; Enkinet al., 1992; Okay, 1993; Kröner et al., 1993; Li, 1994; Li et al., 1993; Ameset al., 1996; Hacker et al., 1998; Zhai et al., 1998; Meng and Zhang, 1999;Faure et al., 2001; Ratschbacher et al., 2003; Tseng et al., 2009). Duringthe last decade, extensive investigations revealed the existence ofanother suture zone, called the Mianlue suture, along the southernmargin of the Qinling–Dabie belt (Zhang et al., 1995a,b, 1996, 2000; Liet al., 1996; Liu et al., 2001; Xu et al., 2002; Li et al., 2007a). Therefore, theQinling–Dabie orogen and its surrounding area can be structurallysubdivided, from north to south, into the Southern North China block(S-NCB), the North Qinling terrane (NQT), the Shangdan suture (SDS),the South Qinling microcontinent block (SQB), the Mianlue suture (MLS)and the South China block (SCB) (Zhang et al., 1995a,b, 2000). A numberof models for the tectonic evolution of the Qinling terrane have been

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proposed (Meng and Zhang, 1999; Faure et al., 2001; Ratschbacher et al.,2003). Controversies, however, still exist, in particular on the timing ofthe collision and the processes of convergence between the North andSouth China blocks along the Shangdan suture zone.

Some authors suggested an Early Paleozoic collision between theNorth and South China blocks (Mattauer et al., 1985; Xu et al., 1988; Renet al., 1991;Kröner et al., 1993; Zhai et al., 1998),whereasGaoet al. (1995)argued that the geochemistry of Devonian fine-grained sediments in thesouthern Qinling belt indicates a collision of Silurian-Devonian age. Yinand Nie (1993) argued that the collision between the North and SouthChinablocksbeganby the interdigitationof thenorth-easternSouthChinablock into the south-easternNorthernChinablock in the LatePermianandthe process continued until Late Triassic times. Based on the formation ofultrahigh-pressure metamorphic rocks in the easternmost part of theQinling–DabieBelt at ~230Ma(e.g., Li et al., 1993;Okay, 1993;Ameset al.,1996; Hacker et al., 1998; Katsube et al., 2009), various Late Triassiccontinent–continent collision models have been proposed for the region,as well as for establishing correlations with the adjacent regions (Hsuet al., 1987; Li, 1994; Oh et al., 2009; Seo et al., 2010). Paleomagnetic datafavoured a Late Triassic–Middle Jurassic collision of the North China andSouth China blocks (Zhao andCoe, 1987; Enkin et al., 1992). Ratschbacheret al. (2003, 2006), Li et al.(2010a,b) and Liu et al. (2011) favoured aPaleozoic and a Mesozoic ages of collisions along the Shangdan andMianlue sutures, respectively. Most researchers believe that the collision

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882 Y. Dong et al. / Gondwana Research 19 (2011) 881–893

between the North and the South China blocks occurred after the closureof the Shangdan Ocean. However, an increasing body of data is notconsistent with simple collision models.

This paper reports new U-Pb ages of zircon and 40Ar/39Ar ages ofhornblende and biotite from magmatic and metamorphic rocks fromthe NQT. Based on this new dataset, we discuss the exhumation andcooling history of the NQT in order to advance the understanding ofthe tectonic processes during convergence between the North andSouth China blocks along the Shangdan zone. It shows that cooling ofthe North Qinling terrane was an unusually slow process, whichcannot be explained by classical exhumation models operative withinthe other centre of continent-continent collisional orogens.

2. Geological setting

The Qinling mountain range lies between the North and SouthChina blocks (Fig. 1), bounded on the north by the Lushan fault and onthe south by the Mianlue–Bashan–Xiangguang fault (Dong et al.,2008a). The Lushan fault is an intra-continental thrust along whichthe North Qinling terrane was thrust onto the southern margin of theNorth China block and formed during the Mesozoic–Cenozoic. TheMianlue–Bashan–Xiangguang fault is also an overthrust along whichthe South Qinling belt was emplaced onto the South China block(Dong et al., 2008a). The existence of two sutures is well documented,i.e. the Shangdan suture in the north and the Mianlue suture in thesouth (Zhang et al., 1995b; Li et al., 2009, 2010a,b). A large number ofgeochemical and geochronological studies suggest that the Mianluesuture zonewas evolved due to the closure of a northern branch of thePaleo-Tethyan Ocean, which separated the South Qinling micro-continental block from the South China block during Devonian toMiddle Triassic times (Zhang et al., 1995a,b, 2000; Li et al., 1996;2007a; Xu et al., 2002; Dong et al., 1999, 2004). Subsequently, it wasoverprinted by the overthrust of the Mianlue–Bashan–Xiangguangfault during the Late Jurassic–Early Cretaceous (Zhang et al., 2000;Dong et al., 2008a).

2.1. Southern sectors of the North China block

The southern sectors of the North China block consist mainly ofamphibolite facies metamorphosed Archean–Palaeoproterozoic base-ment complexes (Zhang et al., 2000) and weakly metamorphosed basicvolcanic and sedimentary cover sequences ranging in age fromMesoproterozoic to Mesozoic. Mesozoic Granitic intrusions are abun-dant, and the region also underwent intra-continental deformationduringMesozoic–Cenozoic times (Zhang, 1989; Zhang et al., 1995a; Xu etal., 1988; Ren et al., 1991).

2.2. North Qinling terrane

The North Qinling terrane is bounded by the Luonan–Luanchuan fault(LLF) on the north and the Shangdan suture (SDS) on the south (Fig. 1). Itcomprises predominantly several lenticular Palaeoproterozoic crystallinebasement units, overlying Meso-Neoproterozoic volcano-sedimentaryrocks, late Mesoproterozoic ophiolites, and Neoproterozoic–Paleozoicvolcano-sedimentary assemblages. These units underwent amphibolitefaciesmetamorphism at ~1.0 Ga, followed by retrogression to greenschistfacies conditions at ~400 Ma (Chen et al., 1991; Liu et al., 1993; Zhanget al., 1994a,b) and are locally covered unconformably by Carboniferous-Permian clastic deposits. From north to south, the main rock units in thisbelt are the Kuanping, Erlangping, and Qinling Groups, and theSongshugou ophiolite, which are separated from each other by thrustfaults or ductile shear zones (Fig. 1).

The Kuanping Groupmainly comprises greenschists, amphibolites,quartz-micaschists, gneisses and marbles. The protoliths of both thegreenschists and amphibolites were tholeiitic basalts with N-MORBand T-MORB geochemical characteristics (Zhang and Zhang, 1995).

Sm-Nd whole-rock isochron ages of these metabasalts range from0.94 to 1.2 Ga (Zhang et al., 1994a,b).

The Erlangping Group is composed of an ophiolitic unit, clasticsedimentary successions and carbonates. The ophiolitic unit containssparse ultramafic rock, massive basalt, pillow basalt, and rareintercalations of radiolarian chert. The geochemistry of the basaltsuggests formation in a back-arc basin setting (Sun et al., 1996). Thefindings of Lower to Middle Ordovician radiolarians within the chertsconfirm that the back-arc basin existed during Early Palaeozoic times(Wang et al., 1995).

The Qinling Group is composed of gneiss, marble and amphibolite,whose protoliths were clastic rock, limestone (You and Suo, 1991) andinterlayer of continental tholeiitic lavas, respectively (Zhang et al., 1994a,b).U-Pb isotopic agesof zircon fromgneisses range from2172 to2267 Ma,whereas the Sm-Nd whole-rock isochron age of the amphibolites (meta-tholeiites) is 1987±49Ma (Zhang et al., 1994a,b). Further age dataindicates that the Qinling Group is a Paleoproterozoic complex, whichunderwent amphibolite facies metamorphism at 990±0.4 Ma and agreenschist facies metamorphic overprint at ca. 425±48Ma (Chen et al.,1991).

The Songshugou ophiolite consists primarily of amphibolite faciesmafic and ultramafic rocks. The geochemistry suggests predominantE- and T-MORB affinities, indicative of the initial stage of an oceanicspreading centre (Dong et al., 2008b). Combined with regionalcorrelations, abundant isotopic age data reveal that the ocean evolvedbetween 1.4 and 1.0 Ga (Li et al., 1993; Zhang et al., 1994a,b; Chenet al., 2002; Dong et al., 2008b).

2.3. Shangdan suture

The Shangdan suture zone is defined by a linear, patchy distributionof tectonic and ophiolitic melanges and arc-related volcanic rocks. Theseunits were overprinted by a series of ductile shear zones and brittle faultsystems, and were intruded by subduction- and collision-relatedgranitoids. The ophiolite- and subduction-related volcanic rocks are themost important members of the Danfeng Group, which were over-printed by greenschist to lower amphibolite facies metamorphicassemblages. The geochemistry indicates that the metamorphosedcalc-alkaline basalts, some massive and some pillow lavas were formedin a typical intra-oceanic island-arc setting (Zhang et al., 1994a,b). Themetamorphosed tholeiitic basalts were generated at a mid-ocean ridge,as evidenced by a slight depletion of light rare earth elements (Donget al., 2010). The U-Pb zircon ages of gabbros from ophiolites within thewestern part of the suture range from 530 to 471 Ma (Yang et al., 2006;Pei et al., 2007; Li et al., 2007b). These isotopic ages consistent with theOrdovician to Silurian age of radiolarian from the interlayer cherts withinthe Danfeng ophiolite in the Guojiagou area (Cui et al., 1996).

2.4. South Qinling block

Unlike the thick-skinned structures of the North Qinling terrane,the South Qinling belt is of thin-skinned nature, characterized bysouth-vergent thrusts and folds showing an imbricated thrust-foldsystem (Zhang et al., 2000). The basement of the South Qinling beltcontains several Precambrian complexes (e.g. Xiaomoling, Douling,Tongbai-Dabie, Foping and Yudongzi complexes), all of which containMeso- to Neoproterozoic rift-type volcano-sedimentary assemblagesmetamorphosed under greenschist facies conditions (Zhang et al.,1995a). The sedimentary cover includes Sinian clastic and carbonaterocks, Cambrian-Ordovician limestones, Silurian shales, and Devonianto Carboniferous clastic rocks and limestones. A few remnants ofUpper Palaeozoic–Lower Triassic clastic sedimentary rocks are alsopresent in the northern part of the South Qinling belt (Zhang et al.,2000).

Fig. 1. (a) Simplified geological map of the Northern Qinling belt. (b) Insert shows location within China.

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3. Sample descriptions

All the samples presented in this study are from the same outcropof the Qinlingmetamorphic complex unit of the NQT in the Qingyouhearea (Fig. 1). At this location, gneisses of the Qinling Group areexposed (Fig. 2a) intercalated with lenticular amphibolites, andgarnet-amphibolites distributed parallel to the foliation of the bandedgneisses (Fig. 2b). The gneisses were migmatitized as indicated byhighly deformed lenticular bodies of granitoids within the foliation(Fig. 2a). Granitic dykes intruded the gneiss, which are now parallel tothe foliation of the gneiss, and folded together with the gneisses(Fig. 2c). These dykes can be distinguished from the first generationgranitoids by their sharp contacts. The third generation of granitoidsintruded into the gneiss and cut the foliation of the migmatitic gneiss(Fig. 2d).

Samples Qy-01 and 027 N-275 were taken from the amphibole-gneiss,whereas sample 027 N-266and027 N-271were collected fromalenticular garnet-amphibolite body within the migmatitic gneiss.Sample Qy-04 came from first generation granitoids (migmatiticgranite) within the migmatitized gneiss, whereas samples Qy-02 and027 N-289 were from the host gneisses. The second and thirdgenerations of granite intrusions were represented by samples Qy-06and Qy-03, respectively.

4. Analytical techniques

High spatial resolutionU-Th-Pb andREEwere obtained using the laserablation inductively coupled plasma mass spectrometer (LA-ICP-MS) atthe State Key Laboratory of Continental Dynamics, Department ofGeology, Northwest University, Xi'an, China. Each sample consisted of20 kg of material was crushed into powder, and then washed and dried.Zircons were separated by heavy-liquid and magnetic methods and thenhandpicked under a binocular microscope. The internal texture of zirconswas examined using cathodoluminescence (CL) images. Zircons weredated in-situ on the LA-ICP-MS. Trace element (REE, Lu, Hf, Ta, Nb, Th, Ti

and P) compositions were simultaneously collected from the same laserablation spot. The laser-ablation system used is a GeoLas 200 M equippedwith a 193 nm ArF-excimer laser, and a homogenizing and imagingoptical system(MicroLas,Göttingen,Germany). Analyseswereperformedon the ELAN 6100 ICP-MS from Perkin Elmer/SCIEX (Canada) with adynamic reaction cell (DRC). The laser ablation spot size is approximately40 μm. 207Pb/206Pb, 206Pb/238U, 237Pb/235U and 208Pb/232Th ratios werecalculated using GLITTER 4.0 (Macquarie University), and were correctedfor both instrumental mass bias and depth-dependent elemental andisotopic fractionation using Harvard zircon 91500 as the externalstandard. The ages were calculated using ISOPLOT 3 (Ludwig, 2003).Thedetailed analytical procedureof age and trace elementdeterminationsof zircons canbe found inYuanet al. (2004). CommonPbcorrectionsweremade following the method of Anderson et al. (2002).

Laser-probe 40Ar/39Ar analysis was carried out at the ARGONAUTLaboratory of the Geology Division at the University of Salzburg. Fouramphibole and three biotite concentrates were separated from themetamorphosed Qinling Group, and were irradiated in the MTAKFKIreactor (Debrecen, Hungary). 40Ar/39Ar analysis was carried out usingan UHV Ar–extraction line equipped with a combined MERCHANTEK™UV/IR laser ablation facility, and a VG–ISOTECHTM NG3600 massspectrometer. Stepwise heating was performed using a defocused(~1.5 mm diameter) 25W CO2–IR laser operating in Tem00 mode atwavelengths between 10.57 and 10.63 μm. Isotopic ratios, ages anderrors for individual steps were calculated following the suggestions ofMcDougall and Harrison (1999) and applying decay constants reportedby Steiger and Jaeger (1977). Definition and calculation of plateau ageshas been carried out using ISOPLOT/EX (Ludwig, 2003). Correctionfactors for interfering isotopes have been calculated from 10 analysesof two Ca-glass samples and 22 analyses of two pure K-glass samples,and are: 36Ar/37Ar(Ca)=0.00026025, 39Ar/37Ar(Ca)=0.00065014, and40Ar/39Ar(K)=0.015466. Variations in the neutron flux weremonitoredwith DRA1 sanidine standard for which a 40Ar/39Ar plateau age of25.03±0.05 Ma has been reported (Wijbrans et al., 1995; vanHinsbergen et al., 2008), and errors on the ages are 1σ inter-laboratory.

Fig. 2. Field photographs showing mutual geological relationships of data rocks. (a) the migmatitic granite was foliated together with the host gneisses and the samples locations;(b) migmatite intercalated with lenticular amphibolites and garnet-amphibolites, which distribute parallel to the foliations of the banded gneisses; (c) granitic dyke intruded thegneiss, parallel to the foliation of the gneiss, and folded together with the gneisses; (d) the third generation of granitoids intruded into the gneiss and cut the foliations of themigmatitic gneiss.

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5. Results

5.1. Zircon U-Pb dating

Results of U-Pb single zircondating of three samples are presented ine-component Table 1 and Fig. 3. Trace element compositions of singlezircons analyzed in-situ by LA-ICP-MS are listed in e-component Table 2.The cathodoluminescence (CL) images of representative zircons foreach sample are shown in Fig. 4.

Zircons from the migmatitic granites (sample Qy-04), which werefoliated together with the host gneisses (Fig. 2a), yield several groupsof 206Pb/238U ages. The CL images display rounded, inherited cores andthin recrystallized rims (Fig. 4a). There is an opaque or translucenttransitional belt between the cores and the rims. The rims of zirconsdisplay variable 206Pb/238U ages ranging from 433 to 525 Ma but withtwo discrete populations of grains recording with weighted meanages of 455±4.5 Ma (MSWD=0.46) and 517±14 Ma (MSWD=14)(Fig. 3a).

Zircons from the granitic dyke (second generation granitoids)(sample Qy-06) (Fig. 2c), oriented parallel to the foliation of the hostgneisses, also exhibit well-developed crystal faces with length/widthratios ranging from2:1 to 3:1. Conspicuousmagmatic oscillatory zoningis observed in CL images (Fig. 4b). Some zircons display inherited cores.Eight spot analyses on the rims yield 206Pb/238U ages ranging from 435to 454 Ma with a weighted mean age of 445±4.6 Ma (MSWD=2.9)(Fig. 3b).

Zircons from the third generation granite (sample Qy-03), which cutsthe foliation of the gneisses (Fig. 2d), exhibit well-developed crystal faceswith length/width ratios ranging from 3:1 to 4:1. Transparent totranslucent, colourless to pale pink grains showing a well developedmagmatic oscillatory zoning in CL images are dominant (Fig. 4c). Thesezircons yield 206Pb/238Uages ranging from409 to422 Mawith aweighted

mean age of 417±1.6 Ma (MSWD=0.90), which is in accordancewith alow intercept age of 420±3.6 Ma (MSWD=0.49) (Fig. 3c). Three spotanalyses in the core of the zircons yield 206Pb/238U ages ranging from 902to 915 Ma with a weighted mean age of 908±7.3 Ma (MSWD=1.18).

5.2. Geochemistry of zircons

Element composition of zircons analyzed from the three granitegenerations shows variable Th/U ratios (Table 1 and Fig. 5). All theTh/U data from the rims of sample Qy-04 show lower Th/U ratios(b0.2), and ten of the sixteen measurements show a Th/U ratio b0.1(Fig. 5a), while their 206Pb/238U apparent ages ranging from 433 to523 Ma (Fig. 3a). The Th/U ratios of zircons from sample Qy-04 arelower than that of the sample Qy-03, which should be derived from atypical magmatic origin. In addition to, the zircons from sample Qy-04display clearly higher Hf, Yb and U, and lower Th contents than thirdgeneration granitic intrusion (sample Qy-03) as illustrated in Fig. 6.

Zircons from sampleQy-06with 206Pb/238U apparent ages from435 to454 Ma showTh/U ratios ranging from0.04 to 0.26, andmostly have Th/UratioN0.1 (Fig. 5b). The zircons from the third generation of granite(sample Qy-03) with U-Pb ages ranging from 409 to 422 Ma exhibit Th/UratiosN0.36 (Fig. 5c). Additionally, two of the three 902–915Ma coreshave Th/U ratios N0.2, whereas the other has a Th/U ratio of 0.13 but thegrain is unlikely to be of metamorphic origin.

Zircons with reliable weighted mean U-Pb ages ranging from 502 to525 Ma in sample Qy-04, 435 to 454 Ma in sample Qy-06, and 409 to422 Ma in sample Qy-03 exhibit perfectly positive Ce anomalies andnegative Eu anomalies in C1 chondrite-normalizedREEpatterns (Fig. 7a,c and e). However, zircons from sample Qy-04 with U-Pb ages rangingfrom 597 to 702 Ma (Fig. 7b), and zircons fromQy-03with U-Pb ages of669 and 905 (Fig. 7f) have no obvious positive Ce anomalies and

Fig. 3. U-Pb diagrams of dated zircons. Insets show details of zircon grains used forcalculation of ages.

885Y. Dong et al. / Gondwana Research 19 (2011) 881–893

negative Eu anomalies in C1 chondrite-normalized REE diagrams(Fig. 7b and f).

5.3. 40Ar/39Ar dating

40Ar/39Ar dating of amphibole and biotite concentrates, eachrepresenting 10–15 grains, from the metamorphic rocks wasperformed in order to constrain the exhumation history of the Qinlingmetamorphic complex subsequent to migmatization. The 40Ar/39Ar

analyses for all minerals from the representative samples are listed ine-component Table 3 and are graphically shown in Figs. 8 and 9.

5.3.1. AmphiboleAll amphiboles from the garnet–amphibolite (samples 027 N-266

and 027 N-271) and amphibolite gneiss (samples 027 N-275 and027 N-289) yield plateau ages of about 400 Ma (Fig. 8). Theamphiboles from sample 027 N-271 display two steps with a largespectrum, giving plateau ages of 409±5.5 Ma and 410±26 Ma(Fig. 8a) which are defined by 35% and 50% of total 39Ar released forintermediate- and high-temperature argon release steps, respectively.Their Ca/K (=1.78*37Ar/39Ar) ratios show large variations withsuccessive heating steps (Fig. 8a). The higher apparent age andlower 37Ar/39Ar ratios during the first heating steps show significantexcess 40Ar. An average age of 410±10 Ma is calculated from thesetwo steps.

The amphiboles from the second sample of garnet–amphibolite(027 N-266) yield two plateau ages of 486±16 Ma and 432±3.4 Ma(Fig. 8b) which are separated by particularly old apparent ages inintermediate-energy release steps. The plateau ages are defined by85% of total 39Ar released for successive steps (Fig. 8b).

The amphibole concentrate from the amphibole-gneiss (sample027 N-275) displays a fairly flat age spectrum with a well-definedplateau, giving an age of 397±33 Ma (Fig. 8c) with over 95% 39Arreleased, The Ca/K (=1.78*37Ar/39Ar) ratios show little variationwith successive heating steps (Fig. 8c).

The amphibole from amphibole gneiss sample 027 N-289 has asimilar 40Ar/39Ar age spectrum to sample 027 N-271, which ischaracterized by two plateau ages of 412±32 Ma and 396±12 Mabeing separated by abnormal apparent ages in the intermediate argonrelease steps. An average age of 405±15 Ma was calculated with 80%of total 39Ar released (Fig. 8d).

5.3.2. BiotiteThe biotite from sample 027 N-271 displays a flat age spectrum

with an age of 368±1.6 Ma (Fig. 9a). It is defined by 60% of total 39Arreleased for thirteen successive intermediate- and high-temperaturesteps at 1σ level of uncertainty. The Ca/K (=1.78*37Ar/39Ar) ratiosshow little variation with successive heating steps (Fig. 9a). Asignificant second plateau of intermediate-temperature steps gives aplateau age of about 339±40 Ma. If the first plateau being considered,an average age of 365±4.6 Ma will be calculated (Fig. 9a).

The biotite concentrates of sample 027 N-275 and Qy-02 yield a fairlyflat age spectrumwith well-defined plateaus giving ages of 333±1.4 Ma(Fig. 9b) and 330±1.3 Ma (Fig. 9c), respectively. They are defined by 95and~100%of total 39Ar released formore than fourteen successiveheatingsteps at 1σ level of uncertainty. These ages are internally consistent withthe second 40Ar/39Ar plateau age of biotite of sample 027 N-271.

All the biotites fromvarious lithological units (e.g. sample 027 N-271of garnet-amphibolite, sample 027 N-275 of amphibolite-gneiss, andsample Qy-02 of gneiss) display similar biotite 40Ar/39Ar plateau agesranging from 330 to 340 Ma. Only biotites from the garnet-amphiboliteof sample 027 N-271 yield an older age of 368±1.6 Ma.

6. Discussion

6.1. Timing of tectono-magmatic events and the trace element geochemistryof zircons

U-Pb zircon dating of migmatitic orthogneiss and younger granitoidintrusions provides chronological constraints on tectono-magmaticevents and thus the tectonic evolutionary history of the Qinlingmetamorphic complex. The Th/U ratios are routinely used to distinguishthe origin of zircon (e.g. Maas et al., 1992). Th/U ratios in magmaticzircons from various rocks mostly range from 0.2 to 1.0, while zircons

Fig. 4. Cathodoluminescence images of dated zircon grains. The circles show the analyzed spots, and the numbers give the age results.

886 Y. Dong et al. / Gondwana Research 19 (2011) 881–893

that grew due to metamorphic events exhibit lower Th/U ratios (b0.1)(Williams and Claesson, 1987; Schiøtte et al., 1988; Kinny et al., 1990).

In this study, the zircons from the three different granitegenerations show variable Th/U ratios. The zircons from the thirdgeneration of granite with U-Pb ages ranging from 409 to 422 Ma,which is undeformed and crosscuts the gneissic foliation, exhibit Th/Uratios N0.36 (Fig. 5a and Table 1). These high Th/U ratios areconsistent with the CL images which show magmatic oscillatoryzoning in zircons (Fig. 4a), indicating for magmatic growth. Hence, weinterpret the weighted mean U-Pb age of 417± 1.6 Ma(MSWD=0.90) (Fig. 3a) as the magma crystallization age of thethird granitoid intrusion. Two of the three 902–915 Ma old zirconsalso have Th/U ratiosN0.2, whereas the third measurement yielded a

Th/U ratio of 0.13. However, a metamorphic origin is considered asunlikely, and zircons of unambiguously magmatic origin have beenshown to contain low Th/U ratios (Zeck andWhitehouse, 1999). Sinceall these ages are dated within the inherited cores of zoned zircons(Fig. 4a), it seams reasonable to infer their original growth duringtectono-magmatic events during 902 to 915 Ma.

Zircons from sample Qy-06 (206Pb/238U apparent ages 435–454Ma),separated from foliation-parallel granitic dykes (Fig. 2c) show Th/U ratiosranging from 0.04 to 0.26 (Fig. 5b). Compared to the well-developedcrystalmorphology, light color and high length/width ratios ranging from3:1 to 4:1 in sample Qy-03, the zircons fromQy-06 exhibit a relative darkcolor and a small length/width ratio ranging from 2:1 to 3:1 (Fig. 4b).These characteristics are similar to that of the zircons from themigmatitic

Fig. 5. Th/U vs. Age of dated zircon grains.

Fig. 6. Bivariate geochemical discrimination of dated zircon grains.

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granite sample Qy-04 (Fig. 4c). Eight spot analyses on the rims yieldmagmatic ages ranging from 435 to 454 Mawith aweightedmean age of445±4.6 Ma (MSWD=2.9) (Fig. 3b). Based on mapping of this locality,geological evidence indicates these granitic dykes were folded togetherwith the foliation of the host migmatitic gneiss. Granitic dykes areexposed as lenticular veins or dykes with a slight foliation at theiredges, by even texture and discontinuous boundaries between thedykes and the migmatitic orthogneiss. It is striking that a similar206Pb/238U weighted mean age of 455±4.5 Ma (MSWD=0.46) andlower intercept age of 446±38Ma (MSWD=27) was determined forthe migmatitic granite sample Qy-04 (Fig. 3c). Therefore, we suggestthat the granite dykes (sample Qy-06) formed at the final stage ofmigmatization at 445±4.6 Ma.

The Th/U ratios of zircon rims from sample Qy-04 are lower (b0.2,mostlyb0.1; Fig. 5b) than that of the sample Qy-03, to which amagmatic origin has been ascribed. 206Pb/238U apparent ages rangefrom 433 to 523 Ma (Fig. 5c). The CL images of sample Qy-04 reveal amorphology similar to sample Qy-06, but different from sample Qy-03(Fig. 4). The rims of zircons display two groups of 206Pb/238U agesranging from 433 to 525 Mawithweightedmean ages of 455±4.5 Ma(MSWD=0.46) and 517±14 Ma (MSWD=14) (Fig. 3c). Most ofthese zircons are concordant and yield an average age of 517±14 Ma,which we suggest as time of migmatization. As documented above,the weighted mean age of 455±4.5 Ma and lower intercept age of446±38 Ma of zircons from the migmatitic granite are concordant,within the error range, with the age of 445±4.6 Ma for sample Qy-06.

This is interpreted to represent the time of a last thermal episode ofthe migmatization of the North Qinling metamorphic complex.

Although it is used to discriminate metamorphic and magmaticzircons (Mojzsis and Harrison, 2002), the low Th/U ratio ofmetamorphic zircon can be caused by involvement of competingeffects of Th-rich minerals (e.g. monazite and allanite) (Williamset al., 1996; Rubatto et al., 2001). On the other hand, metamorphiczircons from high-grade metamorphic orthogneisses can show highTh/U ratios (Friend and Kinny, 2001; Möller et al., 2003) which likelypreserve protolith values (Möller et al., 2002). Meanwhile, someauthors also argued for exceptional cases of igneous zircon with lowTh/U ratios (Zeck and Whitehouse, 1999; Hidaka et al., 2002) and itseems difficult to distinguish between igneous and metamorphicorigins of zircons from the Th/U ratios alone (Hidaka et al., 2002). Tofurther elucidate the magmatic vs. metamorphic provenance of thezircons, trace element analyses were carried out. It provides furtherimportant geochemical information on the nature of the zircon sourcematerials (Ireland and Wlotzka, 1992; Maas et al., 1992; Guo et al.,1996; Poller et al., 2001; Wilde et al., 2001; Hidaka et al., 2002;Hokada and Harley, 2004). The Y and Hf contents of zircons can beused to discriminate the tectonic setting of rocks (Pupin, 1992), and

Fig. 7. Rare earth element patterns of dated zircon grains.

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the U, Th and REE concentrations also yield constraints on the age ofthe samples (Ashwal et al., 1999; Hoskin and Black, 2000).

Zircons from the migmatitic granite (Sample Qy-04) clearly showhigher Hf, Yb and U, and lower Th contents than most of the thirdgeneration granitic intrusion (sample Qy-03) (Fig. 6). These patternsare likely the result of competing growth of Th-rich minerals, which isindicated by presence of monazite within thin section.

In this study, most zircons with reliable weighted mean 206Pb/238Uages exhibit strong heavy REE enrichment, variable positive Ceanomalies and negative Eu anomalies in C1 chondrite-normalizedREE patterns (Fig. 7a, c and e). However, there still have some zirconsshowing weak positive Ce anomalies and negative Eu anomalies in C1chondrite-normalized REE diagrams (Fig. 7b and f), such as zirconsfrom sample Qy-04 have U-Pb ages ranging from 597 to 725 Ma(Fig. 7b), and zircons from Qy-03 with U-Pb ages of 615 and 915 Ma(Fig. 7f). Above all, zircons with the U-Pb ages ranging from 502 to525 Ma in sample Qy-04, 435 to 454 Ma in sample Qy-06, and 409 to422 Ma in sample Qy-03 exhibit perfectly positive Ce anomalies andnegative Eu anomalies in C1 chondrite-normalized REE patterns

(Fig. 7a,c,e). Whitehouse et al. (2005) report zircons from Archeanhigh-grade metamorphic rocks mostly without any Ce and Euanomalies in chondrite-normalized REE diagrams. If this is indeedthe case, the data indicate an age dependence of Ce and Eu anomaliesin the zircons, which will be potential tracers of magmatic andmigmatitic zircons.

It is notable that the three samples contain some NeoproterozoicU-Pb zircon ages ranging from 902 to 915 Ma in granite (sampleQy-03), 902 to 928 Ma in migmatitic granite (sample Qy-04) and861 Ma in granite dyke (sample Qy-06). All these ages were derivedfrom the inherited cores of zircons. Their Th/U ratios are mostly higherthan 0.2 (Fig. 5 and Table 1). This is in accordance with severalNeoproterozoic plutons that have been documented in the NorthQinling terrane, such as the Caiwa post-collisional granite (U-Pb zirconage of 889±10Ma; Zhang et al., 2004), the Niujiaoshan granite (U-Pbzircon age of 958±7 Ma; Wang et al., 2003), the Xilaoyu granodiorite(U-Pb zircon age of 955±5 Ma, Chen et al., 2006), the Zhaigen granite(U-Pb zircon age of 914±10 Ma; Chen et al., 2006), the Guanshangranite (U-Pb zircon age of 926±16 Ma; Chen et al., 2006), the Dehe

Fig. 8. 40Ar/39Ar release patterns and K/Ca ratio of amphibole from the Qinling complex.

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granite (U-Pb zircon age of 943±18Ma, Chen et al., 2004a,b), and theFangcheng alkali-syenite (U-Pb zircon age of 844±1.6 Ma; Bao et al.,2008). The migmatites have undergone partial melting, as evidencedby the presence of abundant leucosomes. Therefore, the ages of zirconcores are inferred to be record of Neoproterozoic magmatism (Wanget al., 2001, 2003), that occurred after the Grenville orogen (Dong et al.,2008b).

6.2. Exhumation of the Qinlng complex and tectonic implications

The radiometric geochronometers allow dating processes whichoperate over a wide temperature range from N850 °C (U-Pb zircon ) to40 °C (apatite U-Th/He) (Stuart, 2002). The 40Ar/39Ar isotope datingtechnique is not only widely used to date metamorphic and relateddeformational events, but also one of the most commonly appliedtools for assessing the cooling and exhumation history, and thetectonothermal evolution of orogenic belts. Since different mineralshave different blocking temperatures, it has become possible to datethe last cooling age through a specific blocking temperature of certain

common minerals, which potentially allows the reconstruction of thecooling history of a rock unit (Svenningsen, 2000).

As discussed previously, sample Qy-04 was collected frommigmatitic leucosome granite. In general, migmatization occurs overa wide span of temperatures (625–850 °C), however, the field andtextural evidence indicate that the migmatites in this study weredeformed or overprinted at a relatively low-temperature. For example,the migmatites were derived from the felsic gneisses with interlayeredamphibolites, which are the major lithological unit of the QinlingGroup. A little of leucosomes are parallel to the well-developedfoliation of migmatitic orthogneiss. Based on the thermodynamicmodels and calibrations for the Ti-in-zircon thermometer (Ferry andWatson, 2007), the temperature of migmatization are calculated asabout 630–700 °C. The U-Pb zircon age of 517±14Ma derived fromthe migmatitic leucosomes may present the major episode ofmigmatization at 630–700 °C.

Migmatization is the tectonothermal response of the northwardsubduction of the early Palaeozoic Qinling oceanic plate. Subduction isindicated by the presence of some metamorphosed calc-alkaline

Fig. 9. 40Ar/39Ar release patterns of biotite from the Qinling complex.

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basalts and massive and pillow lavas cropping out in the south of theNQL terrane (Dong et al., 2010). These rocks were shown to haveformed in a typical oceanic island-arc setting (Zhang et al., 1994a,b),as well as other metamorphosed tholeiitic basalts generated from amid-ocean ridge setting with slight depletion of LREE (Dong et al.,2010). The U-Pb zircon ages of the gabbros from the ophiolite in thewestern part of the Shangdan suture range from 530 to 471 Ma (Yanget al., 2006; Pei et al., 2007; Li et al., 2007b), consistent withradiolarian ages of intercalated cherts ranging from Ordovician toSilurian (Cui et al., 1996). Above the subduction zone, there are well-developed early Palaeozoic subduction-related gabbroic-granitoidintrusions, such as the Huichizi granite (450–485 Ma, U-Pb zircon ;Chen et al., 2008) and the Fushui gabbroic-dioritic intrusion(480–514 Ma, U-Pb zircon; Chen et al., 2004a,b; Su et al., 2004; Liet al., 2006) . Adakitic rocks from the North Qinling belt indicate thelower crust thickening after collision at ~430 Ma (Tseng et al., 2009).

The U-Pb age of 445±4.6 Ma from sample Qy-06 (granite dyke),which is consistent with the age of 455±4.5 Ma from sample Qy-04,limits the end of the migmatization. This dyke parallels to and is foldedtogether with the migmatitic foliation, and intruded by 417±1.6 Magranites. Bymeans of the Ti-in-zircon thermometer (Ferry andWatson,2007), we estimate the crystallized temperature of sample Qy-06 at ca.640–680 °C.

Four samples of amphibole from the garnet amphibolite (samples027 N-266 and 027 N-271), amphibolite gneiss (samples 027 N-275 and027 N-289), respectively, yield plateau ages ranging from 410±10Ma to397±33 Ma (Fig. 8). The blocking temperature of argon withinhornblende is well documented at ca. 540 °C (Phillips et al., 2007). It is,therefore, reasonable to interpret that the cooling of the Qinlingmetamorphic complex tobelow ca.540 °Coccurred at about410–397Ma.

Based on the blocking temperature of argon diffusion withinbiotite at ca. 300 °C (Phillips et al., 2007), the biotites from variouslithological units (e.g. sample 027 N-271 of garnet amphibolite,sample 027 N-275 of amphibolite gneiss, and sample Qy-02 of gneiss)display similar biotite 40Ar/39Ar plateau ages, ranging from 330 to340 Ma (Fig. 9) and represent the time of cooling of the Qinlingmetamorphic complex below ca. 300 °C at about 330–340 Ma.However, it is notable that the biotite from garnet amphibolite(sample 027 N-271) displays a fairly flat age spectrum giving an age of368±1.6 Ma (Fig. 9a) with more than 60% of total 39Ar released. Thisage is about 30 Ma older than the others ages obtained from biotite.We assume the age of 368±1.6 Ma to constrain the maximum ageestimate for cooling of the Qinling complex below ca. 300 °C. If indeed,it implies a period of slower cooling and exhumation between 368 and330 Ma.

Taking into account the U-Pb zircon and multiple mineral 40Ar/39Argeochronology, we propose that the main stage of slow cooling andrelated exhumation of the Qinling metamorphic complex has occurredduring Mid Palaeozoic times (Fig. 10). The migmatization at 630–700 °Coccurred sometime after 517 Ma and not later than 445 Ma. After445 Ma, slow cooling occurred until a temperature of 540 °C wasreached. Assuming simple linear cooling from about 630 °C at 445 Ma to540 °C at 410 (amphibole) Ma suggests a time-integrated cooling rateofb2.5 °C/Ma. It was then followed by rapid exhumation, with a coolingrate of 8 °C/Ma from ca. 540 °C at 397 Ma to 300 °C at 368 Ma, andsubsequently slow/non-cooling from 368 to 330 Ma.

These data show a significant delay in exhumation after peakmetamorphic conditions and a long period of tectonic quiescenceafter the suturing of the North China and South China blocks along theShangdan suture during Silurian–Early Devonian. These relationshipsexclude classical exhumation models of the metamorphic cores,which all imply rapid cooling after peak conditions of metamorphism.After a rapid exhumation in Middle Devonian, the North Qinlingterrane evolved into an unusually slow cooling process during LateDevonian andMiddle Carboniferous times, which cannot be explainedby classical exhumation models operative within the other centre of

continent–continent collisional orogens. The investigation along theMianlue suture zone revealed that the opening of the Mianlue Oceanoccurred in the Late Devonian and Middle Carboniferous time (Zhanget al., 2000). Therefore, we propose that the slow cooling process ofthe North Qinling terrane during Late Devonian and MiddleCarboniferous times could be related to the extensional geodynamicsof the Mianlue Ocean.

7. Conclusions

The results of the LA-ICP-MS U-Pb zircon dating from three samplesof granites and migmatitic leucosome (granite), and 40Ar/39Ar datingfrom four amphibole and three biotite concentrates yield the followingconclusions:

(1) U-Pb zircon dating of migmatitic orthogneiss (leucosome)and granite dyke bracket the age of migmatization between517±14 Ma (MSWD=14) to 445±4.6 Ma (MSWD=2.9).

Fig. 10. Cooling path of the Qinling terrane.

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Subsequent intrusion by granites occurred at 417±1.6 Ma(MSWD=0.90).

(2) Four amphibole 40Ar/39Ar plateau ages mostly range from410±10 Ma to 397±33 Ma and constrain the cooling of theQinling complex below ca. 540 °C at about ca. 400 Ma. Threebiotite 40Ar/39Ar ages indicate cooling of the Qinling complexexhumation below ca. 300 °C at about 330–340 or 368 Ma.

(3) On account of the U-Pb zircon and multiple mineral 40Ar/39Ardating, an exhumation model is proposed as: slow/non-exhumation during 517–445 Ma, a time-integrated coolingwith ratesb2.5 °C/Ma during the 445–410 Ma period, rapidexhumation with cooling rate of 8 °C/Ma from 397 Ma to368 Ma, and subsequently slow/non-cooling from 368 to330 Ma.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.gr.2010.09.007.

Acknowledgments

Yunpeng Dong would like to thank Brigitte Winklehner, GottfriedTichyandYongSun for their kindhelp. Financial support for this studywasjointly provided by the National Natural Science Foundation of China(grants: 40772140,40972140), the Eurasia-Pacific Uninet, the AustrianAcademic Exchange Service (OEAD), and MOST Special Fund from theState Key Laboratory of Continental Dynamics, Northwest University.

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