39Ar geochronological constraints on

GR-00542; No of Pages 13 Gondwana Research xxx (2010) xxx–xxx

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U-Pb and 40Ar/39Ar geochronological constraints on the exhumation history of the North 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 b c

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

a r t i c l e

i n f o

Article history: Received 22 July 2010 Received in revised form 16 September 2010 Accepted 17 September 2010 Available online xxxx Handling Editor: W.J. Xiao Keywords: U-Pb and 40Ar/39Ar geochronology Cooling age Exhumation North Qinling terrane

a b s t r a c t 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 of migmatization at 517 ± 14 Ma and 445 ±4.6 Ma. A subsequent granite intrusion occurred at 417 ± 1.6 Ma. The 40 Ar/39Ar plateau ages of amphibole ranging from 397 ± 33 Ma to 432 ± 3.4 Ma constrain the cooling of the Qinling complex below ca. 540 °C and biotite 40Ar/39Ar ages at about 330–368 Ma below ca. 300 °C. The ages are used to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling at a rate b 2.5 °C/Ma during the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture. These relationships exclude classical exhumation models of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peak conditions 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 South China blocks along the Shangdan suture (e.g. Mattauer et al., 1985; Hsu et al., 1987; Zhao and Coe, 1987; Xu et al., 1988; Zhang et al., 1991; Enkin et al., 1992; Okay, 1993; Kröner et al., 1993; Li, 1994; Li et al., 1993; Ames et 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). During the last decade, extensive investigations revealed the existence of another suture zone, called the Mianlue suture, along the southern margin of the Qinling–Dabie belt (Zhang et al., 1995a,b, 1996, 2000; Li et al., 1996; Liu et al., 2001; Xu et al., 2002; Li et al., 2007a). Therefore, the Qinling–Dabie orogen and its surrounding area can be structurally subdivided, 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 number of models for the tectonic evolution of the Qinling terrane have been ⁎ Corresponding author. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China. Tel.: +86 29 88303028; fax: +86 29 88303531. E-mail address: [email protected] (Y. Dong).

proposed (Meng and Zhang, 1999; Faure et al., 2001; Ratschbacher et al., 2003). Controversies, however, still exist, in particular on the timing of the collision and the processes of convergence between the North and South China blocks along the Shangdan suture zone. Some authors suggested an Early Paleozoic collision between the North and South China blocks (Mattauer et al., 1985; Xu et al., 1988; Ren et al., 1991; Kröner et al., 1993; Zhai et al., 1998), whereas Gao et al. (1995) argued that the geochemistry of Devonian fine-grained sediments in the southern Qinling belt indicates a collision of Silurian-Devonian age. Yin and Nie (1993) argued that the collision between the North and South China blocks began by the interdigitation of the north-eastern South China block into the south-eastern Northern China block in the Late Permian and the process continued until Late Triassic times. Based on the formation of ultrahigh-pressure metamorphic rocks in the easternmost part of the Qinling–Dabie Belt at ~230 Ma (e.g., Li et al., 1993; Okay, 1993; Ames et al., 1996; Hacker et al., 1998; Katsube et al., 2009), various Late Triassic continent–continent collision models have been proposed for the region, as well as for establishing correlations with the adjacent regions (Hsu et al., 1987; Li, 1994; Oh et al., 2009; Seo et al., 2010). Paleomagnetic data favoured a Late Triassic–Middle Jurassic collision of the North China and South China blocks (Zhao and Coe, 1987; Enkin et al., 1992). Ratschbacher et al. (2003, 2006), Li et al.(2010a,b) and Liu et al. (2010) favoured a Paleozoic and a Mesozoic ages of collisions along the Shangdan and Mianlue sutures, respectively. Most researchers believe that the collision

1342-937X/$ – see front matter © 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2010.09.007

Please cite this article as: Dong, Y., et al., U-Pb and 40Ar/39Ar geochronological constraints on the exhumation history of the North Qinling terrane, China, Gondwana Res. (2010), doi:10.1016/j.gr.2010.09.007

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Y. Dong et al. / Gondwana Research xxx (2010) xxx–xxx

between the North and the South China blocks occurred after the closure of the Shangdan Ocean. However, an increasing body of data is not consistent with simple collision models. This paper reports new U-Pb ages of zircon and 40Ar/39Ar ages of hornblende and biotite from magmatic and metamorphic rocks from the NQT. Based on this new dataset, we discuss the exhumation and cooling history of the NQT in order to advance the understanding of the tectonic processes during convergence between the North and South China blocks along the Shangdan zone. It shows that cooling of the North Qinling terrane was an unusually slow process, which cannot be explained by classical exhumation models operative within the other centre of continent-continent collisional orogens. 2. Geological setting The Qinling mountain range lies between the North and South China blocks (Fig. 1), bounded on the north by the Lushan fault and on the south by the Mianlue–Bashan–Xiangguang fault (Dong et al., 2008a). The Lushan fault is an intra-continental thrust along which the North Qinling terrane was thrust onto the southern margin of the North China block and formed during the Mesozoic–Cenozoic. The Mianlue–Bashan–Xiangguang fault is also an overthrust along which the 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 the south (Zhang et al., 1995b; Li et al., 2009, 2010a,b). A large number of geochemical and geochronological studies suggest that the Mianlue suture zone was evolved due to the closure of a northern branch of the Paleo-Tethyan Ocean, which separated the South Qinling microcontinental block from the South China block during Devonian to Middle Triassic times (Zhang et al., 1995a,b, 2000; Li et al., 1996; 2007a; Xu et al., 2002; Dong et al., 1999, 2004). Subsequently, it was overprinted by the overthrust of the Mianlue–Bashan–Xiangguang fault 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 of amphibolite facies metamorphosed Archean–Palaeoproterozoic basement complexes (Zhang et al., 2000) and weakly metamorphosed basic volcanic and sedimentary cover sequences ranging in age from Mesoproterozoic to Mesozoic. Mesozoic Granitic intrusions are abundant, and the region also underwent intra-continental deformation during Mesozoic–Cenozoic times (Zhang, 1989; Zhang et al., 1995a; Xu et al., 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). It comprises predominantly several lenticular Palaeoproterozoic crystalline basement units, overlying Meso-Neoproterozoic volcano-sedimentary rocks, late Mesoproterozoic ophiolites, and Neoproterozoic–Paleozoic volcano-sedimentary assemblages. These units underwent amphibolite facies metamorphism at ~1.0 Ga, followed by retrogression to greenschist facies conditions at ~400 Ma (Chen et al., 1991; Liu et al., 1993; Zhang et al., 1994a,b) and are locally covered unconformably by CarboniferousPermian clastic deposits. From north to south, the main rock units in this belt are the Kuanping, Erlangping, and Qinling Groups, and the Songshugou ophiolite, which are separated from each other by thrust faults or ductile shear zones (Fig. 1). The Kuanping Group mainly comprises greenschists, amphibolites, quartz-micaschists, gneisses and marbles. The protoliths of both the greenschists and amphibolites were tholeiitic basalts with N-MORB and T-MORB geochemical characteristics (Zhang and Zhang, 1995).

Sm-Nd whole-rock isochron ages of these metabasalts range from 0.94 to 1.2 Ga (Zhang et al., 1994a,b). The Erlangping Group is composed of an ophiolitic unit, clastic sedimentary successions and carbonates. The ophiolitic unit contains sparse ultramafic rock, massive basalt, pillow basalt, and rare intercalations of radiolarian chert. The geochemistry of the basalt suggests formation in a back-arc basin setting (Sun et al., 1996). The findings of Lower to Middle Ordovician radiolarians within the cherts confirm 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) and interlayer of continental tholeiitic lavas, respectively (Zhang et al., 1994a, b). U-Pb isotopic ages of zircon from gneisses range from 2172 to 2267 Ma, whereas the Sm-Nd whole-rock isochron age of the amphibolites (metatholeiites) is 1987±49 Ma (Zhang et al., 1994a,b). Further age data indicates that the Qinling Group is a Paleoproterozoic complex, which underwent amphibolite facies metamorphism at 990±0.4 Ma and a greenschist facies metamorphic overprint at ca. 425±48 Ma (Chen et al., 1991). The Songshugou ophiolite consists primarily of amphibolite facies mafic and ultramafic rocks. The geochemistry suggests predominant E- and T-MORB affinities, indicative of the initial stage of an oceanic spreading centre (Dong et al., 2008b). Combined with regional correlations, abundant isotopic age data reveal that the ocean evolved between 1.4 and 1.0 Ga (Li et al., 1993; Zhang et al., 1994a,b; Chen et al., 2002; Dong et al., 2008b).

2.3. Shangdan suture The Shangdan suture zone is defined by a linear, patchy distribution of tectonic and ophiolitic melanges and arc-related volcanic rocks. These units were overprinted by a series of ductile shear zones and brittle fault systems, and were intruded by subduction- and collision-related granitoids. The ophiolite- and subduction-related volcanic rocks are the most important members of the Danfeng Group, which were overprinted by greenschist to lower amphibolite facies metamorphic assemblages. The geochemistry indicates that the metamorphosed calc-alkaline basalts, some massive and some pillow lavas were formed in a typical intra-oceanic island-arc setting (Zhang et al., 1994a,b). The metamorphosed tholeiitic basalts were generated at a mid-ocean ridge, as evidenced by a slight depletion of light rare earth elements (Dong et al., 2010). The U-Pb zircon ages of gabbros from ophiolites within the western 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 the Ordovician to Silurian age of radiolarian from the interlayer cherts within the 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 by south-vergent thrusts and folds showing an imbricated thrust-fold system (Zhang et al., 2000). The basement of the South Qinling belt contains several Precambrian complexes (e.g. Xiaomoling, Douling, Tongbai-Dabie, Foping and Yudongzi complexes), all of which contain Meso- to Neoproterozoic rift-type volcano-sedimentary assemblages metamorphosed under greenschist facies conditions (Zhang et al., 1995a). The sedimentary cover includes Sinian clastic and carbonate rocks, Cambrian-Ordovician limestones, Silurian shales, and Devonian to Carboniferous clastic rocks and limestones. A few remnants of Upper Palaeozoic–Lower Triassic clastic sedimentary rocks are also present in the northern part of the South Qinling belt (Zhang et al., 2000).



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

3. Sample descriptions All the samples presented in this study are from the same outcrop of the Qinling metamorphic complex unit of the NQT in the Qingyouhe area (Fig. 1). At this location, gneisses of the Qinling Group are exposed (Fig. 2a) intercalated with lenticular amphibolites, and garnet-amphibolites distributed parallel to the foliation of the banded gneisses (Fig. 2b). The gneisses were migmatitized as indicated by highly deformed lenticular bodies of granitoids within the foliation (Fig. 2a). Granitic dykes intruded the gneiss, which are now parallel to the foliation of the gneiss, and folded together with the gneisses (Fig. 2c). These dykes can be distinguished from the first generation granitoids by their sharp contacts. The third generation of granitoids intruded into the gneiss and cut the foliation of the migmatitic gneiss (Fig. 2d). Samples Qy-01 and 027 N-275 were taken from the amphibolegneiss, whereas sample 027 N-266 and 027 N-271 were collected from a lenticular garnet-amphibolite body within the migmatitic gneiss. Sample Qy-04 came from first generation granitoids (migmatitic granite) within the migmatitized gneiss, whereas samples Qy-02 and 027 N-289 were from the host gneisses. The second and third generations of granite intrusions were represented by samples Qy-06 and Qy-03, respectively. 4. Analytical techniques High spatial resolution U-Th-Pb and REE were obtained using the laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) at the State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an, China. Each sample consisted of 20 kg of material was crushed into powder, and then washed and dried. Zircons were separated by heavy-liquid and magnetic methods and then handpicked under a binocular microscope. The internal texture of zircons was examined using cathodoluminescence (CL) images. Zircons were dated 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 laser ablation spot. The laser-ablation system used is a GeoLas 200 M equipped with a 193 nm ArF-excimer laser, and a homogenizing and imaging optical system (MicroLas, Göttingen, Germany). Analyses were performed on the ELAN 6100 ICP-MS from Perkin Elmer/SCIEX (Canada) with a dynamic reaction cell (DRC). The laser ablation spot size is approximately 40 μm. 207Pb/206Pb, 206Pb/238U, 237Pb/235U and 208Pb/232Th ratios were calculated using GLITTER 4.0 (Macquarie University), and were corrected for both instrumental mass bias and depth-dependent elemental and isotopic fractionation using Harvard zircon 91500 as the external standard. The ages were calculated using ISOPLOT 3 (Ludwig, 2003). The detailed analytical procedure of age and trace element determinations of zircons can be found in Yuan et al. (2004). Common Pb corrections were made following the method of Anderson et al. (2002). Laser-probe 40Ar/39Ar analysis was carried out at the ARGONAUT Laboratory of the Geology Division at the University of Salzburg. Four amphibole and three biotite concentrates were separated from the metamorphosed Qinling Group, and were irradiated in the MTAKFKI reactor (Debrecen, Hungary). 40Ar/39Ar analysis was carried out using an UHV Ar–extraction line equipped with a combined MERCHANTEK™ UV/IR laser ablation facility, and a VG–ISOTECHTM NG3600 mass spectrometer. Stepwise heating was performed using a defocused (~1.5 mm diameter) 25 W CO2–IR laser operating in Tem00 mode at wavelengths between 10.57 and 10.63 μm. Isotopic ratios, ages and errors for individual steps were calculated following the suggestions of McDougall and Harrison (1999) and applying decay constants reported by Steiger and Jaeger (1977). Definition and calculation of plateau ages has been carried out using ISOPLOT/EX (Ludwig, 2003). Correction factors for interfering isotopes have been calculated from 10 analyses of 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, and 40 Ar/39Ar(K) = 0.015466. Variations in the neutron flux were monitored with DRA1 sanidine standard for which a 40Ar/39Ar plateau age of 25.03 ± 0.05 Ma has been reported (Wijbrans et al., 1995; van Hinsbergen et al., 2008), and errors on the ages are 1σ inter-laboratory.


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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 the gneiss, 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 the migmatitic gneiss.

5. Results 5.1. Zircon U-Pb dating Results of U-Pb single zircon dating of three samples are presented in e-component Table 1 and Fig. 3. Trace element compositions of single zircons analyzed in-situ by LA-ICP-MS are listed in e-component Table 2. The cathodoluminescence (CL) images of representative zircons for each sample are shown in Fig. 4. Zircons from the migmatitic granites (sample Qy-04), which were foliated together with the host gneisses (Fig. 2a), yield several groups of 206Pb/238U ages. The CL images display rounded, inherited cores and thin recrystallized rims (Fig. 4a). There is an opaque or translucent transitional belt between the cores and the rims. The rims of zircons display variable 206Pb/238U ages ranging from 433 to 525 Ma but with two discrete populations of grains recording with weighted mean ages 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 host gneisses, also exhibit well-developed crystal faces with length/width ratios ranging from 2:1 to 3:1. Conspicuous magmatic oscillatory zoning is observed in CL images (Fig. 4b). Some zircons display inherited cores. Eight spot analyses on the rims yield 206Pb/238U ages ranging from 435 to 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 cuts the foliation of the gneisses (Fig. 2d), exhibit well-developed crystal faces with length/width ratios ranging from 3:1 to 4:1. Transparent to translucent, colourless to pale pink grains showing a well developed magmatic oscillatory zoning in CL images are dominant (Fig. 4c). These zircons yield 206Pb/238U ages ranging from 409 to 422 Ma with a weighted

mean age of 417±1.6 Ma (MSWD=0.90), which is in accordance with a low intercept age of 420±3.6 Ma (MSWD=0.49) (Fig. 3c). Three spot analyses in the core of the zircons yield 206Pb/238U ages ranging from 902 to 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 granite generations shows variable Th/U ratios (Table 1 and Fig. 5). All the Th/ U data from the rims of sample Qy-04 show lower Th/U ratios (b 0.2), and ten of the sixteen measurements show a Th/U ratio b0.1 (Fig. 5a), while their 206Pb/238U apparent ages ranging from 433 to 523 Ma (Fig. 3a). The Th/U ratios of zircons from sample Qy-04 are lower than that of the sample Qy-03, which should be derived from a typical magmatic origin. In addition to, the zircons from sample Qy-04 display clearly higher Hf, Yb and U, and lower Th contents than third generation granitic intrusion (sample Qy-03) as illustrated in Fig. 6. Zircons from sample Qy-06 with 206Pb/238U apparent ages from 435 to 454 Ma show Th/U ratios ranging from 0.04 to 0.26, and mostly have Th/U ratioN 0.1 (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/U ratiosN 0.36 (Fig. 5c). Additionally, two of the three 902–915 Ma cores have Th/U ratios N0.2, whereas the other has a Th/U ratio of 0.13 but the grain is unlikely to be of metamorphic origin. Zircons with reliable weighted mean U-Pb ages ranging from 502 to 525 Ma in sample Qy-04, 435 to 454 Ma in sample Qy-06, and 409 to 422 Ma in sample Qy-03 exhibit perfectly positive Ce anomalies and negative Eu anomalies in C1 chondrite-normalized REE patterns (Fig. 7a, c and e). However, zircons from sample Qy-04 with U-Pb ages ranging from 597 to 702 Ma (Fig. 7b), and zircons from Qy-03 with U-Pb ages of 669 and 905 (Fig. 7f) have no obvious positive Ce anomalies and



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analyses for all minerals from the representative samples are listed in e-component Table 3 and are graphically shown in Figs. 8 and 9. 5.3.1. Amphibole All amphiboles from the garnet–amphibolite (samples 027 N-266 and 027 N-271) and amphibolite gneiss (samples 027 N-275 and 027 N-289) yield plateau ages of about 400 Ma (Fig. 8). The amphiboles from sample 027 N-271 display two steps with a large spectrum, 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 for intermediate- and high-temperature argon release steps, respectively. Their Ca/K ( = 1.78*37Ar/39Ar) ratios show large variations with successive heating steps (Fig. 8a). The higher apparent age and lower 37Ar/39Ar ratios during the first heating steps show significant excess 40Ar. An average age of 410 ± 10 Ma is calculated from these two 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 in intermediate-energy release steps. The plateau ages are defined by 85% of total 39Ar released for successive steps (Fig. 8b). The amphibole concentrate from the amphibole-gneiss (sample 027 N-275) displays a fairly flat age spectrum with a well-defined plateau, giving an age of 397 ± 33 Ma (Fig. 8c) with over 95% 39Ar released, The Ca/K ( = 1.78*37Ar/39Ar) ratios show little variation with successive heating steps (Fig. 8c). The amphibole from amphibole gneiss sample 027 N-289 has a similar 40Ar/39Ar age spectrum to sample 027 N-271, which is characterized by two plateau ages of 412 ± 32 Ma and 396 ± 12 Ma being separated by abnormal apparent ages in the intermediate argon release steps. An average age of 405 ± 15 Ma was calculated with 80% of total 39Ar released (Fig. 8d).

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

5.3.2. Biotite The 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 39Ar released for thirteen successive intermediate- and high-temperature steps at 1σ level of uncertainty. The Ca/K ( = 1.78*37Ar/39Ar) ratios show little variation with successive heating steps (Fig. 9a). A significant second plateau of intermediate-temperature steps gives a plateau 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 fairly flat age spectrum with well-defined plateaus giving ages of 333±1.4 Ma (Fig. 9b) and 330±1.3 Ma (Fig. 9c), respectively. They are defined by 95 and~100% of total 39Ar released for more than fourteen successive heating steps at 1σ level of uncertainty. These ages are internally consistent with the second 40Ar/39Ar plateau age of biotite of sample 027 N-271. All the biotites from various lithological 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 to 340 Ma. Only biotites from the garnet-amphibolite of sample 027 N-271 yield an older age of 368 ± 1.6 Ma. 6. Discussion

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

5.3.

40

Ar/39Ar dating

40 Ar/39Ar dating of amphibole and biotite concentrates, each representing 10–15 grains, from the metamorphic rocks was performed in order to constrain the exhumation history of the Qinling metamorphic complex subsequent to migmatization. The 40Ar/39Ar

6.1. Timing of tectono-magmatic events and the trace element geochemistry of zircons U-Pb zircon dating of migmatitic orthogneiss and younger granitoid intrusions provides chronological constraints on tectono-magmatic events and thus the tectonic evolutionary history of the Qinling metamorphic complex. The Th/U ratios are routinely used to distinguish the origin of zircon (e.g. Maas et al., 1992). Th/U ratios in magmatic zircons from various rocks mostly range from 0.2 to 1.0, while zircons


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

that grew due to metamorphic events exhibit lower Th/U ratios ( b 0.1) (Williams and Claesson, 1987; Schiøtte et al., 1988; Kinny et al., 1990). In this study, the zircons from the three different granite generations show variable Th/U ratios. The zircons from the third generation of granite with U-Pb ages ranging from 409 to 422 Ma, which is undeformed and crosscuts the gneissic foliation, exhibit Th/U ratios N0.36 (Fig. 5a and Table 1). These high Th/U ratios are consistent with the CL images which show magmatic oscillatory zoning in zircons (Fig. 4a), indicating for magmatic growth. Hence, we interpret the weighted mean U-Pb age of 417 ± 1.6 Ma (MSWD = 0.90) (Fig. 3a) as the magma crystallization age of the third granitoid intrusion. Two of the three 902–915 Ma old zircons also have Th/U ratios N 0.2, whereas the third measurement yielded a

Th/U ratio of 0.13. However, a metamorphic origin is considered as unlikely, and zircons of unambiguously magmatic origin have been shown to contain low Th/U ratios (Zeck and Whitehouse, 1999). Since all these ages are dated within the inherited cores of zoned zircons (Fig. 4a), it seams reasonable to infer their original growth during tectono-magmatic events during 902 to 915 Ma. Zircons from sample Qy-06 (206Pb/238U apparent ages 435–454 Ma), separated from foliation-parallel granitic dykes (Fig. 2c) show Th/U ratios ranging from 0.04 to 0.26 (Fig. 5b). Compared to the well-developed crystal morphology, light color and high length/width ratios ranging from 3:1 to 4:1 in sample Qy-03, the zircons from Qy-06 exhibit a relative dark color 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 the migmatitic



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

granite sample Qy-04 (Fig. 4c). Eight spot analyses on the rims yield magmatic ages ranging from 435 to 454 Ma with a weighted mean age of 445±4.6 Ma (MSWD=2.9) (Fig. 3b). Based on mapping of this locality, geological evidence indicates these granitic dykes were folded together with the foliation of the host migmatitic gneiss. Granitic dykes are exposed as lenticular veins or dykes with a slight foliation at their edges, by even texture and discontinuous boundaries between the dykes and the migmatitic orthogneiss. It is striking that a similar 206 Pb/238U weighted mean age of 455 ± 4.5 Ma (MSWD= 0.46) and lower intercept age of 446 ± 38 Ma (MSWD = 27) was determined for the migmatitic granite sample Qy-04 (Fig. 3c). Therefore, we suggest that the granite dykes (sample Qy-06) formed at the final stage of migmatization at 445 ± 4.6 Ma. The Th/U ratios of zircon rims from sample Qy-04 are lower ( b 0.2, mostly b 0.1; Fig. 5b) than that of the sample Qy-03, to which a magmatic origin has been ascribed. 206Pb/238U apparent ages range from 433 to 523 Ma (Fig. 5c). The CL images of sample Qy-04 reveal a morphology similar to sample Qy-06, but different from sample Qy-03 (Fig. 4). The rims of zircons display two groups of 206Pb/238U ages ranging from 433 to 525 Ma with weighted mean ages of 455 ± 4.5 Ma (MSWD = 0.46) and 517 ± 14 Ma (MSWD = 14) (Fig. 3c). Most of these 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 of 446 ± 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.

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

This is interpreted to represent the time of a last thermal episode of the migmatization of the North Qinling metamorphic complex. Although it is used to discriminate metamorphic and magmatic zircons (Mojzsis and Harrison, 2002), the low Th/U ratio of metamorphic zircon can be caused by involvement of competing effects of Th-rich minerals (e.g. monazite and allanite) (Williams et al., 1996; Rubatto et al., 2001). On the other hand, metamorphic zircons from high-grade metamorphic orthogneisses can show high Th/U ratios (Friend and Kinny, 2001; Möller et al., 2003) which likely preserve protolith values (Möller et al., 2002). Meanwhile, some authors also argued for exceptional cases of igneous zircon with low Th/U ratios (Zeck and Whitehouse, 1999; Hidaka et al., 2002) and it seems difficult to distinguish between igneous and metamorphic origins of zircons from the Th/U ratios alone (Hidaka et al., 2002). To further elucidate the magmatic vs. metamorphic provenance of the zircons, trace element analyses were carried out. It provides further important geochemical information on the nature of the zircon source materials (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 be used to discriminate the tectonic setting of rocks (Pupin, 1992), and


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

the U, Th and REE concentrations also yield constraints on the age of the samples (Ashwal et al., 1999; Hoskin and Black, 2000). Zircons from the migmatitic granite (Sample Qy-04) clearly show higher Hf, Yb and U, and lower Th contents than most of the third generation granitic intrusion (sample Qy-03) (Fig. 6). These patterns are likely the result of competing growth of Th-rich minerals, which is indicated by presence of monazite within thin section. In this study, most zircons with reliable weighted mean 206Pb/238U ages exhibit strong heavy REE enrichment, variable positive Ce anomalies and negative Eu anomalies in C1 chondrite-normalized REE patterns (Fig. 7a, c and e). However, there still have some zircons showing weak positive Ce anomalies and negative Eu anomalies in C1 chondrite-normalized REE diagrams (Fig. 7b and f), such as zircons from 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 to 525 Ma in sample Qy-04, 435 to 454 Ma in sample Qy-06, and 409 to 422 Ma in sample Qy-03 exhibit perfectly positive Ce anomalies and negative Eu anomalies in C1 chondrite-normalized REE patterns

(Fig. 7a,c,e). Whitehouse et al. (2005) report zircons from Archean high-grade metamorphic rocks mostly without any Ce and Eu anomalies in chondrite-normalized REE diagrams. If this is indeed the case, the data indicate an age dependence of Ce and Eu anomalies in the zircons, which will be potential tracers of magmatic and migmatitic zircons. It is notable that the three samples contain some Neoproterozoic U-Pb zircon ages ranging from 902 to 915 Ma in granite (sample Qy-03), 902 to 928 Ma in migmatitic granite (sample Qy-04) and 861 Ma in granite dyke (sample Qy-06). All these ages were derived from the inherited cores of zircons. Their Th/U ratios are mostly higher than 0.2 (Fig. 5 and Table 1). This is in accordance with several Neoproterozoic plutons that have been documented in the North Qinling terrane, such as the Caiwa post-collisional granite (U-Pb zircon age of 889 ± 10 Ma; Zhang et al., 2004), the Niujiaoshan granite (U-Pb zircon 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 Guanshan granite (U-Pb zircon age of 926 ± 16 Ma; Chen et al., 2006), the Dehe



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

granite (U-Pb zircon age of 943 ± 18 Ma, Chen et al., 2004a,b), and the Fangcheng alkali-syenite (U-Pb zircon age of 844 ± 1.6 Ma; Bao et al., 2008). The migmatites have undergone partial melting, as evidenced by the presence of abundant leucosomes. Therefore, the ages of zircon cores are inferred to be record of Neoproterozoic magmatism (Wang et 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 which operate over a wide temperature range from N850 °C (U-Pb zircon ) to 40 °C (apatite U-Th/He) (Stuart, 2002). The 40Ar/39Ar isotope dating technique is not only widely used to date metamorphic and related deformational events, but also one of the most commonly applied tools for assessing the cooling and exhumation history, and the tectonothermal evolution of orogenic belts. Since different minerals have different blocking temperatures, it has become possible to date the last cooling age through a specific blocking temperature of certain

common minerals, which potentially allows the reconstruction of the cooling history of a rock unit (Svenningsen, 2000). As discussed previously, sample Qy-04 was collected from migmatitic leucosome granite. In general, migmatization occurs over a wide span of temperatures (625–850 °C), however, the field and textural evidence indicate that the migmatites in this study were deformed or overprinted at a relatively low-temperature. For example, the migmatites were derived from the felsic gneisses with interlayered amphibolites, which are the major lithological unit of the Qinling Group. A little of leucosomes are parallel to the well-developed foliation of migmatitic orthogneiss. Based on the thermodynamic models and calibrations for the Ti-in-zircon thermometer (Ferry and Watson, 2007), the temperature of migmatization are calculated as about 630–700 °C. The U-Pb zircon age of 517 ± 14 Ma derived from the migmatitic leucosomes may present the major episode of migmatization at 630–700 °C. Migmatization is the tectonothermal response of the northward subduction of the early Palaeozoic Qinling oceanic plate. Subduction is indicated by the presence of some metamorphosed calc-alkaline


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basalts and massive and pillow lavas cropping out in the south of the NQL terrane (Dong et al., 2010). These rocks were shown to have formed in a typical oceanic island-arc setting (Zhang et al., 1994a,b), as well as other metamorphosed tholeiitic basalts generated from a mid-ocean ridge setting with slight depletion of LREE (Dong et al., 2010). The U-Pb zircon ages of the gabbros from the ophiolite in the western part of the Shangdan suture range from 530 to 471 Ma (Yang et al., 2006; Pei et al., 2007; Li et al., 2007b), consistent with radiolarian ages of intercalated cherts ranging from Ordovician to Silurian (Cui et al., 1996). Above the subduction zone, there are welldeveloped early Palaeozoic subduction-related gabbroic-granitoid intrusions, 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; Li et al., 2006) . Adakitic rocks from the North Qinling belt indicate the lower 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 folded together with the migmatitic foliation, and intruded by 417 ± 1.6 Ma granites. By means of the Ti-in-zircon thermometer (Ferry and Watson, 2007), we estimate the crystallized temperature of sample Qy-06 at ca. 640–680 °C. Four samples of amphibole from the garnet amphibolite (samples 027 N-266 and 027 N-271), amphibolite gneiss (samples 027 N-275 and 027 N-289), respectively, yield plateau ages ranging from 410±10 Ma to 397 ± 33 Ma (Fig. 8). The blocking temperature of argon within hornblende is well documented at ca. 540 °C (Phillips et al., 2007). It is, therefore, reasonable to interpret that the cooling of the Qinling metamorphic complex to below ca. 540 °C occurred at about 410–397 Ma. Based on the blocking temperature of argon diffusion within biotite at ca. 300 °C (Phillips et al., 2007), the biotites from various lithological 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 to 340 Ma (Fig. 9) and represent the time of cooling of the Qinling metamorphic 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 of 368 ± 1.6 Ma (Fig. 9a) with more than 60% of total 39Ar released. This age 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 age estimate for cooling of the Qinling complex below ca. 300 °C. If indeed, it implies a period of slower cooling and exhumation between 368 and 330 Ma. Taking into account the U-Pb zircon and multiple mineral 40Ar/39Ar geochronology, we propose that the main stage of slow cooling and related exhumation of the Qinling metamorphic complex has occurred during Mid Palaeozoic times (Fig. 10). The migmatization at 630–700 °C occurred sometime after 517 Ma and not later than 445 Ma. After 445 Ma, slow cooling occurred until a temperature of 540 °C was reached. Assuming simple linear cooling from about 630 °C at 445 Ma to 540 °C at 410 (amphibole) Ma suggests a time-integrated cooling rate ofb 2.5 °C/Ma. It was then followed by rapid exhumation, with a cooling rate of 8 °C/Ma from ca. 540 °C at 397 Ma to 300 °C at 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. These data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture during Silurian–Early Devonian. These relationships exclude 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 Qinling terrane evolved into an unusually slow cooling process during Late Devonian and Middle Carboniferous times, which cannot be explained by classical exhumation models operative within the other centre of

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

continent–continent collisional orogens. The investigation along the Mianlue suture zone revealed that the opening of the Mianlue Ocean occurred in the Late Devonian and Middle Carboniferous time (Zhang et al., 2000). Therefore, we propose that the slow cooling process of the North Qinling terrane during Late Devonian and Middle Carboniferous times could be related to the extensional geodynamics of the Mianlue Ocean. 7. Conclusions The results of the LA-ICP-MS U-Pb zircon dating from three samples of granites and migmatitic leucosome (granite), and 40Ar/39Ar dating from four amphibole and three biotite concentrates yield the following conclusions: (1) U-Pb zircon dating of migmatitic orthogneiss (leucosome) and granite dyke bracket the age of migmatization between 517 ± 14 Ma (MSWD = 14) to 445 ± 4.6 Ma (MSWD = 2.9).



Fig. 10. Cooling path of the Qinling terrane.

Subsequent intrusion by granites occurred at 417 ± 1.6 Ma (MSWD = 0.90). (2) Four amphibole 40Ar/39Ar plateau ages mostly range from 410 ± 10 Ma to 397 ± 33 Ma and constrain the cooling of the Qinling complex below ca. 540 °C at about ca. 400 Ma. Three biotite 40Ar/39Ar ages indicate cooling of the Qinling complex exhumation below ca. 300 °C at about 330–340 or 368 Ma. (3) On account of the U-Pb zircon and multiple mineral 40Ar/39Ar dating, an exhumation model is proposed as: slow/nonexhumation during 517–445 Ma, a time-integrated cooling with rates b 2.5 °C/Ma during the 445–410 Ma period, rapid exhumation with cooling rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma.

Supplementary materials related to this article can be found online at doi:10.1016/j.gr.2010.09.007. Acknowledgments Yunpeng Dong would like to thank Brigitte Winklehner, Gottfried Tichy and Yong Sun for their kind help. Financial support for this study was jointly provided by the National Natural Science Foundation of China (grants: 40772140,40972140), the Eurasia-Pacific Uninet, the Austrian Academic Exchange Service (OEAD), and MOST Special Fund from the State Key Laboratory of Continental Dynamics, Northwest University. References Ames, L., Zhou, G.Z., Xiong, B.C., 1996. Geochronology and isotopic character of ultrahigh-pressure metamorphism with implications for collision of the Sino-Korean and Yangtze cratons, central China. Tectonics 15, 472–489. Anderson, B.R., Gemmell, J.B., Nelson, D.R., 2002. Lead isotope evolution of mineral deposits in the Proterozoic Throssell Group, western Australia. Economic Geology 97, 897–911. Ashwal, L.D., Tucker, R.D., Zinner, E.K., 1999. Slow cooling of deep crustal granulites and Pb-loss in zircon. Geochimica et Cosmochimica Acta 63, 2839–2851. Bao, Z.W., Wang, Q., Bai, G.D., Zhao, Z.H., Song, Y.W., Liu, X.M., 2008. Geochronology and geochemistry of the Fangcheng Neoproterozoic alkali-syenites in East Qinling orogen and its geodynamic implications. Chinese Science Bulletin 53, 2050–2061.

11

Chen, D.L., Liu, L., Zhou, D.W., Luo, J.H., Sang, H.Q., 2002. Genesis and 40Ar-39Ar dating of clinopyroxene megacrysts in ultramafic terrain from Songshugou, east Qinling Mountain and its geological implication. Acta Petrologica Sinica 18, 355–362 (In Chinese with English abstract). Chen, J.L., Xu, X.Y., Wang, H.L., Wang, Z.Q., Zheng, Z.X., Li, P., Wang, C., 2008. Geochemical Characteristics and Petrogenesis of Early Paleozoic Rock in the west Segment of North Qinling. Acta Geological Sinica 82, 475–484 (In Chinese with English abstract). Chen, N.S., Hun, Y.Q., You, Z.D., 1991. Whole-rock Sm-Nd, Rb-Sr and single grain zircon Pb-Pb dating of the complex rocks from the interior of the Qinling orogenic belt, western Henan and its crustal evolution. Geochemistry 20, 219–227 (In Chinese with English abstract). Chen, Z.H., Lu, S.N., Li, H.K., Song, B., Li, H.M., Xiang, Z.Q., 2004a. The age of the Dehe biotite monzogranite gneiss in the North Qinling: TIMS and SHRIMP U-Pb zircon dating. Geological Bulletin of China 23, 136–141 (In Chinese with English abstract). Chen, Z.H., Lu, S.N., Li, H.K., Zhou, H.Y., Xiang, Z.Q., Guo, J.J., 2004b. Age of the Fushui intermediate–mafic intrusive Complex in the Qinling orogen: new zircon U-Pb and Whole- rock Sm and Nd isotope chronological evidence. Geological Bulletin of China 23, 322–328 (In Chinese with English abstract). Chen, Z.H., Lu, S.N., Li, H.K., Li, H.M., Xiang, Z.Q., Zhou, H.Y., Song, B., 2006. Constraining the role of the Qinling orogen in the assembly and break-up of Rodinia: Tectonic implications for Neoproterozoic granite occurrences. Journal of Asian Earth Sciences 28, 99–115. Cui, Z.L., Sun, Y., Wang, X.R., 1996. A discovery of Radiolaria from Danfeng ophiolites, North Qinling and its tectonic Significance. Chinese Science Bulletin 41, 916–919. Dong, Y.P., Zhang, G.W., Lai, S.C., Zhou, D.W., Zhu, B.Q., 1999. An ophiolitic tectonic melange firstly discovered in Huashan area, south margin of Qinling Orogenic Belt, and its tectonic implications. Science in China (Series D) 42, 292–302. Dong, Y.P., Zhang, G.W., Zhao, X., Yao, A.P., Liu, X.M., 2004. Geochemistry of the subduction-related magmatic rocks in the Dahong Mountains, northern Hubei Province: Constraint on the existence and subduction of the eastern Mianlue oceanic basin. Science in China (series D) 47, 366–377. Dong, Y.P., Zha, X.F., Fu, M.Q., Zhang, Q., Yang, Z., Zhang, Y., 2008a. Characteristics of the Dabashan fold-thrust nappe structure at the southern margin of the Qinling. China. Geological Bulletin of China 27, 1493–1508 (In Chinese with English abstract). Dong, Y.P., Zhou, M.F., Zhang, G.W., Zhou, D.W., Liu, L., Zhang, Q., 2008b. The Grenvillian Songshugou ophiolite in the Qinling Mountains, Central China: Implications for the tectonic evolution of the Qinling orogenic belt. Journal of Asian Earth Sciences 32, 325–335. Dong, Y.P., Zhang, G.W., Hauzenberger, C., Neubauer, F., Yang, Z., Liu, X.M., 2010. Paleozoic tectonics and evolutionary history of the Qinling orogen: evidence from geochemistry and geochronology of ophiolite and related volcanic rocks. Lithos in revision. Enkin, R.J., Yang, Z.Y., Chen, Y., Courtillot, V.E., 1992. Paleomagnetic constraints on the geodynamic history of the major blocks of China from the Permian to the present. Journal of Geophysical Research 97 (B10), 13953–13989. Faure, M., Lin, W., Le Breton, N., 2001. Where is the North China–South China block boundary in eastern China. Geology 29, 119–122. Ferry, J.M., Watson, E.B., 2007. New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers. Contributions to Mineralogy and Petrology 154, 429–437. Friend, R.L., Kinny, P.D., 2001. A reappraisal of the Lewisian gneiss complex: geochronological evidence for its tectonic assembly from disparate terranes in the Proterozoic. Contributions to Mineralogy and Petrology 142, 198–218. Gao, S., Zhang, B.R., Gu, X.M., Xie, X.L., Gao, C.L., Gu, X.M., 1995. Silurian–Devonian provenance changes of South Qinling basins: implications for accretion of the Yangtze (South China) to the North China Cratons. Tectonophysics 250, 183–197. Guo, J.F., O'Reilly, S.Y., Griffin, W.l., 1996. Zircon inclusions in corundum megacrysts; I, Trace element geochemistry and clues to the origin of corundum megacrysts in alkali-basalts. Geochimica et Cosmochimica Acta 60, 2347–2363. Hacker, B.R., Ratschbacher, L., Webb, L., Ireland, T., Walker, D., Dong, S.W., 1998. U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling–Dabie Orogen, China. Earth and Planetary Science Letters 161, 215–230. Hidaka, H., Shimizu, H., Adachi, M., 2002. U-Pb geochronology and REE geochemistry of zircons from Palaeoproterozoic paragneiss clasts in the Mesozoic Kamiaso conglomerate, central Japan: evidence for an Archean provenance. Chemical Geology 187, 279–293. Hokada, T., Harley, S.L., 2004. Zircon growth in UHT leucosome: constraints from zircon-garnet rare earth elements (REE) relations in Napier Complex, East Antarctica. Journal of Mineralogical and Petrological Sciences 99, 180–190. Hoskin, P.W.O., Black, L.P., 2000. Metamorphic zircon formation by solid-state recrystallization of protolith igneous zircon. Journal of Metamorphic Geology 18, 423–439. Hsu, K.J., Wang, Q., Li, J., Zhou, D., Sun, S., 1987. Tectonic evolution of Qinling Mountains, China. Eclogae Geologicae Helvetiae 80, 735–752. Ireland, T.R., Wlotzka, F., 1992. The oldest zircons in the solar system. Earth and Planetary Science Letters 109, 1–10. Katsube, A., Hayasaka, Y., Santosh, M., Li, S.Z., Terada, K., 2009. SHRIMP zircon U-Pb ages of eclogite and orthogneiss from Sulu ultrahigh-pressure zone in Yangkou area, eastern China. Gondwana Research h15, 168–177. Kinny, P.D., Wijbrans, J.R., Froude, D.O., Williams, I.S., Compston, W., 1990. Age constraints on the geological evolution of the Narryer Gneiss Complex, Western Australia. Australian Journal of Earth Sciences 37, 51–69. Kröner, A., Zhang, G.W., Zhuo, D.W., Sun, Y., 1993. Granulites in the Tongbai area, Qinling belt, China: geochemistry, petrology, single zircon geochronology and implications for tectonic evolution of eastern Asia. Tectonics 12, 245–255.


12


Li, H.M., Chen, Z.H., Xiang, Z.Q., Li, H.K., Lu, S.N., Zhou, H.Y., Song, B., 2006. Difference in U-Pb Isotope ages between baddeleyite and zircon in metagabbro from the Fushui complex in the Shangnan-Xixia area, Qinling orogen. Geological Bulletin of China 25, 653–659 (In Chinese with English abstract). Li, S.G., Xiao, Y.L., Liou, D.L., Chen, Y.Z., Ge, N.J., Zhang, Z.Q., Sun, S.S., Zhang, R.Y., Hart, S.R., Wang, S.S., 1993. Collision of the North China and Yangtze Blocks and formation of coesite-bearing eclogites. Chemical Geology 109, 89–111. Li, S.G., Sun, W.D., Zhang, G.W., 1996. Chronology and geochemistry of metavolcanic rocks from Higouxia valley in Mian-Lue tectonic belt, South Qinling: evidence for a Paleozoic oceanic basin and its close time. Science in China (Series D) 39, 300–310. Li, S.Z., Kusky, T.M., Zhao, G.C., Liu, X.C., Zhang, G.W., Kopp, H., Wang, L., 2010a. Two-stage Triassic exhumation of HP-UHP terranes in the Dabie orogen of China: constraints from structural geology. Tectonophysics 490, 267–293. Li, S.Z., Zhao, G.C., Zhang, G.W., Liu, X.C., Dai, L.M., Jin, C., Liu, X., Hao, Y., Liu E.S., Wang, T., 2010b. Not All Folds and Thrusts in the Yangtze Foreland Belt are related to the Dabie-Sulu Orogen: Insights from Mesozoic Deformation South of the Yangtze River. Geological Journal 45, 650–663. Li, S.Z., Kusky, T.M., Liu, X.C., Zhang, G.W., Zhao, G.C., Wang, L., Wang, Y.J., 2009. Two-stage collision-related extrusion of the western Dabie HP-UHP metamorphic terranes, central China: evidence from quartz c-axis fabrics and microstructures. Gondwana Research 16, 294–309. Li, S.Z., Kusky, T.M., Wang, L., Lai, S.C., Liu, X.C., Dong, S.W., Zhao, G.C., 2007a. Collision leading to multiple-stage large-scale extrusion in the Qinling orogen: insights from the Mianlue suture. Gondwana Research 12, 121–143. Li, W.Y., Li, S.G., Pei, X.Z., Zhang, G.W., 2007b. Geochemistry and zircon SHRIMP U-Pb ages of the Guanzizhen Ophiolite complex, the Western Qinling Orogen, China. Acta Petrologica Sinica 23, 2836–2844 (In Chinese with English abstract). Li, Z.X., 1994. Collision between the North and South China blocks: a crustal-detachment model for suturing in the region east of the Tanlu fault. Geology 22, 739–742. Liu, S.F., Zhang, G.W., Dai, S.W., 2001. Evolution of Qinling Mianlue Belt: evidence from sedimentology and tectonics of the northern Yangtze, China. Gondwana Research 4, 690–691. Liu, X., Li, S.Z., Suo, Y.H., Liu, X.C., Dai, L.M., Santosh, M., 2010. Structural anatomy of the exhumation of high-pressure rocks: constraints from the Tongbai collisional orogen and surrounding units. Geological Journal 45, in press. Liu, Y.Y., Yang, W.R., Morinaga, H., Adachi, Y., Yaskawa, K., Yang, Z.H., 1993. Some paleomagnetic results of North China, Qinling and Yangtze blocks. Earth Science-Journal of China University of Geosciences 18, 635–642 (In Chinese with English abstract). Ludwig, K.R., 2003. User's Manual for Isoplot/Ex v3.0, a geochronology toolkit for Microsoft Excel. Berkeley Geochronological Center Special Publications 4, 25–31. Maas, R., Kinny, P.D., Williams, I.S., Froude, D.O., Compston, W., 1992. The Earth's oldest known crust: a geochronological and geochemical study of 3900–4200 Ma old detrital zircons from Mt. Narryer and Jack Hills, Western Australia. Geochimica et Cosmochimica Acta 56, 1281–1300. Mattauer, M., Mattle, P., Malavieille, J., Tapponnier, P., Masluski, H., Xu, Z.Q., Li, Y.L., Tang, Y.Q., 1985. Tectonics of Qinling Belt: build-up and evolution of Eastern Asia. Nature 317, 496–500. McDougall, I., Harrison, T.M., 1999. Geochronology and thermochronology by the 40Ar/39Ar Method. Oxford University Press, UK. 1–269. Meng, Q.R., Zhang, G.W., 1999. Timing of collision of the North and South China blocks: controversy and reconciliation. Geology 27, 123–126. Mojzsis, S.J., Harrison, T.M., 2002. Establishment of a 3.83-Ga magmatic age for the Akilia tonalite (southern West Greenland). Earth and Planetary Science Letters 202, 563–576. Möller, A., O'Brien, P.J., Kennedy, A., Kröner, A., 2002. Polyphyase zircon in ultrahigh-temperature granulites (Rogaland, NW Norway): constraints for Pb diffusion in zircon. Journal of Metamorphic Geology 20, 727–740. Möller, A., O'Brien, P.J., Kennedy, A., Kröner, A., 2003. Linking growth episodes of zircon and metamorphic textures to zircon chemistry: an example from the ultrahigh-temperature granulites of Rogaland (SW Norway). Geological Society London Special Publications 220, 65–81. Oh, C.W., Choi, S.-G., Seo, J., Rajesh, V.J., Lee, J.H., Zhai, M., Peng, P., 2009. Neoproterozoic tectonic evolution of the Hongseong area, southwestern Gyeonggi Massif, South Korea: implications for the tectonic evolution of Northeast Asia. Gondwana Research 16, 272–284. Okay, A.I., 1993. Petrology of a diamond and coesite-bearing metamorphic terrain: Dabie Shan, China. European Journal of Mineralogy 5, 659–675. Pei, X.Z., Ding, S.P., Zhang, G.W., Liu, H.B., Li, Z.C., Li, G.Y., Liu, Z.Q., Meng, Y., 2007. The LA-ICP-MS zircons U-Pb ages and geochemistry of the Baihua basic igneous complexes in Tianshui area of West Qinling. Science in China (series D) 50(suppl, 264–276. Phillips, G., Wilson, C.J.L., Phillips, D., Szczepanski, S.K., 2007. Thermochronological (40Ar/39Ar) evidence of Early Palaeozoic basin inversion within the southern Prince Charles Mountains, East Antarctica: implications for East Gondwana. Journal of the Geological Society 164, 771–784. Poller, U., Huth, J., Hoppe, P., Williams, I.S., 2001. REE, U, Th, and Hf distribution in zircon from Western Carpathian Variscan granitoids; a combined cathodoluminescence and ion microprobe study. American Journal of Science 301, 858–876. Pupin, J.P., 1992. Les Zircons des granites oceaniques et continentaux: couplage typologie-geochimie des elements en traces. Bulletin de la Societe Geologique de France 163, 495–507. Ratschbacher, L., Hacker, B.R., Calvert, A., Webb, L.E., Grimmer, J.C., McWilliams, M.O., Ireland, T., 2003. Tectonics of the Qinling (Central China): tectonostratigraphy, geochronology, and deformation history. Tectonophysics 366, 1–53.

Ratschbacher, L., Franz, L., Enkelmann, E., Jonckeere, R., Poerschke, A., Hacker, B.R., Dong, S.W., Zhang, Y.Q., 2006. The Sino-Korean-Yangtze suture, the Huwan detachment, and the Paleozoic-Tertiary exhumation of (ultra)high-pressure rocks along the Tongbai-Xinxian-Dabie Mountains. Geological Society of America Special Publication 403, 45–75. Ren, J.S., Zhang, Z., Niu, B.G., Liu, Z.G., 1991. On the Qinling orogenic belt—integration of the Sino-Korean and Yangtze blocks. A selection of Papers, presented at the Conference on the Qinling Orogenic Belt. Publishing House of Northwest University, Xi'an, pp. 99–110 (In Chinese with English abstract). Rubatto, D., Williams, I.S., Buick, I.S., 2001. Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia. Contributions to Mineralogy and Petrology 140, 458–468. Schiøtte, L., Compston, W., Bridgwater, D., 1988. U-Th-Pb ages of single zircons in Archean supercrustals from Nain Province, Labrador, Canada. Journal of Earth Science 26, 2636–2644. Seo, J., Choi, S.-G., Oh, C.W., 2010. Petrology, geochemistry and geochronology of the post-collisional Triassic mangerite and syenite in the Gwangcheon area, Hongseong Belt, South Korea. Gondwana Research 18, 479–496. Steiger, R.H., Jaeger, E., 1977. Subcommission on geochronology; convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36, 359–362. Stuart, F.M., 2002. The exhumation history of orogenic belts from 40Ar/39Ar ages of detrital micas. Mineralogical Magazine 66, 121–135. Su, L., Song, S.G., Song, B., Zhou, D.W., Hao, J.R., 2004. SHRIMP zircon U-Pb ages of garnet pyroxenite and Fushui gabbroic complex in Songshugou region and constraints on tectonic evolution of Qinling Orogenic Belt. Chinese Science Bulletin 49, 1307–1310. Sun, Y., Lu, X., Han, S., Zhang, G.W., 1996. Composition and formation of Paleozoic Erlangping ophiolitic slab, North Qinling: evidence from geology and geochemistry. Science in China (Series D) 39, 50–59. Svenningsen, O.M., 2000. Thermal history of thrust sheets in an orogenic wedge: 40Ar/39Ar data from the polymetamorphic Seve Nappe Complex, northern Swedish Caledonides. Geological Magazine 137, 437–446. Tseng, C.Y., Yang, H.J., Yang, H.Y., Liu, D.Y., Wu, C.L., Cheng, C.K., Chen, C.H., Ker, C.M., 2009. Continuity of the North Qilian and North Qinling orogenic belts, Central Orogenic System of China: evidence from newly discovered Paleozoic adakitic rocks. Gondwana Research 16, 285–293. van Hinsbergen, D.J.J., Straathof, G.B., Kuiper, K.F., Cunningham, W.D., Wijbrans, J.R., 2008. No vertical axis rotations during Neogene transpressional orogeny in the NE Gobi Altai: coinciding Mongolian and Eurasian early Cretaceous apparent polar wander paths. Geophysical Journal International 173, 105–126. Wang, T., Zhang, G.W., Pei, X.Z., Wang, X.X., 2001. Neoproterozoic orogeny in the core of the Qinling Orogenic Belt (China) and its implications for assembly of the North and South China blocks. Gondwana Research 4, 815–816. Wang, T., Wang, X.X., Zhang, G.W., Pei, X.Z., Zhang, C.L., 2003. Remnants of a Neoproterozoic Collisional Orogenic Belt in the Core of the Phanerozoic Qinling Orogenic Belt (China). Gondwana Research 6, 699–710. Wang, X.R., Hua, H., Sun, Y., 1995. A Study on microfossils of the Erlangping Group in Wantan Area Xixia County, Henan Province. Journal of Northwest University (Natural Science Edition) 25, 353–358 (In Chinese with English abstract). Whitehouse, M.J., Kamber, B.S., Fedo, C.M., Lepland, A., 2005. Integrated Pb- and S-isotope investigation of sulphide minerals from the early Archaean of southwest Greenland. Chemical Geology 222, 112–131. Wijbrans, J.R., Pringle, M.S., Koppers, A.A.P., Scheveers, R., 1995. Argon geochronology of small samples using the Vulkaan Argon Laserprobe. Proceedings of the Koninklijke Nederlandse Akademie van Wetenschappen-Biological Chemical Geological Physical and Medical Sciences 98, 185–218. Williams, I.S., Claesson, S., 1987. Isotope evidence for the Precambrian province and Caledonian metamorphism of high grade paragneiss from the Seve Nappes, Scandinavian Caledonian. II. Ion microprobe zircon U-Th-Pb. Contribution to Mineralogy and Petrology 97, 205–217. Williams, I.S., Buick, I.S., Cartwright, I., 1996. An extended episode of early Mesoproterozoic metamorphic fluid flow in the Reynolde Range, central Austrlia. Journal of Metamorphic Geology 14, 29–47. Wilde, S.A., Valley, J.W., Peck, W.H., Graham, C.M., 2001. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409, 175–178. Xu, J.F., Castillo, P.R., Li, X.H., Yu, X.Y., Zhang, B.R., Han, Y.W., 2002. MORB-type rocks from the Paleo-Tethyan Mian-Lueyang northern ophiolite in the Qinling Mountains, central China: implications for the source of the low 206 Pb/204 Pb and high 143 Nd/144 Nd mantle component in the Indian Ocean. Earth and Planetary Science Letters 198, 323–337. Xu, Z.Q., Lu, Y.L., Tang, Y.Q., Zhang, Z.T., 1988. Formation of the Composite East Qinling. China Environmental Science Press, Beijing. 1–193 (In Chinese). Yang, Z., Dong, Y.P., Liu, X.M., Zhang, J.H., 2006. LA-ICP-MS zircon U-Pb dating of gabbro in the Guanzizhen ophiolite, Tianshui, West Qinling. Geological Bulletin of China 25, 1321–1325 (In Chinese with English abstract). Yin, A., Nie, S.Y., 1993. An indentation model for the north and south China collision and the development of the Tan–Lu and Honam fault systems, eastern Asia. Tectonics 12, 801–813. You, Z.D., Suo, S.T., 1991. Metamorphic process and structural analysis of the core complex of an orogenic belt: example from the eastern Qinling Mountain. China University of Geosciences Press, Wuhan. 73–90 (In Chinese). Yuan, H.L., Gao, S., Liu, X.M., Li, H.M., Guenther, D., Wu, F.Y., 2004. Accurate U-Pb age and trace element determinations of zircon by laser ablation-inductively coupled


Y. Dong et al. / Gondwana Research xxx (2010) xxx–xxx plasma-mass spectrometry. Geostandards and Geoanalytical Research 28, 353–370. Zeck, H.P., Whitehouse, M.J., 1999. Hercynian, Pan-African, Proterozoic and Archean ion-microprobe zircon ages for a Betic-Rif core complex, Alpine belt, W. Mediterranean: consequences for its P-T- t path. Contributions to Mineralogy and Petrology 134, 134–149. Zhai, X.M., Day, H.W., Hacker, B.R., You, Z.D., 1998. Paleozoic metamorphism in the Qinling orogen, Tongbai Mountains, central China. Geology 26, 371–374. Zhang, C.L., Zhang, G.W., Lu, X.X., 1994a. Characteristics and Origin of Kuanping Granite Body in the East Qinling. Northwest Goescience 15, 27–34. Zhang, C.L., Liu, L., Zhang, G.W., Wang, T., Chen, D.L., Yuan, H.L., Liu, X.M., Yan, Y.X., 2004. Determination of Neoproterozoic post-collisional granites in the north Qinling Mountains and its tectonic significance. Earth Science Frontiers 11, 33–42. Zhang, G.W., 1989. The major sutrue zone of the Qinling belt. Journal of Southeast Asian Earth Sciences 3, 63–76. Zhang, G.W., Zhou, D.W., Yu, Z.P., Guo, A.L., Cheng, S.Y., Li, T.H., Zhang, C.L., Xue, F., Kröner, A., Reischmann, T., Atenberger, V., 1991. Composition, structure and evolution of the lithosphere of the Qinling Orogenic belt. 4 Selection of Papers, presented at the Conference on the Qinling Orogenic Belt (in China). Publishing House of Northwest University, Xian, pp. 121–138.

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Zhang, G.W., Zhang, Z.Q., Dong, Y.P., 1995a. Nature of main tectono-lithostratigraphic units of the Qinling orogen: implications for the tectonic evolution. Acta Petrologica Sinica 11, 101–114 (In Chinese with English abstract). Zhang, G.W., Meng, Q.R., Lai, S.C., 1995b. Structure and tectonics of the Qinling Orogenic belt. Science in China (series B) 38, 13–29. Zhang, G.W., Meng, Q.R., Yu, Z.P., Sun, Y., Zhou, D.W., Guo, A.L., 1996. Orogenesis and dynamics of the Qinling Orogen. Science in China (series D) 39, 225–234. Zhang, G.W., Yu, Z.P., Dong, Y.P., Yao, A.P., 2000. On Precambrian framework and evolution of the Qinling belt. Acta Petrologica Sinica 16, 11–21 (In Chinese with English abstract). Zhao, Z.C., Coe, R.S., 1987. Paleomagnetic constraints on the collision and rotation of north and south China. Nature 327, 141–144. Zhang, Z.Q., Liu, D.Y., Fu, G.M., 1994b. Isotopic Geochronology of Metamorphic Strata in North Qinling. China, Geological Publishing House, Beijing. 1–191 (In Chinese with English abstract). Zhang, Z.Q., Zhang, Q., 1995. Geochemistry of metamorphosed Late Proterozoic Kuanping ophiolite in the Northern Qinling, China. Acta Petrologica Sinica 11 (suppl.), 165–177 (In Chinese with English abstract).
