1. Introduction
It has been considered that the Precambrian basement of the North China Craton was consolidated during the Palaeoproterozoic through the amalgamation of the Eastern Block and the Western Block along the nearly N–S-striking Trans-North China Orogen (referred to as TNCO hereafter) (Fig. 1a; Zhao et al. Reference Zhao, Wilde, Cawood and Lu1998, Reference Zhao, Sun, Wilde and Li2005). Metamorphic and geochronologic investigations of different metamorphic terranes in the northern and middle parts of the TNCO suggest that all of these terranes record clockwise pressure–temperature–time (P–T–t) paths including retrograde isothermal decompression (ITD) segments (e.g. Wang, Chen & Chen, Reference Wang, Chen and Chen1991; Zhai et al. Reference Zhai, Guo, Yan, Han and Li1992; Guo, Bian & Shi, Reference Guo, Bian, Shi and Zhai1996; Liu, Reference Liu1996; Guo et al. Reference Guo, Zhai, Li and Yan1998, Reference Guo, Zhai, Li and Li1999; Liu & Shen, Reference Liu and Shen1999; Zhao, Cawood & Lu, Reference Zhao, Cawood and Lu1999; Zhao et al. Reference Zhao, Cawood, Wilde and Sun2000a ,Reference Zhao, Wilde, Cawood and Lu b , Reference Zhao, Cawood, Wilde and Lu2001a ,Reference Zhao, Wilde, Cawood and Sun b , Reference Zhao, Wilde, Cawood and Sun2002; Guo, O'Brien & Zhai, Reference Guo, O'Brien and Zhai2002), and the ages of the metamorphic peaks were determined at around 1.85 Ga by different geochronologic methods (e.g. Zhao et al. Reference Zhao, Wilde, Cawood and Sun2002, Reference Zhao, Wilde, Sun, Li, Li and Zhang2008, Reference Zhao, Wilde, Guo, Cawood, Sun and Li2010; Wang et al. Reference Wang, Fan, Zhang and Guo2003a ,Reference Wang, Fan, Zhang and Guo b , Reference Wang, Fan, Zhang, Guo, Zhang and Peng2004; Guo et al. Reference Guo, Sun, Chen and Zhai2005; Kröner et al. Reference Kröner, Wilde, O'Brien, Li, Passchier, Walte and Liu2005, Reference Kröner, Wilde, Zhao, O'Brien, Sun, Liu, Wan, Liu and Guo2006; Liu, S. W. et al. Reference Liu, Zhao, Wilde, Shu, Sun, Li, Tian and Zhang2006, Reference Liu, Zhang, Liu, Li, Lü, Yu, Tian and Feng2007; Wan et al. Reference Wan, Wilde, Liu, Yang, Song and Yin2006; Trap et al. Reference Trap, Faure, Lin and Monié2007, Reference Trap, Faure, Lin, Monié and Bruguier2008, Reference Trap, Faure, Lin, Monié, Meffre and Melletton2009). Present studies suggest that the TNCO was involved in Palaeoproterozoic orogenic processes characterized by subduction and collision and later rapid uplift. However, most of the available data were retrieved from the northern and middle metamorphic terranes (e.g. Huai'an, Hengshan, Wutai, Fuping, Lüliang metamorphic complex; Fig. 1b) of the TNCO, whereas data derived from the southern part of the TNCO (e.g. Mei, Reference Mei1994; Wang et al. Reference Wang, Fan, Guo and Li2003a ,Reference Wang, Fan, Zhang and Guo b ; Wan et al. Reference Wan, Wilde, Liu, Yang, Song and Yin2006; Trap et al. Reference Trap, Faure, Lin, Monié, Meffre and Melletton2009; Xiao et al. Reference Xiao, Jiang, Wan, Wan, Wang and Wu2011a ) are fewer, especially from the Zanhuang metamorphic complex, which in turn, limits our understanding of the tectonic evolution of the TNCO: was the whole TNCO, from northern to southern parts, a unified orogenic belt resulting from Palaeoproterozoic orogenic processes?
Figure 1. (a) Sketch map of the North China Craton (modified after Zhao et al. Reference Zhao, Wilde, Cawood and Lu1998, Reference Zhao, Sun, Wilde and Li2005). (b) The Zanhuang metamorphic complex in the Trans-North China Orogen (modified after Zhao et al. Reference Zhao, Wilde, Cawood and Lu1998, Reference Zhao, Sun, Wilde and Li2005). (c) Geological sketch map of the Archaean–Palaeoproterozoic Zanhuang metamorphic complex (modified after Yang et al. Reference Yang, Du, Ren, Song, Wan, Xie and Liu2011). Sample locations are shown on the map.
We attempt to solve this problem through metamorphic and geochronologic investigations of the Zanhuang metamorphic complex, which is located in the south-middle region of the TNCO (Fig. 1b) – an appropriate window from which the Precambrian tectonometamorphic processes of the TNCO can be deciphered. According to our earlier study, amphibolites and metapelites exposed in this terrane record clockwise P–T paths including retrograde ITD processes (Xiao et al. Reference Xiao, Jiang, Wan, Wan, Wang and Wu2011a ,b), suggesting that the Zanhuang metamorphic complex was evolved in the collisional orogenic processes which led to the amalgamation of the Eastern Block and Western Block along the TNCO to form the coherent basement of the North China Craton. In this contribution, we have carried out high-resolution U–Pb dating of zircons separated from metapelites and amphibolites of the Zanhuang metamorphic complex in order to provide new and accurate chronologic data for the tectonic evolution of the TNCO.
2. Geological setting
The Zanhuang metamorphic complex is exposed in the lowland on the eastern slope of Mount Taihangshan in Hebei Province, northern China. The complex is made up of diverse Trondhjemite-Tonalite-Granodiorite (TTG) gneisses and a supracrustal assemblage and anatectic granites of Precambrian age, which were previously considered as a uniform stratigraphic sequence named the ‘Zanhuang Group’. Research studies in recent years indicate that the TTG gneisses and granites (e.g. the Xuting Granite) are well exposed, whereas the supracrustal assemblage named the ‘Neoarchaean Zanhuang Group’ is only locally exposed in this terrane (Fig. 1c; Yang et al. Reference Yang, Du, Ren, Song, Wan, Xie and Liu2011). Neoarchaean to Palaeoproterozoic metamorphic rocks in this complex are mainly composed of metapelites, amphibolites, amphibole-plagioclase gneisses, TTG gneisses, monzogranite gneisses, quartzites, marbles and metamorphosed sandstones, which unconformably underlie unmetamorphosed sedimentary rocks of the Mesoproterozoic Changcheng Group and Palaeoproterozoic Gantaohe Group (Fig. 1c). The amphibolites occur as layers or lens-shaped boudins within the metapelites and felsic gneisses, and geochemical features suggest that the protoliths of the amphibolites were sub-alkaline basaltic rocks, possibly formed in volcanic arcs within a continental margin. Intercalated amphibolites and metapelites were metamorphosed during the same tectonothermal event as evidenced by the consistent gneissosity recorded in these rocks. Three generations of metamorphic mineral assemblages have been identified in both of the metapelites and amphibolites. The prograde assemblages (M1) are inclusion minerals preserved in garnet porphyroblasts, the metamorphic peak assemblages (M2) are garnets and matrix minerals, and the retrograde assemblages (M3), nicknamed ‘white-eye socket’ (Ma & Wang, Reference Ma, Wang, Qian and Wang1994; O'Brien, Walte & Li, Reference O'Brien, Walte and Li2005), are symplectite minerals enclosing embayed garnets. The coronitic symplectites are characterized by mineral intergrowths of plagioclase + hornblende + quartz (Fig. 2b, d) in the amphibolites or plagioclase + biotite + quartz (Fig. 2c) in the metapelites surrounding garnet porphyroblasts, reflecting decomposition of the garnets during the tectonic denudation and fast uplifting of the terrane (Xiao et al. Reference Xiao, Jiang, Wan, Wan, Wang and Wu2011a ,b). Consistent clockwise P–T paths were obtained both by applying thermobarometric computation and thermodynamic forward modelling for the metapelites and the amphibolites in this terrane. The retrieved P–T paths are clockwise with ITD retrograde processes (Xiao et al. Reference Xiao, Jiang, Wan, Wan, Wang and Wu2011a ,b) and are typical of the western Alpine type (Ernst, Reference Ernst1988), which are believed to have resulted from a subduction-collisional orogeny followed by fast post-orogenic uplift.
Figure 2. Photomicrographs of metapelites and amphibolites. (a) Metapelite Sample HB34. Inclusions Pl + Bt + Qtz concentrated in the cores of garnets. (b) Amphibolite Sample HB32. Coronitic symplectite assemblage Pl + Hbl + Qtz constituting a ‘white-eye socket’ domain surrounding the garnet porphyroblast. (c) Amphibolite Sample HB154. The ‘white-eye socket’ symplectitic assemblage mainly consists of Pl + Bt + Hbl + Qtz. (d) Amphibolite Sample HB161. The garnet porphyroblasts are broken. (e) Amphibolite Sample HB175.
3. Sample selection and analytical methods
U–Pb ion microprobe dating of zircons of one representative metapelite (HB34) and four amphibolite (HB32, HB154, HB161 and HB175) samples are reported in this contribution. Sample HB34 was obtained from a quarry site in Xiaohe Village (Fig. 1c), and is a coarse-grained garnet-bearing gneiss with the metamorphic peak mineral assemblage of quartz (30%) + plagioclase (25%) + biotite (25%) + garnet (15%) + opaque minerals (5%). Garnet porphyroblasts contain inclusions (Fig. 2a) formed during the prograde stage and the garnet porphyroblasts preserve prograde chemical zoning except for the outer rim where the Fe no. (= Fe/(Fe+Mg)) gradually increases (Xiao et al. Reference Xiao, Jiang, Wan, Wan, Wang and Wu2011a ) owing to post-peak Fe–Mg re-exchange between the garnet and matrix biotite, similar to metapelites elsewhere (e.g. Tracy, Robinson & Thompson, Reference Tracy, Robinson and Thompson1976; Spear, Hickmott & Selverstone, Reference Spear, Hickmott and Selverstone1990) and as numerically modelled (Spear et al. Reference Spear, Kohn, Florence and Menard1990). A clockwise P–T path was derived from this sample (Xiao et al. Reference Xiao, Jiang, Wan, Wan, Wang and Wu2011a ). Four amphibolite samples (HB32, HB154, HB161 and HB175) were collected from different lenses at Xiaohe, Huamu, Bainai and Hujia'an villages in the south-middle part of the Zanhuang metamorphic complex (Fig. 1c), and these rocks were divided into two groups. Group 1 amphibolites include samples HB32, HB154 and HB161, and these amphibolites are medium–coarse-grained meta-basalt dykes/veins with the metamorphic peak mineral assemblage of plagioclase + quartz + hornblende + garnet + opaque minerals + biotite. Garnet porphyroblasts in samples HB32 and HB154 preserve chemical zoning and the Pl + Hbl + Qtz (Fig. 2b) or Pl + Bt + Hbl + Qtz (Fig. 2c) ‘white-eye socket’ symplectite enclosed garnets (Xiao et al. Reference Xiao, Jiang, Wan, Wan, Wang and Wu2011a ,b). Garnet grains in Sample HB161 are the smallest (less than 0.3 mm), and some garnet grains have been completely consumed and become symplectitic intergrowths of plagioclase + hornblende + biotite (Fig. 2d). The Group 2 amphibolite is Sample HB175, which only preserves the metamorphic peak mineral assemblage, and mainly consists of hornblende (85%) with minor amounts of apatite (7%), magnetite (5%), plagioclase (2%) and quartz (1%) (Fig. 2e).
All these samples were processed by conventional heavy mineral separation techniques to concentrate non-magnetic, heavy fractions. Then zircon grains were hand-picked, mounted in epoxy mounts which were then polished to section the crystals in half and gold/carbon-coated. Cathodoluminescence (CL) imaging of zircon grains was carried out prior to analysis. Zircon isotopic analyses of samples HB154, HB161 and HB175 were performed on the Cameca IMS-1280 secondary ion mass spectrometer (SIMS) at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. The spot size was 20 × 30 μm for SIMS measurements. The zircon targets were mounted with zircon standards Plésovice (337 Ma, Sláma et al. Reference Sláma, Košler, Condon, Crowley, Gerdes, Hanchar, Horstwood, Morris, Nasdala, Norberg, Schaltegger, Schoene, Tubrett and Whitehouse2008) and Qinghu (159.5 Ma, Li et al. Reference Li, Liu, Li, Guo and Chamberlain2009). U–Th–Pb ratios of the isotopes were calibrated to the Plésovice standard zircon, and an in-house zircon standard Qinghu was analysed as an unknown to monitor the external uncertainties of SIMS U–Pb measurements. Isotopes and trace elements of zircons from samples HB32 and HB34 were analysed by laser ablation inductively couple plasma mass spectrometry (LA-ICP-MS) at Northwest University, Xi'an. The spot size was 20 μm in diameter for LA-ICP-MS measurements. Zircon standard 91500 (Wiedenbeck et al. Reference Wiedenbeck, Alle, Corfu, Griffin, Meier, Oberli, Vonquadt, Roddick and Speigel1995) with the U concentration of 81 ppm was used for correction of U–Pb fractionation during LA-ICP-MS analyses, and the reference value of the weighted mean 206Pb–238U age was 1065.4 ± 0.6 Ma (Wiedenbeck et al. Reference Wiedenbeck, Alle, Corfu, Griffin, Meier, Oberli, Vonquadt, Roddick and Speigel1995). In determining the element concentration, Si was used as an internal standard and NIST 610 was used as an external standard. The analytical procedures are the same as those described by Li et al. (Reference Li, Liu, Li, Guo and Chamberlain2009, Reference Li, Li, Liu, Tang, Yang and Zhun2010) for SIMS and by X. M. Liu et al. (Reference Liu, Zhang, Liu, Li, Lü, Yu, Tian and Feng2007) for LA-ICP-MS analyses. All reported ages (see Table S1 in the online Supplementary Material at http://journals.cambridge.org/geo) were calculated using the recommended values of Steiger & Jäger (Reference Steiger and Jäger1977). Measured compositions were corrected for common Pb using non-radiogenic 204Pb, and an average of present-day crustal composition (Stacey & Kramers, Reference Stacey and Kramers1975) was used for the common Pb. Uncertainties on individual analyses are reported at the 1σ level and the weighted mean ages for pooled 207Pb/206Pb analyses are quoted at the 95% confidence level. The analytical data were reduced and plotted using the Isoplot/Ex v. 3.23 program (Ludwig, Reference Ludwig2003).
4. Geochronological results
4.a. Metapelite
Zircons extracted from Sample HB34 are light yellow, semi-transparent and ranging from 50 to 200 μm in length. The zircon grains are prismatic in shape, and the surfaces are bumpy under the transmission microscope. Their components show great variation in U and Th contents and Th/U ratios, ranging, respectively, from 20 to 453 ppm, 2 to 325 ppm and 0.08 to 1.18, which are ascribed to be of typical detrital origin. The zircons show no core–rim texture and variable luminescence, and they preserve weak–indistinct oscillatory zoning in CL images (Fig. 3a). Twenty-seven spot analyses of 21 zircon grains yield 207Pb–206Pb ages ranging from 1885 to 2504 Ma, and most of these ages are > 2300 Ma, except for the analytical spots HB34–12 (2089 Ma), HB34–19 (1885 Ma) and HB34–25 (2058 Ma). Variable luminescence and residual oscillatory zoning indicate these ages are mixed, resulting from the recrystallization of detrital zircon under hydrothermal conditions. Nine core analyses are located on the concordant curve, yielding a weighted mean age of 2504 ± 8 Ma (Fig. 4a); they possibly represent the minimum deposit age and pre-date the maximum metamorphic age. Because the heavy rare earth elements (HREEs) prefer to fractionate into garnet during the metamorphic process, the REE distribution patterns show that the HREEs change insignificantly (Fig. 5a). Such HREE patterns of the zircons imply that there were no metamorphic zircons formed during the metamorphism.
Figure 3. The CL images of typical zircon grains of samples from the Zanhuang metamorphic complex. (a) Metapelite Sample HB34. (b) Amphibolite Sample HB32. (c) Amphibolite Sample HB161. (d) Amphibolite Sample HB154. (e) Amphibolite Sample HB175. The black and white circles represent the spots of SIMS or LA-ICP-MS measurements. The numbers refer to the analytical data listed in Table S1 in the online Supplementary Material at http://journal.cambridge.org/geo.
4.b. Amphibolite
Zircons separated from the Group 1 amphibolites (Samples HB32, HB154 and HB161) are light brown, round and range from 10 to 120 μm in length. These zircon grains are anhedral and characterized by being internally homogeneous with high luminescence in the CL images. The Th/U ratios (0.01–0.41) are mostly lower than 0.1, except for the analytical spots on HB154–1 (0.21), HB154–7 (0.36), HB154–10 (0.34), HB154–11 (0.36) and HB154–14 (0.41), indicative of a metamorphic origin. U and Th contents range, respectively, from 4 to 222 ppm and 0 to 63 ppm. The HREEs in zircons from Sample HB32 show insignificant change in their distribution patterns (Fig. 5b). Forty-nine spots were analysed on 49 zircon grains from these three samples. As shown on the concordia diagrams, most of the analytical spots are concordant, yielding weighted mean 207Pb–206Pb ages of 1860 ± 27 Ma (n = 21; 95% confidence; MSWD = 1.6; Fig. 4b) for Sample HB32, 1845 ± 8 Ma (n = 13; 95% confidence; MSWD = 0.33; Fig. 4d) for Sample HB154 and 1840 ± 6 Ma (n = 15; 95% confidence; MSWD = 1.2; Fig. 4c) for Sample HB161. These data imply that the amphibolites might have been involved in the metamorphic event of ~ 1.85 Ga in the late Palaeoproterozoic.
Figure 4. The U–Pb concordia diagrams showing analytical data for zircons from the Zanhuang metamorphic complex. (a) Metapelite Sample HB34. (b) Amphibolite Sample HB32. (c) Amphibolite Sample HB161. (d) Amphibolite Sample HB154. (e) Amphibolite Sample HB175.
Figure 5. Chondrite-normalized REE distribution patterns of zircons in Sample HB34 (a) and Sample HB32 (b).
Zircons from the Group 2 amphibolite (Sample HB175) are light-coloured, subhedral, long-prismatic and range from 90 to 250 μm in length. The grains are generally characterized by wide concentric oscillatory zoning and variable luminescence (Fig. 3e). The components show minor variation in U and Th contents ranging, respectively, from 81 to 274 ppm and 46 to 356 ppm, and the Th/U ratios (0.44–1.30) are higher than 0.1. The CL images show that the outer peripheral parts (< 5 μm) of most of the zircons have high luminescence. The 13 spot analyses yield a weighted mean 207Pb–206Pb age of 2594 ± 4 Ma (95% confidence; MSWD =1.1; Fig. 4e). There is not sufficient evidence to confirm that these zircons are igneous in origin, and these ages are interpreted to be inherited and not the crystallization age of the protolith of the amphibolite.
5. Discussion and conclusion
There is ongoing debate about the timing of the final assembly of the North China Craton. Some workers believe that the basement of the North China Craton was finally fixed at ~ 2.5 Ga (e.g. Zhai, Bian & Zhao, Reference Zhai, Bian and Zhao2000), whereas others believe that it was finally amalgamated at ~ 1.85 Ga (e.g. Zhao et al. Reference Zhao, Wilde, Cawood and Lu1998, Reference Zhao, Sun, Wilde and Li2005) through collision between the Western Block and the Eastern Block along the TNCO. Geochronological data suggest that the high-pressure granulites in the northern and central segments of the TNCO record a ubiquitous metamorphic event in the Palaeoproterozoic (e.g. Zhao et al. Reference Zhao, Wilde, Cawood and Sun2002, Reference Zhao, Wilde, Sun, Li, Li and Zhang2008, Reference Zhao, Wilde, Guo, Cawood, Sun and Li2010; Guo et al. Reference Guo, Sun, Chen and Zhai2005; Kröner et al. Reference Kröner, Wilde, O'Brien, Li, Passchier, Walte and Liu2005, Reference Kröner, Wilde, Zhao, O'Brien, Sun, Liu, Wan, Liu and Guo2006; Liu, S. W. et al. Reference Liu, Gao, Diwu, Yuan and Hu2007; Trap et al. Reference Trap, Faure, Lin and Monié2007, Reference Trap, Faure, Lin, Monié and Bruguier2008). Metamorphic ages obtained from the Zanhuang metamorphic complex are also in accordance with the ~ 1.85 Ga collision model, whereas few studies have been done on the metamorphic complexes in the southern terranes (e.g. the Zhongtiao, Dengfeng and Taihua complexes) of the TNCO. Furthermore, Wang et al. (Reference Wang, Fan, Guo and Li2003a ,Reference Wang, Fan, Zhang and Guo b ) concluded that the major tectonothermal event influencing the Zanhuang complex had occurred between 1870 and 1793 Ma, according to the main deformation ages determined by 40Ar–39Ar dating of biotite and K-feldspar in mylonites. Trap et al. (Reference Trap, Faure, Lin, Monié, Meffre and Melletton2009) dated metamorphic monazites within the matrix of a mica-schist to be 1824 Ma by electron microprobe Th–U–Pb dating and obtained an age of 1800 Ma determined by 40Ar–39Ar dating of hornblendes from amphibolite and orthogneiss. Also, Xiao et al. (Reference Xiao, Wu, Zhao, Guo and Ren2011 a) found that the metamorphic domains of zircons separated from a metapelite sample (HB135) gave a weighted mean 207Pb–206Pb age of 1821 ± 17 Ma.
Thus, the deposition age of the metapelites and the crystallization age of the amphibolites in the Zanhuang metamorphic complex are, respectively, younger than ~ 2500 Ma and ~ 2600 Ma, and the metapelites and the amphibolites have experienced upper-amphibolite-facies metamorphism, which suggest that the metamorphic age should be < 2500 Ma. As described above, zircon U–Pb chronological analyses indicate that metamorphic zircons were formed during 1840–1860 Ma. Petrographic observations suggest that there are no zircons in the symplectites of the amphibolites and metapelites, and tiny zircon inclusions in the garnet porphyroblasts cannot be separated from the garnets; thus it can be inferred that these zircons were extracted from the matrix assemblages in the rocks and the metamorphic ages (~ 1850 Ma) are possibly close to the peak of metamorphism. These metamorphic ages are in accordance with the previous few chronological data obtained from the Zanhuang complex and other complexes in the central and northern segments of the TNCO (Zhao et al. Reference Zhao, Wilde, Cawood and Sun2002, Reference Zhao, Wilde, Sun, Li, Li and Zhang2008, Reference Zhao, Wilde, Guo, Cawood, Sun and Li2010; Wang et al. Reference Wang, Fan, Guo and Li2003a ,Reference Wang, Fan, Zhang and Guo b , Reference Wang, Fan, Zhang, Guo, Zhang and Peng2004; Guo et al. Reference Guo, Sun, Chen and Zhai2005; Kröner et al. Reference Kröner, Wilde, O'Brien, Li, Passchier, Walte and Liu2005, Reference Kröner, Wilde, Zhao, O'Brien, Sun, Liu, Wan, Liu and Guo2006; Wan et al. Reference Wan, Wilde, Liu, Yang, Song and Yin2006; Liu, S. W. et al. Reference Liu, Gao, Diwu, Yuan and Hu2007; Trap et al. Reference Trap, Faure, Lin and Monié2007, Reference Trap, Faure, Lin, Monié and Bruguier2008, Reference Trap, Faure, Lin, Monié, Meffre and Melletton2009; Xiao et al. Reference Xiao, Jiang, Wan, Wan, Wang and Wu2011a ).
Therefore, from the evidence from reaction textures, metamorphic P–T paths and the metamorphic ages of ~ 1850 Ma, it is inferred that the Zanhuang metamorphic complex was involved in the amalgamation of the Eastern Block and the Western Block along the N–S-striking TNCO during the late Palaeoproterozoic (~ 1.85 Ga), similar to the complexes in the central and northern terranes of the TNCO. Moreover, the widely occurring undeformed and unmetamorphosed Palaeoproterozoic sedimentary rocks (Liu et al. Reference Liu, Zhao, Liu, Sun, Zhang and Yin2012) and mafic dykes (~ 1.77 Ga) (Wang et al. Reference Wang, Fan, Zhang, Guo, Zhang and Peng2004, Reference Wang, Zhao, Cawood, Fan, Peng and Sun2008; Peng et al. Reference Peng, Zhai, Zhang and Guo2005, Reference Peng, Zhai, Ernst, Guo, Liu and Hu2008, Reference Peng, Liu, Zhai and Guo2012; Peng, Zhai & Guo, Reference Peng, Zhai, Guo, Hanski, Mertanen, Rämö and Vuollo2006) throughout the TNCO, postdate the collision events of the orogenic belt.
Acknowledgements
Prof. Dr Xian-Hua Li helped the authors in analysing the isotopes on the Cameca IMS 1280. This work was financed by the National Natural Science Foundation of China (40672129), the Chinese National 973 Programme (2012CB416603), the Chinese Academy of Sciences (XDA08110000) and the Geology Survey of China (1212011120150). We also thank Professor Simon Wilde and an anonymous referee for their corrections, which greatly improved both the science and the English of the original manuscript.
