1. Introduction
The North China Craton is the largest and oldest continental block in China, with rocks up to 3.8 Ga old (Liu et al. Reference Liu, Nutman, Compston, Wu and Shen1992; Song et al. Reference Song, Nutman, Liu and Wu1996). This craton has traditionally been considered to have a relatively uniform Archaean to Palaeoproterozoic basement. However, recent studies have applied terrane accretion and collisional models to the craton (Li et al. Reference Li, Wang, Wang, Liu and Zhao1990; Li, Hart & Wu, Reference Li, Hart and Wu1990; Bai, Reference Bai, Ma and Bai1996; Wang et al. Reference Wang, Li, Hao, Li and Zhou1996; Wu & Zhong, Reference Wu and Zhong1998; Zhao et al. Reference Zhao, Wilde, Cawood and Lu1998, Reference Zhao, Wilde, Cawood and Lu1999a, Reference Zhao, Cawood, Wilde and Sun2000a, Reference Zhao, Cawood, Wilde and Lu2001a, Reference Zhao, Wilde, Cawood and Sunb, Reference Zhao, Sun, Wilde and Li2005; Wilde, Zhao & Sun, Reference Wilde, Zhao and Sun2002; Kusky & Li, Reference Kusky and Li2003; Zhai & Liu, Reference Zhai and Liu2003; Kusky, Li & Santosh, Reference Kusky, Li and Santosh2007). A Palaeoproterozoic collisional belt, named the Trans-North China Orogen, was recognized. Along this belt, two discrete Archaean to Palaeoproterozoic continental blocks, called the Eastern and Western blocks, were amalgamated to form a coherent basement of the North China Craton at c. 1.85 Ga (Fig. 1; Zhao et al. Reference Zhao, Wilde, Cawood and Lu1998, Reference Zhao, Wilde, Cawood and Lu2000b, Reference Zhao, Cawood, Wilde and Lu2001a, Reference Zhao, Sun, Wilde and Li2005; Wilde et al. Reference Wilde, Cawood, Wang and Nemchin1997, Reference Wilde, Cawood, Wang and Nemchin1998, Reference Wilde, Cawood, Wang, Nemchin, Zhao, Malpas, Fletcher, Aitchison and Ali2004a, Reference Wilde, Zhao, Wang and Sunb, Reference Wilde, Cawood, Wang and Nemchin2005; Wilde, Zhao & Sun, Reference Wilde, Zhao and Sun2002; Guo & Zhai, Reference Guo and Zhai2001; Guo, O'Brien & Zhai, Reference Guo, O'Brien and Zhai2002; Guo et al. Reference Guo, Sun, Chen and Zhai2005; Kröner et al. Reference Kröner, Wilde, Li and Wang2005a, Reference Kröner, Wilde, O'Brien, Li, Passchier, Walte and Liub, Reference Kröner, Wilde, Zhao, O'Brien, Sun, Liu, Wan, Liu and Guo2006; Faure et al. Reference Faure, Trap, Lin, Monié and Bruguier2007; Trap et al. Reference Trap, Faure, Lin and Monié2007). There is now a clear outline of the timing and tectonic processes involved in the amalgamation of the North China Craton, probably coinciding with assembly of the supercontinent Columbia (Rogers & Santosh, Reference Rogers and Santosh2002; Zhao et al. Reference Zhao, Cawood, Wilde and Sun2002a, Reference Zhao, Sun, Wilde and Li2003, Reference Zhao, Sun, Wilde and Li2004). However, the crustal evolution history of this craton before amalgamation remains poorly understood.
Figure 1. Tectonic subdivision of the North China Craton and spatial distribution of the Palaeoproterozoic supracrustal rocks (after Zhao et al. Reference Zhao, Sun, Wilde and Li2005). CD – Chengde; ZH – Zhahuang.
Palaeoproterozoic metasedimentary rocks of different metamorphic grades, which preserve the history of the exposed continental crust at the time of their deposition, are widely distributed in the North China Craton, including the Liaohe, Laoling, Ji'an, Jingshan and Fenzishan groups in the Jiao-Liao-Ji Belt within the Eastern Block and Jining, Daqingshan–Wulashan, Helanshan–Qianlishan complexes within the Khondalite Belt in the Western Block (Fig. 1). In the Trans-North China Orogen, metasedimentary rocks are exposed in the Shangtaihua, Hutuo groups and the Fuping, Lüliang complexes (Fig. 1). Previous U–Pb studies on detrital zircons have shown that all these rocks within the three discrete mobile belts experienced the late Palaeoproterozoic metamorphism (Luo et al. Reference Luo, Sun, Zhao, Li, Xu, Ye and Xia2004, Reference Luo, Sun, Zhao, Ayers, Li, Xia and Zhang2008; Lu et al. Reference Lu, Wu, Guo, Wilde, Yang, Liu and Zhang2006; Xia et al. Reference Xia, Sun, Zhao and Luo2006a, Reference Xia, Sun, Zhao, Wu, Xu, Zhang, He and Zhangb, Reference Xia, Sun, Zhao, Wu, Xu, Zhang and Luoc; Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a, Reference Wan, Wilde, Liu, Song, Yang and Yinb), but there are some important differences.
Firstly, the metamorphic overgrowth zircon rims or zircons with metamorphic origin in the Khondalite Belt and the Jiao-Liao-Ji Belt yield some ages at about 1.92 Ga and 1.93 Ga, respectively (Luo et al. Reference Luo, Sun, Zhao, Li, Xu, Ye and Xia2004, Reference Luo, Sun, Zhao, Ayers, Li, Xia and Zhang2008; Lu et al. Reference Lu, Wu, Guo, Wilde, Yang, Liu and Zhang2006; Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a; Santosh, Wilde & Li, Reference Santosh, Wilde and Li2007), while 1.88–1.82 Ga metamorphic ages are recorded in the Trans-North China Orogen (Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a, Reference Wan, Wilde, Liu, Song, Yang and Yinb). This suggests that the continental collision between the Western and Eastern blocks might have happened slightly later than the collision between the Yinshan and Ordos terranes along the Khondalite Belt, as well as the closure of the rift along the Jiao-Liao-Ji Belt. The second difference is the ages of rocks in their source region. Both Archaean (c. 2.5 Ga) and Palaeoproterozoic rocks are abundant in the metasedimentary source rocks from the Jiao-Liao-Ji Belt of the Eastern Block (Luo et al. Reference Luo, Sun, Zhao, Li, Xu, Ye and Xia2004, Reference Luo, Sun, Zhao, Ayers, Li, Xia and Zhang2008; Wan et al. Reference Wan, Wilde, Liu, Song, Yang and Yin2006b) and the Shangtaihua, Hutuo groups and the Fuping Complex in the Trans-North China Orogen (Xia et al. Reference Xia, Sun, Zhao, Wu, Xu, Zhang, He and Zhang2006b; Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a, Reference Wan, Wilde, Liu, Song, Yang and Yinb), while Palaeoproterozoic rocks dominate the source region for those metasedimentary rocks from the Khondalite Belt of the Western Block (Xia et al. Reference Xia, Sun, Zhao and Luo2006a, Reference Xia, Sun, Zhao, Wu, Xu, Zhang and Luoc; Wan et al. Reference Wan, Wilde, Liu, Song, Yang and Yin2006b).
The Lüliang Complex is located in the central segment of the Trans-North China Orogen (Fig. 1). Of particular significance is that some metasedimentary rocks in this complex are similar to those from the Khondalite Belt, not those from the Trans-North China Orogen. Therefore, the Lüliang Complex is considered to have been thrust from the west and subsequently incorporated into the Trans-North China Orogen during the collision between the Eastern and Western blocks. However, they have not been investigated, especially in terms of detrital zircon U–Pb ages and Hf isotope compositions. In this study, we carried out an integrated study of U–Pb and Hf isotope on the detrital zircons separated from the Lüliang khondalites to test this idea. Our results provide important constraints not only on their provenance, but also on controversial and ambiguous issues regarding the tectonic evolution of the North China Craton.
2. Geological background
The Lüliang Complex in the Trans-North China Orogen is located in an area adjacent to the eastern margin of the Western Block (Fig. 1) and mainly consists of metasedimentary rocks intruded by granitoid plutons (Geng et al. Reference Geng, Wan, Shen, Li and Zhang2000; Wan et al. Reference Wan, Geng, Shen and Zhang2000b; Zhao et al. Reference Zhao, Wilde, Sun, Li, Li and Zhang2008). The metasedimentary rocks of the complex have been divided into the Jiehekou, Lüliang and Yejishan/Heichashan groups (Fig. 2; Yu, Wang & Wang, Reference Yu, Wang and Wang1997; Geng et al. Reference Geng, Wan, Shen, Li and Zhang2000; Wan et al. Reference Wan, Geng, Shen and Zhang2000b). The metasedimentary rocks from the Jiehekou Group share common lithological, mineralogical and geochemical features with those of the Khondalite Belt and are referred to as khondalites in the Chinese literature (Qian & Li, Reference Qian and Li1999; Li, Qian & Liu, Reference Li, Qian and Liu2000; Wan et al. Reference Wan, Geng, Liu, Shen, Liu and Song2000a, Reference Wan, Geng, Shen and Zhangb). These rocks are composed of subequal amounts of quartz, plagioclase, K-feldspar and garnet. Biotite and sillimanite are important in most samples. Trace minerals include zircon, apatite, iron oxides and sphene. They are characterized by large ion lithophile elements (LILE) enrichment, and lower abundances of Zr, Hf and Sr. Their rare earth element (REE) distribution has significant LREE enrichment and a negative Eu anomaly. They are mainly exposed in the northwestern part of the complex (Fig. 2) and the major rock types are amphibolite- to granulite-facies graphite-bearing pelitic gneiss/schist, quartzite, felsic paragneiss, marble, calc-silicate rock and minor amphibolite/granulite. Single khondalite layers commonly range in thickness from tens to hundreds of metres. These khondalites show multiple phases of deformation, and commonly occur as strongly foliated, transposed and folded tectonic slices in the field, as manifested by repeated lithological sequences (Li, Qian & Liu, Reference Li, Qian and Liu2000). Locally, a gradational sequence consisting of fine-grained paragneiss, schist and gneiss can be observed (Li, Qian & Liu, Reference Li, Qian and Liu2000). In some locations, leucocratic bands and minor garnet-bearing granitic bodies are present in the khondalites, which were avoided in the field sampling.
Figure 2. Geological map of the Lüliang Complex and sample locality (after Zhao et al. Reference Zhao, Wilde, Sun, Li, Li and Zhang2008). TTG – tonalitic–trondhjemitic–granodioritic.
The Lüliang Group crops out only in the Jinzhouyu area (Fig. 2) and is made up of predominantly basalt, andesite, dacite, rhyolite, wacke, sandstone, banded iron formation and argillaceous rocks metamorphosed in greenschist facies. Coarse-grained amphibolites with preserved gabbroic texture and fine-grained metabasite derived from basalt or diabase crop out as lenses intercalated within sandstone. The metre size of the mafic blocks and their scattering in the terrigeneous rocks suggest that they might be olistoliths (Faure et al. Reference Faure, Trap, Lin, Monié and Bruguier2007). This group is considered to have formed in a magmatic arc and/or a back-arc basin environment (Faure et al. Reference Faure, Trap, Lin, Monié and Bruguier2007; Trap et al. Reference Trap, Faure, Lin and Monié2007). Regionally, the Lüliang Group is not in direct contact with the Jiehekou Group (Fig. 2), but both groups are tectonically overlain by the Yejishan/Heichashan Group, which occurs along a narrow, NE–SW-trending belt that extends from Dongmafang, through Yejishan and Heichashan, to Fangshan (Fig. 2). The Yejishan/Heichashan Group consists of sub-greenschist-facies metamorphosed conglomerates, quartz wackes, sandstones and minor limestones, lithologically and metamorphically similar to the Hutuo Group that overlies the Wutai Group in the Wutai area (Fig. 1). Widespread mafic volcanic rocks in this group were dated by the zircon U–Pb method at 2124 ± 38 Ma (Geng et al. Reference Geng, Wan, Shen, Li and Zhang2000). A foreland basin has been proposed as the formation environment for this group (Bai, Wang & Guo, Reference Bai, Wang and Guo1992; Wu & Zhong, Reference Wu and Zhong1998; Wilde et al. Reference Wilde, Zhao, Wang and Sun2004b; Zhang et al. Reference Zhang, Zhao, Sun, Liu, Xia and He2006a).
3. Analytical procedures
The samples were crushed and subsequently separated by a heavy liquid and magnetic method. Zircons were hand picked and mounted on an adhesive tape, enclosed in epoxy resin and then polished to about half their size. Cathodo-luminescence (CL) imaging of zircon grains was taken in order to observe the inner structure and choose potential target sites for U–Pb dating and Hf analyses at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing, using a CAMECA SE-50 microprobe.
3.a. U–Pb dating
A VG PQ Excel ICP-MS machine equipped with a New Wave Research LUV213 laser ablation system, at the University of Hong Kong, was utilized to analyse the U–Pb isotope compositions of zircons. The laser system delivers a beam of 213 nm UV light from a frequency-quintupled Nd:YAG laser. Analytical settings include a beam diameter of ~ 40 μm, 10 Hz repetition rate, and energy of 0.6–1.3 mJ per pulse. This gave a 238U signal of 3 × 104 to 100 × 104 counts, depending on U contents. A typical ablation time was 30–60 s, resulting in pits 20–40 μm deep. Helium carrier gas was used to transport the ablated sample materials from the laser-ablation cell to the ICPMS torch after mixing with argon gas. No common lead correction has been made because the common lead concentration is generally below the detection limit of the ICP-MS. More detailed instrumental settings and analytical procedures have been described by Xia et al. (Reference Xia, Sun, Zhao, Li and Zhou2004).
3.b. Hf isotope analyses
Hf isotope analyses were conducted by a Neptune multi-collector (MC) ICPMS coupled with an ArF excimer laser ablation system at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing. Ablated material in the laser-ablation cell was transported to the ICPMS torch by helium carrier gas. Analytical settings include a beam diameter of ~ 63 μm, 10 Hz repetition rate, and energies density of 15 J/cm2, which yielded a signal intensity of ~ 10 V at 180Hf for the standard zircon 91500 in situ analysis. During analysis, raw count rates for 172Yb, 173Yb, 175Lu, 176(Hf+Yb+Lu), 177Hf, 178Hf, 179Hf, 180Hf and 182W were collected. Interference of 176Lu on 176Hf was corrected by measuring the intensity of the interference-free 175Lu isotope and using the recommended 176Lu/ 175Lu ratio of 0.02655 (Chu et al. Reference Chu, Taylor, Chavagnac, Nesbitt, Boella, Milton, German, Bayon and Burton2002). The 176Yb/172Yb value of 0.5887 and mean βYb value obtained during Hf analysis on the same spot were applied for the interference correction of 176Yb on 176Hf. The sites of spot Hf analysis are adjacent to those of U–Pb analysis within the same cathodoluminescence domain of zircons. More detailed instrumental settings and analytical procedures have been described by Xu et al. (Reference Xu, Wu, Xie and Yang2004).
4. Results
4.a. CL images of zircons
The CL images show that most detrital zircons from the Lüliang khondalites have concentric oscillatory zoning (Fig. 3b–e, h, i), indicating their igneous origin. A minor portion of zircons have discernable cores mantled by an oscillatory zoning rim (Fig. 3a, f, g, j), which are interpreted to be inherited from the source of the precursor magma.
Figure 3. Representative cathodoluminescence images for zircons from the khondalites in the Lüliang Complex (see text for details).
4.b. U–Pb and Hf isotopic results
A total of 189 U–Pb and 146 Hf isotope analyses have been conducted for the three samples in this study. The analytical data are presented in Table 1 and Appendix Table A1, the latter available as supplementary material online at http://journals.cambridge.org/geo.
Table 1. Hf-isotope analyses for detrital zircons from the khondalites of the Lüliang Complex
4.b.1. Sample LL006
This sample is a meta-quartzose sandstone, collected from 200 m west of Dongnan Village, about 40 km south of Xingxian (N38° 09.033′; E111° 18.517′; Fig. 2). The sample consists of more than 90% quartz and minor flint and silicic lithic fragments. Most detrital zircons from the sample are euhedral or subeuhedral and have concentric oscillatory zoning (Fig. 3b, c), with Th/U ratios ranging from 0.26 to 6.93, typical of an igneous origin. Some zircon grains have concentric oscillatory zoned cores, which can be recognized by discordant zoning from the major parts of the zircons (Fig. 3a). Zircon U–Pb isotopic results are presented in online Appendix Table A1 at http://journals.cambridge.org/geo and in a concordia diagram (Fig. 4a). Most of the 58 analyses are discordant, with a wide range of 207Pb–206Pb ages between 1.9 and 2.6 Ga. Three age populations are defined at 2.0 Ga, 2.2 Ga and 2.4 Ga by concordant or nearly concordant data on the concordia diagram. A number of age peaks mainly at 1976, 2100, 2182, 2288, 2400 and 2544 Ma appear on the histogram based on 207Pb–206Pb age data with discordance between −20% and 20% (Fig 4a, inset). These zircons have εHf values from −11.1 to +8.2 with a peak at +5 (Fig. 5a), suggesting that while old crust was involved in the magmatism, a juvenile source was very significant. The Hf model ages (TCDM) range from 2.17 to 3.14 Ga with a dominant peak at 2.4–2.6 Ga on the histogram (Fig. 5d).
Figure 4. Concordia diagram of LA-ICPMS U–Pb zircon analytical results. The insets are histograms for the distribution of 207Pb–206Pb ages. (a) Sample LL006; (b) sample LL008; (c) sample LL009.
Figure 5. Probability plots of εHf values and Hf model ages (TCDM) for (a) sample LL006, (b) sample LL008 and (c) sample LL009. The vertical dotted line indicates CHUR composition.
4.b.2. Sample LL008
This sample is a sillimanite-bearing biotite gneiss collected from west of Hejiawan Village, about 50 km west of Loufan (N38° 09.493′; E111° 16.619′; Fig. 2). The sample contains 25–40% plagioclase, 25–30% quartz, 15–20% biotite and 2–10% garnet and sillimanite. Biotite defines a marked foliation, and sillimanite generally occurs in foliation planes. Most detrital zircons from the sample are euhedral or subeuhedral and have obviously concentric oscillatory zoning (Fig. 3d, e). Their Th/U ratios range from 0.1 to 2.1, indicating an igneous origin. Some zircon grains have distinct luminescent cores (Fig. 3f, g) with 207Pb–206Pb ages differing from their oscillatorily zoned mantles (LL008–46 and LL008–47, online Appendix Table A1), suggesting the core is inherited in origin.
Three highly disconcordant analytical results, out of a total of 78, for inherited cores give 207Pb–206Pb ages up to 3.4 Ga (Fig. 4b). The other 75 analyses mostly give concordant or nearly concordant ages between 2000 and 2600 Ma. The 207Pb–206Pb age histogram for nearly concordant data (with discordance between −20% and 20%; Fig. 4b, inset) has multiple age peaks from 2030 to 2300 Ma. Hf isotope analyses for these zircons yield εHf values from −16.0 to +9.2 (Fig. 5b), suggesting derivation of their precursor magmas from both old crust and juvenile materials. The TDMC model ages are quite diverse, ranging from 2.18 to 3.90 Ga and showing a bimodal distribution on the histogram, with peaks at 2.29 and 2.61 Ga (Fig. 5e).
4.b.3. Sample LL009
This sample is a metamorphosed pebble-bearing quartzose sandstone, collected from the Yangpo Reservoir in Donghui Village, about 40 km south of Xingxian (N38° 10.453′; E111° 12.225′; Fig. 2). Granular quartziferous or granitic pebbles are usually 4–10 mm in size, and more than 90% of the composition of this sample is quartz. Similar to those in the above two samples, zircons from this sample are mostly subhedral to euhedral, with concentric oscillatory zoning (Fig. 3h, i), and Th/U ratios range from 0.20 to 2.49, indicating an igneous provenance. Some zircon grains have concentric oscillatory zoned cores, which can be recognized by discordant zoning with the major parts of zircons (Fig. 3j), which are interpreted as inherited or xenocrystic cores.
Forty-seven out of 54 U–Pb analyses define a good discordant line with an upper intercept age of 2066 ± 10 Ma (MSWD = 6.0, Fig. 4c, open ellipse). These zircons yield a range of 207Pb–206Pb ages between 1943 and 2140 Ma. The other seven analytical results (Fig. 4c, solid ellipse) show varied degrees of discordance and give ages from 2180 to 2512 Ma. On the histogram for 207Pb–206Pb age data with discordance between −20% and 20%, a dominating sharp peak appears at 2075 Ma (almost the same as the intercept age) (Fig. 4c, inset). The Hf-isotope analyses give εHf values ranging from +0.24 to +7.69, mostly at +5 (Fig. 5c), showing that their source was mainly juvenile. The TCDM model ages range from 2.15 to 2.63 Ga with a single sharp peak at 2.31 Ga (Fig. 5f).
5. Discussion
5.a. Age of source rocks and crustal evolution history in the source area
The detrital zircons from the Lüliang khondalites have 207Pb–206Pb ages ranging from 1.9 Ga to 2.3 Ga, mostly at 2.0 Ga (Fig. 4), indicating that Palaeoproterozoic rocks were dominant in the source region. Minor portions of concordant or nearly concordant Archaean ages are also obtained, and one highly discordant data point gives a suspicious 207Pb–206Pb age up to 3.4 Ga. However, these ages are mainly obtained from the xenocrystic or inherited cores.
The composite plot of εHf values versus 207Pb–206Pb ages for all the analysed zircons (including detrital zircon and their inherited cores) shows that positive εHf values are dominant for the detrital zircons from the Lüliang khondalites and the maximum εHf values occur for some c. 2.1–2.2 Ga zircons, which plot on or near to the depleted mantle evolutionary line (Fig. 6). Thereafter, the highest εHf values decrease with age, corresponding to a 176Lu/177Hf ratio of 0.007, almost equal to that of the upper continent (0.008; Taylor & McLennan, Reference Taylor and McLennan1985; Fig. 6). This means that a large volume of magmas from a juvenile source were incorporated into the crust during Palaeoproterozoic times at 2.1 to 2.2 Ga. About 95% of Hf isotopic data plot above the c. 2.6 Ga crust reference line (Fig. 6), suggesting that there was a 2.6 Ga pre-existing lower crust underlying their source region.
Figure 6. Composite plots of εHf values v. 207Pb–206Pb age for all analysed zircons in this study.
5.b. Constraints on the sedimentary provenance of the Lüliang khondalites
Due to their high mechanical and chemical resistance, accessory minerals such as zircons have been used extensively as markers of provenance in sediments and (meta-) sedimentary rocks (Bodet & Scharer, Reference Bodet and Scharer2000; Condie et al. Reference Condie, Beyer, Belousova, Griffin and O'Reilly2005; Veevers et al. Reference Veevers, Saeed, Belousova and Griffin2005). The early zircon U–Pb studies of sediment provenance involved conventional U–Pb analyses of bulk zircon samples which gave only the average age of a number of grains. This restricted their application to areas where all grains were believed to have a single provenance, or to zircon populations which could be subdivided based on their morphology (e.g. Gaudette, Vitrac-Michard & Allègre, Reference Gaudette, Vitrac-Michard and Allègre1981). Recent studies have adopted spot analyses on zircon to avoid mixing of different generations for a single grain, by use of sensitive high-resolution ion micro-probe (SHRIMP) or LA-ICPMS (e.g. Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a, Reference Wan, Wilde, Liu, Song, Yang and Yinb and Xia et al. Reference Xia, Sun, Zhao and Luo2006a, Reference Xia, Sun, Zhao, Wu, Xu, Zhang and Luoc), thus the population assemblages of more complex sedimentary systems can be assessed. Besides U–Pb ages, the Lu–Hf isotope systematics of zircons provide additional insights into the geological history of their provenance, since they provide crust formation ages for the igneous source rocks. Recent studies show that combined U–Pb ages and Hf isotopic data from detrital zircons can elucidate the source of sediments more precisely (e.g. Veevers et al. Reference Veevers, Saeed, Belousova and Griffin2005).
Detrital zircons analysed in this study for the Lüliang khondalites have 207Pb–206Pb ages ranging from 1.9 Ga to 2.3 Ga, mostly at 2.0 Ga. This age pattern is much different from that of the Eastern Block and the other sedimentary rocks of the Trans-North China Orogen. The Eastern Block is known to have basements ranging from the early Archaean (up to c. 3.8 Ga: Liu et al. Reference Liu, Nutman, Compston, Wu and Shen1992; Song et al. Reference Song, Nutman, Liu and Wu1996), through middle Archaean (3.4–2.9 Ga: e.g. Jahn et al. Reference Jahn, Auvray, Cornichet, Bai, Shen and Liu1987; Song et al. Reference Song, Nutman, Liu and Wu1996) to late Archaean (2.9–2.5 Ga: e.g. Wu et al. Reference Wu, Geng, Shen, Liu, Li and Zhao1991; Kröner et al. Reference Kröner, Cui, Wang and Nemchin1998). Late Archaean rocks are dominant in these areas (see review by Zhao et al. Reference Zhao, Sun, Wilde and Li2005 and references therein), but there are very few Archaean zircons in the Lüliang khondalites. Hf isotopic data also do not favour Archaean rocks as an important component in the source region. Most of the zircons from the Lüliang khondalites have Hf model ages from 2.2 to 2.6 Ga with peaks at 2.3, 2.4 and 2.6 Ga for different samples (Fig. 5d–f). This is in contrast with available Hf isotopic data for detrital zircons from other metasedimentary rocks in the Trans-North China Orogen and Eastern Block, which recorded a Hf model ages peak at 2.6–2.8 Ga and a minor peak at 2.8–3.0 Ga (Xia et al. Reference Xia, Sun, Zhao, Wu, Xu, Zhang, He and Zhang2006b; Luo et al. Reference Luo, Sun, Zhao, Ayers, Li, Xia and Zhang2008; see also below).
In the Western Block, rocks from the Yinshan Terrane are also dominated by 2.5–2.6 Ga high-grade tonalitic–trondhjemitic–granodioritic (TTG) gneiss (Wang, Reference Wang1993), but the age of the Ordos basement rocks remains unknown due to the coverage of Mesozoic to Cenozoic sedimentary rocks. Previous detrital zircon U–Pb age data for the khondalitic rocks along its northern margin indicate a quite similar age pattern with that for the Lüliang khondalites (Xia et al. Reference Xia, Sun, Zhao and Luo2006a, Reference Xia, Sun, Zhao, Wu, Xu, Zhang and Luoc). Additionally, the Hf isotopic data show that these khondalitic rocks (both in the Khondalite Belt and in the Lüliang Complex) share a common crustal evolution history in their provenance: the crustal material was largely separated from the mantle c. 2.6 Ga ago, and significant addition of juvenile material occurred at about 2.1 Ga in their source region (data in this study and Xia et al. Reference Xia, Sun, Zhao, Wu, Xu, Zhang and Luo2006c, Reference Xia, Sun, Zhao, Wu, Xu, Zhang and He2008). In summary, our detrital zircon U–Pb and Hf isotopic data presented in this paper may suggest that the Lüliang khondalites share a common provenance with those khondalitic rocks in the Western Block. Hence our data are consistent with previous considerations that the Lüliang khondalites were deposited on a stable continental margin of the Ordos Terrane (Condie et al. Reference Condie, Boryta, Liu and Qian1992; Zhao et al. Reference Zhao, Sun, Wilde and Li2005; Xia et al. Reference Xia, Sun, Zhao and Luo2006a, Reference Xia, Sun, Zhao, Wu, Xu, Zhang and Luoc) and were thrust from the west and subsequently incorporated into the Trans-North China Orogen during the collision between the Eastern and Western blocks (Zhao et al. Reference Zhao, Wilde, Sun, Li, Li and Zhang2008).
In addition, it is worth noting that the age pattern of the Lüliang khondalites contained in the Jiehekou Group is also in strong contrast to that for the Palaeoproterozoic supracrustal rocks in the Lüliang and Yejishan/Hechashan groups, which show two main age peaks of 2.5 and 2.1 Ga (Liu & Zhao, Reference Liu, Zhao, Xiao, Zhai, Li and Liu2008). This means that the Lüliang Complex cannot be a sedimentary succession and that the subdivision into ‘groups’ is inappropriate. These supracrustal rocks in the Lüliang Complex are different in metamorphic grade, sedimentary environment and detrital source. They were juxtaposed together by subsequent tectonic disturbance.
5.c. U–Pb and Hf model age frameworks of the khondalites source and its comparison with that of the Eastern Block and the Trans-North China Orogen
A total of 636 detrital zircon 207Pb–206Pb ages for the khondalites from present and previous studies (Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a; Xia et al. Reference Xia, Sun, Zhao and Luo2006a, Reference Xia, Sun, Zhao, Wu, Xu, Zhang and Luoc) are plotted in Figure 7a. It displays a quasi-unimodal distribution with a peak at 1.9–2.0 Ga. We consider that this age pattern reflects, to some extent, a very important tectonothermal event which occurred during the period 1.9–2.0 Ga in their source region. This age pattern is in strong contrast to that of other Palaeoproterozoic metasedimentary rocks of the Trans-North China Orogen (Fig. 7b) and Eastern Block (Fig. 7c), which is characterized by two zircon age peaks at 2.1 and 2.5 Ga, corresponding to their voluminous Archaean tonalitic–trondhjemitic–granodioritic gneisses and Palaeoproterozoic granite (n = 289 and 530, respectively; data sources: Guan et al. Reference Guan, Sun, Wilde, Zhou and Zhai2002; Zhao et al. Reference Zhao, Wilde, Cawood and Sun2002b; Luo et al. Reference Luo, Sun, Zhao, Li, Xu, Ye and Xia2004, Reference Luo, Sun, Zhao, Ayers, Li, Xia and Zhang2008; Wilde et al. Reference Wilde, Zhao, Wang and Sun2004b; Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a, Reference Wan, Wilde, Liu, Song, Yang and Yinb; Xia et al. Reference Xia, Sun, Zhao, Wu, Xu, Zhang, He and Zhang2006b). The khondalite source region also shows a quasi-unimodal distribution in the detrital zircon Hf model ages (TCDM), peaking at c. 2.3 Ga. We interpret this peak age as the mean crustal residence age of the crustal materials extracted from the depleted mantle in the khondalite source region. This is also in strong contrast with that for other rocks of the Trans-North China Orogen and the Eastern Block, that is, the former displays a unimodal distribution in detrital zircon Hf model ages (TCDM) with a peak at c. 2.6 Ga (Fig. 8b) and the latter shows a major peak at c. 2.6–2.8 Ga (Fig. 8c).
Figure 7. Histograms for the distribution of detrital zircon 207Pb–206Pb ages from Palaeoproterozoic meta-sedimentary rocks in (a) Western Block (data sources: Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a; Xia et al. Reference Xia, Sun, Zhao and Luo2006a, Reference Xia, Sun, Zhao, Wu, Xu, Zhang and Luoc and this study); (b) Trans-North China Orogen (data sources: Guan et al. Reference Guan, Sun, Wilde, Zhou and Zhai2002; Zhao et al. Reference Zhao, Wilde, Cawood and Sun2002b; Wilde et al. Reference Wilde, Zhao, Wang and Sun2004b; Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a, Reference Wan, Wilde, Liu, Song, Yang and Yinb and Xia et al. Reference Xia, Sun, Zhao, Wu, Xu, Zhang, He and Zhang2006b); (c) Eastern Block of the North China Craton (data sources: Luo et al. Reference Luo, Sun, Zhao, Li, Xu, Ye and Xia2004, Reference Luo, Sun, Zhao, Ayers, Li, Xia and Zhang2008 and Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a).
Figure 8. Histograms for the distribution of detrital zircon Hf model ages (TCDM) from Palaeoproterozoic meta-sedimentary rocks in: (a) Western Block; (b) Trans-North China Orogen; (c) Eastern Block of the North China Craton. Data sources: Luo et al. (Reference Luo, Sun, Zhao, Ayers, Li, Xia and Zhang2008); Xia et al. (Reference Xia, Sun, Zhao, Wu, Xu, Zhang, He and Zhang2006b, Reference Xia, Sun, Zhao, Wu, Xu, Zhang and Luoc) and this study.
5.d. Tectonic implications
Recent studies have led to a general consensus that the North China Craton formed by the collision between the Eastern and Western blocks along the Trans-North China Orogen. However, uncertainties remain. The first issue regards timing of the amalgamation of the two blocks, with some researchers arguing that the final collision between the Western and Eastern blocks was at c. 2.5 Ga (Wang et al. Reference Wang, Li, Hao, Li and Zhou1996; Li et al. Reference Li, Kröner, Qian and O'Brien2000; Li & Kusky, Reference Li and Kusky2007; Zhai, Bian & Zhao, Reference Zhai, Bian and Zhao2000; Kusky & Li, Reference Kusky and Li2003; Polat et al. Reference Polat, Kusky, Li, Fryer, Kerrich and Patrick2005, Reference Polat, Herzberg, Muenker, Rodgers, Kusky, Li, Fryer and Delaney2006), whereas others believe the collision occurred at c. 1.85 Ga (Wilde et al. Reference Wilde, Cawood, Wang and Nemchin1997, Reference Wilde, Cawood, Wang and Nemchin1998, Reference Wilde, Cawood, Wang, Nemchin, Zhao, Malpas, Fletcher, Aitchison and Ali2004a, Reference Wilde, Zhao, Wang and Sunb, Reference Wilde, Cawood, Wang and Nemchin2005; Wilde, Zhao & Sun, Reference Wilde, Zhao and Sun2002; Guo et al. Reference Guo, Sun, Chen and Zhai2005; Guan et al. Reference Guan, Sun, Wilde, Zhou and Zhai2002; Zhao et al. Reference Zhao, Wilde, Cawood and Lu1999a, Reference Zhao, Cawood, Wilde and Sun2000a, Reference Zhao, Wilde, Cawood and Lub, Reference Zhao, Wilde, Cawood and Sun2002b, Reference Zhao, Sun, Wilde and Li2005, Reference Zhao, Wilde, Sun, Li, Li and Zhang2008; Kröner et al. Reference Kröner, Wilde, Li and Wang2005a, Reference Kröner, Wilde, O'Brien, Li, Passchier, Walte and Liub, Reference Kröner, Wilde, Zhao, O'Brien, Sun, Liu, Wan, Liu and Guo2006; Liu et al. Reference Liu, Zhao, Wilde, Shu, Sun, Li, Tian and Zhang2006). The latter interpretation has received wide support from recently published geochronological data for metamorphic zircons from various lithologies of the Trans-North China Orogen, including high-pressure granulites and amphibolites, charnockites and reworked gneisses (Wilde et al. Reference Wilde, Cawood, Wang and Nemchin1998, Reference Wilde, Zhao, Wang and Sun2004b; Guo, Zhai & Xu, Reference Guo, Zhai and Xu2001; Guo et al. Reference Guo, Wang, Sang and Zhai2001, Reference Guo, Sun, Chen and Zhai2005; Guo & Zhai, Reference Guo and Zhai2001; Guan et al. Reference Guan, Sun, Wilde, Zhou and Zhai2002; Zhao et al. Reference Zhao, Wilde, Cawood and Sun2002b; Kröner et al. Reference Kröner, Wilde, Li and Wang2005a, Reference Kröner, Wilde, O'Brien, Li, Passchier, Walte and Liub, Reference Kröner, Wilde, Zhao, O'Brien, Sun, Liu, Wan, Liu and Guo2006; Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a, Reference Wan, Wilde, Liu, Song, Yang and Yinb). The Lüliang khondalites experienced medium- to high-grade metamorphism with a clockwise P–T–t path (Zhao et al. Reference Zhao, Cawood, Wilde and Sun2000a), indicating their deposition before the continental collision. The Palaeoproterozoic detrital zircon ages presented in this study do not support the c. 2.5 Ga collision model, but may provide further evidence for the c. 1.85 Ga collision.
The second debated issue is the direction of the subduction. Some researchers suggested a westward subduction along the east margin of the Western Block (Kusky & Li, Reference Kusky and Li2003; Polat et al. Reference Polat, Kusky, Li, Fryer, Kerrich and Patrick2005, Reference Polat, Herzberg, Muenker, Rodgers, Kusky, Li, Fryer and Delaney2006; Faure et al. Reference Faure, Trap, Lin, Monié and Bruguier2007; Trap et al. Reference Trap, Faure, Lin and Monié2007), whereas other researchers proposed that the subduction was eastward along the western margin of the Eastern Block (Zhao et al. Reference Zhao, Wilde, Cawood and Lu1998, Reference Zhao, Wilde, Cawood and Lu1999b, Reference Zhao, Wilde, Cawood and Lu2000b, Reference Zhao, Sun, Wilde and Li2005, Kröner et al. Reference Kröner, Wilde, Li and Wang2005a, Reference Kröner, Wilde, O'Brien, Li, Passchier, Walte and Liub, Reference Kröner, Wilde, Zhao, O'Brien, Sun, Liu, Wan, Liu and Guo2006; Liu et al. Reference Liu, Zhao, Wilde, Shu, Sun, Li, Tian and Zhang2006; Zhang et al. Reference Zhang, Zhao, Li, Sun, Liu, Wilde, Kröner and Yin2007). In both models, the Trans-North China Orogen represents a late Archaean to Palaeoproterozoic magmatic arc, which is characterized by dominant Neoarchaean to Palaeoproterozoic arc-related juvenile crust with minor reworked basement rocks, fragments of ancient oceanic crust, mélanges, high-pressure granulites, retrograded eclogites, crustal-scale ductile shear zones, and linear fold belts with sheath folds (Li et al. Reference Li, Wang, Wang, Liu and Zhao1990; Li, Hart & Wu, Reference Li, Hart and Wu1990; Zhai, Guo & Yan, Reference Zhai, Guo and Yan1992; Wang et al. Reference Wang, Li, Hao, Li and Zhou1996, Reference Wang, Fan, Zhang and Guo2003, Reference Wang, Fan, Zhang, Guo, Zhang and Peng2004, Reference Wang, Zhao, Fan, Peng, Sun and Xia2007; Wu & Zhong, Reference Wu and Zhong1998; Zhao, Cawood & Lu, Reference Zhao, Cawood and Lu1999, Zhao et al. Reference Zhao, Wilde, Cawood and Lu2000b; Guo, O'Brien & Zhai, Reference Guo, O'Brien and Zhai2002; Zhang et al. Reference Zhang, Zhao, Sun, Wilde, Li and Liu2006b). As mentioned above, detrital zircons from other rocks of the Trans-North China Orogen, such as those from the Wanzi supracrustal rocks of the Fuping Complex (Guan et al. Reference Guan, Sun, Wilde, Zhou and Zhai2002; Zhao et al. Reference Zhao, Wilde, Cawood and Sun2002b; Xia et al. Reference Xia, Sun, Zhao, Wu, Xu, Zhang, He and Zhang2006b), the Shangtaihua Group in the Taihua Complex (Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a, Reference Wan, Wilde, Liu, Song, Yang and Yinb) and the Hutuo Group in the Wutai Complex (Wilde et al. Reference Wilde, Zhao, Wang and Sun2004b; Zhang et al. Reference Zhang, Zhao, Sun, Liu, Xia and He2006a), share common characteristics. Their U–Pb zircon ages show bimodal distribution at 2.1 and 2.5 Ga and their zircon Hf model ages have a dominant peak at 2.6–2.8 Ga. The coevally deposited meta-sedimentary rocks in the interior areas of the Eastern Block, such as the Liaohe Group in the Liaoning Province (Luo et al. Reference Luo, Sun, Zhao, Li, Xu, Ye and Xia2004, Reference Luo, Sun, Zhao, Ayers, Li, Xia and Zhang2008; Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a) and the Jingshan and Fenzishan groups in the Shandong Province (Wan et al. Reference Wan, Song, Liu, Wilde, Wu, Shi, Yin and Zhou2006a), have similar U–Pb and Hf model ages. These characters are in contrast with those for the khondalites source region. If the hypothesis that the khondalites were derived from the Ordos Terrane stands, our results preclude the possibility that the Trans-North China Orogen represents an active continental margin system on the eastern margin of the Western Block, and hence the eastward subduction model is favoured. In this model, the Lüliang khondalites were deposited on the eastern margin of the Western Block in a passive continental margin environment and were thrust eastward later during collision with the Eastern Block. Other metasedimentary rocks in the Trans-North China Orogen, such as the Wanzi Supracrustal rocks, were deposited on the western margin of the Eastern Block in a back-arc basin (Fig. 9). If the hypothesis does not stand, it requires an unknown exotic source area between the Western Block and the Trans-North China Orogen. This scenario does not support the westward subduction model either. Other evidence for the affinity of the Trans-North China Orogen to the Eastern Block includes the fact that large numbers of 2.7–2.8 Ga inherited or xenocrystic zircon ages are found in c. 2.5 Ga augen gneiss, granitic gneiss and tonalitic–trondhjemitic–granodioritic gneisses located in the Trans-North China Orogen (Guan et al. Reference Guan, Sun, Wilde, Zhou and Zhai2002; Zhao et al. Reference Zhao, Wilde, Cawood and Sun2002b). These data are similar to those from rocks in the Eastern Block, but rare in the khondalite rocks from both the Lüliang Complex and the Khondalite Belt. Nd isotope data for the c. 2.1 Ga Nanying granitic gneiss in the Trans-North China Orogen show that it represents reworked 2.7–2.8 Ga old crustal material, and also suggest that the Trans-North China Orogen has an affinity to the Eastern Block (H. Guan, unpub. Ph.D. thesis, Univ. Hong Kong, 2000; Liu et al. Reference Liu, Pan, Li, Li and Zhang2002) because Palaeoproterozoic magmatic activities in the Western Block are characterized by juvenile crustal addition. This scenario is also consistent with the geochemical characters of the khondalites and other supracrustal rocks in the Trans-North China Orogen. The former have chemical index of alteration (CIA) values comparable with that of post-Archaean average Australian shale (PAAS), while the chemical index of alteration (CIA) values for the Wanzi Supracrustal rocks are not as high as shale, indicating a significant amount of immature sediments, possibly volcanic-sedimentary rocks (H. Guan, unpub. Ph.D. thesis, Univ. Hong Kong, 2000).
Figure 9. Schematic sections showing proposed tectonic settings for the Lüliang khondalite and the Wanzi supracrustal assemblage.
We know that the early crust of the Earth formed by vertical melt segregation, and then lateral accretion of juvenile material along cratonic margins became more important (through accretion of fragments of ocean floor, volcanic arc sequences, and intrusion of tonalitic–trondhjemitic–granodioritic-type granite, etc.). In this regard, the Palaeoproterozoic juvenile crustal growth in the khondalites source area may suggest an active continental margin along the Western Block. However, geochemical characters of the khondalites have well established its cratonic or passive margin type deposition environment (Condie et al. Reference Condie, Boryta, Liu and Qian1992; Li, Qian & Liu, Reference Li, Qian and Liu2000; Wan et al. Reference Wan, Geng, Liu, Shen, Liu and Song2000a, Reference Wan, Geng, Shen and Zhangb). Recent studies on the Central Asian Orogenic Belt (CAOB) have shown that intraplate magmatism involving plume melting, basalt underplating and differentiation or remelting of a basaltic source is an important process of continental growth even in the Phanerozoic (e.g. Jahn, Wu & Hong, Reference Jahn, Wu and Hong2000). Hence we propose that the protoliths of the khondalites were generated by intraplate magmatism.
6. Conclusions
U–Pb and Hf isotopic data for detrital zircons show that the Lüliang khondalites mainly came from a 1.9–2.1 Ga Palaeoproterozoic source area, similar to that of the khondalite rocks within the Western Block. The Hf isotope data suggest significant crustal growth in Palaeoproterozoic times and the involvement of c. 2.6 Ga lower crust in the Palaeoproterozoic magmatic activity. These and previously published data for the khondalites are in contrast with other coevally deposited and locally derived supracrustal rocks in the Trans-North China Orogen, precluding the possibility that the Trans-North China Orogen developed on the eastern margin of the Western Block. Instead, coeval detrital zircons in the Trans-North China Orogen and the Eastern Block share many common characteristics, suggesting that they came from a source area with both Palaeoproterozoic and Archaean rocks. These results demonstrate that the Trans-North China Orogen represents a continental margin developed at the western margin of the Eastern Block, thus supporting the eastward subduction model for the Palaeoproterozoic tectonic evolution of the North China Craton.
Acknowledgements
This study was supported by research grants from the Chinese National 973 Program (2007CB411307), the Small Project Funding from the University of Hong Kong (200607176156), Chinese NSFC Grant (40730315), Hong Kong RGC (7048/03P, 7055/05P, 7066/07P), the Germany/Hong Kong Joint Research Scheme sponsored by Hong Kong RGC and German Academic Exchange Service of Germany (G_HK030/07), and the CAS/SAFEA International Partnership Program for Creative Research Teams. Simon A. Wilde read an earlier version and provided constructive comments. We thank Yueheng Yang and Yanbing Zhang for laboratory assistance. A postgradate studentship and a post-doctoral fellowship from HKU to X. P. Xia are greatly acknowledged.
