Multistage late Neoarchaean crustal evolution of the North China Craton, eastern Hebei

Precambrian Research 189 (2011) 43–65

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Multistage late Neoarchaean crustal evolution of the North China Craton, eastern Hebei Allen P. Nutman a,b,∗ , Yusheng Wan a , Lilin Du a , Clark R.L. Friend a,c , Chunyan Dong a , Hangqiang Xie a , Wei Wang a , Huiyi Sun a , Dunyi Liu a a

Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, PR China School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia c 45 Stanway Road, Headington, Oxford OX3 8HU, UK b

a r t i c l e

i n f o

Article history: Received 3 December 2010 Received in revised form 19 April 2011 Accepted 26 April 2011 Available online 5 May 2011 Keywords: North China Craton SHRIMP U/Pb zircon dating Granulite facies Archaean crustal evolution Archaean arc magmatism

a b s t r a c t The eastern part of the North China Craton in eastern Hebei Province contains metamorphosed and deformed Neoarchaean to earliest Palaeoproterozoic rocks (∼2550–2490 Ma) with some older Archaean rocks. Numerous precise U–Pb zircon ages, structural observations, Nd and Hf isotopic data and whole rock geochemistry (our new data with reassessment and integration of previous work) show that there is not a single protracted event at the end of the Archaean lasting ∼60 million years, but that from east to west there are several separate events with their own unique character. The eastern coastal region is dominated by weakly deformed 2530–2510 Ma granites with subordinate granodiorites, diorites and magnesian-gabbros. This suite has inclusions of older 2550–2540 Ma plutonic rocks. ∼80 km inland to the west, there are amphibolite facies biotite-rich schists, lenses of BIF, siliceous fuchsite bearing rocks with 3880–3540 Ma zircons, peridotites and orthogneisses, all intruded by younger granites and monzonites. Three amphibolite–granulite facies gneisses and schists with volcano-sedimentary protoliths have 2548–2534 Ma igneous zircon populations, rare inheritance back to >3600 Ma and 2506 ± 6 Ma zircon equated with granulite facies metamorphism. Older tonalitic to granitic polyphase orthogneisses contain 3287 ± 11 Ma to ∼2940 Ma igneous components. Late kinematic high Fe/Mg gabbros, monzonites and granites have ages of 2499 ± 7 to 2491 ± 13 Ma. 160–100 km inland, gneissose 2550–2530 Ma quartz-diorite to tonalitic rocks occur as intrusions into, or intercalations with, mafic rocks. These were affected by granulite facies metamorphism but then widely retrogressed. Abundant syn-granulite facies neosome and later shear zones disrupt the early geological relationships. The granulite facies metamorphism is dated at 2503 ± 5 Ma from several samples, including a syn-granulite facies pegmatite. The Neoarchaean igneous rocks show marked negative Nb, Ti anomalies, LIL enrichment and enrichment of the light REE relative to the heavy REE across the entire ultramafic, basaltic, tonalitic and granitic compositional spectrum: 2550–2530 Ma tonalites and quartz diorites are most important in the west and 2530–2510 Ma granites are most important in the east. Their composition resembles those formed at various stages of magmatic arc evolution at a convergent plate boundary, rather than being plume products. The eastern Hebei rocks thus formed in a complex arc with distinct pulses of plutonism at 2550–2540 and 2530–2510 Ma, with local incorporation of older continental crust. The 2500–2490 Ma thermal event with its associated higher Fe/Mg magmas involved interaction of underplated fractionated gabbroic melt with deep crustal melts, possibly during extension of an already ∼50 million years old arc. © 2011 Elsevier B.V. All rights reserved.

1. Introduction For North China Craton (NCC) Neoarchaean crust development in eastern Hebei (Fig. 1), we examine the case for plume (Geng et al.,

∗ Corresponding author at: School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia. Tel.: +61 2 4298 1347. E-mail address: [email protected] (A.P. Nutman). 0301-9268/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2011.04.005

2006; Yang et al., 2008) or convergent plate boundary processes (e.g., Polat et al., 2006a,b). This study is undertaken using new data, integrated with our reappraisal of that already published (Table 1). We contend that a plate boundary model for two episodes of crust production at 2550–2530 Ma and 2530–2510 Ma best fits all the available data, and further observe that the event at 2500–2490 Ma with low pressure granulite facies metamorphism and coeval intrusions affecting large areas of the eastern NCC is separate from the two slightly earlier crust-producing magmatic events. For this

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Fig. 1. Geological sketch map of the Santunying–Beidaihe portion of the North China Craton, eastern Hebei Province, China. A colour version of this map is given in the electronic supplement.

youngest event crustal underplating during extension of an already 50 million year old arc is regarded as most likely.

2. Geological overview of the eastern North China Craton, eastern Hebei 2.1. Setting and history of investigation In eastern Hebei Province, east of Beijing, the NCC is overlain by Proterozoic to Cenozoic platform cover rocks and is intruded by Mesozoic granites (Fig. 1). Early Archaean (>3400 Ma) rocks and zircons have been reported from eastern Hebei (e.g., Jahn et al., 1987; Liu et al., 1992, 2007) but are extremely limited in extent and poorly preserved due to their reworking in subsequent tectonothermal events and their inclusion in younger granitoids. Neoarchean rocks are extensive, making up >95% of the total NCC basement exposure in eastern Hebei. Predominant are 2600–2500 Ma gneisses and granitic rocks with lesser amounts of supracrustal rocks (e.g., Jahn et al., 1987; Kröner et al., 1988, 1998; Jahn and Ernst, 1990; Wang et al., 1990; Fang et al., 1998; Geng et al., 2006; Yang et al., 2008). The supracrustal rocks were intruded by a variety of igneous rocks ranging from pyroxenite through gabbro, diorite, tonalite, trondhjemite, granodiorite to granite. Most of these have chemical affinities typical of Archaean TTG suites (Jahn and Zhang, 1984; Fang et al., 1998). Throughout much of the NCC strong ductile deformation produced foliations in most rocks, and has led to the obliteration of much of the magmatic, volcanic and sedimentary structures and any earlier fabrics. Most lithological contacts are now parallel to this foliation, but in rare cases, intrusive contacts and volcanic and sedimentary structures can still be recognized. Weakly foliated pink granite occurs as invasive bodies up to several tens of

metres across and intrudes all the rock types mentioned above. There are also rocks previously described as syntectonic charnockites and granites formed at ∼2500 Ma (Compston et al., 1983; Jahn and Zhang, 1984; Kröner et al., 1998; Liu et al., 1990). In the west, there is widespread occurrence of ∼2500 Ma granulite facies metamorphism (e.g., Compston et al., 1983; Fang et al., 1998; this paper), but in the east granulite facies metamorphism is rarer, even though the 2530–2510 Ma granitic rocks (Yang et al., 2008; this paper) there are old enough to have been affected by this metamorphism.

3. Analytical methods For whole rock analysis, samples were crushed to 200-mesh size for analysis. Major oxides, together with trace elements and REE (supplementary Table 1) were analysed by XRF and ICP-MS at the Institute of Geological Analysis, Chinese Academy of Geological Sciences. Uncertainties depend upon the concentration in the sample, but generally for XRF and ICP-MS are estimated at ±3–5% and ±3–8%, respectively. Whole rock geochemical data were plotted using the ‘GeoPlot’ ExcelTM plug-in of Zhou and Li (2006). Zircons were separated by panning and passing through an isodynamic separator. Hand-picked zircons were cast into epoxy resin discs which were polished to reveal cross sections of the grains. Cathodoluminescence (CL) images to the zircons were acquired, to select sites for analysis. Zircon dating was carried out using the SHRIMP II ion microprobe at the Beijing SHRIMP Center, Chinese Academy of Geological Sciences. The analytical procedures are described by Williams (1998) and Wan et al. (2005a). The intensity of the primary O2− ion beam was 4–6 nA and spot sizes were 25–30 ␮m, with each site rastered for 120–200 s prior to analysis. Five or six scans through the mass stations were made for each age

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Table 1 Compilation of mostly U–Pb zircon age data from eastern Hebei Archaean rocks. Sample

Location/unit

Position GPS WGS84

Lithology

Event dated

Method

Used 207 Pb/206 Pb age

Monzogranite Tonalite Monzogranite Mafic granitoid

plut plut plut plut

SHRIMP SHRIMP SHRIMP SHRIMP

2512 ± 12 2546 ± 6 2525 ± 10 2531 ± 3

Diorite

plut met plut met plut plut plut met plut met

LAICPMS

2528 ± 3 2500 ± 10 2521 ± 8 2497 ± 7 2527 ± 4 2523 ± 6 2524 ± 8 2490 ± 4 2522 ± 4 2499 ± 5

Granodiorite

plut

EVAP

2551 ± 6

Quartzite Tonalitic gneiss Tonalitic gneiss Tonalitic gneiss Granitic gneiss Granulite gneiss

SHRIMP SHRIMP SHRIMP SHRIMP SHRIMP SHRIMP

Tonalitic gneiss Leptynite Monzonite

det plut plut plut plut ig/volc met plut volc plut

SHRIMP SHRIMP

3540–3890 3228 ± 110 3287 ± 11 ≥3129 2936 ± 34 2548 ± 7 2506 ± 6 2548 ± 13 2534 ± 8 2491 ± 13

Ferrogabbro Monzonite Granite

plut plut plut

IDTIMS IDTIMS IDTIMS

2499 ± 8 2495 ± 1 2494 ± 2

40◦ 22.05 N; 119◦ 09.90 E 40◦ 22.05 N; 119◦ 09.90 E 40◦ 21.65 N; 119◦ 09.49 E 40◦ 21.65 N; 119◦ 09.49 E 40◦ 21.65 N; 119◦ 09.49 E 40◦ 22.30 N; 119◦ 07.83 E

Metasedimentary rock Metasedimentary rock Tonalite cobble Granite cobble Conglomerate matrix Felsic volcanic?

det det det det det det

SHRIMP SHRIMP SHRIMP SHRIMP SHRIMP SHRIMP

2504 ± 7 2540 ± 6 2515 ± 9 2510 ± 10 2512 ± 7 2512 ± 7

40◦ 24.24 N; 118◦ 23.850 E 40◦ 19.80 N; 118◦ 16.18 E 40◦ 19.80 N; 118◦ 16.18 E

Granulite gneiss Granulite pegmatite Granulite gneiss

plut plut/met ig met met

SHRIMP SHRIMP SHRIMP SHRIMP SHRIMP

2525 ± 20 2499 ± 9 2537 ± 9 2498 ± 6 2503 ± 5

Granulite diorite

ig

SHRIMP

2551 ± 12

Qinhuangdao-Beidaihe eastern coastal region This paper Beidaihe 39◦ 48.67 N; 119◦ 29.08 E J08/12 Beidaihe 39◦ 48.79 N; 119◦ 29.51 E J08/15 Beidaihe 39◦ 48.79 N; 119◦ 29.51 E J08/16 Qinhuangdao 39◦ 59.03 N; 119◦ 37.71 E J08/18 From Yang et al. (2008) FW04-28 Jiekouling FW04-42

Qinhuandao

Granodiorite

FW04-51 FW04-54 FW04-84

Qinhuandao Qinhuandao Anziling

Granodiorite Granite Leucodiorite

FW04-85

Anziling

Monzodiorite

From Geng et al. (2006) LL07 Yinmahe Qian’an-Qinlong central region This paper Caozhuang J06/01 Caozhuang J06/04 J08/02 Caozhuang J08/04 Caozhuang J06/02 Caozhuang J06/10 Caozhuang J91/11 Caozhuang J00/33 Caozhuang J00/31 Caozhuang From Liu et al. (1990) CF85-73 Mangshan CF85-62 Caozhuang Mangshan CF85-71 From Sun et al. (2010) Dantazi Group J09/08 J09/09 Zhuzhangi Group J09/10-1 Zhuzhangi Group Zhuzhangi Group J09/10-2 J09/10-3 Zhuzhangi Group J09/11 Zhuzhangi Group Santunying western (granulite) region This paper J08/05 Western granulites J08/09 Western granulites Western granulites J08/10

39◦ 56.07 N; 118◦ 33.49 E 39◦ 55.75 ; 118◦ 33.68 E 39◦ 55.77 N; 118◦ 33.89 E 39◦ 55.70 N; 118◦ 33.80 E 39◦ 55.81 N; 118◦ 33.55 E 40◦ 04.47 N; 118◦ 34.46

Composite granulite facies metamorphism age from Geng et al. (2006) Longwan TP1

LAICPMS LAICPMS LAICPMS LAICPMS LAICPMS

TP22 ZH10

Yuhuzhai Qiuhuayu

Granulite diorite Granulite tonalite

ig ig

EVAP SHRIMP

2550 ± 2 2543 ± 6

QX05

Xiaoguanzhuang

Granulite tonalite

ig

SHRIMP

2542 ± 20

Granulite gabbro Granulite tonalite

ig ig

EVAP EVAP

2536 ± 1 2492 ± 5

Metagabbro

met

IDTIMS

2505 ± 2

Chromitites

ig

Re-Os

2547 ± 10

Qingyangshu QX03 Cuizhangzi CZ02 Kusky et al. (2001) Donwanzi NC2022-1,2 Kusky et al. (2004) Zunhua

Ages of Geng et al. (2006)

2517 ± 11 (MSWD = 4.6) 2513 ± 13 (MSWD = 11.6) 2493 ± 25 (MSWD = 7.2)

U–Pb zircon methods: SHRIMP = SHRIMP, IDTIMS = isotope dilution thermal ionisation mass spectrometry, EVAP = thermal evaporation event dated: det = detrital provenance, plut = plutonism, volc = volcanism, ig = igneous (undifferentiated), met = metamorphism error on ‘used ages’ is at the 95% confidence level.

determination. Reference zircons SL13 with a U content of 238 ppm (Williams, 1998) and TEMORA with 206 Pb/238 U age of 417 Ma (Black et al., 2003) were used for elemental abundance and calibration of 206 Pb/238 U, respectively. The common lead correction was applied using measured 204 Pb abundances and model Pb compositions of Cumming and Richards (1975). Data processing and assessment was carried out using the SQUID and ISOPLOT programs (Ludwig,

1997, 2001). Data are summarised in supplementary Table 2. The uncertainties quoted in supplementary Table 2 are 1, whereas the errors for weighted mean ages given in the text are quoted at the 95% confidence level. Samarium and Nd isotope compositions were determined by the isotope dilution technique in the Key Laboratory of Isotope Geology, Ministry of Land and Resources, China. Details of the procedures

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have been described by Zhang and Ye (1987). The separation and purification of Sm and Nd were carried out by standard ion exchange procedures using AG 50Wx8 and HDEHP, respectively. Samarium and Nd isotope analyses were performed using a multicollector Finnegan MAT 261 mass spectrometer. The measured 143 Nd/144 Nd ratios were normalized to 0.7219. The 143 Nd/144 Nd ratios of J.M. Nd2 O3 and BCR-1, measured in the laboratory, were 0.511132G14 (2) and 0.512644G9 (2), respectively. The total blanks for Sm and Nd were 5 × 10−11 g. Summary of new data is presented in supplementary Table 3. 4. Coastal region (Qinhuangdao and Beidaihe) 4.1. Geological description and previous work The Archaean geology of the eastern coastal region is dominated by granitic ‘bodies’ (the term ‘bodies’ is used here to denote granitic sensu lato rocks which, from earlier Provincial Survey mapping studies, had been designated as separate plutons). These bodies vary from amphibolite–granulite facies orthogneisses with little primary structure preserved, to weakly deformed, essentially meta-igneous rocks, whose igneous textures and intrusive relationships are preserved (Yang et al., 2008). The Qinhuangdao, Anziling and Jiekouling, bodies are the three largest (Fig. 1). To the west they were intruded into orthogneisses, amphibolites and garnet–biotite–plagioclase gneisses (Yang et al., 2008 and references therein). From all three bodies these authors obtained indistinguishable U–Pb zircon ages between 2528 ± 3 and 2521 ± 8 Ma for 6 samples ranging in composition from metamelagabbro (their hornblendite) to granite. Biotite-granite is the major phase of the Qinhuangdao body and is medium- to coarse-grained. Granophyric intergrowths of microcline and quartz (indicating shallow emplacement) are common and plagioclase occurs as a fine-grained interstitial phase. Lesser phases within the granites are hornblendite (meta-melagabbro), diorite–monzodiorite and granodiorite. These occur as both inclusions and irregular dyke-like bodies. These bodies appear to intermingle with granite, which also back-veins into them (Fig. 2a). As observed by Yang et al. (2008), these relationships suggest coeval emplacement of different magmas. Also present are volumetrically minor inclusions of layered gneisses and schists, representing the country rocks to the Qinhuangdao body. On clean coastal exposures of these rocks at Beidaihe, there is no evidence of superimposed granulite facies metamorphism. However, away from the coast granodiorites locally contain orthopyroxene (Yang et al., 2008). The Anziling body (Fig. 1) crops out immediately west of the Qinhuangdao body and comprises granodioritic and tonalitic amphibolite- to granulite-facies gneisses. Zircon overgrowths in two samples of this body gave indistinguishable ages of 2499 ± 5 and 2490 ± 4 Ma, also equated with high grade metamorphism (Yang et al., 2008). This contrasts with the Qinhuangdao body where overall deformation is lower and there is only local evidence of high grade metamorphism. The Jiekouling body forms the western side of the coastal region (Fig. 1) and consists of deformed diorite–monzodiorite with minor granite and hornblendite. Zircon overgrowths in a diorite gave an age of 2500 ± 10 Ma, equated with high grade metamorphism (Yang et al., 2008). 4.2. Eastern Qinhuangdao and Beidaihe coastal region—new data and observations 4.2.1. Granite at Beidaihe (sample J08/12) This sample was taken from a beach at Beidaihe (WGS84 reference 39◦ 48.67 N; 119◦ 29.08 E). The granites have a weak foliation

and are overall homogeneous. Zircons from J08/12 are elongate to stubby prisms and show fine scale oscillatory zoning interrupted by recrystallisation domains in CL images (Fig. 4). Seven analyses on 7 zircons have U contents and Th/U ratios ranging from 531 to 36 ppm and 1.02 to 0.54 (supplementary Table 2). Two spots show strong lead loss. The remaining have close to concordant ages, plotting near concordia (Fig. 5a) and give a weighted mean 207 Pb/206 Pb age of 2512 ± 12 Ma (MSWD = 1.1). 2512 ± 12 Ma is interpreted as the intrusion age of the granite.

4.2.2. Inclusion of granodiorite/tonalite (sample J08/15) in granite at Beidaihe This sample was taken from another beach at Beidaihe (39◦ 48.79 N; 119◦ 29.51 E). The granodiorite is volumetrically minor, shows a gneissic/foliated structure and is cut by granite (sample J08/16, below). It is composed of plagioclase, quartz, epidote, chlorite, biotite and titanite. Epidote formed by alteration of plagioclase, and chlorite and titanite by alternation of biotite (and hornblende?). The zircons are elongate to stubby prisms and show fine scale oscillatory zoning (Fig. 4). Ten analyses were undertaken on 9 zircons and have U contents and Th/U ratios of 244–77 ppm and 0.66–0.15 (supplementary Table 2). All of them, except analyses 3.2 (showing strong lead loss) and 7.1 (showing large analytical error), have close to concordant ages (Fig. 5b) and yielded a weighted mean 207 Pb/206 Pb age of 2546 ± 6 Ma (MSWD = 1.0), which is interpreted as the age of the granodiorite.

4.2.3. Granite at a Beidaihe beach (sample J08/16) The granite cuts the granodiorite/tonalite (J08/15) mentioned above, and was taken from the same outcrop. It is only very weakly deformed and shows no evidence in the field of high grade metamorphism. It is composed of plagioclase, K-feldspar (microcline and perthite), quartz and a minor biotite. The zircons are elongate to stubby prisms, and in CL images (Fig. 4) they show oscillatory zoning disrupted by recrystallisation domains. Some grains have concordant dark rims which are considered to be late magmatic or hydrothermal origin (e.g., grain 2, Fig. 4). Eighteen analyses on 18 zircons show large variations in U contents and Th/U ratios from 4406 to 42 ppm and 1.00 to 0.06 (supplementary Table 2). The analyses with low Th/U ratios are on the dark rims. Many analyses show an appreciable component of common Pb and show strong lead loss (Fig. 5c). Five analyses with close to concordant ages yielded a weighted mean 207 Pb/206 Pb age of 2525 ± 10 Ma (MSWD = 1.5). This age is considered to be the intrusion age of the granite. This is consistent with its intrusive relationship with the 2546 ± 6 Ma granodiorite/tonalite inclusion (sample J08/15).

4.2.4. Mafic granitoid near Qinhuangdao (sample J08/18) This sample was taken from a low road-cut inland near Qinhuangdao (39◦ 59.03 N; 119◦ 37.71 E). The mafic granitoid occurs locally as inclusions within granite. It contains abundant hornblende, with plagioclase, microcline, quartz and some epidote, but without biotite. It could be a diorite K-metasomatised during emplacement of the surrounding granites, or an igneous monzonite. The zircons are elongate prisms and in CL images show fine scale oscillatory zoning locally disrupted by recrystallisation domains (Fig. 4). Seventeen analyses were undertaken on 15 zircons, and their U contents and Th/U ratios range from 599 to 133 ppm and 1.11 to 0.21 (supplementary Table 2). Some analyses show strong lead loss. Eleven analyses with close to concordant ages (Fig. 5d) yielded a weighted mean 207 Pb/206 Pb age of 2531 ± 3 Ma (MSWD = 1.6). This is interpreted as the igneous age of the mafic granitoid.

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Fig. 2. (a) Magma-mingling between 2530 and 2520 Ma granitic (gr) and dioritic (di) components on the Beidaihe coastline at 39◦ 48.66 N; 119◦ 29.02 W (WGS-84 datum). Pen for scale – centre-left. (b) Tectonically disrupted leptynites (lep) with lenses of pegmatite (peg), behind the east end of houses at Huangbaiyu hamlet 39◦ 56.00 N; 118◦ 33.66 W. Pen for scale – centre left. (c) Coarse grained granitic gneiss (gn) with disrupted bodies of metadoleritic amphibolite (a) on track west of Huangbaiyu at 39◦ 55.81 N; 118◦ 33.55 W. Sample J06/02 comes from near to the pen – top left of the image. (d) Granitic (gr) component of the granulite facies 2499 ± 8 Ma granitic to ferrogabbroic (Liu et al., 1990) plutonic rocks at Mangshan showing inclusions of previously deformed gneisses (gn) at 40◦ 06.29 N; 118◦ 45.87 W. End of trimming hammer in top right for scale (e) Blebby texture (blebby – not lichen encrustation!) preserved in 2560–2550 Ma Santunying gneisses, affected by ∼2500 Ma granulite facies metamorphism. This indicates retrogression under amphibolite facies conditions following granulite facies metamorphism (see text for further explanation). Pen for scale – bottom right. This outcrop is at the J08/05 sampling locality at Xiaohongmiao (40◦ 24.24 N; 118◦ 23.85 W). (f) Deformed conglomerates of the Zhuzhangzi Group. Granitic cobbles (gr) in a fine-grained matrix of granitic pebbles and mica schist, were intruded by dolerite dykes (a) prior to ductile deformation under amphibolite facies conditions. Pen for scale – bottom right. Southeast of Qinlong at 40◦ 06.29 N; 118◦ 45.87 W.

5. ∼80 km west from coast (Qian’an-Huangbaiyu) 5.1. Geological description and previous work The Qianxi Complex of the Qian’an-Huangbaiyu region (Figs. 1 and 3) is a diverse suite of amphibolite to granulite

facies rocks, all Archaean in age (Liu et al., 1990; Wang et al., 1990 and publications in Chinese therein). Near Huangbaiyu hamlet (Figs. 1 and 3), the Complex contains rare fuchsite-bearing siliceous rocks (informally known as the Caozhuang quartzite) that have been interpreted as meta-quartzite rocks of detrital origin. These quartzites contain only > 3500 Ma zircons, with

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Fig. 3. Geological sketch map of the Caozhuang-Huangbaiyu area, adapted from Chen (1988). A colour version of this map is given in the electronic supplement.

grains up to ∼3880 Ma present (Liu et al., 1992; Wu et al., 2005; Wilde et al., 2008; this paper). Amphibolite lenses in nearby orthogneiss migmatites have yielded an imperfect (MSWD = 11.6) 3522 ± 230 Ma Sm–Nd isochron (recalculated using Isoplot from data in Jahn et al., 1987). On both these counts, Huangbaiyu has been argued to be one of the few localities in China that contains Palaeo-Eoarchaean crust. The Caozhuang quartzite occurs with larger volumes of biotite-rich schists (known as ‘leptynites’, that are generally interpreted as sedimentary rocks derived from intermediate to felsic volcanic sources – e.g., Liu et al., 1990). Associated with the leptynites are disrupted lenses up to 500 m long and 100 m broad of magnetite-bearing BIF (Fig. 3). The leptynites are disrupted by early pegmatite and granite (Fig. 2b), formed by local in situ partial melting and also by intrusion of veins. Locally this granite and pegmatite has been strongly deformed under amphibolite facies conditions, such as near the Caozhuang quartzite occurrences (Fig. 2b). Clearly there is no depositional stratigraphy preserved within the leptynites, the Caozhuang quartzite and BIFs. Orthogneisses also occur near Huangbaiyu, but are extensively disrupted by tracts of younger foliated granite (Chen, 1988). South of Huangbaiyu, orthogneisses crop out sporadically within a ∼1 km long by 200 m broad area (Fig. 3). Heterogeneous coarsegrained granitic gneisses predominate at the unit’s northern edge (sampled as J06/02, Figs. 2c and 3), which also contain lenses

of homogeneous, biotitised amphibolite isolated within granitic neosome (locality for sample BJ-83-3 of Jahn et al., 1987). In the unit’s southern side there are finer-grained tonalitic (plagioclase + quartz + biotite ± hornblende) banded gneisses (sampled as J06/04) and granite (sampled as J08/02, Fig. 3), which also contain homogeneous amphibolite lenses. In the orthogneisses and amphibolites south of Huangbaiyu, there is no textural or mineralogical evidence of granulite facies metamorphism. Orthogneisses also occur 1–0.5 km northeast of Huangbaiyu, for which we report one age determination (sample J91/11, Fig. 3). This complex assemblage of leptynite, BIF, orthogneisses and early deformed granite is intruded discordantly by bodies of less-deformed granitoid which, near Huangbaiyu are monzonitic and granitic in composition (e.g., samples J00/31 and J00/03, Fig. 3). Approximately 2 km northeast of Huangbaiyu, there are migmatitic orthogneiss and granitic rocks with lenses of amphibolite. These have domains with relict metamorphic orthopyroxene as evidence of granulite facies metamorphism (e.g., sample J06/10, Fig. 3). Rocks adjacent to those with relict orthopyroxene show textural evidence such as amphibole ± biotite replacements after orthopyroxene (c.f. McGregor and Friend, 1997) to indicate that granulite facies assemblages were once more widespread but there has been extensive retrogression under amphibolite facies conditions. This is in contrast to at Huangbaiyu and further south, where muscovite is common (e.g., in the Caozhuang quartzite and

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Fig. 4. Representative cathodoluminescence images of dated zircons from the Beidaihe-Qinhuangdao coastal region plutonic rocks.

deformed granites), and where in both the orthogneisses and mafic rocks there is no textural or mineralogical evidence for granulite facies metamorphism. To the east, centred on Qian’an, the Qian’an gneisses are late kinematic intrusions of gabbro, monzonite and granite into the Qianxi Complex (Liu et al., 1990 and publications in Chinese therein). These are well exposed at Mangshan (Fig. 1), where they are at granulite facies, are weakly foliated and contain diverse inclusions of banded gneiss, amphibolite and calc silicate rocks. Some inclusions were strongly deformed before being incorporated into the Qian’an gneisses (Fig. 2d). From Mangshan, Liu et al. (1990) obtained IDTIMS zircon U–Pb ages of 2499 ± 8 Ma for ferrogabbro (sample CF85-73) and 2494 ± 2 Ma for granite (sample CF85-71)

from close to concordant multiple zircon fractions (whole rock analyses for SI/120 and SI/126, equivalent to these samples, are reproduced in supplementary Table 1). To the southeast, in the Lulong area (Fig. 1), migmatitic biotiterich rocks resembling the leptynites near Huangbaiyu are known as the Luanxiang Group (Geng et al., 2006 and publications in Chinese therein) and contain discrete bodies of orthogneiss. One of these known as the Yinmahe gneiss is granodioritic in composition. These rocks were strongly deformed and locally developed a cataclastic texture, prior to intrusion of non-deformed granite dykes. An evaporation Pb zircon age of 2551 ± 6 Ma for the granodiorite protolith of a Yinmahe gneiss (Geng et al., 2006) is integrated with our data (see below).

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Fig. 5.

238

U/206 Pb–207 Pb/206 Pb plots of SHRIMP zircon analyses from Beidaihe-Qinhuangdao region plutonic rocks. Analytical errors are depicted at the 2 level.

5.2. New data and observations 5.2.1. Caozhuang quartzite (sample J08/01) The Caozhuang quartzite crops out near Huangbaiyu hamlet (Fig. 3). It contains some fuchsitic mica, but ordinary muscovite is more common, and the quartzite is disrupted by strongly deformed pegmatite. Within the same tract there are leptynites and lenses of mafic rocks, with the latter possibly being the Cr source to form the fuchsite. The Caozhuang quartzite sample J06/01 (39◦ 56.007 N; 118◦ 33.499 E) yielded abundant large zircons, with complex internal structure (Fig. 6a). Igneous oscillatory zoning is widespread. In most grains with high contrast in CL images, this layering is widely disrupted by recrystallisation domains and can be truncated near grain margins by domains that appear brighter and less structured in CL. Many of the grains are dull in CL images, but nonetheless low-contrast oscillatory zoning can be discerned. In such grains the zoning is generally concordant to the grain exteriors (e.g., grains 36 and 42, Fig. 6a). Fifty nine SHRIMP U–Pb analyses were undertaken on 56 grains (supplementary Table 2). As it was already known that >3650 Ma zircons are common in the Caozhuang quartzite (Liu et al., 1992; Wilde et al., 2008), our study paid attention to those that held the possibility of being the youngest zircons, and hence could constrain better the history of

the rock. Thus we focussed on euhedral grains that have oscillatory zoning parallel to grain exteriors, and that appear dull and of low contrast in the CL images. These grains show moderate to high U content with high Th/U (supplementary Table 2) and are thus likely to be igneous in origin. Most analyses of them yielded strongly discordant <3400 Ma U–Pb ages. However, 4 sites gave close to concordant ages, with a weighted mean 207 Pb/206 Pb age of 3547 ± 11 Ma (MSWD = 1.3) and form a discrete population from the next oldest grains which have ages of 3700–3650 Ma (Fig. 7a). Presently, 3547 ± 11 Ma is the most robust age determination yet on the youngest zircons from the Caozhuang quartzite. Three grains (#1, 15 and 32, supplementary Table 2) yielded 207 Pb/206 Pb ages of ∼3880 Ma, and are the oldest ones yet found in this unit. This matches the age of a single zircon xenocryst in Eoarchaean orthogneiss sample 05FW035 (Wu et al., 2008) from the Anshan area, ∼350 km to the northeast in Liaoning province. Together, these zircons are the oldest recognised materials in the North China Craton. 5.2.2. Amphibolite facies orthogneisses, Huangbaiyu (samples J06/02, J06/04, and J08/02) Sample J06/02 (39◦ 55.81 N; 118◦ 33.55 E) was taken from a finer-grained more biotite-rich portion of migmatites, because this appeared in the field to be an older component (Fig. 2c). It yielded

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Fig. 6. Representative cathodoluminescence images of dated zircons from Huangbaiyu area pre-Neoarchaean rocks.

prismatic zircons, with mostly high U content (supplementary Table 2), and which are largely metamict with most oscillatory zoning destroyed (Fig. 6). The ten U–Pb analyses undertaken concentrated on domains with remnants of the igneous oscillatory zoning. Most sites yielded strongly discordant U–Pb ages, with only two being concordant within error, but whose 207 Pb/206 Pb ages do not agree (2968 ± 12 and 2879 ± 9 Ma at 2; Fig. 7b). Regression of all the data yielded a concordia upper intercept of 2936 ± 34 Ma (MSWD = 1.7). The dated zircons appear to be part of the main igneous population, but slightly better preserved. Therefore we estimate the age of the granitic protolith of this gneiss as ∼2940 Ma. At another location 39◦ 55.75 N; 118◦ 33.68 E, there are scattered large blocks of locally derived, banded tonalitic gneiss and foliated granite, unearthed in mineral exploration. Sample J06/04 of

tonalitic gneiss yielded prismatic zircons which, like those in J06/02 were largely metamict, with only locally preserved oscillatory zoning. The zircons have moderate to high U content, with those with the oldest apparent 207 Pb/206 Pb ages having generally highest Th/U (supplementary Table 2). The data formed a scattered discordant array, with a concordia upper intercept of 3228 ± 110 Ma (MSWD = 3.1), with three sites with the oldest 207 Pb/206 Pb ages at the end of this array having a weighted mean age of 3154 ± 41 Ma (MSWD = 3.3; Fig. 7b). At 39◦ 55.77 N; 118◦ 33.89 E there are foliated tonalitic gneisses (J08/02 in supplementary Table 2) with amphibolite lenses, both infiltrated by concordant foliated granite sheets. Tonalite sample J08/02 yielded similar zircons to J06/04, but less metamict (Fig. 6). Eight U–Pb analyses were undertaken on 5 zircons (supplementary

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Fig. 7.

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238

U/206 Pb–207 Pb/206 Pb plots of SHRIMP zircon analyses from pre-Neoarchaean Huangbaiyu rocks with >2600 Ma ages. Analytical errors are depicted at the 2 level.

Table 2, Fig. 7c). The data form a scattered discordant array, with a concordia upper intercept of 3267 ± 62 Ma (MSWD = 6.6). Two data points on different grains, concordant within error and with indistinguishable 207 Pb/206 Pb ratios, yielded a 207 Pb/206 Pb weighted mean age of 3287 ± 11 Ma (MSWD = 0.2), which is taken as the best estimate of the age of the tonalite protolith. At a nearby exposure (39◦ 55.70 N; 118◦ 33.80 E: J08/03 in supplementary Table 1), lenses of amphibolite locally preserve an amphibolitised gabbroic texture in their interiors. They are interpreted as dykes intruded into the tonalites, which then strongly deformed and boudinaged. Adjacent tonalitic gneiss J08/04 gave a poor yield of highly metamict zircons. Attempted SHRIMP U–Pb analyses on these zircons encountered large amounts of 204 Pb and were aborted. Of two analyses completed with lower 204 Pb content, grain #2 gave close to concordant ages, with a 207 Pb/206 Pb age of 3129 Ma, whereas ages on grain #1 were younger and discordant (supplementary Table 2). This result points to a Mesoarchaean age for this gneiss, indicating that the disrupted amphibolite dyke represented by sample J08/03 must be younger. A likely reason for the poor preservation of the (rather high-U) igneous zircons in these samples is their location <50 m below the unconformity with the overlying Proterozoic sedimentary rocks, and thus lie in an ancient weathering zone.

Sample J91/11 was collected as an amphibolite facies orthogneiss enclave within (∼2500 Ma) granitic rocks ∼500 m northeast of Huangbaiyu. The exact position of this 1991 sample is unknown, and it has been marked arbitrarily at the south-western edge of those gneisses (Fig. 3). The gneiss is biotite bearing, with no textural evidence of having been retrogressed from granulite facies. Zircons from J91/11 were dated by SHRIMP in the early 1990s, prior to the use of CL imaging to characterise the grains and choose analysis sites. The grains appear from optical microscopy to be a single population of oscillatory zoned prisms. Nine analyses were undertaken on 9 grains (supplementary Table 2, Fig. 9b), and show generally high Th/U with moderate to high U contents. Five analyses yielded close to concordant mutually indistinguishable ages, with a weighted mean 207 Pb/206 Pb age of 2548 ± 12 Ma (MSWD = 0.12) which is interpreted as the age of the igneous protolith of J91/11. The remaining 4 analyses are mostly higher U and discordant, having suffered ancient loss of radiogenic Pb. 5.2.3. Granulite facies rocks northeast of Huangbaiyu (sample J06/10) Sample J06/10 is from a ∼1 m wide kernel of non-retrogressed granulite facies orthogneiss (at 40◦ 04.47 N; 118◦ 34.46 E), in migmatites consisting of orthogneiss and amphibolite lenses

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Fig. 8. Representative cathodoluminescence images of dated zircons from Huangbaiyu area Neoarchaean rocks.

within foliated granite. All these rocks show evidence of previous granulite facies metamorphism by hornblende ± biotite replacements after orthopyroxene grains, and more rarely relict orthopyroxene, particularly in mafic rocks. J06/10 yielded abundant zircons with complex internal structure visible in CL images (Fig. 8). Most grains consist primarily of oscillatory zoned zircon, with the zoning partly obliterated by recrystallisation domains. At the margins of the grains are partial shells of highly luminescent structureless zircon, which either formed by recrystallisation of oscillatory-zoned zircon or by new zircon overgrowth. Twenty seven U–Pb analyses were undertaken on 23 grains (supplementary Table 2), all of which gave ages concordant within error (Fig. 9a). Two analyses (4.1 and 4.2) of a structureless zircon both gave ages of ∼2650 Ma, and it is interpreted as a rare xenocrystic grain. Ten out of 12 analyses of high Th/U oscillatory zoned zircon that dominates the grains yielded a weighted mean 207 Pb/206 Pb age of 2548 ± 7 Ma (MSWD = 0.8), with the two rejected analyses interpreted to have undergone small amounts of ancient radiogenic Pb loss. All 12 analyses of the exterior structureless domains bight in CL images have very low U content, and yielded a weighted mean 207 Pb/206 Pb age of 2506 ± 6 Ma (MSWD = 0.6). 2548 Ma is interpreted as the

age of the protolith and 2506 Ma as the timing of granulite facies metamorphism. Note that the protolith age of J06/10 is indistinguishable from that of gneiss J91/11 collected < 1.5 km to the southwest, but which shows no sign of granulite facies metamorphism. 5.2.4. Leptynite, Huangbaiyu (sample J00/33) This strongly deformed amphibolite facies sample was taken from the supracrustal rocks approximately 200 m east-northeast of Huangbaiyu hamlet (Fig. 3). It is composed of plagioclase, quartz, K-feldspar and biotite and is considered to be derived from a dacite or a dacitic volcanic sedimentary rock. The zircons are prismatic, and show fine scale oscillatory zoning in CL images (Fig. 8). Ten analyses with U contents and Th/U ratios range from 74–314 ppm and 0.59–1.39 (supplementary Table 2) yielded close to concordant ages (Fig. 9), with a weighted mean 207 Pb/206 Pb age of 2534 ± 8 Ma (MSWD = 0.56). This is interpreted as the eruption age of the dacitic protolith. Six other analyses show lead loss to different degrees (Fig. 9b) and are commonly higher in U and Th contents (supplementary Table 2). Their disposition on the concordia diagram (Fig. 9b) suggests that they formed at 2700–2650 Ma. Analysis

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Fig. 9.

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U/206 Pb–207 Pb/206 Pb plots of SHRIMP zircon analyses for Huangbaiyu area Neoarchaean rocks. Analytical errors are depicted at the 2 level.

6.1 shows strong lead loss but still gives an old 207 Pb/206 Pb age of ∼3620 Ma, suggesting the presence of Eoarchaean material in the provenance area(s).

5.2.5. Monzonite ( Qian’an gneisses), Huangbaiyu area (sample J00/31) Schlieric, coarse-grained discordant bodies of monzonite occur northwest of Huangbaiyu. These are foliated but not intensely deformed, and show neither textural nor mineralogical evidence of having experienced granulite facies metamorphism. Liu et al. (1990) reported an IDTIMS multigrain U–Pb zircon age of 2495 ± 1 Ma from quartz monzonite sample CF85-62. Sample J00/31 was a recollection of the same intrusion. The zircons have prismatic habit and in CL images show fine scale oscillatory zoning (Fig. 8). Twelve analyses made on 12 zircons have U contents and Th/U ratios of 76–16 ppm and 2.28–1.25 (supplementary Table 2). The data have close to concordant ages and form a tight cluster on a concordia diagram (Fig. 9c). They yield a weighted mean 207 Pb/206 Pb age of 2491 ± 13 Ma (MSWD = 0.87). This is interpreted as the intrusion age of the monzonite and agrees with the age determination of Liu et al. (1990).

6. 160–100 km west from coast (Santunying region) 6.1. Geological description and previous work The western Santunying region is dominated by orthogneisses with an N–S to NE–SW structural trend, interrupted by belts of supracrustal rocks that are assigned together as the Zunhua Group (Fang et al., 1998; Geng et al., 2006; Polat et al., 2006a,b and references therein). These belts are up to several kilometres broad and consist mostly of amphibolites derived from basaltic protoliths (Geng et al., 2006 and publications in Chinese therein; Polat et al., 2006a,b). The orthogneisses retain scattered domains of granulite facies assemblages. Where there is no orthopyroxene, there is abundant textural evidence such as amphibole + quartz ± biotite replacements of coarse orthopyroxene grains, either distributed throughout the rocks to give a ‘blebby texture’ or in ‘spotty pegmatite’ neosome veins formed by dehydration melting (described using the terminology of McGregor and Friend, 1997; Fig. 2e). The whole assemblage is dissected by generally SW–NE trending greenschist facies shear zones. Following Fang et al. (1998) we consider that much of the Santunying region Archaean basement rocks have been affected by granulite facies metamorphism, but were variably

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retrogressed, either statically or associated with amphibolite to greenschist facies deformation. The Santunying orthogneisses are mostly dioritic–tonalitic in composition, but with some magnesiangabbro and granodiorite varieties (supplementary Table 1; Fang et al., 1998; Geng et al., 2006). They range in character from well foliated, and banded to nebulitic, structureless and massive rocks, with little discernable fabric or compositional layering. This lithological diversity has spawned many local names for these rocks. We contend that the entire U–Pb zircon geochronological data set (including reassessment of the data in Geng et al., 2006) render this terminology superfluous, because all dated examples of these rocks have the same U–Pb zircon protolith ages of 2550–2540 Ma, within analytical error. Therefore we use only the most general name ‘Santunying gneisses’ for all these rocks. The protolith supracrustal rocks of the Zunhua Group were mostly basalts, intermediate-acid tuffs, tuffaceous greywacke and BIF (Tan et al., 1983; Zhao et al., 2001). Zunhua belt picritic amphibolites are characterized by light rare earth element (LREE) – enriched patterns and negative high field strength element (HFSE: Nb, Zr, and Ti) anomalies (Polat et al., 2006a,b). Similar chemical trends are shown by associated pyroxenite and dunite bodies with podiform chromite lenses from near Zunhua. These geochemical signatures have been interpreted to indicate a suprasubduction zone geodynamic setting (Polat et al., 2006a,b). Kusky et al. (2001) reported a Neoarchaean ophiolite complex in the Dongwanzi area, a northerly extension of the Santunying region (Fig. 1). Some sheared, amphibolitised gabbros regarded as part of this purported ophiolite yielded sparse (metamorphic?) zircons dated at 2505 ± 2 Ma (Kusky et al., 2001), but other maficultramafic rocks have igneous zircon U–Pb ages of ∼300 Ma (i.e., Carboniferous; Zhao et al., 2007). Therefore, the Archaean ophiolite in the form proposed by Kusky et al. (2001) is invalid and we discuss it no further here. 6.2. New data and observations 6.2.1. Partially retrogressed granulite facies orthogneiss (sample J08/05) Low road cuttings (40◦ 24.24 N; 118◦ 23.850 ) comprise buffcoloured, polyphase, schlieric orthogneisses with small amphibolite lenses. The older tonalitic–trondhjemitic component is infiltrated by domains of coarser grained neosome probably formed by in situ melting, that are discordant to weak compositional layering in the palaeosome with a well-developed blebby texture of hornblende ± biotite. Some relict metamorphic orthopyroxene is preserved, rimmed by hornblende is found, particularly in neosome. These features clearly show the rocks were metamorphosed to granulite facies, and then subsequently retrogressed under amphibolite facies conditions. The textural evidence suggests that the retrogression here was largely static. Sample J08/05 is of tonalitic gneiss, which contains much neosome. It yielded abundant zircons with complex structure, as seen in CL images (Fig. 10). Most grains consist primarily of oscillatory zoned zircon, with the zoning partly obliterated by recrystallisation domains. At the margins of the grains are shells of highly luminescent structureless zircon, which either formed by recrystallisation of oscillatory-zoned zircon or by new zircon overgrowth. Locally developed between the highly luminescent shells and the oscillatory zoned zircons are inner mantles that appear dull and structureless in CL images. Twenty five U–Pb analyses were undertaken on 20 zircons, all of which yielded ages concordant within error (supplementary Table 2, Fig. 11a). Analyses of the oscillatory zoned zircon showed a bimodal dispersion in 207 Pb/206 Pb ages beyond analytical error, from ∼2543 to <2500 Ma. This distribution

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indicates that two ages of zircons are present, rather than a single older one, from which variable amounts of radiogenic Pb were lost during superimposed events. The 4 analyses of oscillatoryzoned zircon with the oldest 207 Pb/206 Pb ages (analyses 1.1, 2.1, 7.1 and 8.1) give a weighted mean 207 Pb/206 Pb age of 2529 ± 21 Ma (MSWD = 0.2), which is interpreted as giving the (minimum) age of the tonalitic palaeosome. All remaining analyses of oscillatory zoned zircon show a younger weighted mean 207 Pb/206 Pb age of 2492 ± 17 Ma (MSWD = 0.4). The outer dull and bright in CL mantles and shells yielded a weighted mean 207 Pb/206 Pb age of 2507 ± 7 Ma (MSWD = 0.4). The latter two ages are interpreted as the timing of ∼2500 Ma granulite facies metamorphism with in situ dehydration melting, giving rise to new igneous oscillatory zoned zircon.

6.2.2. Granulite facies mafic rocks, orthogneisses and granite, Sa River (samples J08/09 and J08/10) This locality is at the south end of the bridge over the Sa River (40◦ 19.80 N; 118◦ 16.18 E). Coarse-grained mafic rocks (J08/08 in supplementary Table 1) are retrogressed from granulite facies, and contain large (>1 cm) garnets, with complex symplectitic coronas of plagioclase + amphibole. Given the retrogression, no orthopyroxene was detected in the coronas, but it does occur out in the groundmass, extensively altered to amphibole + quartz. Orthopyroxene is widely preserved in the tonalitic orthogneisses, with little if any development of new fabrics during slight retrogression under amphibolite facies conditions. Domains of in situ (granitic) melt locally disrupt the tonalitic gneisses, and coalesce to form some coarse-grained pegmatites with large orthopyroxene grains. Sample J08/10 is of granulite facies quartz-dioritic orthogneiss (supplementary Table 1) as free as possible of neosome, and yielded abundant prismatic zircons. Most grains consist primarily of oscillatory zoned zircon, with the zoning partly obliterated by recrystallisation domains. The grain margins are shells of highly luminescent structureless zircon, which either formed by recrystallisation of oscillatory-zoned zircon or by new zircon overgrowth. In some grains these have replaced most of the original oscillatory zoned zircon. Locally developed between the highly luminescent shells and the oscillatory zoned zircons are inner mantles that appear dull and structureless in CL images (Fig. 10). Twenty one U–Pb analyses were undertaken on 16 grains (supplementary Table 2; Fig. 11b). Analyses of the oscillatory zoned zircon showed a dispersion in 207 Pb/206 Pb ages beyond analytical error, from ∼2550 to <2500 Ma. This is interpreted as due to ancient radiogenic Pb-loss from a single aged zircon population. Six analyses with the oldest 207 Pb/206 Pb ages and interpreted as the least disturbed yielded a weighted mean 207 Pb/206 Pb age of 2537 ± 9 Ma (MSWD = 0.8), which is interpreted as giving the age of the quartz-diorite protolith. All 9 analyses of outer dull and bright in CL mantles, shells and replacement domains yielded a weighted mean 207 Pb/206 Pb age of 2498 ± 6 Ma (MSWD = 0.6), which is interpreted as the timing of granulite facies metamorphism with in situ dehydration melting. Coarse-grained granulite facies granitic pegmatite sample J08/09 with large orthopyroxene megacrysts (now largely replaced by orthoamphibole) yielded large (up to 500 ␮m long) zircons with sector zoning or weakly developed oscillatory zoning (Fig. 10). Ten U–Pb analyses were undertaken on 8 grains, and all gave ages concordant within error (supplementary Table 2, Fig. 11c). After rejecting one analysis which might have suffered slight ancient loss of radiogenic Pb, the remaining 9 yielded a weighted mean 207 Pb/206 Pb age of 2503 ± 11 Ma (MSWD = 1.0). This is interpreted as the time of pegmatite formation coeval with granulite facies metamorphism.

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Fig. 10. Representative cathodoluminescence images of dated zircons from the Santunying gneisses.

7. Dantazi and Zhuzhangzi Groups, Qinlong area 7.1. Geological description In the Qinlong area, the Dantazi and Zhuzhangzi Groups of metasedimentary and volcanic rocks occur between the predominantly mafic volcanic Zunhua Group to the west and granitic rocks of the Jiekouling and Anziling bodies to the east (Fig. 1). The Dantazi group consists of interlayered pelitic and more felsic/siliceous units. The Zhuzhangzi Group has basal units of conglomerates consisting of matrix-supported clasts of granite and foliated tonalite in a mica schist matrix (Sun et al., 2010, and references in Chinese therein). Other Zhuzhangzi Group units are layered sedimentary rocks resembling those in the Dantazi Group and more massive

units of mafic and felsic schists, which might be of volcanic or of volcanic-sedimentary origin (Sun et al., 2010). In both the Groups, metamorphic grade ranges from greenschist facies to lower amphibolite facies, and strain is variable, and locally high. In some localities, such as southeast of Qinlong at 40◦ 06.29 N; 118◦ 45.87 W, the conglomerates of the Zhuzhangzi Group had been intruded by dolerite dykes prior to strong deformation under amphibolite facies conditions (Fig. 2f). 7.2. Age of the Dantazi and Zhuzhangzi Groups Based on earlier provincial survey mapping noting the presence of the metaconglomerates (that are anomalous for the Archaean geology of the region), the Zhuzhangzi Group was interpreted to

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Fig. 11. 238 U/206 Pb–207 Pb/206 Pb plots of SHRIMP zircon analyses from the Santunying gneisses (J08/05, J08/09, and J08/10) and a Zhuzhangzi Group metapelitic conglomerate matrix (J09/10-3). Analytical errors are depicted at the 2 level.

be Proterozoic, and to rest unconformably on the Dantazi Group of Archaean age (references in Chinese to Provincial Survey work, in Sun et al., 2010). However, this would imply Proterozoic metamorphism locally to middle amphibolite facies, for which throughout the region there is no other evidence. In order to resolve this issue, Sun et al. (2010) undertook SHRIMP U–Pb zircon dating of metasedimentary rocks from the Dantazi and Zhuzhangzi Groups. Separate granite and tonalite cobbles from Zhuzhangzi conglomerates, and finer grained rocks from both groups all yielded 2540–2510 Ma ages (Sun et al., 2010). None of the samples contain Palaeoproterozoic zircons (detrital or metamorphic), and only one older detrital zircon (>3000 Ma) was found, in a Dantazi Group sample. Data for the matrix of a Zhuzhangzi Group conglomerate (sample J09/10-3) is reproduced here in Fig. 11d. These results indicate that the Zhuzhangzi Group was deposited in the late Neoarchaean, rather than later in Proterozoic. This is in keeping with the lack of evidence elsewhere in the region for high grade metamorphism after 2490 Ma and that rocks of this group do not contain Proterozoic detritus. Therefore, Sun et al. (2010) interpreted the Dantazi and Zhuzhangzi Groups to have been deposited in a late Neoarchaean intra-arc basin at the end of the Neoarchaean. Furthermore, these ages match those determined for higher meta-

morphic grade, migmatised, leptynites found in the Qian’an region (e.g., sample J00/33). This suggests the present ‘stratigraphic’ division of the eastern Hebei early Precambrian volcanic and related sedimentary rocks into several ‘Groups’ needs thorough revision. 8. Interpretation of U–Pb zircon ages 8.1. Data treatment U–Pb zircon data from several previous publications have been integrated with our new results and are summarised in Table 1. For previously published data we have applied ‘filters’, and in the case of Geng et al. (2006) recalculated some of their ages. Liu et al. (1990) reported U–Pb zircon ages based on IDTIMS analyses of bulk zircon fractions of different size and magnetic properties. In some cases all data yielded strongly discordant ages, leading to considerable uncertainty in the concordia upper intercept ages and data interpretation. More recent ion microprobe and LA-ICPMS dating with CL imaging has revealed age complexity in eastern Hebei Archaean zircons, again complicating the interpretation of published strongly discordant U–Pb ages. Thus from the early data, we only use samples that had zircons with close to concordant U–Pb ages, increasing

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confidence in age determinations from them. In such cases when similar rocks have been dated by ion microprobe they give the same ages within error, increasing our confidence in using some of this data. Kusky et al. (2001) reported a U–Pb zircon age of 2505 ± 2 Ma for sparse zircons from a sheared, amphibolite facies metagabbro. Given the tectonothermal history of this rock, we consider this is most likely to denote the time of ductile shearing during high grade metamorphism, rather than this being an igneous age. Yang et al. (2008) reported high quality, well-documented LA-ICPMS U–Pb zircon data on 6 samples from the east of the region (Table 1). We use these ages unmodified. Likewise, the ages obtained on the Zhuzhangzi and Dantazi Groups (Sun et al., 2010) are incorporated without revision. Geng et al. (2006) reported both stepwise thermal evaporation–deposition 207 Pb/206 Pb and SHRIMP U–Pb zircon data on rocks from mostly the western part of the region. The thermal evaporation–deposition data are taken unmodified (Table 1). Geng et al. (2006) showed CL images of representative zircons analysed by the SHRIMP U–Pb method. These resemble zircons from our Santunying gneiss samples, in which igneous oscillatory zircon is variably replaced and mantled by domains that in CL are bright and structureless. These latter are interpreted to have developed under high grade metamorphism, and give ages of ∼2500 Ma. For the oscillatory zoned zircon expected to give the igneous age of the protoliths, Geng et al. (2006) reported weighted 207 Pb/206 Pb mean ages with high MSWD of 2515–2495 Ma, derived with little data filtering. These ages were significantly younger than the 2550–2540 Ma thermal evaporation-deposition 207 Pb/206 Pb ∼2550 Ma ages on their other samples, and they concluded that components of the western gneisses had been emplaced over a ∼60 million year period. We have reassessed SHRIMP U–Pb data on oscillatory zoned zircon in Geng et al. (2006), using a model of variable loss of radiogenic Pb during the ∼2500 Ma granulite facies metamorphism. Thus starting from the ‘youngest’ 207 Pb/206 Pb ages, data were culled, until a population indistinguishable within analytical error remained. In this way we revise the igneous protolith ages upwards as follows: sample TP19 2551 ± 12 Ma (formerly 2519 ± 10 Ma), ZH10 2543 ± 6 Ma (formerly 2515 ± 12 Ma) and QX05 2542 ± 20 Ma (formerly 2495 ± 28 Ma). These revised ages now coincide with the stepwise thermal evaporation–deposition 207 Pb/206 Pb ones on different samples. Thus most granitoids in the western region affected by subsequent high grade metamorphism were intruded between 2550 and 2540 Ma. An exception might be sample CZ02 of Geng et al. (2006), which comes from massive tonalitic rocks lying north of the 2500–2490 Ma Qian’an gneisses (Fig. 1). Thermal evaporation of two zircons from CZ02 yielded rather high common Pb, but a maximum 207 Pb/206 Pb age of 2492 ± 5 Ma was encountered with no detectable 204 Pb in one grain (Geng et al., 2006). Thus it is possible that the body represented by CZ02 is a northern continuation of the Qian’an gneisses igneous suite (with ages of 2499 ± 8, 2495 ± 1, 2494 ± 2 and 2491 ± 13; Liu et al., 1990, this paper). 8.2. Magmatic and metamorphic U–Pb zircon ages Gabbros-tonalites in the west of the region have igneous ages of 2550–2535 Ma (apart from sample J08/05 at 2529 ± 21 Ma with its larger analytical error and possibly incorporation of some neosome zircon within this result). In this group we include the 2548 ± 7 Ma granulite facies gneiss J06/10 and the 2548 ± 12 amphibolite facies gneiss J91/11 northeast of Huangbaiyu. Additionally, in the western Santunying region, podiform chromitites within Zunhua Group dunites have yielded a Re-Os age of 2547 ± 10 Ma (Kusky et al., 2004). This confirms the formation between 2550 and 2535 Ma of an ultramafic to felsic complex (Fig. 12). In contrast, focussed

in the coastal region to the east, melagabbros (hornblendites) to granites dated by Yang et al. (2008) and us mostly have intrusion ages of ∼2525 Ma, i.e., younger than the western rocks. Exceptions are 2546 ± 6 Ma tonalitic/granodioritic inclusion J08/15 in granite at Beidaihe and perhaps marginally older 2531 ± 3 Ma J08/18 mafic granitoid inclusion in granite near Qinhuangdao. Altered granite J08/12 gave an age of 2512 ± 12 Ma. This might be marginally younger than the ∼2525 Ma group, or it could be of the same age, but its zircons are more disturbed. Younger still are variably deformed ferrogabbro, monzonite and granite samples from the central Qian’an region and possibly extending north to embrace sample CZ02 of Geng et al. (2006), which all have ages between 2499 ± 8 and 2491 ± 13 Ma (Liu et al., 1990, this paper). Ages of zircon formed during high grade mostly granulite facies metamorphism range from 2490 ± 4 Ma to 2507 ± 7 Ma, barely distinguishable from each other and showing no systematic change for >100 km west-east across geological structure. Furthermore, the age of granulite facies metamorphism is indistinguishable from the young ferrogabbro, monzonite and granite intrusions, and distinct (50–25 million years younger) from the other orthogneisses and granitoids of the region (Figs. 1 and 12). This in contrast to Geng et al. (2006) who proposed than there was a 60 million year continuum of igneous and metamorphic events from ∼2550 to 2490 Ma. Instead, we consider that there are three discrete igneous events, with only the last being coeval with granulite facies metamorphism (Fig. 12). In addition to this dominant Neoarchean magmatism and metamorphism, this paper reports the most reliable ages yet for Mesoarchaean rocks (>2900 Ma) from the central Qian’an region, near Huangbaiyu. This comprises three samples of banded orthogneiss, which have yielded U–Pb zircon protolith ages between 3287 ± 11 Ma and ∼2940 Ma.

8.3. Interpretation of ∼3500 Ma whole rock Sm–Nd isochron ages for Caozhuang area amphibolites Before the discovery of the > 3500 Ma zircons in the Caozhuang quartzite (Liu et al., 1990), ∼3500 Ma whole rock Sm–Nd isochron ages for Caozhuang area amphibolites were proposed as evidence for ancient rocks in the area (as discussed in most detail by Jahn et al., 1987). For example, Jahn et al. reported whole rock Sm–Nd data for amphibolites scattered across a >3 km broad area west of Caozhuang (their Fig. 3), which gave a best fit line with a slope equivalent to an age of 3470 ± 107 Ma (εNd (T) = + 2.7; MSWD = 3.18). This was regarded as the true age of the rocks (Jahn et al., 1987). We now present our reasons for considering that the ∼3500 Ma Sm–Nd isochrons (Jahn et al., 1987) for Huangbaiyu area amphibolite bodies are not giving true rock ages. Ages derived from Sm–Nd ‘isochrons’ on mafic Archaean rocks have often fuelled considerable controversy. A notable example was the Sm–Nd age of the Kambalda volcanic rocks in Western Australia. Claoué-Long et al. (1984) reported a Sm–Nd isochron of ∼3250 Ma for the Kambalda volcanic rocks, which they interpreted as the time of eruption. This sparked considerable controversy because this was ∼500 million years older than the expected ∼2750 Ma age of these rocks, based upon all the then available geochronological and geological information. Subsequently it was demonstrated by SHRIMP U–Pb zircon dating combined with a reinvestigation of the Sm–Nd systematics of these rocks, that their true eruption age was ∼2750 Ma, but that they were contaminated by older crustal material, including Eoarchaean zircon xenocrysts (Chauvel et al., 1985; Compston et al., 1986). We contend that the Caozhuang amphibolites have a similar origin; i.e., they are younger mafic rocks intruded into and contaminated by older crust.

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} } } }

central region volcanic and sedimentary rocks

Santunying western region igneous rocks

encl encl

encl 2480

2500

central region plutonic rocks

coastal eastern region plutonic rocks

2520 2540 2560 age (Ma, 95% confidence)

age of high grade metamorphism age of main detrital zircon population and granite boulders in sedimentary rocks age of plutonic rocks (encl = volumetrically minor enclaves) Fig. 12. Summary geochronology for Neoarchaean crustal evolution in eastern Hebei, based on age compilation in Table 1.

At Jahn et al.’s amphibolite sample locality BJ-83-3, the host orthogneisses are granitic in composition and have a U–Pb zircon age of ∼2950 Ma (sample J06-02 reported here). The margins of the amphibolites have diffuse contacts with adjacent neosome and can be rimmed by biotite selvedges. Most samples show LREE enrichment over HREE (Jahn et al., 1987). Furthermore, near the locality of our gneiss samples J08/02 and J08/04 with zircon U–Pb ages of 3287 ± 11 Ma and ≥3130 Ma, locally better-preserved amphibolite bodies are found to be dykes intruded into the gneisses (and then deformed and metamorphosed together) and locally in their centres preserve an amphibolitised gabbroic texture with small plagioclase phenocrysts. We interpret these amphibolites as dykes into ≥2950 Ma rocks, which have been subsequently tectonically disrupted, invaded by younger foliated granite and metasomatised. The youngest time that this could have occurred was coeval with major injection of granites at ∼2500 Ma, with deformation and high grade metamorphism that affected all Caozhuang area rocks. At 2500 Ma, the Jahn et al. amphibolites had εNd values ranging from +2 to −5 (Fig. 13a). Plotted on the same figure is the εNd evolution for gneiss sample J00/02 (see below), collected from the same tract south of Huangbaiyu as the old tonalitic gneisses J06/04 and J08/02 and granitic gneiss J06/02. At 2500 Ma J00/02 (with 147 Sm/144 Nd = 0.1085) had ε Nd = –10. Contamination at 2500 Ma of a positive εNd (2500) mantle-derived gabbroic melt modelled as εNd + 4 (with 147 Sm/144 Nd = 0.2) by such old crustal materials could give rise to an Sm–Nd mixing line with slope equivalent to an age of 3550 Ma. These input parameters should be regarded as one of a family of solutions, to illustrate the feasibility of such a contamination model to explain the ∼3500 Ma reference line of Jahn et al. (1987). Note however, even if none of these amphibolites themselves are ∼3500 million years old as originally proposed, they

nonetheless might act as a field marker for the presence of older crustal components hidden in the migmatites in the region, such as the 3287–2940 Ma orthogneisses south of Huangbaiyu that are reported in this paper. 9. Integration of zircon U–Pb ages with whole rock chemistry and Nd–Hf isotopic data New Sm–Nd isotopic and whole rock chemical data from the Huangbaiyu area (supplementary Tables 1 and 3) are integrated with data presented by Yang et al. (2008) for eastern Qinhuangdao and by Fang et al. (1998) and Geng et al. (2006) for the western Santunying regions. 9.1. Eastern Qinhuangdao coastal region We address the eastern region first, because the Archaean rocks there are overall least deformed, they have not undergone such high degrees of metamorphism and thus they still widely preserve igneous relationships. This increases the possibility of detecting subtle variations within the suite that can be attributed to igneous processes. The comprehensive data set presented by Yang et al. (2008) from the eastern region contained rocks constrained to a single age of ∼2525 by U–Pb zircon geochronology. The samples were described as ranging in composition from hornblendite to Kgranite. Inspection of the hornblendite chemical data (11–12 wt% Al2 O3 ) indicates their protoliths were melagabbros – where maybe an amphibolitisation reaction between clinopyroxene and plagioclase gave rise to copious hornblende, giving the ‘hornblendites’. Judging from the chemical analyses, there are some inconsistencies in their nomenclature. Thus gneiss sample FW04-84 described

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Fig. 14. (a) SiO2 –MgO (wt%) and (b) Y–Sr/Y plots eastern Hebei 2550–2490 Ma plutonic rocks. Field for <3000 Ma Archaean TTG is from Martin et al. (2005).

Fig. 13. Sm–Nd isotopic data for eastern Hebei rocks. (a) εNd – age diagram, with initial values calculated using the zircon U–Pb age of the samples. (b) Histogram of εNd (2525 Ma) values for coastal region rocks, classified according to bulk composition.

as ‘granodioritic’ more likely had a leucodioritic protolith. Finally, there are no clear major element distinctions between samples described as ‘monzogranite’ and ‘Kf granite’ thus these are pooled together here simply as ‘granite’. Thus the 2525 Ma suite ranges in composition from gabbro, through diorite–monzodiorite, to granodiorite and granite. Compositionally, this suite forms a swathe across the field of arc-related plutonic rocks in Sr/Y versus Y, La/Yb versus Yb, Mg# versus SiO2 diagrams (Fig. 12 of Yang et al. 2008; representative data is reproduced here in our Fig. 14a). Furthermore, in primitive mantle normalised (McDonough and Sun, 1995) trace element diagrams the whole suite forms a coherent set, with consistent negative anomalies for Th, Nb + Ta, P, Zr + Hf and Ti (Fig. 9 of Yang et al., 2008; representative data is reproduced here along with additional analyses of ours in Fig. 15a). Rather than judging the Nd isotopic signatures of these rocks based on their TDM model ages as done by Yang et al. (2008), we instead assess it via their εNd (2525 Ma), i.e., at the time when they formed (Fig. 13b). This shows that the gabbros (and single leucodiorite) have the highest εNd (2525 Ma) values of +1.2 to +2.2, whereas the monzodiorite to granodiorite and granite generally have lower εNd (2525 Ma) values of 0.0 to +1.4 (plus granite FW04-20 with an even less radiogenic value of −2.3). In terms of their whole rock major, trace element and Nd isotopic systematics, these resemble an arc-related suite, consisting of mantle derived components represented by the gabbros that interacted with and melted older crust. We would interpret that the gabbros with their positive εNd (2525 Ma) values up to +2 (highest in the suite) and consistent negative anomalies for Th, Nb + Ta, P, Zr + Hf and Ti as due to flux melting of a depleted mantle wedge (Fig. 15a). The more granitic vari-

eties, particularly shown by anomalous sample FW04-20 with εNd (2525 Ma) of −2.3 and TDM = 2926 Ma involved melting of somewhat older crustal material. This mixing process was observed by Yang et al. (2008) in their zircon Hf isotopic data, where igneous zircons within individual samples could show scatter in εHf (2525 Ma) beyond analytical error. These Nd isotopic data are supporting evidence for the involvement of Mesoarchaean rocks in the genesis of the 2525 Ma igneous suite. The presence of tonalitic/granodioritic inclusion J08/15 with an age of 2546 ± 6 Ma in granite at Beidaihe indicates also the involvement/incorporation of slightly older Neoarchaean plutonic rocks (matching the age of rocks dominant in the Santunying region in the west) in the ∼2525 Ma magma genesis. We propose that the coastal suite has geochemical and isotopic characters like those found in Andean arcs – where sub-mantle to deep crustal melting-assimilation-reaction processes gives rise to a coeval plutonic suite ranging from gabbro to granite (e.g., Hildreth and Moorbath, 1988). This will involve cannibalisation of slightly older arc rocks by new crustal additions, plus assimilation of any older pre-arc basement rocks that are present. Our arc-interpretation differs from Yang et al. (2008) who regarded these coastal region rocks as the product of a mantle plume (i.e., essentially anhydrous decompression melting in the mantle). This is discussed below. 9.2. Western Santunying region Assessment of the geochemistry of the western Santunying gneisses is not as straightforward as the eastern coastal gneisses, because (a) there is not a single modern coherent data set and (b) the rocks have been affected by granulite facies and therefore overall they are more deformed and modified. A pivotal point in our reassessment and collation of western data is that the span of protolith ages is much more restricted than proposed by Geng

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Fig. 15. Primitive mantle normalised trace element (spider) diagrams for 2550–2490 Ma granitic plutonic rocks.

et al. (2006; with the exception of sample CZ02, see above), being mostly from ∼2550–2530 Ma (Fig. 1 and Table 1), and thereby are older than the eastern 2530–2510 Ma coastal suite. As shown by whole rock analyses in Geng et al. (2006) and Fang et al. (1998), this 2550–2530 Ma suite consists largely of gabbro, diorite, quartzdiorite and tonalite, and thus is overall less potassic than the younger eastern suite. The MgO–SiO2 variation of the western Santunying gneisses (Fig. 14a) mostly coincides with the field of <3000 Ma arc rocks,

that are considered to be the products melts produced by fluxing of a depleted mantle wedge combined with slab melting (e.g., Martin et al., 2005). In the Sr/Y versus Y discrimination diagram they mostly fall within the field of normal arc rocks, with only a few having adakitic characteristics (Fig. 14b). In a primitive mantle normalised trace element diagram the data form a coherent set, with marked enrichment of LILE and light REE versus heavy REE (Fig. 15b). There are pronounced negative Ti and Nb anomalies (supplementary Table 1; Nb data was not presented by Fang

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Fig. 16. Primitive mantle normalised trace element (spider) diagrams for 2550–2490 Ma mafic–ultramafic plutonic rocks. The ∼2550 Ma samples are from Polat et al. (2006a,b).

et al., 1998), as also observed in the eastern coastal 2530–2520 Ma suite. The same trace element patterns are seen in associated picritic metabasalts, pyroxenites and peridotites in the Zunhua Group (Polat et al., 2006a,b) and our amphibolite sample J08/08, namely depletion of Nb and Ti, and enrichment of the LILE and light REE relative to the heavy REE (Fig. 16). The Santunying gneisses show consistent Rb depletion relative to Ba, not seen in the eastern coastal 2530–2520 Ma suite (Fig. 14). This is attributed to Rb-depletion of the Santunying gneisses caused by the ∼2500 granulite facies metamorphism. 9.3. 2500–2490 Ma plutonism and granulite facies metamorphism The 2500–2490 Ma gabbro–quartz monzonite–granite suite overall shows Fe-enrichment relative to 2550–2530 Ma and 2530–2510 Ma plutonic rocks (depressed MgO compared to the other rocks in Fig. 14a). Their emplacement was coeval with medium–low pressure granulite facies metamorphism, with peak conditions estimated at 800–700 ◦ C and <8–7 kbar; based on the absence of garnet and coexisting orthopyroxene + clinopyroxene + plagioclase + quartz in mafic rocks at granulite facies. The compositional span from gabbro to granite and the more positive εNd (2500) values of mafic samples (Fig. 13a) indicate a hybrid origin, by variable mixing of gabbroic magma derived from the mantle, with granitic magma derived from partial melting of sialic crust with older (>2500 Ma) components. The enrichment of Ti, Fe versus Mg and P2 O5 in the more mafic members of the suite indicates fractional crystallisation of the gabbroic magma prior to its emplacement in the crust and then variable mingling with granitic crustally derived magmas. The fractionated mafic rocks are thus likely a manifestation of a crust-mantle boundary or a deep crustal mafic underplate, whose heat was responsible for the contemporaneous low pressure granulite facies metamorphism. A Phanerozoic analogue of such a process is in the Cretaceous Kohistan arc (e.g., Petterson, 2010). 10. Discussion 10.1. Neoarchaean plate boundaries or Plume? When plume processes were refined by fluid dynamics laboratory experiments (e.g., the Australian National University plume model; Campbell et al., 1989), there was considerable debate about

the relative importance of plate boundary versus plume magmatism in the Archaean. The ∼2750–2650 Ma crustal evolution of the Yilgarn Craton was then taken as a classic example of such plume–crust interaction (Campbell and Hill, 1988). Subsequently, this plume model has been adopted to explain the evolution of other Neoarchaean terranes, such the North China Craton in eastern Hebei (e.g., Geng et al., 2006; Yang et al., 2008). In contrast, the Yilgarn craton where the Archaean plume model was established, is now regarded to contain an archetypal example of crustal development at a Neoarchaean convergent plate boundary (Czarnota et al., 2010 and references therein). Since the 1980s, there has been a great advancement in the understanding of Archaean crustal evolution, from whole rock trace element geochemistry, tectonic studies and from large amounts of precise and accurate U–Pb zircon geochronology. Thus the diverse chemical signatures of Archaean basalts are better understood, with the demonstration that compositions resembling modern island arc tholeiites and boninites (Dilek and Polat, 2008) are volumetrically more important than the komatiites (ostensibly of plume origin) that featured so strongly in 1970s and 1980s studies. Also, tectonic investigations increasingly recognise that in Archaean basement terranes, there are early layer parallel tectonic breaks that developed prior to the upright basin and dome style folding that is their most obvious structural feature (e.g., Friend et al., 1987, 1988; Nutman et al., 1989; Lacroix and Sawyer, 1995; Bleeker and van Breemen, 2010). In many cases these early tectonic breaks bring older rocks to tectonically overlie younger ones, or higher metamorphic grade ones to overlie lower metamorphic grade ones – pointing to thrusting in compressional tectonic settings. The large number of reliable U–Pb zircon dates now allows routine resolution of Archaean geological events at a ±10–5 million year scale, as opposed to a ±100–50 million year scale that was still routine in 1980s studies, when there was much greater reliance on Rb–Sr and Sm–Nd whole rock isochron geochronology. These new advances aid discrimination between Archaean crust formation by a plume magmatism (the root cause of which is largely anhydrous decompression melting of the mantle within a single transient event) or at convergent plate boundaries (the root cause of which is largely flux-melting of the mantle or subducted slab, typically in several events following on from each other in short succession). For eastern Hebei, mafic underplating from a plume head, which partially melted capping crustal rocks, was favoured by Geng et al. (2006) and Yang et al. (2008). Alternatively, these rocks have been interpreted as arc-related suites, with the mafic rocks produced by

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flux-melting of the mantle (Polat et al., 2006a,b). These different explanations can be tested by the trace element geochemistry of these rocks. The MORB and OIB decompression mafic melts of the mantle (the latter being the products of modern plumes) do not show negative Nb and Ti anomalies. To illustrate this, average NMORB, E-MORB and OIB (Sun and McDonough, 1989) are plotted with the eastern Hebei 2550–2490 Ma mafic samples (Fig. 16). The ≥2550–2540 Ma picrites and other mafic rocks in the Santunying region devoid of crustal contamination, hornblendites (mafic metagabbros) of the 2530–2520 Ma suite concentrated in the coastal region and ∼2500 Ma Mangshan suite gabbroic sample SI/120 (Wang et al., 1990) have pronounced negative Ti and Nb anomalies (Nb was not reported in the Wang et al., 1990 data). Such geochemical features are seen in arc-related mafic rocks (e.g., Polat et al., 2006a,b and references therein), but not N-MORB, E-MORB, OIB or intraoceanic plateaux basalts (Floyd, 1989) formed by decompression melting. Furthermore, komatiites and related tholeiites, the classic products of Archaean plumes, also do not show Nb and Ti depletions relative to La and Gd, respectively (Jochum et al., 1990), as long as they are not contaminated by continental crust during their ascent (e.g., Polat et al., 2006b). Within the plume model, there should be a temporal bimodal transition from older mafic rocks, to younger granites, as the early hot mantle melts triggered melting of the older sialic crust, to give a suite of predominantly bimodal composition. From the assembled age data this trend is not apparent (Table 1; Fig. 12). Instead, mafic compositions span a 60 million year range from ∼2550 to ∼2490 Ma, with granitic sensu stricto rocks being formed at both ∼2525 Ma and 2500–2490 Ma. Furthermore, tonalites, quartz diorites and trondhjemites are as abundant as granites sensu stricto in the eastern Hebei Neoarchaean igneous record. Thus the Neoarchaean rocks are not only complex in their age distribution, but also they are not bimodal (mafic–granitic) in composition. 10.2. Precambrian analogues A well-exposed example of rocks resembling the eastern Hebei igneous suites is in the Ketilidian orogen on the southern tip of Greenland. This is the site of a Palaeoproterozoic volcanicplutonic arc on southern edge the Greenland Archaean craton, constructed as a result of northerly directed oblique subduction (Garde et al., 2002 and references therein). In the Ketilidian, the 1850–1800 Ma early Julianehaab batholith arc rocks are less ironenriched than 1750–1730 Ma gabbroic to granitic rocks of a later so-called rapakivi suite (Garde et al., 2002 and references therein). The rapakivi suite shows strong Fe-enrichment, light REE over heavy REE enrichment and Ti and Nb depletion, and it is coeval with low pressure granulite facies metamorphism. It appears to be a discrete event late in the history of the arc, related to a new pulse of mantle-derived magma due to change of stress regime, rather than being a continuity with the main magmatic arc development 50–100 million years earlier (Garde et al., 2002 and references therein). Although the details of this event are debated, there was clearly significant emplacement of mafic magma into an existing arc crust, which caused crustal fusion and a low pressure pyroxene granulite facies metamorphic overprint. Thus as a whole, the 2500–2490 Ma eastern Hebei late suite has similar characteristics, both in terms lithological development and timing, as compared to composite mafic-granitic complexes formed late in the development of the Ketilidian – a well-exposed, early Precambrian arc complex. It has been argued that the ovoid pattern of eastern Hebei Neoarchaean plutonic bodies militates against an arc origin (Geng et al., 2006). However, this is not true. In the western Santunying part of the region there is a distinct linear arrangement of plutonic versus volcanic rocks, on essentially the same scale as

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seen in other arc related terranes of the same age, such as southern India (e.g., Chadwick et al., 2007). Therefore we do not agree that the size and the shape of the observed Neoarchaean plutonic bodies in eastern Hebei must indicate that they have to be plume-derived. 10.3. Neoarchaean reconstruction of the North China Craton Because of the scattered exposure of the Archaean rocks in the eastern NCC, it is hard to make a wider reconstruction. However, to the northeast in Liaoning province in the Anshan area (Fig. 1 inset), a complex history of older granitic (sensu lato) crustal development from ∼3800 to 2970 Ma is preserved, with these rocks unconformably overlain by Neoarchaean BIF-rich sedimentary sequences and then intruded by ∼2500 Ma granites without regional high temperature metamorphism (Liu et al., 1992, 2007; Song et al., 1996; Wan et al., 2005b). This might represent an older continental hinterland whose edge became involved in ∼2500 Ma arc development. The ages of Anshan 3800–2970 Ma rocks and the ∼3880 Ma zircon xenocryst reported by Wu et al. (2008) match that for Huangbaiyu area old gneiss components and metaquartzite zircon ages. Therefore, these old rock and zircon components in the Qian’an geology might be related to the older continental fragment that is better preserved in the Anshan area. In the Qian’an region amphibolite facies leptynites (sample J00/33) contain sparse 2700–2650 Ma zircons, distinctly older than the 2540–2530 Ma main population in these rocks. 2700–2650 Ma rocks are unknown in the Anshan region, but ∼2700 Ma rocks are known from Shandong province, in the southern part of the NCC (Fig. 1 inset; Jahn et al., 1988; Cao, 1996; Wan et al., 2011). The ∼2700 Ma Shandong rocks consist of an early suite of metavolcanic and tonalitic rocks, strongly overprinted by ∼2500 Ma granite injection and high grade metamorphism (Wan et al., 2011). These Shandong ∼2700 Ma components might equate with the rare 2700–2650 Ma zircons found in the eastern Hebei ∼2500 Ma rocks. Thus the eastern Hebei ∼2500 Ma rocks might contain minor components derived from different unrelated blocks of older crust (∼2700 Ma and 3880–2970 Ma), perhaps brought together during protracted 2550–2500 Ma arc development with consumption of intervening oceanic crust. 11. Conclusions (1) U–Pb zircon geochronology reveals that eastern Hebei Neoarchaean magmatism was not a single protracted event of uniform character, but was marked by temporally, geographically and geochemically distinct pulses of igneous activity at 2550–2535, 2530–2520 Ma and then at 2500–2490 Ma, with the latter accompanied by granulite facies metamorphism. (2) In the Huangbaiyu area, ancient crustal components ranging up to 3880 Ma old zircons in the Caozhuang quartzite to 3280–2950 Ma orthogneisses are preserved relicts of an ancient continental crustal contaminant. Their ages coincide with those in the Anshan old crustal nucleus in Liaoning province, ∼400 km to the northeast. (3) The geochemical characteristics of the 2550 to 2490 Ma rocks resemble those in arc-related suites, especially long-lived arcs developed at the fringe of older continental crust. Particularly important is the presence of negative Nb and Ti anomalies in all studied mafic rocks (see also Polat et al., 2006a,b) and that tonalites and quartz-diorites are volumetrically important. (4) An extensional setting with crustal thinning and renewed influx of hot asthenosphere would be consistent with low pressure granulite facies metamorphism associated with the 2500–2490 Ma magmatism.

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(5) The complexity of the ∼2500 Ma eastern Hebei arc magmatism matches that found in long-lived arc systems which involved older continental crust, such as along the Andean margin of South America (Hildreth and Moorbath, 1988), the Palaeoproterozoic of South Greenland (Garde et al., 2002) or Neoarchaean crustal development in southern India (Chadwick et al., 2007). Acknowledgments A.P. Nutman acknowledges support from the Chinese Academy of Geological Sciences and the University of Wollongong. Y. Wan acknowledges support from the Ministry of Land and Resources of the Peoples’ Republic of China (1212010711815, 1212010811033). C.R.L. Friend acknowledges support from the Chinese Academy of Geological Sciences for his contribution to this work. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.precamres.2011.04.005. References Black, L.P., Kamo, S.L., Allen, C.M., Aleinikoff, J.K., Davis, D.W., Korsch, R.J., Foudoulis, C., 2003. TEMORA 1: a new zircon standard for Phanerozoic U–Pb geochronology. Chemical Geology 200, 155–170. Bleeker, W., van Breemen, O., 2010. The fundamental architecture of the southcentral Abitibi greenstone belt, superior craton, Canada, and the localization of world-class Au deposits. In: Tyler, I.M. and Knox-Robinson, C.M. (Eds.), Fifth International Archean Symposium Abstracts: Geological Survey of Western Australia, Record 2010/18, pp. 153–154. Campbell, I.H., Hill, R.I., 1988. A two-stage model for the formation of the granitegreenstone terrains of the Kalgoorlie-Norseman area, Western Australia. Earth and Planetary Science Letters 90, 11–25. Campbell, I.H., Griffiths, R.W., Hill, R.I., 1989. Melting in an Archaean mantle plume: heads it’s basalts, tails it’s komatiites. Nature 339, 697–699. Cao, G.Q., 1996. Early Precambrian Geology of Western Shandong. Geological Publishing House, Beijing, pp. 1–210. Chadwick, B., Vasudev, V.N., Hegde, G.V., Nutman, A.P., 2007. Structure and SHRIMP U/Pb zircon ages of granites adjacent to the Chitradurga Schist Belt: implications for Neoarchaean convergence in the Dharwar Craton, southern India. Journal of the Geological Society of India 69, 5–24. Chauvel, C, Dupre, B., Jenner, G.A., 1985. The Sm–Nd age of Kambalda volcanics is 500 Ma too old! Earth and Planetary Science Letters 74, 315–324. Chen, T., 1988. The geology and petrological characteristics of granitic rocks in the Caozhuang-Bailonggang area, Qian’an, Hebei. Bulletin of the Institute of Geology. Chinese Academy of Geological Science 18, 82–97. Claoué-Long, J.C., Thirlwall, M.F., Nesbitt, R.W., 1984. Revised Sm–Nd systematics of Kambalda greenstones, Western Australia. Nature 307, 697–701. Compston, W., Zhang, F.P., Foster, J.J., Collerson, K.D., Bai, J., Sun, D.Z., 1983. Rubidium–strontium geochronology of Precambrian rocks from the Yanshan Region, North China. Precambrian Research 22, 175–202. Compston, W., Williams, I.S., Campbell, I.H., Gresham, J.J., 1986. Zircon xenocrysts from the Kambalda volcanics: age constraints and direct evidence for older continental crust below the Kambalda-Norseman greenstones. Earth and Planetary Science Letters 76, 299–311. Cumming, G.L., Richards, J.R., 1975. Ore lead isotope ratios in a continuously changing earth. Earth and Planetary Science Letters 28, 155–171. Czarnota, K., Champion, D.C., Goscombe, B., Blewett, R.S., Cassidy, K.F., Henson, P.A., Groenewald, P.B., 2010. Geodynamics of the eastern Yilgarn Craton. Precambrian Research 183, 175–202. Dilek, Y., Polat, A., 2008. Suprasubduction zone ophiolites and Archean tectonics. Geology 36, 431–432. Fang, L., Friend, C.R.L., Li.F Q., Li, S.Z., Liu, W., Powell, D., Thirwall, M.F., Yang, Z.S., Zhang, Q.H., 1998. Geology of the Santunying Area of Eastern Hebei Province. Geological Publication House, Beijing, 134 pp. Floyd, P.A., 1989. Geochemical features of intraoceanic plateau basalts. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in the Ocean Basins, Vol. 42. Special Publications of the Geological Society, London, pp. 215–230. Friend, C.R.L., Nutman, A.P., McGregor, V.R., 1987. Late Archaean tectonics in the Færingehavn—Tre Brødre area, Budsefjorden, southern West Greenland. Journal of the Geological Society of London 144, 369–376. Friend, C.R.L., Nutman, A.P., McGregor, V.R., 1988. Late Archaean terrane accretion in the Godthåb region, southern West Greenland. Nature 335, 535–538. Garde, A.A., Hamilton, M.A., Chadwick, B., Grocott, J., McCaffrey, K.J.W., 2002. The Ketilidian orogen of South Greenland: geochronology, tectonics, magmatism, and fore-arc accretion during Paleoproterozoic oblique convergence. Canadian Journal of Earth Sciences 39, 765–793.

Geng, Y.S., Liu, F.L., Yang, C., 2006. Magmatic event at the end of the Archean in eastern Hebei Province and its geological implication. Acta Geologica Sinica (English Version) 80, 819–833. Hildreth, W., Moorbath, S., 1988. Crustal contributions to arc magmatism in the Andes of Central Chile. Contributions to Mineralogy and Petrology 98, 455–489. Jahn, B.M., Zhang, Z.Q., 1984. Archean granulite gneisses from eastern Hebei Province, China: rare earth geochemistry and tectonic implications. Contributions to Mineralogy and Petrology 85, 224–243. Jahn, B.M., Ernst, W.G., 1990. Late Archean Sm–Nd isochron age for mafic–ultramafic supracrustal amphibolites from the Northeastern Sino-Korean Craton, China. Precambrian Research 46, 295–306. Jahn, B.M., Auvray, B., Cornichet, J., Bai, Y.L., Shen, Q.H., Liu, D.Y., 1987. 3.5 Ga old amphibolites from eastern Hebei province, China: field occurrence, petrography, Sm–Nd isochron age and REE chemistry. Precambrian Research 34, 311–346. Jahn, B.M., Auvray, B., Shen, Q.H., Liu, D.Y., Zhang, Z.Q., Dong, Y.J., Ye, X.J., Zhang, Q.Z., Cornichet, J., Macé, J., 1988. Archean crustal evolution in China: the Taishan Complex, and evidence for juvenile crustal addition from long-term depleted mantle. Precambrian Research 38, 381–403. Jochum, K.P., Arndt, N.T., Hofmann, A.W., 1990. Nb-Th-La in komatiites and basalts: constraints on komatiite petrogenesis and mantle evolution. Earth and Planetary Science Letters 107, 272–289. Kröner, A., Compston, W., Zhang, G.W., Guo, A.L., Todt, W., 1988. Ages and tectonic setting of Late Archean greenstone-gneiss terrain in Henan Province, China, as revealed by single grain zircon dating. Geology 16, 211–215. Kröner, A., Cui, W.Y., Wang, S.Q., Wang, C.Q., Nemchin, A.A., 1998. Single zircon ages from high-grade rocks of the Jianping Complex, Liaoning Province, NE China. Journal of Asian Earth Science 16, 519–532. Kusky, T.M., Li, J.H., Tucker, R.D., 2001. The Archean Dongwanzi ophiolite complex, North China Craton: 2.505-billion-year-old oceanic crust and mantle. Science 292, 1142–1145. Kusky, T.M., Li, J.H., Raharimahefa, T., Carlson, R.W., 2004. Re-Os isotope chemistry and geochronology of chromite from mantle podiform chromitites from the Zunhua ophiolitic mélange belt, N. China: correlation with the Dongwanzi ophiolite. In: Kusky, T.M. (Ed.), Precambrian Ophiolites and Related Rocks: Developments in Precambrian Geology, 13. Elsevier, Amsterdam, pp. 275–282. Lacroix, S., Sawyer, E.W., 1995. An Archean fold-thrust belt in the northwestern Abitibi Greenstone Belt: structural and seismic evidence. Canadian Journal of Earth Science 32, 97–112. Liu, D.Y., Shen, Q.H., Zhang, Z.Q., Jahn, B.M., Auvray, B., 1990. Archean crustal evolution in China: U–Pb geochronology of the Qianxi Complex. Precambrian Research 48, 223–244. Liu, D.Y., Nutman, A.P., Williams, I.S., Compston, W., Wu, J.S., Shen, Q.H., 1992. Remnants of >3800 Ma crust in the Chinese portion of the Sino-Korean Craton. Geology 20, 339–342. Liu, D.Y., Wan, Y.S., Wu, J.S., Wilde, S.A., Zhou, H.Y., Dong, C.Y., Yin, X.Y., 2007. Eoarchean rocks and zircons in the North China Craton. Developments in Precambrian Geology, ‘Earth’s Oldest Rocks’, vol. 15. Elsevier, pp. 251–273. Ludwig, K. 1997. Isoplot/Ex. Berkeley Geochronology Center, Publication 1. Ludwig, K.R., 2001. Squid 1.02: A User’s Manual. Berkeley Geochronology Centre, Special Publication 2, pp. 1–19. Martin, H., Smithies, R.H., Rapp, R., Moyen, J.-F., Champion, D., 2005. An overview of adakite, tonalite-trondhjemite-granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos 79, 1–24. McDonough, W.F., Sun, S.-S., 1995. The composition of the Earth. Chemical Geology 120, 223–253. McGregor, V.R., Friend, C.R.L., 1997. Field recognition of rocks totally retrogressed from granulite facies: an example from Archaean rocks in the Paamiut region. South-West Greenland. Precambrian Research 86, 59–70. Nutman, A.P., Friend, C.R.L., Baadsgaard, H., McGregor, V.R., 1989. Evolution and assembly of Archean gneiss terranes in the Godthåbsfjord region, southern west Greenland: structural, metamorphic and isotopic evidence. Tectonics 8, 573–589. Petterson, M. G., 2010. A Review of the geology and tectonics of the Kohistan island arc, north Pakistan. Geological Society, London, Special Publication 338, 287–327, doi:10.1144/SP338.14. Polat, A., Herzberg, C., Münker, C., Rodgers, R., Kusky, T., Li, J., Fryer, B., Delaney, J., 2006a. Geochemical and petrological evidence for a suprasubduction zone origin of Neoarchean (ca. 2.5 Ga) peridotites, central osorgenic belt, North China Craton. Bulletin of the Geological Society of America 118, 771–784. Polat, A., Li, J., Fryer, B., Kusky, T., Gagnon, J., Zhang, S., 2006b. Geochemical characteristics of the Neoarchean (2800–2700 Ma) Taishan Greenstone Belt, North China Craton: evidence for plume–craton interaction. Chemical Geology 230, 60–87. Song, B., Nutman, A.P., Liu, D.Y., Wu, J.S., 1996. 3800 to 2500 Ma crustal evolution in the Anshan area of Liaoning Province, northeastern China. Precambrian Research 78, 79–94. Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in the Ocean Basins, 42. Special Publications of the Geological Society, London, pp. 313–345. Sun, H.Y., Dong, C.Y., Xie, H.Q., Wang, W., Ma, M.Z., Liu, D.Y., Nutman, A., Wan, Y.S., 2010. The formation age of the Neoarchean Zhuzhangzi and Dantazi Groups in the Qinglong area, eastern Hebei province: evidence from SHRIMP U–Pb zircon dating. Geological Review 56, 16.

A.P. Nutman et al. / Precambrian Research 189 (2011) 43–65 Tan, Y.J., Li, S.X., Zhao, W.X., 1983. On the characteristics and tectonics and its evolution of Zunhua Group in the east Hebei Province. Earth Science 3, 60–67 (in Chinese with English abstract). Wan, Y.S., Li, R.W., Wilde, S.A., Liu, D.Y., Chen, Z.Y., Yan, L., Song, T.R., Yin, X.Y., 2005a. UHP metamorphism and exhumation of the Dabie Orogen: evidence from SHRIMP dating of zircon and monazite from a UHP granitic gneiss cobble from the Hefei Basin. Geochimica et Cosmochimica Acta 69, 4333–4348. Wan, Y.S., Liu, D., Song, B., Wu, J., Yang, C., Zhang, A., Geng, Y., 2005b. Geochemical and Nd isotopic compositions of 3.8 Ga meta-quartz dioritic and trondhjemitic rocks from the Anshan area and their geological significance. Journal of Asian Earth Science 24, 563–575. Wan, Y., Liu, D.Y., Wang, S., Yang, E., Wang, W., Dong, C.H., Zhou, H.Y., Yang, Y.H., Diwu, C.R., 2011. ∼2.7 Ga juvenile crust formation in the North China Craton (Taishan-Xintai area, western Shandong Province): further evidence of an understated event from zircon U–Pb dating and Hf isotopic composition. Precambrian Research 186, 169–180. Wang, K.Y., Windley, B.F., Sills, J.D., Yan, Y.H., 1990. The Archaean gneiss complex in E. Hebei province, north China: geochemistry and evolution. Precambrian Research 48, 245–265. Wilde, S.A., Valley, J.W., Kita, N.T., Cavosie, A.J., Liu, D.-Y., 2008. SHRIMP U–Pb and CAMECA 1280 oxygen isotope results from ancient detrital zircons in the Caozhuang quartzite, Eastern Hebei, North China Craton: evidence for crustal reworking 3. 8 Ga ago. American Journal of Science 308, 185–199. Williams, I.S., 1998. U–Th–Pb geochronology by ion microprobe. Applications of microanalytical techniques to understanding mineralizing processes. In: McK-

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ibben, M.A., Shanks III, W.C.P., Ridley, W.I. (Eds.), Soc. Econ. Geol. Short Course, vol. 7. Wu, F.Y., Yang, J.H., Liu, X.M., Li, T.S., Xie, L.W., Yang, Y.H., 2005. Hf isotopes of the 3.8 Ga zircons in eastern Hebei Province, China: implications for early crustal evolution of the North China Craton. Chinese Science Bulletin 50, 2473–2480. Wu, F.-Y., Zhang, Y.-B., Yang, J.-H., Xie, L.-W., Yang, Y.-H., 2008. Zircon U–Pb and Hf isotopic constraints on the Early Archean crustal evolution in Anshan of the North China Craton. Precambrian Research 167, 339–362. Yang, J.H., Wu, F.Y., Wilde, S.A., Zhao, G.C., 2008. Petrogenesis and geodynamics of Late Archean magmatism in eastern Hebei, eastern North China Craton: geochronological, geochemical and Nd-Hf isotopic evidence. Precambrian Research 167, 125–149. Zhang, Z.Q., Ye, X.J., 1987. Mass-spectrometric isotope dilution analysis of REE and precise measurement of 143 Nd/144 Nd ratios. Bulletin of the Institute of Geology, Chinese Academy of Geological Sciences 1, 108–128 (in Chinese with English abstract). Zhao, G.C., Wilde, S.A., Cawood, P.A., Sun, M., 2001. Archean blocks and their boundaries in the North China Craton: lithological, geochemical, structural and P-T path constraints and tectonic evolution. Precambrian Research 107, 45–73. Zhao, C.C., Wilde, S.A., Li, S.Z., Sun, M., Grant, M.L., Li, X.P., 2007. U–Pb zircon age constraints on the Dongwanzi ultramafic-mafic body, North China, confirm it is not an Archean ophiolite. Earth and Planetary Science Letters 255, 85–93. Zhou, J., Li, X., 2006. GeoPlot: an Excel VBA program for geochemical data plotting. Computers and Geosciences 32, 554–560.