SHRIMP and LA-ICP-MS zircon geochronology of the Xiong’er ... · SHRIMP and LA-ICP-MS zircon geochronology of the Xiong’er volcanic rocks: Implications for the Paleo-Mesoproterozoic

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Precambrian Research 168 (2009) 213–222

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Precambrian Research

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HRIMP and LA-ICP-MS zircon geochronology of the Xiong’er volcanic rocks:mplications for the Paleo-Mesoproterozoic evolution of the southern

argin of the North China Craton

anhong He, Guochun Zhao ∗, Min Sun, Xiaoping Xiaepartment of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong

r t i c l e i n f o

rticle history:eceived 10 April 2008eceived in revised form4 September 2008ccepted 26 September 2008

eywords:HRIMPA-ICP-MSiong’er volcanic rocksorth China Cratonaleo-Mesoproterozoicolumbia supercontinent

a b s t r a c t

The Paleo-Mesoproterozoic Xiong’er volcanic rocks along the southern margin of the North China Cratonare lithologically and geochemically similar to those formed in subduction-related, continental marginvolcanic arcs. The volcanic rocks are primarily composed of basaltic andesites and andesites, with minordacites and dacitic rhyolites. Traditionally, the Xiong’er volcanic rocks have been divided from lower toupper into the Xushan, Jidanping and Majiahe Formations, but the ages of volcanic rocks in these forma-tions have not been well constrained, which has hindered further understanding the tectonic significanceof the Xiong’er volcanic belt at the southern margin of the North China Craton. SHRIMP and LA-ICP-MS U-Pbzircon analyses, combined with cathodeluminescence (CL) images, have enabled resolution of xenocrys-tic and magmatic zircons that can be directed toward determination of the ages of the Xiong’er volcanicrocks. SHRIMP and LA-ICP-MS U-Pb analyses on magmatic zircons from two basaltic andesite samples,one dacite sample and one rhyolite sample of the Xushan Formation, known as the lowest sequence ofthe Xiong’er volcanic rocks, indicate that the volcanic eruption of this Formation occurred at ∼1.78 Ga,but most xenocrystic/inherited zircons in these samples yielded 207Pb/206Pb ages ranging from 2.55 Gato 1.91 Ga. Of three samples collected from the Jidanping Formation, two rhyolite samples (05XE015 and05XE100) yielded weighted mean 207Pb/206Pb ages of 1778 ± 5.5 Ma and 1751 ± 14 Ma, respectively, simi-lar to the ages of the volcanic rocks in the Xushan Formation, whereas one dacite sample (05XE066) gavea weighted mean 207Pb/206Pb age of 1450 ± 31 Ma, which is the youngest age obtained from the Xiong’ervolcanic rocks. One andesite sample (06XS012) collected from the Majiahe Formation yielded two majorage populations, with the older one at 1850 ± 5.9 Ma, interpreted as the age of the xenocrystic/inheritedzircons, and the younger one at 1778 ± 6.1 Ma, interpreted as the age of the volcanic eruption to formthe Majiahe andesite, coeval with the formation of most volcanic rocks from the Xushan and JidanpingFormations. These new SHRIMP and LA-ICP-MS U-Pb zircon data indicate that the traditional stratigraphic

subdivision of the lower, middle and upper sequences of the Xiong’er volcanic rocks is not viable and thatmost of the Xiong’er volcanic rocks formed at 1.78–1.75 Ga, with minor felsic volcanic rocks erupting at∼1.45 Ga. Similar-aged arc-related volcanic belts have also been found in the southern margin of NorthAmerica, Greenland and Baltica, the western margin of the Amazonia Craton, the southern and easternmargins of the North Australia Craton, and the eastern margin of the Gawler Craton, which are consideredto represent long-lived (1.8–1.3 Ga), subduction-related growth via accretion at key continental margins

ic Co

F

of the Paleo-Mesoproterzo

. Introduction

There is now a broad agreement that the assembly of theolumbia (Nuna) supercontinent was completed by global-scale.1–1.8 Ga collisional events (see an overview by Zhao et al., 2002a).

∗ Corresponding author. Tel.: +852 28572192; fax: +852 25176912.E-mail address: [email protected] (G. Zhao).

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301-9268/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2008.09.011

lumbia (Nuna) supercontinent.© 2008 Elsevier B.V. All rights reserved.

ollowing its final assembly at ∼1.8 Ga, the supercontinent under-ent a long-lived, subduction-related outgrowth along some of

ts continental margins, forming a number of accretionary zones,ncluding the huge 1.8–1.3 Ga magmatic accretionary belt border-

ng the present southern margin of North America, Greenland andaltica; the 1.80–1.45 Ga Rio Negro-Juruena Belt and 1.45–1.30 Gaondonian-San Ignacio Belt along the western margin of Southmerica; and the 1.8–1.5 Ga Arunta, Musgrave, Mt. Isa, Georgetownnd Coen inliers along the southern and eastern margins of the

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214 Y. He et al. / Precambrian Research 168 (2009) 213–222

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ig. 1. (a) Simplified geological map of the Xiong’er volcanic rocks. (b) Tectonic subB = Western Block; EB = Eastern Block; TNCO = Trans-North China Orogen; K-Bel

inling-Dabie Orogen.

orth Australian Craton (Zhao et al., 2004a and reference therein).he fragmentation of this supercontinent began about 1.6 Ga ago,n association with development of Mesoproterozoic continentalifting along the western margin of Laurentia (Wernecke, Muskwa,elt, Purcell, Uinta, Unkar and Apache supergroups or groups),outhern margin of Baltica (Telemark Supergroup), southeasternargin of Siberia (Riphean aulacogens) and northwestern margin

f South Africa (Kalahari Copper belt). The final breakup of theolumbia (Nuna) supercontinent is marked by the emplacement of.3–1.2 Ga mafic dyke swarms (the McKenzie, Sudbury, Seal Lake,arp and Mealy) and coeval eruption of flood basalts (e.g. Copper-ine River basalts) in North America and their equivalents in other

ratonic blocks.The North China Craton is one of the oldest cratonic blocks in the

orld. A new tectonic model for the evolution of the North Chinaraton envisages discrete Eastern and Western Blocks that devel-ped independently during the Archean and early Paleoproterozoicnd collided along the Trans-North China Orogen to form a coher-nt craton at ∼1.85 Ga (Zhao et al., 1998, 2001b, 2005). Availableata also show that along the northern margin of the North Chinaraton is a late Mesoproterozoic rift belt, named the Zhaertai-Bayanbo-Huade-Weichange rift zone, which extends up to 1000 km longnd is considered to have resulted from fragmentation of the cra-on from some other cratonic block during the break-up of theolumbia (Nuna) supercontinent (Zhao et al., 2003a). Thus, likeost other cratons, the North China Craton records the history

f the assembly and breakup of the Columbia (Nuna) supercon-inent. However, little is known about whether the North Chinaraton, like North America, Greenland, Baltica, Amazonia and other

ratons, underwent a long-lived, subduction-related outgrowth inhe Paleo-Mesoproterozoic during the existence of the ColumbiaNuna) supercontinent.

Along the southern margin of the North China Craton is aarge Paleo-Mesoproterozoic volcanic belt, traditionally named the

Xahr2

on of the North China Craton, modified from by Zhao et al. (2005). Abbreviations:Khondalite Belt; JLJ-Belt = the Jiao-Liao-Ji Belt; QLS-QL-DB Orogen = Qilianshan-

iong’er Group or Xiong’er volcanic belt (Fig. 1(a); Zhao et al.,002a, 2003a), which is most likely to have resulted from theutgrowths of the Columbia (Nuna) supercontinent (He et al.,008). The belt is bounded to the North China Craton by theiangxian–Lintong Fault in the northwest and the Luoyang–Baofengault in the northeast, and is separated from the late Meso-roterozoic Kuanping Ophiolite Complex in the south by theuonan–Luanchun Fault (Fig. 1(b)). The major lithologies of theiong’er Group are dominant andesites and basaltic andesites withinor dacites, rhyolites and interlayered sedimentary rocks, which

verlie the pre-1.8 Ga basement rocks of the North China CratonFig. 1). These volcanic rocks were long considered as products ofMesoproterozoic intracontinental rifting event (Sun et al., 1981;hang, 1989; Yang, 1990; Zhao et al., 2002a; Peng et al., 2005).he major evidence seeming to support the rift model is that theruption of most volcanic rocks in the belt was coeval with themplacement of the mafic dyke swarms within the North Chinaraton. However, lithological assemblages and geochemical data

or these volcanic rocks suggest that they formed under a tectonicetting similar to an Anden-type continental margin arc (Jia, 1985;hen and Fu, 1992; Zhao et al., 2003a; He et al., 2008). The lines ofvidence supporting the continental margin arc model include (1)alc-alkaline basaltic andesite–andesite–dacite–rhyolite assem-lages; (2) geochemical data showing that these volcanic rocks haveffinities to magmatic arcs, with few having an affinity to within-late environments (Jia, 1985; Chen et al., 1992; He et al., 2008);nd (3) the presence of the Mesoproterozoic Kuanping ophioliteomplex to the south of the Xiong’er volcanic belt (Chen et al., 1992).

Although geological, petrological and geochemical data of the

iong’er volcanic rocks suggest that these volcanic rocks formed incontinental margin arc, the precise ages of these volcanic rocksave not been well constrained. Available isotopic age data are onlyestricted to volcanic rocks in the Waifangshan area (Zhao et al.,004b), whereas the ages of the volcanic rocks in other areas (e.g.

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hongtiaoshan, Xiong’ershan, Xiaoshan, etc.) have not been welletermined. Thus, it still remains uncertain whether the Xiong’erolcanic rocks along the southern margin of the North China Cratonere coeval with the outgrowth of the supercontinent Columbia

Nuna) in the period 1.8–1.3 Ga. In addition, the Xiong’er volcanicocks are traditionally divided into the lower, middle and upperequences, named the Xushan, Jidanping and Majiahe Formations,espectively, but such stratigraphic subdivisions have not beeneochronologically tested because of lack of reliable ages. In thisontribution, we have applied SHRIMP and LA-ICP-MS U-Pb zirconating techniques to determine the ages of different lithologicalnits of the Xiong’er volcanic rocks, which will not only examinehether the traditional subdivision of the Xiong’er volcanic rocks

s viable, but also provide important insights into understandinghe Paleo-Mesoproterozoic history of the southern margin of theorth China Craton and its significance to the reconstruction of the

upercontinent Columbia (Nuna).

. Regional setting

The Precambrian basement of the North China Craton can beivided into the Archean to Paleoproterozoic Eastern and West-rn Blocks, separated by the Trans-North China Orogen (TNCO)Fig. 1(a)). Detailed lithological, geochemical, structural, metamor-hic and geochronological differences between the basement rocksf the Eastern and Western Blocks and the TNCO have been sum-arized by Zhao et al. (1998, 1999, 2000, 2001a). Recently, this

hreefold subdivision of the North China Craton has been fur-her refined and modified using new structural, petrological andeochronological data obtained over the last few years (Zhao et al.,005). These new data suggest that the Western Block formed bymalgamation of the Ordos Terrane in the south and the Yinshanerrane in the north along the east-west-trending Khondalite Beltt 1.90–1.95 Ga, about 50–100 Ma earlier than the collision of theestern and Eastern Blocks (Fig. 1(a); Zhao et al., 2005; Santosh

t al., 2006, 2007a,b). The data also suggest that the Eastern Blocknderwent Paleoproterozoic rifting along its eastern continentalargin in the period 2.2–1.9 Ga, forming the Jiao-Liao-Ji Rift Belt

Fig. 1(a); Li et al., 2005, 2006; Li and Zhao, 2007).The Xiong’er volcanic belt occurs along the southern mar-

in of the North China Craton with an area of approximately5,000 km2, and is mainly exposed in the Zhongtiaoshan, Xiaoshan,iong’ershan and Waifangshan areas (Fig. 1(b)). In the Chinese lit-rature, the Xiong’er volcanic rocks in the Zhongtiaoshan area arelso called as the Xiyanghe Group or Xiyanghe volcanic rocks (Sunt al., 1991). The Xiong’er volcanic belt is bounded to the north-est by the Jiangxian–Lintong Fault and to the northeast by the

uoyang–Baofeng Fault, and is separated from the North Qinlingrogenic Belt in the south by the Luonan–Luanchun Fault (Fig. 1(b)).

Late Archean to Paleoproterozoic basement unconformablynderlies the Xiong’er volcanic belt. In the Zhongtiaoshan area,he metamorphic basement consists of TTG gneisses, syn-tectonicranites, and supracrustal rocks including amphibolites, peliticchists, felsic paragneisses, calc-silicate rocks and conglomeraticocks metamorphosed at greenschist to amphibolite facies (Tian etl., 2006; Sun et al., 1991; Zhao et al., 2000). Geochronologic datandicate that metamorphic basement rocks in the Zhongtiaoshanrea formed at ∼2.55 Ga, ∼2.35 Ga, ∼2.15 Ga and ∼2.07 Ga (Sun etl., 1991). In the Xiaoshan, Xiong’ershan and Waifangshan areas,he basement is dominated by the late Archean (2.84–2.80 Ga)

TG gneisses and amphibolites (Shangtaihua Group) and Paleopro-erozoic (2.26–1.84 Ga) graphite-bearing gneisses, marbles, bandedron formation (BIF) and amphibolites (Xiataihua Group), which

ere all metamorphosed from the upper amphibolite facies toranulite facies (Kroner et al., 1988; Wan et al., 2006). These

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rch 168 (2009) 213–222 215

etamorphic rocks, together with other late Archean to Paleo-roterozoic complexes in the Trans-North China Orogen, representlements of a single late Archean to Paleoproterozoic magmatic arcystem that has been subsequently tectonically disrupted and jux-aposed during the collision of the Eastern and Western Blocks at1.85 Ga, which resulted in the final assembly of the North Chinaraton (Zhao et al., 2001b, 2005).

The North Qinling Orogenic Belt is dominated by the two Pro-erozoic metamorphic complexes, named the Qinling and Kuanpingomplexes. The Qinling Complex consists of amphibolite to green-chist facies metagraywackes and marbles, of which detrital zirconsrom the metagraywackes yielded a 207Pb/206Pb evaporation agef 2172 ± 5 Ma (Zhang et al., 1994), interpreted as their maximumepositional age. Geochemical and isotopic properties suggest thathe Qinling Complex was derived from an allochthonous terraino the North China Craton (Ouyang and Zhang, 1996; Zhang etl., 1996). The Kuanping Complex was previously considered asMesoproterozoic ophiolite complex (Zhang and Zhou, 2000),

epresented by ultramafic rocks and tholeiites metamorphosedn greenschist and amphibolite facies in the lower sequence and

etagraywackes (turbidite) and marbles with minor siliceous rocksn the upper sequence. Inherited zircon ages (Yang et al., 2003)nd Sm–Nd data (Zhang et al., 1994; Liu et al., 1995) suggest thathe Kuanping Ocean developed in the period 1400–1000 Ma. Geo-hemical data suggest that the Kuanping Complex formed within aontinental margin sea basin between the North China Craton andhe Qinling Complex (Gao et al., 1996; Zhang et al., 1995).

The Xiong’er volcanic rocks are dominated by basaltic andesite,ndesite, dacite and rhyolites with minor intermediate to sili-ic tuff and mafic to felsic sub-volcanic rocks. The volcanicocks are conventionally subdivided from bottom to top into theushan, Jidanping and Majiahe Formations (Fig. 2), with sedimentseposited locally between the volcanic rocks and the Archean toaleoproterozoic metamorphosed basement. The Xushan Forma-ion is composed primarily of basaltic andesites and andesites,ith minor dacites and dacitic rhyolites. The Jidanping Forma-

ion consists of dacites and rhyolites interleaved with basalticndesites and andesites. The Majiahe Formation is composition-lly similar to the Xushan Formation. It is dominated by basalticndesites and andesites, but contains voluminous sedimentaryocks and pyroclastic rocks, unconformably overlain by the Neo-roterozoic terrigenous sediments, limestones and calc-silicateocks. However, the subdivision of the Xiong’er volcanic rocksnto the lower (Xushan Formation), middle (Jidanping Formation)nd upper (Majiahe Formation) sequences has never been geo-hornologically tested due to the lack of reliable age data.

Previous dates of the Xiong’er volcanic rocks were predom-nantly based on K–Ar and Rb–Sr analyses of minerals and

hole-rock samples and conventional multigrain U–Pb zircon anal-ses, with some Sm–Nd analyses (Qiao et al., 1985; Zhang etl., 1994; Ren and Li, 1996). Most of the ages scatter between700–1400 Ma (Bai et al., 1993; Ren et al., 2000 and referencesherein). These ages are viewed as unreliable because of their largerrors and mobilization of K–Ar and Rb–Sr during later geologi-al processes. Most recently, using SHRIMP technique, Zhao et al.2004b) obtained magmatic zircon ages of ∼1.78 Ga from a numberf samples from the Xiong’er volcanic rocks, but all these samplesere collected from the Waifangshan area. The ages of the Xiong’er

olcanic rocks in other areas still remain unknown.

. Sample selection and analytical methods

In this study, we selected eight samples from the Xushan,idanping and Majiahe Formations of the Xiong’er Group. Of theight samples, four samples (05ZT073, 05XE006, 05XE106 and

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216 Y. He et al. / Precambrian Research 168 (2009) 213–222

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ig. 2. Representative stratigraphic sections of the Xiong’er volcanic rocks from wellocations of the studied samples are shown in the figure.

5XE115) were collected from the Xushan Formation; three sam-les (05XE015, 05XE066 and 05XE100) were collected from the

idanping Formation; and one sample (06XS012) was from the Maji-he Formation. Sample sites and their corresponding formationsre shown in Figs. 1 and 2.

Zircons were separated by the conventional heavy liquid andagnetic techniques. Zircons from the nonmagnetic fractions were

andpicked and mounted on adhesive tapes, embedded in epoxyesin and then polished to about half their thickness and pho-ographed in reflected and transmitted light. CathodoluminescenceCL) images, carried out at Guangzhou Institute of Geochemistrynd the Beijing Ion Probe Center, were taken to define the internaltructures of the zircons.

Samples 05ZT073, 05XE006, 05XE066 and 05XE115 were ana-yzed using the SHRIMP II ion microprobe at the Beijing SHRIMPenter. The SHRIMP analytical procedures were similar to thoseescribed by Williams (1998). Mass resolution during the analyt-

cal sessions was ∼5000 (1% definition), and the intensity of therimary ion beam was 5–8 nA. Primary beam size was 25–30 �m,nd each site was rastered for 120–200 s prior to analysis. Five scanshrough the mass stations were made for each age determination.tandards used were SL13 and TEM (Williams, 1998; Black et al.,

003). Decay constants used for age calculation are those recom-ended by the Subcommission on Geochronology of IUGS (Steiger

nd Jaeger, 1977). Measured 204Pb was applied for the common leadorrection, and data processing was carried out using the Squid andsoplot 3.23 programs (Ludwig, 2001). The uncertainties for individ-

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sed areas (after Zhao et al., 2002b). Correlated sequences are linked by dashed lines.

al analyses are quoted at the 1� confidence level, whereas errorsor weighted mean ages are quoted at 2�. Individual analyses ofach sample are listed in Supplementary Table 1.

The LA-ICP-MS U–Th–Pb analyses of samples 06XS012, 05XE015,5XE100 and 05XE106 were performed on a Nd:YAG 213 laserblation system (Microprobe2, New Wave Research, U.S.A.) cou-led with VG PQ Excell ICP-MS, housed in the Department of Earthciences, the University of Hong Kong. The detailed analytical pro-edures and parameters have been described by Xia et al. (2004).ypical ablation time was 30–60 s, resulting in ablation pit diame-er at ∼40 �m. In our analysis, the repetition rate was 10 Hz, and thencident pulse energy was about 0.08–0.1 mJ. Data reduction, iso-ope ratios and apparent age calculations were carried out with theLITTER software (van Achterbergh et al., 2001) with zircon 91500s an external standard. Data processing was carried out using thesoplot 3.23 programs (Ludwig, 2001). The uncertainties for individ-al analyses are quoted at the 1� confidence level, whereas errorsor weighted mean ages are quoted at 2�. Individual analyses ofach sample are listed in Supplementary Table 2.

. Analytical results

The rocks sampled in this study are composed of basalticndesites, andesites, dacites and rhyolites. Zircons from theseiong’er volcanic rocks show complex morphologies and a varietyf internal structures as revealed by CL images (Fig. 3). Zir-ons from the basaltic andesites and andesites are dominated

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Y. He et al. / Precambrian Research 168 (2009) 213–222 217

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ig. 3. Cathodoluminescence images of selected zircon grains from the studied sampc) grain from Sample 05XE115; (d) grain from Sample 05XE100; and (f) grain from

y inherited or xenocrystic zircons with minor co-magmatic zir-ons. The inherited or xenocrystic zircons comprise primarilyounded or prismatic grains with rounded edge terminations. In CLmage, they are characterized by complex oscillatory zoning withntensity-different luminescence and multi-stage overgrowthsFig. 3(a–b)). The co-magmatic zircons in the basaltic andesitesnd andesites are in small amounts, and are colorless and euhe-ral grains with well-developed prismatic crystal shapes. Mostf the co-magmatic zircons exhibit broad banding-like zoningsith low luminescence (Fig. 3(c)), evidently of crystallization from

he silicate-undersaturated magmas. Zircons from rhyolites areolorless, euhedral and concentrically oscillatory zoning in CLFig. 3(d–f)). In order to determine the ages of the Xiong’er vol-anic rocks, we tried to date more co-magmatic zircons from theamples.

.1. Ages of the Xushan Formation

Sample 05ZT073 is a basaltic andesite collected from the Xushanormation in the Zhongtiaoshan area. It is composed mainly oflagioclase (∼55%) and minor clinopyroxene (∼10%) with aphyricroundmass (∼35%). Part of the glassy groundmass has altered toe sericite + chlorite + epidote + ferrous clay minerals. Most of theircons in the sample are inherited zircons, which are prismatic orounded in shape, ∼50 to ∼100 �m long, with complex, low lumi-escent oscillatory zoning in CL images (Fig. 3(a)). A few grains areo-magmatic zircons that are colorless and prismatic grains withroad oscillatory zoning in CL images (Fig. 3(g)). SHRIMP U-Pb anal-ses were made on 26 zircon grains, and their U–Pb isotopic resultsre listed in Supplementary Table 1 and plotted in Fig. 4(a). Theserains have low to moderate U (40–655 ppm) and Th (21–342 ppm)ontents, with Th/U ratios ranging from 0.17 to 1.52. As shown inig. 4(a), three co-magmatic zircons give 207Pb/206Pb apparent agesuch younger than those of the inherited/xenocrystic zircons, and

ield a weighted mean 207Pb/206Pb age of 1767 ± 47 (MSWD = 0.29).e interpret this age to reflect the approximate eruption time of

he basaltic andesite in the Zhongtiaoshan area. Of 21 analyses onhe inherited/xenocrystic zircons, except one grain (spot C-7.1) thatives a nearly concordant age at 3411 ± 8 Ma (see Supplementary

iaFoi

) and (g) grains from the Sample 05ZT073; (b) and (e) grains from Sample 05XE106;le 05XE015. Detailed descriptions of the studied zircons are given in the text.

able 1), which is the oldest age founded in the Trans-Northhina Orogen, all others have 207Pb/206Pb apparent ages scatteringetween 2282 ± 9 Ma and 2573 ± 10 Ma, interpreted as the crystal-

ization ages of xenocrystic/inherited zircons. As shown in Fig. 4(a),ne concordant population of inherited zircons yields a weightedean 207Pb/206Pb age of 2530 ± 7.6 Ma (MSWD = 1.3), similar to the

ges of the basement TTG gneisses in this region (Tian et al., 2006;un et al., 1991).

Sample 05XE006 is a basaltic andesite collected from theushan Formation in Xiaoshan. It shows a porphyritic-like tex-

ure in which plagioclase (20%) and clinopyroxene (10%) occurss phenocrysts. The groundmass consists of fine-grained pla-ioclase, clinopyroxene, Mg–Fe oxides and glasses. The skeletalexture formed by the plagioclase and clinopyroxene indicateshat super-cooling processes were involved in the volcanism. Not

any zircon grains have been separated from this sample, andost of them are inherited/xenocrystic grains showing light to

ark brown color and prismatic crystal shape with edge-roundederminations. These zircons possess complex oscillatory zoning,ndicating multi-stage zircon growths. A small group of zircons areolorless broken fragments, characterized by broad zonings andow luminescence, typical of co-magmatic zircons from silicate-ndersaturated magmas. SHRIMP U–Pb zircon analyses were maden seven co-magmatic zircons and four inherited/xenocrystic zir-on grains. These zircons have variable U (101–525 ppm) and Th168–1205 ppm) contents and Th/U ratios (0.27–1.54). As shown inig. 4(b), seven analyses on co-magmatic zircons define a discor-ia line with the upper intercept at 1783 ± 20 Ma (MSWD = 1.06),pproximated to be the eruption age of the basaltic andesite in theiaoshan area. All four analyses on inherited zircons are stronglyiscordant, probably due to some Pb loss, with 207Pb/206Pb appar-nt ages ranging from 2031 Ma to 1862 Ma.

Sample 05XE106 is a rhyolite from the Xushan Formation inhe Waifangshan Mountain. It is a porphyritic texture, consist-

ng of anhedral quartz (25%) and plagioclase (15%) as phenocrystsnd microcrystalline groundmass of feldspar, quartz and minore–Ti oxide minerals. Zircons separated from this sample are col-rless and prismatic grains with lengthways parallel fracturesn the surface. They have variable Th/U (0.2–2.23) ratios. Forty

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218 Y. He et al. / Precambrian Research 168 (2009) 213–222

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ig. 4. Concordia diagrams of U–Pb zircon analytical results for samples from the Xu5XE115. Samples 05ZT073, 05XE006 and 05XE115 were analyzed using the SHRIM

ine analyses give mainly three intercept age populations witheighted mean 207Pb/206Pb ages of 1778 ± 8 Ma, 1914 ± 12 Ma and

195 ± 16 Ma (Fig. 4(c)). Zircons with apparent ages around 1.78 Gahow typically igneous oscillatory zoning (Fig. 3(e)), interpreted aso-magmatic zircons and thus their weighted mean 207Pb/206Pbge of 1778 ± 8 Ma is considered as the approximate age of theolcanism to form the rhyolite of the Xushan Formation. In con-rast, inherited/xenocrystic zircons with ages around ∼1.9 Ga areesorbed, dark or nebulously concentric zoning, and those zirconsith ages of >2.1 Ga are convolutedly zoned cores embayed by sub-

equent euhedral overgrowths (Fig. 3(b)).Sample 05XE115 is dacite collected from the Xushan Forma-

ion in the Waifangshan Mountain, and consists of plagioclasehenocrysts and microcrystalline quart + feldspar + Fe–Ti oxideroundmass. Zircons separated from the sample comprise pre-ominantly sub-rounded grains which are colorless to dark brown,nd subordinated euhedral broken fragments which are clean andolorless. CL images show that the sub-rounded zircon grains pos-ess concentric oscillatory zoning, whereas the euhedral brokenircon fragments have broad banding-like zoning (Fig. 3(c)). Theub-rounded zircon grains yield ages scattering from 2700 Mao 1800 Ma, interpreted to be of inherited/xenocrystic origins,hereas the euhedral zircon fragments yield a weighted mean

07Pb/206Pb age of 1783 ± 13 Ma (MSWD = 1.4; Fig. 4(d)), interpreted

s the crystallization age of the dacite of the Xushan Formation.nherited/xenocrystic zircons show lower U (<200 ppm) and Th<100 ppm) contents and Th/U ratios (<1.0) than the co-magmaticircons with U of 102–406 ppm, Th of 92–517 ppm and Th/U of.93–1.22.

sata(

Formation. (a) Sample 05ZT073; (b) Sample 05XE006; (c) Sample 05XE106; and (d)nique, and Sample 05XE106 was dated using the LA-ICP-MS method.

.2. Ages of the Jidanping Formation

Sample 05XE015 is a rhyolite collected from the Jidanping For-ation in the Xiaoshan area, and contains phenocrysts of corroded

uartz (20%) and K-feldspar (15%) in a fine-grained groundmass.ost zircons separated from this sample are colorless and euhedral

o subhedral prism with highly luminescent oscillatory zoning inL images, typical of igneous zircons (Fig. 3(f)). Few zircons possesshomogenous core overgrown by a narrow rim. The Th/U ratios

f the zircons range from 0.6 to 1.8, typical of igneous origin. Of9 analyses, 25 analyses yielded a weighted mean 207Pb/206Pb agef 1778 ± 5.5 Ma (MSWD = 0.70), interpreted as the crystallizationge of the rock (Fig. 5(a)). The other four analyses gave an olderge group with a weighted mean 207Pb/206Pb age of 1912 ± 14 MaMSWD = 0.32), interpreted as the ages of xenocrystic/inherited zir-ons.

Sample 05XE066 is a dacite collected from the upper sequencef the Jidanping Formation in the Xiong’ershan area. The sampleonsists of plagioclase (35%) and quartz (<10%) phenocrysts andntergranular groundmasses of feldspar and quartz microlites (45%)nd minor Fe–Ti oxides (10%). Zircons in this sample are also domi-ated by inherited/xenocrystic zircons, which are colorless to lightrown fractured fragments of large crystals or prismatic grainsith consistently rounded terminations. Some of these zircons

how core-rim textures. The co-magmatic zircons in the samplere extremely dark in CL images and thus their internal struc-ures are hard to recognize. Chemically, the co-magmatic zirconsre characterized by extremely higher U (1066–2687 ppm) and Th1787–4016 ppm) contents and Th/U (1.24–2.38) ratios than those

Page 7

Y. He et al. / Precambrian Research 168 (2009) 213–222 219

Fig. 5. Concordia diagrams of U–Pb zircon analytical results for samples from theJSa

i9ThmTowc

F0

zYaL2

eictL

maadzzdromim

4

srgzfmzaraa

idanping Formation. (a) Sample 05XE015; (b) Sample 05XE066; and (c) 05XE100.ample 05XE066 was analyzed using the SHRIMP technique, and Samples 05XE015nd 05XE100 were analyzed using the LA-ICP-MS method.

nherited/xenocrystic zircons that contain U contents ranging from2 ppm to 858 ppm, Th contents from 68 ppm to 345 ppm, andh/U ratios between 0.23–0.77. Using the SHRIMP technique, weave dated six inherited/xenocrystic zircons and the cores of 16 co-agmatic zircon grains, whose results are listed in Supplementary

able 1. The co-magmatic zircons have 207Pb/206Pb apparent agesf 1.45 Ga, much younger than the inherited/xenocrystic zirconshose ages range from 2352 ± 11 Ma to 1917 ± 10 Ma. On a con-

ordia plot (Fig. 5(b)), most of the analyses on the co-magmatic

it1a

ig. 6. Concordia diagrams of LA-ICP-MS U–Pb zircon analytical results for Sample6XS012 from the Majiahe Formation.

ircons are discordant but strongly correlated. Using IsoplotEx, aork regression through 15 data-points defines an upper interceptge of 1450 ± 31 Ma. The three most concordant data points (spots-3.1, S-4.1 and S-7.1) of the co-magmatic yielded a weighted mean07Pb/206Pb age of 1445 ± 14 Ma, which is interpreted as the beststimate of the eruption age of the rock. To our knowledge, thiss the youngest zircon age obtained so far for the Xiong’er vol-anic rocks, though similar ages were previously obtained fromhese rocks using the Rb–Sr whole-rock dating method (Hu andin, 1988).

Sample 05XE100 is a rhyolite collected from the Jidanping For-ation in the Waifangshan Mountain. This sample is porphyritic

nd consists of quartz (20%) and K-feldspar (∼5%) phenocrysts withphanitic groundmass. It contains a population of colorless, euhe-ral and prismatic zircons. CL images reveal a large uniform centralone surrounded by fine oscillatory-zoned bands (Fig. 3(d)). Someircon grains have microfractures probably resulting from rapidecompression before the eruption. The Th/U ratios have a nar-ow range from 0.8 to 1.2. Eighteen analyses made on the zirconsf this sample yielded concordant ages clustering at a weightedean 207Pb/206Pb age of 1751 ± 14 Ma (MSWD = 0.30) (Fig. 5(c)),

nterpreted as its eruption age of the rhyolite in the Jidanping For-ation.

.3. Age of the Majiahe Formation

Sample 06XS012 is an andesite collected from the lowestequence of the Majiahe Formation in the Xiaoshan area. Theock consists of plagioclase phenocrysts (∼35%) with a pilotaxiticroundmass which has been oxidized to reddish purple color. Mostircons are euhedral and prismatic grains with repressed crystalaces. Based on the CL images, zircons can be grouped into two

orphological types: (1) grains with unzoned patterns, and (2)ircons with broad banding-like zoning. A few unzoned zirconsre embayed by a narrow, relatively higher luminescent rim. Th/Uatios show a variant range of 0.8–2.50. On a concordia plot (Fig. 6),nalytical data form two age groups, with the older group yieldingh weighted mean 207Pb/206Pb age of 1851 ± 5.9 Ma (MSWD = 0.20),

nterpreted as the age of inherited or xenocrystic zircons, andhe younger group giving a weighted mean 207Pb/206Pb age of778 ± 6.5 Ma (MSWD = 0.15), interpreted to be the crystallizationge of the volcanic rocks. In addition, one zircon grain yields a

Page 8

2 Resea

cp

5

lcXtC

5s

antapmwoipXaapttdMsaJ(Ft2

(tsntbFvwizF

5

tttpo1r

attXubdoettpzwtobtcPcNtCt

RMtwgrt

2

3

f2

20 Y. He et al. / Precambrian

oncordant apparent 207Pb/206Pb age of 1987 Ma, and is also inter-reted as an inherited or xenocrystic zircon.

. Discussion and implications

Results of this study, in conjunction with our previously pub-ished geochemical data (He et al., 2008), enable us to placeonstraints on a number of controversial issues regarding theiong’er volcanic belt and the Paleo-Mesoproterozoic tectonic set-

ing and evolution of the southern margin of the North Chinaraton.

.1. On the stratigraphic subdivisions of the Xiong’er volcanicequences

As mentioned earlier, the Xiong’er volcanic rocks have tradition-lly been subdivided into the lower, middle and upper sequences,amed the Xushan, Jidanping and Majiahe Formations, respec-ively, with the Xushan Formation being dominated by basalticndesites and andesites, more mafic in chemical composition com-ared with the Jidanping and Majiahe Formations, which containore dacites, rhyolites, pyroclastic rocks and sedimentary rocks asell as basaltic andesites and andesites. However, such a scheme

f stratigraphic subdivisions for the Xiong’er volcanic sequencess not supported by new SHRIMP and LA-ICP-MS U-Pb zircon dataresented in this study (Fig. 7). For examples, four samples from theushan Formation yielded consistent weighted mean 207Pb/206Pbges around 1.78–1.76 Ga, similar to weighted mean 207Pb/206Pbges of 1.78–1.75 Ga obtained for the volcanic rocks of the Jidan-ing and Majiahe Formations, indicating that the volcanism ofhe Xushan Formation did not occur at some time earlier thanhat of the Jidanping and Majiahe Formations. Similarly, our newata show that the Jidanping Formation is not older than theajiahe Formation, and that the volcanism of both formations

tarted at ∼1.78 Ga, as indicated by weighted mean 207Pb/206Pbges of 1778 ± 5.5 Ma for a rhyolite sample (05XE015) from theidanping Formation and 1778 ± 6.5 Ma for an andesite sample06XS012) from the Majiahe Formation. Moreover, the Jidanpingormation contains some volcanic rocks younger than those of bothhe Xushan and Majiahe Formations, as shown by weighted mean07Pb/206Pb ages of 1751 ± 14 Ma obtained from a rhyolite sample05XE100) and 1445 ± 14 Ma for a dacite sample (05XE066). All ofhese data demonstrate that the traditional scheme of stratigraphicubdivisions of the Xiong’er volcanic sequences as shown in Fig. 2 isot viable. As illustrated in the inset in Fig. 7, most volcanic rocks inhe Xushan, Jidanping and Majiahe Formations formed in the periodetween 1.78 Ga and 1.75 Ga, though minor dacites in the Jidanpingormation erupted at 1445 ± 14 Ma, indicating that the Xiong’erolcanic rocks at the southern margin of the North China Cratonere not the products of a single magmatic event. This conclusion

s also supported by Ren et al. (2000) who obtained single-grainircon ages of ∼1.65 Ga from the volcanic rocks of the Jidanpingormation in the Waifangshan area.

.2. Tectonic setting of the Xiong’er volcanic rocks

Controversy has surrounded the timing and tectonic setting ofhe Xiong’er volcanism for a long time. Sun et al. (1981) proposedhat the Xiong’er volcanic rocks developed under an intracontinen-

al rifting environment in the Mesoproterozoic, but they did notrovide precise constraints on the timing of the volcanism. Basedn Rb-Sr whole-rock isochron ages of 1710 ± 73.6 Ma, 1439 ± 35 Ma,459 ± 48 Ma and 1454 ± 36 Ma obtained for the Xiong’er volcanicocks and limited geochemical data showing that the rocks have

af2zi

rch 168 (2009) 213–222

ffinities to arc-related volcanic rocks, Hu and Lin (1988) arguedhat the Xiong’er volcanic rocks formed in an Andes-type con-inental margin arc setting. Sun et al. (1991) suggested that theiong’er volcanic rocks in the Zhongtiaoshan area were the prod-cts of a continental rifting event that occurred about ∼1830 Ma,ased a single-grain zircon age of 1826 ± 32 Ma obtained for aacite collected from the Xiyanghe volcanic rocks (the equivalentf the Xiong’er volcanic rocks in the Zhongtiaoshan area). Chent al. (1992) also suggested that the volcanic rocks in the Zhong-iaoshan area formed under a continental rifting environment, buthey argued that the volcanic rocks in the Xiong’er area were theroducts of an active continental margin arc during Mesoprotero-oic time. The continental rift model for the Xiong’er volcanic rocksas also advocated by Zhao et al. (2001c, 2002b, 2004b), who ini-

ially thought that the rift event took place at about 1.95 Ga, basedn a LA-ICP-MS U–Pb zircon age of 1.95–1.75 Ga (Zhao et al., 2001c);ut later proposed that this rifting event happened no more earlierhan 1.78 Ga since they obtained a number of SHRIMP U–Pb zir-on ages around 1.78–1.75 Ga (Zhao et al., 2004b). Most recently,eng et al. (2005, 2007, 2008) suggested that both the Xiong’er vol-anic rocks and the mafic dyke swarms in the central part of theorth China Craton were derived from a mantle plume that led

o the fragmentation and breakup of the Paleo-Mesoproterozoicolumbia supercontinent. Thus, hot debates still remain as to theiming and tectonic setting of the Xiong’er volcanic rocks.

To resolve these controversial issues, we undertook a Hong KongGC project entitled ‘Ages and tectonic setting of the Southernargin Volcanic Belt of the North China Craton: constraints on

he reconstruction of a Paleo-Mesoproterozoic supercontinent’, inhich we carried out extensive field, petrological, geochemical and

eochronological investigations on the Xiong’er volcanic rocks. Theesults from our petrological and geochemical studies have led tohe following conclusions (He et al., 2008):

1. The Xiyanghe volcanic rocks in the Zhongtiaoshan area and theXiong’er volcanic rocks in the Waifangshan, Xiong’ershan andXiaoshan area have similar rock assemblages; they are all com-posed primarily of basaltic andesites and andesites, with minordacites and dacitic rhyolites, which are lithologically similar torock associations in modern continental margin arcs, but dis-tinctly different from those of continental rifts or mantle plumes.

. In the primitive mantle normalized trace-element diagrams,the Xiong’er volcanic rocks show enrichments in the LILE andLREE, and negative anomalies on Nb–Ta–Ti, similar to arc-relatedvolcanic rocks produced by the hydrous melting of the metasom-atized mantle wedge. The relatively high HFSE and Fe–Ti oxidesof the Xiong’er volcanic rocks suggest that these volcanic rocksmay have been sourced from mantle source of the Nb-enrichedbasalts (NEB). Residual amphibole in the mantle, similar to themantle source of the NEB implies the Xiong’er volcanic rockswere derived from a hydrous magma.

. Nd-isotope compositions of the Xiong’er volcanic rocks sug-gest that 5–15% older crust has been involved into the upperlithospheric mantle by the subduction-related crustal recyclingduring Archean to Paleoproterozoic time.

All these conclusions support that the Xiong’er volcanic rocksormed under an active continental margin arc setting (He et al.,008).

SHRIMP and LA-ICP-MS U–Pb zircon data presented in this study

re also inconsistent with the mantle plume-driven rifting modelor the Xiong’er volcanic rocks. Previous published ages (Ren et al.,000; Zhao et al., 2004b) and new SHRIMP and LA-ICP-MS U–Pbircon ages reported in this study have revealed that the volcan-sm that formed the Xiong’er volcanic rocks erupted intermittently

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Y. He et al. / Precambrian Research 168 (2009) 213–222 221

ting U

o1∼ttlplpiRzord

5

gCs∼lmhzSs1S(tmZpi

Aws((foH

A

(ggwspr

A

i

R

B

B

C

C

Fig. 7. (a) Geological map of the Xiong’er volcanic rocks showing the exis

ver a protracted interval from 1.78 Ga, through 1.76–1.75 Ga and.65–1.45 Ga, though a major phase of the volcanism occurred at1.78 Ga. Such a long-lived intermittent volcanism is inconsis-

ent with the volcanism of a mantle plume-driven rifting eventhat generally lasts for less than 50 Ma. In contrast, similar long-ived, intermittent, volcanism is not uncommon in ancient andresent-existing continental margin arcs. A present-day example of

ong-lived continental margin arcs is the Andes, where the Pacificlate has been subducting under the west coast of South Amer-

ca for ∼500 Ma since the Cambrian (Howell, 1995; Dalziel, 1997;ivers and Corrigan, 2000). Therefore, SHRIMP and LA-ICP-MS U–Pbircon data presented in this study further support our conclusionf petrological and geochemical studies that the Xiong’er volcanicocks formed in an Andean-type continental margin arc that bor-ered the southern margin of the North China Craton.

.3. Implications for the reconstruction of Columbia (Nuna)

Recognition of the Xiong’er volcanic belt as a continental mar-in arc is very important for reconstruction of the North Chinaraton in the proposed Paleo-Mesoproterozoic Columbia (Nuna)upercontinent. As mentioned early, following its final assembly at1.8 Ga, the Columbia (Nuna) supercontinent underwent a long-

ived, subduction-related outgrowth along some of its continentalargins, forming a number of accretionary zones, including the

uge 1.8–1.3 Ga magmatic accretionary belt that extends from Ari-ona through Colorado, Michigan, southern Greenland, Scotland,weden and Finland to western Russia, bordering the presentouthern margin of North America, Greenland and Baltica; the.80–1.45 Ga Rio Negro-Juruena Belt and 1.45–1.30 Ga Rondonian-an Ignacio Belt along the western margin of South AmericaAmazonia); and the 1.8–1.5 Ga Arunta, Musgrave, Mt. Isa, George-

own, Coen and Broken Hill inliers along the southern and eastern

argins of the North Australian Craton (Karlstrom et al., 2001;hao et al., 2002a, 2004a; Rogers and Santosh, 2002). The dataresented in this study and our recent petrological and geochem-

cal studies indicate that like North America, Greenland, Baltica,

D

G

–Pb ages. (b) Possible chronostratigraphy for the Xiong’er volcanic rocks.

mazonia and North Australia, the North China craton also under-ent subduction-related, continental margin accretion along its

outhern margin during the period of the supercontinent ColumbiaNuna). Thus, in any configurations of the supercontinent ColumbiaNuna), the southern margin of the North China Craton must haveaced an open ocean, but should not have been connected to anyther continental blocks like a configuration recently proposed byou et al. (2008).

cknowledgements

This research was funded by the Chinese National 973 Program2007CB411307), Chinese NSFC Grant (40730315), Hong Kong RGCrants (7063/06P, 7055/05P,7066/07P and 7053/08P), and 111 Pro-ram (B07011) from the China Ministry of Education. The studyas also partly supported by the CAS/SAFEA International Partner-

hip Program for Creative Research Teams. The final version of thisaper benefited from constructive comments from two anonymouseviewers and the Editor Peter A. Cawood.

ppendix A. Supplementary data

Supplementary data associated with this article can be found,n the online version, at doi:10.1016/j.precamres.2008.09.011.

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