Lithos 114 (2010) 253–264

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Geochronological and geochemical constraints on the petrogenesis of alkalineultramafic dykes from southwest Guizhou Province, SW China

Shen Liu a,b,c,d,⁎, Wenchao Su a, Ruizhong Hu a, Caixia Feng a,d, Shan Gao b, Ian M. Coulson d, Tao Wang a,Guangying Feng a, Yan Tao a, Yong Xia a

a State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, Chinab State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, Chinac State Key Laboratory of Continental dynamics, Northwest University, Xi'an 710069, Chinad Solid Earth Studies Laboratory, Department of Geology, University of Regina, Regina, Saskatchewan, Canada S4S 0A2

⁎ Corresponding author. State Key Laboratory of Ore Dof Geochemistry, Chinese Academy of Sciences, Guiyang5895187; fax: +86 851 5891664.

E-mail address: liushen@vip.gyig.ac.cn (S. Liu).

0024-4937/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.lithos.2009.08.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 June 2009Accepted 24 August 2009Available online 8 September 2009

Keywords:Ultramafic dykesGeochronologyGeochemistryPetrogenesisSW Guizhou Province

Geochronological, geochemical and whole-rock Sr–Nd isotopic analyses have been completed on a suite ofalkaline ultramafic dykes from southwest (SW) Guizhou Province, China with the aim of characterising theirpetrogenesis. The Baiceng ultramafic dykes have a LA-ICP-MS zircon 206Pb/238U age of 88.1±1.1 Ma (n=8),whereas two phlogopites studied by 40Ar/39Ar dating methods give emplacement ages of 85.25±0.57 Ma and87.51±0.45 Ma for ultramafic dykes from Yinhe and Lurong, respectively. In terms of composition, these LateMesozoic ultramafic dykes belong to the alkalinemagma series due to their high K2O (3.31–5.04 wt.%) contents.The dykes are characterised by enrichment of light rare earth element (LREE) and large-ion lithosphile elements(LILEs) (Rb and Ba), negative anomalies in high field strength elements (HFSEs), such as, Nb, Ta and Ti relative toprimitive mantle, low initial 87Sr/86Sr ratios (0.7060–0.7063) and positive εNd(t) values (0.3–0.4). Such featuressuggest derivation from low degree (b1%) partialmelting of depleted asthenosphericmantle (garnet-lherzolite),and contamination to various degrees (∼10%) by interaction with upper crustal materials.

eposit Geochemistry, Institute550002, China. Tel.: +86 851

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Alkaline ultramafic rocks, while small in volume, are common intra-plate igneous rocks in Proterozoic and Archaean cratonic regions.Emplacement ages range from the Proterozoic to Tertiary and provideevidence for post-stabilisation thermal perturbations in the underlyingmantle (Graham et al., 2002). Alkaline ultramafic rocks are a commonexpression ofmantle-derivedmagma generation that is associatedwithcontinental rifting or deep fault zones (e.g., Huang et al., 1995; Liu et al.,2005). The ultramafic dykes, in this circumstance, provide importantinformation for understandingnot onlymagmagenesis fromthemantlebut also tectonic evolution in the study area. At the same time, alkalineultramafic rocks may also carry a range of lithospheric mantle xenolithsand xenocrysts. Alkaline ultramafic rocks therefore provide us with adirect link to understanding how the continents were formed and howtheyhaveevolved throughgeological time, aswell as informationon theevolutionof the lithospheric and asthenosphericmantles (Grahamet al.,2002).

Alkaline ultramafic magmatism within China is limited in its extentand such bodies occur only as small, hypabyssal intrusions that are

mainly distributed in the Shandong, Shanxi, Hubei, Sichuan and Yunnanprovinces (Liu et al., 2005). There are many (N40) alkaline ultramaficdykes, however, that intrude Lower Permian to Middle Triassicsedimentary strata fromZhenfeng to Ziyun, in the southwest of GuizhouProvince (Mei, 1973) (Fig. 1b). Studies of these ultramafic intrusions areimportant in understanding the Late Mesozoic tectonic evolution ofsouthwest (SW) Guizhou. However, no geochemical data are availablefor these dykes. Therefore, the emplacement ages and origin of theultramafic dykes are poorly understood, and the nature of their mantlesources is still unknown. A detailed investigation of the geochemicalcharacter of these rocks should provide information on their origin andpossible mantle sources and, moreover, the development of this part ofAsia during theMesozoic. In this paperwepresent newphlogopite 40Ar–39Ar and zircon U–Pb geochronology, major and trace element and Sr–Nd isotopic compositions for the alkaline ultramafic dykes from SWGuizhou Province (Fig. 1b, c). We use this comprehensive dataset toconstrain the precise emplacement age(s), as well as to investigate thefactors controlling thepetrogenesis of theultramafic dykes, suchas theirsource(s), degree of partial melting and subsequent crustal contamina-tion and crystal fractionation.

2. Geological setting and petrology

SW Guizhou lies on both sides of the join between the YangtzeCraton and Youjiang orogenic belt (Huang, 1978; Wang et al., 1995;

Fig. 1. (a) Distributions of the major terranes in China (modified after Chung and Jahn, 1995); (b) geological map of the SW Guizhou area and the distribution of the sedimentaryrocks, Permian basalts, tectonic elements, ultramafic dykes and gold deposits; and (c) simplified geological distribution of the ultramafic dykes (modified after Zhang et al., 2003).

254 S. Liu et al. / Lithos 114 (2010) 253–264

Fig. 2. Representative photomicrographs showing the petrographic features of the studied ultramafic dykes. The ultramafic samples are strongly porphyritic and dominated byphenocrysts of diopside and phlogopite. Large diopside (Di) with phlogopite (phl) and magnetite (Mt) inclusions (BC-2, LR-5, YH-10 and LR-8); diopside (Di) rimmed by phlogopite(phl) (BC-3 and YH-5).

255S. Liu et al. / Lithos 114 (2010) 253–264

Zhang, 2002; Liu et al., 2006). Here Mesoproterozoic Yangtzecrystalline basement is overlain mainly by shallow-marine platformaldeposits of Devonian to Triassic age (Huang, 1978; BGMRGP, 1987).

Several deep fault or fault zones transect the area, such as the NE-trending Mile-Shizhong fault to the northwest, the NW-trendingZiyun-Bama fault to the northeast (Zhang et al., 2003), the ENE-

Fig. 3.Mol% for clinopyroxene andmica phenocrysts fromSWGuizhou ultramafic dykes.

Table 1Microprobe data for representative clinopyroxenes and micas from SW Guizhou ultramafic

Clinopyroxenes YH-8-1 YH-8-2 YH-8-3 YH-9-1

SiO2 52.6 52.35 48.93 47.13TiO2 0.41 0.3 0.46 0.3Al2O3 2.99 3.08 5.58 2.97CaO 23.46 23.29 24.09 21.15MgO 16.14 16.56 13.42 7.19MnO 0.01 0.03 0.11 0.69Na2O 0.28 0.33 0.21 1.72K2O – 0.02 0.01 –

FeO 3.10 3.18 6.73 17.82P2O5 0.63 0.69 0.6 0.53Total 99.62 99.83 100.41 100.47Si 1.92 1.91 1.84 1.96Ti 0.01 0.01 0.01 0.01Al 0.13 0.13 0.25 0.15Ca 0.92 0.91 0.97 0.92Mg 0.88 0.9 0.75 0.45Mn – – – 0.02Na 0.02 0.02 0.02 0.14K – – – –

Fe 0.09 0.10 0.20 0.59P 0.02 0.02 0.02 0.02Mg# 90.7 90.0 78.8 43.3

Micas YH-8-1 YH-8-2 YH-8-3 YH-9-1

SiO2 34.54 35.65 37.54 37.31TiO2 2.82 2.48 1.83 1.73Al2O3 17.19 17.77 19.93 19.6CaO 0.01 0.09 0.08 –

MgO 18.37 18.71 20.43 20.14MnO 0.09 – 0.03 0.13Na2O 0.3 0.28 0.36 0.28K2O 8.45 8.61 9.14 9.1FeO 8.8 9.28 6.53 7.06P2O5 0.13 0.14 0.06 –

Total 90.7 93.01 95.93 95.35Si 2.87 2.89 2.9 2.91Ti 0.18 0.15 0.11 0.1Al 1.68 1.7 1.81 1.8Ca 0 0.01 0.01 0Mg 2.27 2.26 2.35 2.34Mn 0.01 0 0 0.01Na 0.05 0.04 0.05 0.04K 0.9 0.89 0.9 0.9Fe 0.61 0.63 0.42 0.46P 0.01 0.01 0 0Mg# 78.8 78.2 84.8 83.6

Mg#=100⁎Mg/(Mg+Fe); –: not analysed.

256 S. Liu et al. / Lithos 114 (2010) 253–264

trending Nanpanjiang fault to the south and the NS-trending Puding-Ceyang fault (Fig. 1b), which is a deep lithospheric fault believed toextend to depths of some 80 km (Wang et al., 1994).

During the Devonian to Triassic, following the Caledonian tec-tonic cycle, this area subsided to form a large basin now filled by athick sequence of Devonian, Carboniferous, Permian, and Triassicsedimentary strata. Jurassic and Cretaceous sediments also crop outsporadically in the area (BGMRGP, 1987). Furthermore, Permian andTriassic strata are regarded as the primary sources for the dissemi-nated gold deposits in SW Guizhou (Zhang et al., 2003).

Devonian strata comprise black shale, limestone, dolomite,argillaceous limestone, siliceous limestone, and marl of deep orintermediate marine facies. Carboniferous sedimentary rocks mainlyinclude light-grey limestone and sandstone, dark grey or blacklimestone and chert. The upper Permian involves calcareous siltstone,siltstone, shale and limestone. In contrast, dolomitic limestone,dolomite, chert, marl and shale are found in the lower Permian. Theupper Triassic consists of claystone, calcareous claystone, finesandstone and siltstone. The middle Triassic contains limestone,marl, shale, dolomite and shale with tuff, whereas the lower Triassic iscomposed of bioclastic limestone, dolomite, breccia, shale with marl,siltstone and sandy shale with tuff.

dykes.

YH-9-2 YH-9-3 LR-3-1 LR-3-2 BC-23-1

51.03 47.4 50.82 51.99 52.590.67 0.53 0.22 0.11 0.093.21 6.43 3.65 2.59 2.24

24.26 23.54 24.35 23.79 23.615.59 11.42 16.46 16.76 17.130.02 0.08 0.01 0.1 0.030.19 0.89 0.43 0.48 0.370.01 – 0.01 0.04 –

4.06 9.35 2.56 2.58 2.300.61 0.89 0.96 0.98 0.73

99.76 101.09 99.72 99.53 99.121.89 1.83 1.88 1.91 1.930.02 0.02 0.01 – –

0.14 0.29 0.16 0.11 0.10.96 0.97 0.97 0.94 0.930.86 0.66 0.91 0.92 0.94

– – – – –

0.01 0.07 0.03 0.03 0.03– – – – –

0.13 0.28 0.07 0.08 0.070.02 0.03 0.03 0.03 0.02

87.1 69.9 92.6 92.3 93.2

YH-9-2 YH-9-3 LR-3-1 LR-3-2 BC-23-1

36.9 36.97 29.11 31.89 36.863.06 2.37 2.96 2.74 2.28

18.35 17.33 18.44 18.15 19.170.01 0.09 0.26 0.04 0.17

19.18 17.7 18.05 16.93 18.750.06 0.13 0.07 0.08 0.050.44 0.15 0.11 0.52 0.538.98 10.33 6.16 6.16 8.698.49 10.91 6.82 10.09 8.61

– – – – 0.195.47 95.98 81.98 86.6 95.212.9 2.95 2.64 2.77 2.90.18 0.14 0.2 0.18 0.131.7 1.63 1.97 1.86 1.780 0.01 0.03 0 0.012.25 2.1 2.44 2.19 2.20 0.01 0.01 0.01 00.07 0.02 0.02 0.09 0.080.9 1.05 0.71 0.68 0.870.56 0.73 0.52 0.73 0.570 0 0 0 0.01

80.1 74.2 82.4 75.0 79.4

257S. Liu et al. / Lithos 114 (2010) 253–264

Basalt, dolerite, vitric tuff, and alkaline ultramafic dykes crop outin SW Guizhou (Fig. 1). Several sequences of basalt from EarlyCarboniferous to Late Permian age are found in the northwestern partof the area. Early Carboniferous andEarly Permianbasalts are foundonlylocally within shallow-marine sequences, whereas Late Permian,Emeishan flood basalts are widely exposed across northwestern cornerof SW Guizhou (Fig. 1b). Moreover, based on the results of petroleumexploration drilling, voluminous Late Permian Emeishan basalts weredetected beneathXinyi, to thewest of SWGuizhou (Huang, 1986). Apartfrom this, magnetic and gravitational anomalies imply that voluminousmafic-ultramafic intrusions lie beneath SW Guizhou (Zhang et al.,2003). The Emeishan basalts consist mainly of basaltic lava, pyroclasticrocks, and breccia (Mei, 1980). Because the Early and Late Permianbasalts have a uniform chemical composition, it is believed that theywere all erupted in a tectonically stable (i.e., a within-plate type)environment (Mei, 1980; Dobretsov, 2005; Ali et al., 2005). LatePermian dolerite sills and dykes occur in localised areas (Mei, 1980),while felsic, vitric tuffs are found within Triassic dolomite and shale.Many alkaline ultramafic veins and dykes intrude the Lower Permian toMiddle Triassic strata (Fig. 1b and c), and they are considered to becontrolled by a deep lithospheric fault zone (e.g., Puding-Ceyang fault)or fracture (Yang et al., 1992). These alkaline ultramafic dykes aremainly composed of altered, porphyritic olivine-bearing biotite pyrox-enite with brecciated structures and xenoliths of country rocks. Thesedykes contain relatively low SiO2 and MgO contents, but higher CaO,Al2O3, K2O,Nb, Sr, Ba, As, and Pb than typical peridotite (BGMRGP, 1987;Li et al., 1989). Biotite K-Ar ages of these dykes range from 77.5 Ma to97 Ma (BGMRGP, 1987), which coincides with the Yanshanian orogeniccycle (ca. 90 Ma) (Zhou et al., 2002; Deng et al., 2007).

Ultramafic dykes for this study were taken from the Baiceng, Yinheand Lurong (Fig. 1b and c) regions, at the northwestern edge of Youjiangorogenic belt (Liu et al., 2006), where several gold deposits associatedwith about ten of these dykes are present (e.g., BGMRGP, 1987; Zhanget al., 2003). Ultramafic dyke orientations are NE-, N-S- and E-W-striking (Fig. 1b and c). Single dyke spans can range between 1.0 and8.0 m inwidth and 30 m and 1 km in length. The ultramafic samples arestrongly porphyritic (35–45%) and dominated by phenocrysts ofdiopside or salite (30–70%, 0.5–2.0 mm), phlogopite (10–40%, 0.8–5.0 mm) (Figs. 2 and 3) and minor olivine, entirely pseudomorphed bymixtures of serpentine and carbonate (0–10%). The groundmass (55–

Table 2Summary of Argon isotopic results.

T (°C) 40Ar/39Ar 36Ar/39Ar 37Ar/39Ar 40Ar⁎/39Ar 39Arcu

YH-9 phlogopite (J=0.011031, W=66.05 mg)400 29.686 0.0929 0.4354 2.2598 0.6500 24.2014 0.075 0.4809 2.0622 1.4600 8.8443 0.0161 0.1268 4.0889 3.9700 5.9513 0.0048 0.0155 4.5152 11.6800 4.8447 0.0016 0.0078 4.3532 39.0900 5.0031 0.002 0.0062 4.3593 71.71000 5.8821 0.0056 0.0059 4.2261 83.41100 8.2961 0.0136 0.0321 4.2875 86.21200 9.1009 0.0157 0.0165 4.4669 94.91300 10.3103 0.0194 0.0199 4.5791 98.01400 10.6979 0.0213 0.1713 4.4196 100

LR-100 phlogopite (J=0.011330, W=69.75 mg)500 4.0469 0.005 0.0315 2.5627 0.6600 13.1237 0.0381 0.8649 1.9271 0.9700 9.9185 0.0197 1.6743 4.2048 1.6800 8.7266 0.014 3.8046 4.8702 2.3900 5.6891 0.0035 2.5186 4.8356 7.01000 5.0874 0.0019 0.0747 4.5181 15.41100 5.0485 0.0022 0.0289 4.3924 34.71200 5.122 0.0025 0.0149 4.3709 73.61300 6.6031 0.0075 0.0139 4.3943 83.41400 6.9009 0.0085 0.0413 4.4022 99.21450 9.6814 0.0183 0.075 4.2747 100

65%) consists of fine-grained (b0.5 mmgrain size) diopside, phlogopite,carbonate and minor feldspar (e.g., anorthite, K-feldspar). Accessoryminerals include perovskite, magnetite, ilmenite, apatite, rutile, titanite,aegirine, aegirine-augite and analcime. Accordingly, the ultramaficrocks can be termed porphyritic phlogopite pyroxenites (Table 4).

3. Analytical method

3.1. Mineral compositions

Representative clinopyroxene and mica compositions were deter-mined at Nanjing University on a JXA-8800 M electron microprobe,using a 15 kV beam current and 100 s count time. Themineral data aregiven in Table 1.

3.2. 40Ar–39Ar and LA-ICP-MS U–Pb dating

Phlogopite grains from samples YH-9 and LR-100were extracted for40Ar–39Ar dating. Samples were crushed to a grain size of 60–80mesh,and mineral separates were obtained using magnetic separation andhand picking. The minerals were then hand-picked under a binocularmicroscope for purity. The minerals, together with the internationalmuscovite standard Bern-4M, were irradiated in channel H8 of atomicreactor 49-2 at the China Institute of Atomic Energy for fast neutronirradiation. The irradiation duration and neutron flux were 59 h and6.0×1012 n/cm2/s, respectively. The 40Ar–39Ar analyseswereperformedusing anMM-1200Bmass spectrometer at theNational Research Centerof Geoanalysis, Chinese Academy of Geosciences; analytical proceduresfollowed those described in Chen et al. (2006). Data were processedusing the ISOPLOT program (Ludwig, 2003). The analytical results forphlogopite are presented in Table 2 and Fig. 4, inwhich all errors shownrepresent the analytical precision to 1σ. Blanks were measuredbefore each sample analysis with typical background values for 40Ar,39Ar, 37Ar and 36Ar of 2.43×10−14, 5.81×10−16, 4.62×10−16 and5.43×10−16mol, respectively. The inter-laboratory correction factors atthe time of the analysis were as follows: (36Ar/37Ar)Ca=2.39×10−4,(40Ar/39Ar)K=4.78×10−3 and (39Ar/37Ar)Ca=8.06×10−4.

Zircon was separated from one ultramafic sample (N40 kg)(BC01) using conventional heavy liquid and magnetic techniques atthe Langfang Regional Geological Survey, Hebei Province, China.

m (%) 39Ar (×10−14mol) ⁎40Ar (%) Age (Ma) 1σ (Ma)

9 16.41 0.36 44.4 11.07 18.4 0.37 40.6 18.77 59.36 2.36 79.6 9.63 181.51 7.96 87.7 1.99 650.99 27.53 84.6 0.91 773.41 33.02 85.4 1.05 278.32 11.43 82.2 2.57 66.9 2.79 83.4 3.67 206.23 8.95 86.8 1.68 73.88 3.29 88.9 9.4

45.46 1.95 85.9 9.2

5 29.29 0.38 51.6 4.59 15.06 0.15 39 7.03 28.7 0.61 84 2.7

29.89 0.74 96.9 3.31 211.06 5.18 96.2 1.07 378.4 8.68 90.1 0.96 863.83 19.27 87.6 0.95 1740.9 38.65 87.2 0.98 440.1 9.82 87.7 0.93 704.91 15.76 87.8 0.9

34.47 0.75 85.3 2.6

Fig. 4. Phlogopite 40Ar–39Ar plateau ages (a, b), and representative cathodolumines-cence (CL) images and the LA-ICP-MS concordia ages (c) for the zircon grains from theultramafic dykes. The numbers correspond to the spot analyses given in Table 1.

258 S. Liu et al. / Lithos 114 (2010) 253–264

Representative zircon grains were hand-picked under a binocularmicroscope and mounted in an epoxy resin disc, and then polishedand coated with a gold film. Zircons were documented with

transmitted and reflected light as well as cathodoluminescence (CL)to reveal their external and internal structures at the State KeyLaboratory of Continental Dynamics, Northwest University, China.Laser ablation techniques were used for zircon age determinations(Table 3). The analyses were performed with an Agilent 7500a ICP-MS equipped with 193 nm excimer lasers, which is housed at theState Key Laboratory of Geological Processes and Mineral Resources,China University of Geoscience in Wuhan, China. Zircon 91500 wasused as a standard and NIST 610 was used to optimise the results. Thespot diameter was 24 μm. Prior to LA-ICP-MS zircon U–Pb dating, thesurfaces of the grain mounts were washed in dilute HNO3 and purealcohol to remove any potential lead contamination. Analyticalmethodology is described in detail in Yuan et al. (2004). Common-Pb corrections were made using the method of Andersen (2002).Data were processed using the GLITTER and ISOPLOT (Ludwig, 2003)programs. Errors on individual analyses by LA-ICP-MS are quoted atthe 95% (1σ) confidence level.

3.3. Major and trace elements analysis

Fourteen representative samples were collected to carry out majorand trace element determinations and Sr–Nd isotopic analysis.Whole-rock samples were trimmed to remove altered surfaces, andwere cleaned with deionized water, crushed, and powdered with anagate mill.

Major elements were analysed with a PANalytical Axios-advance(Axios PW4400) X-ray fluorescence spectrometer (XRF) at the StateKey Laboratory of Ore Deposit Geochemistry, Institute of Geochem-istry, Chinese Academy of Sciences (IGCAS). Fused glass discs wereused and the analytical precision as determined on the ChineseNational standard GSR-3was better than 5% (Table 4). Loss on ignition(LOI) was obtained using 1 g of powder, heated to 1100 °C for 1 h.

Trace elements were analysed using a Perkin-Elmer Sciex ELAN6000 ICP-MS at the IGCAS. The powdered samples (50 mg) weredissolved in high-pressure Teflon bombs using a HF+HNO3 mixturefor 48 h at ca. 190 °C (Qi et al., 2000). Rh was used as an internalstandard to monitor signal drift during counting. The internationalstandard, GBPG-1 was used for analytical quality control. Theanalytical precision was generally better than 5% for all elements.Analyses of international standards OU-6 and GBPG-1 are inagreement with recommended values (Table 4).

3.4. Sr–Nd isotopic analyses

For Rb–Sr and Sm–Nd isotopic analysis, sample powders werespiked with mixed isotope tracers, dissolved in Teflon capsules withHF+HNO3 acids, and separated by conventional cation-exchangetechniques. Isotopic measurements were performed on a FinniganMAT-262 thermal ionizationmass spectrometer (TIMS) at the IsotopicGeochemistry Laboratory of Yichang Institute of Geology andMineralsresources. Procedural blanks were b200 pg for Sm and Nd andb500 pg for Rb and Sr. The mass fractionation corrections for Sr andNd isotopic ratios were based on 86Sr/88Sr=0.1194 and 146Nd/144Nd=0.7219, respectively. Analyses of standards during the periodof analysis are as follows: NBS987 gave 87Sr/86Sr=0.710246±16(2σ; n=10); La Jolla gave 143Nd/144Nd=0.511863±8 (2σ; n=10).Our analytical results for Sr–Nd isotopes are presented in Table 5.

4. Results

4.1. Petrography and mineral chemistry

The studied clinopyroxenes are unaltered, and form euhedralcrystals with dimensions generally in the range of 3–5 mm. Probeanalyses of unaltered pyroxene show they have a narrow rangecovering diopside and salite (Fig. 3a). Mica phenocrysts are fresh and

Table 3LA-ICP-MS U–Pb isotopic data for zircon from the ultramafic sample (BC01) of southwestern Guizhou.

BC01 Isotopic ratios Age(Ma)

Spot Th (ppm) U (ppm) Pb (ppm) Th/U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ

1 392 112 5.91 3.49 0.0481 0.0206 0.0876 0.002 0.0138 0.0003 102 27 85 2 88 22 143 63 3.80 2.26 0.0482 0.0245 0.0875 0.002 0.0138 0.0003 109 30 85 2 88 23 687 288 7.67 2.39 0.0493 0.0023 0.0878 0.003 0.0139 0.0003 160 35 85 2 89 24 1421 634 14.5 2.24 0.0485 0.0051 0.0892 0.002 0.0137 0.0002 124 30 87 2 88 15 467 137 6.96 3.41 0.0483 0.0372 0.0882 0.002 0.0139 0.0004 114 28 86 2 89 26 702 263 8.59 2.66 0.0496 0.0053 0.0887 0.002 0.0136 0.0003 177 25 86 2 87 27 452 131 8.11 3.44 0.0485 0.0415 0.0875 0.002 0.0135 0.0003 124 29 85 2 86 28 1637 542 16.9 3.02 0.0493 0.0014 0.0895 0.002 0.0135 0.0003 160 27 87 2 87 29 144 613 262 0.24 0.1533 0.0069 6.0901 0.2589 0.2881 0.004 2383 78 1989 37 1632 20

Table 4Major oxides (wt.%) and trace elements (ppm) concentrations of the ultramafic dykes from SW Guizhou Province.

Sample BC-2 BC-3 BC-6 BC-8 BC-22 BC-23 YH-8 YH-9 YH-11 YH-12 YH-13 LR3 LR5 LR8 GSR3R GSR3M

Locality Baiceng Baiceng Baiceng Baiceng Baiceng Baiceng Yinhe Yinhe Yinhe Yinhe Yinhe Lurong Lurong Lurong

Rock type Porphyritic phlogopite pyroxenite Porphyritic phlogopite pyroxenite

SiO2 37.31 36.51 37.02 37.77 35.92 36.93 37.14 38.07 38.38 37.66 36.87 35.96 34.54 35.22 44.64 44.63TiO2 0.76 0.75 0.76 0.78 0.77 0.79 0.78 0.79 0.79 0.76 0.78 0.80 0.77 0.78 2.37 2.36Al2O3 15.13 14.33 14.34 14.28 13.76 14.40 14.30 14.60 14.77 14.42 14.15 15.27 13.84 14.50 13.83 13.79Fe2O3 9.41 8.49 9.08 9.08 9.06 9.29 9.37 9.48 9.45 9.27 9.08 8.99 8.98 9.05 13.40 13.38MnO 0.18 0.16 0.18 0.18 0.18 0.18 0.18 0.19 0.19 0.19 0.17 0.17 0.18 0.16 0.17 0.16MgO 7.32 7.32 7.38 6.94 7.06 7.37 7.57 7.65 7.62 7.41 7.55 6.88 6.93 6.97 7.77 7.76CaO 14.50 14.93 14.81 15.10 15.51 14.88 15.08 14.59 15.08 14.28 14.95 14.24 16.27 13.62 8.81 8.80Na2O 0.29 0.86 0.92 1.71 0.83 0.24 0.56 0.47 0.56 1.29 0.60 0.86 0.44 0.14 3.38 3.39K2O 4.71 4.74 4.27 3.31 5.04 4.10 4.34 4.27 4.03 3.99 4.91 4.44 3.85 4.74 2.32 2.31P2O5 2.00 1.96 1.99 1.82 2.13 1.99 2.04 1.95 1.96 1.93 2.16 2.04 1.93 2.06 0.95 0.94LOI 8.11 8.88 8.15 8.27 9.22 9.33 8.12 7.15 7.00 7.72 8.42 9.92 12.00 12.30 2.15 2.24Total 99.71 98.92 98.90 99.24 99.48 99.49 99.47 99.22 99.84 98.91 99.65 99.57 99.72 99.55 99.78 99.76Mg# 63 65 64 63 63 64 64 64 64 64 65 63 63 63 GBPG-1 (RV⁎) GBPG-1 (MV⁎)Sc 21.6 21.0 22.7 20.4 25.7 24.2 28.8 28.6 28.9 30.1 29.7 15.2 16.1 15.1 13.9 14.3V 242 250 172 223 223 236 227 231 210 224 230 190 217 202 96.5 101Cr 156 142 156 144 142 136 322 316 375 390 321 36.4 35 35 181 180Co 35.1 32.4 34.4 31.9 39.6 40.9 42.2 42.4 41.5 42.7 44 30.9 32.0 32.0 19.5 20.1Ni 59.8 52.7 59.9 55.1 63.9 61.8 93.2 91.8 91.1 96.5 93.7 28.4 35.7 33.1 59.6 59.7Ga 15.8 17.2 15.4 14.8 17.2 17.1 16.6 17.3 16.6 15.5 17.3 16.2 15.5 15.5 18.6 19.2Rb 159 158 115 162 173 182 229 159 176 163 148 138 152 242 56.2 55.8Sr 1490 1840 1440 1550 1700 1646 1818 2219 1812 1377 2291 1908 1890 1883 364 367Y 28.5 27.4 26.4 26.6 28.9 30.3 31.1 33.1 33.7 33.4 31.8 36.8 37.9 36.7 18.0 18.4Zr 256 258 256 246 264 272 307 347 320 281 335 284 277 261 232 232Nb 227 226 194 191 228 228 167 194 153 150 183 136 133 131 9.93 10.2Ba 4640 3660 4750 4950 5870 6446 6397 10,351 4340 5237 8392 6183 6210 5879 908 916La 286 233 287 296 279 294 244 286 256 261 271 222 224 218 53.0 55.0Ce 529 411 533 543 495 521 480 525 475 484 509 447 460 459 103 98.7Pr 49.2 37.1 44.0 49.9 47.5 50.4 49.6 54.1 49.7 50 52.5 51.9 51.4 49.3 11.5 11.9Nd 158 120 156 161 157 167 173 190 173 174 183 186 185 184 43.3 43.1Sm 19.4 14.2 19.2 19.6 19.4 20.7 23.5 25.4 23.5 23.4 24.9 25 24.8 23.9 6.79 6.97Eu 4.67 3.34 4.72 4.68 5.2 5.59 6.17 7.23 5.74 5.86 6.66 6.38 6.42 6.29 1.79 1.84Gd 14.4 10.4 16.6 14.6 13.3 13.9 16 17.1 16.6 16 16.7 17.5 17.6 16.8 4.74 4.75Tb 1.67 1.27 1.81 1.75 1.39 1.48 1.71 1.79 1.71 1.72 1.79 1.81 1.80 1.79 0.60 0.63Dy 6.12 4.81 6.53 6.45 5.9 6.26 6.83 7.09 7.2 7.09 7.04 7.53 7.56 7.48 3.26 3.18Ho 1.12 0.85 1.19 1.16 1.05 1.04 1.12 1.2 1.16 1.12 1.13 1.3 1.31 1.26 0.69 0.71Er 3.13 2.33 3.29 3.26 2.66 2.87 2.83 3.16 3.18 2.89 2.9 3.3 3.31 3.26 2.01 2.03Tm 0.35 0.27 0.36 0.36 0.33 0.37 0.33 0.36 0.36 0.33 0.37 0.42 0.38 0.35 0.30 0.31Yb 2.24 1.65 2.37 2.34 2.18 2.19 2.14 2.38 2.35 2.2 2.36 2.48 2.48 2.39 2.03 2.10Lu 0.32 0.24 0.33 0.34 0.3 0.32 0.31 0.31 0.29 0.28 0.3 0.34 0.35 0.34 0.31 0.31Hf 4.52 3.42 5.02 4.91 5.24 5.16 8.11 8.6 8.19 8.32 8.3 5.8 5.76 5.74 6.07 5.92Ta 9.99 7.14 9.78 9.98 7.53 7.52 4.9 5.89 4.41 4.06 5.53 5.74 5.69 5.71 0.40 0.39Pb 38.2 28.5 60.6 36.8 27.6 25.4 33.8 27.8 28.7 58.7 33.2 11.8 12.0 11.9 14.1 13.6Th 71.1 62.5 64.1 74.4 55 60.6 63.1 75.6 72.2 65.8 72.2 66.2 65.7 65.4 11.2 11.4U 14.3 10.5 12.9 12.1 8.33 8.9 10.1 13.7 10.6 9.52 13.1 7.88 8.0 8.2 0.90 0.87(La/Yb)n 86.3 95.4 81.8 85.5 86.5 90.7 77.0 81.2 73.6 80.2 77.6 60.5 61.0 61.6Eu/Eu⁎ 1.00 0.85 1.02 1.00 0.98 1.01 1.08 1.12 1.09 1.08 1.11 1.12 1.12 1.09Ta/La 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.02 0.03 0.03 0.03

LOI = loss on ignition. RV⁎: recommended values; MV⁎: measured values; the values for GSR-3 fromWang et al. (2003).Mg# = 1004Mg = ðMg + ∑FeÞ atomic ratio. The values forGBPG-1 from Thompson et al. (2000).

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Table 5Sr–Nd isotopic ratios for the ultramafic dykes from SW Guizhou Province.

Sample BC-2 BC-3 BC-6 BC-8 BC-23 YH-8 YH-9 YH-11 YH-12 YH-13 LR-3 LR5 LR8

Rb 161 157 116 165 179 235 161 176 163 148 140 152 242Sr 1486 1838 1442 1546 1653 1827 2226 1812 1377 2291 1899 1890 1883Nd 162 122 158 163 160 177 187 173 174 183 190 185 184Sm 19.2 14.1 19.4 19.8 18.9 25.1 24.9 23.5 23.4 24.9 26.1 24.8 23.987Rb/86Sr 0.3131 0.2482 0.2308 0.3021 0.3262 0.3719 0.2116 0.2807 0.3421 0.1867 0.2121 0.2324 0.3714147Sm/144Nd 0.0716 0.0715 0.0744 0.0736 0.0747 0.0826 0.0797 0.0821 0.0813 0.0823 0.0815 0.0810 0.078587Sr/86Sr 0.706432 0.706335 0.706376 0.706521 0.706452 0.706501 0.706275 0.706433 0.706359 0.706544 0.706271 0.706412 0.7064212σ 16 13 13 16 18 14 25 13 13 14 21 11 14143Nd/144Nd 0.512579 0.512586 0.512587 0.512584 0.512589 0.512595 0.512592 0.512588 0.512593 0.512596 0.512593 0.512589 0.5125862σ 12 8 10 10 7 13 8 10 9 9 10 10 9(87Sr/86Sr)i 0.706040 0.706024 0.706087 0.706143 0.706044 0.706051 0.706019 0.706093 0.705945 0.706318 0.706007 0.706123 0.705959(143Nd/144Nd)i 0.512538 0.512545 0.512544 0.512542 0.512546 0.512549 0.512548 0.512542 0.512548 0.512550 0.512546 0.512543 0.512541εNd(t) 0.3 0.4 0.4 0.3 0.4 0.4 0.4 0.3 0.4 0.4 0.4 0.3 0.3

Chondrite Uniform Reservoir (CHUR) values (87Rb/86Sr=0.0847, 87Sr/86Sr=0.7045, 147Sm/144Nd=0.1967 143Nd/144Nd=0.512638) are used for the calculation.λRb=1.42×10−11 year−1

(Steiger and Jäger, 1977); λSm=6.54×10−12 year−1 (Lugmair and Harti, 1978).

260 S. Liu et al. / Lithos 114 (2010) 253–264

can reach up to 10 mm in diameter but are generally in the range of2.0–5.0 mm. Probe analysis shows that they also vary within a narrowrange (i.e., phlogopite; Fig. 3b).

4.2. Phlogopite 40Ar–39Ar and zircon U–Pb ages

Phlogopite phenocrysts from two samples of ultramafic dykesfrom the Yinhe and Lurong areas (YH-9 and LR-100) yield similar40Ar/39Ar plateau ages of 85.25±0.57 Ma (2σ) (increments 4–11; 94%of 39Ar released) and 87.51±0.45 Ma (2σ) (increments 3–11; 98% of39Ar released), respectively (Table 2; Fig. 4a and b). These plateau agesare interpreted to represent the crystallisation age of the dykes.

Zircon is relatively lacking in the ultramafic dykes; only limitedzircon grains (b30) could be extracted from one ultramafic sample,BC01. Zircons from BC01 are euhedral, colourless and transparent,mostly ranging up to 100 µm in diameter. The minority exhibitoscillatory or sector zoning under cathodoluminescence (CL), a typicalfeature of magmatic zircon. Selected zircon CL images are given inFig. 4c. The studied grains all have relatively high Th/U ratios (2.24–3.49) also suggestive of a magmatic origin. The U–Pb zircon data arepresented in Table 3. Analyses of zircon grains with oscillatorystructures were concordant and yielded the weighted mean 206Pb/238U age of 88.1±1.1 Ma (n=8) for BC01 (Fig. 4c). This age isinterpreted as the crystallisation age for this dyke. One analysis (spot9.1) yielded a 206Pb/238U age of 1632 Ma, and is interpreted to be dueto inheritance.

In conclusion, the studied ultramafic dykes intruded at 85–88 Ma,which is consistent with the existing “biotite” K-Ar results of between77.5 Ma and 97 Ma (BGMRGP, 1987).

Fig. 5. Rock classification plot based on whole-rock SiO2 vs. Na2O+K2O (in oxide wt.%).The division of alkaline and sub-alkaline (green line) is after Katsube et al. (2009).

4.3. Major and trace elements

Ultramafic dykes from SWGuizhou Province have low SiO2 contentsranging from 34.54 to 38.38 wt.% and MgO (6.88–7.65 wt.%) with Mg#

of 63–65. They have relatively high K2O (3.31–5.04 wt.%) and low Na2Ocontents (0.14–1.71 wt.%) (Table 4) and therefore plot in field of high-Kalkaline magmas (Fig. 5). The dykes have essentially constant TiO2

(0.75–0.80 wt.%), Al2O3 (13.76–15.27 wt.%), Fe2O3 (8.49–9.48 wt.%),CaO (13.62 to 15.51 wt.%) and P2O5 (1.82 to 2.16 wt.%) contents.

All of these rocks exhibit LREE enrichment ((La/Yb)N=60.5–95.4)with most samples displaying slight Eu anomalies (Eu/Eu*=0.85–1.12) (Table 4; Fig. 6). In the primitive mantle-normalised traceelemental diagram, the dykes are characterised by enrichment inLILEs (Rb and Ba) and depletion in HFSEs (Ta, Nb, Zr, Hf and Ti), withweakly negative Sr and P and strongly positive Th, U and Pb anomalies(Fig. 7).

4.4. Sr and Nd isotope compositions

The ultramafic dykes from the Baiceng, Yinhe and Lurong haverelatively constant Sr and Nd isotope compositions with initial 87Sr/86Sr ratios ranging from 0.7060 to 0.7063, initial 143Nd/144Nd from0.51254 to 0.51255, and εNd(t) values from +0.3 to +0.4 (Table 5),which suggest a common source region.

5. Discussion

5.1. Crustal contamination

Although the sampledultramafic dykes in SWGuizhouhave low andconstant initial 87Sr/86Sr ratios and positive εNd(t) values (Fig. 8), theyexhibit positive Pb and negative Ta and Ti anomalies when normalisedto primitive mantle values (Fig. 7). They are further characterised byrelatively higher Nb (131–228 ppm), Zr (246–347 ppm), Th (55–75.6 ppm), U (8.0–14.3 ppm), Rb (115–242 ppm) and Pb (25.4–60.6 ppm) compared to upper crust (e.g., Nb=25 ppm, Zr=190 ppm,Th=10.5 ppm, U=2.7 ppm, Rb=84 ppm, Pb=20 ppm, Rudnick andFountain, 1995; Rudnick and Gao, 2003), suggesting that crustalcontamination was significant in these rocks. Significant upper crustalassimilation is also supported by the occurrence of inherited zircon

Fig. 6. Chondrite-normalised rare earth element patterns for the ultramafic dykes in theSW Guizhou Province. REE abundances for chondrites are after Sun and McDonough(1989).

Fig. 7. Primitive mantle-normalised ‘spider’-diagrams for the ultramafic dykes from SWGuizhou. Trace element abundances for primitive mantle are after Sun and McDonough(1989).

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(spot 9 with age of 1632 Ma; Table 3) and elevated LILE and low MgOcontents. Moreover, all dyke samples have lower Ta/La ratios (0.02–0.03) (Table 4) than the primitive mantle (Ta/La=0.06; Wood et al.1979), that has been cited as indicating significant crustal contamina-tion (Song et al., 2006).

Thedegree of crustal contaminationof themagmas canbe illustratedusing (Th/Yb)PM ratios, where the rock values have been normalised tothe relevant trace element contents of the primitive mantle (Qi andZhou, 2008). Samples from the alkaline ultramafic dykes have muchhigher (Th/Yb)PM ratios (146–220) than the upper crust (28; Taylor

and Mclennan, 1985), consistent with a high-degree of crustal con-tamination. In summary, the ultramafic dykes experienced extensivecrustal assimilation prior to emplacement.

5.2. Nature of source region and partial melting

The SW Guizhou alkaline ultramafic dykes have low SiO2 (34.5–38.4 wt.%) (Table 4), suggesting that they were derived from a picrite/

Fig. 8. Initial 87Sr/86Sr vs. εNd(t) diagram for the ultramafic dykes from SW GuizhouProvince. DMM (depleted MORB mantle), EM1, EM2 and HIMU are after Zindler andHart (1986). OIB data are from Wilson (1989). The Yangtze upper/middle crust andlower crust data are from Gao et al. (1999), Ma et al. (2000), and Chen and Jahn (1998).The numbers indicate the percentages of participation of the crustal materials. Thecalculated parameters of Nd (ppm), εNd(t), Sr (ppm) and (87Sr/86Sr)i are 4.4, +7, 102and 0.704 from astenosphere mantle as parental magmas (after Qi and Zhou, 2008); 20,−22, 220, 0.715 and 20, −10, 220, 0.715 as two components of the Yangtze middle/upper crust.

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peridotite mantle source. Low initial 87Sr/86Sr ratios (0.7060–0.7063)and positive εNd(t) values (Table 5; Fig. 8) for the ultramafic dykesfurther indicate that these rocks originated by partial melting of anasthenospheric mantle source with a depleted Sr–Nd isotopiccomposition similar to present day MORB, rather than from anenriched lithospheric mantle source.

Whole-rock REE content is mainly controlled by mantle compo-sition and degree of partial melting and as such they have beenwidelyused to determine the origin of the magmas, and the degree of, andvariation in mantle melting (e.g., Gurenko and Chaussidon, 1995;Johnson, 1998; Münker, 2000; Green, 2006; Zhao and Zhou, 2007).The REE are moderately incompatible during melting of mantleperidotite according to their partitioning coefficient (Johnson, 1998),

Fig. 9. Plots of Sm/Yb vs. Sm showingmelt curves (or lines) obtained using the non-modal bamode and melt mode of ol0.530+opx0.270+cpx0.170+sp0.030 and ol0.060+opx0.280+cpx0.67mode of ol0.600+opx0.200+cpx0.100+gt0.100 and ol0.030+opx0.160+cpx0.880+gt0.090; respcompilation of McKenzie and O'Nions (1991, 1995); PM, N-MORB and E-MORB compositionsdegrees of partial melting for a given mantle source.

and thus, their concentrations and ratios are not greatly affected bymantle depletion and fluid influx (Pearce and Peate, 1995; Münker,2000). In addition, Yb (Dgarnet/melt=6.6) content in primary melts ismainly buffered by residual garnet during the melting of mantleperidotite (Johnson, 1998). Melts produced by partial melting ofmantle peridotite with garnet residue thus have low Yb concentrationand high LREE (e.g., La and Sm)/Yb ratio; garnet has a high partitioncoefficient for Sm (Dgarnet/melt=0.22) relative to La (Dspinel/melt=0.01) (McKenzie and O'Nions, 1991), and for Yb (Dgarnet/melt=6.6)relative to Sm (Dgarnet/melt=0.25) (Johnson, 1998). Partial meltsderived from spinel-lherzolite sources, however, should bring arelatively flat melting trend in terms of REE patterns defined bydepleted and enriched source compositions (Green, 2006), sincespinel has similar partition coefficients for La (Dspinel/melt=0.01), Sm(Dspinel/melt=0.01) and Yb (Dspinel/melt=0.01) (McKenzie andO'Nions, 1991). Based on the above discussion, in the plot of Sm/Ybvs. Sm (Fig. 9), the studied ultramafic dykes have Sm/Yb ratios higherthan the spinel-lherzolite melting curve, but close to those of thegarnet-lherzolite melting trend. Because minor crustal contaminationwill enhance the Sm/Yb and La/Sm ratios, the original melts fromwhich these samples were derived should plot close to garnet-lherzolite melting curve, implying a garnet-lherzolite mantle source.This is further supported by their high La/Yb (90–141). In addition,our calculations from trace element geochemistry suggest that thestudied ultramafic dykes are formed from melts that underwent lowdegree (b1%) partial melting of the source (Fig. 9). This low degree ofpartial melting is also supported by high La/Sm (9.0–16.4) and (La/Yb)N ratios (60.5–95.4) of these rocks, as La/Sm and La/Yb will bestrongly fractionated when the degree of melting is low. We concludethat the studied ultramafic dykes were generated by low degreepartial melting of a depleted, garnet-bearing lherzolite mantle source.

5.3. Fractional crystallisation

The low MgO contents and Mg number (Mg#) of the ultramaficdykes indicate fractionation of clinopyroxene (cpx) and olivine (ol),which is also supported by the presence of cpx and ol phenocrysts inthe ultramafic dykes. Negative Nb, Ta and Ti anomalies (Fig. 7) areconsidered to be related to fractionation of Ti-bearing phases (e.g.,rutile, ilmenite, titanite) and negative P anomalies result from apatiteseparation. The near-normal behaviour of Eu (i.e., weak negative to

tch melting equations of Shaw (1970). Melt curves are drawn for spinel-lherzolite (with0+sp0.110; respectively; Kinzler, 1997) and for garnet-lherzolite (with mode and meltectively; Walter, 1998). Mineral/matrix partition coefficients and DMM are from theare from Sun andMcDonough (1989). Tick marks on each curve (or line) correspond to

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positive anomalies: Eu/Eu*: 0.85–1.12), Sr and Ba (Figs. 6 and 7)indicate that the fractionation and/or accumulation of plagioclase andK-feldspar were not important.

5.4. Petrogenetic model

From the above perspective, we consider that the ultramafic dykesrepresent depleted asthenospheric melts contaminated by uppercrustal materials. The foregoing interpretation is consistent with themodelling curves defined for the studied ultramafic dykes in the Sr–Nd isotope diagram (Fig. 8), in which the mantle picrite (Qi and Zhou,2008) is treated as proxy for the depleted end-member componentsof the ultramafic dykes. A simple mass balance calculation of two-component mixing based on Sr and Nd isotopes, shows that mixing of∼10% upper crustal melts with asthenospheremantle-derivedmagmacan produce the observed Sr–Nd isotopic ratios found in theultramafic dykes.

McKenzie and Bickle (1988) demonstrated that dry asthenospheremantle cannot melt unless the lithosphere is sufficiently thinned toallow this mantle to rise to depths where it can undergo melting(b70–80 km). Therefore, this demands a dynamic mechanism todecipher the origin of the alkaline ultramafic dykes in southwest ofGuizhou Province in the context of the Yanshanian (Youjiang)orogeny. The alkaline ultramafic dykes in this study intruded alonga deep lithospheric fault zone and fracture, which indicates that theywere derived in an extensional tectonic setting so allowing decom-pression melting of the asthenosphere to occur. During EarlyPalaeozoic times (Late Ordovician-early Silurian) (Wu, 2000), theentire Guizhou area was turned into a continental setting due to theeffect of Gugangxi Movement (BGMRGP, 1987). Then, in the EarlyCretaceous when the Yanshanian (Youjiang) orogeny occurred, thereappeared numerous rift basins as the vertical movement of blocks inSW Guizhou continent from later period of early Cretaceous toTertiary (Su, 2002). Meanwhile, vertical movement of the blocksresulted in the formation of folds, faults and fractures. As aconsequence, a relatively extensive, extensional setting may havedeveloped in the study area. This extension, in turn, induced anupwelling of hot asthenosphere along the existing faults or fractures,and it was the high heat flow from this asthenospheric mantle thattriggered low degree partial melting in the lithospheric mantle,producing small volume (alkaline) ultramafic magmas. Subsequently,these mantle-derived magmas (mantle picrites) ascended alongfractures and faults to underplate the upper crust, and extensivecrustal assimilation (∼10%) occurred prior to emplacement.

6. Conclusions

Alkaline ultramafic igneous activities represented by dykesoccurring in SW Guizhou Province were emplaced between 85 and88 Ma. The alkaline ultramafic dykes were derived from low degree(b1%) partial melting of a garnet-bearing lherzolite mantle source.Subsequently, the alkaline ultramafic magma experienced ∼10%contamination with upper crustal materials and underwent extensivefractionation of cpx, ol, Ti-bearing phases (e.g., rutile, ilmenite,titanite) and apatite, prior to emplacement that was aided by regionaldeep lithospheric faults or fractures.

Acknowledgements

The authors would like to thank Andrew Kerr, Franco Pirajno andone anonymous reviewer for their constructive reviews on an earlierversion of this manuscript. We also thank Ruud Koole for quickeditorial handling of the manuscript. This research was supported byChinese 973 program (2007CB411402), the National Nature ScienceFoundation of China (40673029, 40773020, 90714010, 40634020 and40521001) and the Chinese Ministry of Education (B07039). We are

grateful to Gui-xiang Yu for helping in analysing Sr and Nd isotopes,and Yong-sheng Liu and Zhao-Chu Hu are thanked for their help withthe zircon U–Pb dating. The authors also thank Hu-jun Gong for thehelp with CL image-handling.

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