Nb–Ta fractionation induced by fluid-rock interaction insubduction-zones: constraints from UHP eclogite- and vein-hostedrutile from the Dabie orogen, Central-Eastern China

J . HUANG,1 Y. XIAO,1 Y. GAO,2 Z. HOU1 AND W. WU3

1 CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Scienceand Technology of China, Hefei 230026, China (ylxiao@ustc.edu.cn)2Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA (ygao@mail.uh.edu)3Geological Survey of Anhui Province, Hefei 230001, China

ABSTRACT Niobium and Ta concentrations in ultrahigh-pressure (UHP) eclogites and rutile from these eclogitesand associated high pressure (HP) veins were used to study the behaviour of Nb–Ta during dehydrationand fluid-rock interaction. Samples were collected through a �2 km profile at the Bixiling complex inthe Dabie orogenic belt, Central-Eastern China. All but one eclogite away from veins (EAVs) displaynearly constant Nb ⁄Ta ratios ranging from 16.1 to 19.2, with an average of 16.9 ± 0.8 (2 SE), similar tothat of their gabbroic protolith from the Yangtze Block. Nb ⁄Ta ratios of rutile from the EAVs rangefrom 12.7 to 25.3 among different individual grains, with the average values close to those of thecorresponding bulk rocks. These observations show that Nb and Ta were not significantly fractionatedby prograde metamorphism up to eclogite facies when no significant fluid-rock interaction occurs. Incontrast, Nb ⁄Ta ratios of rutile from eclogites close to veins (ECVs) are highly variable from 17.8 to49.8, which are systematically higher (by up to 17) than those of rutile from the veins. Theseobservations demonstrate that Nb and Ta were mobilized and fractionated during localized fluid flowand intensive fluid-rock interaction. This is strongly supported by Nb ⁄Ta zoning patterns in single rutilegrains revealed by in situ LA-ICP-MS analysis. Ratios of Nb ⁄Ta in the ECV-hosted rutile decreasegradually from cores towards rims, whereas those in the EAV-hosted rutile are nearly invariable.Furthermore, the vein rutile shows Nb ⁄Ta zoning patterns that are complementary to those in rutilefrom their immediate hosts (ECVs), suggesting an internal origin for the vein-forming fluids. The Nb ⁄Taratios of such fluids evolved from low values at the early stage of subduction to higher values at latersupercritical conditions with increased temperature and pressure. Quantitative modelling was conductedto constrain the compositional evolution of metamorphic fluids during dehydration and fluid-rockinteraction focusing on Nb–Ta distribution. The modelling results based on our proposed multistagefluid phase evolution path can essentially reproduce the natural observations reported in the presentstudy.

Key words: eclogite; fluid-rock interaction; Nb ⁄Ta fractionation; rutile; vein.

INTRODUCTION

Niobium and Ta have been widely used as indicatorsof geological processes and in global Earth models.They have long been thought to display similar geo-chemical behaviour during the evolution of the crust-mantle system because of their identical valence state(+5) and similar effective ionic radii (Shannon, 1976).As a result, the Nb ⁄Ta ratio has long been regarded asconstant in major silicate reservoirs of the Earth (e.g.McDonough & Sun, 1995; Jochum et al., 2000).However, later high-precision data determined usingisotope dilution (ID) and solution ICP-MS methodshave revealed that the average Nb ⁄Ta ratios of reser-voirs such as continental crust (12–13, Barth et al.,2000), depleted mantle (15.5, Jochum et al., 2000;

Workman & Hart, 2005), mid-ocean ridge basalts(MORBs) (14.2, Munker et al., 2003) and ocean islandbasalts (OIBs) (15.9, Pfander et al., 2007) are variableand below that of chondrites (�17.5, McDonough &Sun, 1995; Jochum et al., 2000; or 19.9, Munker et al.,2003). The hypotheses proposed to account for theterrestrial Nb–Ta paradox include the storage of the�missing Nb� in the metallic core (Wade & Wood, 2001;Munker et al., 2003) or in metasomatic domains withinthe subcontinental lithospheric mantle (Aulbach et al.,2008; Pfander et al., 2012) and hidden reservoirs ofsubducted eclogite or Early Enriched Crust in the deepmantle with suprachondritic Nb ⁄Ta (e.g. McDonough,1991; Kamber & Collerson, 2000; Rudnick et al., 2000;Kamber et al., 2003; Nebel et al., 2010). Experimen-tally determined partition coefficients between rutile

J. metamorphic Geol., 2012, 30, 821–842 doi:10.1111/j.1525-1314.2012.01000.x

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and melts (DNb ⁄DTa £ 1) (see compilation of Xiong etal., 2011) suggest that the residual rutile-bearingeclogites after partial melting should not have Nb ⁄Tahigher than their protolith (e.g. MORBs). This isconsistent with data from OIBs, whose sources maycontain various proportions of eclogites (Hofmann,1997), but they do not have high Nb ⁄Ta ratios as ex-pected from partial melting of eclogite-bearing sources(Pfander et al., 2007).

As the dominant carrier of Nb and Ta, rutile con-tains more than 90% of the Nb and Ta budget ofeclogites (Rudnick et al., 2000; Zack et al., 2002;Spandler et al., 2003; Schmidt et al., 2009) and meta-somatic, rutile-bearing peridotites (Kalfoun et al.,2002). Fluids in equilibrium with rocks (mainlyeclogites) containing rutile as an accessory phase areexpected to be depleted in Nb and Ta due to the lowsolubility of rutile in aqueous fluids (Ayers & Watson,1993; Audetat & Keppler, 2005; Tropper & Manning,2005) and the large partition coefficients for Nb and Tabetween rutile and aqueous fluids (Brenan et al., 1994;Green & Adam, 2003). By comparing HP-UHPmetamorphic rocks with their equivalent MORB orOIB protolith, many studies have demonstrated thatNb and Ta behave conservatively during progrademetamorphism up to eclogite facies and display nosignificant fractionation in the subducted oceanic crust(e.g. Chalot-Prat et al., 2003; Miller et al., 2007;Schmidt et al., 2009).

On the other hand, experimental results have shownthat Nb, Ta and Ti are highly soluble in aqueous fluidsin the absence of rutile (Stalder et al., 1998), or in thepresence of chemical species such as Na–Al–Si, Cl andF (Antignano & Manning, 2008; Manning et al., 2008;Rapp et al., 2010), or in supercritical fluids (Kessel etal., 2005a; Hayden & Manning, 2011). In addition, thepresence of vein rutile enriched in Nb and Ta in deeplysubducted rocks (Castelli et al., 1998; Franz et al.,2001; Rubatto & Hermann, 2003; Spandler & Her-mann, 2006; Gao et al., 2007; John et al., 2008; Zhanget al., 2008; Beinlich et al., 2010) indicates that Ti to-gether with Nb–Ta may be soluble and transportedwithin deep regions of subduction zones. Moreimportantly, systematically lower Nb ⁄Ta in synmeta-morphic and internally derived veins than in theiradjacent host eclogitic rocks suggests that Nb and Tawere mobilized and fractionated by fluids derived frommineral breakdown reactions (Kamber & Collerson,2000; Zhang et al., 2008). Similar suggestions wereproposed based on highly variable Nb ⁄Ta in singlerutile grains from UHP elcogites in the Dabie-Suluorogenic belt (Xiao et al., 2006; Ding et al., 2009;Liang et al., 2009).

The contrasting views of Nb–Ta mobility andfractionation might be explained if fluid flow andfluid-rock interaction were highly localized. Indeed,localization of metamorphic fluids is widely inferredbased on field, petrological and geochemical data(Spandler &Hermann, 2006; Beinlich et al., 2010; Ague,

2011). Trace element transport in the slab might bemore significant in local areas where there are high fluidfluxes and chemical gradients between fluids and rock,and where fluids are channelized into pathways withhigh fluid extraction rates (Spandler et al., 2003, 2004;Spandler & Hermann, 2006; Gao et al., 2007; Zack &John, 2007; John et al., 2008; Beinlich et al., 2010).In this study, Nb and Ta concentrations in rutile

from HP veins, elcogites close to and away from veins,and in whole rocks were obtained. Using these data,we evaluate the effect of fluid activity and fluid-rockinteraction on Nb–Ta mobility and fractionation. Theresults show that remarkable fractionation of Nb–Tacan be caused by highly localized fluid flow andintensive fluid-rock interaction, but no significantfractionation of Nb–Ta occurs in zones where there isno evidence for significant fluid activity.

GEOLOGICAL SETTINGS AND SAMPLELOCATIONS

The Dabie-Sulu orogenic belt in Central-Eastern Chi-na was formed during the Triassic collision betweenthe North China and Yangtze Blocks (Fig. 1a). Itrepresents the largest occurrence of UHP metamorphicrocks so far recognized worldwide and has become thetarget of intense geological and geochemical researchwith respect to UHP metamorphism (Liu & Li, 2008;Zheng, 2008).As the largest coesite-bearing mafic-ultramafic body

in the Dabie-Sulu orogenic belt, the Bixiling complexoccurs as a tectonic block with an area of �1.5 km2

within foliated quartz-feldspathic gneiss (Fig. 1b). Itconsists predominantly of banded eclogites with manylenticular bodies of garnet-bearing ultramafic rocks(Zhang et al., 1995). Coesite relicts and quartz pseud-omorphs in eclogites, and magnesite and Ti-clinohu-mite in meta-ultramafic rocks suggest that the Bixilingcomplex had been subducted to depths of >100 km(Zhang et al., 1995; Chavagnac & Jahn, 1996; Xiao etal., 2000). Field observation, petrological study, traceelement and Sr–Nd isotope evidence all suggest thatthe protolith of the Bixiling complex is gabbro cumu-late differentiated from a mantle-derived magmasource with only slight modification by crustal con-tamination (Zhang et al., 1995; Chavagnac & Jahn,1996, Tang et al., 2007). Garnet-clinopyroxene Sm–Ndand Lu–Hf geochronology, and zircon U–Pb datingsuggest that the eclogite, garnet peridotite and sur-rounding gneiss experienced Triassic metamorphism atc. 220 Ma (Chavagnac & Jahn, 1996; Chavagnac et al.,2001; Zheng et al., 2004, 2008; Schmidt et al., 2008). Inaddition, zircon U–Pb dating shows that their proto-liths emplaced during the middle Neoproterozoic withages of 760–720 Ma (Cheng et al., 2000; Chavagnac etal., 2001; Zheng et al., 2004, 2008).High-pressure (HP) veins are common in UHP

eclogites of the Dabie-Sulu orogenic belt (e.g. Cong,1996; Zheng et al., 2003). Detailed petrological, fluid

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inclusion and oxygen isotope studies indicate that theseveins formed by fluids from an internal source due todehydration reactions of the host rocks (Castelli et al.,1998; Franz et al., 2001; Zheng et al., 2007; Xiao et al.,2011a). However, these veins may have formed duringeither subduction (Castelli et al., 1998; Franz et al.,2001) or exhumation (Zheng et al., 2007).

In this study, 15 eclogitic and four HP vein sampleswere collected at Bixiling in the Dabie orogenic belt ina �2 km south-to-north profile across the complex(Fig. 1b). Among them, three vein-and-host eclogitesample pairs were collected to investigate if any Nband Ta fractionation occurred between them.According to different mineral assemblages, the fourveins were divided into three groups: two quartz veins,one epidote vein and one clinozoisite vein, respectively.They range from 5 to 18 cm wide and 25 to 42 cmlong, showing sharp contacts with the host eclogites(Fig. 2a,b). Eclogite sample 08bxl11 was collected5 cm from an epidote-quartz vein (08bxl11v), 08bxl0310 cm from a quartz vein (08bxl03v), and 08bxl0720 cm from another quartz vein (08bxl07v). Here, thethree eclogite samples are referred to as �eclogite closeto vein (ECV)�, whereas the others as �eclogite away

from vein (EAV)�. It is noted that no adjacent eclogitichost rock was sampled near the clinozoisite-quartz vein(80bxl09v).

ANALYTICAL METHODS

Mineral chemistry

Major element compositions of minerals were deter-mined using a JEOL JXA-8100 electron microprobeequipped with wavelength dispersive spectrometers(WDS) and an energy dispersive spectrometer (EDS) atthe Institute of Geology and Geophysics, ChineseAcademy of Sciences. Analytical conditions wereaccelerating voltage of 15 kV, beam current of 10 nAand electron beam diameter of 1 lm. Both silicates andpure oxides were used as reference standards: diopsidefor SiO2 and CaO, synthetic TiO2 crystal for TiO2,jadeite for Al2O3 and Na2O, synthetic Cr2O3 crystal forCr2O3, hematite for FeO, bustamite for MnO, olivinefor MgO, and sanidine for K2O.

In situ trace element concentrations of rutile weredetermined at the University of Science and Technol-ogy of China (USTC), using an ArF excimer laser

Young diabasic dykeGneissEclogiteUltramafic rock

N

1km

3 km

Qian

ShuiR

iver

Sample localities

250 m

08bxl11

08bxl14

08bxl13

08bxl09v

08bxl04

08bxl12

08bxl18

08bxl02

08bxl15 08bxl16

08bxl01

08bxl0308bxl05

08bxl07v08bxl06

08bxl08

Prof

ileSo

uth

toN

orth

08bxl03v

08bxl07

08bxl11v

Bixiling

Weihai

Su-Lu

Beijing

Dabie

120km

Hong’an

N

Yangtze Block

North ChinaBlock

High P belt blueschistAmphibolite EclogiteUltrahigh P beltMigmatite complexGS Subgreenschist

(a) (b) Bixiling UHP complex

Tanl

ufa

ult

Road

2km

Fig. 1. (a) Geological sketch map of the Dabie-Sulu orogenic belt in Central-Eastern China showing major geological features; (b)Geological sketch map of the Bixiling mafic-ultramafic complex with sample localities along a �2 km profile from south to north.Three vein-and-host rock sets (red-and-white circle pairs) are also shown in Fig. 1b. Eclogite sample 08bxl11 was collected 5 cm froman epidote-quartz vein (08bxl11v), 08bxl03 10 cm from a quartz vein (08bxl03v), and 08bxl07 20 cm from another quartz vein(08bxl07v). The vein sample 08bxl09v is a clinozoisite-quartz vein. Maps are modified after Zhang et al. (1995).

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ablation system (GeoLas Pro, 193 nm wavelength)coupled to a quadrupole ICP-MS (PerkinElmer ElanDRCII). The instrumental parameters for both thelaser and ICP-MS are listed in Table 1. Helium wasused as ablation carrier gas, mixed with argon as ma-keup gas before entering to the plasma torch of theICP-MS. Measurements for elements of interest werecarried out using time resolved analyses operating inpeak jump mode. Each five unknown spots werebracketed by one analysis on NIST 610 glass standard(recommended values from GeoRem, Jochum &Nehring, 2006) for external calibration. Each analysisincorporates a �30 s background acquisition followedby a �40 s data acquisition. Rutile analyses were car-ried out on polished thin sections (�30 lm) for eclogitesamples and thick sections (�2 mm) for vein samples.Laser ablation spot size was set to 60 lm, except forcross-section analyses (30–60 lm, depending on rutile

300 μm

10 cm

300 μm

EclogiteRutile

Quartz

Czo

Ep

Rt

Bi

Chl

Phg

Rt

Ep

CpxBi

Ep

Chl

Bi

Rt

Ep

(a)

(c) (d)

1 cm

(b)

Fig 2b

Eclogite

Qtz-vein

Fig. 2. Photographs of the HP veins. (a) Field relationship between the eclogite and vein. Quartz vein (08bxl03v) predominantlyconsists of quartz with minor rutile and phengite. Red circle represents eclogite sample (08bxl03) drilled at a distance of 10 cm from thequartz vein; (b) Enlarged rutile grain in Fig. 2a. It has the length and width of �1.0 and 0.4 cm, respectively; (c) Coarse-grainedclinozoisite and epidote coexisting with rutile showing analytical points in the clinozoisite-quartz vein sample (08bxl09v); (d) high-pressure minerals (e.g. clinopyroxene and phengite) as well as minor retrograde minerals (e.g. biotite and chlorite) occur in theclinozoisite-quartz vein sample (08bxl09v).

Table 1. LA-ICPMS details and operating parameters.

Laser ablation system ICP-MS

Model GeoLas Pro Model

PerkinElmer ⁄ SCIEX,

Elan DRCII

Type ArF Excimer Type Magnetic sector field

Wavelength 193 nm Forward

power (W)

1350 W

Energy density 10 J cm)2 Plasma gas

flow (Ar)

15 L min)1

Repetition rate 10 Hz Auxiliary gas

flow (Ar)

1.21 L min)1

He carrier gas flow 0.3 L min)1 Make-up (Ar) 0.7 L min)1

Laser warm up �30 s Scanning mode Peak hopping

Ablation time 40 s

Ablation style Single spot

Washout time >120 s Isotopes

measured

27Al, 29Si, 43Ca,49Ti, 57Fe,

90Zr, 93Nb, 180Hf,181Ta, 238U

Laser spot size 30–60 lm

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size). Ti concentrations in rutile were determined bythe electron microprobe and used as internal stan-dardization (isotope mass 49).

During the analytical session, another internationalNIST-612 glass standard, as an unknown sample, wasrepeatedly measured five times. The trace elementconcentrations measured for the NIST-612 glass stan-dard relative to the NIST-610 glass standard agree wellwithin error with the recommended values (Jochum &Nehring, 2006). Duplicate analyses of the NIST-612glass standard suggest that the precision (RSD) was6.9% (Zr), 3.8% (Nb), 4.6% (Hf), 4.6% (Ta), and theaccuracy (RE) 5.0% (Zr), 9.3% (Nb), 1.0% (Hf), 9.7%(Ta). The precision and accuracy estimated in thisstudy are consistent with those estimated based onrepeated analyses of the fused glasses of USGS rockstandards (i.e. W-2a, BHVO-2 and GSP-2) (He et al.,2011).

Bulk rock chemistry

For bulk rock analyses, the samples were crushed in acorundum jaw crusher to 60 mesh, and then �60 g ofeach crushed sample was powdered in an agate ringmill to <200 mesh in size. Major elements were anal-ysed by ICP-AES (Thermo Jarrell Ash Atomscan 25)at the University of Houston (USA) and detailedprocedures were described in Gao et al. (2009). Accu-racy and precision for major elements are generallybetter than 2% based on replicate analyses of certifiedUSGS rock standards.

Trace elements were determined using an ELANDRCII ICP-MS at the USTC. Analytical procedureswere discussed in detail by Hou &Wang (2007) and aresimilar to those reported in Liu et al. (2008). Some�50 mg bulk rock powders were precisely weighed intoa Teflon bomb, and moistened with a few drops ofultrapure water. After adding a mixture of 1.5 mlultrapure HNO3 + 1.5 ml ultrapure HF + 0.01 mlultrapure HClO4 acid, the solution was heated andevaporated to dryness at �110 �C. Then, 1.5 mlultrapure HNO3 + 1.5 ml ultrapure HF were addedagain, and the sealed bomb was heated for 48 h in anoven at 190 �C. Again, the solution was evaporated todryness followed by adding 3 ml ultrapure HNO3, andthen to the state of wet salt. The resultant salt was re-dissolved by adding 3 ml 50% HNO3 and then heatedfor at least 12 h with closed caps in the oven at 150 �C.The final solution was diluted to 80 g with a mixture of1 ml internal-standard solution of the single elementRh which has a dilution factor of 1 ⁄ 1250 in 2%ultrapure HNO3. The diluted solution was then anal-ysed by ICP-MS. In this study, the USGS basalticstandard (BHVO-2) was analysed repeatedly to eval-uate the analytical precision and accuracy (Table 4).The reproducibility (i.e. precision) is expressed as RSD(relative standard deviation) and the accuracy is rep-resented using RE (relative error between measuredand recommended values) (Table 4). The precision for

most elements is better than 5%, except for Ni (5.0%),Zn (6.0%), Co (6.2%), Be (7.1%), Sc (7.1%), Li(7.9%), Cs (8.7%), Sr (15%) and Pb (7.3%). Theaccuracy is generally better than 10%, with many ele-ments agreeing to within 2% of the recommendedvalues. The exception is Zn, which differs by up to10.8%. In particular, for HFSEs (e.g. Zr, Nb, Hf, Ta),the precision is 1.5% for Zr, 1.4% for Nb, 0.4% for Hfand 1.3% for Ta, and the accuracy is 0.4% for Zr,2.4% for Nb, 0.5% for Hf and 4.7% for Ta. Moreimportantly, the range of Nb ⁄Ta for the EAVs (14.0–19.2) is very similar to that in the Dabie eclogites(16.0–20.8) determined using the ID method (Schmidtet al., 2009). In addition, the average Nb ⁄Ta ratio is16.9 ± 0.8 (2 SE n = 12), similar to the Nb ⁄Ta ratioobtained using the ID method for eclogite from thesame outcrop (eclogite DB05 with Nb ⁄Ta ratio of 18.1,Schmidt et al., 2008, 2009). These observations there-fore lend support that our data obtained using thesolution ICP-MS method are precise, as also revealedby repeated analyses of reference materials.

RESULTS

Petrological observation and mineral chemistry

Typical mineral assemblages and modal abundances ofthe samples as well as average chemical compositionsof representative minerals are presented in Tables 2 &3, respectively. The mineral abbreviations used arefrom Kretz (1983), except for amphibole (Amp),phengite (Phg) and symplectite (Sym). The EAVs(08bxl01, 02, 04, 05, 06, 08, 12, 13, 14, 15, 16 & 18) areessentially composed of garnet and omphacite. Garnetfrom the EAVs shows large compositional variationswith pyrope, grossular, almandine and spessartine

Table 2. Mineral assemblage and modal contents of the studiedeclogites and veins.

Sample Rock Minerals and their volume content (%)

08bxl01 Eclogite Grt 52, Omp 35, Phg 1, Qtz 8, Rt 3, Ky 1, Chl (T)

08bxl02 Eclogite Grt 58, Omp 35, Rt 4, Amp 2, Sym 1

08bxl03 Eclogite Grt 60, Omp 36, Rt 3, Qtz 1, Phg (T), Ky (T)

08bxl04 Eclogite Grt 55, Omp 24, Phg 3, Qtz 10, Rt 5, Amp 3, Pg (T),

Chl (T), Ilm (T)

08bxl05 Eclogite Grt 45, Omp 25, Phg 10, Qtz 12, Rt 4, Amp 4, Ttn (T), Bi (T)

08bxl06 Eclogite Grt 68, Omp 24, Phg 3, Qtz 1, Rt 3, Amp 1, Bi (T)

08bxl07 Eclogite Grt 30, Omp 54, Phg 10, Rt 3, Sym 2, Amp 1, Pg (T), Ky (T)

08bxl08 Eclogite Grt 25, Omp 54, Phg 2, Pg 1, Qtz 15, Rt 1, Sym 2

08bxl11 Eclogite Sym 90, Ilm 1, Amp 3, Bi 2, Grt 1, Rt 1, Ep 1, Chl 1, Omp (T)

08bxl12 Eclogite Grt 45, Omp 44, Phg 2, Qtz 3, Rt 3, Sym 3

08bxl13 Eclogite Grt 55, Omp 38, Rt 4, Amp 2, Ky 1, Bi(T)

08bxl14 Eclogite Grt 45, Omp 37, Qtz 14, Rt 4

08bxl15 Eclogite Grt 40, Omp 46, Phg 2.5, Qtz 10, Rt 1, Ilm 0.5

08bxl16 Eclogite Grt 40, Omp 45, Phg 2, Qtz 6, Rt 3, Sym 2, Bi 0.5, Ilm (T)

08bxl18 Eclogite Grt 50, Omp 40, Sym 9, Ilm 1

08bxl03v Qtz vein Qtz 99, Rt 0.5, Phg 0.5

08bxl07v Qtz vein Qtz 99, Rt 1

08bxl09v Czo-Qtz

vein

Rt 30, Czo 50, Ep 10, Qtz 10, Phg (T), Chl (T), Bi (T),

Cpx (T)

08bxl11v Ep-Qtz

vein

Ep 40, Ab 10, Rt 15, Qtz 35, Ttn (T), Bi (T), Chl (T)

T means trace.

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Table

3.Averagemajorelem

entcompositionsofrock-form

ingandaccessory

mineralsfrom

thestudiedeclogites

andveins(w

t%).

Sample

08bxl-01

08bxl-03

08bxl-04

08bxl-05

08bxl-06

08bxl-07

08bxl-08

08bxl-11

08bxl-13

08bxl-01

08bxl-03

08bxl-04

08bxl-05

08bxl-06

08bxl-07

08bxl-08

08bxl-13

08bxl-09v

08bxl-11v

08bxl-04

08bxl-05

08bxl-06

Location

EAV

ECV

EAV

EAV

EAV

ECV

EAV

ECV

EAV

EAV

ECV

EAV

EAV

EAV

ECV

EAV

EAV

Vein

Vein

EAV

EAV

EAV

Mineral

Grt

Grt

Grt

Grt

Grt

Grt

Grt

Grt

Grt

Omp

Omp

Omp

Omp

Omp

Omp

Omp

Omp

Cpx

Ab

Phg

Phg

Phg

N10

48

79

57

64

11

46

66

26

43

34

62

SiO

240.14

40.20

38.60

38.64

38.63

39.86

39.62

38.37

38.79

55.86

55.18

54.87

55.57

55.09

56.13

56.01

54.58

43.30

67.89

52.80

53.51

53.34

TiO

20.04

0.07

0.06

0.05

0.02

0.04

0.08

0.04

0.04

0.03

0.07

0.05

0.06

0.06

0.08

0.03

0.03

0.55

–0.38

0.39

0.43

Al 2O

322.67

22.15

21.56

21.62

21.65

22.48

22.52

21.47

21.63

9.52

8.15

7.12

9.72

7.54

11.26

12.18

5.14

11.53

19.66

23.20

23.02

23.05

Cr 2O

30.07

0.04

0.01

0.02

0.01

0.06

0.04

0.06

0.01

0.12

0.01

0.01

0.01

–0.07

0.06

0.03

0.03

0.01

0.02

0.04

0.03

FeO

17.74

22.80

26.14

24.90

26.32

19.36

19.16

24.25

26.97

2.57

4.70

7.44

6.15

7.68

3.12

2.65

8.96

16.93

0.04

2.88

2.92

3.21

MnO

0.40

0.34

0.48

0.51

0.49

0.53

0.41

1.01

0.46

0.01

0.03

0.02

0.01

0.03

0.02

0.02

0.02

0.40

0.01

–0.01

0.02

MgO

11.17

7.92

4.97

4.96

4.81

7.41

7.82

3.21

4.92

10.09

9.87

8.80

7.82

8.46

8.76

8.22

9.57

10.15

0.01

4.29

4.30

4.20

CaO

7.73

8.07

8.27

9.26

8.21

10.98

10.76

11.67

7.90

14.43

14.75

14.05

12.33

13.58

13.12

12.37

15.41

10.45

0.41

0.01

0.02

0.01

Na2O

0.03

0.03

0.03

0.06

0.03

0.04

0.04

0.03

0.04

6.27

6.13

6.36

7.38

6.65

7.14

7.55

5.39

2.53

11.66

0.31

0.24

0.23

K2O

0.01

0.01

0.01

0.01

–0.01

0.01

0.02

0.01

0.01

0.02

0.01

0.02

0.01

0.01

0.01

0.01

0.94

0.12

10.36

10.44

10.26

Total

99.99

101.62

100.14

100.02

100.19

100.76

100.46

100.12

100.76

98.92

98.90

98.74

99.06

99.10

99.71

99.09

99.14

96.81

99.80

94.24

94.89

94.77

Si

3.00

3.03

3.01

3.01

3.01

3.01

2.99

3.00

3.01

1.99

1.98

1.99

2.00

1.99

1.99

1.99

2.00

1.66

2.97

3.55

3.57

3.56

Ti

––

––

––

––

––

––

––

––

–0.02

–0.02

0.02

0.02

Al

2.00

1.97

1.98

1.98

1.99

2.00

2.00

1.98

1.98

0.40

0.35

0.31

0.41

0.32

0.47

0.51

0.22

0.52

1.01

1.84

1.81

1.81

Cr

––

––

––

––

––

––

––

––

––

––

––

Fe3

+–

––

0.01

––

–0.01

0.01

0.04

0.11

0.15

0.11

0.15

0.04

0.03

0.17

0.36

0.04

0.09

0.08

0.11

Fe2

+1.10

1.44

1.70

1.61

1.72

1.22

1.21

1.57

1.74

0.04

0.03

0.07

0.08

0.08

0.05

0.04

0.11

0.19

–0.07

0.09

0.07

Mn

0.02

0.02

0.03

0.03

0.03

0.03

0.03

0.07

0.03

––

––

––

––

0.01

––

––

Mg

1.24

0.89

0.58

0.57

0.56

0.83

0.88

0.37

0.57

0.54

0.53

0.48

0.42

0.46

0.46

0.43

0.52

0.58

–0.43

0.43

0.42

Ca

0.62

0.65

0.69

0.77

0.69

0.89

0.87

0.98

0.66

0.55

0.57

0.55

0.47

0.53

0.50

0.47

0.60

0.43

0.02

––

–

Na

––

–0.01

–0.01

0.01

–0.01

0.43

0.43

0.45

0.51

0.47

0.49

0.52

0.38

0.19

0.99

0.04

0.03

0.03

K–

––

––

––

––

––

––

––

––

0.05

0.01

0.89

0.89

0.87

Cations

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

8.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

4.00

5.04

6.93

6.92

6.91

Prp

0.42

0.30

0.19

0.19

0.19

0.28

0.29

0.13

0.19

Jd0.41

0.35

0.31

0.42

0.32

0.48

0.51

0.22

Ab

0.97

Grs

0.21

0.22

0.23

0.26

0.23

0.30

0.29

0.33

0.22

Ae

0.04

0.11

0.15

0.11

0.15

0.04

0.03

0.17

An

0.02

Alm

0.37

0.48

0.57

0.54

0.57

0.41

0.40

0.53

0.58

Di

0.52

0.54

0.48

0.41

0.45

0.46

0.43

0.51

Or

0.01

Sps

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.02

0.01

Hd

0.04

0.03

0.07

0.07

0.08

0.05

0.04

0.10

8 26 J . H U A N G E T A L .

� 2012 Blackwell Publishing Ltd

Table

3.(Continued)

Sample

08bxl-0708bxl-0808bxl-0708bxl-0408bxl-1108bxl-11v

08bxl-09v

08bxl-11v08bxl-11v

08bxl-0508bxl-0108bxl-0308bxl-0408bxl-0508bxl-0608bxl-0708bxl-07v

08bxl-0808bxl-09v08bxl-1108bxl-11v08bxl-1308bxl-11v08bxl-11

Location

ECV

EAV

ECV

EAV

Vein

ECV

Vein

Vein

Vein

EAV

EAV

ECV

EAV

EAV

EAV

ECV

Vein

EAV

Vein

ECV

Vein

EAV

Vein

ECV

Mineral

Pg

Pg

Amp

Amp

Ep

Ep

Ep

Czo

Bi

Bi

Rt

Rt

Rt

Rt

Rt

Rt

Rt

Rt

Rt

Rt

Rt

Rt

Ttn

Ilm

N3

42

511

28

43

21

34

33

32

32

33

36

2

SiO

246.67

47.56

50.84

51.68

37.66

37.67

38.04

37.41

37.05

35.83

0.05

0.01

0.02

0.02

0.03

0.03

0.07

0.04

0.03

0.02

0.03

0.01

30.40

0.04

TiO

20.12

0.12

0.21

0.19

0.17

0.19

0.16

0.12

1.77

3.20

99.56

100.07

99.02

99.00

98.35

99.51

99.91

99.13

98.55

96.47

99.78

99.50

37.30

49.60

Al 2O

338.60

39.47

10.32

4.74

24.59

24.70

24.35

25.03

14.62

17.14

–0.01

0.01

0.01

0.02

0.01

0.04

0.03

0.01

0.01

0.02

0.01

1.64

0.01

Cr 2O

30.08

0.06

0.05

0.01

0.02

0.06

0.02

0.09

0.03

0.05

0.39

0.03

0.04

0.01

0.02

0.17

0.40

0.18

0.04

0.12

0.19

0.02

0.01

0.04

FeO

0.52

0.39

7.74

13.57

10.17

9.65

10.41

8.96

19.72

16.13

0.31

0.33

0.39

0.43

0.36

0.42

0.40

0.40

0.56

0.86

0.55

0.86

0.55

46.72

MnO

–0.02

0.07

0.13

0.19

0.11

0.08

0.11

0.20

0.06

–0.02

–0.01

0.02

–0.01

0.02

0.01

0.01

0.02

0.02

0.05

2.54

MgO

0.19

0.18

15.34

13.89

0.09

0.07

0.06

0.11

11.56

11.20

–0.01

–0.01

0.01

0.01

0.01

0.01

0.01

0.01

–0.01

0.01

0.20

CaO

0.36

0.40

7.69

9.34

21.74

20.47

21.01

19.15

0.02

0.34

0.04

0.01

0.02

0.01

0.01

0.01

0.03

0.01

0.02

0.05

0.01

0.02

28.14

0.02

Na2O

6.68

7.00

3.96

2.23

0.02

0.02

0.03

0.02

0.06

0.19

0.02

0.01

0.02

0.04

0.02

0.01

0.02

0.02

0.04

0.03

0.02

0.01

0.04

0.03

K2O

0.99

0.84

0.18

0.30

0.01

––

–9.75

8.33

––

0.02

––

0.01

0.01

0.01

0.01

0.01

0.02

–0.01

–

Total

94.21

96.05

96.39

96.07

94.66

92.93

94.16

91.02

94.77

92.47

100.38

100.49

99.54

99.54

98.83

100.17

100.90

99.85

99.29

97.58

100.65

100.46

98.16

99.21

Si

3.00

3.00

7.14

7.50

3.05

3.10

3.10

3.14

2.85

2.76

––

––

––

––

––

––

1.00

–

Ti

0.01

0.01

0.02

0.02

0.01

0.01

0.01

0.01

0.10

0.19

0.99

1.00

1.00

1.00

1.00

1.00

0.99

1.00

1.00

0.99

0.99

0.99

0.92

0.94

Al

2.92

2.93

1.71

0.81

2.34

2.40

2.34

2.47

1.33

1.56

––

––

––

––

––

––

0.06

–

Cr

––

0.01

––

––

0.01

––

––

––

––

––

––

––

––

Fe3

+0.15

0.14

0.54

0.55

0.55

0.38

0.46

0.23

0.14

0.30

––

––

––

––

––

––

0.09

0.11

Fe2

+–

–0.37

1.09

0.14

0.29

0.25

0.40

1.13

0.74

––

––

––

––

0.01

0.01

0.01

0.01

–0.88

Mn

––

0.01

0.02

0.01

0.01

0.01

0.01

0.01

––

––

––

––

––

––

––

0.05

Mg

0.02

0.02

3.21

3.01

0.01

0.01

0.01

0.01

1.33

1.28

––

––

––

––

––

––

–0.01

Ca

0.02

0.03

1.16

1.45

1.88

1.80

1.83

1.72

–0.03

––

––

––

––

––

––

0.99

–

Na

0.83

0.86

1.08

0.63

––

––

0.01

0.03

––

––

––

––

––

––

––

K0.08

0.07

0.03

0.05

––

––

0.96

0.82

––

––

––

––

––

––

––

Cations

7.04

7.04

15.27

15.14

8.00

8.00

8.00

8.00

7.86

7.70

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.01

1.00

1.01

3.08

2.00

EAV

meanseclogites

awayfrom

veins,whereasECV

meanseclogites

close

toveins.N

represents

thenumber

ofanalysedpoints.Elementconcentrationsbelow

thedetectionlimitofelectronmicroprobeanalysisare

denotedas

�–�.

Nb – T a F R A C T I O N AT I O N I N D U C E D B Y F L UI D- R O CK I N TE R AC T I O N 82 7

� 2012 Blackwell Publishing Ltd

components varying in ranges of 17.8–43.4%, 19.5–31.6%, 35.4–59.3% and 0.69–1.25%, respectively. Thejadeite components of omphacite from the EAVs rangefrom 21.7 to 57.8%. The large compositional varia-tions of garnet and omphacite in different eclogitesamples probably result from different bulk composi-tion and mineral assemblage of their host rocks (Ta-bles 2–4). Rutile occurs in the matrix or as inclusionsin other minerals (e.g. garnet, omphacite and phengite)

in all eclogites, except for sample 08bxl18 that containsno rutile. The rutile is present in abundances of 1–5%and has lengths of up to �400 lm. Minor symplectiteof amphibole + plagioclase occurs at the rims ofomphacite. Replacement of garnet by amphi-bole ± plagioclase and breakdown of phengite intofine-grained biotite could be also observed in a fewretrograde eclogites, in which rutile is commonly rim-med by thin ilmenite and ⁄ or titanite layers. The ECVs

Table 4. Bulk rock major and trace element compositions of the eclogites away from veins and analyses of the BHVO-2 standard.

Sample: 08bxl

01

08bxl

02

08bxl

04

08bxl

05

08bxl

06

08bxl

08

08bxl

12

08bxl

13

08bxl

14

08bxl

15

08bxl

16

08bxl

18

BHVO-2

n = 3

RSD(%) Rec. RE(%)

Comment: EAV EAV EAV EAV EAV EAV EAV EAV EAV EAV EAV EAV Avg.

Major element (wt%)

SiO2 50.07 49.68 47.91 48.09 45.68 51.78 48.68 45.13 49.39 48.96 49.60 45.71

TiO2 0.91 0.86 2.69 2.33 2.88 1.00 0.65 2.67 0.76 0.72 0.66 0.35

Al2O3 14.69 14.34 15.09 14.94 16.87 18.36 21.05 13.77 15.87 16.30 17.58 11.27

Fe2O3 11.74 11.74 16.71 17.78 18.62 8.57 8.30 19.13 9.69 9.67 8.24 14.32

MnO 0.18 0.18 0.24 0.26 0.28 0.14 0.12 0.20 0.16 0.15 0.15 0.20

MgO 10.28 10.79 5.60 4.94 5.12 5.66 7.66 6.54 8.41 8.41 7.99 22.35

CaO 9.70 10.11 8.70 8.79 8.11 10.85 10.30 10.39 13.68 13.60 13.08 4.97

Na2O 2.24 2.44 2.58 1.59 2.01 2.95 2.71 2.09 2.03 1.95 2.55 0.76

K2O 0.04 0.04 0.22 0.52 0.42 0.28 0.40 0.05 0.02 0.21 0.02 0.03

P2O5 0.16 0.26 0.25 0.75 BD 0.40 0.11 0.01 BD 0.04 0.14 0.04

LOI 0.46 1.10 0.42 0.17 0.31 0.74 0.90 0.16 0.62 0.64 0.69 1.29

Total 100.46 100.66 100.42 100.17 100.31 100.74 100.90 100.16 100.62 100.64 100.69 101.29

Mg# 63.44 64.54 39.89 35.51 35.27 56.65 64.65 40.40 63.22 63.30 65.77 75.57

Trace element (ppm)

Li 5.28 BD 6.49 5.25 5.84 11.8 11.6 6.22 5.69 5.38 4.88 3.10 4.47 7.86 4.8 -6.79

Be 0.38 0.29 0.38 0.24 0.29 0.43 0.32 0.32 0.29 0.25 0.25 0.17 1.07 7.13 1.0 6.61

Sc 27.1 57.3 47.2 40.3 50.9 32.2 13.1 51.3 35.6 38.5 40.4 11.4 30 7.14 32 -7.61

V 168 639 695 302 587 209 113 801 208 229 184 94.9 309 3.28 317 -2.41

Cr 424 3.31 4.65 32.1 9.15 221 244 7.09 352 286 555 1979 270 4.10 280 -3.67

Co 42.4 57.0 62.7 41.5 48.5 32.3 40.8 64.3 33.0 37.0 29.4 112 42.2 6.19 45 -6.13

Ni 70.6 14.1 3.54 13.4 4.47 31.4 120 4.67 97.3 72.6 99.2 633 113 5.03 119 -4.74

Cu 53.1 23.7 53.0 33.9 23.6 51.3 72.0 41.0 62.4 61.0 56.3 51.6 125 3.48 127 -1.34

Zn 60.8 102 97.3 95.9 98.7 57.3 47.3 108 51.9 48.3 43.2 70.1 114 6.00 103 10.83

Ga 13.9 19.0 20.6 17.3 18.6 18.7 16.0 19.9 15.5 15.5 14.4 8.89 21.3 3.76 22 -3.22

Rb 0.36 0.62 2.85 8.65 4.44 8.13 9.43 1.46 0.7 5.71 0.29 0.67 9.38 3.72 9.11 3.01

Sr 58.9 38.3 114 78.1 25.6 788 432 55.1 209 243 248 166 430 15.72 396 8.56

Y 17.8 19.6 19.9 38.6 26.7 18.9 11.0 13.3 14.7 12.0 11.1 7.82 25.6 1.14 26 -1.41

Zr 46.1 29.9 29.1 124 46.9 37.1 32.4 9.69 12.2 11.4 18.2 24.1 172.7 1.54 172 0.38

Nb 1.73 1.54 1.31 3.97 1.37 1.45 1.20 0.71 0.42 0.49 0.56 0.65 17.7 1.35 18.1 -2.40

Cs 0.05 BD 0.07 0.17 0.14 0.35 0.59 0.13 0.06 0.49 0.04 0.52 0.10 8.67 0.10 -1.36

Ba 5.92 18.2 55.5 96.7 70.9 73.8 382 1847 9.99 35.2 15.7 37.9 127 3.39 131 -2.80

La 2.08 0.68 3.19 11.6 0.41 16.1 5.45 0.33 2.77 3.29 2.76 1.18 14.6 2.08 15.2 -3.75

Ce 6.16 2.01 8.86 35.3 0.95 35.1 12.0 0.91 6.45 7.50 5.93 3.72 36.5 2.48 37.5 -2.62

Pr 1.12 0.34 1.45 5.60 0.17 4.75 1.66 0.22 0.98 1.12 0.85 0.64 5.30 4.09 5.35 -0.95

Nd 6.50 1.84 8.38 31.1 1.12 22.9 8.42 1.70 5.34 6.12 4.50 3.77 23.4 0.40 24.5 -4.66

Sm 2.09 1.06 2.75 8.21 1.08 5.31 2.02 1.19 1.57 1.87 1.32 1.08 5.95 1.83 6.07 -2.06

Eu 0.76 0.68 1.11 2.50 0.77 1.82 0.87 0.72 0.63 0.78 0.55 0.41 1.96 0.65 2.07 -5.26

Gd 2.47 2.32 3.24 7.33 2.93 5.01 2.14 1.92 1.98 2.05 1.56 1.21 5.84 2.04 6.24 -6.39

Tb 0.46 0.47 0.57 1.06 0.67 0.68 0.33 0.36 0.38 0.35 0.28 0.21 0.89 0.93 0.92 -3.43

Dy 2.90 3.12 3.34 6.32 4.46 3.50 1.91 2.17 2.48 2.09 1.84 1.28 5.21 1.46 5.31 -1.79

Ho 0.65 0.62 0.73 1.45 1.02 0.7 0.42 0.49 0.56 0.46 0.43 0.29 0.92 1.89 0.98 -5.97

Er 1.69 1.74 1.86 3.82 2.61 1.82 1.07 1.23 1.47 1.17 1.15 0.75 2.45 0.62 2.54 -3.48

Tm 0.25 0.23 0.27 0.58 0.39 0.26 0.16 0.18 0.22 0.18 0.18 0.11 0.32 0.99 0.33 -2.28

Yb 1.70 1.54 1.83 4.01 2.67 1.81 1.10 1.26 1.49 1.17 1.21 0.81 1.97 0.64 2.0 -1.45

Lu 0.24 0.22 0.25 0.56 0.37 0.25 0.15 0.17 0.21 0.16 0.17 0.12 0.273 1.23 0.274 -0.33

Hf 0.86 0.74 0.62 2.20 1.01 0.72 0.64 0.29 0.34 0.26 0.42 0.49 4.34 0.40 4.36 -0.46

Ta 0.09 0.09 0.08 0.24 0.08 0.09 0.07 0.04 0.03 0.03 0.03 0.04 1.09 1.34 1.14 -4.74

Pb 0.84 3.65 8.46 13.7 1.45 16.8 2.07 7.06 10.6 0.65 2.23 2.37 1.64 7.29 1.60 2.79

Th 0.19 0.09 0.41 1.50 0.04 1.23 0.28 0.12 0.12 0.18 0.20 0.05 1.12 0.74 1.22 -8.30

U 0.08 0.10 0.09 0.24 0.12 0.25 0.07 0.10 0.03 0.05 0.05 0.02 0.391 2.59 0.403 -2.86

Nb ⁄Ta 19.2 17.1 16.4 16.5 17.1 16.1 17.1 17.8 14.0 16.3 18.7 16.3

Zr ⁄Hf 53.6 40.4 46.9 56.4 46.4 51.5 50.6 33.4 35.9 43.8 43.3 49.2

Total REE 29.1 16.9 37.8 119.4 19.6 100.0 37.7 12.9 26.5 28.3 22.7 15.6

Eu ⁄Eu* 1.02 1.33 1.14 0.99 1.32 1.08 1.28 1.46 1.09 1.22 1.17 1.10

(La ⁄Yb)N 0.88 0.32 1.25 2.07 0.11 6.38 3.55 0.19 1.33 2.02 1.64 1.04

RSD denotes relative standard deviation, while RE denotes the error between measured and recommended values. Recommended values (Rec.) are the preferred values from GeoRem (Jochum

& Nehring, 2006): http: ⁄ ⁄ georem.mpch-mainz.gwdg.de ⁄ sample_query_pref.asp. EAV means eclogites away from veins. Total Fe as Fe2O3; Mg# = 100*Mg ⁄ (Mg + SFe); BD = below

detection limit. Eu ⁄Eu* = EuN ⁄ (SmN · GdN)0.5, where subscript N for each element means normalized by the chondrite values from Sun & McDonough (1989).

8 28 J . H U A N G E T A L .

� 2012 Blackwell Publishing Ltd

show mineral assemblages and petrological character-istics similar to those of the EAVs, but eclogite 08bxl11was subjected to severe retrogression and mainly con-sists of symplectite (�90 vol.%) with minor garnet,omphacite and rutile. Garnet from the ECVs consistsof pyrope (10.9–31.6%), grossular (20.7–34.9%),almandine (34.2–53.8%) and spessartine (0.62–2.95%).The jadeite components of omphacite from the ECVsvary from 34.2% to 50.0%.

The veins have different mineral assemblages andhighly variable modal concentrations. In addition, thedistribution of minerals in the veins is highly heteroge-neous (Table 2). Vein sample 08bxl03v predominantlyconsists of quartz with only one large rutile crystal of�1.0 · 0.45 cm in size and minor phengite (Fig. 2a,b).Vein sample 08bxl07v has only one relatively small rutilegrain, 0.30 cm long and 0.25 cm wide. The clinozoisite-quartz vein (08bxl09v) consists of clinozoisite +epidote + rutile + quartz + phengite + clinopyrox-ene + minor chlorite and biotite. Abundant rutile,clinozoisite, epidote occur as very large crystals(Fig. 2c,d). Minor retrograde minerals including chlo-rite and biotite imply that the vein was subjected toretrograde metamorphism during exhumation. Tworutile grains with sizes of �0.08 · 0.10 cm and�0.35 · 0.50 cm in the vein have been analysed(Fig. 2c). As shown in Table 3, clinopyroxene from veinsample 08bxl09v has relatively low SiO2 (43.3 wt%),CaO (10.4 wt%) and Na2O (2.53 wt%) but high FeO(16.9 wt%) and K2O (0.943 wt%) contents. Phengitehas average K2O and Na2O contents of 9.8 and0.71 wt%, respectively, and an average Si-contentcluster of 3.21 pfu. The Ps content [100 · Fe3 +⁄ (Al + Fe3 + )] of coarse-grained clinozoisite variesfrom 6.7 to 10 with an average value of 8.7. The epidote-quartz vein (08bxl11v) mainly consists of epidote,

quartz, rutile and albite with minor biotite, chloriteand titanite. The average composition of epidotefrom the vein is similar to that from the host eclogite08bxl11 (Table 3). Albite has an average compositionof Ab0.975An0.019Or0.006. One coarse-grained rutile inthe vein up to �1.4 · 1.0 cm in size was analysed. Itis noted that all the four veins contain large rutilegrains which should form at pressures of >1.5 GParevealed by experimental studies (Liou et al., 1998;Xiong et al., 2005). However, they do not containtypical HP eclogite facies minerals such as garnetand omphacite. The occurrence of coarse-grainedminerals in the veins (e.g. epidote and rutile), withsizes distinctly larger than those in the host eclogites,indicates that they were precipitated in situ from thevein-forming fluids rather than being mechanicallytransported from the host eclogites (Gao & Klemd,2001; Spandler & Hermann, 2006).

Nb and Ta in EAV eclogites

The major and trace element data of the EAV eclogitesare presented in Table 4. Only the EAVs were selectedfor whole rock analysis, because the amount of avail-able ECV and vein material is usually insufficient forsuch analysis due to sampling difficulties in the field. Inaddition, the highly heterogeneous mineral distribu-tions and coarse-grained mineral size in the veins im-pede obtaining a representative bulk composition.Anyway, Nb and Ta are dominantly hosted by rutile,as indicated by the observation that Nb ⁄Ta ratios ofthe EAVs show no systematic difference from those ofrutile grains within them (Fig. 3a). Therefore, weconsider that the absence of the bulk rock data of theveins and ECVs does not hamper a straightforwardinterpretation and discussion of the results.

OIBMORB

(a)

OIB

Chondriticratio = 19.9

Dabie eclogite (S09)

Ave. MORB ratio = 14.2

(b)

Gabbros from Yangtze Block

Continental basalt

Chondritic ratio = 19.9

Ave. MORB ratio = 14.2

Gabbros from Yangtze Block

Ave.gabbroNb/Ta = 16.2 Ave.gabbro

Nb/Ta = 16.2

MORB

08bxl12

08bxl13

08bxl14

08bxl15

08bxl16

08bxl18

08bxl01

08bxl02

08bxl04

08bxl05

08bxl06

08bxl080

5

10

15

20

25

30Bulk rockRutile

Nb/

Ta

0.1 1 10 1000

5

10

15

20

25

30Bulk rock

Nb/

Ta

Nb (ppm)

Fig. 3. (a) Comparison of average Nb ⁄Ta values of rutile with Nb ⁄Ta in bulk rock; (b) Plot of Nb (ppm) v. Nb ⁄Ta for the EAVeclogites. For comparison, also shown are data for chondrite, MORBs, OIBs and continental basalts (data from Munker et al., 2003;Pfander et al., 2007, 2012). Data for gabbros (inferred protolith for the investigated eclogites) from the Yangtze Block and the Dabieeclogites determined using the ID method are also displayed (data from Ling et al., 2003; Zhao & Zhou, 2007; Schmidt et al., 2009,Dong et al., 2011). S09 denotes Schmidt et al. (2009).

Nb – T a F R A C T I O N AT I O N I N D U C E D B Y F L UI D- R O CK I N TE R AC T I O N 82 9

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Nb and Ta concentrations in the EAVs range from0.42 to 3.97 ppm and 0.03 to 0.24 ppm, respectively.Most of the EAVs have a limited range of Nb ⁄Tavalues from 16.1 to 19.2, which is higher than those ofMORB (11–16, Munker et al., 2003), except for sample08bxl14 that has slightly lower Nb ⁄Ta (14.0) (Fig. 3).For comparison, Fig. 3b also shows the fields forMORBs (Munker et al., 2003), OIBs (Pfander et al.,2007), continental basalts (Pfander et al., 2012), DabieUHP eclogites analysed by high-precision ID method(Schmidt et al., 2009) and gabbros from the YangtzeBlock (Ling et al., 2003; Zhao & Zhou, 2007; Dong etal., 2011). All but one eclogite plot out of the MORBfield, but most plot within the field for gabbros fromthe Yangtze Block.

Nb and Ta in rutile

HFSE concentrations in rutile from the eclogites andveins are given in Table 5. Rutile from the EAVs hasNb ⁄Ta ratios ranging from 12.7 to 25.3, with Nb andTa concentrations ranging from 19.2 to 179 ppm and1.12 to 11.9 ppm, respectively (Fig. 4a). It is note-worthy that Nb ⁄Ta ratios show much larger variationsamong different individual rutile grains (12.7–25.3)than in the bulk host EAVs (14.0–19.2), indicating thatrutile can fractionate Nb from Ta in a mineral-scale,consistent with the observations of Kalfoun et al.(2002) and Schmidt et al. (2009). Nevertheless, theaverage Nb ⁄Ta of rutile is similar to that in the cor-responding bulk host EAVs (Fig. 3a), implying that itdominates the Nb–Ta budget of eclogite, consistentwith previous studies (Rudnick et al., 2000; Zack et al.,2002; Spandler et al., 2003; Schmidt et al., 2009).

Rutile from the ECVs shows a much larger spread inboth Nb ⁄Ta ratios (17.8–49.8) and Nb–Ta concentra-tions (Nb: 99.5–503 ppm, Ta: 2.04–14.4 ppm). Differ-ent rutile grains within the same ECV also showrelatively large Nb ⁄Ta variations (Fig. 4). Similarly,the vein rutile has highly variable Nb ⁄Ta of 10.9–29.2,with Nb and Ta concentrations from 155 to 2021 ppmand 9.02 to 151 ppm, respectively (Table 5). In general,rutile from the vein samples 08bxl03v and 08bxl11v haslower Nb ⁄Ta ratios and higher Nb–Ta concentrationsthan that from their host eclogites (Fig. 4b,d), whilerutile from the vein sample 08bxl07v displays bothlower Nb ⁄Ta ratios and Nb–Ta concentrations relativeto that from the ECV sample 08bxl07 (Fig. 4c).

Figure 5 shows rim-core-rim profile analyses on fourrutile grains from the EAV sample 08bxl02 and ECVsample 08bxl03. Generally, grains from the EAV havenearly invariable Nb ⁄Ta ratios and Nb–Ta concen-trations, whereas grains from the ECV display distinctrim-core-rim zoning patterns in Nb ⁄Ta ratios, withlow ratios in the rims and higher ratios in the cores(Fig. 5). Apparently, the Nb ⁄Ta zoning is mainlycaused by significant rim-ward increase in Ta concen-trations (Fig. 5). In contrast, rutile from the veinsamples 08bxl03v and 08bxl09v shows lower Nb ⁄Ta

ratios in the cores than in the rims (Fig. 6). The com-plementary zoning patterns of Nb ⁄Ta between thevein- and ECV-hosted rutile suggest that fluidsresponsible for the HP vein-formation are most likelyfrom the immediate hosts.Interestingly, the average Nb ⁄Ta ratios of rutile from

the EAVs throughout the �2 km Bixiling profile arerelatively homogeneous, ranging only from 15.2 to 20.4(Fig. 7) and are consistent with bulk rock Nb–Ta data(14.0–19.2). However, the average Nb ⁄Ta ratios ofrutile from the ECV samples 08bxl03 and 08bxl11 areof 34.4 and 36.4, respectively, much higher than thoseof rutile from the related veins (16.3–20.9, Fig. 7).

Quantitative modelling

To decipher the relative compositional evolution ofaqueous fluids released by dehydration of amphiboliteat the early stage of subduction to supercritical fluidsat UHP conditions in the absence or presence of rutile,the evolution was modelled in three stages: (I) aqueousfluids released by dehydration of amphibolite in theabsence of rutile; (II) aqueous fluids equilibrated withresidual eclogite in the presence of rutile. In this stage,the pressure is thought to be >1.5 GPa (the lowestpressure for rutile stability, Liou et al., 1998; Xionget al., 2005), but <3.4 GPa (the second critical end-point in the basalt-H2O system, Mibe et al., 2011).Thus, the aqueous fluids are not at supercritical con-ditions; and (III) supercritical fluids equilibrated withresidual eclogite in the presence of rutile. In this stage,the pressure is assumed to be >3.4 GPa so that thefluid phase becomes supercritical.Previous studies have documented that the protolith

of the Bixiling eclogites is gabbro cumulate thatformed by differentiation of mantle-derived basalticmagmas (Zhang et al., 1995; Chavagnac & Jahn, 1996;Tang et al., 2007). In addition, zircon U–Pb dating forthe Bixiling metaigneous rocks, including eclogite,gneiss and peridotite, yields protolithic ages of 760–720 Ma (Cheng et al., 2000; Chavagnac et al., 2001;Zheng et al., 2004, 2008), similar to those obtained forUHP metaigneous rocks from other areas in the Da-bie-Sulu orogenic belt (Zheng, 2008 and referencestherein). Therefore, the Neoproterozoic (c. 750 Ma)gabbro cumulate from the Yangtze Block could be theequivalent protolith of the studied eclogites (Linget al., 2003; Zhao & Zhou, 2007; Dong et al., 2011).Hence, the average Nb ⁄Ta ratio of the inferred pro-tolith gabbros was used as starting composition(16.2 ± 0.6, 2 SE n = 78, Ling et al., 2003; Zhao &Zhou, 2007; Dong et al., 2011). A modal compositionof 50% amphibole, 40% plagioclase, 5% garnet and5% quartz for the amphibolite, and 49.5% garnet,49.5% clinopyroxene and 1% rutile for the eclogitewas assumed. Dehydration of amphibolite (stage I)was modelled by a fractional process, and the equationfor modal fractionation is from Shaw (1970):

8 30 J . H U A N G E T A L .

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Table 5. High field strength element concentrations in rutile from the studied eclogites and veins.

Eclogite Sample

Rutile Zr Nb Hf Ta Nb ⁄Ta Zr ⁄Hf Rutile Zr Nb Hf Ta Nb ⁄Ta Zr ⁄Hf Rutile Zr Nb Hf Ta Nb ⁄Ta Zr ⁄Hf

08bxl01

rt1 118 156 6.34 9.08 17.2 18.5 rt2 120 152 5.12 7.39 20.5 23.5 rt8 136 24.1 6.04 1.44 16.8 22.5

rt2 94.0 159 4.65 9.21 17.3 20.2 rt3 112 147 3.74 8.08 18.1 29.8 rt9 99.3 21.3 4.45 1.12 19.0 22.3

rt3 82.4 164 4.50 10.9 15.1 18.3 rt4 103 148 3.93 7.44 20.0 26.2 rt10 123 22.4 5.03 1.43 15.7 24.4

rt4 115 153 6.12 9.06 16.8 18.8 rt5 102 146 3.96 8.68 16.9 25.7 Mean 114 21.3 5.09 1.42 15.2 22.4

rt5 119 161 6.53 9.37 17.2 18.3 rt6 108 145 3.68 7.21 20.1 29.4 08bxl14

rt6 112 155 6.26 9.07 17.1 17.9 rt7 105 143 3.82 8.79 16.2 27.4 rt1 117 51.2 7.29 2.87 17.8 16.0

rt7 123 157 6.33 9.62 16.3 19.4 rt8 105 151 4.24 8.08 18.6 24.7 rt2 115 50.1 6.93 3.15 15.9 16.6

rt8 91.2 153 5.14 8.97 17.0 17.8 rt9 105 146 4.07 7.45 19.5 25.7 rt3 123 51.7 6.81 3.09 16.7 18.1

Mean 107 157 5.73 9.41 16.7 18.7 Mean 109 147 4.17 7.85 18.8 26.4 rt4 95.4 50.4 5.20 3.18 15.8 18.3

08bxl02 08bxl06 rt5 117 50.8 7.49 2.81 18.1 15.6

rt1 104 48.4 4.59 2.60 18.7 22.6 rt1 97.1 41.6 5.16 2.70 15.4 18.8 rt6(R) 101 51.7 5.35 2.89 17.9 18.9

rt2 134 47.3 5.94 2.21 21.4 22.6 rt2 83.8 43.0 4.48 2.29 18.8 18.7 rt6(C) 122 50.7 6.27 2.88 17.6 19.4

rt3 117 47.5 5.52 2.54 18.7 21.3 rt3 101 41.1 4.69 2.57 16.0 21.6 rt7(R) 97.9 51.6 5.97 2.85 18.1 16.4

rt4 109 48.0 4.55 2.47 19.4 23.9 rt4 103 41.5 5.33 2.41 17.2 19.3 rt7(C) 101 49.8 6.42 2.71 18.4 15.7

rt5 100 48.2 4.31 2.18 22.1 23.3 rt5 106 43.1 4.89 2.66 16.2 21.8 Mean 110 50.9 6.42 2.94 17.4 17.2

rt6 99.9 50.4 4.82 2.57 19.6 20.7 rt6 112 42.3 6.00 2.68 15.8 18.6 08bxl15

rt7 108 51.0 4.82 2.66 19.2 22.4 rt7 111 43.3 5.27 2.40 18.1 21.1 rt1 117 61.6 5.99 3.20 19.2 19.6

rt8 127 48.0 5.90 2.41 19.9 21.6 rt8 110 42.2 5.68 2.60 16.2 19.4 rt2 128 61.9 6.30 3.21 19.3 20.4

rt9–1 71.6 49.5 2.96 2.53 19.6 24.2 rt9 118 43.7 6.04 2.44 17.9 19.6 rt3 142 61.5 6.96 3.42 18.0 20.4

rt9–2 76.1 48.4 3.08 2.77 17.4 24.7 rt10 108 42.0 5.43 2.67 15.7 19.8 rt4 130 61.7 6.24 3.16 19.5 20.8

rt9–3 83.7 47.2 3.63 2.28 20.7 23.1 Mean 105 42.4 5.30 2.54 16.7 19.9 rt5 129 60.9 6.25 3.49 17.4 20.7

rt9–4 91.0 55.0 4.19 2.37 23.2 21.7 08bxl07 rt6 119 62.0 6.26 3.17 19.5 19.0

rt9–5 98.2 51.2 4.34 2.81 18.2 22.6 rt1 126 177 6.00 8.04 22.0 21.0 rt7 107 60.4 5.99 3.30 18.3 17.8

rt9–6 99.2 46.6 3.72 2.45 19.0 26.6 rt2 105 187 5.51 9.33 20.1 19.0 rt8 92.7 59.7 4.94 3.21 18.6 18.8

rt10–1 97.3 50.5 5.33 2.72 18.5 18.2 rt3 102 174 4.31 9.63 18.1 23.7 rt9 129 59.9 6.76 3.33 18.0 19.0

rt10–2 104 48.3 5.99 2.04 23.7 17.3 rt4 96.4 178 5.06 8.68 20.5 19.1 rt10 99.3 60.4 5.46 3.43 17.6 18.2

rt10–3 94.2 50.0 5.19 2.04 24.5 18.1 rt5 101 170 5.39 8.63 19.7 18.8 Mean 119 61.0 6.11 3.29 18.6 19.5

rt10–4 90.7 49.2 4.02 1.94 25.3 22.6 rt6 105 173 5.92 8.19 21.2 17.8 08bxl16

rt10–5 87.6 47.0 3.02 2.39 19.6 29.0 rt7 89.2 175 4.62 9.44 18.5 19.3 rt1 89.2 83.1 4.45 4.22 19.7 20.0

rt10–6 84.1 44.4 4.52 2.41 18.4 18.6 rt8 85.6 174 4.39 9.75 17.8 19.5 rt2 107 87.2 5.85 4.48 19.5 18.3

Mean 98.9 48.8 4.5 2.4 20.4 22.3 rt9 96.9 171 3.95 8.87 19.2 24.5 rt3 86.3 84.8 3.66 4.99 17.0 23.6

08bxl03 rt10 121 176 6.18 7.98 22.1 19.6 rt4 121 87.5 5.78 5.45 16.1 20.9

rt1 115 113 5.48 4.29 26.3 20.9 rt11 109 175 5.78 8.93 19.6 19.0 rt5 93.9 87.8 5.11 5.07 17.3 18.4

rt2 120 111 5.93 3.92 28.2 20.3 rt12 115 179 5.95 8.86 20.2 19.3 rt6 91.4 88.0 4.83 4.39 20.0 18.9

rt3 94.5 111 4.82 4.59 24.2 19.6 rt13 128 181 5.98 8.13 22.3 21.4 rt7 81.9 76.1 4.52 4.42 17.2 18.1

rt4 119 108 5.95 3.10 35.0 19.9 rt14 127 178 6.28 8.63 20.7 20.2 rt8 75.9 76.0 3.72 4.52 16.8 20.4

rt5 103 106 4.98 3.49 30.5 20.7 rt15 127 182 5.98 8.23 22.1 21.2 Mean 93.3 83.8 4.74 4.69 17.9 19.8

rt6 86.3 115 3.96 3.61 31.7 21.8 Mean 109 177 5.42 8.75 20.3 20.2

rt7 117 108 5.81 2.98 36.2 20.1 08bxl08

rt8 95.3 111 4.90 4.34 25.5 19.5 rt1 118 136 5.29 8.78 15.4 22.4

rt9 122 111 5.94 2.95 37.5 20.6 rt2 80.6 133 4.49 8.07 16.5 17.9

rt10 94.2 111 4.92 3.70 30.1 19.2 rt3 113 136 5.93 8.47 16.1 19.1

rt11 105 110 5.69 3.61 30.5 18.5 rt4 90.3 139 3.98 9.08 15.3 22.7

rt12–1 83.2 100 4.40 3.13 32.0 18.9 rt5 101 134 5.53 8.31 16.1 18.2

rt12–2 87.6 102 5.66 2.16 47.3 15.5 rt6 86.8 133 5.15 9.05 14.7 16.9

rt12–3 85.3 103 4.32 2.81 36.6 19.7 rt7 97.3 133 5.52 8.06 16.5 17.6

rt12–4 92.7 99.5 5.21 3.05 32.6 17.8 rt8 83.9 136 4.73 8.64 15.7 17.7

rt12–5 99.4 106 5.63 3.26 32.6 17.7 rt9 108 137 5.87 8.41 16.3 18.4

rt13–1 88.3 107 3.80 5.00 21.5 23.2 Mean 97.8 135 5.17 8.54 15.9 19.0

rt13–2 82.4 104 3.64 3.50 29.7 22.7 08bxl11

rt13–3 73.7 99.5 3.42 2.66 37.4 21.6 rt1 95.7 445 4.49 13.5 32.9 21.3

rt13–4 74.7 100 4.02 2.16 46.5 18.6 rt2 91.8 503 4.70 14.4 34.9 19.5

rt13–5 82.8 101 4.09 2.04 49.4 20.2 rt3 79.8 472 4.29 11.4 41.6 18.6

rt13–6 79.2 102 4.39 2.30 44.2 18.0 Mean 89.1 473 4.49 13.1 36.4 19.8

rt13–7 83.5 107 4.33 3.63 29.6 19.3 08bxl12

rt13–8 86.5 116 3.64 4.17 27.9 23.8 rt1 80.0 169 3.43 9.78 17.3 23.3

rt14(R) 91.0 109 3.75 3.41 31.9 24.3 rt2 85.0 168 4.21 9.36 17.9 20.2

rt14(M) 86.7 111 3.64 2.46 45.0 23.8 rt3 86.4 169 3.72 9.63 17.5 23.2

rt14(C) 89.9 114 3.14 2.30 49.8 28.6 rt4 90.8 170 3.96 8.92 19.1 22.9

Mean 94.0 107 4.65 3.28 34.4 20.5 rt5 90.0 179 3.93 11.9 15.1 22.9

08bxl04 rt6 121 167 5.43 9.96 16.8 22.3

rt1 115 45.8 6.20 3.04 15.0 18.6 rt7 92.2 170 4.92 10.2 16.6 18.8

rt2 107 47.8 5.92 3.32 14.4 18.0 rt8 83.5 143 4.18 8.74 16.3 20.0

rt3 94.4 46.8 4.35 2.81 16.6 21.7 rt9 93.5 143 4.91 7.97 18.0 19.1

rt4 96.6 47.1 4.86 2.89 16.3 19.9 Mean 91.4 164 4.30 9.60 17.2 21.4

rt5 117 47.6 6.13 3.23 14.7 19.1 08bxl13

rt6 106 47.5 5.81 2.91 16.3 18.2 rt1 107 19.2 4.64 1.52 12.7 23.1

rt7 101 50.2 4.87 3.52 14.3 20.8 rt2 107 21.5 4.48 1.38 15.5 23.8

rt8 107 49.1 5.08 3.28 15.0 21.1 rt3 132 19.3 6.28 1.30 14.9 21.1

Mean 106 47.7 5.40 3.12 15.3 19.7 rt4 101 20.4 4.59 1.59 12.8 21.9

08bxl05 rt5 113 20.9 5.54 1.47 14.2 20.4

rt1 123 148 4.94 7.52 19.6 24.9 rt6 108 23.1 4.63 1.50 15.4 23.3

rt7 114 20.9 5.27 1.42 14.7 21.6

Nb – T a F R A C T I O N AT I O N I N D U C E D B Y F L UI D- R O CK I N TE R AC T I O N 83 1

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Table 5. (Continued)

Vein Sample

Rutile Zr Nb Hf Ta Nb ⁄Ta Zr ⁄Hf Rutile Zr Nb Hf Ta Nb ⁄Ta Zr ⁄Hf Rutile Zr Nb Hf Ta Nb ⁄Ta Zr ⁄Hf

08bxl03v

)1 62.4 254 2.77 12.6 20.1 22.5 rt1-17 98.3 186 4.31 13.4 13.9 22.8 M5 118 545 4.85 26.6 20.5 24.4

)2 62.3 255 3.00 12.3 20.7 20.8 rt1-18 97.6 177 4.41 13.1 13.5 22.1 M6 113 701 4.83 29.2 24.0 23.3

)3 62.6 251 2.97 12.4 20.2 21.1 rt1-19 100 177 4.39 13.3 13.3 22.8 M7 116 1047 5.00 71.1 14.7 23.2

)4 65.0 245 3.05 13.0 18.9 21.3 rt1-20 99.1 191 4.32 13.5 14.2 22.9 M8 115 813 4.88 48.5 16.7 23.5

)5 66.9 245 2.92 13.0 18.9 22.9 rt1-21 96.6 223 4.36 15.0 14.8 22.1 M9 116 557 4.69 26.7 20.9 24.6

)6 67.2 248 3.25 15.3 16.2 20.6 rt1-22 104 219 4.39 13.4 16.4 23.6 M10 115 528 4.44 26.5 19.9 26.0

)7 73.9 239 3.33 12.3 19.5 22.2 rt2-1 99.3 225 4.19 13.8 16.2 23.7 M11 116 488 4.52 24.3 20.1 25.7

)8 71.4 228 3.14 11.3 20.1 22.8 rt2-2 102 196 4.47 17.2 11.4 22.8 M12 117 524 4.75 24.0 21.9 24.6

)9 74.6 252 3.32 15.9 15.9 22.5 rt2-3 108 191 4.85 17.5 10.9 22.3 M13 116 1193 5.20 81.4 14.7 22.2

)10 72.1 261 3.45 16.2 16.1 20.9 rt2-4 99.4 222 4.23 15.1 14.7 23.5 M14 108 976 4.73 57.4 17.0 22.9

)11 66.3 227 3.21 12.2 18.6 20.6 rt2-5 113 223 4.52 14.1 15.8 25.0 M15 100 1810 4.33 151.2 12.0 23.1

)12 67.3 225 3.02 12.0 18.8 22.3 Mean 103 214 4.54 15.5 14.0 22.8 C1 115 503 4.69 25.5 19.8 24.5

)13 65.0 244 2.96 12.6 19.3 22.0 08bxl11v C2 119 507 4.69 25.5 19.9 25.3

)14 67.0 248 3.00 13.0 19.1 22.3 )1 106 761 4.60 27.0 28.2 23.0 C3 121 499 4.70 24.6 20.3 25.7

)15 66.1 249 3.01 13.0 19.2 21.9 )2 97.7 479 4.51 22.3 21.5 21.7 C4 118 497 4.62 24.3 20.5 25.5

)16 63.1 250 2.89 12.3 20.3 21.8 )3 109 466 4.39 22.2 21.0 24.8 Mean 110 881 4.78 49.6 19.4 23.1

)17 61.5 248 2.79 12.2 20.3 22.0 )4 110 461 4.33 22.0 20.9 25.5

)18 61.6 258 2.83 12.4 20.8 21.8 )5 111 479 4.58 22.5 21.3 24.3

)19 63.3 261 2.95 11.7 22.3 21.5 )6 108 460 4.49 22.3 20.6 24.1

)20 65.6 263 3.06 11.3 23.3 21.5 )7 110 467 4.36 22.4 20.9 25.3

)21 64.2 260 3.02 11.8 22.1 21.3 )8 112 465 4.37 22.4 20.8 25.6

)22 65.5 261 2.96 12.3 21.2 22.1 )9 112 478 4.63 23.1 20.7 24.2

R1 76.0 257 3.15 10.7 24.0 24.1 )10 111 471 4.55 22.6 20.8 24.3

R2 67.9 261 3.03 10.9 23.9 22.4 )11 110 479 4.68 23.2 20.6 23.5

R3 66.5 260 3.01 11.0 23.6 22.1 )12 109 471 4.52 23.0 20.4 24.2

R4 63.0 256 2.88 12.5 20.5 21.9 )13 111 476 4.47 22.7 20.9 24.9

R5 63.0 258 2.82 12.9 20.0 22.4 )14 110 473 4.51 23.0 20.6 24.3

R6 64.3 257 2.89 12.8 20.1 22.3 )15 110 476 4.47 22.7 21.0 24.5

M1 64.3 262 2.94 11.8 22.2 21.9 )16 110 498 4.47 23.9 20.9 24.5

M2 65.0 255 3.01 11.1 23.1 21.6 )17 113 483 4.47 23.4 20.6 25.3

M3 67.4 257 3.10 11.3 22.7 21.7 )18 112 477 4.47 23.5 20.3 25.0

M4 63.7 253 3.00 11.4 22.3 21.3 )19 112 480 4.51 23.6 20.3 24.8

M5 61.6 260 2.84 10.6 24.5 21.7 )20 113 485 4.63 24.0 20.2 24.3

M6 60.7 255 2.88 10.6 24.0 21.0 )21 114 497 4.77 24.3 20.4 23.9

M7 61.5 248 2.79 12.2 20.3 22.0 )22 114 489 4.58 24.1 20.3 24.9

C1 61.3 256 2.93 11.4 22.5 20.9 )23 113 494 4.58 24.0 20.6 24.6

C2 62.3 252 3.06 10.5 24.1 20.4 )24 109 479 4.47 23.9 20.0 24.3

C3 62.2 253 2.78 10.5 24.2 22.3 )25 111 482 4.48 24.2 19.9 24.8

Mean 65.4 252 3.0 12.2 20.9 21.8 )26 111 504 4.49 25.2 20.0 24.7

08bxl07v )27 105 800 4.68 37.7 21.2 22.4

)1 90.6 164 4.36 10.5 15.6 20.8 )28 98.6 1184 4.80 53.6 22.1 20.5

)2 92.7 165 4.51 10.5 15.7 20.6 )29 92.0 936 4.08 73.0 12.8 22.6

)3 95.5 167 4.68 10.1 16.5 20.4 )30 95.9 720 4.02 42.6 16.9 23.9

)4 98.6 170 4.47 10.3 16.5 22.1 )31 96.4 823 4.32 39.5 20.8 22.3

)5 101 172 4.67 10.6 16.3 21.6 )32 99.2 1239 4.72 83.5 14.8 21.0

)6 101 168 4.46 10.3 16.3 22.8 )33 99.8 1145 4.43 75.0 15.3 22.5

)7 103 171 4.47 10.9 15.7 23.1 )34 101 1093 4.70 69.3 15.8 21.5

)8 101 167 4.56 10.3 16.2 22.1 )35 101 755 4.42 30.7 24.6 22.9

)9 102 169 4.04 10.5 16.1 25.1 )36 103 1106 4.92 59.4 18.6 21.0

)10 103 168 4.48 10.5 15.9 23.0 )37 113 1592 5.51 103 15.5 20.4

)11 101 171 4.44 10.3 16.6 22.8 )38 111 1658 5.21 101 16.4 21.4

)12 100 162 4.61 10.3 15.7 21.7 )39 114 1828 5.53 101 18.1 20.6

)13 98.3 160 4.31 9.8 16.2 22.8 )40 115 1945 5.79 106 18.3 19.9

)14 99.2 157 4.78 9.0 17.5 20.8 )41 113 1953 5.69 119 16.4 19.9

)15 85.9 155 4.72 9.8 15.8 18.2 )42 114 1979 5.82 122 16.2 19.6

)16 85.1 160 3.89 9.5 16.9 21.9 )43 115 1964 5.95 120 16.4 19.3

R 89.2 158 4.43 9.4 16.9 20.1 )44 114 1981 5.71 120 16.5 19.9

Mean 96.9 165 4.46 10.1 16.3 21.7 )45 114 2006 5.59 122 16.5 20.5

08bxl09v )46 114 2021 5.87 123 16.4 19.4

rt1-1 103 214 4.40 13.9 15.4 23.3 )47 111 1958 6.13 122 16.1 18.2

rt1-2 98.4 234 4.21 14.4 16.3 23.4 )48 109 1956 5.86 123 15.9 18.6

rt1-3 96.7 249 4.39 14.9 16.7 22.0 )49 105 1936 5.84 123 15.8 17.9

rt1-4 101 256 4.46 16.7 15.4 22.6 )50 94.5 1827 5.25 124 14.7 18.0

rt1-5 103 254 4.89 20.8 12.2 21.2 )51 90.1 541 3.55 27.6 19.6 25.4

rt1-6 105 250 4.66 20.7 12.1 22.6 )52 84.5 347 4.06 15.3 22.7 20.8

rt1-7 103 246 4.71 20.3 12.1 22.0 R1 100 481 4.34 23.4 20.6 23.1

rt1-8 102 240 4.84 20.8 11.5 21.1 R2 112 485 4.64 24.3 19.9 24.0

rt1-9 105 234 4.81 18.4 12.7 21.7 R3 114 490 4.56 23.7 20.6 25.0

rt1-10 107 204 4.74 14.3 14.3 22.6 R4 106 1102 4.81 37.8 29.2 22.1

rt1-11 109 201 4.69 14.2 14.2 23.2 R5 126 749 5.12 39.1 19.2 24.5

rt1-12 109 200 4.69 14.0 14.3 23.2 R6 127 757 5.17 39.5 19.1 24.6

rt1-13 109 189 4.76 13.9 13.6 22.9 M1 114 498 4.74 24.6 20.2 24.0

rt1-14 111 186 4.80 14.0 13.2 23.1 M2 117 908 4.95 47.1 19.3 23.7

rt1-15 107 191 4.71 14.0 13.6 22.8 M3 116 588 4.73 25.7 22.9 24.6

rt1-16 101 195 4.30 13.8 14.2 23.6 M4 117 534 4.61 26.1 20.4 25.4

Only 08bxl03, 07, 11 are eclogites close to veins, the others are eclogites away from veins. rt = rutile. C, M and R in parentheses represent the geographic core, the middle zone and the rim of rutile, respectively. rt-NO.1-

(NO.2): NO.1 means the number of rutile grains analysed, whereas NO.2 indicates rim-core-rim profile analyses. Continuous numbers (1, 2, 3…) for rutile from the vein samples also indicate rim-core-rim profile analyses.

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CL=C0 ¼ ð1=D0Þ � ð1� FÞðð1=D0Þ�1Þ ð1ÞCL is the element concentration in the fluids, C0 is theaverage element concentration in the starting mate-rial, D0 is the bulk partition coefficient betweenamphibolite and aqueous fluids, and F is the degreeof dehydration (percentage of water loss). The com-positions of the aqueous fluids in stage II and thesupercritical fluids in stage III were modelled assum-ing equilibrium between the fluid phases and residualeclogite:

CLðNb=Ta;stages II&III)¼CSðNb=Ta, stage IÞ=DNb=TaðbulkÞ

ð2Þ

CS (Nb ⁄Ta, stage I), derived from D0*CL (Nb ⁄Ta,stage I), is the remaining element concentration in thesolid residue after dehydration during stage I. DNb ⁄ Ta

(bulk) represents bulk DNb (bulk) over DTa (bulk) in the

residue. Comparison of HP veins with their hostwallrocks in natural systems has shown thatDrutile ⁄ aqueous fluids is greater for Nb than for Ta, andthus Ta preferentially enters into vein-forming aqueousfluids (Kamber & Collerson, 2000; Zhang et al., 2008).This is consistent with several experimental resultswhere DNb ⁄DTa > 1 for rutile ⁄ aqueous fluids wasobserved (Brenan et al., 1994) or in other words thatcompared with Ta, Nb has a lower solubility inaqueous fluids (e.g. Linnen & Keppler, 1997). Al-though Green & Adam (2003) observed that DNb ⁄DTa

for rutile ⁄ aqueous fluids is <1, they pointed out thatthe analysis of quenched solute may only reflect therelative concentration of trace elements in the originalfluids; variable porosity of the quench solute may leadto significant variation in absolute trace element con-tents, so that absolute values for Drutile ⁄ aqueous fluids

were not tightly constrained. In view of these obser-vations, partition coefficients of rutile ⁄ aqueous fluidsfor Nb and Ta (DNb ⁄DTa > 1) from Brenan et al.

Nb (ppm) Nb (ppm)

Nb (ppm)Nb (ppm)10 100 1000

0

10

20

30

40

50

60

19.9

Nb/

Ta

EAVECV

(a)

Nb/

Ta

19.9

0 50 100 150 200 250 3000

10

20

30

40

50

6008bxl03 (ECV)08bxl03v (Vein)

(b)

140 160 180 200 22010

15

20

25

3008bxl07 (ECV)08bxl07v (Vein)

(c)

19.9

0 500 1000 1500 2000 25000

10

20

30

40

50

6008bxl11 (ECV)08bxl11v (Vein)

(d)

19.9

Fig. 4. Plots of Nb (ppm) v. Nb ⁄Ta for rutile. (a) All rutile grains from both the EAVs and ECVs. It is noted that Nb ⁄Ta ratios ofrutile from the ECVs show relatively larger variations compared with those of rutile from the EAVs; (b–d) Comparison of rutilefrom the veins and their host eclogites (ECVs). Rutile from vein samples 08bxl03v and 08bxl11v has lower Nb ⁄Ta ratios and higherNb–Ta concentrations compared with rutile from the related host eclogites, while rutile from vein sample 08bxl07v has both lowerNb ⁄Ta ratios and lower Nb–Ta concentrations. Dash dot lines denote the revised chondritic Nb ⁄Ta ratio (Munker et al., 2003).

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(1994) were chosen for the calculations. Min-eral ⁄ aqueous fluid partition coefficients for garnet andclinopyroxene were taken from experimental data ofStalder et al. (1998). Amphibole ⁄ aqueous fluid parti-tion coefficient for Nb was chosen from Brenan et al.(1995). Because no amphibole ⁄ aqueous fluid partitioncoefficients for Ta are available, we assumed that thepartition coefficient of Ta is equal to that of clinopy-roxene (Stalder et al., 1998), based on the observation

from Xiong (2006) that crystal ⁄melt partition coeffi-cients for the two minerals are actually the same interms of Ta. Thus, the Damphibole ⁄ clinopyroxene ratio ofNb v. Ta is �6.5 (Table 6), within the range of 2.5–8.0in metabasaltic rocks (e.g. Xiao et al., 2006). For Nb–Ta distributions between supercritical fluids andeclogite, the bulk partition coefficients were taken fromKessel et al. (2005a) (considering a residual mineralassemblage of garnet + clinopyroxene + rutile). Pla-

Fig. 5. Rim-core-rim profile analyses of rutile grains from both the EAV and ECV. All scale bars represent 150 lm.

Fig. 6. Rim-core-rim profile analyses of rutile grains from vein samples. Grey bar I denotes the modelled Nb ⁄Ta range for the aqueousfluids released during dehydration of amphibolite at the early stage of subduction (stage I). Grey bar II and III denote the modelledNb ⁄Ta range of the aqueous fluids (stage II) and supercritical fluids (stage III) in equilibrium with the residual eclogite, respectively(see text for Discussion). The modelling results are given in detail in Fig. 8.

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gioclase and quartz are assumed to have no effect onNb–Ta distributions because of very low Nb–Ta con-centrations in these two minerals (in most cases, belowdetection limits) (Xia et al., 2010). A detailed list of D-values for Nb–Ta is given in Table 6.

Modelling results (Fig. 8) show that an aqueousfluid released by dehydration of amphibolite has verylow Nb ⁄Ta ratios (5.7–6.1), assuming a water-lossfrom 0 to 2 wt%. In stage II, the Nb ⁄Ta ratios of anaqueous fluid in equilibrium with the residual eclogitedramatically increase to 14.0–15.0 (Fig. 8c). In stageIII, a supercritical fluid in equilibrium with the residualUHP eclogite has even higher Nb ⁄Ta ratios of 19.4–

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.05

10

15

20

25

30

35

40

45

08bxl07

08bxl03

Chondritic ratio = 19.9

from Yangtze BlockRange for gabbros

08bxl11

Rutile in EAVRutile in ECVRutile in veinResidual fluid

Nb/

Ta

Distance (km)S N

Ave. MORB ratio = 14.2Ave. gabbro Nb/Ta = 16.2

Fig. 7. Average Nb ⁄Ta of rutile from the eclogites and HP veinsin a south-to-north profile at the Bixiling complex. Upper andlower dash dot lines denote the revised chondritic and MORBvalues, respectively (data from Munker et al., 2003). Blue bar inthe left denotes the range of Nb ⁄Ta in gabbros from the YangtzeBlock, and their average is displayed as grey bar (data from Linget al., 2003; Zhao & Zhou, 2007; Dong et al., 2011). Pink starrepresents the average Nb ⁄Ta value of all rutile grains from theEAVs. Pink open squares mean the residual fluids after veinformation, which may metasomatize the vein-hosting elcogitesand escape from the rocks. It is noted that the average Nb ⁄Tavalues of rutile from the EAVs are similar to that of the gabbros,whereas those from the ECVs are much higher. It is also impor-tant to note that rutile from the HP veins has Nb ⁄Ta values muchlower than those of rutile from their immediate hosts (ECVs).

Table 6. Partition coefficients of Nb–Ta used in this study.

Nb Ta Run NO. T (oC) P (GPa) Reference

Mineral ⁄ aqueous fluid partition coefficients for Nb–Ta

clinopyroxene 0.17 0.36 62 1000 3.0 Stalder et al., 1998

garnet 0.20 0.38 53 1000 3.0 Stalder et al., 1998

rutile 194 147 mean 900 1.0 Brenan et al., 1994

amphibole 1.10 0.36 125 900 2.0 Brenan et al., 1995

Bulk eclogite (garnet + clinopyroxene + rutile) ⁄ supercritical fluid partition coefficients for

Nb–Ta

ecl. ⁄ super.fluid

0.36 0.43 RK 214 1000 6.0 Kessel et al., 2005a

Because no partition coefficient data for Ta are available, DTa between amphibole and

aqueous fluids is assumed to be equal to DTa between clinopyroxene and aqueous fluids,

based on the observation from Xiong (2006) that crystal ⁄melt partition coefficients for

clinopyroxene and amphibole are actually the same in terms of Ta.

0.000 0.005 0.010 0.015 0.02016.0

16.5

17.0

17.55.6

5.7

5.8

5.9

6.0

6.1

6.2

CL

(Nb/

Ta)

Cs (N

b/Ta

)

F (fraction of fluid in H2O-rock system)

(a)

(b)

0.000 0.005 0.010 0.015 0.02019.0

19.5

20.0

20.5

21.014.0

14.2

14.4

14.6

14.8

15.0(c)

CL

(Nb/

Ta)

CL

(Nb/

Ta)

F (fraction of fluid in H2O-rock system)

(d)

Fig. 8. (a, b): Nb ⁄Ta in aqueous fluids (CL) and residue (CS) after dehydration of amphibolite at the early stage of subduction (stage I,F = 0–2%). Amphibolite dehydration was modelled by a modal fractional process, and the equation CL ⁄C0 = (1 ⁄D0)*(1–F)((1 ⁄ D

0))1) from Shaw (1970) was used to calculate Nb–Ta concentrations in the fluid (CL) and residue (CS). C0 is the average

element concentrations in the starting material (gabbros from the Yangtze Block, Nb ⁄Ta = 16.2 ± 0.6, 2 SE, n = 78, Ling et al.,2003; Zhao & Zhou, 2007; Dong et al., 2011), D0 is the bulk partition coefficient between amphibolite and aqueous fluids, and F is thedegree of dehydration (percentage of water loss); (c) Nb ⁄Ta in aqueous fluids in equilibrium with the residue (e.g. eclogite) (stage II);(d) Nb ⁄Ta in supercritical fluids (stage III) at pressures above the second critical endpoint in the basalt-H2O system (e.g. �3.4–5.0 GPa, Kessel et al., 2005b; Mibe et al., 2011). The compositions of aqueous fluids in Stage II and supercritical fluids in Stage III weremodelled assuming equilibrium between the fluid phases and residual eclogite: CL (Nb ⁄Ta, stages II & III) = CS (Nb ⁄Ta, stageI) ⁄DNb ⁄ Ta (bulk). CS (Nb ⁄Ta, stage I), derived from D0*CL (Nb ⁄Ta, stage I), is the remaining element concentration in the solid residueafter dehydration during stage I. DNb ⁄ Ta (bulk) represents bulk DNb (bulk) over DTa (bulk) in the residue.

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20.7 (Fig. 8d). Meanwhile, the modelling results showthat even a 2 wt% water-loss through amphibolitedehydration can only shift the initial Nb ⁄Ta ratio by�6–7% (e.g. from 16.2 to 17.3) (Fig. 8b).

DISCUSSION

Nb–Ta behaviour during subduction without intensive fluid-rock interaction

The EAV eclogites probably did not experienceintensive fluid-rock interaction, as no fluid-relatedveins were found near the sampling localities. Inaddition, the heterogeneity of structural H2O contentsand the preservation of highly variable salinity inclu-sions in UHP eclogite facies minerals (e.g. garnet,omphacite and kyanite) from the Bixiling eclogites alsosuggest that fluid activity during UHP metamorphismwas very limited (Xiao et al., 2000; Xia et al., 2005;Sheng et al., 2007). The low and limited fluid-rockinteraction is further evidenced by the preservation oflow presubduction d18O values (as low as )1.3&, i.e.meteoric H2O-like, Zheng et al., 1999; Xiao et al.,2000). Therefore, a direct comparison of the EAVeclogites with unmetamorphosed protolithic rockswould qualitatively evaluate Nb–Ta mobility andfractionation during subduction without intensivefluid-rock interaction.

The EAVs display bulk Nb ⁄Ta ratios ranging from14.0 to 19.2, which fall in the range defined by thegabbros (the likely protolith of them) from the Yan-gtze Block (Fig. 3). Their average Nb ⁄Ta ratio of16.9 ± 0.8 (2 SE n = 12) is very similar to that of thegabbros (16.2 ± 0.6, 2 SE n = 78; Ling et al., 2003;Zhao & Zhou, 2007; Dong et al., 2011). Furthermore,most of the EAVs have Nb concentrations at the lowerend of the gabbro range (Fig. 3b). From trace elementconcentrations of basaltic lavas, Sims & DePaolo(1997) deduced log-log concentration plots, in whichonly two trace elements with identical partition coef-ficients during mantle melting will yield a linear cor-relation line with a slope of 1. The log-logconcentration-correlation test basically should workfor the studied EAV eclogites, whose protolith wassubjected to partial melting, magma differentiation,Triassic subduction, and subsequent eclogitisation.The slope of 0.965 ± 0.038 for Nb and Ta is veryclose to 1 (Fig. 9) and suggests negligible Nb ⁄Tafractionation during the processes mentioned above.The same observation has been reported by Schmidt etal. (2009). They determined Nb–Ta concentrations ofboth continental- and oceanic-type eclogites by highprecision ID method and obtained a slope of �1 forthe element pair Nb and Ta in the log-log plot. Thesefindings suggest that neither significant Nb–Tamobility nor their fractionation occurred during thetransformation of mafic rocks (i.e. gabbro andMORB) to eclogites during subduction withoutintensive fluid-rock interaction. This conclusion is

consistent with results obtained from geochemicalstudies of HP eclogitic rocks in other orogenic belts(e.g. Chalot-Prat et al., 2003; Spandler et al., 2004;Miller et al., 2007). However, trace elements are mostlikely to be removed from the slab in regions of hightemperature or in zones of intensive fluid-rock inter-action (Spandler et al., 2004). Actually, significant Ti,Nb and Ta mobility has been observed due to highrates of fluid extraction and intensive fluid-rockinteraction (e.g. Gao et al., 2007; John et al., 2008;Beinlich et al., 2010).

Nb ⁄ Ta fractionation induced by intensive fluid-rockinteraction

Significant fluid-assisted Nb ⁄Ta fractionation hasbeen reported by numerous previous studies (Kamber& Collerson, 2000; Xiao et al., 2006; Zhang et al.,2008; Ding et al., 2009; Liang et al., 2009), on thebasis of Nb ⁄Ta variations in single rutile grains andlower Nb ⁄Ta in synmetamorphic HP ⁄UHP veinsrelative to their host rocks. Our data on the ECVs andveins provide new evidence for the fractionation ofNb from Ta by intensive fluid-rock interaction atdifferent stages of metamorphism during continentalcollision.

Origin of HP veins within ECVs

Here epidote from the vein and ECV eclogite hassimilar compositions (Table 3), the ECV- and vein-hosted rutile shows nearly complementary Nb ⁄Tazoning patterns (Figs 5 & 6), and the veins have smallsizes (Fig. 2a,b). These are typical features for veinsthat form by fluids from their immediate host rocks (aninternal source) (Becker et al., 1999; Gao & Klemd,2001; John et al., 2008). Previous petrological, fluid

Fig. 9. Plot after Sims & Depaolo (1997) of log (Nb) vs. log (Ta)for the bulk rock composition of the EAVs. The slope of Nb–Ta(0.965) is very close to 1, indicating negligible fractionation be-tween Nb and Ta during eclogitisation (see text for Discussion).

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inclusion and oxygen isotope studies on HP veinswithin eclogites in the Dabie-Sulu orogenic belt havedemonstrated that all were precipitated from internallyderived fluids (e.g. Castelli et al., 1998; Franz et al.,2001; Zheng et al., 2007; Zhang et al., 2008; Xiao et al.,2011a). The preservation of primary high salinityinclusions in the Bixiling eclogites indicates a nearlyclosed fluid system and a limited fluid flow duringUHP metamorphism (Xiao et al., 2000). Thus, weconclude that the investigated rutile-bearing veinsoriginated from an internal fluid source.

Theoretically, veins can form synmetamorphically atdifferent times in an orogenic cycle. The pronouncedNb ⁄Ta zoning patterns in the vein rutile (Fig. 6)indicate that it did not form in one single stage ofprecipitation. The highly variable compositions of thevein rutile probably resulted from a circulating fluid,which dissolved elements in the host eclogite andtransported them over short distances (Rubatto &Hermann, 2003; Spandler & Hermann, 2006). Thus,the fluid may be locally derived from dehydrationreactions at the amphibolite-to-eclogite transition andevolved over time at different stages of metamorphism.The evidence for the formation of vein mineralsexperiencing a multi-stage evolution also comes fromthe significant zoning patterns of major elements (i.e.Mg, Mn and Ca) and HREE in garnet from HP veinswithin eclogites from New Caledonia (Spandler &Hermann, 2006). As rutile is believed to be stable atpressures of >1.5 GPa (Liou et al., 1998; Xiong et al.,2005), the vein rutile cores with low Nb ⁄Ta ratios areexcepted to form at a relatively early stage of sub-duction, whereas the rims with high Nb ⁄Ta ratios mayform from a supercritical fluid during initial exhuma-tion in response to the decrease in pressure (see Dis-cussion below). In this regard, the formation of theveins may have experienced prograde via peak to ret-rograde metamorphism.

Nb–Ta fractionation during intensive fluid-rock interaction

The HP veins represent precipitates or cumulates ofextensive fluid-rock interaction rather than the fluiditself (e.g. Rubatto & Hermann, 2003; Spandler &Hermann, 2006). The presence of coarse-grained rutilein the HP veins suggests that HFSEs, especially Ti, Nband Ta are mobilized during extensive fluid-rockinteraction. Lower Nb ⁄Ta ratios of the vein- thanECV-hosted rutile as well as the significant Nb ⁄Tazoning patterns of them (Figs 4–7), indicates thatmajor fractionation between the two elements occurredwhere fluid activity is significant. However, for theEAVs, Nb ⁄Ta ratios in both bulk rock and rutile arewithin the range defined by the protolith gabbros(Figs 3 & 7). These features strongly suggest that Nb–Ta mobility and fractionation are associated withintensive fluid-rock interaction during metamorphismbut only occurred locally.

Nb ⁄Ta ratios in rutile from the ECV samples08bxl03 and 08bxl11 are much higher than those ingabbroic protoliths (Fig. 7). On the contrary, Nb ⁄Taratios in rutile from the ECV sample 08bxl07 aresimilar to those in protolithic gabbros (Fig. 7). Aseclogitic samples 08bxl11, 08bxl03 and 08bxl07 werecollected at distances of 5, 10 and 20 cm from the HPveins, respectively, the Nb ⁄Ta fractionation seems tobe limited to length scales of <20 cm. This inter-pretation is generally consistent with the scales offluid flow (<20–50 cm) inferred from O isotope dis-tributions for the continuous vertical profiles of themain hole of the Chinese Continental Scientific Dril-ling program in the Sulu orogenic belt (Chen et al.,2007). The spatial variability due to localized fluidflow may largely explain the contrasting viewsregarding the nature and intensity of element masstransfer during devolatilization of a subducting slabassociated with eclogite facies metamorphism (e.g.Chalot-Prat et al., 2003; Spandler et al., 2003, 2004;Spandler & Hermann, 2006; Gao et al., 2007; Milleret al., 2007; John et al., 2008; Schmidt et al., 2009;Beinlich et al., 2010).

Given that rutile dominates the Nb–Ta budget ofhigh-grade metamorphic rocks, it has been proposed toplay a large role in Nb–Ta mobility and fractionationduring dehydration and fluid-rock interaction (Mein-hold, 2010; Xiao et al., 2011b; Zheng et al., 2011 andreferences therein). Therefore, the presence or absenceof rutile and its solubility in metamorphic fluids are theimportant factors that control Nb–Ta behaviour dur-ing metamorphism. It has been experimentally shownthat Nb and Ta are highly soluble in aqueous fluids inthe absence of rutile (Stalder et al., 1998), whileimmobile in the presence of rutile (Ayers & Watson,1993; Brenan et al., 1994; Green & Adam, 2003; Au-detat & Keppler, 2005; Tropper & Manning, 2005). Inaddition, Na–Al–Si-, Cl- and F-bearing aqueous fluidsand supercritical fluids have been proven to be mediafor mobilizing Nb and Ta (Kessel et al., 2005a; An-tignano & Manning, 2008; Manning et al., 2008). Onthe basis of these experimental results together with thelowest pressure limit stability field of rutile (>1.5 GPa;Liou et al., 1998; Xiong et al., 2005), possible expla-nations for Nb–Ta mobility and fractionation include:(I) dehydration at the early stage of subduction beforethe appearance of rutile, where amphibole controlsNb–Ta fractionation (Xiao et al., 2006; Ding et al.,2009; Liang et al., 2009); (II) the presence of chemicalspecies such as F) (or Cl)) and Na–Al–Si which sig-nificantly enhance the solubility of Nb–Ta in aqueousfluids (Gao et al., 2007; Beinlich et al., 2010); and (III)at conditions where pressures are above the secondcritical endpoint in a rock-H2O system, and hence thefluid phase becomes supercritical, which has a greatcapacity for transporting Nb and Ta (Zheng et al.,2011 and references therein).

Here, except for one vein sample 08bxl11v thatcontains �10 vol.% albite, the other three veins lack

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Na- and F- rich minerals, indicating that the fluidsfrom which the vein minerals precipitated have negli-gible Na and F contents. Therefore, complex-formingligands (e.g. Na–Si–Al polymers and F)) cannot ex-plain the abundant rutile in the veins. On the otherhand, the significant Nb ⁄Ta zoning patterns in the veinrutile, with cores being lower than rims (Fig. 6), sug-gest that Nb ⁄Ta ratios in the fluids are low during therutile core growth but higher during the rim growth.Major dehydration of the subducted slab is thought tooccur at depth where the wet-blueschist ⁄ amphiboliteconverts to dry-eclogite (Liu et al., 1996; Schmidt &Poli, 1998). At the early stage of dehydration, whererutile is absent, amphibole is the main carrier of Nband Ta in most cases (e.g. Ionov & Hofmann, 1995;Foley et al., 2002; Zack et al., 2002; Rapp et al., 2003).In addition to amphibole, titanite has been proposed tohave a high affinity for Nb and Ta in the amphibo-lite ⁄ blueschist (John et al., 2011). However, somestudies showed that, compared with amphibole, themodal influence of titanite on Nb–Ta budget andNb ⁄Ta of shallow slab fluids is minor (Zack et al.,2002; Rapp et al., 2003). Furthermore, no titaniteinclusions in rutile grains were identified in our eclogitesamples, and titanite often appears as retrogradeproduct of rutile (e.g., Carswell et al., 1996; this study).Thus, amphibole is likely the main phase that frac-tionates Nb from Ta during the early stage of sub-duction (Foley et al., 2002; Xiao et al., 2006; Konig &Schuth, 2011). As low-Mg# amphibole favours Nbover Ta (Tiepolo et al., 2000; Foley et al., 2002),aqueous fluids released by early-stage dehydration ofamphibolite should have low Nb ⁄Ta ratios (Xiaoet al., 2006; Ding et al., 2009; Liang et al., 2009). Thelow Nb ⁄Ta aqueous fluids are probably transportedinto and stored in fractures and ⁄ or pore spaces, owingto the general absence of fluid connectivity in maficrocks (Mibe et al., 2003). Consequently, the storedfluids will at least partly be delivered to greater depthsalong with ongoing subduction. Rutile in the ECVsprobably first formed when the pressure exceeded1.5 GPa (Liou et al., 1998; Xiong et al., 2005). How-ever, at this time, TiO2 was highly likely unsaturated inthe stored aqueous fluids and hence no vein rutilecrystallized. At this stage (stage II in Quantitativemodelling), however, because of high fluid ⁄ rock ratios(or fluid activity), the stored aqueous fluids wouldintensively interact with rutile in surrounding eclogites.The intensive fluid-rock interaction together with highrate of fluid extraction would cause the newly formedECV-hosted rutile cores to have high Nb ⁄Ta ratios, asTa has a higher solubility in aqueous fluids comparedwith Nb (Brenan et al., 1994; Linnen & Keppler, 1997).With further subduction, at certain conditions TiO2

saturation might occur in the stored aqueous fluids andthus the vein rutile cores started to crystallize. Quan-titative calculations show that the aqueous fluids haverelatively low Nb ⁄Ta ratios (�6.0–15, stages I & II,Fig. 6) before they become supercritical. The vein ru-

tile cores crystallized from such early-stage aqueousfluids hence should have low Nb ⁄Ta ratios as observed(Fig. 6). The source of Ti for the vein rutile coregrowth includes Ti released from amphibolite dehy-dration before the appearance of rutile in the ECVs(stage I) and that extracted from the ECV-hosted rutileby aqueous fluids (stage II). When the pressure in-creased above the second critical point in the basalt-H2O system (�3.4–5.0 GPa, �770–1000 �C, Kesselet al., 2005b; Mibe et al., 2011), the aqueous fluidscould transform into supercritical fluids. Actually, theaction of supercritical fluids has been recognized in theDabie-Sulu UHP metamorphic rocks (Ferrando et al.,2005; Frezzotti et al., 2007; Zhang et al., 2008; Xiaet al., 2010). Previous studies showed that the peakpressure for the Bixiling eclogites would have exceeded4.0 GPa at �1000 �C (Xiao et al., 2000), which con-curs with the P-T estimates for the coexisting meta-ultramafic rocks (Zhang et al., 1995). Therefore,supercritical fluids were likely to be present in theBixiling eclogites. They have high solubility of variousfluid-immobile elements including HREE and HFSE(Kessel et al., 2005a; Xia et al., 2010; Hayden &Manning, 2011) and could further extract Nb–Ta–Tifrom Ti-bearing phases (i.e. rutile) in the host eclogites.The supercritical fluids favour Nb over Ta (Kessel etal., 2005a) and thus have relatively high Nb ⁄Ta ratiosas revealed by quantitative calculations (�20, Figs 6 &8). Therefore, high Nb ⁄Ta ratios in the rims of the veinrutile can be attributed to precipitation from super-critical fluids (Fig. 6). As minerals probably onlycrystallize from supercritical fluids when pressure de-creases (Zheng et al., 2011), the rims of the vein rutilepossibly formed at decompression during initialexhumation of the subducted continental crust. It isnoted that one of the 22 points in the rutile from thevein 08bxl03v (the sixth spot, Fig. 6) deviates from theNb ⁄Ta zoning patterns, which is a consequence of highTa concentration. This is likely caused by local changesin the Ta concentration of metamorphic fluids duringrutile precipitation.

Residual fluids after vein formation

Given that the veins formed by internally derived flu-ids sourced from their host mafic rocks, if they entirelyrepresent the fluid composition, complementary aver-age Nb ⁄Ta ratios between the vein- and ECV-hostedrutile are expected. However, Nb ⁄Ta ratios of rutilefrom both the veins and ECVs (e.g. 08bxl03 &08bxl03v; 08bxl13 & 08bxl13v) are higher than thoseof the protolith of the eclogites (Fig. 7). From themass balance view, this indicates that an additionalNb–Ta fractionation probably occurred during theprecipitation of vein minerals (e.g. rutile), and thatparts of the fluids carrying Nb and Ta have escapedafter vein formation. In a case study on hydrous HPveins within alpine eclogites, Becker et al. (1999)pointed out that bulk vein composition is not simply

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equivalent to the released fluids during dehydration oftheir host eclogites, implying that a substantial volumeof the fluids together with some elements (e.g. Rb, Pb,Cs and Ba) must have escaped from the rocks. In thisregard, the vein rutile here did not retain the entiredissolved Nb and Ta in the fluids that originally re-leased during fluid-rock interaction. Concentrationgradients of Nb and Ta in the studied samples wereobserved, with the highest contents in the vein-, themiddle in the ECV- and the lowest in the EAV-hostedrutile (Table 5). Such a feature indicates that the ECV-hosted rutile was probably metasomatized by a fluidwith high Nb–Ta concentrations. To explain this fea-ture, we envisage that with the complete crystallizationof vein rutile, the majority of Nb–Ta in the fluidswould be deposited in the vein, and a fraction of Nb–Ta appears to go back to the vein-surrounding eclog-ites. In this way, the ECV-hosted rutile can addition-ally incorporate Nb–Ta from the residual fluids (i.e.fluids metasomatizing vein-hosting elcogite plus fluidsescaping from the rock after vein formation) andhence has higher Nb–Ta contents relative to the EAV-hosted rutile. In addition, assuming that the residualfluids are in equilibrium with the vein rutile, the esti-mated Nb ⁄Ta ratios of the residual fluids are between10.5 and 15.7 (Fig. 7), using the average concentra-tions of Nb–Ta in the vein rutile and the partitioncoefficients for Nb–Ta between rutile and aqueousfluids from Brenan et al. (1994) (Table 6). Metaso-matism of rutile by such fluids with relatively lowNb ⁄Ta ratios can possibly explain the low Nb ⁄Taratios in the rims of rutile from the ECVs (Fig. 5).

CONCLUSIONS

(1) Similar Nb–Ta concentrations and Nb ⁄Ta ratiosbetween the EAV eclogites which did not experi-ence intensive fluid-rock interaction as revealed byfield observations and their inferred gabbroic pro-toliths indicate insignificant Nb–Ta mobility andfractionation during prograde metamorphism upto eclogite facies.

(2) Veins within the Bixiling UHP eclogites wereformed by internally derived fluids from their hostrocks and represent products of complex fluid-rock interaction from prograde through peak toretrograde metamorphism. The presence ofcoarse-grained rutile in the veins and the distinctNb ⁄Ta fractionation in the ECV- and vein-hostedrutile indicate that Nb and Ta can be significantlymobilized and fractionated during localized fluidflow and intensive fluid-rock interaction in sub-duction-zones. However, the fractionation of themmay be limited to length scales of <20 cm.

(3) Ratios of Nb ⁄Ta in the vein rutile increase grad-ually from the cores towards the rims, which arecomplementary to that from their immediate hosts(ECVs). The Nb ⁄Ta zoning patterns of the veinrutile might have been caused by changes in

Nb ⁄Ta ratios of the vein-forming fluids at multi-stage metamorphism. The Nb ⁄Ta ratios of thevein-forming fluids evolved from low values at anearly stage of subduction to higher values at latersupercritical conditions with increased temperatureand pressure.

ACKNOWLEDGEMENTS

This study was financially supported by the ChineseMinistry of Science and Technology (2009CB825002),the National Foundation of Science of China(40921002, 41090372), the Knowledge InnovationProject of the Chinese Academy of Sciences (KZCX1-YW-15–3), and the Hundred Talent Program to Y.L.Xiao. Thanks are due to S. Guo, Q. Mao and Y.G. Mafor their help with the electronmicroprobe analysis, andR.H. Gao and L.J. Xu for their help with the solutionICP-MS analysis. We thank J. Hoefs, G. Worner, J.Hora, S.A. Liu and Y.X. Chen for helpful discussion onan early draft of this paper. We also gratefullyacknowledge J. A. Pfander, B.S. Kamber and X. Dingfor thorough and constructive reviews of the manu-script, and D. Robinson for the editorial handling andcomments. We specially thank J.A. Pfander and D.Robinson for polishing our written English.

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Received 27 October 2011; revision accepted 18 June 2012.

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