Cheng Hao,2009,Lithos

Lithos110(2009)327–342

ContentslistsavailableatScienceDirect

Lithos

journalhomepage:/locate/lithos

TransitionaltimeofoceanictocontinentalsubductionintheDabieorogen:

ConstraintsfromU–Pb,Lu–Hf,Sm–NdandAr–Armultichronometricdating

HaoCheng

a,b,

⁎

,

c

,EizoNakamura

b

,rt

c

,Yong-FeiZheng

d

,TsutomuOta

b

,

Yuan-BaoWu

e

,KatsuraKobayashi

b

,Zu-YiZhou

a

a

StateKeyLaboratoryofMarineGeology,TongjiUniversity,Shanghai200092,China

InstituteforStudyoftheEarth'sInterior,OkayamaUniversityatMisasa,Tottori682-0193,Japan

c

SchoolofEarthandEnvironmentalSciences,WashingtonStateUniversity,Pullman,Washington99164,USA

d

CASKeyLaboratoryofCrust-MantleMaterialsandEnvironments,SchoolofEarthandSpaceSciences,UniversityofScienceandTechnologyofChina,Hefei230026,China

e

StateKeyLaboratoryofGeologicalProcessesandMineralResources,FacultyofEarthSciences,ChinaUniversityofGeosciences,Wuhan430074,China

b

articleinfoabstract

Weinvestigatedtheoceanic-typeXiongdianhigh-pressureeclogitesinthewesternpartoftheDabieorogen

withcombinedU–Pb,Lu–Hf,Sm–NdandAr–roupsofweighted-mean

206

Pb/

238

U

agesat315±5,373±4and422±rast,Lu–Hfand

Sm–Ndisochrondatesyieldidenticalagesof268.9±6.9and271.3±teandamphiboleAr–Ar

totalfusionanalysesgiveNeoproterozoicapparentages,whicharegeologicallymeaninglessduetothe

presenceofexcess

40

claseinclusionsinzirconcoressuggestthattheSilurianageslikelyrepresent

protolithages,whereastheCarboniferousagescorrespondtoprogrademetamorphism,basedonthe

eweakly-preservedprogrademajor-andtraceelementzoningin

garnet,acombinedtexturalandcompositionalstudyrevealsthattheconsistentLu–HfandSm–Ndagesofca.

270Marecordalatereventofgarnetgrowthandthusmarktheterminationofhigh-pressureeclogite–facies

U–Pb,Lu–HfandSm–Ndagessuggestamodelofcontinuousprocessesfrom

oceanictocontinentalsubduction,pointingtotheonsetofprogrademetamorphismpriortoca.315Mafor

thesubductionofoceaniccrust,whilethepeakeclogite–faciesmetamorphicepisodeisconstrainedto

,theinitiationofcontinentalsubductionisnotearlierthanca.270Ma.

©htsreserved.

Articlehistory:

Received22August2008

Accepted9January2009

Availableonline8February2009

Keywords:

Continentalsubduction

Dabie

Eclogite

Geochronology

Oceanicsubduction

Tectonictransition

uction

Subductionzonesareessentialtothedynamicevolutionofthe

earth'tionofoceanicand

continentalcrusteventuallyleadstoclosureofbackarcbasinsandarc-

continentandcontinent-continentcollisions(O'Brien,2001;Ernst,

2005;Zhengetal.,2008),formingvarioustypesofhigh-pressure(HP)

andultrahigh-pressure(UHP)tionof

oceaniclithospherecausesacomplexcontinuumofdiageneticand

metamorphicreactions;manykilometresofoceaniclithosphereare

ultimatelyconsumedpriortothesubsequentcontinentalslab

tedcontinentalslabsthatdetach

fromtheoceaniclithospherethatwasdraggingthemintothemantle

areexpectedtorapidlyrisetoMohodepthsbecauseoftheirpositive

,studyingsubductedoceaniccrustinsubductionzones

canprovidecluestotheincorporationrateofsupercrustalmaterial

⁎eyLaboratoryofMarineGeology,TongjiUniversity,

Shanghai200092,.:+862165982358;fax:+862165984906.

E-mailaddress:chenghao@().

0024-4937/$–seefrontmatter©htsreserved.

doi:10.1016/.2009.01.013

intothemantleandcanshedlightontheinitiationofsuccessive

iningageochronologicalframework

fordeterminingthesequenceanddurationofoceanictocontinental

subductionandHPandUHPmetamorphismplaysanessentialrolein

thisrespect.

Zirconhaslongbeenrecognizedasapromisinggeochronometerof

theU–Pbdecaysystembecauseofitsrefractorynature,commonly

preservedgrowthzonesandmineralinclusionswithinasinglegrain.

Recentdevelopmentsinanalyticaltechniquesallowustounravela

wealthofinformationcontainedinzirconswithrespecttotheir

growthhistoryandthustheprogradeandretrogrademetamorphic

evolutionofthehostrock(Gebauer,1996;Wuetal.,2006;Zhenget

al.,2007).TheLu–Hfgarnettechniquehasbeenappliedtoconstrain

theprogradeandhigh-temperaturehistoriesofmetamorphicbelts

(e.g.,Duchêneetal.,1997;Blichert-ToftandFrei,2001;Anczkiewiczet

al.,2004,2007;Lagosetal.,2007;Kylander-Clarketal.,2007;Chenget

al.,2008a)becauseofitshighclosuretemperature(Dodson,1973;

Schereretal.,2000)andthefactthatgarnetstronglypartitionsLu

overHf,resultinginahighparent/daughterratio(Otamendietal.,

2002).CombinedwithSm–Ndagedetermination,theLu–Hfgarnet

geochronometercanpotentiallybeusedtoestimatethedurationof

tal./Lithos110(2009)327–342

fiedgeologicmapoftheHuwanmélangearea(b)insouthernDabieorogen(a),modifiedafterYeetal.(1993)andLiuetal.(2004b),showingthesamplelocalitiesforthe

nces:asterisk,thisstudy;[1],Ratschbacheretal.(2006);[2],Jahnetal.(2005);[3],Liuetal.(2004a);[4],Eideetal.(1994);[5],Webbetal.(1999);[6],Xu

etal.(2000);[7],Yeetal.(1993);[8],Sunetal.(2002);[9],Jianetal.(1997);[10],Jianetal.(2000);[11],Gaoetal.(2002);[12],Lietal.(2001);[13],Wuetal.(2008).amp—

amphibole;brs—barroisite;phen—phengite;zrn—zircon.

tal./Lithos110(2009)327–342329

garnetgrowth,whicheitherreflectsearlyprogrademetamorphism

(Lapenetal.,2003),exhumation(Chengetal.,2009)oraparticular

garnetgrowthstage(Skoraetal.,2006).Datingtheexhumationof

high-pressure(HP)andultra-high-pressure(UHP)metamorphic

rocksbyconventionalstep-heatingAr–Artechniquewaslargely

hamperedanddiscreditedduetothepresenceofexcess/inherited

argon(Lietal.,1994;Kelley,2002).However,theAr–Argeochron-

ometerremainsirreplaceableinconstrainingtheexhumationofHP/

UHPmetamorphicrocksbecauseofitsintermediateclosuretempera-

heless,timingmustbeintegratedwithtexturesand

petrologyinordertoquantifythedynamicsofgeologicalprocesses,

whichevergeochronologicalmethodisused.

Duringthepasttwodecades,considerableprogresshasbeenmade

inconstrainingtheprogrademetamorphismandexhumationofHP/

UHPmetamorphismoftheDabie–Suluorogenbyavarietyof

geochronologicalmethods,indicatingaTriassiccollisionbetweenthe

,Eideetal.,1994;Amesetal.,

1996;Rowleyetal.,1997;Hackeretal.,1998;Lietal.,2000,2004;

Zhengetal.,2004).Theinitiationofcontinentalsubductionispinned

toca.245Ma(Hackeretal.,2006;Liuetal.,2006a;Wuetal.,2006;

Chengetal.,2008a),

otherhand,thefingerprintsofearlycontinentalsubductionmaynotbe

preservedincontinental-typemetamorphicrocksduetothesucces-

a-

tively,thetimingofinitiationofcontinentalsubductionsubsequentto

theterminationofoceanicsubductionmayberegisteredintheHP/

UHPeclogites,tly,the

onlyoutcroppingcandidateistheXiongdianHPeclogiteinthewestern

partoftheDabieorogen(Lietal.,2001;Fuetal.,2002).However,U–Pb

zirconagesrangingfrom216±4to449±14Mahavebeenobtained

fortheXiongdianeclogite(Jianetal.,1997;Sunetal.,2002;Gaoetal.,

2002);thegeologicalsignificanceofthisagespreadiscontroversial.

EffortstoclarifythegeochronologicalevolutionoftheXiongdian

eclogitewerehamperedbyamucholderSm–Ndgarnet-whole-rock

isochronof533±13Ma(Yeetal.,1993)andthefactthatfurtherSm–

NdandRb–Sranalysesfailedtoproducemineralisochrons(Lietal.,

2001;Jahnetal.,2005),althoughoxygenisotopicequilibriumwas

largelyattained(Jahnetal.,2005).

Here,wepresentacombinedU–Pb,Lu–Hf,Sm–Nd,Ar–Arand

oxygenmulti-isotopicandmineralchemicalstudyoftheXiongdian

ferencesinthesesystems,inconjunctionwith

chemicalprofilesingarnetporphyroblastsandzircons,providea

windowintothetime-scalesoftheoceanicsubductionandsub-

sequentexhumation.

onologicalbackgroundandsampledescriptions

TheQinling–Dabie–

Suluorogenineast-centralChinamarksthe

junction

betweentheNorthandSouthChinaBlocks(Cong,1996;

Zhengetal.,2005).ThewesternpartoftheDabieorogen,usually

termedtheWestDabieandsometimestheHong'anterrane,is

separatedfromtheTongbaishaninthewestbytheDawuFaultand

fromtheEastDabiebytheShangmafaultintheeast(Fig.1a).It

containsaprogressivesequenceofmetamorphiczonescharacterized

byincreasingmetamorphicgrade,fromtransitionalblueschist–

greenschistinthesouth,throughepidote–amphiboliteandquartz

eclogite,,Zhouetal.,1993;Hacker

etal.,1998;Liuetal.,2004b,2006b).TheXiongdianeclogitescropout

inthenorthwesterncorneroftheWestDabie,intheXiongdian

mélangewithintheHuwanmélangeafterthedefinitionofRatschba-

cheretal.(2006),inanalogytothetermsoftheSujiahemélange(Jian

etal.,1997)andHuwanshearzone(Sunetal.,2002).TheHuwan

mélangeconsistsofeclogite,gabbro,amphibolite,marble,and

ogiticmetamorphicpeakfortheXiongdianeclogite

isestimatedat600–730°C,1.4–1.9GPa(Fuetal.,2002),550–570°C,

∼2.1GPa(Liuetal.,2004b)and540–600°C,∼2.0GPa(Ratschbacher

etal.,2006),followedbyretrogressionat530–685°Cand∼0.6GPa

(Fuetal.,2002).

ExceptfortheXiongdianeclogite,consistentTriassicmetamorphic

ageshavebeenobtainedforothereclogitesacrosstheWestDabie

(Webbetal.,1999;Sunetal.,2002;Liuetal.,2004a;Wuetal.,2008).

ThisindicatesthatWestDabieislargelyacoherentpartofanHP–UHP

beltelsewhereintheDabie–onological

debateislimitedtotheXiongdianeclogite(Fig.1b).U–Pbzircon

agesrangingfromca.216toca.449Mahavebeenobtainedfor

al.(1997)reportedca.400,ca.373and

301±0.6MaagesbyID–ed-meanSHRIMPages

rangefrom335±2to424±5Ma(Jianetal.,2000).TheSilurianU–Pb

zirconageswereinterpretedastheageoftheprotolith,whilethe

Carboniferousagesmarkhigh-pressuremetamorphism(Jianetal.,

1997,2000).Weighted-mean

206

Pb/

238

USHRIMPU–Pbzirconsagesof

433±9,367±10and398±5Mawereinterpretedastheprotolith

age,while323±7and312±5Malikelydatethehigh-pressure

metamorphism(Sunetal.,2002).ATriassicageof216±4Matogether

with449±14and307±14Maweighted-mean

206

Pb/

238

USHRIMP

U–PbzirconagesappeartoarguefortheinvolvementoftheTriassic

subductionintheXiongdianeclogite(Gaoetal.,2002).Agarnet-

whole-rockSm–Ndisochronof533±13Ma(Yeetal.,1993)was

interpretedtorefll

Table1

ChemicalcompositionsoftheXiongdianeclogitefromthewesternDabie.

SamplenumberDB17DB18

(Majoroxidesin%)

SiO

2

54.5452.45

TiO

2

0.370.43

Al

2

O

3

14.6212.35

Fe

2

O

3

8.7710.15

MnO0.150.16

MgO6.669.91

CaO10.3510.26

Na

2

O2.882.65

K

2

O0.600.28

P

2

O

5

0.060.05

Cr

2

O

3

⁎6601118

NiO⁎137247

L.O.I0.871.28

Total99.95100.11

(Traceelementsinppm)

Li27.627.0

Be0.560.47

Rb9.7813.8

Sr178130

Y12.612.7

Cs0.893.67

Ba86552.4

La2.211.77

Ce5.975.12

Pr0.880.80

Nd4.354.10

Sm1.251.26

Eu0.470.39

Gd1.531.52

Tb0.280.29

Dy1.831.91

Ho0.410.42

Er1.141.19

Tm0.190.19

Yb1.311.34

Lu0.200.20

Pb6.441.85

Th0.050.07

U0.110.06

Zr28.828.2

Nb1.191.77

Hf0.870.88

Ta0.050.08

⁎Inppm.

tal./Lithos110(2009)327–342

350Mahavebeenexplainedastheretrogrademetamorphicage(Xu

etal.,2000).The310±3Maphengite

40

Ar/

39

Arage(Webbetal.,1999)

islikelygeologicallymeaninglessduetotheconcave-upwardage

spectrum,tively,

existinggeochronologyprovidesanapparentlyconflictingpicturefor

ingoftheoceaniccrustsubduction

andexhumationessentiallyremainstoberesolved.

Thetwoeclogitesexaminedinthisstudywereselectedbasedon

theirmineralassemblages,inclusiontypesandgeologicalcontext

(Fig.1).Theone(DB17)fromtheeastbankoftherivertotheeastof

Xiongdianvillageisacoarse-grainedandstronglyfoliatedbanded

eclogite,composedmainlyofgarnet,d

(DB18)eclogitewassampledabout50mtothenorthofDB17andis

stronglyfoliatedwithasimilarmineralogyassemblagebutsmaller

garnetgrains.

s

Samplepreparation,mineralseparationandchemicalprocedures

forisotopeanalysis,instrumentationandstandardreferencematerials

usedtodeterminewholerockandbulkmineralcompositions,insitu

majorandtraceelementanalyses(InstituteforStudyoftheEarth's

Interior,OkayamaUniversityatMisasa,Japan),zirconU–Pbisotope

andtraceelementanalyses(ChinaUniversityofGeosciencesin

Wuhan),Lu–HfandSm–Ndisotopeanalyses(WashingtonState

University),Ar–Arisotopeanalyses(GuangzhouInstituteofGeo-

chemistry,ChineseAcademyofSciences)andoxygenisotopeanalyses

(UniversityofScienceandTechnologyofChina)aredescribedinthe

Appendix.

ockchemicalanalysisdata.(a)Chondrite-normalizedREEdistribution

patternsoftheXiongdianeclogites.(b)Primitive-mantle-normalizedspidergramsof

theXiongdianeclogites.

s

emicalcomposition

Sm–NdandRb–Sranalysesfailedtoproducesisochrons(Lietal.,2001;

Jahnetal.,2005),whichwasbelievedtobeduetounequilibrated

isotopicsystemsdespitethefactthatoxygenisotopicequilibriumwas

largelyattained(Jahnetal.,2005).Phengite

40

Ar/

39

Aragesofca.430–

TheXiongdianeclogitesaremainlyofbasalticcomposition,but

theyshowawiderangeofmajorandtraceelementabundances.

DespitethehighSiO

2

(52–58%)andlowTiO

2

(0.32–0.43%)contents,

attered-electronimagesandrim-to-rimmajor-elementcompositionalzoningprofi—

amphibole;Ap—apatite;Cal—calcite;Cpx—clinopyroxene;Zo—zoisite;Phen—phengite;Omp—omphacite;Qtz—quartz;Zrn—zircon.

tal./Lithos110(2009)327–342331

theyhaveMgO=5.1–9.9%,Cr=430–1118ppm,Ni=88–247ppm

(Table1;Lietal.,2001;Fuetal.,2002;Jahnetal.,2005).Incontrastto

existingLREE-enrichedchondriticREEpatterns,oursampleshave

ratherflatREEpatternsaroundtentimesmorechondriticabundances

withsmall,bothnegativeandpositiveEuanomalies(Fig.2a).

RubidiumisdepletedandSrdisplaysenrichmentwithrespecttoCe.

BothnegativeandnoNbanomaliesrelativetoLawereobserved

(Fig.2b).TheN-MORB-normalizedvalueofThisaround0.5,lower

thanpreviousreportedvaluesofupto25(Lietal.,2001).

raphyandmineralcomposition

TheXiongdianeclogitesoccurasthinlayersintercalatedwith

dolomite–plagioclasegneissandphengite–quartzschist(Fuetal.,

2002),mainlyconsistingofgarnet,omphacite,epidote(clinozoisite),

phengiteandminoramphibole,quartzandkyanite(Fig.3).Zircons

wereobservedbothasinclusionsingarnetporphyroblastsandinthe

pleshavesimilarmineralassemblages,butdifferin

ite(X

Jd=0.46–0.48

)te

has3.30–3.32Siapfuand∼0.4wt.%TiO

2

.Garnetsrangeinsizefrom

0.5to5mmindiameter,eitherasporphyroblastsorascoalesced

polycrystals,mostlywithidioblasticshapeswithinclusionsofquartz,

calcite,apatiteandomphacite(Fig.3).Garnetislargelyhomogeneous

(Prp

24–25

Alm

49–50

Grs

24–25

Sps

1.5–1.9

),butshowsaslightlyMn-

enrichedcore(Fig.3d;Table2).HREEsinlargegarnetporphyroblasts,

suchasYbandLu,displayweakbutcontinuousdecreasesin

concentrationfromcoretorim(Fig.4a),mimickingtheMnOzoning

pattern,whichcouldbeexplainedbytheirhighaffinityforgarnetand

likelyarisesfromanoverallRayleighdistillationprocessduringearly

garnetgrowth(Hollister,1966;Otamendietal.,2002).

However,thelimitedvariationinMREEconcentrations,suchasSm

andNd,ingarnetwithrespecttotheweakzoninginHREE(Fig.4a)

mightbeexplainedbygrowthinanenvironmentwhereMREEsare

notlimitedandcontinuouslysuppliedbythebreakdownofother

mhasafairlyflatprofile(Table3),reflectingits

incompatiblecharacteringarnetandabsenceofHf-competing

tinctdomainscanbe

definedinthelargegarnetporphyroblastsbasedonthechemical

onesareaninclusion-

richcorewithricherMnandHREEandaninclusion-freerimwith

poorerMnandHREE(Fig.3d).Theinclusion-freerimforindividual

garnethasarathersimilarwidthof200–250μm(Fig.3).Although

concentrationsofNd(0.22–0.41ppm)andSm(0.33–0.48ppm)vary

withinsinglegarnetgrains,theSm/Ndratios(0.8–2.2)areconsistent

Table2

Representativemajor-elementdataofthegarnets,omphacites,phengites,amphibolesandzoisites.

(wt.%)

Grt

Rim

SiO

2

TiO

2

Al

2

O

3

FeO

⁎

MnO

MgO

CaO

Na

2

O

K

2

O

Total

O.N.

Si

Al

Ti

Fe

2+

Mn

Mg

Ca

Na

K

38.68

0.05

21.92

22.98

0.68

6.37

9.10

0.03

0.00

99.80

12

2.986

1.994

0.003

1.486

0.044

0.733

0.753

0.004

0.000

Phn

Rim

SiO

2

TiO

2

Al

2

O

3

FeO

⁎

MnO

MgO

CaO

Na

2

O

K

2

O

Total

O.N.

Si

Al

Ti

Fe

2+

Mn

Mg

Ca

Na

K

48.86

0.40

29.03

1.99

0.00

2.79

0.01

0.93

10.00

94.02

11

3.302

2.313

0.020

0.112

0.000

0.282

0.001

0.122

0.862

49.09

0.41

28.68

1.99

0.00

2.77

0.01

0.92

9.91

93.78

11

3.323

2.288

0.021

0.113

0.000

0.280

0.001

0.121

0.855

Core

49.33

0.41

29.01

2.00

0.00

2.78

0.01

0.92

9.81

94.28

11

3.318

2.300

0.021

0.113

0.000

0.279

0.001

0.120

0.842

49.01

0.40

29.19

1.97

0.01

2.80

0.01

0.91

9.78

94.09

11

3.304

2.319

0.020

0.111

0.000

0.282

0.001

0.119

0.841

38.64

0.06

21.94

23.05

0.72

6.38

8.94

0.03

0.00

99.77

12

2.984

1.997

0.003

1.491

0.047

0.735

0.740

0.005

0.000

38.66

0.05

22.07

23.06

0.79

6.28

9.02

0.03

0.00

99.96

12

2.981

2.006

0.003

1.489

0.052

0.722

0.745

0.005

0.000

38.53

0.05

21.99

23.16

0.88

6.31

8.92

0.03

0.00

99.87

12

2.975

2.001

0.003

1.499

0.058

0.726

0.738

0.005

0.000

Core

38.65

0.05

21.99

23.05

0.75

6.36

9.03

0.03

0.00

99.91

12

2.980

1.999

0.003

1.489

0.049

0.731

0.746

0.005

0.000

Amp

Rim

47.08

0.22

12.66

11.60

0.10

12.20

9.97

2.79

0.48

97.09

23

6.831

2.164

0.024

1.407

0.012

2.639

1.550

0.784

0.089

47.07

0.22

12.81

11.48

0.09

12.47

10.09

2.77

0.47

97.49

23

6.800

2.182

0.024

1.387

0.012

2.686

1.562

0.777

0.087

Core

46.72

0.22

12.58

11.46

0.09

12.44

10.07

2.82

0.47

96.88

23

6.799

2.158

0.024

1.394

0.012

2.699

1.570

0.795

0.088

46.75

0.22

12.62

11.36

0.09

12.30

10.10

2.83

0.47

96.76

23

6.809

2.167

0.024

1.383

0.012

2.670

1.577

0.798

0.088

38.66

0.05

21.84

23.11

0.68

6.35

8.99

0.03

0.00

99.71

12

2.988

1.990

0.003

1.496

0.044

0.732

0.744

0.005

0.000

Inclusions-in-zircon

37.86

0.05

21.68

24.42

0.99

4.23

10.57

0.02

0.00

99.82

12

2.958

1.997

0.003

1.596

0.066

0.493

0.885

0.003

0.000

37.75

0.08

21.86

24.33

0.93

4.74

9.50

0.01

0.00

99.21

12

2.962

2.021

0.005

1.599

0.062

0.554

0.798

0.002

0.000

Omp

Rim

55.93

0.12

11.26

4.25

0.03

8.15

13.22

6.65

0.00

99.60

6

1.996

0.474

0.003

0.127

0.001

0.434

0.506

0.460

0.000

Zo

Mantle

39.05

0.13

28.55

6.01

0.05

0.07

24.10

0.00

0.00

97.96

12.5

3.008

2.592

0.007

0.387

0.004

0.007

1.989

0.000

0.000

38.92

0.13

28.21

6.01

0.05

0.06

23.86

0.00

0.00

97.24

12.5

3.019

2.579

0.007

0.390

0.004

0.007

1.983

0.000

0.000

Core

39.02

0.13

28.73

6.03

0.06

0.07

24.13

0.00

0.00

98.16

12.5

3.000

2.603

0.007

0.388

0.004

0.007

1.988

0.000

0.000

39.02

0.12

28.62

6.07

0.05

0.07

24.14

0.00

0.00

98.09

12.5

3.003

2.596

0.007

0.390

0.004

0.008

1.990

0.000

0.000

56.12

0.11

11.22

4.23

0.02

8.02

13.36

6.41

0.00

99.60

6

2.010

0.473

0.003

0.127

0.001

0.428

0.513

0.445

0.000

56.13

0.11

11.33

4.32

0.03

7.96

13.32

6.39

0.00

99.70

6

2.010

0.478

0.003

0.129

0.001

0.425

0.511

0.443

0.000

Core

56.20

0.11

11.26

4.27

0.02

8.13

13.34

6.42

0.00

99.87

6

2.007

0.474

0.003

0.128

0.001

0.433

0.511

0.445

0.000

⁎

Totaliron;concentrationsreportedaswt.%.

tal./Lithos110(2009)327–342

shellsdominatethevolumeofLu(Chengetal.,2008a).The0.90–

0.93ppmLucontentsbyID-MC-ICPMSapparentlyresemblethoseof

thegarnetrim,whichcouldbereadilyexplainedbythespherical

r,weinterpretthiswithcautionbecause

individualgarnetporphyroblastscouldhavedifferentzoningpatterns

andtheindividualLuprofilemightnotberepresentativeofthe

populationofgarnetgrains,althoughthechemicalzoningcenter

(nucleationsite)coincideswiththegeometriccenter(Fig.3d),

tion,biasedmineral

hand-pickingshouldbeconsidered(Chengetal.,2008a,b).Moreover,

sincethethin-sectionpreparationmethodforthisstudycannot

ensurethattherealcenterofthegarnetwasexposed,theobserved

zoningherelikelyonlyrepresentsaminimumzoningofparticular

garnetporphyroblasts.

tionofP–Tconditions

MetamorphicpeakP–Tconditionsof2.2GPaand620°CfortheDB17

Xiongdianeclogite(Fig.6)areevaluatedonthebasisofrecentcali-

brationsoftheassemblagegarnet+omphacite+phengite+kyanite+

quartz,accordingtothedatasetofHollandandPowell(1998).Higher

P–Tvaluesof2.4GPaand650°Carecalculatedwiththecalibrationsof

KroghRavnaandTerry(2004).Whileatemperatureof620±29°Cis

estimatedbyquartz–garnetOisotopethermometer(Zheng,1993),Ti-

in-zirconthermometer(Watsonetal.,2006;FerryandWatson,2007)

givessimilarvalueof695±22°-in-rutilethermometer(Watsonet

al.,2006;FerryandWatson,2007)yieldsalowervalueof634–652°C

andasimilartemperatureof683–701°C(Fig.6)whenusingthe

pressure-dependentcalibrationofTomkinsetal.(2007)at2.2GPa.

Calibration1usesupdatedversionsofthethermodynamicdataset

andactivitymodelsintheprograms

THERMOCALC3.26

andAX(Holland,

Powell,1998;latestupdateddataset;Powelletal.,1998)byusingan

avPTcalculationinthesimplifiedmodelsystemNCKFMASHwith

excessSiO

2

andH

2

ation2usesthermobarometrybasedon

thedatabaseofHollandandPowell(1998)andactivitymodelsfor

garnet(Gangulyetal.,1996),clinopyroxene(HollandandPowell,

1990)andphengite(HollandandPowell,1998).Analysesofgarnet,

omphaciteandphengite(Table2)wereprocessedaccordingtothe

ation3usesmineralOisotopecompositions

(Table4)toestimatetemperaturebasedonthequartz–garnetO

isotopethermometer(Zheng,1993).Calibrations4and5useTi

contentsinzirconbyLA-ICPMSandZrconcentrationofrutilebySIMS

(Table5)totemperatureestimationsbasedontheTi-in-zirconandZr-

in-rutilethermometers,respectively(Watsonetal.,2006;Ferryand

Watson,2007;Tomkinsetal.,2007).

Theassemblageofgarnet–omphacite–kyanite–phengite–quartzis

representativeofmetamorphicpeakconditionsoftheXiongdian

ite-normalizedREEpatterns(SunandMcDonough,1989)ofzircons,garnets

andomphacitefromXiongdianeclogite(a)andREEdistributionpatternsbetweenzircon

andgarnet(b).TheequilibriumD

REE(Zrn/Grt)

valuesofRubatto(2002),Whitehouseand

Platt(2003)andRubattoandHermann(2007)arepresentedforcomparison.

withthoseobtainedbyID-MC-ICPMS(1.9–2.4)withinerror(Fig.5a),

indicatingthattheNdisotopicanalysesinthisstudyareessentially

unaffectedbyMREE-richinclusions,likelyduetoefficientmineral

pickingand/orconcentratedH

2

SO

4

sistentHf

concentrationsof0.10–0.13ppmwithinsinglegrainswiththose

(0.11–0.13ppm)byID-MC-ICPMSindicatestheHf-richphaseswere

essentiallyremovedduringdigestion(Fig.5b).TheoverallLu

concentrationslightlyskewstowardsthegarnetrimbecauseofthe

,theouter

Table3

SIMSSm,Nd,HfandLuconcentrationprofilesofthegarnetsinFigs.4and5.

(ppm)

Rim

Li

Sr

Y

Hf

La

Ce

Pr

Nd

Sm

Eu

Gd

Dy

Er

Yb

Lu

0.93

0.10

45.6

0.11

0.01

0.04

0.01

0.39

0.45

0.27

1.85

5.68

3.74

4.10

0.90

1.14

0.13

46.8

0.13

0.02

0.05

0.02

0.33

0.36

0.27

1.96

5.86

4.13

4.18

0.91

0.88

0.12

46.6

0.12

0.02

0.05

0.03

0.28

0.38

0.27

1.75

5.58

4.04

4.01

0.88

0.84

0.12

47.3

0.12

0.01

0.06

0.02

0.38

0.44

0.28

1.80

6.18

4.25

3.86

0.84

0.89

0.10

46.4

0.11

0.00

0.05

0.02

0.35

0.47

0.30

1.85

5.87

4.23

4.23

0.84

0.98

0.10

47.1

0.11

0.00

0.04

0.02

0.27

0.41

0.24

1.78

5.84

4.16

4.11

0.89

0.75

0.10

48.3

0.12

0.01

0.04

0.02

0.22

0.48

0.28

1.85

5.79

3.76

4.49

1.13

0.52

0.12

50.0

0.12

0.01

0.04

0.02

0.28

0.45

0.28

1.84

6.19

4.15

4.34

1.15

0.99

0.11

52.0

0.12

0.01

0.05

0.02

0.34

0.45

0.25

1.93

6.46

4.65

4.97

1.28

0.58

0.12

53.5

0.10

0.01

0.03

0.02

0.31

0.41

0.30

1.82

6.40

4.99

5.19

1.26

0.69

0.13

53.1

0.11

0.01

0.04

0.03

0.27

0.34

0.29

1.57

5.50

4.53

5.19

1.26

0.87

0.10

55.3

0.10

0.01

0.04

0.02

0.41

0.33

0.24

1.92

6.91

4.98

5.65

1.33

0.67

0.11

54.6

0.10

0.02

0.05

0.02

0.28

0.42

0.25

1.69

6.09

4.63

5.10

1.32

Core

0.75

0.10

57.8

0.10

0.01

0.03

0.02

0.26

0.41

0.22

1.53

6.40

5.20

5.69

1.42

Cpx

22.1

33.5

0.92

0.41

0.02

0.12

0.03

0.36

0.31

0.22

0.65

0.26

0.06

0.12

0.01

tal./Lithos110(2009)327–342333

/NdversusNdandLu/:data

:dataobtainedbyion

—wholerockbybomb-digestion,savWR—wholerockby

arsforbothIMSandIDmethodsaresignificantlysmallerthan

thesymbols.

y-calibratedthermobarometerisdefinedbythethree

reactionsof3Celadonite+1Pyrope+2Grossular=3Muscovite+

6Diopside,2Kyanite+3Diopside=1Pyrope+1Grossular+2Quartz,

and3Celadonite+4Kyanite=3Muscovite+1Pyrope+

intersectionpointof2.2GPaand620°Cisdefinedandtherefore

independentofcommonly-usedFe–

offersanadvantagewithregardstogarnet–clinopyroxene,whichis

pronetoretrogradereactionsandproblemsstemmingfromferric

ironestimationofomphacite(Lietal.,2005).Resultsareplotted

eereactions

andintersectionpointsareshownaccordingtoprogramsofcalibrations

1–5inFig.6.

isotopicdata

TheOisotopecompositionsofmineralsforthetwoeclogitesare

iredwithquartzforisotopegeothermo-

metry,garnet,omphacite,phengite,kyanite,zoisiteandamphibole

yieldtemperaturesof620±29,563±35,567±43,508±31,404±28

and685±39°CforeclogiteDB17,ethese

temperaturesareconcordantwithratesofOdiffusionandthusclosure

temperaturesinthemineralassemblagegarnet+omphacite+

kyanite+phengite+quartz(ZhengandFu,1998),representativeof

metamorphicpeakconditions,acontinuousresettingofOisotopes

inthedifferentmineral-pairsystemsisevidentduringcooling(Giletti,

1986;Eileretal.,1993;Chenetal.,2007).

Quartz–garnetpairsfromeclogiteDB17givetemperaturesof620±

29°C,whichareconsistentwiththosecalibratedbythe

THERMOCALC

–onsofpy+2gr+3cel=6di+

3mu;3di+2ky=py+gr+2q;and3cel+4ky=py+3mu+4qandintersectionpoints

areplottedaccordingtothecalibrationsofHollandandPowell(1998,latestupdated

dataset)insolidlinesandKroghRavnaandTerry(2004)equartz

equilibriumisalsoshown(HollandandPowell,1998).Abbreviations:alm—almandine,

gr—grossular,py—pyrope,cel—celadonite,mu—muscovite,di—diopside,jd—

jadeite,coe—aturesestimatedbyquartz–garnetoxygenisotope

thermometry(Zheng,1993),Ti-in-zirconandZr-in-rutilethermometries(Watsonet

al.,2006;Tomkinsetal.,2007)arealsoshown.

method,indicatingthatOisotopeequilibriumwasachievedand

preservedduringeclogite–faciesrecrystallization(Fig.7a).Thisisalso

evidencedbytheapparentequilibriumfractionationbetweengarnet

andomphacite(Fig.7b).Incontrast,equilibriumfractionationwasnot

calculatedquartz–amphibolepairtemperatureof685±39°Cis

distinctlyhigherthanthe508±31°Cfromthequartz–zoisitepair.

Becauseoxygendiffusioninamphiboleisfasterthaninzoisiteand

kyanite(ZhengandFu,1998),amphiboleexchangesoxygenisotopes

uently,the

Oisotopetemperatureincreasesforthequartz–amphibolepair,

whereasthequartz–zoisitetemperaturedecreasesrelativetothe

regard,theretrogrademetamorphism

ofamphibolite–faciesshouldtakeplaceatatemperaturebetween

∼685and∼508°therhand,thelowquartz–kyanitepair

temperature(404±28°C)couldbeinterpretedasaresultofinfluence

byretrogressivemetamorphismwithoutacleargeologicalmeaning.

Table4

OxygenisotopedataofmineralsfortheXiongdianeclogite.

SamplenumberMineralδ

18

O(‰)PairΔ

18

O(‰)T

1

(°C)T

2

(°C)

DB17Quartz12.86,12.66

Phengite10.26,10.14Qtz–Phn2.57567±43

Garnet8.83,8.85Qtz–Grt3.93620±29605±22

Omphacite9.64,9.56Qtz–Omp3.17563±35574±28

Zoisite9.31,9.43Qtz–Zo3.40508±31494±21

Amphibole9.83,9.60Qtz–Amp3.06685±39

Kyanite9.36,–Qtz–Ky3.41404±28

WR9.85,9.91

DB18Garnet9.74,9.59

Omphacite8.58,8.48Omp–Grt−1.14

WR10.15,9.99

T

1

andT

2

werecalculatedbasedonthetheoreticalcalibrationsofZheng(1993)and

Matthews(1994),respectively,withomphacite(Jd

45

Di

55

).Uncertaintyonthe

temperatureisderivedfromerrorpropagationoftheaveragereproducibilityof

±15‰forδ

18

O(‰)valuesinthefractionationequations.

tal./Lithos110(2009)327–342

Table5

LA–ICPMStraceelementanalysesofzirconandZrcompositionofrutilebySIMSintheXiongdianeclogite.

(ppm)

Label

DB17–1

DB17–2

DB17–3

DB17–4

DB17–5

DB17–6

DB17–7

DB17–8

DB17–9

DB17–10

DB17–11

DB17–12

DB17–13

DB17–14

DB17–15

DB17–16

DB17–17

DB17–18

DB17–19

DB17–20

DB17–21

DB17–22

DB17–23

DB17–24

DB17–25

Rutile–1

Rutile–2

Rutile–3

Rutile–4

Ti

5.95

5.68

4.54

5.33

8.01

6.05

6.55

6.63

6.72

4.96

7.66

6.56

5.29

7.20

6.04

6.04

4.04

4.51

7.47

9.08

4.24

5.09

4.74

4.94

6.45

267

301

313

332

Y

200

169

173

220

115

265

62

219

167

87

141

282

196

83

66

113

60

126

127

109

120

286

160

158

225

–

–

–

–

Nb

0.49

0.16

0.09

0.29

0.08

0.29

0.05

0.65

0.40

0.08

0.43

0.64

0.09

0.08

0.32

0.16

0.43

0.11

0.30

0.34

0.23

0.51

0.41

0.24

0.15

–

–

–

–

La

bD.L.

bD.L.

bD.L.

bD.L.

bD.L.

0.01

bD.L.

bD.L.

bD.L.

bD.L.

bD.L.

bD.L.

bD.L.

0.01

bD.L

bD.L.

0.01

bD.L

bD.L

bD.L

bD.L.

bD.L

bD.L

0.02

bD.L

–

–

–

–

Ce

0.68

0.55

0.83

1.30

0.58

1.42

0.41

1.70

0.99

0.51

0.69

1.72

1.94

0.63

1.08

0.86

0.47

1.53

0.81

1.84

1.06

1.43

1.15

0.70

1.12

–

–

–

–

Pr

bD.L.

bD.L.

0.01

0.01

bD.L.

0.02

bD.L.

bD.L.

0.01

bD.L.

bD.L.

bD.L.

0.02

0.01

bD.L

bD.L.

0.02

0.02

0.03

0.02

0.01

bD.L

0.01

bD.L

bD.L

–

–

–

–

Nd

0.07

0.10

0.13

0.11

0.13

0.43

0.06

0.14

0.20

0.09

0.11

0.23

0.38

0.10

0.09

0.18

bD.L

0.10

0.19

0.23

bD.L.

0.14

0.09

0.13

0.15

–

–

–

–

Sm

0.25

0.20

0.44

0.43

0.17

0.82

0.06

0.36

0.50

0.10

0.21

0.51

0.73

0.15

0.10

0.32

bD.L

0.33

0.31

0.40

0.26

0.50

0.35

0.19

0.39

–

–

–

–

Eu

0.16

0.13

0.25

0.19

0.12

0.36

0.05

0.05

0.22

0.07

0.10

0.12

0.28

0.09

0.10

0.14

0.09

0.17

0.11

0.10

0.09

0.11

0.05

0.05

0.19

–

–

–

–

Gd

2.63

1.53

2.92

2.55

1.47

4.33

0.86

2.14

2.96

0.88

1.78

3.42

3.76

1.02

0.97

1.54

0.84

2.35

1.45

1.71

1.35

3.68

1.36

1.52

2.83

–

–

–

–

Tb

0.98

0.78

1.07

1.18

0.57

1.71

0.31

1.02

0.93

0.46

0.66

1.35

1.30

0.37

0.37

0.58

0.35

0.75

0.64

0.56

0.55

1.50

0.79

0.73

1.17

–

–

–

–

Dy

13.8

11.6

13.2

15.5

7.89

19.5

4.21

15.3

12.7

6.14

9.12

19.9

16.0

5.71

4.76

7.84

4.32

10.0

9.23

7.35

8.11

20.3

10.9

10.1

16.7

–

–

–

–

Ho

6.29

5.42

5.36

6.67

3.42

7.99

1.88

6.67

4.94

2.81

4.21

8.44

6.46

2.60

2.05

3.61

1.71

4.13

3.80

3.04

3.57

8.57

4.60

4.45

7.03

–

–

–

–

Er

32.5

30.8

26.9

33.8

18.1

41.2

10.2

36.0

25.3

14.7

21.8

44.2

30.2

13.4

10.4

17.7

9.86

18.8

19.9

17.2

19.0

42.6

24.6

24.5

36.8

–

–

–

–

Tm

8.05

8.02

6.89

8.30

4.67

9.85

2.71

8.77

6.24

3.87

5.51

10.5

6.69

3.54

2.53

4.60

2.35

4.26

5.15

4.29

4.55

10.0

5.90

6.18

9.35

–

–

–

–

Yb

92

95

78

92

58

109

33

99

70

48

64

115

70

41

28

54

25

45

58

49

54

110

68

74

112

–

–

–

–

Lu

18.3

19.7

16.9

19.6

13.3

23.8

7.30

21.4

14.5

11.0

14.2

24.0

14.0

10.6

6.26

12.4

6.05

9.10

13.1

10.9

12.2

22.9

14.0

16.1

25.4

–

–

–

–

Hf

7238

9475

8353

8929

9119

8448

8095

11643

7502

8598

7951

10187

8629

8627

7159

8844

6342

7462

7728

9839

7701

8307

8379

7340

7787

–

–

–

–

Ta

0.37

0.20

0.10

0.23

0.08

0.18

0.07

0.36

0.22

0.09

0.39

0.24

0.10

0.13

0.21

0.16

0.19

0.11

0.16

0.21

0.24

0.14

0.17

0.06

0.16

–

–

–

–

Th

21.9

35.3

8.22

19.9

39.4

24.3

33.2

7.35

6.19

6.60

10.4

3.67

75.5

3.35

68.1

5.97

5.97

47.4

1.90

8.58

63.2

4.25

2.52

2.21

39.5

–

–

–

–

U

311

457

64.4

81.0

288

84.3

224

61.4

62.6

65.8

121

44.7

65.4

30.1

165

60.7

122

48.8

27.6

139

186

55.3

46.1

39.4

407

–

–

–

–

Th/U

0.07

0.08

0.13

0.25

0.14

0.29

0.15

0.12

0.10

0.10

0.09

0.08

1.15

0.11

0.41

0.10

0.05

0.97

0.07

0.06

0.34

0.08

0.05

0.06

0.10

–

–

–

–

Eu/Eu⁎

0.62

0.73

0.66

0.55

0.71

0.58

0.65

0.18

0.56

0.75

0.49

0.29

0.51

0.69

1.02

0.62

–

0.58

0.47

0.35

0.44

0.24

0.20

0.27

0.56

–

–

–

–

onology

Anoxygenisotopestudyofisochronmineralscanpotentiallyprovide

acriticaltestforthevalidityofmineralSm–Ndchronometers(Zheng

etal.,2002;Xieetal.,2004).Takingintoaccountthedisequilibrium

oxygenisotopefractionationbetweengarnetandomphaciteoftheDB18

eclogite,whichimpliesSm–Nddisequilibria,furthergeochronological

analysisontheDB18eclogitewasnotexecuted.

-ICPMSU–PbisotopeandREEdata

ThezirconsfromtheDB17eclogitearecolourlessandtransparent

curasfinetomedium

grainedcrystals(20–150μmindiameter),withlength/widthratiosof

1:1–2.5:1,rcons

showaCL-darkcoresurroundedbyanovergrowthrimwithweakCL

(Fig.8).Garnetandphengiteinclusionswerefoundwithinzircon,

netincludedinzirconhashigher

MnOandlowerMgOthanthatofthematrix(Fig.3d),usingtheserial

polishingstrategy(Chengetal.,2008b).Phengiteinclusionswerenot

analyzedduetoaninadequatepolishingprocedure.

TwentyfiveLA-ICPMSanalysesyieldedthreegroupsofweighted-

mean

206

Pb/

238

Uagesat315±5Maforsevenanalyses(MSWD=2.2;

groupA),373±4Mafornineanalyses(MSWD=1.1;groupB)and

422±7Mafornineanalyses(MSWD=2.3;groupC)(Fig.8;Table6).

LargebutoverlappingvariationswerefoundinbothUandThcontents

of122–457ppmand6–68ppmforgroupA,39–186ppmand2–

63ppmforgroupB,28–288ppmand2–76ppmforgroupC,

respectively(Table5).GroupAanalyseshavelowTh/Uratiosof0.05–

0.15exceptforonepoint(0.41).Th/Uratiosof0.05–0.34and0.07–

1.16wereobtainedforgroupBandCanalyses,

correlationbetweenU,ThorTh/Uratiosandagewasobserved

(Fig.9),norwerethereanysystematicdifferencesingrainsizeand

heless,the

groupCanalysesarepredominantlyinthecores,andgroupAages

distinguishthemselvesbyhigherUcontentsandgenerallylowerTh/U

izedREEpatternsforthethreegroupsaresimilarand

owrelativeHREE-enrichmentandsmall,both

negativeandpositiveEuanomalies(Fig.4a).Nosystematiccorrela-

r,theREEsare

largelydepletedfromcoretorimwithinasinglezircon.

–HfandSm–Ndisotopicdata

Weanalyzedhandpickedseparatesofgarnet,omphaciteandthe

wholerock(seesummaryofisotopicresultsinTable7andFig.11).The

parent/daughterratiosofLu/HfandSm/Ndforgarnetsare6.9–8.5and

1.9–2.4,suggestingthatgarnetfractionatesLufromHfmorestrongly

thanSmfromNd,whichisconsistentwithexperimentalpartition

coeffi,Greenetal.,2000).Thethreegarnetfractionshavelow

Ndconcentrations(0.22–0.39ppm)and

147

Sm/

144

Ndratiosof∼1.4

(Table7),comparabletotheNdcontents(0.22–0.41ppm)andtheSm/

Ndratios(0.8–2.2)acrosssinglegarnet(Fig.5a).Theconsistencyofbulk

andinsituSm/Ndresultssuggeststhatthehandpickedbulkfractions

arelargelyfreeofMREE-enrichedinclusionsinisotopicdisequilibrium

fractionshaveratherlowHfcontentsof

∼0.1ppm,producinglesspreciseHfisotopeanalysesthanthewhole

valueiswithintherangeoftheinsitudata,contamination

nificantlylowerLu/Hf

ratiosandhigherHfconcentrationsofthebomb-digestedwholerocksas

comparedtoSavillex-digestedsplitsareduetothedissolutionofzircon.

IsochronswereconstrainedbySavillex-digestedwholerock,

bomb-digestedwholerock,garnet,kyaniteandomphacitealiquots.

Thethreegarnetfractionscombinedwithomphacite,kyanite,and

twowholerocksyieldaLu–Hfageof268.9±6.9Ma(MSWD=27)

withaε

Hf

(t)=+8.2andacorrespondingSm–Ndageof271.3±

5.3Ma(MSWD=8.6)withaε

Nd

(t)=+rornotthe

bomb-digestedwholerockshouldbeincludedintheisochron

regressionsdependsonwhetherthezirconinthatsamplewasin

ingthebomb-digested

wholerockimprovestheprecisionoftheregressionandyields

indistinguishableinitialisotopicvalue,suggestingthatbulkzircons

areco-geneticwiththepeakmetamorphicassemblage,althoughthe

mucholderzirconU–Pbagesappeartonotfavorthisinterpretation.

tal./Lithos110(2009)327–342335

rmplotforoxygenisotopefractionationsbetweenquartzandother

mineralsfortheXiongdianeclogitefromthewesternDabie(a).Oxygenisotope

fractionations(Δ

18

O)δ

18

Ovaluesinthe

Xiongdianeclogites(b).Notethatthesamplesbeyondthegreydomainareoutof

oxygenisotopicequilibriumandthelimitedrangeofδ

18

O,indicatingtheirsimilarpre-

onationparametersAareafterZhengetal.

(2003)withomphacite(Jd

45

Di

55

).Ky—kyanite;Phen—phengite;Omp—omphacite;

Zo—zoisite;Grt—garnet;Amp—amphibole.

–Arisotopicdata

Thephengiteandamphibolemulti-grainlaserstep-heating

analysesyieldedinhomogeneouslyreleasedexcess

40

Arwithnon-

interpretabletotalfusionagesof548.8±1.6and861.7±2.6Ma,

respectively(Fig.11),whichareanomalouslyolderthantheages

obtainedbyU–Pbzirconages(Fig.8)andLu–HfandSm–Ndmineral

isochronages(Fig.10).Extremelyhighinitial

40

Ar/

36

Arratiosof11282

and4471areobtainedforphengiteandamphibole,respectively,

indicatingthepresenceofexcess/inherited

40

ybe

inheritedfromtheprotolith(Scaillet,1996)orberelatedtosecondary

fluidactivity(DeJongetal.,2001),fluidinclusions(Giorgisetal.,

2000),diffusionofexcessargonthroughtheminerallattice(Reddyet

al.,1996)orincorporationinsolidinclusions(Bovenetal.,2001).The

excess

40

ArinphengitefromUHPeclogitesintheSuluorogenwas

interpretedasinheritedfromtheprotolith(Lietal.,1994;Giorgis

etal.,2000).

sion

c-typeprotolith

ThehighSiO

2

(52–58%)contentsoftheXiongdianeclogite

resemblethoseofbasalticandesiteandaredistinctfromthemajor

basalticorgabbroiceclogitesacrosstheDabieOrogenicBelt.

Petrologicevidenceofproposedmetasomaticorigin(Jahnetal.,

2005)hCr(430–1118ppm)andNi(88–

247ppm)concentrationsdonotfavoranislandarcbasaltorandesite

r,thenegativeNbandZranomalies(Fig.2b)aretypical

ngdianeclogitesaredistinguishedfromthe

majorityofeclogitesintheDabiebyhighlypositiveinitialε

Nd

(t)and

ε

Hf

(t)values(Lietal.,2001;Fuetal.,2002;Jahnetal.,2005;Fig.10).

Themajorandtraceelementcontentsandthustheelementratiosof

theXiongdianeclogite,suchasBa/Nb(11–729),U/Th(0.3–2.5),and

Pb/La(1.0–9.0),erseelementandisotope

featuresappeartoarguethattheprotolithoftheXiongdianeclogites

wasatransitionaltypebasaltfromormixtureofislandarcandoceanic

crust,whichwasderivedfromthedepletedmantlewithlittle

tion,theoxygenisotope

data(Fuetal.,2002;Jahnetal.,2005;Table4)indicatethatthe

eclogiteprotolithexchangedisotopicallywithancientseawaterprior

toplatesubductionatlowtemperatures,whichisconsistentwiththe

high

87

Sr/

86

Srratios(Jahnetal.,2005)andcontrastsstronglywiththe

majorityofeclogitesfromtheDabieorogen(Zhengetal.,2003).

ThenewzirconU–Pb,garnetLu–HfandSm–Ndages,togetherwith

phengiteandamphiboleAr–Arresultspresentedhere,raiseimportant

questionsregardingthemetamorphichistoryoftheXiongdianeclogites

fromthewesternDabieorogen.

5.2.U–Pbageinterpretation

ZirconinHPmetamorphicrockscanpreserveinheritanceand

crystallizeatdifferentmetamorphicepisodesduringsubductionand

exhumationduetoitstemperatureresistanceandchemicalrobust-

r,theassumptionthatazirconU–Pbagecorrespondsto

peakmetamorphicPTconditionsrequiresverificationbyindependent

methods,suchasshapeandinternalstructure,chemicalcomposition/

elementalratio(HoskinandBlack,2000;Rubatto,2002;Bingenetal.,

2004;Wuetal.,2006),PT-indexedmineralinclusions(Hermannetal.,

2001)andgeothermometers(Watsonetal.,2006).Nevertheless,the

linkbetweenpetrologic,mineralchemical,microstructuralandzircon

agewithaparticularmetamorphicepisodeoftenremainsambiguous.

Threegroupsofweighted-mean

206

Pb/

238

Uagesat315±5Ma

(groupA),373±4Ma(groupB)and422±7Ma(groupC)were

obtainedfortheXiongdianeclogite(Fig.8).Thereisnosignificant

U–PbisotopicanalysesandrepresentativeCLimagesofzirconfromthe

icaluncertaintiesarewithin1σ.

tal./Lithos110(2009)327–342

Table6

ZirconU–PbisotopicdataobtainedbyLA-ICPMSfortheXiongdianeclogite.

SpotElement(ppm)

Th

DB17-1

DB17-2

DB17-3

DB17-4

DB17-5

DB17-6

DB17-7

DB17-8

DB17-9

DB17-10

DB17-11

DB17-12

DB17-13

DB17-14

DB17-15

DB17-16

DB17-17

DB17-18

DB17-19

DB17-20

DB17-21

DB17-22

DB17-23

DB17-24

DB17-25

22

35

8.2

20

39

24

33

7.4

6.2

6.6

10

3.7

76

3.4

68

6.0

6.0

47

1.9

8.6

63

4.3

2.5

2.2

39

U

311

457

64

81

288

84

224

61

63

66

121

45

65

30

165

61

122

49

28

139

186

55

46

39

407

Th/U

0.07

0.08

0.13

0.25

0.14

0.29

0.15

0.12

0.10

0.10

0.09

0.08

1.16

0.11

0.41

0.10

0.05

0.97

0.07

0.06

0.34

0.08

0.05

0.06

0.10

Isotopicratios

207

206

Age(Ma)

207

235

Pb/

Pb

±1σ

0.00151

0.00095

0.00227

0.00178

0.00149

0.00178

0.00125

0.00212

0.00211

0.00413

0.00183

0.00529

0.00362

0.00585

0.00160

0.00435

0.00317

0.00300

0.00533

0.00160

0.00150

0.00296

0.00357

0.00343

0.00115

Pb/

U

±1σ

0.00967

0.00676

0.02080

0.01472

0.01342

0.01433

0.00859

0.01716

0.01911

0.03767

0.01667

0.04191

0.03479

0.04722

0.01088

0.04193

0.02179

0.02784

0.05083

0.01022

0.01225

0.02718

0.02787

0.02746

0.00688

206

238

Pb/

U

±1σ

0.00059

0.00058

0.00092

0.00075

0.00081

0.00074

0.00058

0.00077

0.00086

0.00130

0.00086

0.00132

0.00120

0.00140

0.00061

0.00110

0.00083

0.00098

0.00148

0.00054

0.00068

0.00098

0.00097

0.00091

0.00050

208

232

Pb/

Th

±1σ

0.00018

0.00043

0.00124

0.00054

0.00058

0.00044

0.00040

0.00110

0.00137

0.00245

0.00109

0.00345

0.00051

0.00481

0.00029

0.00278

0.00333

0.00045

0.00415

0.00018

0.00038

0.00214

0.00337

0.00321

0.00015

207

206

Pb/

Pb

±1σ

64

40

92

69

61

69

51

86

84

165

71

210

139

239

67

176

105

87

173

69

39

93

111

107

48

207

235

Pb/

U

±1σ

7

5

14

10

9

10

6

12

13

25

11

29

23

33

8

29

16

18

33

8

9

18

20

19

5

206

238

Pb/

U

±1σ

4

4

6

5

5

4

4

5

5

8

5

8

7

9

4

7

5

6

9

3

4

6

6

6

3

0.05348

0.05363

0.05507

0.05702

0.05500

0.05678

0.05516

0.05518

0.05584

0.05580

0.05709

0.05598

0.05729

0.05483

0.05403

0.05510

0.05247

0.05756

0.05571

0.05361

0.05503

0.05464

0.05546

0.05600

0.05482

0.37490

0.38067

0.51114

0.47704

0.50143

0.46201

0.38108

0.45117

0.51129

0.51942

0.52685

0.45189

0.55943

0.45021

0.37030

0.44942

0.36875

0.54419

0.54069

0.36662

0.45957

0.51116

0.44233

0.45634

0.37286

0.05084

0.05148

0.06732

0.06067

0.06612

0.05902

0.05011

0.05930

0.06641

0.06751

0.06693

0.05855

0.07082

0.05955

0.04971

0.05930

0.05098

0.06858

0.07040

0.04960

0.06059

0.06787

0.05786

0.05911

0.04933

0.01592

0.01925

0.02121

0.01903

0.01715

0.01721

0.01637

0.01848

0.01978

0.03321

0.02228

0.02891

0.02300

0.03210

0.01484

0.02284

0.05295

0.02209

0.03778

0.01552

0.01965

0.02612

0.02359

0.02143

0.01540

349

356

415

492

412

483

419

420

446

444

495

452

503

405

372

416

306

513

441

355

413

398

431

452

405

323

328

419

396

413

386

328

378

419

425

430

379

451

377

320

377

319

441

439

317

384

419

372

382

322

320

324

420

380

413

370

315

371

414

421

418

367

441

373

313

371

321

428

439

312

379

423

363

370

310

differenceinU,ThandREEcontentsforthegrainsofthethreeage

elationbetweenU,ThorTh/Uratiosandagesandno

systematiccorrelationbetweenREEpatternsandagescouldbefound.

AllthezircondomainsshowparallelandlargelyoverlappingHREE-

enrichment,implyingthatzircongrowthisnotincommunication

withgarnetand/orzirconformedinan‘open’system,preventingany

changeinzirconcompositionduetothecrystallizationofgarnetor

tion,thecalculatedzircon-garnetREEdistribu-

tionappearstosuggestthatallthezircondomainsdidnotattainREE

equilibriumwiththematrixgarnets(Fig.4b),eventhoughthereis

uncertaintyoverwhichD

REE

(Zrn/Grt),

Harleyetal.,2001;Rubatto,2002;WhitehouseandPlatt,2003;

RubattoandHermann,2007).Moreover,Ti-in-zircongeotherm-

ometercalculationsyield667–706°CforgroupAzircondomains,

688–732°CforgroupBand671–718°CforgroupC,muchhigherthan

thepetrologicestimatepeakeclogite–faciesmetamorphismof

∼620°hattheTiinzirconisinequilibriumwithrutile,all

kofa

flatREEpattern,thesimilargrowthtemperature,andtheindistinct

Th/Uratiosappeartoargueforamagmaticoriginforallthreegroups

r,alloftheolderages(groupC)comefromthe

coresindarkCLwithplagioclaseinclusions,whiletheovergrowthrim

positionsofthe

garnetinclusionsinzirconmimicthoseofthematrixgarnetcores,

distinguishingthemfromthematrixgarnetrimbyahigherMnO

content(Fig.3d).Therefore,the315±5Maageclustersareprobablya

maximumestimateofpeakeclogite–faciesmetamorphismandlikely

skewedtoearliergarnetgrowthontheprogradeP–rce

ageswithaweighted-meanvalueof422±7Mamayrepresentan

arentconcordantzircon

datesof∼370Macouldhavebeenproducedbypartialrecrystalliza-

,Pidgeon,1992),orrepresentearlierprogradeorasecond

ssicageswererecordedinoursample,reflecting

eitherincompletepreservationorinsufficientinvestigation.

Therefore,itmostlikelythatthepeakmetamorphictemperature

fortheXiongdianeclogitewasnohigherthan700°C,asestimated

fromTi-in-zirconandZr-in-rutile,andthe∼620°Cisaretrograde

temperature,asrecordedinthegarnetriminoxideionsandoxygen

equilibriumzircon-garnetREErelationshipsobserved

intheXiongdianeclogiteindicatethattheeclogite–facieseventthat

ledtothemajorityofgarnetformationandthustheomphacite–garnet

assemblageisnotthesameonethatledtothechemicalmodification

metamorphiceventsthatformedandlaterpartiallyreplacedthe

garnetarenotregisteredinthezirconagedataasobtainedfrom

modifigeochronologythereforerecordsthe

later,post-peakcoolinghistory.

–HfandSm–Ndageinterpretation

TheconsistentgarnetisochronLu–HfandSm–Ndagesofca.

270Mamostlikelyreflectaresettingeventwhenfluid-assisted

recrystallizationofgarnetoccurredduringHPeclogite–faciesmeta-

lydatingtheeclogiticassemblageofgarnetand

omphaciteisfarmorestraightforwardthanfortheaccessoryminerals,

suchaszirconandmonazite,whichmaynotreflecttheonlyeclogitic

r,theacquiredagedeterminedusinggarnet

ationbetweenzircon

206

Pb/

238

UageandTh/UratiofortheXiongdianeclogite.

tal./Lithos110(2009)327–342

Table7

Lu–HfandSm–NdisotopedatafortheXiongdianeclogite.

Sample

bombWR

savWR

Grt1

Grt2

Grt3

Omp1

Omp2

Lu(ppm)

0.242

0.193

0.917

0.927

0.904

0.007

0.007

Hf(ppm)

2.20

0.362

0.113

0.110

0.131

0.448

0.448

176

337

Lu/

177

Hf

176

Hf/

177

HfSm(ppm)

1.21

1.21

0.920

0.541

0.532

0.250

0.272

Nd(ppm)

3.97

4.00

0.392

0.222

0.285

0.826

0.941

147

Sm/

144

Nd

143

Nd/

144

Nd

0.0156

0.0756

1.148

1.201

0.9765

0.0023

0.0023

0.282435±8

0.283216±23

0.288512±17

0.288973±17

0.287722±23

0.282892±9

0.282773±10

0.1836

0.1832

1.419

1.474

1.130

0.1832

0.1749

0.512792±9

0.512798±6

0.514989±22

0.515083±21

0.514426±16

0.512765±9

0.512738±9

bombWR—wholerockbybombdigestion,savWR—wholerockbysavillexdigestion,Grt—garnet,Omp—omphacite.

Decayconstantsusedfor

147

Smand

176

Luis6.54×10

−12

yr

−1

and1.867×10

−11

yr

−1

(Schereretal.,2001;Söderlundetal.,2004).

Ageerrorsare95%confidencelevelandwerecalculatedusingIsoplotv.3.5(Ludwig,2003).Errorscalculatedforagesarebasedsolelyonexternalreproducibilityofspikedstandards

andwhole-rocksamples(errorsforindividualanalysesarenegligible):

147

Sm

/144

Nd=0.5%,

143

Nd/

144

Nd=0.0035%,

176

Lu/

177

Hf=0.5%,

176

Hf/

177

Hf=0.01%.

Lu–Hfand/orSm–Ndgeochronologydependsonhowaccuratelythe

analyzedgarnetseparatesreflectthecompletegarnetchemistry

rmore,inordertospecifythegeological

meaningsoftheseages,thegarnetgrowthhistoryandmetamorphic

pathmustbeunraveled.

Biasesproducedduringmineralseparationwillresultinbiased

ages(Chengetal.,2008a).Ideally,thoroughanalysesofparent-

daughterzoningprofilesforgarnetsofdifferentsizesandthecrystal

sizedistributionofthegarnetgrainswouldclarifytheoveralleffect.

However,er,

althoughX-raycomputedtomographyorserialsectioningmaybe

usedtolocatetheexactgeometriccentersofthegarnetporphyro-

blasts,suchmethodsdonotnecessarilyidentifygarnetnucleation

centers,

approachisnotpracticalforoursamples,inwhichgarnetsarelargely

polycrystalswithmultiplenucleationcenters(Fig.3).Inaddition,

zoningprofilesofasinglegarnetmaynotberepresentativeofmulti-

ore,constraintsfrominsitu

–NdandLu–—garnet,Omp—omphacite,savWR—whole

rockbySavillex-digestion,bombWR—arsare

significantlysmallerthanthesymbols.

zoninganalysesareusefulonlyasguidestowhateventanisotopicage

heless,theclosuretemperatureforthespecific

geochronologicalsystemissubjecttovariousfactors,includinggrain

size,coolingrate,mineralmodeandfluid(Dodson,1973;Chenetal.,

2007).Linkinganisotopicagetoaspecificgarnetgrowthepisodeis

rtodeterminewhethertheseagesreflect

progradegarnetgrowth,peakmetamorphism,orcooling,the

observedelementalzoningpatternsingarnetmustbeinterpreted.

Thepartially-preservedgrowthzoninginmajorelementsandSm,

Nd,LuandHfingarnetporphyroblastsfortheXiongdianeclogite

(DB17)indicatethatpotentialdiffusiveresettingofagesisnot

significantinthissample,pointingtoalowerpeakmetamorphic

temperaturethanLu––Hfageof268.9±6.9Ma

thereforemostlikelyapproximatesgarnetgrowthonaprogradeP–T

ghdatingtheprograde-zoning-preservedgarnetsdoes

notnecessarilyproduceanagethatisskewedtowardstheearlystage

ofprogradegarnetgrowth,,

Lapenetal.,2003),itmaybiastowardsaparticularmetamorphic

episode(Chengetal.,2008b).

Thegarnethasaninclusion-richcoresurroundedbyasharply

definedinclusion-freeidiomorphicrim,indicatingchangingcondi-

tionsduringgrowth,suchasthediffusionandcrystalgrowthrates.

Thisfeaturemayalsobeduetoachangeintheporphyroblast-forming

reaction,actionbecameactive,consumingthemineralthat

formsinclusions(PasschierandTrouw,1996).Whateverthecause,the

ghgarnet

porphyroblastsareoftenisolatedfromeachother,grainsthat

impingedorcontactedeachotherinlategarnetgrowtharerecognized

asclustersofmultiplegrains(Fig.3).Theformationandprevalenceof

polycrystalsimpliesthatgarnetnucleiimpingedoneachotherand

r,theabsenceofroundededgesonsmallgarnet

crystals,whichwouldbeexpectedgiventhecoarseningoflargegarnet

grainsattheexpenseofsmallergrains,andtherestrictedwidth(200–

250μm)oftheinclusion-freeovergrownrimsregardlessofgarnetsize,

soopposesthesize-

ore,coalescence(Ostwald

ripening)waslikelyadominantfeatureduringearlygarnetgrowth

otonous

decreaseinMnOandHREEspro

files

fromcoretorimsuggeststhatthe

thermallyaccelerated,diffusion-controlled(TADC)nucleationand

growthmechanism(Carlson,1999)wasnotnecessaryasamechanism

forgarnetgrowth,andfavorsasurface-kinetics-controlledgrowth

mechanismwithconstantradiusgrowthrateforthelategarnet

growth(Chengetal.,2008b).Nevertheless,flatMnOprofilesforsmall

garnetgrains,andtheidenticalchemistryforinclusion-freesmall

garnetswithlargegarnetrims,indicatethatmostsmallgarnetgrains

andtheouterrimsofthelargegarnetporphyroblastsformedina

distinctgrowthepisoderatherthanacontinuationoftheearlier

growthstageofthelargegarnetcores(Chengetal.,2009).

Electronbackscatterdiffraction(EBSD)analysesofthesepoly-

crystals,presentedinaseparatestudy(Chengetal.,inpreparation),

revealthatgarnetpolycrystalsarecomprisedoftwoormoredistinct

tal./Lithos110(2009)327–342

polygonalfabricofgarnetpolycrystalsmaybeformedbyfluid-aided

staticrecrystallizationinresponsetograinboundaryareareduction.

Theoccurrenceofomphaciteinthetransitionzonefrominclusion-

richtoinclusion-freeingarnetimpliesthattheouterrimdidnotgrow

eralseparationbiasedthe

bulkLucompositionstowardsthoseofthegarnetrim(Fig.5b),likely

ore,TheLu–Hfageof

268.9±6.9Maisskewedtoreflectingthetimingofinclusion-free

garnetgrowthratherthanearlygarnetgrowthoranaverageestimate

respondingandidenticalSm–Nd

ageof271.3±5.3Maisskewedtowardslatergarnetgrowthbecausea

muchgreaterproportionofSmispresentwithintheoutershells

heeclogitic–facies

P–Tconditionsestimatedfromthegarnetrimcompositionof2.2GPa

and621°C,ca.270Mashouldbeaminimumestimateoftheendof

peakmetamorphism,andlikelydatesearlyexhumationafterpeak

pressuremetamorphism.

EclogitesthatrecrystallizedaboveLu–HfandSm–Ndclosure

temperaturesforlongenoughre-equilibratedthetwochronometric

systemswiththematrix,andre-initializedthetwogeochronometers.

DifferencesintheLu–HfandSm–Ndages,therefore,shouldprovide

insightintothetime-scalesofsuccessivecoolingratherthanskewsto

nticalLu–HfandSm–Ndagesforthe

Xiongdianeclogiteprobablyonlyrecordashortperiodoffinalgarnet

growthanddonotreflectthecompletegarnetgrowthhistoryas

servedMnOandLuzoning

suggestthattheblockingtemperatureoftheLu–HfandSm–Ndsystems

intheXiongdianeclogiteareapparentlygreaterthanthemetamorphic

temperatureof∼620°essscatterobservedinthegarnetLu–Hf

regressionline(MSWD=27)deriveseitherfromincompleteHfisotope

equilibrationduringeclogite–faciesmetamorphismand/orresultsfrom

betterseparationofdifferentstagesofgarnetgrowth(t

mineralseparationactuallyresultsintheworststatistics)ifgarnet

growthoccursoverMyrtotensofMyr(Kohn,2009).

Thequartz–garnetpairsgiveaconsistentOisotopetemperatureof

∼620°Cwithapetrologicestimateof∼621°CbytheTHERMOCALC

method,indicatingthatOisotopeequilibriumwasachievedand

ervationofhigh-salinityaqueousinclusionsin

epidotecoexistingwithsolidphasesindicatesthatinsolubledaughter

mineralsweretrappedintheHPmetamorphicfluid(Fuetal.,2002),as

shownbygarnetandomphaciteinclusionsbutnotamphibole(Jahn

etal.,2005),incontrasttothelow-salinityaqueousretrogradefluid.

Therefore,weproposethattheformergarnetgrainsimpingedon

eachother,coalescedbytheaidofhigh-salinityaqueousfl

uid,largely

resulting

fluidisnotonlyan

importantmediumthatfacilitatedgarnetovergrowthbutitalso

affectedtheLu–HfandSm–Ndisotopere-equilibrationduringHP

eclogite–faciesrecrystallization,resultinginlargelyconsistentiso-

chronages.

–Arageinterpretation

TheanomalousNeoproterozoicphengiteandamphibole

40

Ar/

39

Ar

agesaregeologicallymeaninglessandwereattributedtoexcess/

40

Arhasbeenwelldocumented

for

40

Ar/

39

,Lietal.,

1994;ArnaudandKelley,1995).The

40

Ar/

39

Aragedeterminationson

thesephengiteandamphiboleseparatesapparentlyhavebroadand

concordantageplateaus(Fig.11).However,theundefinedisochron

andtheextremelyhighinitial

40

Ar/

36

Arratiosof11282and4471

deviatesignificantlyfromthatofthemodernatmosphere,suggesting

abundantexcess

40

Wijbrans(2006)introducedtheupper

andlowerreferencelinesintheAr–Arnormalisochronplotto

constraintheprotolithandmetamorphicages,r,

thebasisofthereferencelinesisgeologicallyproblematicandsuch

referencelinescannotbedefi

agesof555.25±6.19and489.19±15.17Ma,correspondingtothe

upperandlowerreferencelinesforthephengite,aresignificantlyolder

logicalsignificanceoftheolderreference

heless,alltheAr–Aragesarehundredsof

millionsofyearsolderthantheoldestzirconU–Pbageaswellasthe

Lu–HfandSm–ges,therefore,aregeologically

meaningless,althoughthesourceofexcess

40

Arhasyettobe

edinsituintra-grainprofilingofphengitesand

amphibolesandotherco-geneticmineralsmayshedlightonthisissue.

icalimplications

Thesymmetricalthermobaricpattern(Liuetal.,2006b),in

conjunctionwithstructural(Hackeretal.,1998)andconsistent

Triassicgeochronologicaldata(Liuetal.,2004a;Liuetal.,2006a;Wu

etal.,2008)formajorityofeclogitesfromwesternDabie,demon-

stratesthatthewesternDabiebelongstothesameHPsliceoverlying

theUHPsliceandisacoherentpartoftheTriassicDabieandSulu.

Threegroupsofweighted-mean

206

Pb/

238

Uagesatca.315,ca.373and

ca.422MaoftheXiongdianeclogitearebroadlyconsistentwith

previouslyreportedU–Pbzirconages(Jianetal.,1997,2001;Sunetal.,

2002;Gaoetal.,2002).Incontrast,coupledLu–HfandSm–Nddates

insunclear

whethertheexacttimingofHPmetamorphismoccurredinthe

Ordovician(Jianetal.,1997,2001;Gaoetal.,2002),Carboniferous

(Sunetal.,2002;Ratschbacheretal.,2006)orTriassic(Gaoetal.,

2002),andwhethermultipleHPmetamorphiceventsareresponsible

forthespreadages(Jianetal.,1997,2001;Gaoetal.,2002;Sunetal.,

2002;Ratschbacheretal.,2006)orwhetherthePaleozoicagesare

essentiallymeaningless(Jahnetal.,2005).

Collectively,theagesofOrdovician,Silurian,Devonian,Carbonifer-

ousandTriassichavebeenrecordedintheU–Pbzircongeochronol-

ovician(Gaoetal.,2002)toSilurian(Jianetal.,1997,

2001;Gaoetal.,2002;Sunetal.,2002;thisstudy)U–Pbzirconage

clusterscanbereadilyinterpretedastheprotolithagebasedonthe

steepREEpatterns,negativeEuanomaliesandplagioclaseinclusions

(Figs.4and8).TheCarboniferousU–PbzirconageslikelydatetheHP

metamorphismbasedontheflatREEpatternsandtheHP-indexed

mineralinclusionswithintheanalyzedspotdomains(Sunetal.,

2002),andmayreflectHPmetamorphismduringaprogradepath

ratherthanthepeakconditionsbasedonthecompositionalsimilarity

betweenthegarnetinclusionandmatrixgarnetcore(Fig.3d).This

suggestsdecoupledgrowthbetweenzirconsandthematrixgarnet.

TheDevonian(Jianetal.,1997,2001;Gaoetal.,2002;Sunetal.,2002;

thisstudy)ageseitherreflectadiverseoriginoftheprotolithorjust

assicageapparentlymarksmajor

subductionacrossthewholeDabie–Suluorogenicbelt(Gaoetal.,

2002)andthuswitnessestheeffectoftheTriassiccontinental

r,numerousTriassicmuscovite

40

Ar/

39

Arages(Yeet

al.,1993;Xuetal.,2000;Ratschbacheretal.,2006)inthisareaand

Rb/Srmineral-whole-rockisochronages(Yeetal.,1993)appearto

argueforaTriassicoverprintingeventratherthandistinctHP

metamorphism(Gaoetal.,2002).Thesignificanceofbarroisite/

phengite

40

Ar/

39

Aragesofca.430–350Ma(Yeetal.,1993;Xuetal.,

2000)cannotbepracticallyinterpretedduetothesaddle-shaped

steppedheatingreleasespectrumandtheolderthanU–Pbzircon

.400MaRb/Srwhole-rockageislikelymeaningless

becauseofthelarge-scaleisotopicdisequilibrium(Jahnetal.,2005).

ThepresentLu–HfandSm–NddatingfillthePermiangapandare

interpretedtorepresentlategarnetgrowthduringdecompressionsoon

–Pbzirconageofca.315Ma

decoupledfromtheLu–Hf/Sm–Ndageofca.270Mabracketsthepeak

eclogite–er-

pretationthattheconsistentLu–Hf/Sm–Ndageofca.270Marepresents

theterminationofgarnetgrowthindicatesthateclogite–facies

tal./Lithos110(2009)327–342339

Fig.11.

40

Ar/

39

Aragespectraand

39

Ar

K

/

37

Ar

Ca

ofphengiteandamphibolecrystalsby

step-heatinganalysis.

metamorphismoccurredaslateas270Ma,andthusthemaximum

peakHPmetamorphismisbracketedbetweenca.315andca.270Ma.

Combinedwiththeminimumestimationoftheinitiationofcontinental

subductionofpriortoca.245Ma(Wuetal.,2006;Chengetal.,2008a),

ourdatafavoramodelofcontinuousoceanic-continentalsubduction:

oceanicsubductionstartedpriortoca.315Ma,reachingHPmeta-

morphicconditionsatca.315–270Ma,andsomeHProckswerescraped

offwithcontinuoussubductionduetocrustaldetachment,likethe

greenschist–faciesrocksinthenorthernmarginoftheDabie–Sulu

orogenicbelt(Zhengetal.,2005).Thehigh-grademetamorphicrocks

ncontinentalsubduction

startedpriortoca.245MaandreachedpeakHP/UHPconditionsduring

235–225Ma,followedbyfastexhumation,producingwidespread

eclogitefaciesmetamorphismthroughouttheDabie–

wholesubductionofoceanictocontinentalcrustspansabout100Myr

fromCarboniferoustoTriassic.

EclogitesintheHuwanmélangeshowacomplexoriginanddiverse

,Sunetal.,2002;Liuetal.,2004a;Jahnetal.,2005;

Liuetal.,2006b;Ratschbacheretal.,2006;thisstudy).TheXiongdian

eclogiteinthesouthernareahasanoceanic-basalttoisland-arcorigin

withouttectonicaffinitywiththeSouthChinaBlock,reachingpeakHP

metamorphicconditionsatca.315–orth,theHujiawan

eclogitebearsaSilurian–DevonianislandarcaffinityandsufferedHP

metamorphismatca.300Ma(Liuetal.,2001).Incontrast,the

Qianjinhepengeclogiteatthenorthernmarginhasitsoriginfromthe

SouthChinaBlockwithNeoproterozoicprotolithages(Sunetal.,

2002),showinganaffinitywiththeQinlingmicrocontinentwith

Paleozoicages(Liuetal.,2004a).TheHPmetamorphictimingofthe

QianjinhepengeclogiteisconstrainedtotheTriassic(Liuetal.,2004a),

consistentwiththemajorityofeclogitesfromwesternDabie.

TheoverallHuwanmélangehasaPermian–Triassiccoolingages

(Yeetal.,1993;Webbetal.,1999;Xuetal.,2000;Ratschbacheretal.,

2006).Inthisregard,weproposethattheHuwanmélangerepresents

amixtureofoceanicbasaltsandSouthChinaBlockbasement

materialsthatstucktogetherduringtheTriassicafterHPmetamorph-

boniferous–

PermianHPeclogites,ngdianeclogite,overlaidthose

Triassiceclogitesandwereexhumedandsuccessivelyexposedinthe

pothesisalsopredictsthatCarboniferous–Permian

HProcksanalogoustotheXiongdianeclogitemaybediscoveredtothe

southofwesternDabie.

sions

Thepresentstudyhelpsresolvesomeimportantquestions

concerningthetimingofoceanictocontinentalsubductionandthe

ngdianeclogiteinthewestern

partoftheDabieorogenyieldsconsistentLu–HfandSm–Ndisochron

agesofca.270Ma,recordingprocessesfrompeakHPmetamorphism

unctionwithzirconU–Pb

dates,theonsetofoceanicsubductionisconstrainedtothe

CarboniferousandpeakHPmetamorphismoccurredpriortoca.

nationofU

–Pb

zirconandLu–Hf/Sm–Ndgarnet

geochronologieswithinsituelementalanalysesallowsabetter

interpretationofthesenewradiometricagesandanimproved

understandingofthechronologyofsubduction-zonemetamorphism

ultsaddafurtherargument

infavouroftheviewthattheoceanicsubductionreachedHP

conditionsduringthelateCarboniferoustoPermianinWestDabie.

ThesignificanceoftheDevoniandatesisnotyetresolved;theyeither

indicateadiverseprotolithorreflectanearlierprogrademetamorph-

ductedoceanicbasaltswerelargelyconsumed,butafew

HPmetamorphicrocksreachedpeakconditionsnoearlythanca.

rescrapedoffofthedowngoingoceaniccrustbefore

ca.270Ma,stucktogetherwiththeTriassicHProcksandwere

adonotsupportthatthe

rall

oceanictocontinentalsubductioncouldspanaperiodaslongas

∼100Myr,butspecificepisodesofzirconandgarnetgrowthmayonly

occurinanarrowtimeintervalwithoutgeochronologicalregistration.

Acknowledgements

,wafortheirhelpwiththe

lediscussionswithH.L.

Qiu,

ightsof

reviewersIanBuick,GrayBeboutandananonymouswerehelpfulin

search

wassupportedbytheChinese‘973’project(2009CB825006),Program

forYoungExcellentTalentsinTongjiUniversity,COE-21programto

ra,NSFgrantsEAR-0609856andEAR-0711326toJeff

VervoortandsponsoredbyProgramofShanghaiSubjectChiefScientist

(08XD14042).

AppendixA

icalmethods

ockmajorandtraceelementanalysis

Wholerockmajorelementcompositions,alongwithCrandNi,

weredeterminedbyX-rayfluorescencespectrometer(XRF),onglass

beadsfluxedfrommixing∼100mgofpowderedsampleand∼5gof

Li

4

B

2

O

7

.Analyticalerrors(RSD%)were1–2%(1σ)

elementconcentrationsweredeterminedbyanAgilent7500cs

ICPMS,hostedattheInstituteforStudyoftheEarth'sInterior(ISEI),

OkayamaUniversityatMisasa,Japan,throughconventionalmethods

(MakishimaandNakamura,1997;Makishimaetal.,1999).Analytical

tal./Lithos110(2009)327–342

errorsfortraceelementsaregenerallybetterthan3%forhighfield

strength(HFS)elementsand5%forotherelements.

mineralmajorandtraceelementanalysis

Mineralmajorandtraceelementanalyseswereobtainedatthe

ISEI,OkayamaUniversityatMisasa,Japan,usingaJEOL8800electron

microprobeandCAMECAims5fsecondary-ionmassspectrometer.

-

mentaldetailscanbefoundinChengetal.(2007).Analyticalerrors

(RSD%)formajorelementswere1–2%(1σ)orbetterand5–10%(1σ)

stakenduringmicroprobeanalysesto

avoidmicroinclusionsandfracturesusingavisualstagesystem.

–HfandSm–Ndisotopeanalysis

Wholerockswerecrushedwithasteeljawcrusherandpartsofthe

rockchipswerepowderedinanagatemillwhileotherswerehand

pickedtoobtaincleanmineralseparatesunderabinocularmicroscope.

Amagneticseparatorwasnotusedsoastoavoidsplittingthegarnet

intodifferentmagneticfractions(Lapenetal.,2003)andbiasingthe

al

preparationsandSm/NdandLu/Hfisotopicanalyticalproceduresare

identicaltothoseofChengetal.(2008a),exceptthatconcentrated

H

2

SO

4

leachingofgarnetfractionswasperformedinthisstudyto

removephosphateinclusions(asrecommendedbyAnczkiewiczand

Thirlwall,2003),asphosphateinclusionswereobservedwithingarnet

porphyroblasts(Fig.3c).Lu–HfandSm–Ndisotopicdatawere

analyzedusingtheThermoFinniganNEPTUNEatWashingtonState

roducibilityof

149

Sm/

152

Sminthisstudywas

0.516908±0.000037(2σ;n=25).Theoverallexternaluncertainties

appliedtomeasureddataare0.5%for

176

Lu/

177

Hfor

147

Sm/

144

Nd,

0.01%for

176

Hf/

177

Hfand0.0035%for

143

Nd/

144

Nd,asencountered

–Hf

ageswerecalculatedusingthe

176

Ludecayconstantvalueof1.867×10

−11

(Schereretal.,2001;Söderlundetal.,2004).Lu–HfandSm–Nd

isochronswereproducedwiththeprogramIsoplot/Ex(Ludwig,2003)

witherrorsatthe95%confidencelevel.

U–Pb,REELA-ICPMSanalysis

Theseparatedzircongrainsfromtherocksamplesbymagneticand

heavy-liquidtechniqueswerethenpickedfromsievedfractionsunder

entativezirconsweremountedin

epoxyresinandpolisheddowntoexposegraincentresforcathodo-

luminescence(CL)ingwereobtainedbyscanning

electronmicroscopy(SEM)usinganFEIPHILIPSXL30SFEGinstrument

witha2minscanningtime,operatingatconditionsof15kVand

-ICPMSzirconU–PbanalysiswascarriedoutattheChina

ailedanalyticalprocedure

followsYuanetal.(2004).AGeoLas2005Mlaser-ablationsystem

equippedwitha193nmArF-excimerlaserinconnectionwithan

wasusedasthecarriergasto

enhancethetransporteffiicalspot

diameterwasaround35μsurementswereperformedusing

awereprocessedusing

rdsilicateglassNISTSRM610,withworking

valuesrecommendedbyPearceetal.(1996),wasusedtocalculateU

deproductionratewastunedtoless

than0.5%rageanalyticalerrorrangedfromaround10%

(1σ)forlightrareearthelements(LREE)toabout5%(1σ)fortheother

monPbcorrectionwascarriedoutusingtheEXCEL™

programofAndersen(2002),assumingt=270MafortimeofPbloss.

ThedatawereproducedwiththeprogramIsoplot/Ex(Ludwig,2003).

isotopeanalysis

Themineraloxygenisotopeanalysiswasconductedbythelaser

fluorinationtechniqueusinga25WMIR-10CO

2

laseratUniversityof

ScienceandTechnologyofChinainHefei(Zhengetal.,2002).Oxygen

gaswasdirectlytransferredtotheFinniganDelta+massspectro-

metertomeasure

18

O/

16

Oand

17

O/

16

isotopedataare

reportedaspartsperthousanddifferences(‰)fromthereference

standardVSMOWintheδ

18

erencemineralswere

used:δ

18

O=5.8‰forgarnetUWG-2(Valleyetal.,1995)and

δ

18

O=5.2‰forSCO-1olivine(Eileretal.,1995).Reproducibilities

forrepeatmeasurementsofeachstandardandoursamplesonagiven

daywerebetterthan0.1‰(1σ)forδ

18

O.

–Arisotopeanalysis

40

Ar/

39

AragesweremeasuredusingaGVInstruments®5400mass

spectrometeratGuangzhouInstituteofGeochemistry,ChineseAcademy

sandmonitorstandardDRA1sanidine(Wijbrans

etal.,1995)withanassumedageof25.26±0.07Mawereirradiatedat

the49–tionfactorsforinterfering

argonisotopesderivedfromCaandKare:(

39

Ar/

37

Ar)

Ca

=8.984×10

−4

,

(

36

Ar/

37

Ar)

Ca

=2.673×10

−3

and(

40

Ar/

39

Ar)

K

=5.97×10

−3

.The

crusherconsistsofa210mmlong,28mmborediameterhigh-

temperatureresistantstainlesssteeltube(T

max

∼1200°C).The

extractionandpurificationlineswerebakedoutforabout10hat

150°Cwithheatingtapeandthecrusherat250°Cwithanexternaltube

mentaldetailscanbefoundinQiuandWijbrans(2006).

Theblanksare:

36

Ar(0.005–0.008)mV,

37

Ar(0.0001–0.0004)mV,

38

Ar

(0.0001–0.0029)mV,

39

Ar(0.0005–0.0090)mVand

40

Ar(0.34–1.9)mV.

Thesample/blankratiosof

39

Arrangefrom703to22008forphengite

awereprocessedusingtheEXCEL™

programofKoppers(2002).

mentarydata

Supplementarydataassociatedwiththisarticlecanbefound,in

theonlineversion,atdoi:10.1016/.2009.01.013.

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