LW8lw8

TheAuthor(s)2011.
ThisarticleispublishedwithopenaccessatSpringerlink.
comcsb.
scichina.
comwww.
springer.
com/scp*Correspondingauthors(email:yangdb@jlu.
edu.
cn;xuwl@jlu.
edu.
cn)ArticleGeologyFebruary2012Vol.
57No.
6:651659doi:10.
1007/s11434-011-4852-xRepeatedmodificationoflithosphericmantleintheeasternNorthChinaCraton:ConstraintsfromSHRIMPzirconU-PbdatingofdunitexenolithsinwesternShandongYANGDeBin1,2*,XUWenLiang1*,GAOShan3,XUYiGang2&PEIFuPing11CollegeofEarthSciences,JilinUniversity,Changchun130061,China;2CASKeyLaboratoryofIsotopicGeochronologyandGeochemistry,GuangzhouInstituteofGeochemistry,ChineseAcademyofSciences,Guangzhou510640,China;3StateKeyLaboratoryofGeologicalProcessesandMineralResources,ChinaUniversityofGeosciences,Wuhan430074,ChinaReceivedJune7,2011;acceptedOctober10,2011;publishedonlineNovember21,2011FourdunitexenolithsfromtheTietonggouintrusionofwesternShandong,China,weresubjectedtoSHRIMPzirconU-PbdatingtoconstraintimingoftheNorthChinaCraton(NCC)destruction,atopicofmuchcontroversy.
Cathodoluminescenceimagesrevealedthat15ofthe18zircongrainsfromthexenolithsdisplaystripedabsorption.
Therestshowedoscillatorygrowthzoni-ation.
AllthezirconshadvariablecontentsofTh(49–3569ppm;average,885ppm)andU(184–5398ppm;average,1277ppm),andvariableTh/Uratios(0.
15–2.
04).
Thesezirconcharacteristicsindicateamagmaticorigin.
Thezirconagedatacanbedividedintofivegroups:131–145,151–164,261–280,434–452,and500–516Ma.
GroupI(131–145Ma)isconsistentwithtimingofformationoftheTietonggouhigh-Mgdiorites.
GroupII(151–164Ma)issimilarinagetoMiddle-LateJurassicmagmatismintheeasternNCC,whichincludedbothmantle-derivedandintensivecrust-derivedmagmatism.
GroupIII(261–280Ma)issimilarinagetotheEmeishanlargeigneousprovince,andGroupIV(434–452Ma)issimilarinagetoPaleozoichigh-silicamagmatismintheeasternNCC.
GroupV(500–516Ma)maycorrespondtotheglobalPan-Africanevent.
ResultsindicaterepeatedmodificationoflithosphericmantleintheeasternNCC,andsuggestthatthemostintensivemodificationoccurredinthelateMesozoic(131–164Ma).
dunite,SHRIMPzircondating,lithosphericmantle,multiplemodification,NorthChinaCratonCitation:YangDB,XuWL,GaoS,etal.
RepeatedmodificationoflithosphericmantleintheeasternNorthChinaCraton:ConstraintsfromSHRIMPzirconU-PbdatingofdunitexenolithsinwesternShandong.
ChinSciBull,2012,57:651659,doi:10.
1007/s11434-011-4852-xTheArcheanNorthChinaCraton(NCC)isanidealsettinginwhichtoinvestigatethedestructionofastablecraton.
ThelithosphericmantlebeneaththeNCCunderwentadra-maticchangefromanancient,cold,and>200-km-thicklithosphericmantleintheEarlyPaleozoictoayoung,hot,and60–80-km-thicklithosphericmantleintheCenozoic[1–11].
Thischangeinthicknesshasbeenreferredtoaslithosphericthinningorcratondestruction.
However,thereareuncertaintiesandcontroversyregardingthetimingofthislithosphericthinning,thegeodynamiccontextofthisevent,andthemechanismofthethinning.
PreviousstudieshaveproposedthatNCCdestructionoccurredduringtheMesozoic[7,12–14],theLateMesozoic[15–18],ortheMesozoicandCenozoic[19],basedmainlyonanalysesofMesozoicandCenozoicmagmatismintheeasternNCCandintheDabie-Suluorogenicbelt.
However,fewgeochrono-logicaldatahavebeenreportedforthemodifiedlithosphericmantleinthisregion.
EarlyCretaceoushigh-MgdioritesinwesternShandongcontainharzburgitexenolithswithArcheanRe-depletionmodelagesandabundantdunitexenoliths[20–24].
Traceelementdataofmineralsandwhole-rockSr-Nd-OsisotopicdataoftheTietonggouperidotitexenoliths(fromEarlyCretaceoushigh-Mgdiorites)revealthattheharzburgites652YangDB,etal.
ChinSciBullFebruary(2012)Vol.
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6representtheresidueofancientlithosphericmantle;where-as,thedunitesformedviaareactionbetweenmantleperi-dotiteandmeltderivedfromdelaminatedlowercontinentalcrust[24,25].
Inthiscase,zirconsinthexenolithsarelikelytohaveformedduringmodificationofthelithosphericman-tlebyasilicate-richmelt.
ThepresentstudyaimstoconstraintimingoftheNCCdestruction,basedondetailedpetrographicstudiesoffourdunitexenolithsentrainedbyEarlyCretaceoushigh-MgdioritesinwesternShandong,aswellasSHRIMPzirconU-Pbagedata.
Thedataconstrainthetimingofmodifica-tionoflithosphericmantlebeneaththeeasternNCC.
1GeologicalbackgroundandsampledescriptionsTheNCC–surroundedbytheCentralAsianOrogenicBelt(CAOB),theQinling-Dabie-Suluorogenicbelt,andtheYangtzeCraton(YC)(Figure1(a))–issubdividedintotheEasternBlock,WesternBlock,andinterveningTrans-NorthChinaOrogen(TNCO)/CentralOrogenicBeltbasedontheageandlithologicalassociationsofmetamorphicrocks,tectonicevolution,andtheP-T-tpathofmetamorphism[26].
WesternShandong,locatedintheEasternBlockoftheNCC(Figure1(a)),isdominatedbytheArcheanTaishanGroup,CambrianandLower-MiddleOrdovicianseries,andCarboniferous-Permiansequences.
Mesozoicstrataaredom-inantandconsistmainlyofsedimentaryrocksingrabens,whileCenozoicstrataconsistmainlyofalluvialandlacus-trinesediments[27].
InadditiontoPrecambrianigneousrocks,voluminousMesozoicintrusiverocksarewidespreadthroughoutwesternShandong.
TheTietonggouintrusion,whichisexposedoveranareaofapproximately5km2(Figure1(b)),islocatednearYanzhuangtowninLaiwucity(117°52′E,36°05′N),andconsistsmainlyofearlynorite-gabbroandlaterpyroxene-diorite.
Resultsoflaserablation-inductivelycoupledplasma-massspectrometry(LA-ICP-MS)zirconU-PbdatingandbiotiteAr-ArdatingindicatethattheTietonggoupyroxene-dioriteformedintheEarlyCretaceous(131–135Ma)[28,29].
PeridotitexenolithsareabundantintheTietonggouin-trusion,andaregenerallyellipsoidal,ranginginsizefrom3cm*2cm*1cmto8cm*6cm*4cm(Figure2(a)).
Basedontheircontentsofolivine,orthopyroxene,andcli-nopyroxene,thexenolithscanbeclassifiedintochromite-bearingdunite,spinel-bearingharzburgite,andchromite-bearingwehrlite.
Theduniteisdominant[24].
Thisstudyisfocusedexclusivelyonchromite-bearingdunitesfromtheTietonggouintrusion.
Thechromite-bearingdunitesaregreenincolorandareequigranularand/orporphyroclastic,ormassive,andconsistofolivine(~93%),chromite(~3%),orthopyroxene(~3%),andphlogopite(~1%)(Figure2(b)–(d)).
Olivinescanbesubdividedintotwogroups,basedontheirsize.
GroupIconsistsofporphyroclasticolivineswithkinkbands,rang-inginsizefrom1.
0to4.
0mm;GroupIIconsistsofun-strainedrecrystallizedolivinesranginginsizefrom0.
3to0.
6mm.
Thedunitesarecutbyveinsoforthopyroxene±phlogopite.
Secondaryclinopyroxenesoccurlocallyaroundchromitewithinthedunites.
Themineralogyandpetrogra-phyoftheduniteshavebeendescribedpreviously[24].
2MethodsToavoidcontaminationofdunitexenolithsbythehostrocks,adetailedpetrographicstudywasperformedinitially.
TheweatheredsurfacesofthexenolithsandreactionrimsbetweenFigure1GeologicalsketchmapofthewesternShandong(modifiedafter[29]).
1,Quaternarysystem;2,Paleogene-Neogenesystem;3,MesozoicErathem;4,PaleozoicErathem;5,ArcheanEonothem;6,Mesozoicvolcanicrocks;7,gabbro;8,diorite;9,fault;10,samplinglocation.
YangDB,etal.
ChinSciBullFebruary(2012)Vol.
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6653Figure2PhotographsshowingfieldoccurrenceandtexturesofdunitexenolithsfromthewesternShandong.
thexenolithsandhostrockswereremovedusingadiamondsaw.
Theremainingrockwasmanuallycrushedto100–120meshandwashedwithethanol.
Aftermagneticseparation,zirconswereconcentratedusingheavyliquid,andfinallyhand-pickedunderabinocularmicroscope.
Exceptforthemagneticseparationdevice,newtoolswereusedtoavoidcontaminationofthesamplesduringtheseparationofzir-con.
Zircongrainsweremountedinepoxy,polished,andcoatedwithgold.
Thegrainswereexaminedundertrans-mittedandreflectedlightusinganopticalmicroscope,andcathodoluminescence(CL)imageswereobtainedusingaJEOLscanningelectronmicroscopehousedattheBeijingIon-probeCenter,ChineseAcademyofGeologicalSciences,Beijing,China,torevealtheirinternalstructuresandtose-lectsuitablesitesforSHRIMPanalyses.
ThezirconswereanalyzedusingaSHRIMPIIattheBeijingIon-probeCenter,ChineseAcademyofGeologicalSciences,Beijing,China.
Detailsoftheexperimentalconditionsandprocedureshavebeendescribedpreviously[30–32].
Ageswerecalibratedagainstareferencezircon(TEM)withanageof417Ma[33].
U,Th,andPbconcentrationsweremeasuredusingthereferencezirconSL13(ageof572Ma;Ucontentof238ppm).
DatawerecalculatedusingSQUID1.
0andISOPLOT3.
0programs[34].
CommonPbwascorrectedbasedonthemeasured208Pb.
Ablationpitsweregenerallyabout25m*30minarea.
3ResultsWeobtained7zircongrainsfrom~300gofsampleLW8-42A,5grainsfrom~280gofsampleLW8-42B,5from~250gofsampleLW8-45,and2from~270gofsampleLW10-2.
AnalyticalresultsforthesamplesaregiveninTable1.
ZirconsfromsampleLW8-42AweretransparentandTable1SHRIMPzirconU-Pbdatingresultsforthedunitexenolithesa)SpotNo.
206Pbc(%)U(ppm)Th(ppm)Th/U206Pb*(ppm)206Pb/238Uage(Ma)207Pb*/206Pb*±%207Pb*/235U±%206Pb*/238U±%Err.
corr.
SampleLW8-42A10.
203152920.
9611.
2261.
0±110.
05703.
30.
3244.
80.
04133.
50.
72122.
68335490.
157.
46151.
3±6.
10.
05339.
50.
175100.
02383.
50.
34730.
293881910.
5124.
5452.
0±170.
05532.
80.
5544.
50.
07273.
40.
77140.
90136813661.
0324.
2130.
6±5.
40.
05392.
20.
1524.
10.
02053.
50.
84352.
23116115491.
3823.
9145.
3±7.
00.
04844.
40.
1525.
60.
02283.
50.
62160.
58358712990.
3768.
6141.
4±5.
00.
05012.
90.
1534.
40.
02223.
40.
76272.
506993300.
4914.
0143.
6±5.
70.
05137.
20.
1608.
00.
02253.
50.
437SampleLW8-42B13.
529512340.
2522.
7158.
2±6.
80.
0518150.
178150.
02493.
50.
23620.
49539813010.
25103140.
6±4.
80.
05121.
70.
1563.
70.
02213.
30.
89131.
63210213980.
6948.
3164.
3±6.
50.
05245.
10.
1876.
10.
02583.
40.
55840.
4614608120.
5727.
5139.
1±5.
10.
05312.
30.
1604.
10.
02183.
40.
825SampleLW8-4510.
86198029251.
5341.
0152.
1±6.
90.
06914.
20.
2275.
40.
02393.
40.
63020.
74180435692.
0433.
2139.
0±7.
60.
07603.
20.
2284.
90.
02183.
70.
75230.
868243590.
4560.
2514.
0±18.
00.
05882.
70.
6734.
30.
08303.
40.
78243.
142372120.
9211.
6322.
0±18.
00.
0700210.
490220.
05124.
20.
19251.
60234880.
399.
59280.
0±12.
00.
05446.
50.
3337.
50.
04453.
60.
486SampleLW10-210.
782282401.
0916.
2500.
3±9.
90.
0657100.
731100.
08071.
50.
14920.
3510114270.
4472.
5515.
8±3.
80.
06121.
20.
7031.
40.
08330.
720.
50031.
991841660.
9311.
2433.
6±6.
90.
06205.
60.
5955.
80.
06961.
30.
229a)Errorsin1;PbcandPb*indicatethecommonandradiogenicportions,respectively.
ErrorinStandardcalibrationwas1.
78%;commonPbcorrectedusingmeasured208Pb.
654YangDB,etal.
ChinSciBullFebruary(2012)Vol.
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6possessedellipticalorirregularshapes.
Thegrainswere35–110minlengthandhadlength/widthratiosof1.
1–2.
0(Figure3(a)).
The206Pb/238Uagesobtainedfor4of7ana-lyticalspotsfromsampleLW8-42Arangedfrom131to145Ma.
ThesezirconsdisplayedstripedabsorptioninCLimages(Figure3(a)),similartothosereportedformaficigneousrocksandthehostdiorite[29].
TheThandUcon-tentsofthezirconsvariedfrom330to1549ppmandfrom699to3587ppm,respectively,andtheirTh/Uratiosrangedfrom0.
37to1.
38(Figure4).
Thespot2zircon,whichyieldedanageof151Ma,showedtypicalmagmaticoscil-latorygrowthzonation.
Thespot1and3zircons,whichhadstripedabsorptionandTh/Uratiosof0.
95and0.
51,yielded206Pb/238Uagesof261±11and452±17Ma,respectively(Figure5(a)).
ZirconsfromsampleLW8-42B,whichwerecolorless/transparentandprismatictoellipticalinshape,were40–100mlongandhadlength/widthratiosof1.
5–2.
5(Figure3(b)).
Twoofthe4analyzedspotsinthissampleyield206Pb/238Uagesof139±5(spot4)and141±5Ma(spot2)(Figure5(b)),consistentwiththeyoungestagesofzirconsfromsampleLW8-42A.
ThesetwozirconsshowedstripedabsorptioninCLimages(Figure3(b))andhadhighUcontents(1460and5398ppm)andhighTh/Uratios(0.
57and0.
25)(Figure4),indicatingamagmaticorigin.
Theothertwozir-consyielded206Pb/238Uagesof158±7and164±7Ma,showedweaklystripedabsorptioninCLimages,hadhighcontentsofTh(234and1398ppm)andU(951and2102ppm),andhighTh/Uratios(0.
25and0.
69)(Figure4).
ZirconsfromsampleLW8-45werecolorless/transparentandstubbytoacicularorirregularinshape.
ThegrainswereFigure3Cathodoluminescence(CL)imagesofzirconsfordunitexeno-liths.
Ellipsesindicateanalysissites.
Numbersindicateanalyticalspotnumberand206Pb/238Uage.
Figure4PlotsofzirconTh/Uratiosvs.
theirU-Pbagesfordunitexenoliths.
40–70mlongandhadlength/widthratiosof1.
3–2.
0(Fig-ure3(c)).
All5oftheanalyzedspotsplottedonornearaconcordiacurve(Figure5(c)).
Twoofthegrainsyielded206Pb/238Uagesof139±8and152±7Ma,showedstripedabsorptioninCLimages(Figure3(c)),hadhighcontentsofTh(3569and2925ppm)andU(1804and1980ppm),andhighTh/Uratios(2.
04and1.
53)(Figure4),indicatingamagmaticorigin.
Theotherthreespots(Spots3–5)yielded206Pb/238Uagesof514±18,322±18,and280±12Ma,re-spectively(Figure5(c)).
ThesezirconswerecharacterizedbyrelativelylowcontentsofTh(88–359ppm)andU(234–824ppm),andlowTh/Uratios(0.
39–0.
92)(Figure4).
ZirconsselectedfromsampleLW10-2were70–150minlength,colorless/transparent,andprismaticorirregularinshape(Figure3(d)).
Threespotswereanalyzedontwozir-congrainsfromthesample.
Thecoreandrimofonegrainyielded206Pb/238Uagesof500±10and434±7Ma,respec-tively(Figure5(d)).
ThezirconshowedstripedabsorptioninCLimages(Figure3(d))andyieldedTh/Uratiosforthecoreandrimof1.
09and0.
93,respectively(Figure4).
Theotherzircongrain(spot2)fromthesampleisstructurelessinaCLimage,hadahighUcontent(1011ppm)andahighTh/Uratio(0.
44),andyieldeda206Pb/238Uageof516±4Ma.
4Discussion4.
1OriginofzirconindunitexenolithsZircon(ZrSiO4)growsunderSiO2-oversaturatedconditions.
PrimaryzircondoesnotreadilyforminmantleperidotitebecauseoftheextremelylowZrandSicontentsofthisrocktype.
However,zirconhasbeenreportedfromultrahigh-pressuregarnetperidotitesandmantle-derivedperidotitexenoliths[35–41].
Thegrowthofzirconsinsuchxenolithsmayberelatedtolate-stagemodificationofmantleperido-titebysilica-richmelts[35,36].
ThusitisrelevantwhetherdunitexenolithsfromtheTietonggouhigh-Mgdioritesweremodifiedbysilica-richmelt.
InthecaseoftheTietonggouperidotitexenoliths,evidenceofsuchmodificationmaybeobtainedfrompetrographicstudies,insitumineraltraceYangDB,etal.
ChinSciBullFebruary(2012)Vol.
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6655Figure5U-PbConcordiadiagrams(a)–(e),andrelativeprobabilitydiagram(f)oftheSHRIMPzirconU-Pbfordunitexenoliths.
elementdata,andwhole-rockSr-Nd-Osisotopicdata.
Inthesedunitexenoliths,metasomatizedorthopyroxeneandorthopyroxene+phlogopiteoccurasveinsorzoningaroundchromite,suggestingthatthexenolithswereindeedmodi-fiedbysilica-richmelt[22,24].
Inaddition,theorthopy-roxenethatoccurredinveinsandaroundchromitehadhighercontentsoftraceearthelementsthanprimaryortho-pyroxenefromtheharzburgitexenoliths.
Secondaryclino-pyroxeneinwehrlitexenolithswasstronglyenrichedinlightrareearthelementsanddepletedinheavyrareearthelements[24].
Finally,thedunitexenolithswerecharacter-izedbyhighinitial87Sr/86Srratios(0.
7058–0.
7212),lowNd(t)values(19.
59to+0.
18),andclearReaddition.
Theselinesofevidencesuggestthatthedunitexenolithsweremodifiedbysilica-richmelt[24,25].
CombinedwiththeexistenceofharzburgitexenolithswithArcheanRe-depletionmodelagesinthesameintrusion,andtraceelementabundancesofolivinesfromthedunitexenoliths[22,24],weconcludethatthedunitexenolithsoriginatedinthelithosphericmantle,butwerestronglymodifiedbymeltderivedfromthedelaminatedcontinentalcrust[24].
Zirconsinthedunitexenolithscouldbeattribut-edtomodificationbysuchasilica-richmelt.
Inotherwords,thedifferentgroupsofzirconagesmayrepresenteventsin656YangDB,etal.
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6whichsilica-richmeltsmodifiedthelithosphericmantle.
4.
2OriginofzirconsindunitexenolithsTheSHRIMPzirconU-Pbagedatafromthefourdunitexenoliths,exceptforspot4insampleLW8-45-4(322±18Ma),whichyieldedalargeerror,canbesubdividedintofivegroups:131–145Ma(n=7),151–164Ma(n=4),261–280Ma(n=2),434–452Ma(n=2),and500–516Ma(n=3)(Figure5(e),(f)).
Group1(131–164Ma)wasdominantindunitexenolithsandwassimilarinagetotheintrusiveageofthehostrocks(Tietonggouintrusion;131–134Ma)[28,29],whichmayindicatestronginteractionbetweenmeltderivedfromthedelaminatedlowercontinentalcrustandmantleperidotite.
PreviousstudieshavereportedthatdelaminationofthelowercontinentalcrustbeneaththeNCCwaspossiblyre-latedtocollisionbetweentheNCCandYangtzeblocksduringtheTriassic(220–240Ma)[14,24,42],andthatin-teractionbetweensilica-richmeltandmantleperidotiteoc-curredafterthistime.
Inthiscase,thequestionwouldre-mainregardingtheoriginofzirconsinthedunitesxenolithsthatyieldedagesof261–280,434–452,and500–516Ma.
Therearetwopossiblezirconoriginsoftheseages:(1)theyoriginatedfromthedelaminatedlowercontinentalcrustoftheNCCand/orthesubductedslaboftheYC;or(2)theywerederivedfromrepeatedmodificationofthelithosphericmantlebysilica-richmelts.
Inthefirstcase,wewouldhaveexpectedtofindzirconswithagesof2500Ma,1850Ma(typicaloftheNCC)and/or700–900Ma(typicaloftheYC).
However,zirconswiththeseageswerenotfoundinthedunitexenoliths.
Thus,weconcludethatthesezirconsrec-ordrepeatedmodificationoflithosphericmantle.
Thisin-terpretationgivesrisetothequestionofwhethercoevalmagmatism,similartotheagesofzirconsinthedunitexen-oliths,existedintheeasternNCC.
Magmaticzirconsaregenerallydistinguishedfrommeta-morphiczirconsbasedoncathodoluminescence(CL)imag-esandThandUcontentsofzircon,aswellastheirTh/Uratios[43].
Typically,magmaticzirconsshowoscillatorygrowthzonation(forfelsicigneousrocks)orstripedabsorp-tion(formaficigneousrocks)inCLimages,andhavehighThandUcontents,andhighTh/Uratios(>0.
4).
Conversely,metamorphiczirconsarestructurelessorshowpuddingtex-tureonCLimagesandhavelowThandUcontents,aswellaslowTh/Uratios(LW8-42AhadalowTh/Uratio(0.
15),itshowedtypicaloscillatorygrowthzonation,againindicatingamagmaticorigin.
Basedonthe-sefindings,weconcludethatthezirconswithagesof151–164Maareofmagmaticorigin.
Theseages(151–164Ma)areconsistentwiththeSHRIMPzirconU-PbageoftheHuaziyumaficlamprophyreineasternLiaoningprovince(155±4Ma)[56],andwithzirconU-PbagesoftheJingshangranitoidsintheBengbuarea,theDuogushanandWendenggranitoidsinthenorthernsectionoftheSuluultrahigh-pressuremetamorphicbelt(155–160Ma)[54,57],theLinglongandLuanjiahegranitoidsineasternShandong(155–160Ma)[53],andLateJurassicgranitoidsineasternLiaoning[58].
LateJurassicmagmatismintheeasternNCCwasgenerallycharacterizedbyintensivefelsicmagmaticevents,whereaslittlemaficmagmatismoccurredatthistime(e.
g.
theHuaziyulamprophyreintheeasternLiaoning).
Thezirconswithagesof261–280Maweresubhedraloranhedral,showedstripedabsorptioninCLimages,andhadhighTh/Uratios(0.
39–0.
96),suggestingamagmaticorigin.
Theagegroupof261–280MawassimilartotheageoftheEmeishanlargeigneousprovince(259–262Ma)[59,60]andcorrespondswithtimingofthemassextinctioneventattheendofthePermian[61].
PermianigneousrockshavenotbeenreportedfromtheeasternNCC,exceptforasmallquantityofdetritalzircons(agesof273–282Ma)extractedfromJurassicsandstonesintheMengyinandZhoucunba-sins[62].
SHRIMPandLA-ICP-MSzirconU-Pbagedataforfelsicintrusiverocks,volcanictuff,andmafic-ultra-maficrocksindicatemagmatismat254–285MaalongthenorthernmarginoftheNCC[63–66].
TheseresultssuggestthattheglobalPermianeventaffectednotonlythenorthernmarginoftheNCC,butalsothelithosphericmantlebeneaththeeasternNCC.
Zirconsofthepresentstudywithagesof434–452MashowedstripedabsorptioninCLimagesandhadhighTh/Uratios(0.
51–0.
93),suggestingamagmaticorigin.
TheseagesareconsistentwiththeLA-ICP-MSU-Pbagesofcap-turedzirconsfromtheXiachangzhuangmagnetite–amphiboliteintrusiverock(450–484Ma)[67],SIMSU-Pbagesofcap-turedmagmaticzirconswithoscillatorygrowthzonationYangDB,etal.
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57No.
6657fromCenozoicbasaltineasternLiaoning(419–487Ma)[68],andtheU-PbageofperovskitesfromtheMengyinkimberliteinwesternShandongProvince(456±8Ma)[69],aswellasphlogopiteRb-SrandAr-AragesobtainedfortheFuxiankimberliteintheLiaoningregion(463–466Ma)[70,71].
TheseresultspointtotheoccurrenceofPaleozoicmagmaticeventsintheeasternNCC.
Theultramaficnatureofkimberliteshamperstheformationofzircon.
However,thepresenceofzirconsthatgrewwithinkimberliticmagmainwesternShandongandeasternLiaodongindicatestheoccurrenceofasilica-richmagmaticeventintheeasternNCC,inadditiontoearlyPaleozoicsilica-poorultramaficmagmatism.
EarlyPaleozoiczirconsfromdunitexenoliths,asanalyzedinthepresentstudy,mayhaveresultedfrommetasomatismofasilica-richmelt.
Asmentionedabove,zirconswithagesof500–516Mawereofmagmaticorigin.
TheseagesweresimilartothoseofthePan-Africantectono-thermalevents,indicatingthatthelithosphericmantleunderneaththeeasternNCCwasaffectedbythisevent.
Untilnow,thisperiodofmagmatismhadonlybeenreportedincapturedzirconsfromtheXia-changzhuangmagnetite–amphiboliteintrusion(505±10Ma)[67]andindetritalzirconsfromCretaceoussedimentaryrocks(497±13Ma)inthePingyiBasinofwesternShan-dong[62].
4.
3RepeatedmodificationofthelithosphericmantleintheeasternNCCResultsofSHRIMPzirconU-PbdatingoftheTietonggoudunitexenolithsindicatethatlithosphericmantleinwesternShandongrecordsmultipleepisodesofmantlemagmatismranginginagefromtheearlyPaleozoictothelateMesozoic(131–516Ma).
Thisobservationindicatesthatthelitho-sphericmantlewassubjectedtovariousdegreesofmelt-relatedmodification,andthatthemostintensivemodifica-tionoccurredinthelateMesozoic(131–164Ma).
RecentstudiesofperidotitexenolithsfromPaleozoicdiamond-bearingkimberlitesandCenozoicbasaltshaverevealedthatthelithosphericmantleintheNCChasexpe-riencedacomplexevolutionaryprocess[72–75].
Forexam-ple,Li,Sr,andNdisotopicdataforperidotitexenolithsfromtheHannuoba,Fanshi,andHebiCenozoicbasaltswithintheNCCsuggestthatlithosphericmantleintheNCCexperiencedmultipleinteractionsbetweenmelt/fluidandperidotite[72].
TherepeatedmodificationoflithosphericmantleintheNCCisindicatedbyzirconU-Pbdating,traceelementdata,andHfisotopicdataforgarnet/spinelpyroxe-niteveinsthatformedviareactionsbetweenasilica-richmeltandperidotiteintheCenozoicHannuobabasalts[73],andbyinsituRe-Osisotopicdataonsulfidesfromperido-titexenolithsinthesebasalts[74].
Petrographicandmineralchemicaldataforpyroxenesfromgarnetperidotitexeno-lithsintheMengyinkimberlitesrevealedthattheancientlithosphericmantleintheeasternNCChasbeenrepeatedlyoverprinted[75].
ResultsreportedhereinsuggestthatthelithosphericmantleintheeasternNCChasbeenrepeatedlymodifiedandthatthemostintensivemodificationoccurredinthelateMesozoic(131–164Ma).
5Conclusions(1)ZirconsfromdunitexenolithsintheTietonggouintru-sionofwesternShandongformedduringrepeatedmodifica-tionofthelithosphericmantlebysilica-richmelt.
(2)SHRIMPzirconU-Pbagedataindicatethatallthezirconsareofmagmaticorigin,andyieldagesthatdefinefivegroupings:131–145,151–164,261–280,434–452,and500–516Ma,consistentwiththeoccurrenceofmultiplemagmatic-thermaleventsintheeasternNCC.
(3)ThelithosphericmantleintheeasternNCCwassub-jectedtorepeatedmodification,withthemostintensivemodificationoccurringinthelateMesozoic(131–164Ma).
WeappreciatetheassistanceofDirectorLiLinqingoftheLangfangGeo-logicalSurveyofHebeiProvincefortechnicalsupportduringsamplecollection.
WethankSongBiaoandLiuDunyifortechnicalsupportduringSHRIMPIIanalyses.
Weappreciatecommentsbytwoanonymousreview-erswhichimprovedthemanuscript.
ThisworkwassupportedbytheNa-tionalBasicResearchProgramofChina(2009CB825005),theNationalNaturalScienceFoundationofChina(90814003,90714010,91014004and41002018).
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