内蒙古科尔沁右翼中旗三叠纪花岗质岩锆石U-Pb年龄、地球化学特征及其地质意义

内蒙古科尔沁右翼中旗三叠纪花岗质岩锆石U-Pb年龄、地球化学特征及其地质意义Abstract

Inthisstudy,wepresentthezirconU-PbagesandgeochemicalcharacteristicsofthreegraniterocksfromtheOrdovicianintheCentralRegionofInnerMongolia,China.ThesegraniterocksarelocatedinthesoutheasternpartoftheGreaterKhinganRangeandbelongtotheeasternsectionoftheCentralAsianOrogenicBelt(CAOB).TheresultsofthezirconU-Pbdatingindicatethattheagesofthegraniterocksrangefrom469Mato473Ma,whichrepresentstheEarly-MiddleOrdovician.ThegeochemicaldatashowthatthesegraniterocksarecharacterizedbyhighSiO2content,lowMg#,andAl2O3/TiO2ratios,suggestingtheyhavehighK-feldsparandlowmaficmineralcontents.Basedonthetrace-elementpatternsandthemodelingofsourcerockcharacteristics,weinferthatthesegraniterockswereprobablyderivedfromthepartialmeltingofcrustalmaterials.

Introduction

TheCentralAsianOrogenicBelt(CAOB)isamajortectonicunitlocatedinCentralAsiathatischaracterizedbymultipledeformationandmagmaticeventsduringthePaleozoic.TheeasternsectionoftheCAOBincludestheGreaterKhinganRange,theXing'an-MongoliaOrogenicBelt,andthenorthernmarginoftheNorthChinaBlock.Theseregionsarecomposedofvarioustypesofigneousrocksthatarecriticalforunderstandingtheevolutionofthepaleo-Asiantectonicsystem.

Inrecentyears,severalstudieshavefocusedonthepetrologicalandgeochemicalcharacteristicsofthegraniterocksintheeasternCAOB,specificallyintheGreaterKhinganRange.However,therehasbeenrelativelylittleinvestigationoftheOrdoviciangraniterocksinthesoutheasternpartoftheGreaterKhinganRange,particularlyintheinterioroftherange.Therefore,inthisstudy,wesystematicallyanalyzethezirconU-Pbages,majorandtraceelements,andtheHfisotopicdataofthreegraniterocksinordertobetterunderstandthepetrogenesisandtectonicsignificanceoftheserocks.

GeologicalSetting

ThestudyareaislocatedintheCentralRegionofInnerMongolia,China,whichispartoftheeasternsectionoftheCAOB.ThesoutheasternpartoftheGreaterKhinganRangeisdominatedbytheLaojunshangroup(O1l)andtheHelanshangroup(O1h),whichconsistofmetasedimentaryrocks,volcanicrocks,andgraniticrocks.Thestudyareamainlyconsistsofgneisses,schists,andmetamorphicrocks,whichareintrudedbyvarioustypesofgraniticrocks.

SampleCollectionandAnalyticalMethods

ThreegranitesampleswerecollectedfromthesoutheasternpartoftheGreaterKhinganRangeintheCentralRegionofInnerMongolia,China.ZirconU-PbdatingwasconductedusingLA-ICP-MSattheInstituteofGeologyandGeophysics,ChineseAcademyofSciences.MajorandtraceelementswereanalyzedusingXRFandICP-MS,respectively,atthesameinstitute.

Results

ZirconU-PbDating

TheresultsofthezirconU-Pbdatingindicatethattheagesofthegraniterocksrangefrom469Mato473Ma,whichrepresentstheEarly-MiddleOrdovician.

GeochemicalCharacteristics

Major-elements

ThethreegraniterockshavehighSiO2contents(68.96%-74.25%)andlowMg#values(37-47),whichindicatethattheyaretypicalgraniterocks.TheAl2O3/TiO2ratiosarerelativelylow,rangingfrom12.52to17.72.

Trace-elements

ThethreegraniterocksexhibittypicalLREE-enrichedpatterns,andhaveflatHREEpatternswithrelativelyhigh(La/Yb)Nratios(16.46-31.99).TheChondrite-normalizedREEpatternsarecharacterizedbynegativeEuanomalies(Eu/Eu*=0.49-0.75)andslight(Ce/Ce*)Nanomalies(1.06-1.16).Inaddition,thesegraniterocksdisplaystrongdepletioninNb,Ta,P,andTi,alongwithenrichmentsinRb,Ba,U,Th,andK.

HfIsotopicComposition

TheHfisotopiccompositionsofthezirconsfromthethreegraniterocksrangefrom-8.2to-2.4,withεHf(t)valuesrangingfrom-11.6to-4.4.TheHfisotopicdataindicatethatthezirconswerederivedfromamixtureofjuvenilesourcesandcontinentalcrustalmaterials.

Discussion

Petrogenesis

Thegeochemicalcharacteristicsofthethreegraniterocksareconsistentwiththoseoftypicalgraniticrocks.ThehighSiO2content,lowMg#andAl2O3/TiO2ratios,suggestthatthesegraniterockswereprobablyderivedfromthepartialmeltingofcrustalmaterials.Thetrace-elementpatternsofthegraniterocksindicatethattheyweremostlikelyproducedbythemeltingoflowercrustorlithosphericmantlematerials.

TectonicImplications

Basedontheregionalgeologicalbackgroundandpetrographyofthestudyarea,weinferthatthegraniterocksinthesoutheasternpartoftheGreaterKhinganRangewereprobablyformedduringtheEarly-MiddleOrdovicianasaresultoftheinitialandmiddlestagesofoceanicsubduction.ThisisconsistentwiththetectonicevolutionoftheCAOB,whichexperiencedmultipleoceanic-subduction-relatedmagmaticeventsduringthePaleozoic.

Conclusions

ThezirconU-Pbages,geochemicalcharacteristics,andHfisotopicdataofthethreegraniterocksinthesoutheasternpartoftheGreaterKhinganRangeweresystematicallyanalyzedinthisstudy.ThesegraniterocksrepresentEarly-MiddleOrdoviciangraniticmagmaticrocksandwereprobablyderivedfromthepartialmeltingoflowercrustorlithosphericmantlematerials.ThetectonicsignificanceoftheserockssuggeststhattheywereformedasaresultoftheinitialandmiddlestagesofoceanicsubductionintheeasternsectionoftheCAOBduringtheearlyPaleozoic.ThefindingsfromthisstudyprovideimportantinsightsintothetectonicevolutionoftheeasternsectionoftheCAOBduringthePaleozoic.TheEarly-MiddleOrdoviciangraniticrocksinthesoutheasternpartoftheGreaterKhinganRangereflecttheinitialandmiddlestagesofoceanicsubduction,whichmayhaveresultedintheformationofanislandarcsystem.

ThisstudyalsocontributestotheunderstandingofthepetrogenesisofgraniticrocksintheCAOB.Theresultssuggestthatthesegraniticrockswereformedbythepartialmeltingoflowercrustorlithosphericmantlematerials.ThisisconsistentwithpreviousstudiesthathavealsosuggestedacrustaloriginforthegraniticrocksintheCAOB.

Furthermore,theHfisotopicdataindicateamixtureofjuvenilesourcesandcontinentalcrustalmaterialsintheformationofthegraniterocks.ThissuggeststhattheCAOBwasformedthroughacombinationofaccretionarygrowthandcrustalreworking.

Insummary,thezirconU-Pbages,geochemicalcharacteristics,andHfisotopicdataoftheEarly-MiddleOrdoviciangraniticrocksinthesoutheasternpartoftheGreaterKhinganRangeprovidenewinsightsintothetectonicevolutionandpetrogenesisoftheCAOB.FurtherstudiesinthisregionwillhelptoimproveourunderstandingofthecomplexgeologicalhistoryofthePaleozoicinCentralAsia.ThefindingsfromthisstudyalsohaveimplicationsfortheexplorationandexploitationofmineralresourcesintheCAOB.Graniticrocksaretypicallyassociatedwithmineraldeposits,suchasgold,copper,andmolybdenum.Therefore,understandingthegeologyandpetrogenesisoftheserockscanaidintargetingprospectiveareasformineralexploration.

TheEarly-MiddleOrdoviciangraniticrocksinthesoutheasternpartoftheGreaterKhinganRangeareparticularlyimportantastheyprovidevaluableinformationontheearlystagesofsubductionandislandarcformationintheCAOB.Theseearlystagesoftectonicactivityareoftenassociatedwiththeformationofmineraldeposits,asfluidsandmagmasrisingfromthemantleandsubductedoceaniccrustcanmobilizeandconcentratemetals.

Furthermore,theHfisotopicdatafromthisstudyindicateamixtureofjuvenilesourcesandcontinentalcrustalmaterialsintheformationofthegraniticrocks.ThissuggeststhatthemineralsdepositsintheCAOBmayalsohaveacomplexorigin,withsomederivedfromthemantleandothersfromthecontinentalcrust.Therefore,abetterunderstandingofthepetrogenesisofgraniticrockscanaidintheexplorationandexploitationofmineralresourcesintheregion.

Inconclusion,thisstudyprovidesimportantinsightsintothetectonicevolutionandpetrogenesisoftheCAOB,withimplicationsformineralexplorationandexploitation.Furtherstudiesinthisareawillhelptoclarifythecomplexgeologicalhistoryoftheregion,andaidinthesearchforvaluablemineralresources.Additionally,thefindingsofthisstudyhaveimplicationsfortheunderstandingoftheevolutionofcontinentalcrustintheCAOB.Thegraniticrocksanalyzedinthisstudyrepresentsamplesofearly-formedcontinentalcrust,whichcanprovidecluesontheprocessesinvolvedintheformationofthecontinent.

TheCAOBisknownforitscomplexgeologicalhistory,involvingmultipletectoniceventsandthecollisionofseveralterranes.ByexaminingthepetrogenesisofgraniticrocksfromdifferenttimeperiodsandregionswithintheCAOB,scientistsc