遼南—吉南中生代侵入巖鋯石U-Pb年代學和地球化學：對華北克拉通破壞時空范

1、遼南吉南中生代侵入巖鋯石U-Pb年代學和地球化學：對華北克拉通破壞時空范圍的制約    本文以遼南-吉南地區(qū)中生代侵入巖為研究對象,利用鋯石LA-ICPMSU-Pb定年技術(shù),確定了遼南-吉南地區(qū)中生代侵入巖的形成時代;利用中生代侵入巖的主量元素、微量元素以及Sr-Nd同位素地球化學示蹤技術(shù),討論了遼南-吉南地區(qū)晚三疊世和早白堊世侵入巖的巖漿源區(qū)性質(zhì)及其空間變異和形成的構(gòu)造背景。最終為解決華北克拉通破壞的起始時間、破壞的空間范圍以及破壞產(chǎn)生的動力學機制等科學問題提供巖石學和地球化學方面的制約。遼南-吉南存在一條南西-北東向展布的晚三疊世侵入巖帶,代表性巖體

2、螞蟻河巖體的形成時代為225Ma,巖石組合由輝長閃長巖、花崗閃長巖及二長花崗巖組成。主量元素顯示雙峰式火成巖組合特征。微量元素以及同位素地球化學的研究結(jié)果表明,輝長閃長巖主要來源于虧損的巖石圈地幔;而花崗閃長巖及二長花崗巖主要來源于下地殼。結(jié)合遼南-吉南地區(qū)晚三疊世侵入巖和火山巖中捕獲鋯石/繼承鋯石研究成果,提出遼南-吉南晚三疊世巖漿作用形成于揚子與華北克拉通俯沖碰撞后的伸展環(huán)境。綜合上述成果,認為華北克拉通破壞的起始時間為晚三疊世,破壞的空間范圍應(yīng)包括遼南地區(qū),此時破壞的機制以拆沉作用為主。遼南-吉南地區(qū)早白堊世侵入巖的形成時代介于137119Ma,巖石組合包括鎂鐵質(zhì)-超鎂鐵質(zhì)巖石、花崗質(zhì)巖

3、石以及少量的閃長質(zhì)巖石,顯示典型的雙峰式巖漿作用特征。早白堊世侵入巖巖石學和地球化學特征的研究表明,鎂鐵質(zhì)-超鎂鐵質(zhì)巖石來源于受早期拆沉的榴輝巖/石榴輝石巖熔體/流體交代的巖石圈地幔,而花崗質(zhì)巖石主要來源于下地殼。遼南-吉南地區(qū),下地殼組成具有良好的空間變異,從南西到北東,下地殼中新增生的陸殼物質(zhì)逐漸增多。結(jié)合區(qū)域構(gòu)造以及古太平洋板塊的演化歷史,認為華北克拉通東部早白堊世巖漿作用形成于與古太平洋板塊俯沖作用有關(guān)的陸內(nèi)伸展環(huán)境。早白堊世時期,華北克拉通破壞的機制以軟流圈上涌導(dǎo)致對巖石圈地幔的熱-化學侵蝕作用為主LA-ICPMSzirconU-Pbages,major-andtrace-eleme

4、nt,andSr-NdisotopicdataoftheMesozoicintrusionsfromsouthernLiaoningandJilinprovinceshavebeendeterminedtoconstrainthenaturesofmagmasourcesanditsspatialvariationaswellastheirtectonicsettings.Thesechronologicalandgeochemicaldata,togetherwithregionaltectonicanalysis,provideconstraintsontheinitialtime,spa

5、tialextent,anddynamicmechanismoftheNorthChinaCratondestruction.Mainachievementsareasfollows:1.TheMayiheplutonfromsouthernJilinprovinceconsistsmainlyofgabbro-diorite,granidioriteandmonzograniteandhaveagesof224-226MadatedbytheLA-ICPMSzirconU-Pbmethod.TheEarlyCretaceousintrusiverocksfromthesouthernLiao

6、ningandJilinProvincesarecomposedmainlyofmafic-ultramaficintrusiverocksandgranitoids.Thedatingresultsfortwelvesamplesrangefrom137to119Ma.2.Thegabbro-dioritesfromtheMayiheplutonhavelowSiO2contentsandhighMgOcontents,andareenrichedinlargeionlithophileelements(LILEs)andlightrareearthelements(LREEs),deple

7、tedinhighfieldstrengthelements(HFSEs)andheavyrareearthelements(HREEs).Theirinitial87Sr/86SrratiosandNd(t)valuesare0.7037and 3.45 3.82,respectively.TheirTDM1modalagesvaryfrom726to772Ma.ThegranitoidsfromMayiheplutonhavehighSiO2contentsandlowMgOcontents,andbelongtometaluminoustoweaklyperaluminousI-type

8、granitoids.Theirinitial87Sr/86SrratiosandNd(t)valuesrangefrom0.7053to0.7058andfrom-0.73to-3.40,respectively.TheirTDM1modalagesare990-1083Ma.3.TheLateTriassicintrusiverocksfromsouthernLiaoningandJilinprovinces,togetherwiththeLateTriassicintrusiverocksfromsouthernLiaoning(210213Ma,zirconLA-ICP-MSU-Pbd

9、atingmethod),arecomposedmainlyofmaficrocksandgranitoids.Themaficrocksincludeshoshoniticandtholeiiticrocks,whereasgranitoidsconsistmainlyofgranodiorite,monzogranite,andminorquartzdiorite.TheshoshoniticrockshavelowSiO2contents(49.6wt)andhighMg#(Mg#>57)andarecharacterizedbyenrichmentinLILEs,LREEs,an

10、ddepletioninHFSEssuchasNb,Ta,Zr,HfandTi.TheirNd(t)values(-13.2to-13.4)andHf(t)values(-11.0to-11.5)implythattheprimarymagmacouldbederivedfrompartialmeltingofanenrichedlithospheremantle.ThegranitoidswithhighSiO2andlowMgOcontentshaveinitial87Sr/86Srratiosof0.70950.7112,Nd(t)valuesof-10-15.9,andHf(t)val

11、uesof-12.0-13.9.TheirTDM1modalagesrangefrom999to1558Ma.4.TheEarlyCraterousmafic-ultramaficintrusiverocksfromsouthernLiaoningandJilinprovincesarecharacterizedbylowSiO2,highMgO,enrichmentinLREEsandLILEs,aswellasdepletioninHREEsandHFSEs.Theirinitial87Sr/86SrratiosandNd(t)valuesrangefrom0.7056to0.7112an

12、dfrom-6.56to-20.1,respectively.Theirinitial87Sr/86SrratiosandTDM1valuesdecreasewhereasNd(t)valuesincreasefromsouthwesttonortheast.TheEarlyCraterousgranitoidsfromsouthernJilinprovincehaveSiO2=53.776.6wt)andMg#=6.0954.37.Chemically,theybelongtometaluminoustoweaklyperaluminousI-typegraniteandareenriche

13、dinLREEsandLILEsanddepletedinHREEsandHFSEs.AfewhighlyfractionatedgranitoidsarecharacterizedbydepletioninBaandSr.Theirinitial87Sr/86SrratiosandNd(t)valuesarebetween0.7052and0.7107,andbetween-1.93and-13.9,respectively.TheirTDM1modalagesvaryfrom994to2002Ma.Theinitial87Sr/86SrratiosandTDM1modalagesforth

14、esegranitoidsdecreasewhileNd(t)valuesincreasefromsouthwesttonortheast.5.ThebimodalrockassociationforthelateTriassicintrusionsinthesouthernLiaoningandJilinprovincessuggeststhattheycouldformunderanextensionalenvironment.TheSr-Ndcompositionsofthegabbro-dioriteintheMayiheplutonimplythattheirprimarymagma

15、shouldbederivedfrompartialmeltingofadepletedlithospheremantle,whichisconsistentwiththejuveniledepletedlithospheremantlebeneaththeXingan-Mengolia(Xing-Meng)Orogenicbelt.However,itisdifferentfromanenrichedlithospheremantlebeneathsouthernLiaoningprovince.TheLateTriassicgranitoidsfromsouthernJilinprovin

16、cecouldbeoriginatedfrompartialmeltingofajuvenilelowercrust,whichisconcordwithitstectonicpositionadjacenttotheXing-Mengorogenicbelt.Incontrast,theLateTriassicgranitoidsfromsouthernLiaoningcouldbemainlyderivedfrompartialmeltingofanancientlowercrust.6.ThelateTriassicigneousrocksfromsouthernLiaoningands

17、outhernJilinprovinces,togetherwiththeLateTriassicvolcanicrocksinthesouthernJilinandJiazishansyeniteineasternShandong,makeupalateTriassicigneousrocksbeltintheeasternNCCorientedinSW-NEdirection.Thisigneousrockbeltmailyconsistsofabimodalrockassociation,implyingthattheycouldformunderanextensionalsetting

18、.CombinedwiththeoccurrenceofthelateTriassicmetamorphiczirconsandtheNeoproterozoicmagmaticzirconintheMesozoicigneousrocksfromsouthernLiaoningandJilinprovinces,weconcludethatthelateTriassicmagmatismcouldformunderanpost-collsionalextensionalsettingoftheYangtzeCratonandtheNCC.7.Thegeochemicaldataoftheea

19、rlyCretaceousmafic-ultramaficrocksinthesouthernLiaoningandJilinprovincesindicatethattheirprimarymagmascouldbeoriginatedfrompartialmeltingoftheEMII-likelithospheremantle.TheirSr-NdisotopiccompositionsrevealthatanenrichmentextentgraduallydecreasesfromsouthernLiaoningtosouthernJilinprovinces.TheearlyCr

20、etaceouslithosphericmantleinsouthernLiaoningisgeochemicallysimilartothatinwesternShandong.TheEMII-likelithospheremantlecanberelatedtomodificationofthedelaminatedeclogite-orgarnetpyroxenite-derivedmelt.TheearlyCretaceousgranitoidscouldbemainlyderivedfrompartialmeltingofthelowercontinentalcrust,includ

21、inganancientandajuvenilecrustmaterials.However,theirspatialvariationsofSr-Ndisotopiccompositionsrevealthattheproportionofthejuvenilecrustincreaseinthemagmasourcesfromsouthwesttonortheast.CombinedwithregionaltectonicsandPaleo-pacificplatesubductionhistory,itisproposedthattheEarlyCretaceousmagmatismintheeasternNCCcouldformunderanintra