念青唐古拉花岗岩热演化历史和山脉隆升过程的热年代学分析
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摘要
念青唐古拉山是青藏高原内部的重要山脉,主体由黑云母二长花岗岩组成,岩体内部发育不同类型的变质岩包体如Lgn、Ygn片麻岩和元古代(Pt)变质岩,岩体东西两侧发育伸展型韧性剪切带。对念青唐古拉黑云母二长花岗岩进行矿物对热年代学分析,良好地揭示了岩浆热演化历史和山脉隆升过程。通过单颗粒锆石离子探针测年,发现65.0～55.0Ma发生早期岩浆侵位事件,形成Lgn、Ygn花岗片麻岩包体;在18.3～11.1Ma期间,在约11km深度的Lgn、Ygn下方发生大规模岩浆侵位和结晶成岩事件,形成念青唐古拉黑云母二长花岗岩(NG)。在11.1～9.3Ma期间,念青唐古拉花岗岩发生快速冷却和隆升过程,平均降温速度约222.2°C/Ma,对应的平均差异隆升速率为5.56mm/a;在9.3～8.6Ma期间,念青唐古拉花岗岩继续发生差异隆升和快速降温,平均降温速率为142.8°C/Ma,对应的差异隆升速率为3.57mm/a;在8.0～5.0Ma期间,念青唐古拉山区发生伸展型韧性剪切变形,导致念青唐古拉花岗岩快速隆升,平均差异隆升速率为3.50mm/a;在5.0～3.7Ma期间,念青唐古拉花岗岩继续发生构造隆升,平均降温速率约92.3°C/Ma,对应的平均差异隆升速率为2.31mm/a。自3.7Ma以来念青唐古拉花岗岩平均降温速度达27.0°C/Ma,平均抬升速度达0.68mm/a。念青唐古拉岩浆集聚、NG花岗岩侵位与INDEPTHII地震深反射亮点揭示的地壳局部熔融存在动力学成因联系,导致上地壳伸展构造变形、NG花岗岩缓慢冷却和念青唐古拉山脉快速隆升。
As an important mountain range in central Tibetan plateau, the Nyainqentanglha Range is world-famous. It is mainly composed of biotite-plagioclase-K-feldspar granite (NG) containing a variety of enclaves such as Lgn and Ygn gneiss and Pt metamorphic rocks. Huge ductile shear zones (NSZ) consisting of extensional mylonite were discovered on both sides of the Nyainqentanglha granite. They gradually change into schistosity belts in the north. SHRIMP U-Pb dating of zircons from Lgn gneiss enclave and NG granite yielded ages of 65.0～55.0 Ma and 18.3～11.1 Ma, representing periods of granitic magma crystallization in Palaeocene and Miocene respectively. Rb-Sr isochronic dating of biotite and K-feldspar from NG granite, K-Ar dating of samples from NG granite and NSZ mylonite and fission-track dating of apatite from NG granite gave time constraints for thermo-tectonic events after the emplacement of the NG granite. Thermal-chronological analyses show that the Nyainqentanglha granite (NG) was emplaced and crystallized during 18.3～11.1 Ma beneath Lgn and Ygn gneiss at an average depth of 11 km, followed by rapid uplift (cooling) processes at the rate of 5.56 mm/a (222.2 ℃/Ma), 3.57 mm/a (142.8 ℃/Ma), 3.50 mm/a, 2.31 mm/a (92.3 ℃/Ma) and 0.68 mm/a (27 ℃/Ma) in 11.1～9.3 Ma, 9.3～8.6 Ma, 8.0～5.0 Ma, 5.0～3.7 Ma and 3.7 Ma to the present respectively. Rapid uplif- ting (cooling) in 8.0～5.0 Ma was caused by extensional ductile shearing of NSZ, and extensional faulting contributed to the uplif- ting of the Nyainqentanglha Range since 3.7 Ma. Magma accumulation and NG granite emplacement related geodynamically to partial melting of the crust detected by INDEPTH-II seismic bright-spots at the depth of 13～20 km caused the upper crust extension of central Tibet, the gradual cooling of NG and the rapid uplift of the Nyainqentanglha Range.
As an important mountain range in central Tibetan plateau, the Nyainqentanglha Range is world-famous. It is mainly composed of biotite-plagioclase-K-feldspar granite (NG) containing a variety of enclaves such as Lgn and Ygn gneiss and Pt metamorphic rocks. Huge ductile shear zones (NSZ) consisting of extensional mylonite were discovered on both sides of the Nyainqentanglha granite. They gradually change into schistosity belts in the north. SHRIMP U-Pb dating of zircons from Lgn gneiss enclave and NG granite yielded ages of 65.0～55.0 Ma and 18.3～11.1 Ma, representing periods of granitic magma crystallization in Palaeocene and Miocene respectively. Rb-Sr isochronic dating of biotite and K-feldspar from NG granite, K-Ar dating of samples from NG granite and NSZ mylonite and fission-track dating of apatite from NG granite gave time constraints for thermo-tectonic events after the emplacement of the NG granite. Thermal-chronological analyses show that the Nyainqentanglha granite (NG) was emplaced and crystallized during 18.3～11.1 Ma beneath Lgn and Ygn gneiss at an average depth of 11 km, followed by rapid uplift (cooling) processes at the rate of 5.56 mm/a (222.2 ℃/Ma), 3.57 mm/a (142.8 ℃/Ma), 3.50 mm/a, 2.31 mm/a (92.3 ℃/Ma) and 0.68 mm/a (27 ℃/Ma) in 11.1～9.3 Ma, 9.3～8.6 Ma, 8.0～5.0 Ma, 5.0～3.7 Ma and 3.7 Ma to the present respectively. Rapid uplif- ting (cooling) in 8.0～5.0 Ma was caused by extensional ductile shearing of NSZ, and extensional faulting contributed to the uplif- ting of the Nyainqentanglha Range since 3.7 Ma. Magma accumulation and NG granite emplacement related geodynamically to partial melting of the crust detected by INDEPTH-II seismic bright-spots at the depth of 13～20 km caused the upper crust extension of central Tibet, the gradual cooling of NG and the rapid uplift of the Nyainqentanglha Range.
引文
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YinAn.2001.GeologicalevolutionoftheHimalayanTebetanorogeninthecontextofPhanerozoicgrowthofAsia.ActaGeoscienticaSinica,22(3):193～230(inChinesewithEnglishabstract).
ZhaoWenjinandINDEPTHTeam.2001.DeepstructureandtectonicsofHimalayaMountainsandYaluzangbusuture.Beijing:GeologicalPublishingHouse,63～255(inChinesewithEnglishabstract).
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胡道功,吴珍汉,叶培盛等.2003.西藏念青唐古拉山闪长质片麻岩锆石UPb年龄.地质通报,22(11～12):936～940.
刘琦胜,吴珍汉,叶培盛等.2003.念青唐古拉花岗岩锆石离子探针UPb同位素测年.科学通报,48(20):2170～2175.
吴珍汉,江万,吴中海等.2002.青藏高原腹地典型盆山构造形成时代.地球学报,23(4):289～294.
尹安.2001.喜马拉雅青藏高原造山带地质演化.地球学报,22(3):193～230.
赵文津及INDEPTH项目组著.2001.喜马拉雅山及雅鲁藏布江缝合带深部结构与构造研究.北京:地质出版社,63～355.
赵文津,薛光琦,赵逊等.2004.INDEPTHIII地震层析成像———藏北印度岩石圈俯冲断落的证据.地球学报,15(1):1～10.
Bierqiuk.1966.AffectoftemperatureonCabalancebetweenplagioclaseandhornblende.BulletinofAcademyofUSSR,60(6):1436～1438(inRussia).
BlisniukMPeter,HackerRBradley,GlodnyJohannes.2001.NormalfaultingincentralTibetsinceatleast13.5Maago.Nature,412:628～632.
BrownLD,ZhaoW,NelsonKD.1996.Brightspots,structureandmagmatisminsouthernTibetfromINDEPTHseismicreflectionprofiling.Science,274:1688～1690.
ChenLeshou,BookerJ,JonesAG.1996.ElectronicallyconductivecrustinsouthernTibetfromINDEPTHmagnetotelluricsurvering.Science,274:1694～1696.
CompstonW,WilliamsIS,KirschvinkJLetal.1992.ZirconUPbagesofearlyCambriantimescale.J.Geol.Soc.,149:171～184.
DodsonM.1973.Closuretemperatureincoolinggeochrolonogicalandpetrologicalsystems.ContributionstoMineralogyandPetrology,40:259～274.
EdwardsMA,HarrisonTM.1997.Whendidtheroofcollapse?LateMiocenenorthsouthextensionintheHighHimalayarevealedbytheThPbmonazitedatingoftheKhulaKangrigranite.Geology,25:543～546.
HarrisN,MasseyJ.1994.DecompressionandanatexisofHimalayanmetapelites.Tectonics,13:1537～1546.
HarrisonTM,ArmstrongRL,NaeserCW.1979.GeochronologyandthermalhistoryofthecoastplutoniccomplexnearPrinceRupert,BritishColumbia.Can.J.EarthSci.,16:400～410.
HarrisonTM,CopelandP,KiddWSFetal.1995.ActivationoftheNyainqentanghlashearzone:implicationsforupliftofthesouthernTibetanPlateau.Tectonics,14:658～676.
HodgesMS,ParrishRR,SearleMP.1996.TectonicevolutionofthecentralAnnapurnarange,NepaleseHimalayas.Tectonics,15:1264～1291.
HuDaogong,WuZhenhan,YePeishengetal.2003.SHRIMPUPbagesofzirconsfromdioriticgneissintheNyainqentanglhaMountains,Tibet.GeologicalBulletinofChina,22(11～12):936～940(inChinesewithEnglishabstract).
LiangTingli,DuoJi,TanQingyuan.1995.ProspectingofdeepgeothermalresourcesinnorthYangbajaingeothermalfield.PressedbyGeothermalBrigadeofTibetGeologicalBureau,24～68(inChinesewithEnglishabstract).
LiuQisheng,WuZhenhan,HuDaogongetal.2004.SHRIMPUPbzircondatingonNyainqentanglhagraniteincentralLhasablock.ChineseScienceBulletin,49(1):76～82.
NelsonKD,ZhaoW,BrownLDetal.1996.PartiallymoltenmiddlecrustbeneathsouthernTibet:SynthesisofProjectINDEPTHresults.Science,174:1684～1688.
LudwigKR.2000.User′smanualforISOPLOT/Exversion2.4GeochronologicaltoolkidforMicrosoftexcel.BerkeleyGeochronologyCenter(specialpublicationno.1a),1～53.
PanY,KiddWSF.1992.Nyainqentanglhashearzone:alateMioceneextensionaldetachmentinthesouthernTibetanplateau.Geology,22:775～778.
PlyusninaLP.1982.Geothermometryandgeobarometryofplagioclasehornblendebearingassemblages.Contrib.MineralPetrol,80:140～146.
SearleMP,ParrishRR,HodgesKVAetal.1997.ShishaPangmaleucogranite,southTibetanHimalaya:fieldrelations,geochemistry,age,originandemplacement.JournalofGeology,105:295～317.
SpicerRA,HarrisNBW,WiddowsonMetal.2003.ConstantelevationofsouthernTibetoverthepast15Millionyears.Nature,421:622～624.
WagnerGA.1992.Fissiontrackdating.Germany:KluwerAcademicPublisher,145～158.
WagnerGA.1998.Agedeterminationofyoungrocksandartifacts.SpringerVerlag,Berlin,219～294.
WilliamsIS,ClaessonS.1987.IsotopeevidenceforthePrecambrianprovinceandCaledonianmetamorphismofhighgradeparagneissfromtheSeveNappes,ScandinavianCaledonides,Ⅱ.IonmicroprobezirconUThPb.Contrib.Mineral.Petrol.,97:205～217.
WuC,NelsonKD,WortmanGetal.1998.YadongcrossstructureandsouthTibetandetachmentintheeastcentralHimalaya.Tectonics,17:28～45.
WuZhenhan,JiangWan,WuZhonghaietal.2002.DatingoftypicalbasinandrangetectonicsincentralTibetanplateau.ActaGeoscienticaSinica,23(4):289～294(inChinesewithEnglishabstract).
WuZhenhan,HuDaogong,YePeishengetal.2004.ThrustingofthenorthLhasablockintheTibetanplateau.ActaGeologicaSinica,78(1):246～259.
XuRH,ScharerU,AllegreCJ.1985.MagmatismandmetamorphismintheLhasablock(Tibet):ageochronologicalstudy.JournalofGeology,93:41～57.
YinAn.2001.GeologicalevolutionoftheHimalayanTebetanorogeninthecontextofPhanerozoicgrowthofAsia.ActaGeoscienticaSinica,22(3):193～230(inChinesewithEnglishabstract).
ZhaoWenjinandINDEPTHTeam.2001.DeepstructureandtectonicsofHimalayaMountainsandYaluzangbusuture.Beijing:GeologicalPublishingHouse,63～255(inChinesewithEnglishabstract).
ZhaoWenjin,XueGuangqi,ZhaoXunetal.2004.SeismicimagingofthesubductingIndialithospherebeneathnorthTibet.ActaGeoscienticaSinica,25(1):1～10(inChinesewithEnglishabstract).
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