地磁学地壳结构

1GEOMAGNETISMGEOMAGNETISM地壳磁性结构的研究Satellitesurveys,performedfromaltitudesofafewhundredkilometers,andshipborneandairbornesurveys,performedalongorneartheEarth’ssurface.Satellitesurveysanalyzemostlytheanomalieswithwavelengthsintheorderofathousandkilometers;Shipborneandairbornesurveysinvestigateanomalieswithshorterwavelengths,uptoafewtensorhundredsofkilometers,andaremainlyaimedatidentifyingthelateralmagnetizationcontrastsconnectedtoregionaltectonicstructures.2GEOMAGNETISMGEOMAGNETISM上个世纪五十年代以来，世界各国相继发射人造地球卫星，它们携带磁力仪进行近地地磁场测量，主要有：（1）1958年，前苏联发射了Sputnik3卫星，平面倾角65°，高度范围226~1881km，携带磁通门(标量)磁力仪，可达到的异常精度100nT。地壳磁性结构的研究3GEOMAGNETISMGEOMAGNETISM（2）1979年，美国发射了一颗与太阳同步的Magsat卫星，平面倾角97°，高度范围325-550km，携带磁通门(矢量)与铯光泵(标量)磁力仪，可达到的异常精度分别为6nT与3nT。（3）2000年，德国发射了一颗圆形轨道的CHAMP卫星，平面倾角87.3°，高度范围300-460km，携带磁通门(矢量)与欧弗豪泽质子(标量)磁力仪，可达到的异常精度分别为2nT与1nT地壳磁性结构的研究4GEOMAGNETISMGEOMAGNETISM磁卫星的分辨率可达150-300km。磁卫星异常与地质构造有较好的相关性，展现了研究全球构造的广阔应用前景。图8.CHAMP卫星和仪器位置前视图

地壳磁性结构的研究5GEOMAGNETISMGEOMAGNETISM地壳磁性结构的研究

磁异常的定性分析及反演视磁化率的分布；

钻孔岩芯磁化率测量；

出露的高级地体和麻粒岩包体的磁化率测量6GEOMAGNETISMGEOMAGNETISM地壳磁性结构的研究ThemagneticcharacteristicsoftheEarth’scrustaresystematicallydifferentoncontinentsandoceans,andprovideindicationsonlargescalegeodynamicprocesses.Wewillconsideronlythecrust,becausethemagneticroleoftheuppermantleiscontroversialandinanycasesecondary.7GEOMAGNETISMGEOMAGNETISM地壳磁性结构的研究Thedifferentstructure,compositionandgeodynamichistoryoftheoceanicandthecontinentalcrustalsoreflectinthemagneticanomaliestheyproduce.OceanicAnomalieshavearegulargeneralpattern,formedbyalternatingpositiveandnegativebands,whosewidthisintheorderoftensofkilometersandwhoselengthcanreach1000km.Hencethelinearmagneticanomalyterm.Fig1.TotalmagneticfieldanomalymapsouthwestofVancouverIsland.Symbols:black/whiteareas=positive/negativeanomaly;straightlines=faultsoffsettingtheanomalypattern(fromRaffandMason1961)8GEOMAGNETISMGEOMAGNETISM地壳磁性结构的研究Abovecontinents,instead,anomalieshaveamarkedindividualnature.Theycanbecorrelatedtoindividualgeologicalbodiesortheycanbearrangedaccordingtolargescaletrendsthatunderlinelargerstructures,whilenotbeingdirectlylinkedtothedetailsoftheirgeometry.9GEOMAGNETISMGEOMAGNETISM地壳磁性结构的研究Thestructureofthetwotypesofcrusthasbeendeducedonthebasisofvariationsinthevelocityofseismicwaves.AccordingtotheRaittmodeltheoceaniccrust,onaverage7kmthick,isformedbythreelayers.Layer1consistsofsediments,Layer2ofbasalticrocks,Layer3,accordingtocurrentinterpretations,ofgabbros(辉长岩).0=seawater;1=sediments;2=basalt;3=gabbro;4=peridotite（橄榄岩）;10GEOMAGNETISMGEOMAGNETISM地壳磁性结构的研究Thecontinentalcrust,withameanthicknessof30-35km,hasafarmorecomplexstructure;assumingasareferencethemodelproposedbyMüllerforthehercynian（海西期）crustofcentralEurope,itissubdividedintoupper(sediments,lowgrademetamorphicrockswithgraniticintrusions（花岗岩侵入体）,graniticlaccoliths（岩盘）,migmatites（混合岩）)andlowercrust(amphibolites（斜长角闪岩）,granulites（麻粒岩）).1=Cenozoic（新生代）sediments;2=MesozoicandPalaeozoic(中古生界)sediments;3=low-grademetamorphics;4=granite;5=migmatites;6=amphibolite;7=granulite;8=ultramafics(超镁铁质岩)11GEOMAGNETISMGEOMAGNETISM地壳磁性结构的研究Lastly,werecallthattheageoftheoldestoceaniccrustislessthan200Ma(middleJurassic),whereastheageofthecratoniccontinentalcrustisgreaterthan500Maandmayreach4Ga(Archean).Theamplitudeoftheoceanicanomalies,intheorderof500nT,andtheirhighgradient,indicatethatthesourcebodiesareclosetothesurfaceandhaveamagnetizationintheorderof5Am–1.Ifthemagnetizationisassumedtobeinduced,Ji=κHΕ,thenmagneticsusceptibilitymustbeintheorderofκ≈105μSI,averyhighvaluebutcompatible,intheory,withthatofoceanicbasalts.12GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Systematicmeasurementsacrossaridgehighlightawide,positiveanomalycenteredontheriftandasequenceofmaximaandminima,i.e.ofpositiveandnegativeanomalies,whoseprofileawayfromtheridgeissymmetricalongthetwosides.Parallelismbetweentheanomaliesandtheridgeaxis,andsymmetryofthemaximaandminimasequencearethebasesofthemodelproposedby

VineandMatthewsandindependentlybyMorleyin1963.Theyarethecombinedeffectofthesea-floorexpansionandthepolarityreversaloftheEarth’smagneticfield.13GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Fig.3a.MagneticanomaliesoverReykjanesRidge,southofIceland;askeletonmagneticmap;symbols:black/whiteareas=positive/negativeanomaly;A=centralanomalyovertheridge;Fig3b.totalfieldmagneticanomalyprofilesprojectedperpendiculartotheridgeaxis(1gamma=1nT)(fromHeirtzleretal.1966)14GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Themodelpresupposesthatnewoceaniccrustisformedintheriftregion,wherebasalticlavasareproducedbytheupwardcurrentofamantleconvectioncell.ThelavascontainTi-magnetiteandastheycoolbelowitsCuriepointtheyemagnetizedinthedirectionoftheEarth’sfield.Asexpansionproceeds,basaltsmoveawayfromtherift,newlavasareerupted,cooldown,andaremagnetized.15GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Ifthefieldreverseditspolarity,themagnetizationofthenewbasaltswillbeoppositetothatofthepreviousones.Thecontinuityoftheexpansionandthesuccessionofthereversalsthusgiverisetoasequenceofsea-floorbandswhoseTRMhasalternativelynormalandreversepolarity.TheflooroftheoceansisthuscomparabletoatapethatrecordsthehistoryoftheEarth’smagneticfield.16GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Fig4.TheVine-Matthews-Morleymodel:examplefortheJuandeFucaRidge(offthePacificcoastofCanada);asea-floorspreadingandmagnetization:black/white=normal/reversepolaritymagnetizationinLayer2;bmagneticanomalymap:black/white=positive/negativeanomaly;Fig4c.totalfieldanomalyprofilemeasuredalongthecentralportionofthemap(1gamma=1nT);dmagnetizationmodelandcomputedtotalfieldanomaly(fromVine1968)17GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Fig5.Anomaliesofmagnetizedcrustalblocksinthenorthernhemisphere(verticalremanenceforsakeofsimplicity);a,bindividualblocks.Normalpolaritymagnetization(blackblock)causesananomalypositiveabovetheblocks,negativeatthesides;reversepolaritymagnetization(whiteblock)causesananomalynegativeabovethecenterandpositiveatthesides.Theshapeoftheanomalydependsontheblock’swidth;coverlapoftheanomaliesofcontiguousblocks.NormalbandscauseananomalousfieldwhichisaddedtotheEarth’spresentfield(positiveanomalies),thereversebandsafieldthatissubtracted(negativeanomalies).18GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Fig6.CorrelationofmagneticanomaliesprofilesacrosstheSouthAtlantic,NorthAtlanticandSouthPacificridges.Thepositionoftheridgeaxisisgivenbytheoriginofthetimescale(0Ma)intheupperpartofthefigure.Thedifferentlengthscaleoftheprofilestakesintoaccountthedifferentspreadingrate(fromMenard1986,simplifiedafterHeirtzleretal.1968)Cross-checkingsbetweendifferentridgesandwithupliftedsectionsconfirmthevalidityoftheVine-Matthews-Morleymodel.19GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Magneticcharacteristicsofthesea-floorrocksThemodelproposestheTRMofbasaltsasthecauseoftheanomaliesandrequiresthatMTRM>>Mi,i.e.thatthecontributionofthemagnetizationinducedbythepresentfieldwouldbenegligible.Experimentaldataconfirmthehypothesis,sinceanalysesonsamplesobtainedfromcoresshowthattheKnigsbergerratioofoceanicbasaltsisalwayshigh,evenhigherthanQ≈100.Q=Mr/Mi

柯尼希斯贝格比(Knigsbergerratio)20GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Inthefirstmodels,themagnetizationsourcewasplacedinLayer2,constitutedbybasaltswithathicknessintheorderof2km.thepillowlavasoftheupperpartofLayergraduallychangewithdepthtodoleritesills(灰绿岩床)andsheeteddikes(席状岩墙,Layer2B);thegabbrosofLayer3arethoughttobeisotropicandpossiblyingreen-schist(绿片岩相)faciesintheupperpart(Layer3A),cumuliticinthelower(Layer3B).21GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Magneticcharacteristicsofthesea-floorrocksThemainsourceoftheanomaliesarethepillowlavas,withathicknessof0.5-0.8kmandamagnetizationintheorderof5Am–1.However,anon-negligiblecontributionisalsobroughtbydolerites,dikesandisotropicgabbros,withmagnetizationsintheorderof0.5-1Am–1.Anadditionalpossiblecontributioncouldcomefromserpentinizedlherzolites(蛇纹石化二辉橄榄岩)attheMohodepth.22GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Magneticcharacteristicsofthesea-floorrocksSerpentinizationisageologicallow-temperature

metamorphic

processinvolvingheatandwaterinwhichlow-silica

mafic

and

ultramaficrocksare

oxidized

and

hydrolyzed

withwaterintoserpentinite.

Peridotite(橄榄岩),including

dunite(纯橄榄岩),atandneartheseafloorandinmountainbeltsisconvertedto

serpentine,

brucite(水镁石),magnetite,andotherminerals—somerare,suchas

awaruite

(铁镍矿，Ni3Fe),andevennative

iron.Intheprocesslargeamountsofwaterareabsorbedintotherockincreasingthevolumeanddestroyingthestructure.23GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Magneticcharacteristicsofthesea-floorrocksLaboratoryanalysesonremanentmagnetizationhaveshownafarmorecomplexpicturethanthehypothesisofaTRMacquiredbyrapidcoolingofpillowlavas.TheTi-magnetiteofoceanicbasaltshasahighTicontentandthusalowCuriepoint(<250°C).Asmentionedbefore,ittendstooxidizeintomaghemite(赤铁矿),characterizedbychemicalmagnetization(CRM),withhigherCuriepointandgreaterstability.However,thisprocessentailsthattheCRMisnolongerstrictlysynchronous（同步的）totheemplacement（侵位）ofthelavas.Therefore,theresultingpictureishighlycomplex:24GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构MagneticcharacteristicsoftheoceanicrocksClearly,thehypothesisofTRMmagnetizedblocksissimplisticandboththephysicochemicalprocessesinfluencingremanenceandthepresenceofothermagnetizedrocksinadditiontopillowbasaltsentailproblemsanddifficultiesthatarefarfrombeingsolved.25GEOMAGNETISMGEOMAGNETISM海洋地壳磁性结构Magneticcharacteristicsofthesea-floorrocksTheseproblemsdonotdiminishthevalueoftheVine-Matthews-Morleymodel,whosetheoreticaleleganceisbuttressedbytheglobalgeodynamicandchronologicalpicture(likeGPTS,etc.)andwhichrepresentsamilestoneinthedevelopmentoftheEarthSciences.Asoftenhappens,theoriginalmodelislikeasketch,drawnbyanartistonthespurofthemoment:completingtheactualpaintingwillrequirealong,patientworktoproducetheharmonyofindividualdetails.26GEOMAGNETISMGEOMAGNETISM大陆地壳磁性结构MagneticcharacteristicsofthecontinentalrocksThecontinentaluppercrustisformedbyverydifferentlithologies,withextremelydiversifiedmagneticproperties.Itisthesiteoflocalandregionalanomalies,whichcanbedirectlycorrelatedtoindividualgeologicalstructures.27GEOMAGNETISMGEOMAGNETISM大陆地壳磁性结构70年代以来,随着科学技术的迅猛发展,人们可以利用磁卫星对全球地磁场进行全面、系统地观测,并与近地表空间范围内的高精度航磁测量及现代数学物理方法与计算技术相结合反演解释磁异常.一方面利用全球区域磁异常(不同波长)与科学创新的地质思维(或地质概念、地质模型等)相结合对区域构造直接进行地质解释;另一方面运用合理的数学-物理模型,反演出区域不同尺度地壳的磁性分布,然后依据磁性与岩石、矿物、地球化学组分的相互关系,推测大陆地壳的结构.28GEOMAGNETISMGEOMAGNETISM大陆地壳磁性结构随着世界范围内多个出露高级地体和麻粒岩包体的岩石学、地球化学研究的深入,人们普遍认为,这些出露的高级地体和麻粒岩包体可以作为窥探地壳深部结构及地球动力学过程的窗口,它们为大陆地壳磁性结构研究提供了一条重要途径。29GEOMAGNETISMGEOMAGNETISM大陆地壳磁性结构磁性结构与地壳构造的相互关系磁性结构与变质相的相互关系磁性结构与地壳物质组成的相互关系30GEOMAGNETISMGEOMAGNETISM磁性结构与地壳构造大陆地壳的磁性结构与地壳构造相互关系主要是：依据构造运动过程中常常伴随物质的分异(如岩浆活动),使得地壳中磁性矿物重新组合；或由于不同深度和不同岩性的前寒武纪基底抬升至地表或区域产生拗陷等作用,在磁卫星或航磁中产生不同尺度分布的磁异常.31GEOMAGNETISMGEOMAGNETISM磁性结构与地壳构造由于卫星磁测飞行高度很高,因而分辨力差,但它覆盖全球,不留空白区,而且能避免由地壳浅部不均匀剩余磁化强度引起的高波数异常的干扰,可以从整体上从宏观上了解地壳深部的磁性变化。32GEOMAGNETISMGEOMAGNETISM磁性结构与地壳构造33GEOMAGNETISMGEOMAGNETISM磁性结构与地壳构造

Arkani-Hamed等人应用磁卫星资料分析了中国部分地区的长波长磁异常,并反演得到视磁化率的分布结果,认为：波长为540km的磁场具有最高的分辨率(在-6~7nT范围内,误差小于1nT),视磁化率异常与中国的区域构造特征吻合很好.34GEOMAGNETISMGEOMAGNETISM磁性结构与地壳构造35GEOMAGNETISMGEOMAGNETISM磁性结构与地壳构造36GEOMAGNETISMGEOMAGNETISM磁性结构与地壳构造37GEOMAGNETISMGEOMAGNETISM磁性结构与地壳构造如高磁性区对应前寒武纪克拉通,而低磁区位于山区、缝合带及西藏高原.这些结果支持了前人的假说,即大陆中等尺度磁异常可能反映了下地壳磁性的横向变化.如果由于山区地壳的增厚,使得下地壳部分插入高温的上地幔可导致岩石热退磁,这可能是造成一些碰撞带区域常呈低磁异常的主要原因.38GEOMAGNETISMGEOMAGNETISM磁性结构与地壳构造39GEOMAGNETISMGEOMAGNETISM磁性结构与地壳构造Continent-continentcollisionzonesofvariousagesarecharacterizedbylowmagneticsusceptibilitiesandhenceweakintensitiesofinducedmagnetization.在该区出露的地壳深源岩石中发生明显的水化作用使得橄长岩（troctolite）中含铬磁铁矿的瓦解,导致磁性减小（from~10-3to10-2(SIunits)to~5x10-4）.Thissusceptibilitycontrastcorrespondstopositionoftheoxidesofthetroctolite(particularlyCr-bearingmagnetite),liberatingFe,Al,Cr,andTitobeincorporatedinamphiboleandrutile(角闪石和金红石).40GEOMAGNETISMGEOMAGNETISM磁性结构与地壳构造41GEOMAGNETISMGEOMAGNETISM磁性结构与变质相人们在分析磁卫星磁异常资料时,注意到它们与岩石的相对变质级的关系,尤其是深部基底岩石变质级的差异.大陆地壳磁性结构与变质相的相互关系主要依赖对出露大陆地壳断面岩石的研究.42GEOMAGNETISMGEOMAGNETISM磁性结构与变质相Fountain等人系统总结了来自意大利Ivrea地区、澳大利亚的Fraser与Musgrave地区、加拿大蒙大拿州的Piwitonei带以及西非的Kasila组的5个出露地壳断面的地质、地球物理与地球化学的研究结果,提出大陆地壳大尺度层状分布特征反映的不是成分差异,而是变质作用的结果.其中上、中、下地壳分别对应绿片岩相、角闪岩相与麻粒岩相.43GEOMAGNETISMGEOMAGNETISM磁性结构与变质相Wasilewski等人首先探讨了意大利北部Ivrea地壳剖面的磁性结构与变质相的关系,其中重点分析了角闪岩相与麻粒岩相过渡带的岩石磁性特征.整个断面自上而下分成A,B,C,D4层.A层代表中-上地壳,为弱磁性;B层为角闪岩相,为中等强度磁性层;C层为泥质片麻岩与镁铁质岗粒岩互层,为弱磁性层;D层为麻粒岩相的斜长二辉麻粒岩、辉石岩、角闪岩与镁铁质岩,其磁性特征为强且变化剧烈.44GEOMAGNETISMGEOMAGNETISM磁性结构与变质相45GEOMAGNETISMGEOMAGNETISM磁性结构与变质相Schlinger综合岩石磁学、矿物学、区域地质及地球物理资料,系统分析了挪威Lofoten与Vesteralen地区深地壳出露地体的磁结构.重点探讨了片麻岩与二长岩在角闪岩相与麻粒岩相条件下岩石的磁性特征及其差异,结果表明,麻粒岩相磁性均普遍大于角闪岩相.46GEOMAGNETISMGEOMAGNETISM磁性结构与变质相河南登封至鲁山地区地壳剖面的磁性结构与变质相关系对应结果为:自绿片岩相,经角闪岩相至麻粒岩相,随着变质等级的增加,岩石磁性呈规律增大（刘庆生等，1993）。华北地台定州-五台的大陆地壳剖面中绿片岩相、角闪岩相及麻粒岩相,岩石磁性也是逐渐增大的（刘庆生等，1996）。47GEOMAGNETISMGEOMAGNETISM磁性结构与变质相巴基斯坦北部Kohistan杂岩代表了年轻的地壳(40~100Ma),以镁铁质岩为例,其磁性结构为自角闪岩相至麻粒岩相的进变质过程,对应于磁性增强;而自辉石麻粒岩退变质至角闪岩相,由于麻粒岩中亚铁磁性矿物铁钛氧化物的破坏而导致磁性明显减弱（Schlingeretal.,1989）。Kola超深井岩石磁性结构的分析结果也表明,岩石中磁铁矿与磁黄铁矿的结晶、转化及磁化强度的分布特征与岩石的变质作用密切相关（Bakhvalov,1984)。48GEOMAGNETISMGEOMAGNETISM磁性结构与地壳物质组成大陆地壳由各类火成岩、变质岩及沉积岩组成.岩石的物理性质是它们的矿物和化学组成、结构构造以及形成时的条件及随后所有变化(尤其是物质组成的变化)的函数,它在相当大的程度上以四维形式决定地质过程的进程.依据岩石磁性的基本理论,地壳物质具有不同的磁学属性.岩石的磁性结构与其矿物,地球化学组分密切相关.49GEOMAGNETISMGEOMAGNETISM磁性结构与地壳物质组成通常情况下,造岩矿物以顺磁性、反磁性结构为主;矿石矿物以亚铁磁性结构为主.含铁矿物(Fe-Ti氧化物及铁镁硅酸盐矿物)以亚铁磁性特征为主.而硅铝矿物则以弱磁性或顺磁性、反磁性为主。以柯拉超深钻结果为例,该井自上而下,从元古界岩石到太古界片麻岩,磁化率κ从10-3SI减小到10-4SI,其中橄榄岩的κ>10-2SI,富含磁铁矿的火山岩层κ>10-3SI两种情况例外.火成岩与沉积岩层平均磁化率的差异随深度的增加而减小.50GEOMAGNETISMGEOMAGNETISM磁性结构与地壳物质组成Schumann等人给出了德国KTB深钻0-7200m岩石的磁性与岩性及含铁矿物的分布结果.结果表明,在2000~2600与6500m附近主要以反铁磁性的磁黄铁矿为主;磁铁矿主要赋存于300,3200~3500和4200~4700m区段,有时也含有少量的赤铁矿与针铁矿.51GEOMAGNETISMGEOMAGNETISM

磁性结构与地壳物质组成紫苏花岗岩岩性（含有斜方辉石的花岗闪长岩，花岗岩，闪长岩）镁铁质--长英质紫苏花岗岩是一类与高级变质作用有成因联系的早前寒武纪含紫苏辉石的特殊中酸性侵入岩或变质岩52GEOMAGNETISMGEOMAGNETISM

磁性结构与地壳物质组成磁性结构与地壳物质组成53GEOMAGNETISMGEOMAGNETISM

古元古代太古代高度熔合岩英闪质片麻岩花岗闪长岩和花岗岩变质火山岩和变质沉积岩54GEOMAGNETISMGEOMAGNETISM

应用实例Bimodal，lognormaldistribution双峰对数正态分布Thetwopeaksinthedistributionreflectthepopulationofpositivemagnetic50anomaly-producingsamples(susceptibilities>0.0025SI)andthegroupofsamplesthatproducebackgroundmagneticfieldvalues.Theformersubpopulationcontainsferrimagneticminerals;thelattercontainsparamagnetic(mostlysilicate)mineral.55GEOMAGNETISMGEOMAGNETISM

应用实例56GEOMAGNETISMGEOMAGNETISM

应用实例57GEOMAGNETISMGEOMAGNETISM

应用实例磁性结构与地壳物质组成58GEOMAGNETISMGEOMAGNETISM

59GEOMAGNETISMGEOMAGNETISM

磁性结构与地壳物质组成60GEOMAGNETISMGEOMAGNETISM

DiscussionMostdiscussionsofLWAsourcerockshavefocusedonmoremaficrocksofmetamorphicorigin,particularlythoseofgranulitefacies(麻粒岩相).Thisis,inpart,duetotheinitialassumptionofalowercrustaloriginforthesourceswhichthenleads(throughmodeling)tosourcemagnetizationssolargethatonlymaficandpossiblyultramariclithologiescanbeconsideredbasedoncurrentphysicalpropertydatabases.Also,generallyobservedlowercrustalvelocitiesof>7km/s,requiremaficcomposition.磁性结构与地壳物质组成61GEOMAGNETISMGEOMAGNETISM

DiscussionIncontrast,thedominantMintoigneouscharnockiticunitsaremorefelsicbutaremoreefficientintheproductionofmagnetitefromtheavailableiron.Additionally,theirlateralextentislargeenoughtoproduceLWAeventhoughtheyoccuratshallowlevelsinthecrust.磁性结构与地壳物质组成62GEOMAGNETISMGEOMAGNETISM

DiscussionThepresenceinthecrustofigneouschanrockiticrockslikethoseintheMintoblockcouldsatisfythesourcerequirementsofmagneticmodelsofLWAatotherlocations.IgneouscharnockitesarecommoncomponentsofgranuliteterranesofArchean,ProterozoicandPhanerozoicage.Thelargecharnockiticbodiesinthesedeeplyeroded(15-30km)terranesmayrepresenttherootzonesoftalc-alkalinegranitoidbatholiths(钙碱性的花岗岩基)andthereforecouldbeanimportantcomponentofPrecambrianandP