湖南道县辉长岩包体的组构与波速各向异性研究及其意义
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摘要
辉长岩包体的主要造岩矿物斜长石、单斜辉石的组构测定结果表明 :条带状辉长岩的斜长石组构为结晶b轴的优选定向 ,推测其成因是在重力作用占主导并伴有低应力的情况下形成的。宏观呈块状的辉长岩 ,在微观组构上仍有定向线理存在。经由组构、岩石密度、矿物组成计算得到的理论波速各向异性与实测波速得到的一致 ,从而表明地震波的各向异性对矿物组构的强烈依赖性。推测道县辉长岩包体的波速各向异性反映了下地壳地震波的一般特征。其组构特征反映了该区中、新生代下地壳的变形特征。
Seismic wave anisotropy of the lower_crust has becrme an important topic in recent years.We can get some information of the lower_crust by such geophysical methods as reflection and refraction,and some work on xenolith samples also gives us much beneficial information,including elastic velocity measurments and rock fabric measurments at U stage.In this paper, fabrics of main rock_forming minerals like plagioclase,clinopyroxene and orthopyroxene in two kinds of gabbro xenoliths have been studied.Fabric figures show that b axis of plagioclase has preferred orientation vertical to lineation.This shape is the same as the bedding gabbro intrusions in Late Paleozoic Panxi rift.Synthetic velocity calculated from fabrics,densities and rock components is consistent with the measurements in laboratory.This implies that anisotropy of the lower crust strongly depends on LPO of main minerals,which can be used to interpret strong reflection in seismic wave reflection of the lower crust.
Seismic wave anisotropy of the lower_crust has becrme an important topic in recent years.We can get some information of the lower_crust by such geophysical methods as reflection and refraction,and some work on xenolith samples also gives us much beneficial information,including elastic velocity measurments and rock fabric measurments at U stage.In this paper, fabrics of main rock_forming minerals like plagioclase,clinopyroxene and orthopyroxene in two kinds of gabbro xenoliths have been studied.Fabric figures show that b axis of plagioclase has preferred orientation vertical to lineation.This shape is the same as the bedding gabbro intrusions in Late Paleozoic Panxi rift.Synthetic velocity calculated from fabrics,densities and rock components is consistent with the measurements in laboratory.This implies that anisotropy of the lower crust strongly depends on LPO of main minerals,which can be used to interpret strong reflection in seismic wave reflection of the lower crust.
引文
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2 王京彬 .道县虎子岩岩体及其包体的地球化学特征和构造意义 .湖南地质 ,1991,10 (1) :5 2～ 5 8.
3 郭 锋 ,吴有林 ,范蔚铭等 .湖南省宁远 -道县中生代玄武岩中辉长岩包体的岩石学初步研究 .大地构造与成矿学 ,1996 ,2 0 (1) :38～ 45 .
4 王方正 ,李红丽 ,朱勤文 .湘南火山岩深源包体及岩石圈岩石学模型 .地质科技情报 ,1997,16 (3) ,1～ 7.
5 北京大学地质系岩矿教研室 .光性矿物学 .北京 :地质出版社 1979.
6 WenkHR ,BungeHJ ,JansenE&PannetierJ.PreferredOrientationofplagioclase neutrondiffractionandU stagedata.Tectonophysics ,1986 ,12 6 ,2 71～ 2 84.
7 ChristensenNI .Poisson’sratioandcrustalseismology .JGeophys.Res.,1996 ,10 1(B - 2 ) :3139～ 315 6
8 KernH ,TubiaJM .PressureandtemperaturedependenceofP andS wavevelocities,seismicanisotropyanddensityofshearedrocksfromtheSieraAlpnjatamassif.EarthPlanetSci.Let .,1993,119,191～ 2 0 5 .
9 ParsonsT ,ChristensenNI ,WilshireHG .VelocitiesofsouthernBasinandRangexenoliths :Insightsonthenatureoflowercrustalreflectivityandcomposition .Geology ,1993,2 3(2 ) :12 9～ 132 .
10SuetnovaE ,CarbonellR ,andSmithsonSB .MagmainlayeringetMohooftheBasinandRangeofNevada .GeophysicalResearchLetters ,1993,2 0 (2 4) :2 945～ 2 948.
11BraileLWandChangCS .TheContinentalMohorivicicDiscontinuity ,ResultsfromNearVeriticalandWide AngleSeismicReflectionStudies.InReflectionSeismology :Aglobalperspective.,editedbyBarazangiMandBrownL ,GeodynamicsSe riesAGU ,1986 ,13,2 5 7～ 2 72 .
12JarcowCMandTompsonGA .TheNatureoftheMohorovicicDiscontinuity .EarthPlanet.Sci.,1989,17,475～ 5 0 6 .
13孔 华 ,金振民 ,林源贤 .道县玄武岩中麻粒岩包体的岩石特征及年代学研究 .长春科技大学学报 ,2 0 0 0 ,30 (2 ) :8～12 .
14EtheridgeMA ,WallVJ ,CoxSF ,etal.Theroleofthefluidphaseduringregionalmetamorphismanddeformation .J.Metamorph .Geol.,1983,1,2 0 5～ 2 2 6 .
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