大别造山带现今地壳结构岩石组成研究
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摘要
通过大别造山带岩石地震波速的高温高压实验测定值与地震测深断面观察值的对比,结合地表出露岩石对深部岩石组成的地球化学示踪,揭示了该造山带现今地壳各结构层的基本岩石组成:上地壳主要由长英质岩石组成,榴辉岩在其中仅占0～20%;中地壳(低速层)主体由长英质片麻岩和花岗岩组成;下地壳上部由80%镁铁质麻粒岩和20%长英质麻粒岩组成;下地壳下部由幔源底侵岩浆作用形成的镁铁质麻粒岩组成。下地壳和上地幔的泊松比为0.265～0.280,明显高于榴辉岩的泊松比,表明其中即使有榴辉岩存在,含量也应极少。各构造单元出露的白垩纪花岗岩具有相似的化学组成,其铅、钕与锶同位素组成均可与北大别的核杂岩相对比,表明花岗岩浆均应以深部核杂岩为源岩。因此,被白垩纪花岗岩侵入的南大别超高压变质岩单元和宿松高压变质岩单元深部均应由核杂岩组成,亦即超高压和高压变质岩应呈拆离的构造岩片覆盖于核杂岩之上。鉴于白垩纪花岗岩的钕模式年龄(tDM)均集中于古元古代(1.8～2.2Ga),表明花岗岩的源区应限于核杂岩中构成原中地壳下部的古元古代结晶基底岩层。这为中地壳主要由长英质片麻岩组成提供了佐证。考虑到大量花岗岩浆的抽出,应留下更大体积的镁铁质岩残留体,后者在地壳流变增厚调整过程中可能因密度增大而下沉,也可能
The rock-constitutions of each crustal layer have been determined from both the geophysical and geochemical constraints. The results indicate that the upper crust is composed dominantly of felsic rocks with 0 ～ 20% eclogites, the middle crust as a low velocity layer mainly of felsic gneisses and granites, the upper part of lower crust of 80% mafic granulites and 20% felsic granulites, and the lower part of lower crust basically of mafic granulites. The Poisson's ratio of the lower crust and the upper mantle ranges between 0.265 and 0.280. This implies that no notable quantity of eclogites exists there because the Poison's ratio values of both the fresh eclogite and the retrograde metamorphosed eclogite have proved to be below 0.265. The Late Cretaceous granites from different tectonic units are similar in geochemical features to each other regardless their intrusion into the core complex, UHPM rocks, or HPM rocks. Their Pb, Nd and Sr isotopic compositions coincide well with those of the exposed core complex consisting of mainly felsic gneisses and amphibolites. This implies that the common source of the granites from various tectonic units should be the core complex in the depth of the crust and the core complex should underlie the UHP-HP metamorphic rock units, as slipping structural sheets. The source area was proved to be composed of the Paleoproterozoic crystalline basement rocks at the lower part of the original middle crust by the dominantly Paleoproterozoic Nd model ages of the granite and from the comparison with the crustal structure-constitution in East Qinling. Because the source rocks must have changed to more voluminous mafic residue after extraction of the granitic magma, the mafic residual rocks were most likely to migrate from the original middle crust to the present lower crust, especially its upper part, owing to the increase in their density or wave velocity. Finally, the possibility that the mafic granulites at the crust bottom were formed through the underplating of the mantle-derived magma at about 1.1 Ga is discussed from geochemistry of the Late Cretaceous basalt in the Dabie orogen.
The rock-constitutions of each crustal layer have been determined from both the geophysical and geochemical constraints. The results indicate that the upper crust is composed dominantly of felsic rocks with 0 ～ 20% eclogites, the middle crust as a low velocity layer mainly of felsic gneisses and granites, the upper part of lower crust of 80% mafic granulites and 20% felsic granulites, and the lower part of lower crust basically of mafic granulites. The Poisson's ratio of the lower crust and the upper mantle ranges between 0.265 and 0.280. This implies that no notable quantity of eclogites exists there because the Poison's ratio values of both the fresh eclogite and the retrograde metamorphosed eclogite have proved to be below 0.265. The Late Cretaceous granites from different tectonic units are similar in geochemical features to each other regardless their intrusion into the core complex, UHPM rocks, or HPM rocks. Their Pb, Nd and Sr isotopic compositions coincide well with those of the exposed core complex consisting of mainly felsic gneisses and amphibolites. This implies that the common source of the granites from various tectonic units should be the core complex in the depth of the crust and the core complex should underlie the UHP-HP metamorphic rock units, as slipping structural sheets. The source area was proved to be composed of the Paleoproterozoic crystalline basement rocks at the lower part of the original middle crust by the dominantly Paleoproterozoic Nd model ages of the granite and from the comparison with the crustal structure-constitution in East Qinling. Because the source rocks must have changed to more voluminous mafic residue after extraction of the granitic magma, the mafic residual rocks were most likely to migrate from the original middle crust to the present lower crust, especially its upper part, owing to the increase in their density or wave velocity. Finally, the possibility that the mafic granulites at the crust bottom were formed through the underplating of the mantle-derived magma at about 1.1 Ga is discussed from geochemistry of the Late Cretaceous basalt in the Dabie orogen.
引文
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[9] 侯青叶,张本仁,张宏飞.大别核部杂岩中斜长角闪岩的地球化学特征及其地质意义[J].地质论评,2003,(4):HouQingye,ZhangBenren,ZhangHongfei.GeochemicalcharacteristicsoftheamphibolitesfromDabieCoreComplexinCentralChinaandtheirgeologicalimplications[J].GeologicalReview,2004,50(1):77-86.(inChinesewithEnglishabstract)
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[12] Yong ShengLiu,ShanGao,Shu YanJin,Sheng HongHu,MinSun,Zu BinZhao,Jia LinFeng.GeochemistryoflowercrustalxenolithsfromNeogeneHannuobaBasalt,NorthChinaCraton:Implicationsforpetrogenesisandlowercrustalcomposition[J].GeochimicaetCosmochimicaActa,2001,65(15):2589-2604.
[13] 张本仁,高山,张宏飞,等.秦岭造山带地球化学[M].北京:科学出版社,2002.187.ZhangBenren,GaoShan,ZhangHongfei,etal.GeochemistryofQinlingorogenicbelt[M].Beijing:SciencePress,2002.187.(inChinese)
[14] 李曙光,聂永红,HartSR,张宗清.俯冲陆壳与上地幔的相互作用———Ⅱ.大别山镁铁 超镁铁质岩的Sr,Nd同位素地球化学[J].中国科学(D辑),1998,28(1):18-22.LiShuguang,NieYonghong,HartSR,ZhangZongqing.Interactionbetweensubductedcontinentalcrustandthemantle—Ⅱ.SrandNdisotopicgeochemistryofthesyncollisionalmaficultramaficintrusionsinDabieMountains[J].ScienceinChina(seriesD),1998,28(1):18-22.(inChinese)
[15] JahnBM,WuFY,LoCH,Chin HoTsai Crust-mantleinteractioninducedbydeepsubductionofthecontinentalcrust:geochemicalandSr Ndisotopicevidencefrompost collisionalmafic ultramaficintrusionofthenorthDabiecomplex,CentralChina[J].ChemicalGeology,1999,157:119-146.
[16] 匡少平,张本仁,凌文黎,韩吟文.大别造山带南坡晚白垩世玄武岩源区地幔特征[J].地质科学,2001,36(2):203-211.KuangShaoping,ZhangBenren,LingWenli,HanYinwen.MantlesourceofLateCretaceousbasaltsinSouthDabieOrogrnicBelt,China[J].ChineseJournalofGeology,2001,36(2):203-211.(inChinesewithEnglishabstract)
[17] 凌文黎,程建萍,王歆华,周汉文.武当地区新元古代岩浆岩地球化学特征及其对南秦岭晋宁期区域构造性质的指示[J].岩石学报,2002,18(1):25-36.LingWenli,ChengJianping,WangXinhua,ZhouHanwen.GeochemicalfeaturesoftheNeoproterozoicigneousrocksfromtheWudangregionandtheirimplicationsforthereconstructionoftheJinningtectonicevolutionalongthesouthQinlingOrogenicBelt[J].ActaPetrologicaSinica,2002,18(1):25-36.(inChinesewithEnglishabstract)
[2] 高山,金振民,金淑燕,KernH,许志琴,杨天楠,董树文.大别超高压榴辉岩高温高压下地震波速和密度的初步实验研究──对造山带地壳深部组成和莫霍面性质的启示[J].科学通报,1997,42:862-866.GaoShan,JinZhenmin,JinShuyan,KernH,XuZhiqin,YangTiannan,DongShuwen.Preliminarystudyoftheseismicvelocityanddensitytheultrahigh pressureeclogitesfromtheDabieregionbyexperimentsunderhightemperatureandhighpressure:ImplicationsforthedeepcrustalconstitutionandtheMohopropertyintheorogenicbelt[J].ChineseScienceBulletin,1997,42:862-
866.(inChinese)
[3] KernH,GaoS,JinZM,TillPopp,JinShuyan.PetrophysicalstudiesonrocksfromtheDabieultrahigh pressure(UHP)metamorphicbelt,centralChina:Implicationsforthecompositionanddelaminationofthelowercrust[J].Tectonophysics,1999,301:191-215
[4] KernH,JinZM,GaoS,TillPopp,ZhiqinXu.Physicalpropertiesofultrahigh pressuremetamorphicrocksfromtheSuluterrain,easterncentralChina:ImplicationsfortheseismicstructureattheDonghai(CCSP)drillingsite[J].Tectonophysics,2002,354:315-330.
[5] GaoS,KernH,JinZM,PoppT,JinSY,ZhangHF,ZhangBR.Poisson'sratioofeclogite:theroleofretrogression[J].EarthandPlanetaryScienceLetters,2001,192:523-531.
[6] HongfeiZhang,ShanGao,ZengqiuZhong,BenrenZhang,LiZhang,ShenhongHu GeochemicalandSr Nd PbisotopiccompositionsofCretaceousgranitoids:ConstraintsontectonicframeworkandcrustalstructureoftheDabieshanultrahighpressuremetamorphicbelt,China[J].ChemicalGeology,2002,186:281-299.
[7] 张理刚,等.东亚岩石圈块体地质[M].北京:科学出版社,1995.252.ZhangLigang,etal.Block geologyofEasternAsialithosphere:Isotopegeochemistryanddynamicsofuppermantle,basementandgranite[M].Beijing:SciencePress,1995 252.(inChinesewithEnglishabstract)
[8] 索书田,桑隆康,韩郁箐,游振东,等.大别山前寒武纪变质地体岩石学与构造学[M].武汉:中国地质大学出版社,1993.259.SuoShutian,SangLongkang,HanYujing,YouZhendong,etal.ThepetrologyandtectonicsinDabiePrecambrianmetamorphicterrain,centralChina[M].Wuhan:PressofChinaUniversityofGeosciences,1993.259.(inChinesewithEnglishabstract)
[9] 侯青叶,张本仁,张宏飞.大别核部杂岩中斜长角闪岩的地球化学特征及其地质意义[J].地质论评,2003,(4):HouQingye,ZhangBenren,ZhangHongfei.GeochemicalcharacteristicsoftheamphibolitesfromDabieCoreComplexinCentralChinaandtheirgeologicalimplications[J].GeologicalReview,2004,50(1):77-86.(inChinesewithEnglishabstract)
[10] MengelK.Evidencefromxenolithsforthecompositionofthelithosphere[A].BlundellD,FreemanR,MuellerS,eds.Acontinentrevealed:TheEuropeangeotraverseChapter[C].1992,4,4.3:91-102
[11] 樊祺诚,刘若新,李惠民,李霓,隋建立,林卓然.汉诺坝麻粒岩的锆石年代学及稀土元素地球化学[J].科学通报,1998,43(2):133-137.FanQicheng,LiuRuoxin,LiHuimin,LiNi,SuiJianli,LinZhuoran.ZirconchronologyandrareearthelementsgeochemistryofthegranulitesfromHannuoba,NorthChina[J].ChineseScienceBulletin,1998,43(2):133-137.(inChinese)
[12] Yong ShengLiu,ShanGao,Shu YanJin,Sheng HongHu,MinSun,Zu BinZhao,Jia LinFeng.GeochemistryoflowercrustalxenolithsfromNeogeneHannuobaBasalt,NorthChinaCraton:Implicationsforpetrogenesisandlowercrustalcomposition[J].GeochimicaetCosmochimicaActa,2001,65(15):2589-2604.
[13] 张本仁,高山,张宏飞,等.秦岭造山带地球化学[M].北京:科学出版社,2002.187.ZhangBenren,GaoShan,ZhangHongfei,etal.GeochemistryofQinlingorogenicbelt[M].Beijing:SciencePress,2002.187.(inChinese)
[14] 李曙光,聂永红,HartSR,张宗清.俯冲陆壳与上地幔的相互作用———Ⅱ.大别山镁铁 超镁铁质岩的Sr,Nd同位素地球化学[J].中国科学(D辑),1998,28(1):18-22.LiShuguang,NieYonghong,HartSR,ZhangZongqing.Interactionbetweensubductedcontinentalcrustandthemantle—Ⅱ.SrandNdisotopicgeochemistryofthesyncollisionalmaficultramaficintrusionsinDabieMountains[J].ScienceinChina(seriesD),1998,28(1):18-22.(inChinese)
[15] JahnBM,WuFY,LoCH,Chin HoTsai Crust-mantleinteractioninducedbydeepsubductionofthecontinentalcrust:geochemicalandSr Ndisotopicevidencefrompost collisionalmafic ultramaficintrusionofthenorthDabiecomplex,CentralChina[J].ChemicalGeology,1999,157:119-146.
[16] 匡少平,张本仁,凌文黎,韩吟文.大别造山带南坡晚白垩世玄武岩源区地幔特征[J].地质科学,2001,36(2):203-211.KuangShaoping,ZhangBenren,LingWenli,HanYinwen.MantlesourceofLateCretaceousbasaltsinSouthDabieOrogrnicBelt,China[J].ChineseJournalofGeology,2001,36(2):203-211.(inChinesewithEnglishabstract)
[17] 凌文黎,程建萍,王歆华,周汉文.武当地区新元古代岩浆岩地球化学特征及其对南秦岭晋宁期区域构造性质的指示[J].岩石学报,2002,18(1):25-36.LingWenli,ChengJianping,WangXinhua,ZhouHanwen.GeochemicalfeaturesoftheNeoproterozoicigneousrocksfromtheWudangregionandtheirimplicationsforthereconstructionoftheJinningtectonicevolutionalongthesouthQinlingOrogenicBelt[J].ActaPetrologicaSinica,2002,18(1):25-36.(inChinesewithEnglishabstract)
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