【作者】 陈仁旭；

【导师】 郑永飞；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2009， 博士

【摘要】 大陆深俯冲和折返的研究已经成为21世纪发展和完善板块构造理论的前沿和核心课题。位于中国中东部的大别-苏鲁造山带出露有世界上规模最大的超高压变质构造单元,是超高压变质和大陆碰撞研究的典型地区。本文利用中国大陆科学钻探（CCSD）工程岩芯样品连续取样的优势,对深钻主孔200～4000m的超高压变质岩,特别是不同岩性接触带,进行了系统的地球化学分析,包括全岩主量和微量元素、Rb-Sr、Sm-Nd和Lu-Hf同位素、矿物氢氧同位素、单矿物结构水含量和全水含量、锆石微量元素、U-Pb和Lu-Hf同位素。结果为大陆俯冲带变质的化学地球动力学提供了新的制约,特别是地表露头样品研究难以解决的科学问题。在锆石岩相学研究的基础上,对不同锆石区域进行了微量元素、U-Th-Pb和Lu-Hf同位素同时原位分析;在对变质锆石进行U-Pb定年分析时,第一次直接将微量元素和Lu-Hf同位素联系起来。通过这种联合研究,我们区分出大陆俯冲带变质过程中原岩锆石的固态重结晶、交代重结晶和溶解重结晶以及变质新生长。榴辉岩相条件下变质生长锆石以谐和的变质作用年龄,典型石榴石效应的平坦HREE配分,低的REE（特别是HREE）、Y、Nb+Ta和Th+U含量,高的Hf含量,低的（Lu/Gd）N、Lu/Hf和Th/U（＜0.1）比值,以及升高的¹⁷⁶Hf/¹⁷⁷Hf比值为特征。这说明变质锆石生长过程中同时存在石榴石效应和流体效应。相反地,变质重结晶不同程度的再造原岩锆石,具体取决于变质过程中流体活动的程度。固态重结晶锆石被再造的程度最小,几乎继承了原岩锆石的所有地球化学特征。它们以不一致线上不谐和的U-Pb年龄,典型的岩浆锆石陡峭的MREE-HREE配分,高的微量元素含量和比值,以及低的¹⁷⁶Hf/¹⁷⁷Hf比值为特征。另一方面,溶解重结晶锆石显示出最大程度的再造,具有近谐和或谐和的变质作用年龄,陡峭的MREE-HREE配分,相比原岩锆石降低的微量元素含量如REE、Th、U、Y、Nb、Ta和Ti,几乎不变的Hf同位素比值。交代重结晶锆石被再造的程度介于这两者之间,结果具有介于它们之间的元素和同位素特征。石榴石和富水流体同时存在条件下的变质生长以亏损HREE平坦配分和低的微量元素含量为特征,富水流体存在条件下的变质重结晶则表现出MREE到HREE逐渐降低而不是平坦的HREE配分。因此,变质锆石的多指标同时原位分析比单独分析在区分变质新生长与不同类型变质重结晶之间具有优势。在变质锆石学研究的基础上,通过识别锆石的形成条件,能够对深钻主孔超高压变质岩锆石U-Pb年龄的地质意义进行制约。深钻主孔超高压变质岩锆石U-Pb不一致线定年得到的上交点年龄为780±22Ma,对应于原岩形成年龄。在超高压变质岩中得到两组变质年龄242±9Ma和221±2Ma,分别对应于进变质高压-超高压转变和退变质榴辉岩相再造过程中流体存在条件下的生长和重结晶。对CCSD深钻主孔200～4000m超高压变质岩系统的岩相学、氧同位素、氢同位素、矿物水含量（结构水和全水）联合研究发现,超高压变质岩原岩在俯冲之前经历了大气降水的高温热液蚀变甚至局部低δ¹⁸O岩浆作用。深钻主孔¹⁸O亏损深度达3300m,结合地表露头岩石达～30000km²的¹⁸O亏损,表明扬子陆块北缘曾发生三维空间巨量物质（＞100000km³）的¹⁸O亏损。深钻主孔超高压变质岩中各矿物显示出巨大的氢氧同位素变化,共存矿物之间的氢氧同位素分馏既有平衡也有不平衡,相同矿物氢氧同位素也表现出不同的行为,指示了超高压变质矿物和同位素在折返过程中不同的退变质效应。根据相邻样品之间的距离、岩相学和δ¹⁸O值,确定出不同岩性之间O同位素不均一性的尺度为20到50cm,对应于大陆碰撞过程中流体活动的最大尺度。TC/EA-MS和FTIR分析都发现,超高压变质岩中名义上无水矿物都含有大量的水以结构羟基和分子水的形式存在。不同岩性接触带的矿物O同位素、H同位素、全水含量以及羟基含量的研究表明,尽管超高压变质岩经历了广泛的退变质作用,退变质流体在稳定同位素组成上是内部缓冲的,且退变质流体主要来自名义上无水矿物中结构羟基和分子水的出溶以及含水矿物的分解。在不同岩性接触带,稳定同位素、水含量及岩相学上出现同步变化,表明不同岩性接触带可能是流体流动最活跃的地方。对于远离榴辉岩-片麻岩接触带的榴辉岩,退变质流体主要来自榴辉岩内部名义上无水矿物中结构羟基和分子水的降压出溶以及含水矿物的分解。对于靠近接触带的榴辉岩,则可能主要受来自比榴辉岩更富水的片麻岩中矿物的脱水作用。定量计算表明,在折返过程中,1m³体积由石榴石和绿辉石组成的榴辉岩结构羟基降压出溶能释放出3.07～3.44kg水,这能形成139.58～156.28kg角闪石。这能为超高压榴辉岩的角闪岩化提供大量的流体来源。系统的矿物水含量测定和计算表明,片麻岩具有比榴辉岩较高的全水含量,在深俯冲陆壳的初期折返过程中,片麻岩能比榴辉岩释放出更多的水。在榴辉岩和片麻岩接触带的降压脱水,所释放的水会从片麻岩流向榴辉岩导致邻近片麻岩的榴辉岩发生显著的水化。对CCSD深钻主孔连续岩芯段超高压变质岩系统的主微量元素和Sr-Nd-Hf同位素研究发现,大陆俯冲带变质过程中确实存在显著的元素活动,特别是水溶性元素LILE和LREE。深钻主孔超高压变质岩连续岩芯段显示出主量和一些微量元素如LILE（K、Rb、Ba、Th和U）和LREE的巨大变化,但水不溶性元素如HFSE和HREE变化较小。虽然富水流体活动可以导致LILE巨大的变化,但一些榴辉岩和角闪岩中SiO₂、LILE和LREE的巨大变化指示,它们受到了深俯冲陆壳折返过程中与长英质超高压片麻岩相关的部分熔融形成的花岗质熔体的交代作用。岩相学观察也显示了部分熔融熔体的存在。在不同岩性接触带发现有元素和岩相学的同步变化,表明超高压岩石折返过程中不同板片组分之间富水流体和/或含水熔体及其相关元素活动的出现。同一岩性的局部某些样品存在部分熔融现象。对有些变质岩,岩相学观察到部分熔融的现象,但是在化学组成上没有反映,指示一种原地的深熔混合岩化。因此,在超高压板片“热”折返过程中,大陆地壳在局部会发生部分熔融,导致俯冲板片内显著的质量传输。然而,元素的运移只发生在很小的尺度内,在不同岩性接触带和裂隙带的有限开放体系中。CCSD深钻主孔锆石U-Pb和Hf同位素研究显示,深钻主孔超高压变质岩原岩记录了两种不同的双峰式岩浆活动。第一种双峰式岩浆活动为中元古代晚期新生地壳的裂谷再造,而第二种双峰式岩浆活动则为古元古代中期古老地壳的裂谷深熔作用。不管怎样,它们的原岩都是由华南陆块北缘新元古代中期不同来源的双峰式岩浆活动所形成,裂谷熔融作用发生在罗迪尼亚超大陆裂解之前或之中。造山带岩石圈的熔融能够产生具有不同来源的双峰式岩浆活动。弧陆碰撞带是聚集新生和古老地壳最好的地方,这两种不同来源的双峰式岩浆活动在超大陆裂解期间沿着华南陆块北缘的弧陆碰撞带出现,裂谷熔融作用是对罗迪尼亚超大陆在780Ma开始裂解的响应。作为大陆岩石圈中的薄弱带,它们会进一步发展成超大陆裂谷,最终导致超大陆在750Ma时裂开。从超大陆裂谷到超大陆裂开的构造演化期间,上涌的镁铁质岩浆会导致大气降水热液蚀变的新生地壳和古元古代地壳的再造。因此,CCSD主孔超高压变质岩原岩的双重双峰式组成有力地支持了华南新元古代中期岩浆岩成因的板块-裂谷模型。更多还原

【Abstract】 The study of continental deep-subduction has been one of the forefront and core subjects to advance the plate tectonic theory in the twenty-first century.Along the Dabie-Sulu orogenic belt in east-central China crops out the largest lithotectonic unit containing ultrahigh-pressure （UHP） metamorphic rocks in the world.Much of our understanding of the world’s most enigmatic processes in continental deep-subduction zones has been deduced from various records in this belt.By taking advantage of having depth profiles from core samples of the Chinese Continental Scientific Drilling（CCSD） project in the Sulu orogen,a series of combined studies,including petrography,whole-rock major and trace elements,whole-rock Sr-Nd-Hf isotopes,mineral O and H isotopes and water content（structural hydroxyl content and total water content）,and zircon trace elements,U-Pb and Lu-Hf isotopes,were carried out for UHP metamorphic rocks from the CCSD main hole（MH） at continuous depths of 200 to 4000 m.The results provide insights into the chemical geodynamics of continental subduction-zone metamorphism,especially on the issues that are not able to be resolved from discrete surface outcrops.Simultaneous in-situ analyses of trace elements,U-Th-Pb and Lu-Hf isotopes were carried out for distinct domains of zircons from the CCSD-MH core samples.For the first time,trace elements are directly linked to Lu-Hf isotopes in metamorphic zircons with reference to their U-Pb dates.This enables methodological integration to distinguish solid-state,replacement and dissolution recrystallizations of protolith zircons from new growth during continental subduction-zone metamorphism.Metamorphically grown zircons under eclogite-facies metamorphism are characterized by concordant U-Pb ages for the metamorphism,flat HREE patterns typical of the garnet effect,low contents of REE（especially HREE）,Y,Nb+Ta and Th+U,high contents of Hf,low（Lu/Gd）_N,Lu/Hf and Th/U（＜0.1） ratios,and elevated ¹⁷⁶Hf/¹⁷⁷Hf ratios relative to solid-state recrystallized zircons.This suggests the effects of both garnet and fluid on the growth of metamorphic zircons.In contrast,metamorphic recrystallization has reset the U-Th-Pb isotope system of protolith zircons to different extents, depending on the extents of fluid action during metamorphism.Solid-state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons,which are characterized by discordant U-Pb ages close to or below the protolith age,steep MREE-HREE patterns typical of magmatic origin,high contents of trace elements and their ratios,and low ¹⁷⁶Hf/¹⁷⁷Hf ratios.On the other hand,dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant U-Pb ages for the metamorphism,steep MREE-HREE patterns,lowered contents of trace elements such as REE,Th,U,Y,Nb,Ta and Ti relative to the protolith zircons,and almost unchanged Hf isotope ratios.Replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes inbetween.While the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of HREE with flat pattern and the low contents of trace elements,the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of MREE to HREE without the flat HREE pattern.Therefore,the simultaneous in-situ analyses of metamorphic zircons have the advantage over single-term analyses in making distinction between the new growth and the different types of recrystallization.Petrography,mineral O and H isotopes,mineral water contents（total water and structural hydroxyl） were systematically investigated for UHP metamorphic rocks in a depth of 200-4000 m from the CCSD-MH.Premetamorphic protolith is deduced to have underwent high-T meteoric hydrothermal alteration and even lowδ¹⁸O magmatism before the continental subduction.Minimum depth of ¹⁸O depletion is up to 3300 m,together with areal ¹⁸O depletion of over 30000 km² in surface outcropped rocks along the Dabie-Sulu orogenic belt, the three-dimensional ¹⁸O depletion of over 100000 km³ occurs along the northern margin of the Yangtze Block.The UHP metamorphic rocks show large variations in H and O isotopes. Both equilibrium and disequilibrium H and O isotope fractionations are observed between coexisting minerals.The same minerals show different H and O isotope behaviors in different lithologies.All these indicate different retrograde effects on the UHP minerals and isotopes during the exhumation of deeply subducted continental crust.Inspection of the relationship between the distance,petrography and ¹⁸O values of adjacent samples shows significant O isotope heterogeneities between the different and same lithologies on scales of 20 to 50 cm, corresponding to the maximum scales of fluid flow during the continental collision.Both TC/EA-MS and FTIR analyses show that nominally anhydrous minerals contain significant amounts of water in the form of structural hydroxyl and molecular water.Despite the widespread retrogression,retrograde fluid was internally buffered in the stable isotope compositions.The retrograde fluid is of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl and molecular water as well as decomposition of hydrous minerals.Changes in mineral O isotope,H isotope and water content occur in eclogite-gneiss contacts,concordant with petrographic changes.The contacts between the different lithologies are thus the most favorable place for fluid action.Fluid for retrogression of the eclogites away from the eclogite-gneiss contact was derived from the decompression exsolution of internal water.For the eclogites adjacent to gneiss,in contrast, the retrograde metamorphism was principally caused by aqueous fluid from the gneiss that is relatively rich in water.A quantitative estimate suggests that a 1 m³ volume of bimineral eclogite composed of garnet and omphacite can release 3.07-3.44 kg of water by decompression exsolution of structural hydroxyl,which can in turn form 139.58-156.28 kg of amphibole during exhumation.This can provide a sufficient amount of water for amphibolitization of eclogite.Systematic measurement and calculation of mineral water contents suggest that the gneiss is capable of storing more water than the eclogite under the same UHP conditions.Thus,the UHP gneiss can release more water than the UHP eclogite during the initial exhumation of deeply subducted slabs.By decompression dehydration at the contact between eclogite and gneiss,the released water could flow from the gneiss to the eclogite and thus result in remarkable hydration of the eclogite adjacent to the gneiss.Whole-rock major-trace elements and Sr-Nd-Hf isotopes were systematically investigated for continuous core segments from the CCSD-MH to decipher element mobility in UHP eclogite-facies metamorphic rocks during subduction and exhumation of continental crust. The UHP metamorphic rocks from continuous core segments exhibits a large variation in major and some trace elements such as LILE（e.g.,K,Rb,Ba,Th and U） and LREE,but a relatively limited range in HFSE and HREE.This suggests high mobility of LILE and LREE but immobility of HFSE and HREE during continental subduction-zone metamorphism. Though aqueous fluid can result in large variation of LILE,large variation of SiO₂ and LREE in some eclogite suggests metasomatism of felsic melt produced by partial melting of the associated gneisses during the exhumation.The petrographic observation also demonstrates the presence of partial melting.Concordant variations of element and petrography at the contact between the different lithologies imply the occurrence of aqueous fluid or hydrous melt and its associated element mobility between the different slab components during exhumation.Partial melting locally occurs in the same lithologies.While the partial melting petrographically occurs in some samples,there is no corresponding change in lithochemistry. This suggests that the partial melts did not escape from the host rocks,resulting in a kind of metatexite migmatites.Therefore,partial melting of continental crust took place locally during "hot" exhumation of the deeply subducted continental crust,causing significant mass transfer within the slab.Nevertheless,the efficient transport of elements only occurs on small scales and is thus limited in local open-systems at the lithological contrasts and fractures. Zircon U-Pb and Lu-Hf isotope studies indicate that the UHP rocks from the CCSD-MH have protolith of contrasting origins.Mid-Neoproterozoic protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting at the northern margin of the Yangtze Block.The first type of bimodal magmatism formed by reworking of juvenile Late Mesoproterozoic crust,whereas the second type of bimodal magmatism was principally generated by rifting anatexis of ancient Middle Paleoproterozoic crust.Melting of orogenic lithosphere has potential to bring about bimodal magmatism with contrasting origins.Because arc-continent collision zones are the best place to accumulate both juvenile and ancient crusts, the contrasting types of bimodal magmatism are proposed to occur in an arc-continent collision orogen during the supercontinental rifting,in response to the attempted breakup of the supercontinent Rodinia at ca.780 Ma.As weak zones in the lithosphere,they would be further developed by the supercontinental rifting,finally transformed to the supercontinental breakup at ca.750 Ma.Underplating of mafic magma during the tectonic advance from supercontiental rifting to breakup results in repetitive reworking of both the meteoric-hydrothermally altered juvenile crust and the Paleoproterozoic ancient crust.In this regard,the dual-bimodal compositions for the CCSD-MH metaigneous protoliths lent support to the plate-rift model for petrogenesis of mid-Neoproterozoic igneous rocks in South China.更多还原