【作者】 陈伊翔；

【导师】 郑永飞；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2013， 博士

【摘要】 自20世纪80年代中期和90年代初期在大陆表壳岩石中发现柯石英和金刚石以来,大陆深俯冲和超高压变质作用一直是固体地球科学研究的前沿和热点问题。中国大别-苏鲁造山带是世界上规模最大、出露最好的超高压变质带之一,是研究大陆深俯冲过程中岩石学和地球化学变化的理想天然实验室。本文对苏鲁造山带中部和西南部超高压变质岩石进行了系统的岩石学和地球化学研究,结果为大陆碰撞过程中地壳深熔作用提供了证据,对大陆俯冲带变质过程中的流体活动与元素迁移、副矿物响应和变质锆石学等提供了新的重要制约,丰富了俯冲带化学地球动力学。对苏鲁造山带中部和西南部的仰口、岚山和青龙山超高压变质花岗片麻岩和石英岩进行了岩石学和锆石学研究,结果表明它们在折返过程中都经历了部分熔融。这些岩石中石英颗粒边界出现拉长的、高度尖锐状的长石,三联点填充有尖锐状的长石,由钾长石和石英构成的显微脉体和钾长石的晶面发育。这些特征表明,钾长石和石英形成于深熔熔体,并继承了熔体假象。根据锆石CL图像、矿物包裹体和REE配分型式,可以区分出深熔锆石和亚固相变质生长锆石。大部分深熔锆石不含柯石英,具有高Th含量、低Th/U比值、陡峭的REE配分型式和显著负Eu异常,U-Pb年龄为217±2-224±2Ma。少数深熔锆石含柯石英,具有相对低U含量、低Th/U比值、陡峭的REE配分型式和显著负Eu异常,U-Pb年龄为221+5-226+3Ma,主要集中在224+2Ma。这表明深熔作用始于深俯冲大陆地壳的早期折返阶段,但仍处于超高压变质域,持续到高压条件下并且熔融程度变大。片麻岩中白云母残晶与尖锐状钾长石共存,指示深熔熔体可能来自白云母脱水分解诱导的部分熔融。不纯石英岩中全部为变质生长锆石,有的含有柯石英、硬玉和金红石等高压矿物包裹体,有的含有石英、白云母、斜长石和钾长石等低压矿物包裹体。锆石中少量多相矿物包裹体由石英+白云母、白云母+钾长石、石英+钾长石和石英+斜长石等组成,总体上呈花岗斑状结构,可能代表了先前的长英质熔体,因此这些锆石域可能结晶自深熔熔体,与其陡峭的REE配分型式和负的Eu异常特点一致。含不同矿物组合的锆石给出了类似的U-Pb年龄（-220Ma）,指示这些超高压岩石经历了较快的折返过程。锆石δ¹⁸O值多集中在-0.6-0.1‰。,少量为-5.2--4.3%o,并且它们具有类似的U-Pb年龄、微量元素和Hf同位素组成,表明深熔熔体来自O同位素不同的两个源区。一个来源可能主要为石英岩自身,另一个来源可能为围岩片麻岩。纯石英岩中含有残留岩浆锆石核和变质生长边。含榴辉岩相矿物包裹体的锆石域表现出平坦的HREE配分型式、无负Eu异常,而含低压矿物包裹体的锆石域显示出陡峭的REE配分型式和显著的负Eu异常。它们的U-Pb年龄类似,均为-220Ma,表明纯石英岩既记录了榴辉岩相条件下亚固相锆石生长,也记录了较低压力下从深熔熔体中的生长,这些岩石也经历了相对较快的折返过程。不纯石英岩中存在两类石榴石,一类富A1,具有较低的LREE和Nb-Zr含量以及陡峭的REE配分型式；另一类具有较低的A1和较高的Fe含量、显著升高的LREE和Nb-Zr含量以及变化的HREE配分型式。后者含有钾长石、斜长石、石英和榍石等矿物包裹体,有时与角闪石共生；锆石包裹体U-Pb年龄为-214Ma。因此,第一类石榴石为金红石稳定域中榴辉岩相的变质生长,而第二类石榴石可能是折返过程中深熔反应产物。榍石和绿帘石等副矿物的微量元素组成也记录了深熔熔体的影响。锆石中Ti含量测温表明,深熔反应可能始于榴辉岩相条件下多硅白云母的脱水分解,然后进一步降压折返时转变为以白云母脱水熔融为主。对苏北东海地区青龙山花岗片麻岩和榴辉岩进行了锆石学研究,发现锆石δ¹⁸O值、U-Pb年龄、Th/U比值和REE配分模式之间存在一定的相关性。总体上残留锆石核具有新元古代U-Pb年龄、正δ¹⁸O值和岩浆成因锆石REE配分型式,它们是在-770Ma时结晶于具有正δ¹⁸O值的岩浆。变质新生锆石具有低Th/U和¹76Lu/¹77Hf比值、低稀土总量、弱或无Eu负异常、三叠纪变质年龄（2044±4-252±7Ma）和负δ¹⁸O值（-10.0--2.2‰）。这些特征表明,变质生长锆石结晶于负δ¹⁸O变质流体,而这种流体来自经过高温热液蚀变的新元古代负δ18O原岩在三叠纪大陆俯冲带榴辉岩相变质条件下的脱水作用。很多锆石颗粒具有残留锆石核和新生锆石边,并且它们具有显著不同的δ¹⁸O值、Th/U比值和U-Pb年龄。锆石粒间和粒内具有巨大氧同位素变化,指示榴辉岩相变质条件下不同锆石域之间或岩浆锆石核与变质流体之间的氧同位素交换极为有限。对变质锆石的综合分析揭示,原岩锆石经历了固态重结晶、交代重结晶和溶解重结晶等改造过程,重结晶程度与负δ¹⁸O变质流体的参与程度有关。固态重结晶仅造成了放射成因Pb丢失,使原岩锆石的U-Pb年龄年轻化,而微量元素和O-Hf同位素组成都几乎没有改变。经历交代重结晶的锆石域显示不同程度的负δ¹⁸O值和部分再造的REE、U-Pb和Lu-Hf同位素体系,在颗粒边缘和裂隙部位出现溶解重结晶。溶解重结晶锆石具有类似变质生长锆石的δ¹⁸O值、几乎完全再造的U-Pb定年体系和部分再造的稀土配分体系。这些结果不仅进一步揭示了青龙山超高压变质岩中负δ¹⁸O锆石成因,而且为区分大陆俯冲带不同类型的锆石重结晶作用提供了新的制约。在青龙山含黝帘石榴辉岩石榴石和绿辉石中发现了三类多相固体包裹体,分别由斜长石+石英、斜长石+石英+钾长石和重晶石+斜长石+钾长石+黝帘石/绿帘石组成。多相固体包裹体周围的寄主矿物大多显示出放射状裂纹。它们总体上含有很低的REE和微量元素,但大离子亲石元素（LILE）如Sr、Ba和Pb的含量较高。其中前两类多相固体包裹体具有高的Si0₂和Na2_O含量,非常低的FeO+MgO+TiO2含量和变化的K₂O含量。这些包裹体的存在表明,含黝帘石榴辉岩发生了部分熔融作用,其主微量元素特征为部分熔融机制提供了制约。这三类多相固体包裹体可能具有不同成因,第一类主要由黝帘石脱水熔融产生,并且有大量绿辉石参与,形成了富Na熔体；第二类由黝帘石和多硅白云母脱水熔融形成,熔体具有变化的K含量；第三类是与熔体有关的高氧逸度富水流体与寄主矿物反应的结果。对青龙山附近白虎山的超高压片麻岩进行了岩石学、锆石学和地球化学研究,揭示了大陆碰撞过程中伴随温压变化和熔／流体作用副矿物的多期生长和改造行为,也为变质过程中的流体性质及其对微量元素迁移的作用提供了新的认识。通过对锆石REE分布型式、矿物包裹体种类和锆石Ti温度计的综合分析,发现大陆碰撞过程中存在三期锆石生长。第一期为进变质榴辉岩相生长,U-Pb年龄为-237Ma；第二期为退变质榴辉岩相生长,U-Pb年龄为-222Ma；第三期为退变质麻粒岩相生长,U-Pb年龄为-205Ma。这三期锆石具有显著不同的稀土配分型式和微量元素含量,指示它们生长于不同性质的变质熔／流体中。第三期锆石具有显著升高的LREE和HFSE含量,因此很可能生长于含水熔体中,与岩相学观察结果一致。三期变质锆石生长的温压条件表明,变质锆石的幕式生长取决于变质熔／流体的幕式释放行为。片麻岩中榍石的微量元素变化很大,根据榍石REE配分型式和微量元素组成,可以区分出残留岩浆榍石、变质生长榍石和变质重结晶榍石。变质生长榍石具有显著降低的LREE和Th含量以及Th/U比值。它们具有不同程度升高的Nb含量和Nb/Ta比值（可达66）,表明榍石生长过程中Nb/Ta发生了显著分异。片麻岩中石榴石的多期生长表现在矿物包裹体类型和主微量元素组成上存在差异。石榴石的主量元素环带表明,它经历了从进变质角闪岩相到退变质榴辉岩相的持续生长。不同环带中石榴石的微量元素组成则指示,在不同温压条件下形成石榴石的基质矿物在相对比例和矿物组成上发生了变化。对东海地区超高压变质岩进行了全岩Sr-Nd同位素、矿物O同位素和锆石学研究,发现其原岩经历了强烈水岩相互作用,不同程度亏损18O,最低可达-10.8‰。。全岩O同位素显示很大的空间变化,千米尺度达13.5%o,同一露头变化达3.7‰。结合前人研究成果,可以推测这些超高压变质岩原岩在新元古代经历了不同程度高温大气降水热液蚀变,在三叠纪大陆俯冲带变质过程中O同位素均一化尺度较小（矿物颗粒尺度）以及折返过程时非常有限的流体活动。然而,高压石英脉的出现以及不同类型岩石中出现部分熔融,表明深俯冲大陆地壳折返时超高压变质岩内部局部出现显著的熔／流体活动。超高压变质岩的全岩和矿物记录了水岩相互作用过程中的地球化学迁移行为。这些岩石亏损LILE,在tl=750Ma表现出异常低的初始87Sr/86Sr比值,但是在t2=230Ma时表现出非常高的87Sr/86Sr比值,指示LILE等水溶性元素在原岩热液蚀变和变质脱水过程中都具有显著的活动性。榴辉岩中高压石英脉富含石英、绿帘石和蓝晶石等矿物,表明流体流动使Na、Al、Si和LREE等元素发生了显著溶解和运移。锆石O同位素显示巨大变化,表明变质过程中岩石O同位素受到了不同程度改造。受流体作用的影响,锆石U-Pb同位素、微量元素和Lu-Hf同位素体系受到不同程度再造,从而显示这些元素和同位素具有不同的活动性。残留锆石εHf（t）值具有双峰式特征,部分接近新元古代亏损地幔值,部分具有变化的负εHf（t）值,指示新元古代长英质岩浆形成时既有新生地壳再造,也有古老陆壳重熔。更多还原

【Abstract】 The study of continental deep subduction and ultrahigh-pressure (UHP) metamorphism has been one of the forefronts and hotspot topics in solid Earth sciences since the groundbreaking findings of coesite and diamond in metamorphic rocks of supracrustal protolith in1980’s and1990’s. The Dabie-Sulu orogenic belt in east-central China is one of the largest and best exposed UHP metamorphic zones in the world, representing an excellent natural laboratory for the study of petrological and geochemical changes during continental subduction-zone metamorphism. This PhD thesis focuses on petrology and geochemistry of UHP metamorphic rocks in the middle and southwestern parts of the Sulu orogen. The results provide evidence for crustal anatexis during continental collision and new constraints on fluid action and element mobility, the behavior of accessory minerals, and metamorphic zirconology. This has great bearing on chemical geodynamics of continental subduction zones.A combined study of petrology and zirconology was carried out for UHP granitic gneiss and quartzite from the Yangkou, Lanshan and Qinglongshan areas. The results indicate that these rocks experienced anatexis during exhumation of deeply subducted continental crust. Petrographic observations show the occurrence of elongated, highly cuspate feldspars in grain boundaries, interstitial cuspate feldspars in triple junctions, felsic veinlets mainly consisting of K-feldspar+quartz, and feldspar crystal faces against quartz. These features indicate that some feldspar and quartz grew from anatectic melts in the granitic gneiss and quartzite, with inheritance in melt microstructure. Anatectic zircon domains grown from the melts are distinguished from metamorphic zircon domains grown at subsolidus conditions based on CL images, mineral inclusions and REE patterns. Some anatectic zircon domains contain coesite inclusions and exhibit relatively low U contents, low Th/U ratios, steep REE patterns with strong negative Eu anomalies. Their U-Pb ages are221±5to226±3Ma, mostly clustering at224±2Ma. Nevertheless, the majority of anatectic zircon domains does not contain coesite inclusion and exhibits high U contents, low Th/U ratios (<0.1), steep REE patterns with strong negative Eu anomalies, and U-Pb ages of217±2to224±2Ma. These results indicate that the UHP rocks experienced incipient melting during the early exhumation but still in the UHP regime, and then extensive anatexis at lower pressures. Muscovite relicts coexist with cuspate feldspars in the granitic gneiss, suggesting that the anatectic melts originate from dehydration melting due to phengite breakdown. Zircon grains from the impure quartzite are all metamorphic growth, and they contain not only eclogite-facies mineral inclusions of coesite, jadeite, rutile but also lower pressure mineral inclusions that include multiphase solid (MS) inclusions composed of two or more phases of muscovite, quartz, K-feldspar and plagioclase. Some MS inclusions have granitic composition and show granophyric texture, representing former hydrous melt. Thus, such zircon domains would have grown from anatectic melts, consistent with their steep REE patterns and negative Eu anomalies. The anatectic zircon domains containing different mineral inclusions exhibit similar U-Pb ages of-220Ma, suggesting that the UHP rock experienced a relatively rapid exhumation. Most zircon domains have similar δ18O values of-0.6to0.1‰, with a few in the range of-5.2to-4.3‰. The all zircon domains exhibit similar U-Pb ages, trace element and Hf isotope compositions. Thus, the anatectic melts probably have two sources with different O isotope compositions. The predominate one may derive from the quartzite itself, whereas the other from the country rocks. Zircon grains from the pure quartzite contain relict cores of magmatic origin and significant metamorphic overgrowth rims. Zircon domains that contain eclogite-facies mineral inclusions exhibit flat HREE patterns, no Eu anomalies and U-Pb ages of-220Ma. Similar U-Pb ages are obtained for domains that contain lower pressure mineral inclusions and exhibit steep REE patterns and marked negative Eu anomalies. These observations indicate that zircon records subsolidus overgrowth at eclogite-facies conditions but suprasolidus growth at lower pressures. There are two types of garnet in the impure quartzite. One has high Al contents, low LREE and Nb-Zr contents, and steep REE patterns; the other exhibits low Al but high Fe contents, significantly elevated LREE and Nb-Zr contents and variable HREE patterns. The latter garnets contain mineral inclusions of K-feldspar, plagioclase, quartz and titanite, sometimes in coexistance with amphibole. Zircon enclosed by the latter garnet gave consistent U-Pb ages of-214Ma, thus such garnet is interpreted as a peritectic product of the anatectic reaction that involves felsic minerals and possibly amphibole and titanite. The REE patterns of epidote and titanite also record their multistage growth and metasomatism by anatectic melts.A combined in-situ study of zircon U-Pb ages, trace elements and O-Hf isotopes was conducted for the UHP granitic gneiss and eclogite in the Qinglongshan area of northern Jiangsu province. The results exhibit correlations between zircon818O values, U-Pb ages, Th/U ratios and REE patterns for the relict cores of magmatic origin and the newly grown rims of metamorphic origin. Generally, the relict magmatic cores with U-Pb ages of769±9Ma have positive δ18O values of0.1to10.1‰, high Th/U and176Lu/177Hf ratios, high REE contents, and steep MREE-HREE patterns with negative Eu anomalies. They are interpreted as crystallizing from positive δ18O magmas during protolith emplacement in the Neoproterozoic. In contrast, the newly grown domains have concordant U-Pb ages of204±4to252±7Ma and negative δ18O values of-10.0to-2.2%o, low Th/U and176Lu/177Hf ratios, low REE contents, and flat HREE patterns with weak to no Eu anomalies. They are interpreted as growing from negative δ18O fluids that were produced by metamorphic dehydration of the Neoproterozoic high-T glacial-hydrothermally altered rocks during the Triassic continental collision. Large O isotope heterogeneities occur in both intergrain and intragrain of zircon, indicating the limited O isotope exchange between different zircon domains in a single grain or between the relict magmatic domains and metamorphic fluids. The results suggest that protolith magmatic zircon underwent three subtypes of metamorphic recrystallization, with the extent of recrystallization depending on the accessibility to negative δ18O fluids. The solid-state recrystallized zircon domains maintained positive δ18O values and the REE and Lu-Hf isotopes of protolith zircon, but their U-Pb ages are somewhat lowered. The dissolution recrystallized zircon domains exhibit negative δ18O values similar to the metamorphic growths, almost completely reset U-Pb ages, and partially reset REE systems. The replacement recrystallized zircon domains show variably negative δ18O values, and partially reset REE, and U-Pb and Lu-Hf isotopic systems, with local dissolution recrystallization along grain boundaries and fractures. Therefore, these results place robust constraints on the origin of negative δ18O zircon in the UHP rocks from the Sulu orogen and provide a methodological framework to distinguish the different types of metamorphic zircons in continental subduction zones.Multiphase solid inclusions in both garnet and omphacite were investigated for zoisite-bearing UHP eclogite in the Qinglongshan area. The results provide petrological evidence for local anatexis of the eclogite during the continental collision. There are three types of MS inclusions, which are composed of plagioclase+quartz, plagioclase+quartz+K-feldspar, and barite+plagioclase+K-feldspar±zoisite/epidote, respectively. They generally have low trace element contents except such large ion lithophile elements (LILE) as Sr, Ba and Pb. The first and second types of MS inclusions have high contents of SiO2(75～90wt%) and Na2O (2.9to7.3wt%), very low FeO+MgO+TiO2contents and variable K2O contents (0to2.4wt%). The host minerals mostly exhibit radial fractures surrounding the MS inclusions. These features suggest different origins for the MS inclusions. The first type of MS inclusion would be primarily derived from dehydration melting of zoisite with involvement of omphacite to form Na-rich melts, whereas the second type of MS inclusion was derived from dehydration melting of both zoisite and phengite in the eclogite to form melts with variable K contents. The third type of MS inclusion is a result of interaction between the aqueous fluid of high oxygen fugacity and the host mineral. While anatexis of the zoisite-bearing eclogite is evident from the occurrence of MS inclusions, the major and trace element compositions of such MS inclusions provide insights into the origin of partial melts in the eclogite and thus into the nature of dehydration melting in the UHP metamorphic zone.An integrated study of petrology, geochronology and geochemistry was performed for UHP gneisses in the Baihushan area close to the Qinglongshan in northern Jiangsu province. The results not only highlight the polyphase growth of such metamorphic minerals as zircon, titanite and garnet in response to P-T changes and fluid/melt action, but also provide insights into the property of metamorphic fluid/melt and their effects on trace element mobility during the continental collision. A combined result from REE patterns, mineral inclusions and Ti-in-zircon temperatures suggests three stages of zircon growth. Prograde growth occurred at-237Ma primarily at eclogite-facies, retrograde growth at-222Ma mostly at eclogite-facies, and the last growth at-205Ma possibly at granulite-facies. The three stages of zircon growth are deciphered by distinct REE patterns and trace element compositions, recording the differences in the property of metamorphic fluid/melt. The episodic growth of metamorphic zircon is primarily dictated by the episodic releasing and focusing of metamorphic fluid/melt. Relict domains of magmatic titanite are distinguished from metamorphosed and metamorphic domains by their distinctive REE patterns and trace element compositions. The metamorphic titanite exhibit variably elevated Nb contents and Nb/Ta ratios, suggesting significant Nb/Ta fractionation during titanite formation. Polyphase growth of garnet from prograde amphibolite-facies to retrograde eclogite-facies is suggested by an integrated analysis of mineral inclusions and major element compositions in large garnet grains. Trace element contents vary in different zones of garnet, which is ascribed to changes in the paragenesis and composition of matrix minerals involved in garnet-forming reactions at different P-T conditions.A comprehensive study was carried out for UHP metamorphic rocks from the Donghai area in northern Jiangsu province for their whole-rock geochemistry, mineral O isotopes and zirconology. The results confirm that protoliths of the UHP rocks experienced strong high-T glacial meltwater-rock interaction in the Neoproterozoic, leading to their variable depletion of18O, with δ18O values as low as-10.8%o. The mineral O isotope composition has a large spatial heterogeneity, with δ18O values differing up to13.5%o on the kilometer scale and3.7%o on a single outcrop. This indicates the primary O isotope heterogeneity that was established during protolith emplacement but not homogenized even at hand-specimen scale during the Triassic UHP metamorphism and very limited fluid flow during the continental subduction-zone metamorphism. However, the fluid flow is locally significant during exhumation, resulting in the formation of quartz veins, symplectites, coronas, and local anatexis in UHP rocks. Geochemical transport due to fluid action is evident in whole-rock geochemistry and mineralogical composition. The UHP rocks exhibit unreasonably low87Sr/86Sr ratios at t1=750Ma but much radiogenic Sr isotopes at t2=230Ma, suggesting the mobility of water-soluble LILE due to both the hydrothermal alteration during protolith emplacement and the metamorphic dehydration during continental collision. Fluid-rock interaction during the continental collision mobilized Na, Al, Si, Ca, LREE and LILE, resulting in the formation of high-pressure veins in the UHP eclogites. The protolith zircon of magmatic origin underwent different types of metamorphic recrystallization in response to fluid-mineral interaction, leading to different extents of mobility in trace elements and O-Hf isotopes. Both positive εHf(t) values close to the depleted mantle of Neoproterozoic age and highly variable negative εHf(t) values occur in the relict magmatic zircon domains, indicating that the protoliths of UHP rocks were formed by reworking of both juvenile and ancient crustal rocks in the middle Neoproterozoic.更多还原