【作者】 夏琼霞；

【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】 板块俯冲和折返过程中的流体活动是碰撞造山带变质作用、同碰撞岩浆作用乃至成矿作用的重要内容之一,也是理解地球演化,包括全球水循环、深俯冲板块的命运、碰撞造山带内的岩浆岩成因,以及洋壳和陆壳再循环等问题的关键。在中国东部大别-苏鲁造山带发现的超高压变质岩,为大陆地壳深俯冲变质的产物,记录了大陆碰撞过程中从地壳俯冲到地幔深度、超高压变质、以及深俯冲地壳折返等三个主要阶段的各种物理化学变化,是研究大陆深俯冲和折返过程中变质脱水和部分熔融的重要对象。前人对于镁铁质高压/超高压变质成因榴辉岩中元素在变质流体中的活动性进行了大量研究,但是对于长英质超高压片麻岩中流体活动的研究才刚刚开始。本学位论文以中国中东部大别造山带南部(简称南大别)低温/超高压变质带内的花岗片麻岩为研究对象,进行了岩相学、全岩主量和微量元素、单矿物氧同位素和微量元素、以及锆石微量元素、U-Pb和Lu-Hf同位素等地球化学研究。研究结果识别出陆壳深俯冲和折返过程中出现了变质脱水和部分熔融,同时还识别出了不同变质流体作用下的锆石变质重结晶和新生长。变质重结晶包括固态重结晶、交代重结晶和溶解重结晶,显示了不同流体(富水流体、含水熔体和超临界流体)条件下的变质生长锆石,包括从富水流体中生长的锆石和从含水熔体中生长的锆石。结果不仅为标识变质锆石的形成机制提供了依据,而且为识别不同类型变质流体在大陆碰撞过程中的作用发现了线索。岩相学观察显示,两组花岗片麻岩样品具有不同的结构和共生矿物组合。组I片麻岩样品为鳞片变晶结构、残留骸晶结构、交代蚕食结构等,帘石类、云母类含水矿物较多。组II片麻岩样品为花岗变晶结构,主要组成矿物为石英和长石,且钾长石含量十分丰富,帘石和云母等含水矿物量少,榴辉岩相变质矿物(石榴石、褐帘石、金红石)缺乏。锆石U-Pb定年结果显示,这些超高压变质花岗岩的原岩年龄和变质年龄分别为778±6 Ma和219±6 Ma。锆石εHf(t)值为-11.5±1.4 ~ -2.2±0.5,对应的Hf模式年龄为古元古代中期(2.4 ~ 1.8 Ga),不同于中大别的正εHf(t)值(1.1±0.6 ~ 8.2±0.7)和中元古代晚期Hf模式年龄(1.3 ~ 1.1 Ga)。因此,华南陆块北缘元古代存在两期新生地壳生长,即中元古代晚期和古元古代中期。两组样品的锆石δ18O值为-2.8 ~ +4.7‰,明显低于正常地幔值5.3±0.3‰,表明原岩在新元古代侵位过程中受到过大气降水的高温热液蚀变。大部分样品在t1 = 780 Ma时的87Sr/86Sr初始比值很低,而在t2 = 230 Ma时的87Sr/86Sr初始比值很高,这暗示了原岩在新元古代热液蚀变和三叠纪变质脱水两个过程中发生了广泛的流体干扰。两组样品具有相似的稀土和微量元素配分模式,但是,组I样品的Nb与LREE(La和Ce)显示很好的相关性,与LILE(Rb、Ba、Pb、Th和U)没有相关性,说明在脱水过程中LILE发生了不同程度的迁移活化。结合岩相学研究,可见组I片麻岩在三叠纪大陆碰撞过程中经历了变质脱水反应。Nb与LREE的相关性继承了原岩的特征,而不是由变质脱水引起的。组II片麻岩的Nb含量与LILE呈线性相关,但与LREE不相关,显示在大陆碰撞过程中变质脱水与LILE迁移发生明显脱耦。另外,组II片麻岩的主量元素FeO+MgO+TiO2含量很低(1.04 ~ 2.08% wt.%),而SiO2含量(75.33 ~ 78.23 wt.%)和全碱Na2O+K2O含量(7.52 ~ 8.92 wt.%)很高,并且与长英质岩石经部分熔融实验产生的熔体组成相似。因此,组II片麻岩在深俯冲陆壳折返初期由于多硅白云母分解引起了脱水熔融。此外,在组II片麻岩中几乎没有发现特征的超高压变质矿物;尽管没有明显的深色体和浅色体,但在片麻岩内部发现了由细粒矿物组成的长英质细脉。因此,组II片麻岩由于变质脱水而诱发了一种低程度原位深熔的混合岩化作用,即长英质熔体并没有从寄主片麻岩中分离出去。锆石U-Pb定年得到的变质年龄与大别-苏鲁造山带超高压榴辉岩相退变质引起高压榴辉岩相重结晶时间一致,对应于深俯冲板片折返初期达到最大温度之前,折返板片中流体/熔体的释放导致了锆石的生长。根据参与锆石改造的变质流体性质,进一步可将变质锆石区分为五种不同类型,即固态重结晶锆石、交代重结晶锆石、溶解重结晶锆石,以及富水流体和含水熔体中的生长锆石。不同类型的变质锆石显示不同的U-Pb年龄、微量元素组成、Th/U比值、176Lu/177Hf比值以及176Hf/177Hf比值。变质生长锆石都具有谐和的三叠纪U-Pb变质年龄,高U含量以及低Th/U比值和176Lu/177Hf比值,以及升高的176Hf/177Hf比值。但是富水流体中生长锆石还具有低的REE、Th以及HFSE等微量元素含量,而从含水熔体中生长锆石的REE、Th以及HFSE等微量元素含量却很高。根据大陆俯冲带变质过程中流体/熔体活动的程度,可以识别变质重结晶锆石记录的原始岩浆锆石所经受的不同程度改造。固态重结晶锆石是在流体相对缺乏的条件下经受变质的,仅锆石内部结构和U-Th-Pb同位素体系受到部分改造,表现清晰的或模糊的岩浆环带结构、新元古代中期到三叠纪的206Pb/238U表观年龄,而微量元素和Lu-Hf同位素体系则保持原岩锆石的特征。交代重结晶锆石受流体改造程度较固态重结晶明显,CL图像显示冷杉叶状分带、弱分带或无分带结构,也具有新元古代中期到三叠纪的206Pb/238U表观年龄。其REE配分特征与原岩锆石相似,但部分LREE呈现富集,除了可能的矿物包裹体影响外,主要与热液富水流体有关。其Lu-Hf同位素体系由于少量石榴石的影响,176Lu/177Hf比值轻微降低,而176Hf/177Hf比值基本保持不变。溶解重结晶锆石受到变质流体改造程度最高,CL图像显示海绵状或多孔状结构,具有近谐和的206Pb/238U年龄;如果溶解重结晶改造得彻底,则有可能得到谐和的三叠纪变质年龄。其微量元素具有普遍富集的特征,即REE、Th、U和HFSE (Nb, Ta和Hf)都明显高于原岩锆石,同时REE配分还显示Ce正异常不明显。由于溶解重结晶锆石的Hf同位素组成主要受原岩锆石控制,因此流体对其初始Hf同位素组成没有影响,176Lu/177Hf和176Hf/177Hf比值与原岩锆石几乎保持不变。对于具有普遍提高的LREE、Th、U、HREE和HFSE (Nb、Ta、Hf)含量的海绵状变质锆石,不可能仅仅与常见的热液流体有关。由于含水流体不能携带大量的HREE、Th、U和HFSE等微量元素,因此要求能携带大量HREE和HFSE等元素的超高压含水熔体或超临界流体。在花岗岩-水体系中,富集微量元素的矿物(如帘石、石榴石、金红石等)在含水熔体区域内稳定存在,正常的高压脱水熔融并不能使这些矿物发生分解并释放出大量的微量元素。但是在峰期超高压变质条件下,能够产生超临界流体,这样可以使帘石、石榴石、金红石等矿物发生不稳定分解,同时释放大量微量元素进入变质流体。原来富集REE-HFSE的岩浆锆石,当遇到具有强溶解能力的超临界流体时,可能会变得不稳定而发生显著的溶解重结晶。在深俯冲板片折返初期,由于突然降压和继续升温,超临界流体会发生相分离形成不混溶的富水流体和含水熔体,流体中富集的难溶性元素会随着压力下降发生出溶并作为副矿物沉淀出来,表现为研究区内花岗片麻岩中呈脉状产出的帘石、榍石和锆石等矿物集合体。因此,超临界流体在低温/超高压变质条件下的活动是迁移各种元素的有效载体。更多还原

【Abstract】 Fluid activity during crustal subduction and exhumation is very important to understand a wide spectrum of phenomena, including ultrahigh-pressure (UHP) metamorphism, syn-collisional magmatism and mineralization. It is also a key to understanding the evolution of the Earth, including the global circulation of water, the fate of deeply subducted slab, the origin of igneous rocks in collision orogenic belts, and the recycling of oceanic and continental crusts. UHP metamorphic rocks have been well recognized to occur in the Dabie-Sulu orogenic belt in east-central China, which is a sound target to study the nature and extent of fluid-rock interactions during pre-, syn- and post-peak UHP metamorphism during the continental collision. While much attention has been paid to fluid action in mafic HP to UHP eclogites in continental and oceanic subduction zones, less is focused on felsic UHP gneiss in continnetal subduction zones.This dissertation presents a combined study of petrography, whole-rock major and trace elements as well as Rb-Sr and Sm-Nd isotopes, mineral O isotopes and trace elements as well as zircon trace elements, U-Pb and Lu-Hf isotopes for two groups of low-T/UHP granitic gneiss in South Dabie orogen. The results demonstrate that metamorphic dehydration and partial melting occurred during subduction and exhumation of deeply subducted continental crust. As a progress in metamorphic zirconology, metamorphic growth and recrystallization of zircon can be distincted with respect to protolith inheritance, fluid and melt effects. Mechanisms of solid-state, replacement and dissolution recrystallization were involved in these reworking processes, manifesting the role of different types of metamorphic fluid (aqueous fluid, hydrous melt, and supercritical fluid). Metamorphic growth proceeds from aqueous fluid or hydrous melt during subduction and exhumation of deeply subducted continent. These results give a clue on not only the formation mechanism of metamorphic zircon, but also the action of different kinds of metamorphic fluid during continental collision.Petrographic observations show that Group I gneiss is lepidoblastic with a metasomatic relict texture. Many UHP metamorphic minerals such as garnet, epidote, phengitic muscovite and rutile remain residual skeletal or zoning textures. In contrast, Group II gneiss is leucogranitic with a granoblastic texture. Quartz and feldspar are major rock-forming minerals, and hydrous minetals such as epidote and muscovite are very limited. UHP metamorphic minerals such as (garnet, epidote and rutile) were not found in Group II gneiss, however, the content of K-feldspar is very high. Zircon U-Pb dating yields two groups of ages at 778±13 Ma and 223±4 Ma, respectively, corresponding to protolith formation in the middle Neoproterozoic and metamorphic modification in the Triassic. There are negativeεHf(t) values of -11.5±1.4 to -2.2±0.5 for the zircon, correspongding to old Hf model ages of 2.4 to 1.8 Ga. These are in contrast to those for UHP metaigneous rocks in Central Dabie that are dominated by positiveεHf(t) values of 1.1±0.6 to 8.2±0.7 and young Hf model ages of 1.0 to 1.3 Ga. Thus, there are two episodes of crustal growth in the late Mesoproterozoic (1.3 to 1.0 Ga) and the middle Paleoproterozoic (2.4 to 1.8 Ga), respectively, in the northern edge of the South China Block. Tectonic evolution from supercontinental rift to breakup is considered as a basic mechanism to cause the reworking of both ancient and juvenile crust at about 780 Ma.Zirconδ18O values of -2.8 to +4.7‰were obtained for the two groups of metagranites, which are lower than normal mantle values of 5.3±0.3‰. This is consistent with 18O depletion of metaigneous protolith due to high-T meteoric-hydrothermal alteration at Neoproterozoic. Most samples have extremely low 87Sr/86Sr ratios at t1 = 780 Ma, but very high 87Sr/86Sr ratios at t2 = 230 Ma. This suggests intensive fluid disturbance due to the hydrothermal alteration of protoliths during the Neoproterozoic magma emplacement and the metamorphic dehydration during the Triassic continental collision. The two groups of gneiss have similar patterns of REE and trace element partition. Group I gneiss displays good correlations between Nb and LREE but no correlations between Nb and LILE (Rb, Ba, Pb, Th and U), indicating differential mobilities of LILE due to dehydration. Together with the petrographic observation, metamorphic dehydration is evident in Group I gneiss during the prograde subduction. Thus the correlation between Nb and LREE is inherited from protolith rather than caused by metamorphic modification.For Group II gneiss, in contrast, Nb correlates with LILE, but not with LREE. This may indicate decoupling between the dehydration and LILE transport during continental collision. In addition, Group II gneiss has extremely low contents of FeO + MgO + TiO2 (1.04 to 2.08 wt.%), high SiO2 contents of 75.33 to 78.23 wt.%, and high total alkali (Na2O + K2O) contents (7.52 to 8.92 wt.%), comparable with compositions predicted from partial melting of felsic rocks by experimental studies. This indicates that the dehydration melting may have occurred in Group II gneiss due to breakdown of muscovite during“hot”exhumation. Almost no UHP metamorphic minerals survived; granoblastic texture occurs locally to result in a kind of metatexite migmatites due to dehydration melting without considerable escape of felsic melts from the host gneiss. The metamorphic ages for zircon from the two groups of gneiss are concordant with those for the transition from UHP to HP eclogite-facies recrystallization in the Dabie-Sulu orogenic belt. It also corresponds to the maximum temperature during the initial exhumation, which made fluid/melt available for the zircon growth.According to the metamorphic fluid involved, metamorphic zircon can be divided into five subtypes: metamorphic recrystallization via solid-state transformation, replacement alteration or dissolution reprecipitation, and metamorphic growth corresponds to chemical precipitation from aqueous fluid or hydrous melt. Different types of metamorphic zircon have their characteristic apparent 206Pb/238U ages, trace element compositions, Th/U ratios, 176Lu/177Hf and 176Hf/177Hf isotope ratios. Metamorphically grown zircons from the aqueous fluid or hydrous melt are characterized by concordant Triassic U-Pb ages, high contents of U, low Th/U and 176Lu/177Hf ratios, and elevated Hf isotope ratios. However, metamorphical zircon grown from the aqueous fluid has low contents of REE, Th and HFSE, whereas zircon grown from the hydrous melt show very high contents of REE, Th and HFSE.Metamorphic recrystallization results in varying degrees of reworking of protolith zircons, depending on the activity of aqueous fluid or hydrous melt during subduction-zone metamorphism. Metamorphic zircons formed by solid-state recrystallization exhibit the lowest degrees of reworking on internal textures and U-Pb isotopes, so that they are characterized by regular or blurred oscillatory zoing and discordant U-Pb ages between Neoproterozoic and Triassic. While the zircon internal texture and U-Pb isotope systems may be partially reworked by solid-state recrystallization, its initial Hf isotopes and trace elements keep unchanged with those for protolith zircons. This indicates that there is a lack of metamorphic fluid during the solid-state recrystallization.Replacement-recrystallized zircons show intenser metamorphic modification than solid-state recrystallization. They display fir-tree zoned, weak zoned or unzoned texture, and discordant U-Pb ages between Neoproterozoic and Triassic. Their REE patterns resemble those for protolith magmatic zircon except LREE enrichment for some zircons. This suggests that they underwent metamorphic reworking by replacement alteration in the presence of metamorphic fluid. The LREE enrichment may partly be caused by the presence of microscale LREE-bearing mineral inclusions (such as apatite, monazite or epidote) in the zircons. The 176Lu/177Hf ratios may decrease a little due to the garnet effect, but the 176Hf/177Hf ratios would remaine unchanged.The dissolution-recrystallized zircons display the highest degrees of modification by metamorphic fluid. They exhibit very weak cathodoluminescence, spongy or porous texture, with nearly concordant U-Pb ages if the metamorphic reworking by dissolution recrystallization is complete. Their trace elements show consistent enrichment of REE, Th, U and HFSE (Nb, Ta and Hf) relative to protolith zircon, with insignificant positive Ce anomalies in their REE patterns. Because the Hf isotope composition of dissolution recrystallized zircon is predominated by the protolith zircon, the fluid effect is insignificant on the change in the initial Hf isotope ratios. As a result, the 176Lu/177Hf and 176Hf/177Hf ratios keep almost unchanged relative to the protolith zircon.The consistent enrichment of trace elements in the sponge-textured zircons relative to the magmatic zircon indicates that a common hydrothermal fluid is not adequate for the replacement alteration because the common high-T/LP aqueous fluid cannot transfer a large amount of trace elements. Thus, the involvement of a special UHP metamorphic fluid such as supercritical fluid or hydrous melt is required that has a strong capacity to extract significant amounts of LREE, HREE Th, U and HFSE from such accessory minerals as allanite, garnet, rutile and zircon. Because these minerals are stable in the field of hydrous melt in granite-water systems, they are not able to be decomposed during the exhumation of deeply subducted continental crust. Instead, the supercritical fluid is suggested to transport the LREE, HREE, Th, U and HFSE in the accessory minerals to recrystallized zircons. While the supercritical fluid is stable in deep subduction zones, it would separate into a hydrous melt and an aqueous fluid due to abrupt depression and continuous heating during the initial“hot”exhumation. As a result, the dissolved components would exsolve from the supercritical fluid, with significant incorporation of the fluid-immobile elements into precipitating minerals. This is indicated by occurrence of vein aggregate of REE-rich accessory minerals (epidote, titanite and zircon) in the fissure of exhumated metagranites. Therefore, the action of supercritical fluid is evident under the low-T/UHP metamorphic conditions.更多还原