【作者】 刘小驰；

【导师】 吴元保；

【作者基本信息】 中国地质大学 ， 地球化学， 2013， 博士

【摘要】 大陆深俯冲和折返的研究是发展和完善板块构造理论的前缘科学问题。对高压-超高压岩石折返与俯冲带深部流体过程等有关科学问题进行深入的研究,是建立大陆深俯冲和折返理论体系的重要方面。中国东部的西大别造山带和西部的柴北缘造山带中的高压-超高压变质岩,为大陆地壳深俯冲变质的产物,同时还残存有先期深俯冲的洋壳,是研究大陆深俯冲-折返机制和流体过程的重要对象。本学位论文以西大别造山带、柴北缘造山带中的高压-超高压岩石为研究内容,进行了系统的锆石原位U-Pb年龄、微量元素、Lu-Hf同位素、O同位素分析。研究主要获得以下认识：(1)西大别浒湾变质带存在两种类型的高压榴辉岩,其中一类榴辉岩中的残留岩浆锆石U-Pb年龄为411±4Ma,锆石具有正的εHf(t)值(可高达14.4),表明这些榴辉岩的原岩来源自亏损的地幔源区,很有可能对应晚志留纪古特提斯洋壳,另一类榴辉岩及花岗质片麻岩中岩浆锆石的U-Pb年龄分别为700±4Ma和738±6Ma,这些岩石的原岩主体应为中-新元古代扬子板块基底岩石,其中榴辉岩中锆石的εHf(t)值变化范围是-1.0至7.4,表明这类榴辉岩的原岩是新元古代新生地壳再造的产物。两类榴辉岩中变质锆石的U-Pb年龄分别为310±3Ma和306±7Ma,并且显示出低的Th/U比值和微量元素含量,平坦的HREEs配分模式、无明显的Eu异常及低的176Lu/177Hf,这些特征表明浒湾变质带的洋壳和陆壳两种类型榴辉岩共同经历了石炭纪约310Ma的榴辉岩相变质作用。浒湾变质带存在的洋-陆耦合俯冲作用表明,低密度的陆壳类型岩石可能对于高密度洋壳岩石的抬升和保存起到了重要的作用。(2)浒湾变质带中变沉积片岩有分散的岩浆锆石U-Pb年龄表明西大别浒湾变质带部分高压陆壳岩石具有不同的原岩形成时代。斜长角闪岩的锆石U-Pb年龄为1806±63Ma,εHf(t)值为-16.3,原岩可能来自古老地壳物质的再造,富石英浅色脉体及斜长角闪岩中的变质锆石U-Pb年龄分别为243±4Ma和241±1Ma,绿辉石和石榴子石等矿物包裹体和锆石微量元素特征表明这些锆石形成于榴辉岩相变质作用。浒湾变质带陆壳高压岩石经历了约243Ma的榴辉岩相高压变质作用。前人研究表明大别造山带洋壳俯冲的持续时间可能到约250Ma。因此大别造山带陆壳岩石高压-超高压变质作用的初始时间应早于243Ma,与原岩为洋壳的岩石的高压变质时间相吻合。西大别高压-超高压板片的俯冲和折返是在不同时间内完成的,而是在三叠纪经历了多板片的俯冲-折返过程。(3)首次在柴北缘造山锡锡铁山地体榴辉岩锆石中发现超高压矿物包裹体柯石英,柴北缘造山带的四个变质地体都应经历过超高压变质作用。利用SIMS和LA-ICPMS方法确定锡铁山地体超高压峰期时代为441±9Ma,整个柴北缘高压-超高压变质带4个地体在460-440Ma共同经历了俯冲、超高压变质、折返循环,并最终形成了统一的柴北缘巨型高压-超高压变质带。柴北缘洋壳类型的岩石与陆壳类型岩石经历超高压变质作用时间上是连续的,都兰超高压变质地体的蛇纹石化方辉橄榄岩中锆石U-Pb年龄为448±9Ma与蓝晶石榴辉岩的锆石U-Pb年龄455±5Ma一致。低密度的蛇纹石化橄榄岩和陆壳岩石可能对高密度镁铁质超高压岩石保存和折返起到了重要的浮力牵引作用。(4)西大别低温-超高压岩石中片麻岩和榴辉岩的石英脉锆石U-Pb年龄为214.5±3.1Ma和213.8±2.4Ma,反映了西大别超高压岩石中流体活动时间约为215Ma。石英脉中锆石与各寄主岩石中的锆石有一致的O同位素,但是Hf同位素出现差异,其中以片麻岩为围岩的石英脉样品09MC05中的锆石相对围岩片麻岩中的锆石有更低的εHf(t)值(-5.92),而以榴辉岩为围岩的石英脉样品09MC12中的锆石的εHf(t)值(-0.97)介于榴辉岩和片麻岩之间,表明这一阶段的流体活动中,流体的规模更为广泛。结合锆石中低的微量元素含梁特征,成脉流体应来自寄主岩石折返降压过程中经含水矿物脱水作用形成的富水流体。出现的该期流体导致了超高压岩石由高压榴辉岩相至角内岩相的退变质作用。(5)柴北缘锡铁山超高压变质地体的一套长英质浅色脉体、寄主榴辉岩和围岩片麻岩中锆石形态学、U-Pb年龄、微量元素、Hf-O同位素的研究揭示：石英脉中自形振荡环带的锆石具有高的U、Li、Th、Nb、Ta和REEs含量,其形成时间为442±6Ma,与超高压变质时间一致。锆石可能形成自近超高压变质峰期的超临界流体。这些振荡环带的锆有着与围岩片麻岩相似的Lu-Hf同位素组成,但是其O同位素介于片麻岩与寄主榴辉岩之间,这表明来自围岩片麻岩的流体为超临界流体,对Lu和Hf有较强的运移能力,而来自榴辉岩的流体为对微量元素运移能力弱的富水流体。长石石英脉中,锆石U-Pb年龄为420Ma,与锡铁山麻粒岩相变质时间一致,微量元素上强烈富集HREEs,锆石结晶自含水熔体。锆石的Hf-O同位素组成介于围岩片麻岩和寄主榴辉岩之间,但片麻岩占主体。因此,锡铁山超高压地体的片麻岩和榴辉岩在折返过程中的麻粒岩相阶段约420Ma都发生了部分熔融作用,发生部分熔融形成的含水熔体主体来自围岩片麻岩,含水熔体能迁移较远距离。(6)大陆深俯冲过程中的洋-陆耦合俯冲-折返过程表明,洋壳类型岩石与陆壳类型岩石经历超高压变质作用时间上是连续的,低密度的陆壳高压-超高压岩石及蛇纹石化橄榄岩可能对于同时期俯冲的高密度洋壳岩石的抬升和保存起到了重要的携带者的作用。大陆深俯冲形成的低温-超高压变质地体的流体以富水流体为主,发生在折返过程中的流体活动是多阶段的,富水流体的出现促进了超高压岩石的退变质作用。中温-超高压变质地体中可以出现超临界流体及含水熔体,超临界流体出现在近超高压变质作用峰期的初始折返阶段,含水熔体出现在麻粒岩相叠加阶段。超临界流体和含水熔体都能脱离寄主岩石,迁移较远的距离,能够运移俯冲板片中的元素和水进入地幔楔,导致了地幔楔发生部分熔融,最终导致具有典型地球化学特征的岛弧岩浆岩出现。更多还原

【Abstract】 The study of deeply subducted continental is forefront subject to advance the plate tectonic theory. It has been realized recently that sufficient attention must be paid to processes of the subduction and exhumation of continental and oceanic crust, and intra-slab fluid flow and chemical changes. HP-UHP metamorphic rocks, including both continental and residual oceanic protolith, have been well recognized to occur in the Western Dabie orogen in the east of China and the North Qaidam orogen in the west of China, which is a wonderful target to investigate exhumation and fluid-rock interaction of subducted slab during the continental collision. By taking advantage of in situ zircon U-Pb age, trace element, Lu-Hf isotope and O isotope composition were carried out. Available results from this study can be summarized as follows:(1) There are two types of HP-eclogites in the Huwan shear zone. U-Pb age of residued magmatic zircon in an eclogite constrain its protolith formation at411±4Ma. The zircon in this sample displays εHf(t) values up to+14.4. The positive εHf(t) value suggests that the protolith was derived from a relatively depleted mantle source, most likely Late Silurian Paleotethyan oceanic crust. Whereas a granitic gneiss and the other eclogite yield protolith U-Pb ages of738±6and700±14Ma, respectively, which are both the Neoproterozoic basement rocks of the Yangtze Block. The zircon in the eclogite has εHf(t) values of-1.0to+7.4and TDMI ages of1294to966Ma, implying prompt reworking of juvenile crust during its protolith formation. Metamorphic zircon in both eclogite samples displays low Th/U ratios, trace element concentrations, relatively flat HREE patterns, weak negative Eu anomalies, and low176Lu/177Hf ratios. All these features suggest that both the oceanic and continental crustal rocks in the Huwan shear zone have experienced Carboniferous eclogite facies metamorphism at ca.310Ma. The HP continental rocks in the Huwan shear zone might have played a key role in the exhumation and preservation of the oceanic rocks through buoyancy-driven uplift.(2) Magmatic zircons in the amphibolite yielded protolith U-Pb age of1806±63Ma. Metamorphic zircons in a quartz-rich leucosome and the amphibolite gave weighted mean206Pb/238U ages of241±1Ma and243±4Ma, respectively. These metamorphic zircons contain mineral inclusions of garnet and omphacite, and are characterized by relatively flat HREE patterns with slight negative Eu anomalies. This indicates that they formed under eclogite facies conditions. The zircons in the amphibolite show a negative εHf(t) value of-16.3, suggesting that the protolith was derived from reworking of the ancient continental crust. The U-Pb age of ca.243Ma is consistent with eclogite-facies metamorphic ages of the juvenile oceanic crust in the western Dabie orogen, suggesting that the initiation of continental subduction for high-pressure metamorphism of the Dabie orogen can be traced back to ca.243Ma. The western Dabie HP-UHP slices might suffer from multi-slice or differential subduction and exhumation during the Triassic.(3) The first record of coesite is reported as an inclusion in a metamorphic zircon, which provides unambiguous evidence for the UHP metamorphism of the Xitieshan terrane. Combined with previous results, the HP-UHP metamorphism of the Xitieshan terrane may have lasted460-440Ma with the peak UHP metamorphism at441±9Ma. A compilation of the reported geochronological data reveals that all four terranes of the North Qaidam orogen might have experienced coeval UHP metamorphism during the early Paleozoic(420-450Ma), and thus may have suffered a coherent subduction, UHP metamorphism, and exhumation cycle. U-Pb age of zircon in an serpentinised harzburgite from Dulan terrane is448±9Ma. It consists with the metamorphic zircon U-Pb age of Ky-eclogite (455±5Ma). Both the oceanic-and continental-type rocks in the North Qaidam belt share the same episode HP-UHP metamorphism, suggesting a model of continuous processes from oceanic to continental subduction. Buoyancy of low density serpentinites and continental rocks likely contributed to the exhumation of higher density mafic oceanic rocks.(4) In situ zircon U-Pb age, trace element, and O-Hf isotope were carried out of difference quartz veins from the Xinxian UHP unit of the west Dabie orogen. The LT-UHP rocks of the west Dabie might experiment two episodes of fluid activity during exhumation stage. The zircons of quartz vein hosting by granitic gnesiss and eclogite have206Pb/238U ages for214.5±3.1Ma and213.8±2.4Ma, respectively. It suggests the second episode fluid flow occurred at ca.215Ma, responding to the stage from HP eclogite-facies to amphibolite-facies during exhumation. The zircons have consistent O isotope compositions with the zircons from their hosting rocks, but variable in Hf isotope. The zircon of quartz vein within gneiss display low εHf(t) value of-5.92. Whereas the zircon of quartz vein within eclogite have εHf(t) value of-0.97. The fluid might be aqueous fluid derived from the decomposition of hydrous minerals of hosting rocks. The pervasive fluid flow results in HP-eclogite-to amphibolite-facies retrogression.(5) Integrated study of in situ U-Pb age, trace element, and O-Hf isotope for zircons from a suite of metamorphic rocks including a quartz vein, a host eclogite, and a surrounding granitic gneiss in the Xitieshan UHP terrane of the North Qaidam orogen decipher: Oscillatory zoning zircons from the quartz vein show euhedral shape, relatively high contents of U, Li, Th, Nb, Ta and REEs, with a formation age of442±6Ma consistent with the timing of UHP metamorphism. This demonstrates that the zircons grew very probably from a channelized supercritical fluid close to the peak of UHP metamorphism. In particular, the oscillatory zoning zircons have similar Lu-Hf isotope compositions to zircons from the surrounding gneiss, but variable O isotope compositions between the host eclogite and the surrounding gneiss. Therefore, it is implied that the fluid from the surrounded gneisses was a supercritical fluid with high transport ability for Lu and Hf, while the fluid from the host eclogites was an aqueous fluid without significantly transport such elements. In contrast, both of them have a great ability to carry O. Both eclogite and gneiss from Xitieshan terrane suffered pervasive partial melting during exhumation to the HP granulite-facies regime. Most hydrous melt derived from country gneiss at this stage. The hydours melt can escape the surrounding granitic gneiss for a long distance.(6) Oceanic-and continental rocks (or serpentinised peridotite) can share coupled subduction-exhumation cycle. It suggests that the low density continental rocks and serpentinised peridotites might have played a key role in the exhumation and preservation of the higher density oceanic rocks through buoyancy-driven uplift. The fluids in the LT/UHP terrane mainly derived from multistage aqueous fluids pulse during deeply subductionof continental crust. Occurrence of aqueous fluids can enhance retrograde metamorphism of UHP rocks. Supercritical fluids and hydrous melts can be found in the MT/UHP terrane. The supercritical fluid formed at the initial exhumation stage of UHP rocks and hydours melt formed at HP granulite-facies regime. The supercritical fluid that incorporates these two types of fluid is fed to the overlying mantle wedge from the subducting slab. This process not only transport elements and water from the subducting slab to the mantle wedge, but also triggers the melting of the mantle wedge, which leads to the generation of island arc magmatism with the typical geochemical signatures.更多还原