【作者】 黄建；

【导师】 肖益林； Gerhand Worner；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2013， 博士

【摘要】 本博士论文第一个研究课题是通过对比研究大别山碧溪岭岩体远离变质脉的榴辉岩(EAV)、靠近变质脉的榴辉岩(ECV)和高压变质脉中金红石的Nb-Ta特征,以期探讨大陆俯冲带水/岩相互作用过程中Nb-Ta的活动性及可能的分异机制和尺度。直接比较EAV榴辉岩和没有经历变质的火成岩原岩—-扬子新元古代辉长岩的地球化学特征,能定性的评估俯冲带水-岩相互作用较弱时,Nb-Ta的地球化学行为。EAV榴辉岩的Nb和Ta含量分别为0.42～3.97ppm和0.03-0.24ppm, Nb/Ta比值为14.0～19.2,落在扬子辉长岩的Nb/Ta比值范围之内(12～21)。其次,两者具有相似的平均Nb/Ta比值,分别为16.9±0.8(2s.e.n=12)和16.2±0.6,(2s.e.n=78)。再次,在log[Nb]对log[Ta]图解上,获得了一条斜率为0.965±0.038直线。这说明在地幔部分熔融、岩浆分异、三叠纪深俯冲和榴辉岩化的整个过程中,Nb-Ta几乎没有发生分异。在EAV榴辉岩中,金红石的Nb和Ta含量分别为19.2～179ppm和1.12～11.9ppm, Nb/Ta比值为12.7～25.3,说明在矿物尺度上Nb-Ta发生了分异。金红石平均的Nb/Ta比值与寄主榴辉岩全岩的Nb/Ta比值相似,说明副矿物金红石控制着全岩的Nb-Ta含量。ECV榴辉岩中的变质脉尺度较小,仅5～18cm宽,25～42cm长；电子探针分析显示它们中的绿帘石具有相似的主量元素组成；LA-ICPMS单颗粒金红石边-核-边剖面分析显示它们中的金红石具有互补的Nb/Ta比值变化环带。这些特征说明形成变质脉的流体是由围岩脱水产生。ECV金红石的Nb和Ta含量分别为99.5～503ppm和2.04～14.4ppm, Nb/Ta比值为17.8～49.8。类似地,脉金红石的Nb/Ta比值变化较大,为10.9～29.2,Nb和Ta含量分别为155～2021ppm和9.02～151ppm。总体来说,脉金红石Nb/Ta比值低于对应的ECV金红石Nb/Ta比值,说明区域性流体活动和强烈的水/岩相互作用能导致显著的Nb-Ta迁移和分异。脉金红石具有显著的核部低边部高的Nb/Ta比值环带,说明它们是在不同的变质阶段多次结晶形成以及成脉流体的Nb/Ta比值由早期的低值演化到晚期的高值。在金红石出现之前,角闪岩早期脱水形成低Nb/Ta比值的流体,这种流体被储存在岩石的裂隙或空隙内,与岩石一起随着俯冲的进行被带到深部。当变质压力高于玄武岩-H20体系的第二临界点压力时,富水流体转化为具有较高Nb/Ta比值的超临界流体。因此,脉金红石核部从早期的富水流体中结晶形成,具有较低的Nb/Ta比值,而边部从晚期的超临界流体中结晶形成,具有较高的Nb/Ta比值。以扬子辉长岩为起始物质,对俯冲脱水和水/岩相互作用过程中Nb-Ta的重新分配进行了模拟,获得的结果与自然观察基本相符。本博士论文第二个研究课题是利用钻机取样的优势,在南大别低温/超高压变质带,沿着垂直于不同岩性接触边界的方向,钻取花岗片麻岩和角闪岩(榴辉岩退变质产物)样品,从全岩主量和微量元素、Sr-Nd-O同位素以及单矿物主量元素出发,结合岩相学证据,探讨大陆深俯冲过程中元素迁移和超临界流体的关系。氧同位素结果显示片麻岩和角闪石的δ18O值分别为+0.59～+1.29‰和-1.11～-2.21‰(SMOW),且逐渐靠近接触带,片麻岩的δ18O值略有降低,而角闪岩的818O值逐渐升高,表明变质流体更易在不同岩性接触带汇聚,不同岩性接触带是变质流体的通道。地球化学分析显示：与其它角闪岩相比,直接靠近岩性接触带的角闪岩具有更高的大离子亲石元素(LILE:K,Rb,Sr,Ba,Pb,Th和U)、高场强元素(HFSE:Zr,Hf,Nb,Ta和Ti)和稀土元素(REE)含量,但是具有类似的SiO2、Cr和Ni含量。富水流体和角闪岩相退变质作用不会导致榴辉岩全岩主量、微量元素的显著变化；而含水熔体的交代会导致SiO2含量明显增加,不符合观察到的事实。岩相学证据表明在南大别变质岩超高压变质阶段存在超临界流体。角闪岩中石榴石内存在不同类型的多相固体包裹体,矿物组合为钾长石+石英+方解石+锆石±角闪石±斜黝帘石±石榴石±磷灰石。它们最可能是在峰期变质条件下由先前捕获的超临界流体结晶形成。理论上,超临界流体只可能在南大别低温/超高压花岗片麻岩中形成,而不易在榴辉岩中出现。这种流体具有极强溶解能力,致使片麻岩中富集HFSE和HREE元素的副矿物(e.g.,金红石、石榴石和褐帘石)分解。在峰期变质条件下,这种富含LILE、HFSE和REE的超临界流体汇聚在不同岩性接触带并交代紧邻的榴辉岩,导致岩性接触带边界角闪岩样品主微量元素含量发生显著的变化。质量平衡计算显示,在南大别片麻岩中出现的超临界流体中,微量元素的活动顺序如下：Ba> K (Li, Cs, Rb, Pb)> U> Th> REE> Nb (Ta)> Zr (Hf)。更多还原

【Abstract】 Niobium (Nb) and tantalum (Ta)are classical high field strength elements with identical valence (+5) and effective radius (0.640A). Whether the mobility and fractionation of Nb and Ta occur significantly during subduction-zone metamorphism is a controversialscientific subject. The first research subject in this doctoral dissertation is to determine Nb and Ta concentrations in whole rocks and in rutile from HP veins, ecolgites close to (ECV) and away from veins (EAV) collected at Bixiling in the mid-T/UHP Central Dabie orogenic belt. Based on these data, we discuss the mechamism and effective distance of Nb-Ta mobility and fractionation during fluid/rock interaction in continental subduction-zone metamorphism.A direct comparison of the EAV eclogites with unmetamorphosed protolithic rocks-Neoproterozoic gabbro from the Yangtze Block, would qualitatively evaluate Nb-Ta mobility and fractionation during subduction without intensive fluid-rock interaction. Nband Ta concentrationsin the EAVsrange from0.42to3.97ppm and0.03to0.24ppm, respectively. The EAVs display bulk Nb/Ta ratios ranging from14.0to19.2, which fall in the range defined by the gabbros from the Yangtze Block. Their average Nb/Ta ratio of16.9±0.8(2s.e. n=12) is very similar to that of the gabbros (16.2±0.6,2s.e. n=78). These observations indicate transformation of gabbro to UHP eclogite did not lead to the fractionation of Nb-Ta. More importantly, in the log[Nb] vs. log[Ta] diagram, the slope of0.965±0.038for Nb and Ta is very close to1and suggests negligible Nb/Ta fractionation during partial melting, magma differentiation, Triassic subduction, and subsequent eclogitisation.Rutile from the EAVs has Nb/Ta ratios ranging from12.7to25.3, with Nb and Ta concentrations ranging from19.2tol79ppm and1.12to11.9ppm,respectively.It is noteworthy that Nb/Ta ratios show much larger variations among different individual rutile grains (12.7to25.3) than that in the bulk host EAVs (14.0to19.2), indicating that rutile can fractionate Nb from Ta at mineral-scale. Nevertheless, the average Nb/Ta ofrutile grains is similar to that in the corresponding bulk host EAVs, implying that rutiledominates the Nb-Ta budget of eclogite.Epidote from the vein and ECV eclogite has similar compositions, the ECV-and vein-hosted rutile shows nearly complementary Nb/Ta zoning patterns and the veins have small sizes, ranging from5to18cm wide and25to42cm long, indicating that the investigated rutile-bearing veins originated from an internal fluid source. Rutile from the ECVs shows a much largerspread inbothNb/Ta ratios(17.8-49.8)and Nb-Ta concentrations (Nb:99.5～503ppm,Ta:2.04～14.4ppm). Similarly, the vein rutile has highly variable Nb/Ta of10.9-29.2, with Nb and Ta concentrations from155to2021ppm and9.02to151ppm, respectively. In general, compared to the ECV-hosted rutile, the vein-hosted rutile has lower Nb/Ta ratios, strongly suggesting that Nb-Ta mobility and fractionation are associated with intensive fluid-rock interaction during metamorphism but only occurred locally.The vein-hosted rutile shows lower Nb/Ta ratios in the cores than in the rims,indicating that it did not form in a single stage precipitation and probably experienced a multi-stage evolution and that the Nb/Ta of the vein-forming fluids evolved from low values at the early stage of subduction to higher values at later supercritical fluids conditions with increased temperature and pressure. At the early stage of subduction, where rutile is absent, aqueous fluids released by early-stage dehydration of amphibolite should have low Nb/Ta ratios. The aqueous fluids were probably transported into and stored in fractures and/or pore spaces. Consequently, the stored fluids will at least partly be delivered to greater depths along with ongoing subduction.When the prsessure increased above the second critical point in the basalt-H2O system, the aqueous fluids could transform into supercritical fluids. Such fluids favour Nb over Ta and thus have relatively high Nb/Ta ratios. The vein rutile cores crystallized from early-stage aqueous fluids hence should have low Nb/Ta ratios, and high Nb/Ta ratios in the rims can be attributed to precipitating from supercritical fluids. Quantitative modelling was conducted to constrain the compositional evolution of metamorphic fluids during dehydration and fluid-rock interaction focusing on Nb-Ta distribution. The modelling results based on our proposed multistage fluid phase evolution path can essentially reproduce the natural observations reported in the present study.This doctoral research has also investigated an in-situ profile drilled through the UHP granitic gneiss and eclogite (retrogressed to amphibolite) contact at Zhujiachong in the low-T/UHP South Dabie orogenic belt. A combined investigation of petrology, mineral chemistry, major and trace elements, and Sr-Nd-0isotopes on them was carried out. The results improved the better understanding of the nature and action of supercritical fluids and the associated element mobility at UHP metamorphic conditions.Oxygen isotopic composition ranges from+0.59to+1.29%o (SMOW) in gneisses and from-1.11to-2.21‰in amphibolite. The gneisses show a slight decrease, while the amphibolites display a progressive increase in δ18O values towards their lithological boundary, suggesting that the contact between different lithologies is the most favorable place for fluid activity. Gechemical analyses show that directly at the contact to the gneiss, the amphibolite has higher concentrations in K, Al, LILE, REE, HFSE, Th and U, but similar SiO2, FeOt and transitional metal element (e.g., Cr and Ni) contents relative to the other amphibolites further away from the boundary.Aqueous fluids and amphibolite-facies retrogression of eclogite are known to have no significant effect on their major and trace elements. Also, Si-rich metasomatism from partially melted gneiss should increase the silica content of the amphibolite at the contact,which is not observed.Multiphase solid minnerals inclusions in garnet from the amphibolite are mainly composed of K-feldspar+quartz+calcite+zircon±hydrous minerals (i.e., amphibole and clinozoisite)±garnet±apatite. They probablycrystallized from early-trapped supercritical fluids under peak metamorphic conditions.Theoretically, supercritical fluids were likely to be present in granitic gneiss, but unlikely in eclogite from the low-T/UHP South Dabie orogenic belt. Such fluids have a very high capacity to dissolve HFSE-and HREE-enriched accessory minerals (e.g., rutile, garnet, and allanite) in granitic gneiss under peak metamorphic conditions. Metasomatism ofsuch fluids enriched in LILE, REE, HFSE, Th and U could lead to the observed geochemical variations at the contact. Mass balance calculation shows that element mobility in the inferred supercritical fluids in the South Dabie granitic gneiss decreases in the following order:Ba> K (Li, Cs, Rb, Pb)> U> Th> REE> Nb (Ta)> Zr (Hf).更多还原