Fluid-Rock Interactions during High-Pressure and Ultrahigh-Pressure Metamorphism

Fluid inclusions and stable-isotope data of HP and UHP terrains from China and the European Alps are used to evaluate fluid-rock interactions during subduction and continental collision. The choice of the European Alps and China for a comparative study is compelling, because the rocks of the Alps exemplify subduction of a cold (mature) oceanic lithosphere, whereas those of China illustrate the responses of continental crust to subduction under relatively higher temperatures representative of the base of a thickened continental wedge. Our study demonstrates that there is no difference between the fluid-rock behavior of continental and oceanic crust during subduction and metamorphism. In both regions, there is evidence for the presence of a grain-boundary fluid phase during HP metamorphism. Recognition of grain- to hand specimen-scale chemical and isotopic heterogeneity attests to local buffering by mineral assemblages of fluid compositions. Assuming a one-dimensional fluid flow perpendicular to rock-layer contacts, we estimated fluid fluxes of 15 to 465 cm³/cm² to as low as 1.6 cm³/cm² in China and Alpine rocks, respectively. The values from China are to be considered as maxima because, in most cases, sampling was at a reconnaissance scale and does not permit evaluation of disequilibrium on a scale of one centimeter.

An important finding, characteristic of the entire range of conditions represented by these rocks, is the retention of recognizable vestiges of pre-subduction fluid-rock interactions, be they a geothermal system with a surface-exposure length scale on the order of 100 kilometers in continental rocks of China, or relics of high- and low-temperature hydrothermal alteration features in oceanic material of the Western Alps. A key issue arising from these results concerns the plausibility of transferring a slab component to the source of arc-magma genesis in a low fluid/rock regime. Possible scenarios are discussed in light of several recent innovative experiments that have dispelled some of the speculation obscuring our understanding of fluid-melt-rock interactions during HP and UHP metamorphism.     

Geospeedometry and Time Scales of High-Pressure Metamorphism

Kinetic theory allows the calculation of a time scale for metamorphic events using the extent of relaxation of garnet growth zoning along a particular P-T trajectory. Eclogitic garnets from the Kokchetav Massif (Kazakhstan), the Great Caucasus (Russia), and the Yukon-Tanana terrane (Canada) experienced different metamorphic P-T histories and display different types of zoning patterns, which allowed testing of a variety of geospeedometric procedures. In all cases, the preservation of sharp compositional gradients and hence the limited degree of diffusive modification of garnet compositions can be explained if associated tectono-metamorphic processes were of very short duration. Results of diffusion modeling indicate rates of temperature and pressure change on the burial and/or the exhumation path on the order of several hundreds of °C/m.y. and several cm/yr, respectively. These extreme exhumation and cooling rates apply for rocks buried to a depth greater than, for example, 20 km, thus arguing for the existence of contrasted velocity fields for eclogitic block exhumation from deep versus shallow levels of the lithosphere.     

Pink manganian phengite in a high P/T meta-conglomerate from northern Syros (Cyclades, Greece)

A new occurrence of Mn-rich rocks was discovered within the high-pressure/low-temperature metamorphic rocks on the Palos peninsula of Syros (Greece). Near the summit of Mount Príonas, a meta-conglomerate consists of calcite (~63 wt%), pink manganian phengite, blue–purple manganian aegirine–jadeite, microcline, albite and quartz. In addition, it contains abundant braunite-rich aggregates (up to ~1.5 cm in diameter) that include hollandite [(Ba_0.98–1.02K_<0.01Na_<0.02Ca_<0.03) (Mn _1.02–1.52 ³⁺ Fe _0.38–0.88 ³⁺ Ti_0.29–0.92Mn _5.11–5.76 ⁴⁺ )O₁₆], barite and manganian hematite. Due to metamorphic recrystallization and deformation, the contacts between clasts and matrix are blurred and most clasts have lost their identity. In back-scattered electron images, many aegirine–jadeite grains appear patchy and show variable jadeite contents (Jd_10–67). These pyroxenes occur in contact with either quartz or albite. Manganian phengite (3.41–3.49 Si per 11 oxygen anions) is of the 3T type and contains 1.4–2.2 wt% of Mn₂O₃. At the known P–T conditions of high-pressure metamorphism on Syros (~1.4 GPa/ 470 °C), the mineral sub-assemblage braunite + quartz + calcite (former aragonite) suggests high oxygen fugacities relative to the HM buffer (+7 ≤ ?f_O2 ≤ + 17) and relatively high CO₂ fugacities. The exact origin of the conglomerate is not known, but it is assumed that the Fe–Mn-rich and the calcite-rich particles originated from different sources. Braunite has rather low contents of Cu (~0.19 wt%) and the concentrations of Co, Ni and Zn are less than 0.09 wt%. Hollandite shows even lower concentrations of these elements. Furthermore, the bulk-rock compositions of two samples are characterized by low contents of Cu, Co and Ni, suggesting a hydrothermal origin of the manganese ore. Most likely, these Fe–Mn–Si oxyhydroxide deposits consisted of ferrihydrite, todorokite, birnessite, amorphous silica (opal-A) and nontronite. Al/(Al + Fe + Mn) ratios of 0.355 and 0.600 suggest the presence of an aluminosilicate detrital component.     

3D geometrical modelling of post-foliation deformations in metamorphic terrains (Syros,Cyclades, Greece)

Superposed to ductile syn-metamorphic deformations, post-foliation deformations affect metamorphic units during their exhumation. Understanding the role of such deformations in the structuration of metamorphic units is key for understanding the tectonic evolution of convergence zones. We characterize post-foliations deformations using 3D modelling which is a first-order tool to describe complex geological structures, but a challenging task where based only on surface data. We propose a modelling procedure that combines fast draft models (interpolation of orientation data), with more complex ones where the structural context is better understood (implicit modelling), allowing us to build a 3D geometrical model of Syros Island blueschists (Cyclades), based on field data. With our approach, the 3D model is able to capture the complex present-day geometry of the island, mainly controlled by the superposition of three types of post-metamorphic deformations affecting the original metamorphic pile: i) a top-to-South ramp-flat extensional system that dominates the overall island structure, ii) large-scale folding of the metamorphic units associated with ramp-flat extensional system, and iii) steeply-dipping normal faults trending dominantly NNW-SSE and EW. The 3D surfaces produced by this method match outcrop data, are geologically consistent, and provide reasonable estimates of geological structures in poorly constrained areas.     

High-Pressure Metamorphism in the SW Tauern Window, Austria: P-T Paths from Hornblende-Kyanite-Staurolite Schists

The hornblende garbenschist horizon of the Lower Schieferhulleseries (LSH) in the SW Tauern Window, Austria, contains theassemblage hornblende + kyanite + staurolite + garnet + biotite+ epidote + plagioclase + ankerite + quartz + rutile + ilmenite,with either chlorite or paragonite present in all samples. Theseassemblages are divariant in the system SiO₂-Al₂O₃-TiO₂-Fe₂O₃-MgO-FeO-MnO-CaO-Na₂O-K₂O-H₂O-CO₂.Garnet-biotite geothermometry yields temperatures of final equilibrationof {small tilde}550 °C, and garnet-plagioclase-kyanite-quartzgeobarometry indicates pressures of 6–8 kb for the matrixassemblage and 9–10 kb for plagioclase inclusions in garnet.Quantitative modelling of zoned garnet, hornblende, and plagioclaseindicates growth and equilibration along a decompression pathfrom {small tilde}530 °C, 10 kb to {small tilde}550 °C,7 kb. Fluid inclusion data constrain the uplift path to havepassed through a point at {small tilde} 375 °C, 1.5 kb. These data permit the construction of a relatively completeP-T loop for metamorphism associated with the Alpine orogeniccycle in the LSH of the SW Tauern Window. The maximum pressureconditions ({small tilde}10 kb at 530 °C) recorded alongthis loop are considerably higher than previous estimates of5–7 kb for the region. Simple overthrust models developedfor the Tauern Window cannot account for pressures of this magnitude;a more likely scenario involves partial subduction of the rocksto a depth of {small tilde}35 km, followed by prolonged heatingin response to decay of the subduction isotherms. Initial upliftappears to have been rapid and occurred along a nearly isothermalpath. Significant cooling did not occur until the rocks werewithin {small tilde}5 km of the surface. Detailed tectonic modelsfor the evolution of the Tauern Window must be able to accountfor the quantitative features of the P-T loop.     

胶东莱西地区高压基性麻粒岩的变质作用

胶东地区莱西－莱阳－栖霞－带晚太古代花岗片麻碉中,出露相当规模的高压基性麻业岩和超镁铁质岩组合,曾被认为它们是与苏鲁－大别超高压碰撞造山作用有关的一套岩石组合,高在性麻粒岩中可同三期变质矿物组合;早期为Ｇｒｔ核部）＋Ｃｐｘ＋Ｐｌ;中期为Ｇｒｔ（边中）＋Ｃｐｘ＋Ｏｐｘ＋Ｐｌ＋Ａｍｐ＋Ｑ＋Ｍｔ;晚期为Ｃｐｘ＋Ｐｌ＋Ａｍｐ＋Ｑ＋Ｍｔ。早期的石榴石含钱铝榴石和镁铝榴石组分较高,单斜辉石含Ａｌ高。应用矿物温     

大别山北部榴辉岩和英云闪长质片麻岩锆石U-Pb年龄及多期变质增生

大别山北部榴辉岩及英云闪长质片麻岩的锆石U-Pb年龄分析表明：北部榴辉岩相峰期变质时代为226～230Ma左右;北部塔儿河一带英云闪长质片麻岩经历过印支期变质事件;大别山北部与南部超高压岩石中一致的（226～230Ma）高压或超高压变质年龄表明,北部镁铁-超镁铁质岩带中部分岩石也曾作为扬子俯冲陆壳的一部分,在印支期发生过高压或超高压变质作用;本区锆石发生过两期变质增生事件,一是印支期高压或超高压变质,另一期是燕山期热变质事件;榴辉岩及英云闪长质片麻岩的原岩形成时代为晚元古代;锆石U-Pb年龄可用多期变质增生模型来解释。     

南秦岭佛坪地区早中生代变质-深熔-变形作用的锆石U-Pb年代学制约

本文通过几何学与运动学等构造变形分析, 勾勒出南秦岭佛坪地区龙草坪混合岩化片麻岩穹隆、佛坪县城片麻岩穹隆以及秧田坝—十里铺走滑剪切带的基本形态。同时, 对该区混合岩、麻粒岩以及变形花岗岩进行了系统的岩石学与年代学研究。结果表明, 混合岩的锆石具有明显的核-边结构, 其中中色体年龄集中在(211.8±1.9) Ma, 浅色体中锆石边部年龄为(203.7±1.6) Ma, 而继承核部则显示了与本地区泥盆纪变质沉积岩中碎屑锆石一致的年龄谱系。麻粒岩的锆石也具有明显的核-边结构, 核与边分别得到(201.5±7.0) Ma和(188.1±1.5) Ma的加权平均年龄。伟晶岩和细粒花岗岩样年龄分别为(200.4±2.0) Ma和(201.1±1.5) Ma, 误差范围内一致。穹隆一带的副片麻岩和走滑剪切带南侧变质沉积岩分别得到(198.1±2.2) Ma和(196.1±2.1) Ma的变质年龄。佛坪县城片麻岩穹隆一带的变形花岗岩年龄为(205.5±3.0) Ma。综合以上构造变形与变质-深熔-岩浆作用的研究, 我们构建了佛坪地区早中生代构造演化序列, 并提出片麻岩穹隆侵位过程中得到了作为勉略带东侧部分的秧田坝—十里铺韧性走滑剪切带活动的促进。并认为南秦岭构造带至少在217~212 Ma之前仍处于俯冲环境, 于211~201 Ma进入同碰撞背景, 并于200~190 Ma完成同碰撞向伸展体制的转变。     

Isograd Patterns and Pressure-Temperature Distributions During Regional Metamorphism

In rocks undergoing regional metamorphism, pressure and temperature gradients are not everywhere parallel, that is, isotherms are not everywhere parallel to isobars. Temperature gradients vary across the metamorphic terrain and decrease with increasing temperature. The angle between the temperature and the pressure gradients and the magnitude of the temperature gradient must be considered when isograd patterns are used to determine the distribution of pressure and temperature in the rocks when the isograds formed.In Val Mesolcina, southeastern Switzerland, post-tectonic isograds corresponding to mineral dehydration reactions dip steeply northward intersecting the more gently dipping isograd corresponding to the kyanite-andalusite transition. Distributions of pressure and temperature compatible with the isograd pattern have values of α (the angle between the pressure and the temperature gradients) of 50–70° and temperature gradients of 12–15 °C/km. These results indicate that at or near the peak of the Tertiary (Lepontine) phase of Alpine metamorphism isothermal surfaces in these rocks dipped steeply northward and isobaric surfaces were horizontal.The westward divergence of Lepontine mineral zone boundaries can be explained by decreasing values of α; that is, the flattening toward the west of a steep-sided thermal dome present in the eastern part of the Lepontine terrain at the time of metamorphism.     

Caledonian Low-Temperature Granulite-Facies Metamorphism in the West Kunlun Orogenic Belt-SHRIMP Chronological Evidence from Zircon

1IntroductionTheWestKunlunorogenicbeltbordersontheTarimplateinthenorth ,linkswiththeKarakorum Qiangtangblockinthesouth ,iscutbytheAltunfaultintheeast,andextendswest wardstothePamirPlateau (BureauofGeologyandMineralResources,Xinjiang ,1 999;JiangChunfaetal.,2 0 0 0 ) .OnthebasisofthediscoveryoftheKudaophioliteandaseriesofstudies(DingDaoguietal.,1 996 ;PanYushengetal.,1 994 ,1 999;WangZhihongetal.,2 0 0 0 ;PanYushengetal.,2 0 0 0 ) ,thepreviousgeologicalworkerscommonlyacceptedthatthereh…     

北京周口店官地杂岩变质-深熔作用的锆石 LA-ICP-MS U-Pb定年

应用LA-ICP-MS技术测得北京周口店官地杂岩深熔成因浅色体和其源岩--斜长角闪岩中的锆石U-Pb年龄分别为(2 548+23/-24) Ma和(2 538+34/-35) Ma.这表明,官地杂岩在新太古代末期发生了高级区域变质作用和深熔作用.定年结果为华北陆块的东部地块新太古代末期的克拉通化事件提供了进一步的变质年代学证据.     

华北太古宙高压基性麻粒岩的两类PT轨迹及其构造意义：矿物化学和？…

在华北克拉通桑干构造带中,分布有许多太古宙高压基性麻粒岩,呈大小不等的岩块产于强烈变形ＴＴＧ片麻岩,二辉麻粒岩、混合岩和花岗岩中。高压麻粒岩富含石榴石斑晶,斑晶内部钙铝榴石含量明显高于边缘,一般可达２５－２８％,许多石榴石含有单斜辉石包体,Ａｌ２Ｏ３含量高达７．４－１１．２％,相应的契尔马克分子比例为１２－１８％。这些成分特征指示了早期相对较主的变质能力。石榴石主昌一或全中晶反应边,是Ｐｌ－Ｏｐｘ     

华北太古宙高压基性麻粒岩的两类PT轨迹及其构造意义:矿物化学和变质作用研究

在华北克拉通桑干构造带中,分布有许多太古宙高压基性麻粒岩,呈大小不等的岩块产于强烈变形的TTG片麻岩、二辉麻粒岩、混合岩和花岗岩中.高压麻粒岩富含石榴石斑晶,斑晶内部钙铝榴石含量明显高于边缘,一般可达25%～28%.许多石榴石含有单斜辉石包体,A12O3含量高达7.4%～11.2%,相应的契尔马克分子比例为12%～18%.这些成分特征指示了早期相对较高的变质压力.石榴石斑晶广泛发育后成合晶反应边,是Pl-Opx-Amp-Mt组合的放射状细粒交生体,邻接的单斜辉石和石榴石斑晶边缘与后成合晶组合近于反应平衡,单斜辉石Al2O3含量小于5%,石榴石边缘的钙铝榴石含量也大大低于核部.这些都显示了压力较低条件下石榴石和单斜辉石的分解.温度压力的计算结果揭示出高压麻粒岩的两类PT轨迹,它们早期的变质作用都表现出较高的压力(1.2～1.45GPa).一部分高压麻粒岩具有顺时针的PT轨迹,并显示升温减压和近等温减压过程.它们很可能形成于晚太古代某种型式的俯冲和碰撞构造过程.另一部分高压麻粒岩具有逆时针的PT轨迹,显示降温降压过程,并且早期变质温度高达1050C.它们很可能来自碰撞之前的岛弧下地壳底部.在碰撞阶段的后期,这些不同成因的高压麻粒岩都被卷入了与碰撞构造直接相关的桑干构造带,经历共同的构造抬升过程.桑干构造带可以认为是构造缝合带.     

Fluid Flow During Contact Metamorphism at Mary Kathleen, Queensland, Australia

Corella marbles in the Mary Kathleen Fold Belt were infiltratedby fluids during low-pressure (200-MPa) contact metamorphismassociated with the intrusion of the Burstall granite at 1730–1740Ma. Fluids emanating from the granite [whole-rock (WR) ¹⁸O=8.1–8.6%]produced Fe-rich massive and banded garnet—clinopyroxeneskarns [¹⁸O(WR)=9.1–11.9%]. Outside the skarn zones, marblemineralogies define an increase in temperature (500 to >575C) and XCO₂ (0.05 to >0.12) towards the granite, andmost marbles contain isobarically univariant or invariant assemblagesin the end-member CaO–MgO–Al₂O₃–SiO₂–H₂O–CO₂system. Marbles have calcite (Cc) ¹⁸O and ¹³C values of 12.3–24.6%and –1.0 to –3.9%, respectively. A lack of down-temperaturemineral reactions in the marbles suggests that pervasive fluidinfiltration did not continue after the thermal peak of contactmetamorphism. The timing of fluid flow probably correspondsto a period of high fluid production and high intrinsic permeabilitiesduring prograde contact metamorphism. The petrology and stableisotope geochemistry of the marbles suggest that these rockswere infiltrated by water-rich fluids. If fluid flow occurredup to the peak of contact metamorphism, the mineralogical andisotopic resetting is best explained by fluids flowing up-temperaturetoward the Burstall granite. However, if fluid flow ceased beforthe peak of regional metamorphism, the fluid flow directioncannot be unambiguously determined. At individual outcrops,marble ¹⁸O(Cc) values vary by several permil over a few squaremetres, suggesting that fluid fluxes varied by at least an orderof magnitude on the metre to tens-of-metre scale. Fluids werefocused across lithological layering; however, mesoscopic fracturesare not recognized. The focusing of fluids was possibly viamicrofractures, and the variation in the degree of resettingmay reflect variations in microcrack density and fracture permeability.The marble—skarn contacts represent a sharp discontinuityin both major element geochemistry and ¹⁸O values, suggestingthat, at least locally, little fluid flow occurred across thesecontacts.     

Accuracy of Laser Ablation U-Pb Zircon Dating: Results from a Test Using Five Different Reference Zircons

Using a state‐of‐the‐art 193 nm‐LA‐MC‐ICP‐MS system and with careful control of analytical procedures, the long term external reproducibility and accuracy of the ages Phanerozoic zircons measured over a period of months using calibrator bracketing for the ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb ages were ca. 1% (2 RSD) if a single reference zircon was used for the matrix‐matched calibration. When different reference zircons were used for the calibration, suspicious systematic shifts in the obtained ages were observed and thus a reduction in the overall accuracy of the dating method became obvious. Such shifts were within a few percent range of the U‐Pb and Pb/Pb ages and seemed to vary independently of zircon age and composition. A “test of accuracy” experiment was conducted reducing instrumental effects as far as possible by analysing five different reference zircons mounted on a single mount eight times during the same session. An identical protocol was used for all analyses, with unchanged instrument parameters and with ion beam intensities kept as identical as possible. For data reduction, every zircon served consecutively as the reference zircon for calibration, with the others in the batch treated as unknowns. The known reference age and the four calculated ages obtained using the four other RMs for calibration were then compared. Even using such a strict analytical protocol, shifts in ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U and ²⁰⁷Pb/²⁰⁶Pb ratios were still present. They varied non‐systematically and ranged from ?4.35% to 3.08% for the investigated age range (1065 Ma to 226 Ma). Assuming the absence of instrumental effects (i.e., memory, dead‐time correction, non‐linearity of ion counters and interdetector calibration, crystallographic orientation, ablation cell geometry and setup, gas flows), the observed shifts were attributed to matrix and/or ablation related effects. It is proposed that non‐spectral matrix effects in the Ar plasma torch resulted in non‐uniform signal enhancement (or depression?) leading to shifts both in elemental and Pb isotopic ratios. Additionally, the ablated particle size distribution could be an important factor controlling plasma conditions and thus mass bias and fractionation. Until such effects are well understood and controlled, it would seem that any LA‐ICP‐MS zircon U‐Pb and ²⁰⁷Pb/²⁰⁶Pb age determination cannot be meaningfully interpreted at below a ca. 3% to 4% (2 RSD) confidence level.     

东昆仑高压变质带榴辉岩年代学、地球化学及其地质意义

近年来在东昆仑造山带中发现出露多处榴辉岩,由夏日哈木-苏海图、大格勒、宗加、尕日当（浪木日上游）、温泉、加当等多个榴辉岩、榴闪岩高压变质地体组成,呈透镜体或条带状产于金水口岩群中,构成了一条长达530 km的高压变质带.从榴辉岩的岩石学、地球化学、同位素年代学等方面进行系统梳理,结果表明岩石类型复杂,主要可分为榴辉岩、退变榴辉岩、榴闪岩,岩石地球化学显示东昆仑榴辉岩SiO₂含量为41.58%~59.00%,平均值为50.19%,Al₂O₃含量为11.27%~18.54%,平均值为14.66%,TiO₂含量为0.76%~1.59%,平均值为1.03%.稀土配分曲线主要为轻稀土富集型,微量元素配分主要介于E-MORB与N-MORB之间.获得加当榴闪岩变质年龄为440±13 Ma,原岩年龄为934±15 Ma,同时结合东昆仑地区榴辉岩锆石年龄对其进行分析,锆石单点206Pb/238U年龄在直方图上显示出丰富的信息,变质峰期年龄出现明显3个年龄峰,分别为451 Ma、432 Ma和412 Ma,原岩年龄出现峰值934 Ma,其中515~440 Ma记录了板块俯冲时段的岩浆热事件;440~420 Ma为陆壳俯冲-碰撞的记录;420~390 Ma是榴辉岩在折返过程中退化变质的反映.东昆仑榴辉岩变质时代与东昆仑原特提斯洋构造演化密切相关.      

大别山碧溪岭深色榴辉岩和片麻状花岗质岩石SHRIMP分析：晋宁期？…

本文报道大别山安徽省岳西县碧溪岭深色榴辉岩和片麻状花岗质岩石的锆石SHRIMP分析结果.深色榴辉岩的原岩为基性凝灰岩,锆石在岩石中主要产于石英、石榴子石和绿辉石内,为高压-超高压变质作用的产物,内部主体形成年龄约为757±7 Ma,代表高压-超高压榴辉岩相变质事件的时代;经受了后期流体改造的锆石边部年龄为223±3 Ma.在片麻状花岗质岩石中,发育韵律环带结构的深熔锆石形成于约727±15 Ma,受后期流体改造的锆石边部年龄为219±3 Ma.综合野外地质关系、区域地质背景和锆石的岩相学特征,笔者提出了碧溪岭榴辉岩相岩石高压-超高压变质作用发生于晋宁期,印支期仅代表一次后期流体改造事件的观点.     

云开地块印支期变质-深熔作用:混合岩、片麻岩锆石U-Pb年代学和Hf同位素证据

云开地块中生代构造演化是华南地区的研究热点之一.通过对云开地块变质基底中的混合岩、片麻岩（5个样品）和花岗岩（1个样品）开展锆石LA-ICP-MS U-Pb定年,获得440.3±3.3 Ma、230.2±2.9 Ma、230.7±1.3 Ma、459.5±2.7 Ma、431.5±4.3 Ma、229.2±5.4 Ma、229.7±2.7 Ma 7组变质（深熔）或岩浆年龄和2组（样品1432-1和ID7-3）碎屑锆石年龄,碎屑锆石年龄范围均为太古代-新元古代,且具有~1.0 Ga年龄主峰,与天堂山岩群和云开群碎屑锆石年龄谱相似.区域资料表明云开地块天堂山岩群和云开群具有相似的物质组成,均形成于早古生代-新元古代,存在变质程度和物质面貌的差异;在加里东期构造-热事件的基础上,广泛叠加了印支期区域变质（深熔）-构造-流体作用影响.4个样品中（1431-1、1432-1、D116-3和ID7-3）锆石原位Lu-Hf同位素组成显示,加里东期变质和深熔锆石Lu/Hf同位素组成基本一致,应继承了原岩的同位素组成特征.印支期变质和深熔锆石Lu/Hf同位素组成不同,可能主要由变质作用和深熔作用的差异所致.以古-中元古代为主的地壳物质参与了加里东期和印支期变质-深熔作用,在加里东期和印支期深熔作用过程中,均有少量幔源物质的加入,印支期幔源物质的贡献相对显著.      

High fO2 Metasomatism During Whiteschist Metamorphism, Zambezi Belt, Northern Zimbabwe

The Kadunguri Whiteschists are a group of talc- and kyanite-bearinglithologies that occur in the Chewore Inliers from the ZambeziBelt of northern Zimbabwe. They crop out on the southern marginof the Chewore Ophiolite Terrane, a Mesoproterozoic ophioliteand island arc, as a 5 km x 1·5 km, southeasterly dipping,semi-continuous block, and contain the second known naturaloccurrence of yoderite. Major element analyses define the whiteschistswithin the relatively simple MFASH system. Major and trace elementanalyses indicate that the whiteschists originate from the metasomaticalteration of alkalic ocean-island-type metabasalts similarto those in the underlying Ophiolite Terrane. Synmetamorphicor metasomatic mineral parageneses indicate peak P–T conditionsof between 13 and 15 kbar at 550–600°C, and the highlyoxidizing nature of all reactions indicates the presence ofa high fO₂ metasomatic fluid. The peak P–T conditionsrequire that this synmetamorphic, exotic metasomatic fluid wasavailable at depths near 55 km. The age of high-pressure metamorphismis constrained within the Pan African tectonothermal cycle at550–520 Ma. Tectonometamorphism in the Zambezi Belt isrelated to a period of extensive crustal thickening possiblyrelated to amalgamation of Gondwanaland. KEY WORDS: Congo Craton; high pressure; Kalahari Craton; metasomatism; whiteschist; Zambezi Belt