冀西北石榴基性麻粒岩中辉石的演化及其地质意义

冀西北石榴基性麻粒岩中的辉石可分为三个世代。第一世代的单斜和斜方辉石包裹于石榴石变斑晶中, 它们形成的温压条件为 T=750～830℃, P=1.0～1.26GPa。第二世代的单斜和斜方辉石分布于基质中, 它们和斜长石常构成120°交角的稳定共生结构, 形成条件 T=780～860℃, P=0.83～0.92GPa。第三世代的辉石产于石榴石的冠状反应边内, 形成条件 T=720～750℃, P=0.554～0.679GPa。从第一世代单斜辉石到第三世代单斜辉石, 它们的Al     

中国东部上地幔岩石组构与稀土配分之间的关系

本文通过对中国东部新生代玄武岩中二辉橄榄岩包体的研究认为,上地幔岩石变形结构和组构类型在不同大地构造单元中的分布是不同的,据此,可划分为华北—东北上地幔弱变形域和东南沿海上地幔强变形域。与变形特征对应的东南沿海地区包体稀土元素配分型式为LREE富集型,华北—东北地区包体稀土元素配分型式则为平坦型和轻微LREE富集型,这表明上地幔流变剪切作用强度与稀土元素富集作用呈正相关,同时反映出上地幔流变状态的差异。根据包体变形特征,我们提出华北—东北地区与东南沿海地区中新生代具有截然不同的大地构造演化特征。     

Effect of Fe on garnet–melt trace element partitioning: experiments in FCMAS and quantification of crystal-chemical controls in natural systems

Garnet–melt trace element partitioning experiments were performed in the system FeO–CaO–MgO–Al₂O₃–SiO₂ (FCMAS) at 3 GPa and 1540°C, aimed specifically at studying the effect of garnet Fe²⁺ content on partition coefficients (D^Grt/Melt). D^Grt/Melt, measured by SIMS, for trivalent elements entering the garnet X-site show a small but significant dependence on garnet almandine content. This dependence is rationalised using the lattice strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454], which describes partitioning of an element i with radius r_i and valency Z in terms of three parameters: the effective radius of the site r₀(Z), the strain-free partition coefficient D₀(Z) for a cation with radius r₀(Z), and the apparent compressibility of the garnet X-site given by its Young's modulus E_X(Z). Combination of these results with data in Fe-free systems [Van Westrenen, W., Blundy, J.D., Wood, B.J., 1999. Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt. Am. Mineral. 84, 838–847] and crystal structure data for spessartine, andradite, and uvarovite, leads to the following equations for r₀(3+) and E_X(3+) as a function of garnet composition (X) and pressure (P):

r₀(3+) [Å]=0.930X_Py+0.993X_Gr+0.916X_Alm+0.946X_Spes+1.05(X_And+X_Uv)−0.005(P [GPa]−3.0)(±0.005 Å)

E_X(3+) [GPa]=3.5×10¹²(1.38+r₀(3+) [Å])^−26.7(±30 GPa)

Accuracy of these equations is shown by application to the existing garnet–melt partitioning database, covering a wide range of P and T conditions (1.8 GPa<P<5.0 GPa; 975°C<T<1640°C). D^Grt/Melt for all 3+ elements entering the X-site (REE, Sc and Y) are predicted to within 10–40% at given P, T, and X, when D^Grt/Melt for just one of these elements is known. In the absence of such knowledge, relative element fractionation (e.g. D_Sm^Grt/Melt/D_Nd^Grt/Melt) can be predicted. As an example, we predict that during partial melting of garnet peridotite, group A eclogite, and garnet pyroxenite, r₀(3+) for garnets ranges from 0.939±0.005 to 0.953±0.009 Å. These values are consistently smaller than the ionic radius of the heaviest REE, Lu. The above equations quantify the crystal-chemical controls on garnet–melt partitioning for the REE, Y and Sc. As such, they represent a major advance en route to predicting D^Grt/Melt for these elements as a function of P, T and X.     

The influence of Cr on the garnet–spinel transition in the Earth''s mantle: experiments in the system MgO–Cr2O3–SiO2 and thermodynamic modelling

The position of the transition from spinel peridotite to garnet peridotite in a simplified chemical composition has been determined experimentally at high pressures and high temperatures. The univariant reaction MgCr₂O₄+2Mg₂Si₂O₆=Mg₃Cr₂Si₃O₁₂+Mg₂SiO₄, has a negative slope in P–T space between 1200 °C and 1600 °C. The experimental results, combined with assessed thermodynamic data for MgCr₂O₄, MgSiO₃ and Mg₂SiO₄ give the entropy and enthalpy of formation of knorringite garnet (Mg₃Cr₂Si₃O₁₂). Thermodynamic calculations in simplified chemical compositions indicate that Cr shifts the garnet-in reaction to much higher pressures than previously anticipated. Moreover, in Cr-bearing systems a pressure–temperature field exists where garnet and spinel coexist. The width of this divariant field strongly depends on the Cr/(Cr+Al) of the system.     

平南幔源包体中橄榄石的显微构造研究及其意义

平南玄武岩中的尖晶石二辉橄榄岩包体的平衡温度为930~980℃, 平衡深度为59~74km, 包体中橄榄石的扭折带滑移系多为(010) [100], 但也有(001) [100]的滑移系类型; 斜方辉石的滑移系为(100) [001], 它们均为高温低应变速率下的滑移系, 说明该区的上地幔主要是在高温低应变速率条件下经历了塑性变形作用.橄榄石位错组态多样, 有自由位错、位错壁、位错弓弯、缠结、{110}滑移带, 反映了上地幔的塑性变形特征.根据位错壁的大小估算, 上地幔差异流动应力为24.5~42.1MPa, 流动速率为2.93×10-17~8.36×10-16s-1, 有效粘度为1.72×1023~2.80×1024 Pa·s, 特征与中国东部新生代上地幔较为一致, 反映同处于拉张环境.      

喜马拉雅造山带新生代花岗岩中两类石榴石的地球化学特征及其在地壳深熔作用中的意义

在喜马拉雅碰撞造山带中,石榴石是变泥质岩的主要造岩矿物,也是花岗岩或淡色体的重要副矿物,保存了有关地壳深熔作用的关键信息,是揭示大型碰撞造山带中-下地壳物质的物理和化学行为的重要载体。在喜马拉雅造山带内,新生代花岗质岩石(淡色花岗岩和混合岩中的淡色体)含两类石榴石,大多数为岩浆型石榴石,自形-半自形,不含包裹体,但淡色体中含有港湾状的混合型石榴石。岩浆型石榴石具有以下地球化学特征:(1)从核部到边部,显示了典型的"振荡型"生长环带;(2)富集HREE,亏损LREE,从核部到边部,Hf、Y和HREE含量降低;(3)显著的Eu负异常;(4)相对于源岩中变质石榴石,Mn和Zn的含量显著增高。岩相学和地球化学特征都表明:变泥质岩熔融形成的熔体(淡色体)捕获了源岩的变质石榴石,熔体与石榴石反应导致大部分元素的特征被改变,只在核部保留了源岩的部分信息。同时,在花岗质熔体结晶过程中,形成少量的岩浆型石榴石。这些石榴石摄取了熔体中大量的Zn,浓度显著升高,在斜长石和锆石同步分离结晶作用的共同影响下,石榴石中Eu为明显负异常,Hf、Y和HREE浓度从核部到边部逐渐降低。上述数据和结果表明,花岗岩中石榴石的矿物化学特征记录了精细的有关花岗岩岩浆演化的重要信息。     

胶北粉子山群石榴云母片岩的三叠纪独居石年龄及其地质意义

胶北地块粉子山群石榴云母片岩中石榴石变斑晶内包裹物迹线明显,保留了岩石形成过程中的多期变质变形信息。电子探针成分面扫描图显示石榴石成分环带明显,可分为核部、幔部和边部。石榴石中MgO、FeO、MnO和CaO含量变化特征表明其核部到边部温度先升高后降低,对应进变质及退变质过程。根据原位独居石Y元素成分面扫描图显示,部分独居石颗粒由核部到边部Y含量呈现逐渐降低趋势,说明测得的232.6±1.1Ma~229.5±3.7Ma的独居石U-Pb年龄,对应石榴石的进变质生长过程。结合1869±72Ma的锆石U-Pb年龄数据,可推断粉子山群石榴云母片岩至少经历了古元古代及三叠纪两期变质事件的改造。粉子山群石榴云母片岩卷入了苏鲁超高压变质带的俯冲碰撞造山事件。电子探针成分分析结果表明粉子山群石榴云母片岩中的石榴石属于铁铝榴石,反映出经受中级区域变质作用的特征。说明粉子山群石榴云母片岩虽然参与了三叠纪苏鲁超高压变质带的俯冲碰撞造山过程,但俯冲深度较浅。这可用大陆俯冲过程中上盘的俯冲剥蚀来解释,并可为陆-陆碰撞俯冲剥蚀模式提出的扬子板片在240~220Ma的深俯冲作用过程中拽动胶北地块向下俯冲又折返的运动过程提供佐证,但胶北地块是否经历了深俯冲超高压变质作用,还需要进一步验证。     

中国东北地区上地幔组成、结构及热状态

本文通过对中国东北地区新生代玄武岩中超镁铁质包体的研究,根据地质压力计所估算的包体形成深度、玄武岩浆及辉石岩浆起源的深度等,建立了中国东北地区上地幔组成模式:1.壳幔过渡带;2.尖晶石二辉橄榄岩层;3.含有痕量硅酸盐熔体的低速层;4.尖晶石方辉橄榄岩层;5.石榴石二辉橄榄岩层。与此同时,根据地质温度计与地质压力计推算资料,作者认为中国东北地区上地幔属大洋地温。与典型克拉通地温相比,低速层位置较浅,上地幔处于过热状态,这与大陆裂谷的发育有关。     

Pressure dependence of color of natural uvarovite: the barochromic effect

The electronic absorption spectra of natural uvarovite containing 62 mole% of the Cr³⁺ end-member were studied at pressures between 10⁻⁴ and ca. 13 GPa using DAC techniques combined with microscope spectrometric device. With increasing pressure, a barochromic effect with change from green to red color of the garnet specimen was observed. This change could be interpreted on the basis of the spectra and the data points derived in an ICE color card. The evaluation of crystal field data from the spectra showed that 10Dq of chromium increases on pressure while the Racah parameter B, and thus the nature of the chemical bond of Cr–O does not change significantly.     

Garnet growth at high- and ultra-high pressure conditions and the effect of element fractionation on mineral modes and composition

In this paper we show that thermodynamic forward modelling, using Gibbs energy minimisation with consideration of element fractionation into refractory phases and/or liberated fluids, is able to extract information about the complex physical and chemical evolution of a deeply subducted rock volume. By comparing complex compositional growth zonations in garnets from high-and ultra-high pressure samples with those derived from thermodynamic forward modelling, we yield an insight into the effects of element fractionation on composition and modes of the co-genetic metamorphic phase assemblage. Our results demonstrate that fractionation effects cause discontinuous growth and re-crystallisation of metamorphic minerals in high pressure rocks. Reduced or hindered mineral growth at UHP conditions can control the inclusion and preservation of minerals indicative for UHP metamorphism, such as coesite, thus masking peak pressure conditions reached in subducted rocks.Further, our results demonstrate that fractional garnet crystallisation leads to strong compositional gradients and step-like zonation patterns in garnet, a feature often observed in high-and ultra-high pressure rocks. Thermodynamic forward modelling allows the interpretation of commonly observed garnet growth zonation patterns in terms of garnet forming reactions and the relative timing of garnet growth with respect to the rock's pressure–temperature path. Such a correlation is essential for the determination of tectonic and metamorphic rates in subduction zones as well as for the understanding of trace element signatures in subduction related rocks. It therefore should be commonplace in the investigation of metamorphic processes in subduction zones.