Evolution of bulk composition, mineralogy, strain style and fluid flow during an HT–LP metamorphic event: sillimanite zone of the Catalan Coastal Ranges Variscan basement, NE Iberia

Metamorphic rocks in the Osor complex (Guilleries massif, NE Iberian Peninsula) show the following structural and compositional features: strong differentiation into quartz-rich gneissic semipelitic and quartz-absent, mica-rich schistose bands, higher density of igneous (both basic and leucogranitic) and quartz veins in the schistose domains and strong strain partitioning in the pelitic bands. Garnet is present in both kinds of lithologies, showing also differential textural and chemical features interpreted to be dependent on bulk composition, deformation and fluid interaction histories. Textures, mineral composition and thermobarometry suggest the operation of concurrent mechanical, mass transfer and thermal phenomena such as: (1) variations in strain style, (2) fluid infiltration, (3) magmatic injection and (4) HT–LP metamorphic and metasomatic episodes. The following sequence of events is suggested: initially the cooling of syntectonic high-T basic quartz diorite sheets promoted high strain rates, low dP/dT thermobaric evolution, incipient anatexis in the pelitic bands and devolatilization through a pervasive to vein-channelized prograde fluid flow. The prograde flow enhanced an ongoing compositional tectono-metamorphic differentiation and produced metasomatism through depletion of the Osor rocks in alkalis and calcium. Later injection and cooling of peraluminous leucogranitoid sheets, preferentially along pelitic bands, increased the ratio of magmatic/metamorphic components in the fluids and strongly enriched them in alkalis producing a second metasomatic episode. During crystallization of quartz and leucogranitoid veins, the pelitic bands were strongly enriched back again in alkalis, promoting the blastesis of big crystals of post-peak muscovite and albite as well as the retrogression of garnet. The metasomatic mica-rich levels must have been the preferred locus for development of a new deformation style dominated by shear band fabrics in metapelites and related to a release of the gravitational instability originated previously due to crustal thickening. The increasing decompressional component of the retrograde P–T path also suggests that this style of deformation was prevalent during, if not responsible for, a phase of exhumation of the metamorphic complex. It is suggested that similar patterns of thermomechanical and mass transfer phenomena could well be a fundamental characteristic common to all HT–LP metamorphic belts.     

Distribution d'un élément trace (Cr) dans un sol développé sur roches métamorphiques : variabilité à l'échelle d'un versantDistribution of chromium in soils developed into metamorphic rocks: variability at the hillslope scale

The influence of pedogenetic factors onto spatial variability of chromium in soil surface horizon was studied in a metamorphic area (Massif Central, France) at the hillside scale. A grid survey was performed. Surface soil horizons were sampled at each point and analysed in Cr. Results show the determinant influence of the rock type on Cr content distribution along the slope. However, surface horizons of these soils exhibit a lower Cr content heterogeneity than the deeper horizons. This was explained by soils' material erosion and deposition along the slope. Tillage and hedges also influence the Cr distribution in surface horizon along the slope. To cite this article: S. Salvador-Blanes et al., C. R. Geoscience 334 (2002) 51–58     

Shock temperature in calcite (CaCO3) at 95-160 GPa

The temperatures induced in crystalline calcite (CaCO₃) upon planar shock compression (95-160 GPa) are reported from two-stage light gas gun experiments. Temperatures of 3300-5400 K are obtained by fitting six-channel optical pyrometer radiances in the 450-900 nm range to the Planck gray-body radiation law. Thermodynamic calculations demonstrate that these temperatures are some 400-1350 K lower than expected for vibronic excitations of the lattice with a 3R/mole-atom specific heat (R is gas constant). The temperature deficit along the Hugoniot is larger than that expected from only melting. In addition to melting, it appears likely that shock-induced decomposition of calcite occurs behind the shock front. We modeled disproportionation of calcite into CaO (solid) plus CO₂ (gas). For temperature calculations, specific heat at constant volume for 1 mole of CO₂ is taken to be 6.7R as compared to 9R in the solid state; whereas a mole of calcite and a mole of CaO have their solid state values 15R and 6R, respectively. Calculations suggest that the calcite decomposes to CaO and CO₂ at ∼110±10 GPa along the Hugoniot. Recent reanalysis of earlier VISAR measurements of particle velocity profiles [1] indicates that calcite shocked to 18 GPa undergoes disproportionation at much lower pressures upon isentropic expansion.     

Cathodoluminescence, electron microscopy, and Raman spectroscopy of experimentally shock-metamorphosed zircon

Thorough understanding of the shock metamorphic signatures of zircon could be the basis for the use of this mineral as a powerful tool for the study of old, deeply eroded, and metamorphically overprinted impact structures and formations. This study of the cathodoluminescence (CL) and Raman spectroscopic signatures of experimentally (20-60 GPa) shock-metamorphosed zircon single crystals contributes to the understanding of high-pressure microdeformation in zircon. For all samples, an inverse relationship between the brightness of the backscattered electron (BSE) signal and the corresponding cathodoluminescence intensity was observed. The unshocked sample shows crosscutting, irregular fractures. The 20 GPa sample displays some kind of mosaic texture of CL brighter and darker domains, but does not exhibit any shock metamorphic features in BSE or CL images. The 40 GPa sample shows a high density of lamellar features, which might be explained by the phase transformation between zircon- and scheelite-structure phases of zircon and resulting differences in the energy levels of the activator elements. The CL spectra of unshocked and shocked (20, 40, and 60 GPa) zircon samples are dominated by narrow emission lines and broad bands in the region of visible light and in the near-UV range. The emission lines result from rare earth element activators and the broad bands might be associated with lattice defects. Raman spectra revealed that the unshocked and 20 GPa samples represent zircon-structure material, whereas the 40 GPa sample yielded additional peaks with relatively high peak intensities, which are indicative of the presence of the scheelite-type high-pressure phase. The 60 GPa sample has a Raman signature that is similar to that of an amorphous phase, in contrast to the observations of an earlier TEM study that the crystalline scheelite-structure phase is stable at this shock pressure. The 60 GPa Raman signature cannot be explained at this stage. The results show a clear dependence of the CL and Raman properties of zircon on shock pressure, which confirm the possible usage of these methods as shock indicators.     

集宁群和迁西群麻粒岩相变质流体的特征和成因

矿物中流体包裹体研究和脱水反应热力学计算结果表明，集宁群和迁西群麻粒岩相变质峰期流体均富碳质,x(CO2 CH4)一般为70%-80%，x(H2O)≈10%-20%，密度一般为1.0-1.1g/cm^3。集宁群中变质流体的成分和H2O活度在空间上分布不均匀，明显受岩性控制。富铝片麻岩、黑云紫苏片麻岩和基性麻粒岩中αH2O值分别为0.10-0.20、0.20-0.40和0.40-0.60，表明其非外源成因，未曾在岩石中均匀渗透，而可能与进变质过程原岩中普遍存在的有机质氧化和各种脱挥发分反应有关，其富碳质还与H2O相对易于逸散有关。迁西群变质峰期流体成分在空间上分布均匀，不受岩性控制，在玉平寨等地区上述3类岩石中αH2O值主要都在0.13-0.25之间，XH2O为0.1-0.2，反映流体似为外源成因，并确曾在岩石中均匀渗透。本区存在幔源基性岩浆底侵的地质条件，这种富碳质流体可能与它们结晶时析出的CO2流上升有关。     

Evaluation of the SNOWPACK model’s metamorphism and microstructure in Canada: a case study

The snow thermodynamic multi-layer model SNOWPACK was developed to address the risk of avalanches by simulating the vertical properties of snow. Risk and stability assessments are based on the simulation of the vertical variability of snow microstructure, as well as on snow cohesion parameters. Previous research has shown systematic error in grain size simulations (equivalent optical grain size) over several areas in northern Canada. To quantify the simulated errors in snow grain size and uncertainties in stability, the snow specific surface area (SSA) was measured with a laser-based instrument. Optical grain size was retrieved to validate the optical equivalent grain radius from SNOWPACK. The two study plots are located in Glacier National Park, BC, and Jasper National Park, AB, Canada. Profiles for density and stratigraphic analysis were obtained as well as grain size profiles, combined with snow micropenetrometer (SMP) measurements. Density analysis showed good agreement with the simulated values (R² = 0.76). Optical grain size analysis showed systematic overestimation of the modeled values, in agreement with the current literature. The error in SSA evolution for a rounding environment was mostly constant, whereas error for conditions driven by a temperature gradient was linked to the size of the facetted grains.     

变质改造型关键金属矿——富集Li-Rb-Cs-Tl-Ga的云母片岩

关键金属矿产是国际上最近提出的资源概念,对战略性新兴产业的发展至关重要,但认知程度较低。洛扎岩浆-变质杂岩体位于喜马拉雅带东部,侵位于藏南拆离系内。在岩体东北侧,云母片岩被含电气石淡色花岗岩捕虏。云母片岩主要由金云母、绿泥石和少量黑云母组成。从全岩地球化学成分来看,云母片岩具有含量较高的Al₂O₃(13.38%～14.32%)、K₂O(6.09%～9.66%)、FeO^*(27.11%～30.09%)、MgO(15.25%～17.21%)、TiO₂(0.09%～0.26%),富集关键金属Li(650×10^-6～1 031×10^-6)、Rb(1 649×10^-6～2 773×10^-6)、Cs(98×10^-6～229×10^-6)、Tl(5.7×10^-6～12.1×10^-6)、Ga(121×10^-6     

走滑断裂对超高压变质岩石折返的贡献及青藏高原北部白垩纪隆升之新思考

青藏高原已识别出柴北缘、南阿尔金和高喜马拉雅三条超高压变质带。这些超高压变质带提供了一个不可多得的研究超高压变质岩石形成和折返的机会。柴北缘超高压变质带位于阿尔金断裂的东边,是柴达木—东昆仑地体与祁连—阿尔金微地体和阿拉善—敦煌地体碰撞的产物,由榴辉岩、石榴石橄榄岩和含柯石英片麻岩组成,榴辉岩形成时代500~440Ma,峰期超高压变质年龄440Ma。南阿尔金超高压变质带位于阿尔金断裂带的西边,以产出榴辉岩和石榴石橄榄岩为特征,榴辉岩形成时代为500Ma。南阿尔金超高压变质带被认为是柴北缘超高压变质带的西延,两者被阿尔金断裂左旋位移约400km。阿尔金断裂是巨大的深度>200km的岩石圈走滑断裂,断裂的活动时代至少早到240~220Ma,认为走滑过程中伴随的隆升作用有可能为柴北缘和南阿尔金超高压变质岩石的折返和出露地表做出了贡献,其中阿尔金断裂起到了类似剪刀型断裂的作用。高喜马拉雅超高压变质带在巴基斯坦和印度被发现,以榴辉岩中含柯石英或金刚石为特征,榴辉岩的超高压变质年龄为46Ma,表明超高压变质岩石发生在雅鲁藏布江缝合线关闭后并快速折返。喀喇昆仑断裂走滑过程中伴随的抬升作用则可能对高喜马拉雅地区超高压变质岩石的折返和出露地表做出贡献。在中国东部出露的大别—苏鲁超高压变质带被巨大郯庐断裂左旋走滑位移约500km,可以看作是走滑作用伴随的抬升运动对超高压变质岩石的最后折返和出露地表做出重要贡献的又一例证。青藏高原的隆升通常被认为是印度板块和欧亚大陆新生代以来的碰撞结果。根据高原北部断裂的时代、火山活动和沉积盆地的形成,我们提出高原的隆升是两次俯冲碰撞的结果。第一次发生在中特提斯班公湖-怒江洋盆在白垩纪时期的关闭,其时由于北部来自塔里木盆地和北中国板块及东部来自太平洋板块俯冲产生的抵柱效应,高原北部开始隆升;第二次发生在印度板块的新生代俯冲碰撞作用,造成高原的整体抬升,由此可以解释高原北部平均海拔(5000m)要高于高原南部(平均海拔4000m)。