Multi-textured garnets from a single growth event: an example from northern Norway

Abstract Textural relationships between porphyroblasts of biotite and garnet in metasediments in the Nordkinn Peninsula area of the Finnmarkian Caledonides of North Norway are apparently complex. There is evidence for two textural zones in both mineral phases and superficially the development of these appears to have overlapped, at least in part, in time and space. This apparently complex porphyroblast growth history can be considerably simplified if only one period of garnet growth occurred and if different inclusion fabrics developed where garnet replaced biotite porphyroblasts and where it overgrew the matrix foliation. The possibility that porphyroblasts with textural evidence for multiphase growth histories actually grew during a single crystallization event is of importance in the interpretation and elucidation of tectonometamorphic relationships.     

Metamorphism of metapelites: calculations of equilibrium assemblages and numerical simulations of the crystallization of garnet

Abstract This work uses a simplified model of equilibrium to predict the assemblage sequence and compositional zoning in garnet that should result from prograde metamorphism of common bulk compositions of pelitic rocks. An internally-consistent set of model thermodynamic data are derived for natural mineral compositions from natural assemblages. Equilibrium assemblages can be calculated for pelitic compositions with excess quartz and either muscovite or K-feldspar at any pressure and water pressure. The compositions and abundances of phases in equilibrium assemblages can be calculated where the elements Mg, Fe and Mn are exchanged among phases. The prograde metamorphic assemblage sequences and the effects of pressure on assemblages, predicted by the simulation method presented here, are similar enough to natural observations to suggest that the simulations can be used to analyse natural equilibrium and growth processes. The calculated phase diagrams at moderate and high crustal pressures explain the mineral assemblage sequence produced by prograde metamorphism in common pelitic compositions. Garnet appears by continuous reaction of biotite and chlorite as the garnet-biotite-chlorite divariant field migrates toward higher Mg/Fe ratios over the bulk composition. Staurolite appears in common bulk compositions when garnet and chlorite become incompatible. An aluminum silicate phase can appear when staurolite and chlorite react. Staurolite breaks down at an extremum point to produce garnet. Continuous reaction of biotite and sillimanite causes growth of abundant garnet. The reaction sequence involving garnet, staurolite and aluminum silicates is probably different at low pressure, but the main reason that staurolite and garnet are rare is the restricted compositional range over which their assemblages exist. Andalusite appears by the divariant reaction of chlorite and cordierite appears at low temperature in low pressure assemblages for common bulk compositions by the extremumpoint breakdown reaction of chlorite. Compositional zoning of garnet and the systematic variation of biotite composition in metamorphic sequences indicate that garnet is probably fractionated during growth. Fractionation of garnet causes garnet-consuming, univariant reactions to become multivariant. The metastable persistence of garnet should reduce the abundance and stability range of staurolite. Fractionation of even small quantities of garnet should deplete the equilibrating bulk composition of Mn, but have little effect otherwise. The simulations show that the prograde assemblage sequence in pelitic rocks can be complex in detail, with some assemblages lasting over temperature intervals of only a few degrees. The major prograde reactions that release water are the breakdown of chlorite to form garnet at low grade and the breakdown of muscovite at high grade. The volume of water released by formation of garnet at high grade is also important. These reactions have the capacity to buffer water pressure. The density of anhydrous pelitic rock increases markedly when chlorite breaks down and by the continuous reaction forming garnet at high grade. The heat content is controlled principally by heat capacity and continuous reactions. Discontinuous reactions have little thermal buffering capacity. Simulations of garnet fractionation show that commonly-observed garnet zoning profiles can be formed by garnet growth in the assemblage garnet-biotite-chlorite in common bulk compositions. A reversal of Fe-zoning in garnet can occur when garnet resumes growth above staurolite grade in the assemblage garnetbiotite-sillimanite. Discontinuities in zoning profiles can be caused only by disequilibrium. The disequilibrium can be due to either metastable persistence during a hiatus in growth or to growth by irreversible reaction. Because the appearance of garnet is controlled by a continuous rather than a discontinuous reaction, the appearance of garnet is very sensitive to bulk composition. The early development of garnet is also sensitive to the pressure and water pressure of metamorphism. As a consequence the first garnet isograd is of limited thermometric value. Metastable persistence of kyanite and manite at high grades could reduce the abundance of garnet and allow biotite to persist. Metastable persistence would also limit the of cordierite formation.     

Contact aureoles as constraints on regional P-T trajectories: an example from the Northern Alabama Piedmont, USA

Abstract Contact aureoles of plutons emplaced into regionally metamorphosed terranes can provide indicators of physical conditions along a portion of regional metamorphic P-T trajectories, thereby allowing reconstruction of more complete P-T loops than would be otherwise possible. In the Northern Alabama Piedmont of the southern Appalachians, Wedowee Group metapelites preserve evidence for two regional metamorphic phases overprinted by contact metamorphism adjacent to the Blakes Ferry Trondhjemite. Textural evidence indicates that an early bt+st+grt assemblage was replaced by bt+chl+grt during the latter stages of regional metamorphism. Changes in AKFM topology, complex Fe-Mg-Ca garnet zoning, and the latestage appearance of epidote indicate that a sequence of continuous reactions (bt+st = grt+ ms followed by chl+ms+Ca-grt+Ca-pl=bt+ (Fe+Mg)-grt+ep) occurred in response to increasing pressure and resulted in the observed changes in mineral assemblage. Pl-ms-bt-grt thermobarometry indicates conditions of 580° 65°C, 8.5±0.8 kbar for equilibration of grt+ bt+chl. Pluton emplacement, subsequent to penetrative deformation, caused textural annealing and mineral re-equilibration by the continuous reaction bt+(Fe+Mg)-grt+ep = chl+ms+Ca-grt+Ca-pl within 50 m of the pluton. Conditions of 510±65°C, 5-7 kbar are inferred. A reconstructed P-T trajectory for this area is characterized by (1) early moderate- T , moderate- P metamorphism; (2) an increase in P to approximately 8.5 kbar; and (3) decompression and slight cooling prior to pluton emplacement. The compressional phase of this path is interpreted to result from underthrusting of the Wedowee metasediments to mid-crustal levels during Palaeozoic crustal thickening. Late-stage decompression prior to intrusion records uplift of these rocks in response to movement on structurally lower thrusts.     

P-T grids for silica-undersaturated granulites in the systems MAS (n+ 4) and FMAS (n+ 3)-tools for the derivation of P-T paths of metamorphism

Abstract Considering the minerals cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si) and corundum (Co) in the system FeO-MgO-Al₂O₃-SiO₂ (FMAS), the stable invariant points are [Co], [Ga], [Cd] and [Sa]. Constraints imposed by experimental data for the system MAS indicate that under low P _H2o conditions the invariant points occur at high temperature (> 900° C) and intermediate pressure (7-10 kbar). This temperature is higher than that commonly advocated for granulite facies metamorphism. In granulites Fe-Mg exchange geothermometers may yield temperatures of 100–150° C below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high-temperature assemblages and by Al-rich cores in orthopyroxene. Application of the FMAS grid to some well-documented granulite occurrences introduces important constraints on their P-T histories. Rocks of different bulk compositions, occurring in close proximity in the field, may record distinct segments of their P-T paths. This applies particularly to rocks with evidence for reaction in the form of coronas, symplectites and zoned minerals. Consideration of curved reaction boundaries and _XMs isopleths may explain apparently contradictory results for the stability of cordierite obtained from low-temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other.     

Glaucophane and pyroxene breakdown reactions in the Pennine units of the Eastern Alps

Abstract New occurrences of crossite and jadeitic pyroxene are described from a thick metabasite unit within the upper levels of the Peripheral Schieferhülle in the Tauern Window, Austria. Unusual textures are preserved which provide evidence for the reactions and mechanisms involved in the breakdown of crossite and jadeitic pyroxene. Zones of albite and chlorite, produced by reaction between crossite and paragonite, have been preserved due to sluggish reaction kinetics during decompression from the blueschist to the greenschist facies. The zonal sequence is interpreted in terms of chemical potential gradients in Na, Mg and Al, which have been established by overstepping the equilibrium boundary. Breakdown textures of jadeite-acmite pyroxene to a symplectite of albite + hematite + actinolite, and of crossite to talc and actinolite are also described.
The occurrence of crossite and jadeitic pyroxene at high levels within the Peripheral Schieferhülle implies that even upper levels of the structural sequence have undergone blueschist facies metamorphism with pressures in excess of 8 kbar during the Alpine collisional event.     

桌面虚拟现实技术研究及其应用

虚拟现实(VirtualReality,VR)技术是一项综合性的、具有广阔应用前景的信息技术。本文介绍了虚拟现实技术的应用领域,阐述了桌面虚拟现实系统的特点,探讨了它的关键技术。     

塔里木河中下游地区气象灾害情况及气象对策

分析了近30年塔里木河中下游地区主要灾害性天气的情况，及对农业、生态和给人们甚至环境带来的严重影响，通过成因分析，提出了一些长期的气象对策和建议。