Petrostructural evolution of ultramafic rocks of the Kalninsky chromite-bearing massif, Western Sayan

The Kalininsky ultramafic massif is a fragment of lower structural zone of the Kurtushiba ophiolitic belt in the extreme northeastern part of the Western Sayan. The massif is composed largely of rocks making up the dunite-garzburgite banded complex. The northeastern part of the massif is composed mainly of dunite with linear NW-trending chromite-bearing zones, the localization of which is controlled by banding of the dunite-harzburgite complex. Harzburgite and dunite are characterized by inhomogeneous structures and textures caused by nonuniform ductile deformation, which is expressed as heterogeneous extinction, kink bands, and syntectonic and annealing recrystallization. The petrostructural patterns of olivine in harzburgite and dunite provide evidence for three stages of ductile deformation. At the first stage under deep mantle-crustal conditions, the ductile flow of ultramafic rocks developed mainly in a regime of axial compression, high temperature (>1000°C), and low strain rate (? < 10^?6 s^?1), which resulted in translational gliding along the (010)[100] and (100)[001] systems in olivine and enstatite, respectively, in combination with a subordinate role of syntectonic recrystallization. Consequently, the rocks acquired a medium-grained (mesogranular) microstructure. At the second stage, related to the thermal effect on ultramafics, the ductile flow developed under the settings of low strain rate (? < 10^?6 s^?1) and rising temperature (>1000°C). The translational gliding in olivine proceeded largely along (010)[100] and was accompanied by diffusion creep. As the temperature rose, ductile deformation gave way to secondary recrystallization of annealing, which facilitated the growth of olivine grains free of dislocations owing to absorption of individual grains oriented adversely relative to the compression axis and deformed grains saturated with dislocations. As a result, dunite and harzburgite with a coarse-grained porphyroblastic microstructure have been formed. The third stage of ductile flow was apparently related to their transport along deep-seated thrust faults under settings of intense shear deformations at a high temperature (～1000°C) and strain rate (? >10^?4 s^?1). The ductile flow in olivine resulted in heterogeneous translational gliding along (010)[100] and accompanied by intense syntectonic recrystallization with the formation of a porphyroblastic microstructure. Chromite mineralization in dunite is controlled by internal banding. Intense ductile flow facilitated the metamorphic separation of linearbanded Cr-spinel segregations. Thus, the results of a petrostructural study show that ultramafic rocks of the Kalninsky massif, ascending to the upper lithosphere, underwent both axial and shear ductile deformations in the mantle and lower crust, and these deformations controlled chromite mineralization.     

Macro‐and Microstructural,Textural Fabrics and Deformation Mechanism of Calcite Mylonites from Xar Moron‐Changchun Dextral Shear Zone,Northeast China

The calcite mylonites in the Xar Moron-Changchun shear zone show a significance dextral shearing characteristics. The asymmetric(σ-structure) calcite/quartz grains or aggregates, asymmetry of calcite c-axes fabric diagrams and the oblique foliation of recrystallized calcite grains correspond to a top-to-E shearing. Mineral deformation behaviors, twin morphology, C-axis EBSD fabrics, and quartz grain size-frequency diagrams demonstrate that the ductile shear zone was developed under conditions of greenschist facies, with the range of deformation temperatures from 200 to 300°C. These subgrains of host grains and surrounding recrystallized grains, strong undulose extinction, and slightly curved grain boundaries are probably results of intracrystalline deformation and dynamic recrystallization implying that the deformation took place within the dislocation-creep regime at shallow crustal levels. The calculated paleo-strain rates are between 10~(–7.87)s~(–1) and 10~(–11.49)s~(–1) with differential stresses of 32.63–63.94 MPa lying at the higher bound of typical strain rates in shear zones at crustal levels, and may indicate a relatively rapid deformation. The S-L-calcite tectonites have undergone a component of uplift which led to subhorizontal lifting in an already non-coaxial compressional deformation regime with a bulk pure shear-dominated general shear. This E-W large-scale dextral strike-slip movement is a consequence of the eastward extrusion of the Xing'an-Mongolian Orogenic Belt, and results from far-field forces associated with Late Triassic convergence domains after the final closure of the Paleo-Asian Ocean.     

Thermodynamic and kinetic constraints on reaction rates among minerals and aqueous solutions. II. Rate constants,effective surface area,and the hydrolysis of feldspar

Analysis of experimental data reported by Lagache (1965, 1976), Evans (1965), Busenberg (1975), Busenberg and Clemency (1976), Holdren and Berner (1979), Siegel and Pfannkuch (1984), and Chou and Wollast (1984) with the aid of irreversible thermodynamics and transition state theory (Aagaard and Helgeson, 1977, 1982) suggests that at temperatures at least up to 650°C, the rate of both congruent and incongruent feldspar hydrolysis in aqueous solutions far from equilibrium at pH ? 10.6 ? (2300/T), where T stands for temperature in kelvins, is a function solely of effective surface area and pH at constant pressure and temperature. At higher pH, the rate is apparently pH-independent up to ~pH 8 at 25°C, where it again becomes pH-dependent at higher pH. Observations of scanning electron micrographs indicate that the cross-sectional area of etch pits on hydrolyzed feldspar grains is of the order of 10^?9 to 10^?8 cm² and that the ratio of the effective to total surface area (which may or may not change with reaction progress) ranges from <0.01 to 1, depending on the grain size, dislocation density, and the extent of comminution damage on the surfaces of the grains. Apparent rate constants retrieved from experimental data reported in the literature for feldspar hydrolysis in the lower pH-dependent range extend from ~10^?13 to ~10^?7 moles cm^?2 sec^?1 at temperatures from 25° to 200°C, which is consistent with activation enthalpies for albite and adularia of the order of 20 kcal mole^?1. In contrast, the apparent rate constants for the pH-independent rate law range from ~10^?16 to ~10^?11 moles cm^?2 sec^?1 at temperatures from 25° to 650°C, which requires an activation enthalpy for adularia of ~ 9 kcal mole^?1. These observations are consistent with surface control of reaction rates among minerals and aqueous solutions. The rate-limiting step in the pH-dependent case apparently corresponds at the lower end of the pH scale to breakdown of a protonated configuration of atoms on the surface of the reactant feldspar, but at higher pH the rate is limited by decomposition of an activated surface complex corresponding in stoichiometry to hydrous feldspar. In highly alkaline solutions, an activated complex containing hydroxyl ions apparently controls the rate of feldspar hydrolysis. Nevertheless, near equilibrium, regardless of pH the rate is proportional to the chemical affinity of the overall hydrolysis reaction.     

Hydrolytic weakening of experimentally deformed Westerly granite and Hale albite rock

In order to determine the effect of water on deformation in the brittle-ductile transition region of crustal rocks, experiments have been conducted on Westerly granite and a polycrystalline albite rock, comparing samples dried at 160°C for 12 h (‘dry’) and samples with about 0.2 wt% water added (‘wet’). The deformation mechanisms and style of deformation of the wet and dry samples, determined using optical and transmission electron microscopy, have been found to depend on temperature, pressure, strain rate, and strain. At 15 kb and 10⁻⁶, the added water reduces the temperature of the transition between microcracking and dislocation glide and climb by about 150–200°C for both quartz and feldspar. However, the penetration of ‘water’ into the grains is slow compared with the time of the experiments and many of the wet samples show evidence of initial microcracking and later dislocation creep. Wet samples deformed at 10 kb show less hydrolytic weakening than wet samples deformed at 15 kb. Because the deformation mechanism and strength of silicates depend so sensitively on trace amounts of water, and because the water content of experimental samples varies with temperature and pressure and thus with time, flow laws for any samples are only meaningful if the water content has been carefully controlled or characterized.     

Fluid record of rock exhumation across the brittle–ductile transition during formation of a Metamorphic Core Complex (Naxos Island,Cyclades, Greece)

Fluid inclusions trapped in quartz veins hosted by a leucogneiss from the southern part of the Naxos Metamorphic Core Complex (Attic‐Cycladic‐Massif, Greece) were studied to determine the evolution of the fluid record of metamorphic rocks during their exhumation across the ductile/brittle transition. Three sets of quartz veins (V‐M2, V‐BD & V‐B) are distinguished. The V‐M2 and V‐BD are totally or, respectively, partially transposed into the foliation of the leucogneiss. They formed by hydrofracturing alternating with ductile deformation accommodated by crystal‐plastic deformation. The V‐B is discordant to the foliation and formed by fracturing during exhumation without subsequent ductile transposition. Fluids trapped during crystal–plastic deformation comprise two very distinct fluid types, namely a CO₂‐rich fluid and a high‐salinity brine, that are interpreted to represent immiscible fluids generated from metamorphic reactions and the crystallization of magmas respectively. They were initially trapped at ～625 °C and 400 MPa and then remobilized during subsequent ductile deformation resulting in various degrees of mixing of the two end‐members with later trapping conditions of ～350 °C and 140 MPa. In contrast, brittle microcracks contain aqueous fluids trapped at 250 °C and 80 MPa. All veins display a similar δ¹³C pointing to carbon that was trapped at depth and then preserved in the fluid inclusions throughout the exhumation history. In contrast, the δD signature is marked by a drastic difference between (i) V‐M2 and V‐BD veins that are dominated by carbonic, aqueous‐carbonic and high‐salinity fluids of metamorphic and magmatic origin characterized by δD between ?56‰ and ?66‰, and (ii) V‐B veins that are dominated by aqueous fluids of meteoric origin characterized by δD between ?40‰ and ?46‰. The retrograde P–T pathway implies that the brittle/ductile transition separates two structurally, chemically and thermally distinct fluid reservoirs, namely (i) the ductile crust into which fluids originating from crystallizing magmas and fluids in equilibrium with metamorphic rocks circulate through a geothermal gradient of 30 °C km^?1 at lithostatic pressure, and (ii) the brittle upper crust through which meteoric fluids percolate through a high geothermal gradient of 55 °C km^?1 at hydrostatic pressure.     

Deformation of dolomite single crystals from 20–800° C

Dolomite single crystals of six different crystallographic orientations were tested in compression under confining pressure at temperatures of 20 to 800° C. The chosen orientations favoured slip or twinning on particular systems. The deformed crystals were analysed by optical and high voltage transmission electron microscopy to determine activated deformation systems, dislocation behaviour, etc., and to assist in interpreting stress-strain data. It is shown that slip on c≡(0001) and on \(f \equiv (\bar 1012)\) ), and twinning on f between 300 and 600° C, are the principal modes of deformation. At low temperatures there is considerable cataclasis, and shear-fracturing must be counted as a significant deformation mechanism. The effects of climb become apparent at temperatures ?600° C. A pronounced increase in strength with testing temperature shown by some orientations of the crystals is largely associated with c slip, but f twinning also shows similar although weaker tendencies. The yield stress for f slip decreases markedly with temperature. Values of critical resolved shear stress are obtained for c and f slip, and for f twinning. Major and minor slip systems, dislocation and twin configurations generated by deformation under different regimes are documented and illustrated. Some of the characteristics of the deformation systems are attributed to the details of atomic displacements. In particular, the increase in strength with temperature for c slip is explained by friction of CO ₃ ^2? groups during dislocation movement, which is unique for c slip. This friction increases with thermal vibration, expansion and rotation of the CO ₃ ^2? groups.     

Unit-cell dimensions and tetrahedral-site ordering in synthetic gallium albite (NaGaSi3O8)

The tetrahedral-site order-disorder transformation in gallium albite (NaGaSi₃O₈) has been investigated using Rietveld structure refinement. Study of gallium-substituted albite (in contrast to pure albite [NaAlSi₃O₈]) is facilitated by a relatively rapid order-disorder transformation and the large difference in X-ray scattering efficiencies of gallium and silicon. High albite-structure NaGaSi₃O₈, grown in a Na₂WO₄ flux, was ordered by hydrothermal annealing below 820° C and dry annealing above 820° C, to avoid melting, using a load pressure of approximately 1 kbar. Equilibration of the order-disorder reaction has been verified by three independent reversals of ordering. The transformation between low gallium albite and high gallium albite occurs over the temperature range 890° C 970° C. The gallium content of the T ₁o site increases continuously with decreasing temperature. The gallium contents of the T ₁m and T ₂m sites decrease smoothly with increasing ordering while the gallium content of the T ₂o site decreases, then increases and then decreases again with decreasing temperature. Unit-cell parameters and the triclinic obliquity vary throughout the order-disorder transformation and undergo abrupt changes at 913±3° C and 937±3° C. These abrupt changes correlate with changes in the gallium content of the T ₂o site, the X and Z ordering parameters and the configurational entropy. The order-disorder transformation in gallium-aluminum albite (NaGa_0.5Al_0.5Si₃O₈) occurs in the temperature range 765° C-850° C, at a temperature intermediate to the transformation in albite (50% order at about 680±20° C) and gallium albite.     

Deformation at low and high stress-loading rates

In an extensional shear zone in the Talea Ori, Crete, quartz veins occur in high-pressure low-temperature metamorphic sediments at sites of dilation along shear band boundaries, kink band boundaries and boudin necks. Bent elongate grains grown epitactically from the host rock with abundant fluid inclusion trails parallel to the vein wall indicate vein formation by crack-seal increments during dissolutionprecipitation creep of the host rock. The presence of sutured high-angle grain boundaries and subgrains shows that temperatures were sufficiently high for recovery and strain-induced grain boundary migration, i.e. higher than 300 -350℃, close to peak metamorphic conditions. The generally low amount of strain accumulated by dislocation creep in quartz of the host rock and most veins indicates low bulk stress conditions of a few tens of MPa on a long term. The time scale of stress-loading to cause cyclic cracking and sealing is assumed to be lower than the Maxwell relaxation time of the metasediments undergoing dissolution-precipitation creep at high strain rates(10^-10 s^-1 to 10^-9 s^-1), which is on the order of hundred years. In contrast, some veins discordant or concordant to the foliation show heterogeneous quartz microstructures with micro-shear zones, sub-basal deformation lamellae, shortwavelength undulatory extinction and recrystallized grains restricted to high strain zones. These microstructures indicate dislocation glide-controlled crystal-plastic deformation(low-temperature plasticity) at transient high stresses of a few hundred MPa with subsequent recovery and strain-induced grain boundary migration at relaxing stresses and temperatures of at least 300 -350℃. High differential stresses in rocks at greenschist-facies conditions that relieve stress by creep on the long term, requires fast stress-loading rates, presumably by seismic activity in the overlying upper crust. The time scale for stress loading is controlled by the duration of the slip event along a fault, i.e. a few seconds to minutes.This study demonstrates that microstructures can distinguish between deformation at internal low stress-loading rates(to tens of MPa on a time scale of hundred years) and high(coseismic) stress-loading rates to a few hundred MPa on a time scale of minutes.     

Fractal analysis of magnetite grains — implications for interpreting deformation mechanism

In the present study, the grain size (d) and shape of 225 magnetite grains, that crystallized at T>600°C in a syntectonic granite (Godhra Granite, India) are evaluated and implications of data to decipher deformation mechanism of magnetite are discussed. Fractal (ruler) dimension (D) analysis of magnetite grains is performed and it is demonstrated that they show fractal behaviour. Smaller magnetite grains tend to be more serrated than the larger ones, which is manifested in the higher fractal (ruler) dimension (D) of the former. Assuming a natural strain rate ranging between 10⁻¹⁰ s⁻¹ and 10⁻¹⁴ s⁻¹, the grain size data fall dominantly in the dislocation creep field of the existing deformation mechanism map of magnetite for 630°C. However, SEM-EBSD studies reveal that subgrains are absent in the magnetite grains and they did not undergo dislocation creep. Thus it is inferred that the shape of magnetite grains was not controlled by dislocation creep. It is concluded that the higher serration and increased fractal dimension of finer magnetite grains implies the importance of diffusion creep as an important deformation mechanism at high-T for magnetite in polymineralic rocks.     

Very low temperature,natural deformation of fine grained limestone: a case study from the Lucania region,southern Apennines,Italy

Abstract

The deformation behavior of fine grained limestones from the Monte Sirino area (Lucania region) of the southern Apennines has been analysed by constraining microstructural observations and crystallographic fabrics with data on the metamorphic conditions of deformation. X-ray and infrared analysis of clay minerals, together with illite ‘crystallinity’ data, suggest that the studied rocks underwent very low grade metamorphism in the deep diagenetic zone. The limestones consist of very fine grained (<10 μm) aggregates of micrite. Elliptically-shaped radiolarians, preserved as moulds with coarser (>20 μm) crystalline fillings, provide common strain markers. Optical microstructures and strain analysis indicate heterogeneous intracrystalline strain in the coarser (>50 μm) calcite. On the other hand, SEM and TEM observations, and crystallographic fabrics determined by X-ray texture goniometry, indicate a deformation involving not only intracrystalline slip, but also an important component of grain boundary sliding in the fine grained matrix. The inferred microscopic deformation mechanisms are compared with constitutive flow laws derived from experimental studies. For the maximum inferred temperature of deformation of 250 °C and geologic strain rates of 10^?13?10^?15 s^?1, deformation mechanism maps for calcite suggest twinning and other glide mechanisms to be active in grains larger than about 5?10 μm. Smaller grains would be mostly deformed by grain size sensitive creep mechanisms, which include both diffusion mass transfer processes and grain boundary sliding. Deformation features observed in the study limestones are compatible with the prediction of such temperature-dependent mechanism maps. © 2001 Éditions scientifiques et médicales Elsevier SAS     

The effect of deformation on the TitaniQ geothermobarometer: an experimental study

We performed high strain (up to 47 %) axial compression experiments on natural quartz single crystals with added rutile powder (TiO₂) and ~0.2 wt% H₂O to investigate the effects of deformation on the titanium-in-quartz (TitaniQ) geothermobarometer. One of the objectives was to study the relationships between different deformation mechanisms and incorporation of Ti into recrystallized quartz grains. Experiments were performed in a Griggs-type solid-medium deformation apparatus at confining pressures of 1.0–1.5 GPa and temperatures of 800–1,000 °C, at constant strain rates of 1 × 10^?6 or 1 × 10^?7 s^?1. Mobility of Ti in the fluid phase and saturation of rutile at grain boundaries during the deformation experiments are indicated by precipitation of secondary rutile in cracks and along the grain boundaries of newly recrystallized quartz grains. Microstructural analysis by light and scanning electron microscopy (the latter including electron backscatter diffraction mapping of grain misorientations) shows that the strongly deformed quartz single crystals contain a wide variety of deformation microstructures and shows evidence for subgrain rotation (SGR) and grain boundary migration recrystallization (GBMR). In addition, substantial grain growth occurred in annealing experiments after deformation. The GBMR and grain growth are evidence of moving grain boundaries, a microstructure favored by high temperatures. Electron microprobe analysis shows no significant increase in Ti content in recrystallized quartz grains formed by SGR or by GBMR, nor in grains grown by annealing. This result indicates that neither SGR nor moving grain boundaries during GBMR and grain growth are adequate processes to facilitate re-equilibration of the Ti content in experimentally deformed quartz crystals at the investigated conditions. More generally, our results suggest that exchange of Ti in quartz at low H₂O contents (which may be realistic for natural deformation conditions) is still not fully understood. Thus, the application of the TitaniQ geothermobarometer to deformed metamorphic rocks at low fluid contents may not be as straightforward as previously thought and requires further research.     

青海可可西里北缘韧性剪切带的显微构造特征

在可可西里北缘发现的糜棱岩化带,经显微构造研究确定为韧性剪切带。此带发育流劈理及拉伸线理。糜棱岩化花岗岩和糜棱岩化石英脉中的石英发育亚晶粒构造、位错构造和动态重结晶,长石主要发育机械双晶。石英c轴组构属韧性剪切带中的典型形式,石英变形以位错蠕变机制和位错滑移机制共存为特征,石英的动态重结晶作用是由亚晶粒旋转机制形成。长石的变形主要是通过机械双晶实现的。此韧性剪切带形成时的温度约400℃,差异应力约30MPa,应变速率约1.9×10^-13s^-1.     

Diffusion of Na,K, Rb and Cs in glasses of albite and orthoclase composition

The measurement of diffusion coefficients for Na, K, Rb and Cs has been realized by the technique of active salt deposits on glasses of albite and orthoclase composition, at normal pressure and in the temperature range 300–1000°C. The values of D are between 10^?6 and 10^?12 cm² s^?1 and, for every type of run, they vary with temperature according to Arrhenius laws, with activation energies ranging from 13 to 68 kcal mole^?1. These important variations are related to the size of the diffusing element (at 700°C in albite glass D_Na/D_K/D_RbD_Cs ～- 10⁷/10⁵/10³/1) and to the size of the major alkali element (for rubidium at 800°C D_or·gl/D_ab·gl ～- 20). By comparison with available data on diffusion in feldspars, we emphasize the influence of the defect density on the diffusion process.     

长乐-南澳韧性剪切带中糜棱岩变形微构造研究

 长乐-南澳韧性剪切带糜棱岩的显微变形机制有粒间滑动、粒内滑动、位错及其滑移和蠕变、应变的局部恢复等。石英丰富的韧性变形和位错特征以及长石的脆一弹性变形说明糜棱岩形成于绿片岩相条件,变形时的温度约为300-350℃。石英塑性变形处于重结晶一软化阶段和热加工一恢复阶段。由位错密度和动态重结晶颗粒粒径计算了差异应力和应变速率,分别为56.8-99.7MPa,(0.35-1.61)×10^-13s^-1和175.8-406.9MPa,(0.7-7.2)×10^-12s^-1,二者较大的差值表明糜棱岩是缓慢上升至地表的,这一结论与应变局部恢复现象一致。该韧性剪切带是闽粤沿海早白垩世末碰撞造山的产物。     

High-temperature stress relaxation of synthetic,polycrystalline galena

Stress relaxation experiments were performed on hot-pressed cylinders of synthetic, polycrystalline galena at temperatures of 500°, 600°, 700° and 800°C at atmospheric pressure after various levels of axial strain, in the range from 2% to 25%, had been imposed at a constant strain rate (?) of 3×10^?4 s^?1. For this study a new apparatus was built which can measure small changes in applied stress (σ), better than ±0.5 × 10⁵ Pa, and which incorporates a facility whereby the composition of the test specimen can be fixed through equilibration with a gas atmosphere of controlled composition. Deformation mechanism maps constructed largely on theoretical grounds predict that, under the conditions studied here, high-temperature dislocation creep, for which ?∝σ⁵, gives way on lowering the applied stress to diffusion creep, for which ?∝σ. The experimental results agree in large measure with these theoretical predictions. However, it is not possible in this preliminary study to conclusively identify the dominant deformation mechanisms.     

Analbite → monalbite transition in a heat treated twinned amelia albite

Amelia albite annealed at > 1080 °C for 3200 hrs by Duba and Piwinskii (1974) shows very fine twin lamellae (～1 μm) after the albite law, suggesting that it once underwent transformation into monalbite. A fragment of this specimen was investigated at 27 °C, 300 °C, 550 °C, 800 °C and 930 °C using the high-temperature precession technique. As the temperature increases, the splitting angle of c ^*-axes (likewise c ^*-axes) of two twin individuals continues to decrease. The photographs taken at 930 °C show that these two splitting angles have converged to 0^o, indicating completion of the transformation into monalbite. The transition point we observe supports the results of MacKenzie (1952) (920±20 °C) and Grundy et al. (1967) (930 °C) rather than those of Sueno et al. (1973) and Prewitt et al. (1974) (> 1080 °C); the discrepancy is most likely due to the differences in the degree of Al-Si disorder of the samples used in the experiments.     

The effect of water,pressure, and strain on Al/Si order-disorder kinetics in feldspar

The disordering kinetics of Al/Si in albite depend on how the samples are dried, and thus on the presence of trace amounts of water. The disordering rate increases with water content and confining pressure. At 10 kb the activation energy is about 67 kcal/mole compared to about 87 kcal/mole for samples disordered in air. Simultaneous plastic deformation increases the disordering rate and the effect is most pronounced below 900° C at 10^?6/s. Some albite ordering and microcline disordering experiments show similar kinetic behavior. These results are significant for interpreting the structural state and the high-temperature deformation of feldspars.     

Microstructures documenting Cenozoic extension processes in the northern continental margin of the South China Sea

ABSTRACT

In order to investigate the thinning process of the northern continental margin of the South China Sea, petrographic and microstructural analysis were carried out on 20 greenschistfacies mylonite samples, which were obtained from Site U1504 of IODP Expedition 367/368 in the Outer Margin High of the region. The mineral assemblage of the greenschist-facies mylonite is chlorite + epidotite + albite (Ab = 94.7–99.9) + quartz, which contains 10-30% gravel components. Microstructural analysis indicates that the greenschist-facies mylonite experienced two episodes of deformation：early ductile deformation followed by a later stage of brittle deformatio. Both episodes of deformation suggest an extensional environment. The extensive development of bulging recrystallization (BLG) of quartz, microscopic fractures and fine granulation of albite suggest that the temperature of ductile deformation is about 300-400°C, compatiable with a ductile shearing at shallow crust levels (~5-10 km). Petrographic features suggest that the greenschist-facies mylonite might originate from volcanic sedimentary rocks or sedimentary rocks affected by the intrusion of mafic magma. Combined with seismic interpretation, we propose that the greenschist-facies mylonite might be formed by crustal exhumation after thick Mesozoic sediments were denuded by a major extension.     

Oxygen isotopic fractionation in the system quartz-albite-anorthite-water

Oxygen isotopic fractionations have been determined between quartz and water, albite and water, and anorthite and water at temperatures from 300 to 825°C, and pressures from 1.5. to 25 kbar. The equilibrium quartz-feldspar fractionation curves can be approximated by the following equations: 1000ln α_Q?PI = (0.46 + 0.55β)10⁶T^?2 + (0.02 + 0.85β) between 500 and 800°C 1000ln α_Q?PI = (0.79 + 0.90β)10⁶T^?2 — (0.43 ? 0.30β) between 400 and 500°C where β is the mole-fraction of anorthite in plagioclase.Application of these isotopic thermometer calibrations to literature data on quartz and feldspar gives temperatures for some metamorphic rocks which are concordant with quartz-magnetite temperatures. Plutonic igneous rocks typically have quartz-feldspar fractionations which are substantially larger than the equilibrium values at solidus temperatures, indicating substantial retrograde exchange effects.     

Rheological Properties of Mantle Peridotites at Yushigou in the North Qilian Mountains and Their Implicationsfor Plate Dynamics

This paper discusses in detail the deformation textures, glide system, petrofabrics and olivine dislocation microstructures of mantle peridotites at Yushigou in the North Qilian Mountains, northwestern China. The peridotites have undergone high-pressure, high-temperature and low-strain rate plastic flow deformation. According to the dynamic recrystallized-grain size of olivine and the average spacing between the dislocation walls as well as the chemical composition of enstatite, the authors calculated the rheological parameters of the ancient upper mantle in the study area as follows: temperatures 1025-1093℃; pressures 3043-4278 MPa; depths 95-132 km; deviatoric stress 28-32 MPa; strain rates 0.2×10-14-2.13×10-14s-1 and equivalent viscosities 0.45×1020-4.65×1020 Pa · s. These parameters suggest that the position where plastic flow took place was correspondent to the low-velocity zone beneath the oceanic lithosphere and that oceanization characterized by middle-velocity (1-3 cm/a) sea-floor spreadi