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1.
The crystallography and geometry of high-angle grain boundaries from dynamically recrystallized quartz have been studied. On the basis of combined electron backscatter diffraction and universal stage measurements, the complete crystallographic orientation of the grain boundaries could be calculated. The u-stage rotation of the grain boundaries to a vertical position reveals that they are never curved but always consist of straight segments. Our results show that these segments preferentially occupy rhombohedral, trapezohedral and bipyramidal orientations, i.e., orientations in a  25–50° girdle to the c-axis. A specific, albeit low, number of segments with special crystallographic orientation, with respect to a neighbouring quartz grain, often shows another special orientation with respect to the other neighbouring grain. Preferred combinations of grain boundary orientations related to both neighbouring grains are (i) low-index rhombohedral and high index trapezohedral, (ii) low-index bipyramidal and low-index trapezohedral or high-index rhombohedral, and (iii) low-index trapezohedral and low or high index trapezohedral. In certain cases, such as at triple junctions, the boundaries occupy specific trapezohedral orientations with a constant angle to the c-axis. This argues for energy isotropy of trapezohedral planes with the same angle to the quartz c-axis. In general, good match coincidence site lattice (CSL) orientations are not preferentially occupied so that most of the studied grain boundaries represent general boundaries. The formation of straight segments in special crystallographic orientations indicates the crystallographic control and implies an energy reduction of certain general boundaries.  相似文献   

2.
Grain boundary migration between strained, substructured grains and newly appearing, strain free grains has been observed during static in-situ annealing of pre-deformed rocksalt in the SEM. With increasing temperature (T) the migration velocity increases and the character of grain boundary migration changes. As temperature increases there is an increase in the length of individual migrating boundary segments that move at similar rates. In addition, the frequency of migrating boundaries that form traces of a {100} boundary plane of at least one of the crystals involved increases, and moving grain boundaries between new and old grains change from highly irregular to smooth, straight boundaries. At the same time there is a decrease in the influence of the substructure of pre-existing strained grains on the grain boundary movement. Resultant microstructures reflect these changes. At  325–350 °C, the deformed-then-annealed microstructure is characterized by very irregular grain boundaries, a high abundance of 5–50 m scale remnants of old, substructured grains within new grains, giving a poikilitic microstructure. At  350–400 °C, grain boundaries often exhibit elongate embayments into the strained grains and most remnants of old, strained grains are located at former grain boundaries. At > 400 °C, grain boundaries between new and old, strained grains are straight to smoothly curved.The grain boundary velocity observations are explained by the effect of temperature on mobility coupled with local driving force variations. Additionally, at low annealing temperature, impurity (solute) drag and driving-force variations are influential, while at high temperature the anisotropy in grain boundary energy with crystallographic orientation becomes more important. Transferring the knowledge from our experiments to geological samples enables us to recognize and interpret similar microstructures in rocks, thereby making it is possible to relate microstructural characteristics to the pre-annealing and post-deformational annealing history.  相似文献   

3.
《Journal of Structural Geology》2002,24(6-7):1125-1137
Dynamically recrystallized and sutured quartz grains from metamorphic rocks with different strain intensities and temperature conditions ranging from ca. 350°C to ca. 700°C have been studied. Universal-stage measurements on quartz–quartz high-angle grain boundaries show that they are never curved but always consist of straight segments which preferentially occupy specific crystallographic orientations in relation to both neighboring crystals. With increasing temperature the segments preferentially concentrate in a decreasing number of orientations, mainly near the rhombohedral {101̄1} planes. The crystallographic data and the observations on grain boundary geometries suggest that: (i) grain boundary orientations are strongly crystallographically controlled, (ii) this control is the main factor on the textural equilibration of quartz–quartz grain boundaries in metamorphic rocks, and (iii) grain boundaries from dynamically recrystallized quartz should be regarded as annealed and equilibrated fabrics that are stable against subsequent annealing as long as the material is not re-deformed.  相似文献   

4.
Quartzofeldspathic ultramylonites from the Alpine Fault Zone, one of the world's major, active plate boundary-scale fault zones have quartz crystallographic preferred orientations (CPO) and abundant low-angle (<10° misorientation) boundaries, typical microstructures for dislocation creep-dominated deformation. Geometrically necessary dislocation density estimates indicate mean dislocation densities of ∼109 cm−2. A significant proportion (∼30%) of grain boundaries (>10° misorientation) are decorated by faceted pores, commonly with uniformly-oriented pyramidal shapes. Only grain boundaries with >10° misorientation angles in polymineralic aggregates are decorated by pores. Mean grain boundary pore densities are ∼5 × 108 cm−2. Grain boundary pores are dissolution pits generated during syn-deformational transient grain boundary permeability, nucleating on dislocation traces at dilatant grain boundary interfaces. They have not been removed by subsequent grain boundary closure or annealing. Pore decoration could have led to grain boundary pinning, triggering a switch in the dominant deformation mechanism to grain boundary sliding, which is supported by evidence of CPO destruction in matrix quartz. Pore-decorated grain boundaries have significantly reduced surface area available for adhesion and cohesion, which would reduce the tensile and shear strength of grain boundaries, and hence, the bulk rock. Grain boundary decoration also significantly decreased the mean distance between pores, potentially facilitating dynamic permeability. Consequently, these microstructures provide a new explanation for strain weakening and evidence of fluid flow along grain boundaries in mylonites at mid-crustal conditions.  相似文献   

5.
The mylonitization of the Pankenushi gabbro in the Hidaka metamorphic belt of central Hokkaido, Japan, occurred along its western margin at ≈600 MPa and 660–700 °C through dynamic recrystallization of plagioclase and a retrograde reaction from granulite facies to amphibolite facies (orthopyroxene + clinopyroxene + plagioclase + H2O = hornblende + quartz). The reaction produced a fine-grained (≤100 μm) polymineralic aggregate composed of orthopyroxene, clinopyroxene, quartz, hornblende, biotite and ilmenite, into which strain is localized. The dynamic recrystallization of plagioclase occurred by grain boundary migration, and produced a monomineralic aggregate of grains whose crystallographic orientations are mostly unrelated to those of porphyroclasts. The monomineralic plagioclase aggregates and the fine-grained polymineralic aggregates are interlayered and define the mylonitic foliation, while the latter is also mixed into the former by grain boundary sliding to form a rather homogeneous polymineralic matrix in ultramylonites. However in both mylonite and ultramylonite, plagioclase aggregates form a stress-supporting framework, and therefore controlled the rock rheology. Crystal plastic deformation of pyroxenes and plagioclase with dominant (100)[001] and (001)1/2 slip systems, respectively, produced distinct shape- and crystallographic-preferred orientations of pyroxene porphyroclasts and dynamically recrystallized plagioclase grains in both mylonite and ultramylonite. Euhedral to subhedral growth of hornblende in pyroxene porphyroclast tails during the reaction and its subsequent rigid rotation in the fine-grained polymineralic aggregate or matrix produced clear shape- and crystallographic-preferred orientations of hornblende grains in both mylonite and ultramylonite. In contrast, the dominant grain boundary sliding of pyroxene and quartz grains in the fine-grained polymineralic aggregate of the mylonite resulted in their very weak shape- and crystallographic-preferred orientations. In the fine-grained polymineralic matrix of the ultramylonite, however, pyroxene and quartz grains became scattered and isolated in the plagioclase aggregate so that they were crystal-plastically deformed leading to stronger shape- and crystallographic-preferred orientations than those seen in the mylonite.  相似文献   

6.
The structure of grain boundaries in a granite-origin ultramylonite, composed mainly of fine-grained feldspar and quartz, was studied by high-resolution electron microscopy (HREM). At most of the boundaries, not only between the same minerals but also between different minerals, lattice fringes in adjacent grains meet at the interface with no other appreciable phases. In these boundaries, some of the straight segments correspond to a low-index plane of one of the connected grains. Boundaries containing voids, with a spheroidal shape elongated along the boundaries, were observed only between quartz grains. It is suggested that these boundaries were formed by healing of microcracks. The structural width of major boundaries, deduced from lattice-fringe imaging, is less than about 0.5 nm. Received: 15 September 1998 / Revised, accepted: 8 April 1999  相似文献   

7.
In the present paper the intensity of serration of quartz grain boundary sutures in the Palaeoproterozoic age Malanjkhand granite (Central India) is quantified using the ruler method of fractal analyses. The northern margin of the granite has proximity to the Central Indian Suture (CIS) that forms the southernmost part of Central Indian Tectonic Zone (CITZ) along which amalgamation of the northern and southern Indian shields took place. The fractal dimension (D) of the quartz sutures is calculated in 13 samples collected at varying distance from the CIS. D values are noted to increase in samples towards the CIS. This demonstrates the influence of the tectonic events along the CIS on fabric development in the Malanjkhand granite. Magmatic fabric defined by preferentially oriented feldspar laths and high-T solid-state deformation fabrics are observed in areas distant from the CIS. In contrast, mylonites and low-T fabrics such as bulging quartz grain boundaries occur in proximity to the CIS. It is inferred that the emplacement and high-T fabric development in the Malanjkhand granite was synchronous with regional accretionary processes that occurred in the region during the Palaeoproterozoic. Two different possibilities, one involving a single tectonic event and the other involving multiple tectonic events are discussed to explain the superimposition of low-T over high-T fabric due to which intensity of quartz grain boundary serration increases towards the CIS.  相似文献   

8.
The kinetics of the calcite to aragonite transformation have been investigated using synthetic polycrystalline calcite aggregates, with and without additional minerals present. The reaction progresses as a function of time were measured at four temperature/pressure conditions: (1) 550 °C/1.86 GPa; (2) 600 °C/2.11 GPa; (3) 650 °C/2.11 GPa, and (4) 700 °C/2.29 GPa. Experiments reveal that Mg-calcite and Fe-calcite transforms to aragonite at considerably slower rates than pure calcite, and that Sr-bearing calcite and calcite + quartz aggregates transform at significantly higher rates than pure calcite. The reaction progresses vs. time data for pure calcite were fitted to Cahns grain-boundary nucleation and interface-controlled growth model. Evidence for interface-controlled growth is provided by petrographic observations of grain boundaries. The activation energy for aragonite growth from the synthetic polycrystalline calcite determined in this study is significantly lower than that previously determined from a natural marble. The discrepancy in rates and activation energy may be attributed to the nature of grain boundaries, to deformational strain or the presence of impurities in the studied samples, and likely to uncertainties in experimental conditions. The results of this study imply that the variation of local petrologic conditions, in addition to temperature, pressure and grain size, may play an important role in determining the rates for the calcite to aragonite transformation in nature.Editorial responsibility: W. Schreyer  相似文献   

9.
In polycrystalline aggregates of olivine with mean grain sizes above 35 μm plus a low basaltic melt fraction, both wetted and melt-free grain boundaries are observed after equilibration times at high pressures and temperatures of between 15 and 25 days. In order to assess a possible dependence of the wetting behaviour on the relative orientation of neighbouring grains, a SEM based technique, electron backscatter diffraction (EBSD), is used to determine grain orientations. From the grain orientations relative orientations of neighbouring grains are calculated, which are expressed as misorientation axis/angle pairs. The distribution of misorientation angles and axes of melt-free grain boundaries differ significantly from a purely random distribution, whereas those of wetted grain boundaries are statistically indistinguishable from the random distribution. The relative orientation of two neighbouring grains therefore influences the character of their common grain boundary. However, no clustering towards special (coincident site lattice) misorientation axes is observed, with the inference that the energy differences between special and general misorientations are too small to lead to the development of preferred misorientations during grain growth. Received: 8 December 1997 / Revised, accepted: 6 April 1998  相似文献   

10.
The microstructure of olivine-olivine grain boundaries has been studied in experimentally deformed (1200–1227 °C, 300 MPa) partially molten olivine and olivine-orthopyroxene rocks. In-situ melting produced ∼1 vol% melt in all samples studied. Grain boundary analyses were carried out using a number of transmission electron microscopy techniques. The grain boundary chemistry in undeformed olivine-orthopyroxene starting material showed evidence for the presence of an intergranular phase along some, but not all, of the olivine-olivine boundaries. In the deformed samples, ultrathin Si-rich, Al- and Ca-bearing amorphous films have been observed along all investigated olivine-olivine grain boundaries. The chemistry of the grain boundaries, which is considered to be indicative for the presence of a thin film, was measured with energy-dispersive X-ray spectroscopy (EDX) and energy-filtering imaging. The amorphous nature of the films was confirmed with diffuse dark field imaging, Fresnel fringe imaging, and high-resolution electron microscopy. The films range in thickness from 0.6 to 3.0 nm, and EDX analyses show that the presence of Al and Ca is restricted to this ultrathin film along the grain boundaries. Because thin melt films have been observed in all the samples, they are thought to be stable features of the melt microstructure in deformed partially molten rocks. The transition from the occasional presence of films in the undeformed starting material to the general occurrence of the films in deformed materials suggests that deformation promotes the formation and distribution of the films. Alternatively, hot-pressing may be too short for films to develop along all grain boundaries. A difference in creep strength between the studied samples could not be attributed to grain boundary melt films, as these have been found in all deformed samples. However, a weakening effect of grain boundary melt films on olivine rheology could not be ruled out due to the lack of confirmed melt-film free experiments. Received: 13 April 1999 / Revised, accepted: 10 February 2000  相似文献   

11.
Microstructural modification processes like dynamic recrystallization and grain growth can have a major effect on the transient and (semi-)steady state flow behaviour of deforming materials. Work on metals and ceramics suggests that deformation-enhanced changes in grain topology and the corresponding increase in fraction of non-hexagonal grains, called cellular defect fraction, can promote grain growth during deformation. The present study tests this hypothesis, by investigating the evolution of the cellular defect fraction during deformation, accompanied by grain growth, of aggregates with distributed grain sizes. For this purpose, we made use of the ELLE 2D microstructural modeling package. We simulated and quantified microstructural evolution under conditions where both surface energy driven grain boundary migration (GBM) and homogeneous deformation or grain size sensitive (GSS) straining were allowed to occur. The simulations show that contemporaneous GBM and simple geometrical straining of grain aggregates with distributed grain size and coordination number lead to extra grain neighbor switching, an increase in defect fraction, and enhanced grain growth. An increase in defect fraction was also found in a selected set of natural calcite mylonites that, with increasing temperature, show an increase in grain size and contribution of GSS creep. Analysis of defect fraction thus appears to be a good microstructural tool to establish whether or not a material has experienced normal static (defect fraction  0.7) or dynamic grain growth (defect fraction  0.8).  相似文献   

12.
Summary ¶The lithology, age, geological setting, structural and metamorphic history of the granitic mylonites from the Mylonite Zone (MZ) in southwestern Sweden have been studied extensively. The deformation history, growth of microstructures, intensity of deformation, changes in mineral compositions, and pressure-temperature conditions of deformation have, however, not been addressed. In this study, powder X-ray diffraction, optical microscopy, electron microprobe analysis and transmission electron microscopy of micas, chlorite, and plagioclase are combined to understand the physical and textural changes experienced by the rocks during mylonitization. It is shown that the occurrence of foliated micas in shear bands, recrystallization of quartz and biotite, and undulatory extinction in quartz grains were not uniform throughout the samples studied. Occurrence of dislocations and low-angle grain boundaries confirm that deformation occurred largely by glide dislocations. The low-angle grain boundaries observed are formed by the re-arrangement of these dislocations during grain size reduction processes. The micas show a high degree of spatial stacking order, but spatial stacking disorder in micas and chlorites has also been found.Ordered stacking faults are formed during low strain while disordered stacking faults are formed under high strain conditions. Occurrence of both ordered and disordered stacking faults indicates that the intensity of deformation was not uniform through the entire MZ. Moreover, the chemical composition of plagioclase shows that the exsolution lamellae observed with optical and electron microscopy are due to Ca-subsolidus reactions during low temperature deformation. Several substitution reactions occurring in the micas indicate that deformation took place between 0.3 and 0.4GPa, at a temperature higher than 500°C.Received October 15, 2001; revised version accepted December 25, 2002 Published online June 2, 2003  相似文献   

13.
Experimental annealing of galena samples with known deformation histories shows that this mineral has the necessary properties to be a valuable source of information about low-grade deformational environments. Annealed galena displays recovery and/or recrystallization features dependent upon the type of texture inherited from the tectonic event, which in turn is closely linked to deformation temperatures.In samples deformed at temperatures less than 200 ° C in the laboratory, later annealing produced subgrains, mosaics of new grains, or rapid grain boundary migration as the annealing temperatures were varied from 200 ° C to 700 ° C. Kink bands maintained characteristic straight simple boundaries inherited from the deformation event. Samples deformed above 300 ° C developed syntectonically recrystallized textures. Kink bands had been converted to elongate grains with complex sutured grain boundaries during deformation, and mosaics of new grains were found in highly deformed regions. These textures were extremely stable through later annealing. Despite our changing annealing temperatures through 500 ° C, we did not produce similar textures from both low and high temperature deformation runs.Examination of polished and etched galena from low-grade tectonic settings may well be worth the effort since galena textures appear to display features indicative of deformational evironments, even after being subjected to considerable post-tectonic thermal perturbations.  相似文献   

14.
Detailed electron microscope and microstructural analysis of two ultrahigh temperature felsic granulites from Tonagh Island, Napier Complex, Antarctica show deformation microstructures produced at  1000 °C at 8–10 kbar. High temperature orthopyroxene (Al 7 wt.% and  11 wt.%), exhibits crystallographic preferred orientation (CPO) and frequent subgrain boundaries which point to dislocation creep as the dominating deformation mechanism within opx. Two different main slip systems are observed: in opx bands with exclusively opx grains containing subgrain boundaries with traces parallel to [010] and a strong coupling of low angle misorientations (2.5°–5°) with rotation axes parallel to [010] the dominating slip system is (100)[001]. Isolated opx grains and grain clusters of 2–5 grains embedded in a qtz–fsp matrix show an additional slip system of (010)[001]. The latter slip system is harder to activate. We suggest that differences in the activation of these slip systems is a result of higher differential stresses imposed onto the isolated opx grains and grain clusters. In contrast to opx, large qtz grains (up to 200 μm) show random crystallographic orientation. This together with their elongate and cuspate shape and the lack of systematic in the rotation axes associated with the subgrain boundaries is consistent with diffusion creep as the primary deformation mechanism in quartz.Our first time detailed microstructural observations of ultrahigh temperature and medium to high pressure granulites and their interpretation in terms of active deformation mechanisms give some insight into the type of rheology that can be expect at lower crustal conditions. If qtz is the mineral phase governing the rock rheology, Newtonian flow behaviour is expected and only low differential stress can be supported. However, if the stress supporting mineral phase is opx, the flow law resulting from dislocation creep will govern the rheology of the rock unit; hence, an exponential relationship between stress and strain rate is to be expected.  相似文献   

15.
The four primary modes of microscopic occurrence of uranium in the Katemcy and Streeter granites of central Texas as revealed by fission track mapping are: 1) in accessory minerals; 2) along grain boundaries; 3) in microfractures; and 4) in quartz and feldspars. Most of the uranium occuring in the first three modes, accounting for more than three-fourths of the total, appears to be readily mobilized during weathering.  相似文献   

16.
Abstract The microstructure of quartz in metacherts of the Ryoke metamorphic belt in central Japan develops from polygonal, through duplex to irregular with increasing metamorphic grade. The polygonal microstructure is composed of small (mostly 90–160 μm), equant, equigranular, polygonal quartz grains, whereas the irregular microstructure is characterized by large (>300 μm) grains with irregular grain boundaries. The duplex microstructure is a mixture of small polygonal and large irregular grains. The development of these microstructures is interpreted as being due to secondary recrystallization. The size of polygonal grains is greatly influenced by the presence of second-phase minerals, such as mica, whereas that of large irregular grains is unaffected by second-phase minerals. There seems to be a critical grain size for quartz to occur as polygonal aggregates: no polygonal aggregates occur in rocks with larger than the critical grain size. The size (about 140 μm) decreases slightly with increasing volume fraction of mica. The mean grain sizes of polygonal quartz ( D ) and coexisting mica ( d ) in the duplex microstructure are systematically related to the volume fraction of mica ( f ) by D = 0.728 d (1/ f )0.629.  相似文献   

17.
Many physical properties of rocks are sensitive to grain size and hence to the structure of grain boundaries. Depending on their properties, such as deformation and transport behaviour, boundaries may be divided into two broad types, namely special and general grain boundaries. Electron backscattered diffraction (EBSD) is used to investigate the misorientation distributions of grain boundaries and, more recently, to determine the population of grain boundary planes. Studies on metals and ceramics suggest that the grain boundary plane, rather than the misorientation, is the key parameter when defining special and general grain boundaries. In this study, the distribution of grain boundary plane orientations has been successfully determined using EBSD for a slightly deformed, synthetic NaCl material containing 22 ppm water. Boundaries showed a preference for {100} planes, which occurred with twice the frequency of a random distribution. The grain boundary plane distributions found in NaCl were largely in agreement with studies on MgO. Grain boundaries, with a coincident site lattice (CSL) misorientation, also showed a preference for {100} planes, rather than the planes of high coincident density associated with the CSL. Three main types of boundary were identified, namely {100} twist boundaries, boundaries with {100}{hkl} planes and general {hkl}{hkl} boundaries. As the properties of these three types of boundary differ, then the transport and creep properties in wet NaCl will depend on the fraction of the different boundary types found in the grain boundary population.  相似文献   

18.
The shape, pattern and crystallographic orientation of grain boundaries represent important characteristics of crystalline material and contain information about its deformation and annealing history. The present study includes measurements of grain boundaries from experimentally annealed analogue material as well as natural foam texture of quartz and plagioclase. The main subject is the relation between the development of a foam texture and the crystallographic orientation of its grain boundaries and their geometry. (1) During annealing, grain sizes stabilize at certain values. On a statistical basis, these values can be applied as a geothermometer. (2) On the light-microscope scale, the grain boundaries in foam textures commonly consist of two or several planar facets. They are preferentially oriented along specific crystallographic planes, namely in relation to both neighbouring crystals; for quartz they tend to be rhombohedral. (3) Even highly misoriented facets and dihedral angles largely deviating from the 'equilibrium angle' of 120° may be stable over long periods of annealing. (4) Parts of single boundaries may migrate, whereas other parts are stationary during annealing. The results of the present study suggest that the anisotropy of surface energy has a considerable influence on the development of foam textures and that modelling of texture development should include the influence of the crystallographic orientation of grain boundaries.  相似文献   

19.
Interfacial energies of quartz/quartz (qz/qz), albite/albite (ab/ab), and quartz/albite (qz/ab) boundaries in low-grade pelitic schist were determined based on measured values of dihedral angles. Three kinds of microstructures were investigated, and the interfacial energies were obtained in two independent ways. (1) Relative values of interfacial energy were calculated from dihedral angles formed at quartz and albite triple junctions. (2) Subgrain boundary energy was calculated using the Read-Shockley theory for a boundary connected to an intergranular pore. Dihedral angles formed at the corners of intergranular pores were measured. From the interfacial tension balance equation, the value of the qz/qz grain boundary energy was then obtained. (3) Dihedral angles formed at intersections of either pericline or albite twin boundaries with either ab/ab or qz/ab boundaries were measured. The twin boundary energy was calculated based on a previously derived equation using Landau potential, twin wall thickness, and critical temperature for a phase transition in albite. With a modified interfacial tension balance equation for a twin boundary fixed to a facet orientation, the interfacial energies of ab/ab and qz/ab boundaries were obtained. Energies obtained by methods of (2) and (3) are in good agreement. The interfacial energies for qz/qz, ab/ab, and qz/ab boundaries obtained in this study are 270뀶, 300끞, and 250끀 mJ/m2, respectively.  相似文献   

20.
The electron backscattering diffraction technique (EBSD) was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates, both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains. For the Tonale mylonites we investigated, a temperature range of 300–380 °C, 0.25 GPa confining pressure, a flow stress range of ~ 0.1–0.2 GPa, and a strain rate of ~ 10− 13 s− 1 were estimated. Experimental samples of Black Hills quartzite were analyzed, which had been deformed in axial compression at 700 °C, 1.2–1.5 GPa confining pressure, a flow stress of ~ 0.3–0.4 GPa, a strain rate of ~ 10− 6 s− 1, and to 44% to 73% axial shortening. Using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development. Our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains. Grain boundary bulges are not only formed by local grain boundary migration, but they also display a lattice misorientation indicative of subgrain rotation. Dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation, i.e. with an orientation unsuitable for easy basal slip. In addition, Dauphiné twins within porphyroclasts are preferred sites for recrystallization. We interpret large misorientation angles in the experimental samples, which increase with increasing strain, as formed by the activity of fluid-assisted grain boundary sliding.  相似文献   

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