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1.
A series of experiments is described in which layered specimens were shortened parallel to the layering. The specimens comprise two salt (NaCl) layers sandwiched between three layers of salt–mica synthetic schist. All specimens were prepared and deformed under the same conditions, except for the amount of shortening, which was varied. The resulting fold-shapes are variable, even where the amounts of shortening are the same. In one specimen, folds are believed to have developed essentially by buckling with very little concomitant bulk homogeneous shortening perpendicular to the axial-plane. The specimen lacks an axial-plane foliation. Other folds are believed to have experienced varying amounts of bulk homogeneous shortening before and during buckling, and all have axial-plane foliations that have developed by grain-scale transposition of the original bedding-parallel mica foliation. The difference in the behaviour of the various specimens is explained in terms of initial perturbations. These irregularities take the form of initial deflections in the almost planar bedding, variation in the degree of preferred orientation of the mica grains, and local compositional variation within individual salt–mica layers.  相似文献   

2.
Large biotite grains, up to 1 metre across, in a pegmatite near Broken Hill, Australia, have been heterogeneously deformed and partly recrystallized in zones of relatively high strain, probably under conditions of the lower amphibolite facies. Many of the new aggregates (which consist mainly of more magnesian biotite, with muscovite, and less abundant ilmenite and albite) have a mica preferred orientation. Some of the oriented mica aggregates have grown in kink-like deformation zones (some of which appear to have involved fracturing) and others have grown in dilatation zones (growth of mica probably keeping pace with the opening of the zones). The shapes and preferred orientation of new mica grains appear to be due to varying contributions by (a) mechanical rotation of slices cleaved parallel to (001), (b) coaxial nucleation and/or growth of new mica on rotated portions of deformed biotite, and (c) preferred nucleation and/or growth of new grains in directions of minimum mechanical constraint and maximum transport of chemical components. This preferred growth can also explain the observed high degree of elongation of the oriented mica grains.  相似文献   

3.
Thin sheets of composite ice-mica have been deformed in order to simulate the development of cleavages in quartz-mica rocks. A strong initial mica preferred orientation was variably oriented to the shortening direction. Deformation parallel to the foliation results in a crenulation type cleavage developing from shear bands initiated after a component of pure shear. Deformation oblique to the foliation produces a differentiated cleavage and involves a large component of shear strain subparallel to the original anisotropy. The strain is accommodated by intra- and intercrystalline processes that produce extensive grain elongation and rearrangement of the ductile matrix, thereby forming ice vs mica rich regions. On the other hand, there is no drastic morphological change when a sample is shortened perpendicular to an original foliation: that is, where the micas lie in the plane of no shear strain. Instead, the mica fabric is strengthened and the grains in the ductile matrix are flattened.Two models are presented for the initiation, propagation and evolution of the observed crenulation versus differentiated cleavage types. These depend on mica stacking and orientation relative to the transverse properties of the sample and also on the direction of anisotropy to the XY plane of the bulk strain ellipsoid. The models invoke shear on planes of high shear strain and rotation of the shear bands and rigid mica grains into a direction approximately parallel to the bulk extension direction.  相似文献   

4.
Phlogopite micas from nodules in South African kimberlites were analyzed for major elements with the electron microprobe and for volatile contents by high temperature mass spectrometry. The micas are from primary- (deformed) and secondary- (undeformed) textured grains in perodotite xenoliths, glimmerites, MARID (mica-amphibole-rutile-ilmenite-diopside) suite nodules and a mica megacryst. The major element and volatile contents of micas exhibiting these modes of occurrence overlap to a greater extent than indicated in previous studies. Concentrations of volatile species occupying structurally defined crystallographic sites (H2O, F, Cl) are greater for many of the micas than predicted on the basis of the mica formula, particularly for the glimmerite and MARID suite samples. A correlation exists between micas with tetrahedral and octahedral cation deficiencies and those with excess H2O, F and Cl. Substitution of H+ for tetrahedral and possibly octahedral cations may be responsible for the excess H2O in these micas. Except for one sample, the major element and volatile data for the peridotite, glimmerite and MARID suite micas indicate that they crystallized at oxygen fugacities below the quartz-fayalite-magnetite buffer. F and K2O are in the correct proportion in the micas to provide the source for these elements in alkali basalts, but not in mid-ocean ridge basalts. Kaersutite amphibole is a more likely source of potassium and fluorine in mid-ocean ridge basalts.  相似文献   

5.
In situ observations of polycrystalline ice deformed in simple shear between −10 and −1°C are presented. This study illustrates the processes responsible for the deformation, the development of a preferred crystallographic orientation and the formation of a preferred dimensional orientation. Intracrystalline glide on the basal plane, accompanying grain rotations and dynamic recrystallization, helps to accommodate the large intragranular strains. These are the most important mechanisms for crystallographic reorientation and produce a stable fabric that favours glide on the basal plane. Localized kinks, developed in grains unfavourably oriented for easy glide, are unstable and are overprinted by dynamic recrystallization. Dynamic recrystallization is a strain softening process with nucleation occurring in the form of equiaxed grains that grow subparallel to pre-existing grain anisotropies and become elongate during deformation. Plots of grain axial ratio against orientation ( ) indicate a weak shape fabric which does not correspond to the theoretical foliation and elongation for the appropriate increment of shear strain. We argue that estimates of the strain magnitude made from orientation of elongate grains are unreliable in high temperature shear zones. These results are applicable to both geological and glacial shear environments.  相似文献   

6.
Within the ‘glimmerite’ nodules occurring within kimberlite pipes we recognize the MARID suite consisting of varying proportions of mica, amphibole, rutile, ilmenite and diopside. Banding of some specimens is interpreted as cumulate layering. All specimens were deformed either before incorporation into the host kimberlite or during intrusion. Compared with minerals in peridotite xenoliths, the MARID ones are lower in Al2O3 and Cr2O3, but richer in total iron. The MARID micas, amphiboles, diopsides, ilmenites and probably rutiles contain substantial Fe2O3 indicative of oxidizing conditions. The amphibole is potassic richterite. Micas of the megacryst suite in kimberlite have less total iron and Fe2O3 than micas of the MARID suite. We suggest that the rocks of the MARID suite crystallized under oxidizing conditions from a magma, chemically similar to kimberlite, within the higher parts of the upper mantle: the presence of amphibole restricts the depth to less than ~ 100 km. A xenolith containing olivine and orthopyroxene as well as minerals similar to but not the same compositionally as MARID-types is interpreted as a metasomite, possibly representing wall-rock of a magma body from which MARID-suite rocks crystallized.  相似文献   

7.
The Moine nappe mylonites of the area of Loch Eriboll are inferred to have deformed predominantly by simple shear. Samples were taken from each limb of a late recumbent fold and the quartz crystallographic preferred orientation and optical microstructure were investigated. The pattern of preferred orientation was found to be related to the local folded orientation of the foliation and stretching lineation for the overturned limb and to the inferred imposed deformation for the normal limb. The mechanical development of the fold was modelled as a similar fold forming in a homogeneous mechanically anisotropic material with linear viscous rheology. A very high mechanical anisotropy was required for active amplification of small initial perturbations under simple shear applied at 10 ° to the initial enveloping surface. The high anisotropy also constrained the local kinematics to approximate simple shear parallel to the plane of easy shear, which contributes to the maintenance of the preferred orientation patterns relative to the local macroscopic fabric elements. The degree of mechanical anisotropy calculated from the preferred orientation assuming the operation of the common quartz slip systems is lower than that required for mechanical amplification of the folds. Consequently, it is suggested that the mechanical anisotropy derives from sliding on preferentially aligned grain boundaries.  相似文献   

8.
藏东及邻区钾玄岩系岩石云母特征及其岩石学意义   总被引：3，自引：3，他引：3  
青藏高原东部及邻区新生代钾玄岩系列岩石包括钾质碱性深成岩、火山岩、煌斑岩和偏酸性的斑岩。云母是钾玄岩系列岩石中的主要暗色矿物，分布亦较广泛。我们用单矿物化学分析方法测定本区钾玄武岩系四种岩石中云母和化学成分，其结果MF＝1．30－1．99，应为金云母－富镁黑云母，属于富碱富镁高铝贫铁类型的云母；云母的红外光谱，在450cm^-1和1000cm^-1附近都有很强的吸收谱带，显示了四种岩石中的云母具有类似的红外光谱特征；X光衍射测定，本区云母的d(060)为15．32nm-15.41nm，它们的晶胞参数变化不大，均为三八面体1M型云母。以上特征表明，本区钾玄岩系四种岩石中云母均形成于高碱度环境，它们与钙碱性系列岩石中云母形成的环境迥然不同。从四种岩石的云母具有相似的化学成分、红外光谱和晶胞参数，说明它们的寄生岩石具有钾岩的特征，其物源来自交代地幔。  相似文献   

9.
A multilayered salt/mica specimen with embedded strain markers was shortened to produce a fold and the distribution of strain was subsequently mapped out over the profile plane. On a fine scale the initial foliation, which is parallel to the undeformed layers, is folded by tight kinks to produce two new foliations; one is defined by the preferred orientation of kink boundaries and the other by the preferred orientation of (001) of mica. In the hinge region of the fold the first of these new foliations is parallel to the local λ1λ2-principal plane of strain whereas the preferred orientation of mica is bimodal and is symmetrical about the λ1λ2-plane. Elsewhere the two new foliations are not parallel to the principal plane of strain and angular divergencies of up to 30–35° are measured. If a March model with initial random mica orientation is assumed for the development of mica preferred orientation then the correct value of strain is predicted but the orientation of the principal plane of strain can be grossly in error. A theoretical analysis of the angular relationships to be expected between kink boundaries and the λ1λ2-plane of strain confirms that for the type of geometries experimentally developed, large divergences of up to 35° should be common. In rocks where the foliation has developed by processes similar to those recorded here, large angular divergencies between the foliation and the λ1λ2-principal plane of strain should be expected as the rule.  相似文献   

10.
The 3D shape, size and orientation data for white mica grains sampled along two transects of increasing metamorphic grade in the Otago Schist, New Zealand, reveal that metamorphic foliation, as defined by mica shape‐preferred orientation (SPO), developed rapidly at sub‐greenschist facies conditions early in the deformation history. The onset of penetrative strain metamorphism is marked by the rapid elimination of poorly oriented large clastic mica in favour of numerous new smaller grains of contrasting composition, higher aspect ratios and a strong preferred orientation. The metamorphic mica is blade shaped with long axes defining the linear aspect of the foliation and intermediate axes a partial girdle about the lineation. Once initiated, foliation progressively intensified by an increase in the aspect ratio, size and alignment of grains, although highest grade samples within the chlorite zone record a decrease in aspect ratio and reduction in SPO strength despite continued increase in grain size. These trends are interpreted in terms of progressive competitive anisotropic growth of blade‐shaped grains so that the fastest growth directions and blade lengths tend to parallel the extension direction during deformation. The competitive nature of mica growth is indicated by the progressive increase in size and resultant decrease in number of metamorphic mica with increasing grade, from c. 1000 relatively small mica grains per square millimetre of thin section at lower grades, to c. 100 relatively large grains per square millimetre in higher grade samples. Reversal of SPO intensity and grain aspect ratio trends in higher grade samples may reflect a reduction in the strain rate or reduction in the deviatoric component of the stress field.  相似文献   

11.
Anomalously large chemical ranges in muscovite-paragonite and muscovite-celadonite systems are observed in white micas from the Piemonte calcschists in the Chisone valley area, internal western Alps. The petrographical and chemical observations on white mica strongly suggest that most mica crystals with high Na/K ratios in the chlorite zone are of detrital origin, and were derived from the pre-Alpine high-temperature metamorphic sequence such the Caledonian and/or Variscan. Submicroscopic muscovite (Ms) - paragonite (Pg) composite aggregates occur in the chlorite zone and their EPMA analyses give an apparent chemical composition range from Ms0.6Pg0.4 to Ms0.2Pg0.8. In the rutile zone, the paragonite content of the white micas is less than 20%, suggesting that the white micas have been homogenized during the Alpine metamorphism even if detrital white micas existed.Metamorphic mica is also very heterogeneous. The total range in Si content becomes wider with increasing of metamorphic grade: 3.22–3.39 pfu for the chlorite zone, 3.07–3.45 pfu for the chloritoid zone and 3.06–3.59 pfu for the rutile zone. This clearly indicates that the micas have experienced significant retrogressive chemical reactions during cooling and exhumations of the host schists.The detrital white mica in the chlorite zone has not reset well in its K-Ar system during the Alpine subduction-related metamorphism. The wide range of the white mica K-Ar ages from 115 to 41 Ma must be due to a mixture of various amounts of detrital white mica in the separates. This feature is also observed in the chloritoid zone though the age variation is not so large as that in the chlorite zone. In contrast, the mica in the rutile zone, which was higher than 450°C, has been reset completely during Alpine HP metamorphism.  相似文献   

12.
Compositional variation of phlogopitic micas in upper mantle peridotites is reviewed. Phlogopitic micas in garnet peridotites are systematically lower in Al (or eastonite component) than those in spinel peridotites. The core of phlogopite megacryst and phenocryst of kimberlite is always lower in Al than the rim. It is apparent that Al/(Al + Si) ratio or eastonite component in phlogopitic micas in ultramafic rocks is controlled by the equilibrium pressure and temperature. In the upper mantle peridotites containing garnet or spinel, the Al/(Al + Si) ratio of phlogopitic mica decreases with increasing pressure at constant temperature. Phlogopitic mica is a potential thermo-barometer in both garnet- and spinel-peridotite facies.  相似文献   

13.
The orientation of the optical indicating surface of vitrinite in reflected light has been determined following deformation at 350 and 500°C, confining pressures of 500 and 800 MPa and a strain rate of 10−5 s−1. High temperature and large strain have facilitated reorientation of the indicating surface, increase in anisotropy (bireflectance) and an increase in maximum vitrinite reflectance. In a specimen deformed at 500°C and 23% axial strain the maximum vitrinite reflectance has been reoriented more than 70° from close to parallel to σ1 in the undeformed state to perpendicular to σ1 following deformation. Orientation of the optical indicating surface of some of the deformed specimens suggests the orientation of the maximum reflectance is a composite product of the original orientation of the indicating surface and an orientation produced during deformation.  相似文献   

14.
Zusammenfassung Es wurden 28 Glimmerproben und 6 Proben anderer mit ihnen assoziierter Minerale aus westmährischen Skarnen chemisch untersucht. Die Glimmerproben stammten aus Skarngesteinen (Mg-Skarne, Amphibol-Biotit-und Cummingtonit-Biotit-Skarne, Almandin-Biotit-Schiefer der Randzonen der Skarnkörper) sowie aus Gneisen der Skarnhüllen und aus Pegmatiten, die die Skarne durchsetzen. Die Mg-Skarne enthalten Phlogopit, die anderen untersuchten Gesteine vorwiegend Siderophyllit-Eastonit. Almandinarme Assoziationen, wie Mg-Skarne und Cummingtonit-Skarne, enthalten aluminiumarme Glimmer, während sich die mit Al übersättigten Gesteine (mit Sillimanit oder Spinell) durch aluminiumreiche Glimmer auszeichnen. Der empfindlichste Indikator, der auf die Assoziation, aus welcher der Glimmer stammt, schließen läßt, ist Titan. Die größte Variabilität unter den Hauptkomponenten der Glimmer weist Fluor auf, und zwar sogar in Glimmern aus derselben Assoziation. Die wichtigste Assoziation mit Biotitführung in westmährischen präkambrischen Skarnen, nämlich die Almandin-Biotit-Schiefer der Ränder der Skarnkörper, entstand durch Austauschreaktionen während der Regionalmetamorphose aus Glimmerschiefern und tonerdereichen glimmerschieferähnlichen Gesteinen, unter Zufuhr von Fe, Mg und Mn aus den Skarnen. Die Glimmer westmährischer Skarngesteine werden mit Skarnglimmern anderer präkambrischer Gebiete der Welt verglichen.
Micas of the metamorphic skarns in Western Moravia
Summary The author has investigated 28 mica specimens and 6 specimens of minerals associated with micas which were all recovered from West-Moravian Precambrian skarns, from their host rocks as well as from pegmatites which intersect the skarns. The magnesium skarns contain phlogopite, whereas the other skarn types, such as amphibole-biotite skarns, cummingtonite-biotite skarns, and almandite-biotite schists, as well as gneisses and pegmatites, contain siderophyllites-eastonites. The constituent micas of assemblages poor in aluminium, such as the magnesium skarns and cummingtonite skarns, are low in aluminium, whereas those associated in rocks with sillimanite and spinel are, on the other hand, rich in aluminium. Titanium is among all the main mica components most sensitive to the character of the assemblage, from which the mica was recovered. Fluorine, again, displays a relatively very broad range of concentrations, even in mica samples originating from the same assemblage. The most important assemblage containing micas are the almandite-biotite schists bordering those skarn bodies, which are situated in aluminium-rich mica schists. They developed in the course of regional metamorphism at the expense of mica schists by the action of iron, magnesium and manganese imported from the skarn. The micas of the West-Moravian skarns are compared with the micas of other Precambrian regions of the world.

Mit 11 Abbildungen  相似文献   

15.
The degree of preferred orientation of clay minerals in a series of fifteen fine-grained sediments has been precisely measured by transmitted X-ray goniometry. This varies systematically with quartz content. By contrast it is apparently unrelated to fissility since the one fissile sample (a ‘paper shale’) occurs at the mid-point of the sample series ranked in order of degree of preferred orientation. In this series fissility corresponds with the presence of fine-scale lamination. Estimates of compaction strain using the theory of March (1932) and assuming random initial clay orientation are consistent with the view that these samples started life with porosities similar to those of present-day muddy sediments. On this basis it is concluded that preferred orientation in clay-rich sediments results almost entirely from compaction strain. The degree to which this can be attained is limited by the presence of nonplaty particles (such as quartz grains) which prevent planar fabric development in their immediate vicinity.  相似文献   

16.
An optical comparison illustrates the difference in behaviour of the two mica minerals biotite and muscovite; their response to deformation, and to chemical processes such as grain dissolution. Non-passive mechanical rotation, segmentation of deformed grains by a recovery-recrystallization type process and syntectonic growth of the phyllosilicates all contribute to the development of a strong tectonic foliation within a deformed pegmatite from the Italian Alps. There are significant mechanical differences between the two micas. Biotite readily deforms by kinking whereas muscovite forms sinusoidal folds and seldom kinks. If kink-like structures (deformation zones) are present in muscovite they are generally accompanied by fracturing. Fracturing and displacements are obvious in most deformed muscovites both parallel to axial surfaces and between (001) cleavages. Fracturing is occasionally recognised in biotite. However, it is often obscured by extensive dissolution and new grain nucleation. Evidence for dissolution processes in biotite is more abundant than in muscovite.  相似文献   

17.
After compressive axialsymmetric deformation galena, halite, fluorite, sphalerite and magnetite show preferred orientation of the lattice. In all minerals the 〈110〉 direction is aligned with the axis of compression, but halite shows a second component with 〈100〉 parallel to the axis of compression. In extension tests of halite the 〈111〉 direction is aligned with the axis of the specimen. All these preferred orientations are consistent with the Taylor-theory of preferred orientation for axialsymmetric flow of metals. In order to make use of the extended and improved versions of the Taylor-theory it is necessary to get more data on the behaviour of single crystals during deformation and to have complete quantitative polefigures of deformed mineral specimens which permit the calculaton of the orientation distribution function and inverse polefigures.  相似文献   

18.
Fine grained dolomite has been deformed in over twenty compression experiments in a Griggs-type piston-cylinder apparatus at various P and T conditions. Preferred orientation determined quantitatively using X-ray techniques and spherical harmonic analysis of the data is presented in inverse pole-figures of — 2/m symmetry. In most cases specimens display strong preferred orientation which varies mainly as a function of temperature. At all conditions it is very different from calcitic limestone. Although there is no significant grain growth even at 1000 °C the simple c-axes maximum fabric above 700 °C might be the result of recrystallization or translation on c. Below 700 °C, the preferred orientation is much weaker and complex. The primary maximum in the inverse pole-figure is near e, a secondary maximum near a high angle positive rhomb, principal minima are at c and f. This inverse pole-figure is consistent with f-twinning and translation gliding on r (t = a ?), two mechanisms which counteract each other. The latter is a new deformation mechanism for dolomite which we propose in order to explain the pattern of preferred orientation. The minimum at c is less pronounced below 100 ° C suggesting that c-translation may be active, but in these fine-grained aggregates it appears to be less important than is expected from single crystal experiments (Higgs and Handin, 1959), at least at low temperatures.  相似文献   

19.
This study focuses on the nature of giant micas occurring at the contact between the ?zvatan (foid-bearing) syenites and the metamorphic basement in Central Anatolia. The studied micas are dark greenish-black in color and crystallized within vein shape like bodies as a narrow lens. The origin and processes responsible for the formation of these independent crystals of the giant micas were investigated by mineralogical, petrographical and geochemical analyses with the use of Confocal Raman Spectroscopy (CRS), Fourier Transform Infrared (FTIR) Spectroscopy, X-Ray Diffraction (XRD), Polarized Energy Dispersive X-Ray Fluorescence Spectrometer (PED-XRF) and Electron Probe Micro Analysis (EPMA). According to XRD, CRS, FTIR and EPMA data, the giant micas are phlogopite. EPMA results reveal that studied mica minerals represent the products of re-equilibrated primary mica characterized by high MgO and FeO and low Al2O3 and TiO2 contents. The trace element concentrations of the giant micas display similar patterns with the upper crust. The giant micas are crystallized within small cubicles from an alkaline magma and their composition is possibly modified by a mixing event between the crust- and mantle-derived magmas and contaminated at varying extent by the basement metamorphic rocks.  相似文献   

20.
The regional development of distinct patterns of preferred orientation of quartz c-axes in the Saxony Granulites has been well documented in the literature. A suite of specimens representative of these fabrics has been examined both by optical universal stage, to determine quartz c-axis orientation, and by X-ray diffraction, to obtain orientation data from r, z, m and a. The data are combined to yield inverse pole figures of schistosity and lineation.The finite strain of the Saxony Granulites is thought to be essentially a flattening and there is no evidence that the deformation path is other than one of continuous flattening. Elongation in the plane of the schistosity is local and not extreme. Because of this apparently simple deformation picture, and because preliminary transmission electron microscopy reveals the presence of dislocation structures similar to those found in deformed metals, an attempt is made to interpret the quartz orientation in terms of dislocation slip mechanisms. There is some evidence that the activation of different mechanisms is perhaps primarily controlled by temperature. At least some of the patterns of preferred orientation of quartz were probably produced by deformation in the field of stability of α-quartz.  相似文献   

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