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1.
Samples of the calcite-rich Shelburne Marble collected at the Pfizer Quarry in Adams, Massachusetts, show an order of magnitude variation in grain size. Calcite grain size ranges from 94 to 1101  μm. Because these calcite marbles share the same pressure, temperature and strain histories, some other factor must be responsible for the grain size variation.
Grain size appears to be controlled by the concentration of impurity or second-phase particles. Large calcite grain size occurs where the volume fraction of second-phase particles is low and grain size decreases as second-phase volume fraction increases. The relationship between calcite grain size ( D ), second-phase grain size ( d ) and second-phase volume fraction (  f  ) can be described by the power law D / d =1.4/ f   0.36, a result that is consistent with models based upon short-term (hours or days) laboratory experiments with metals and ceramics and computer simulations of grain growth. Grain growth appears to be greatly restricted by as little as a few per cent of second-phase particles, with a transition from highly restricted to almost unrestricted grain growth occurring at ≈5% volume of second-phase particles. These results indicate that second-phase particles exercise an important control on grain size and can effectively inhibit grain growth in metamorphic rocks. The behaviour of second-phases in short-term laboratory experiments may closely approximate the behaviour of second-phases in grain growth lasting several orders of magnitude longer in the metamorphic environment.  相似文献   

2.
The microstructural evolution of polymineralic contact metamorphic calcite marbles (Adamello contact aureole) with variable volume fractions of second-phase minerals were quantitatively analyzed in terms of changes in grain size and nearest neighbor relations, as well as the volume fractions, dispersion and occurrences of the second phases as a function of changing metamorphic conditions. In all samples, the calcite grain size is controlled by pinning of grain boundaries by second phases, which can be expressed by the Zener parameter (Z), i.e., the ratio between size and volume fraction of the second phases. With increasing peak metamorphic temperature, both the sizes of matrix grains and second phases increase in dependence on the second-phase volume fraction. Two distinct coarsening trends are revealed: trend I with coupled grain coarsening limited by the growth of the second phases is either characterized by large-sized or a large number of closely spaced-second phase particles, and results finally in a dramatic increase in the calcite grain size with Z. Trend II is manifest by matrix controlled grain growth, which is retarded by the presence of single second-phase particles that are located on calcite grain boundaries. It is supported by grain boundary pinning induced by triple junctions, and the calcite grain size increases moderately with Z. The two different grain coarsening trends manifest the transition between relatively pure polymineralic aggregates (trend II) and microstructures with considerable second-phase volume fractions of up to 0.5. The variations might be of general validity for any polymineralic rock, which undergoes grain coarsening during metamorphism. The new findings are important for a better understanding of the initiation of strain localization based on the activation of grain size dependent deformation mechanisms.  相似文献   

3.
The rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously. The enhanced exchange rate may result from a reduction in grain size (shorter distance for volume diffusion), dissolution and growth of grains by diffusion creep (pressure solution), or the movement of high-angle grain boundaries through strained grains during recrystallization in the dislocation creep regime. The migration of high-angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization. The operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases (An1 and An79) at 900°C, 1 GPa confining pressure, and a strain rate of ∼2x10-6s-1. The polygonal, recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition. At the conditions of these experiments, the volume interdiffusion rate of NaSi/CaAl is too slow to produce any observable chemical change, and microstructural-chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine-grained (2–10 μm) aggregates. These results indicate that strain-induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars, pyroxenes, and amphiboles.  相似文献   

4.
In this study, the chemistry and microstructure of garnet aggregates within a metamorphic vein are investigated. Garnet‐bearing veins in the Sanbagawa metamorphic belt, Japan, occur subparallel to the foliation of a host mafic schist, but some cut the foliation at low angle. Backscattered electron image and compositional mapping using EPMA and crystallographic orientation maps from electron‐backscattered diffraction (EBSD) reveal that numerous small garnet (10–100 μm diameter) coalesce to form large porphyroblasts within the vein. Individual small garnet commonly exhibits xenomorphic shape at garnet/garnet grain boundaries, whereas it is idiomorphic at garnet/quartz boundaries. EBSD microstructural analysis of the garnet porphyroblasts reveals that misorientation angles of neighbour‐pair garnet grains within the vein have a random distribution. This contrasts with previous studies that found coalescence of garnet in mica schist leads to an increased frequency of low angle misorientation boundaries by misorientation‐driven rotation. As garnet nucleated with random orientation, the difference in misorientation between the two studies is due to the difference in the extent of grain rotation. A simple kinetic model that assumes grain rotation of garnet is rate‐limited by grain boundary diffusion creep of matrix quartz, shows that (i) the substantial rotation of a fine garnet grain could occur for the conditions of the Sanbagawa metamorphism, but (ii) the rotation rate drastically decreased as garnet grains formed large clusters during growth. Therefore, the random misorientation distribution of garnet porphyroblasts in the Sanbagawa vein is interpreted as follows: (i) garnet within the vein grew so fast that substantial grain rotation did not occur through porphyroblast formation, and thus (ii) random orientations at the nucleation stage were preserved. The extent of misorientation‐driven rotation indicated by deviation from random orientation distribution may be useful to constrain the growth rate of constituent grains of porphyroblast that formed by multiple nucleation and coalescence.  相似文献   

5.
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6.
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   

7.
Carbonate mylonites with varying proportions of second-phase minerals were collected at positions of increasing metamorphic grade along the basal thrust of the Morcles nappe (Helvetic nappes, Switzerland). Variations of temperature, stress, and strain rate, changes in chemistry of solid and fluid phases, and differing degrees of strain localization and annealing were tracked by measuring the shapes, mean sizes, and size distributions of both matrix and second-phase grains, as well as crystal preferred orientation (CPO) of the matrix. Field structures suggest that strain rate was constant along the fault. The mean and distribution of the calcite grain sizes were affected most profoundly by temperature: Increased temperature, presumably accompanied by decreased stress, correlated with larger mean sizes and wider size distributions. At a given location, the matrix grains in mylonites with more second-phase particles are, on average, smaller, have narrower size distributions, and have more elongate shapes. For example, mylonites with 50 vol.% of second phases have matrix grain sizes half that of pure mylonites. Changes in calcite chemistry and the presence of synkinematic fluids seemed to influence microfabric only weakly. Temporal variations in conditions, such as exhumation-induced cooling, apparently provoke changes in temperature, stress, and strain rate along the nappe. These changes result in further strain localization during retrograde conditions and cause the grain size to be reduced by an additional 50%. The matrix CPO strengthens with increasing temperature or strain, but weakens and rotates with increasing second-phase content. These fabric changes suggest differing rates of grain growth, grain size reduction, and development of CPO owing to variations in the deformation conditions and, perhaps, mechanisms. To interpret natural mylonite structures or to extrapolate mechanical data to natural situations requires careful characterization of the microfabric, and, in particular, second-phase minerals.  相似文献   

8.
Garnet (10 vol.%; pyrope contents 34–44 mol.%) hosted in quartzofeldspathic rocks within a large vertical shear zone of south Madagascar shows a strong grain‐size reduction (from a few cm to ～300 μm). Electron back‐scattered diffraction, transmission electron microscopy and scanning electron microscope imaging coupled with quantitative analysis of digitized images (PolyLX software) have been used in order to understand the deformation mechanisms associated with this grain‐size evolution. The garnet grain‐size reduction trend has been summarized in a typological evolution (from Type I to Type IV). Type I, the original porphyroblasts, form cm‐sized elongated grains that crystallized upon multiple nucleation and coalescence following biotite breakdown: biotite + sillimanite + quartz = garnet + alkali feldspar + rutile + melt. These large garnet grains contain quartz ribbons and sillimanite inclusions. Type I garnet is sheared along preferential planes (sillimanite layers, quartz ribbons and/or suitably oriented garnet crystallographic planes) producing highly elongated Type II garnet grains marked by a single crystallographic orientation. Further deformation leads to the development of a crystallographic misorientation, subgrains and new grains resulting in Type III garnet. Associated grain‐size reduction occurs via subgrain rotation recrystallization accompanied by fast diffusion‐assisted dislocation glide. This plastic deformation of garnet is associated with efficient recovery as shown by the very low dislocation densities (1010 m?3 or lower). The rounded Type III garnet experiences rigid body rotation in fine‐grained matrix. In the highly deformed samples, the deformation mechanisms in garnet are grain‐size‐ and shape‐dependent: dislocation creep is dominant for the few large grains left (>1 mm; Type II garnet), rigid body rotation is typical for the smaller rounded grains (300 μm or less; Type III garnet) whereas diffusion creep may affect more elliptic garnet (Type IV garnet). The P–T conditions of garnet plasticity in the continental crust (≥950 °C; 11 kbar) have been identified using two‐feldspar thermometry and GASP conventional barometry. The garnet microstructural and deformation mechanisms evolution, coupled with grain‐size decrease in a fine‐grained steady‐state microstructure of quartz, alkali feldspar and plagioclase, suggests a separate mechanical evolution of garnet with respect to felsic minerals within the shear zone.  相似文献   

9.
A number of microstructural features indicate a difference in the dominant deformation mechanism between the higher temperature Ryoke and the lower temperature Sambagawa and Shimanto metamorphic belts of Japan. The microstructures of metacherts containing deformed radiolaria are divided into two types: in both the Sambagawa and Shimanto belts the quartz grains are tabular while in the Ryoke belt they are equiaxed. TEM studies of these metacherts revealed that the tabular grains contain abundant subboundaries consisting of large numbers of network dislocations and bowe-out dislocations, while the equiaxed grains contain no subboundaries and have low densities of dislocations which are not bowed-out. There is a corresponding difference in the textures (lattice preferred orientation of quartz): the Ryoke metacherts display randomly distributed c-axes of quartz, while the Sambagawa and Shimanto metacherts show conspicuous crossed girdle patterns with some asymmetry. There is a third difference between these regions: in the metacherts of the Ryoke metamorphic belt, the strain magnitudes determined from deformed radiolaria increase with increasing volume fraction of mica in the same metamorphic P and T conditions, while in the Sambagawa and the Shimanto metamorphic cherts the strain magnitudes decrease with increasing the mica fraction.These microstructures, textures, and rheological behaviours of quartz-mica rocks suggest a change of deformation mechanism between the lower temperature Sambagawa and Shimanto, and the higher temperature Ryoke metamorphic belts. Since random fabrics of c-axes of quartz are inconsistent with lattice rotation due to dislocation glide, the Ryoke metacherts may have deformed by pressure-solution.  相似文献   

10.
Frequency distributions of true interfacial angles in high-grade,regional metamorphic rocks from Broken Hill indicate that interfacialtension varies with relative crystallographic orientation incommon minerals. In some single-phase aggregates (e.g. quartz,feldspar, garnet, and calcite) the grains are equant and polygonal,anisotropy of interfacial tension having only a small effecton the appearance of such aggregates. This is also true of two-phaseaggregates of these phases, but their appearance is complicatedby the presence of inclusions, the relative proportions of ihephases, and the characteristic dihedral angle formed by a grainof one phase in contact with two grains of the other. In phases for which interfacial tension is more orientation-dependent,crystal forms exert some control on the appearance of the microstructure.For example, in hornblende and pyroxene aggregates, some interfacesare parallel to planes of the form {110}, although most areirrational. In still more anisotropic phases, low-energy forms(e.g. {001} of biotite and {110} of sillimanite) are very stable,giving rise to a predominance of planar, rational boundariesover curved, adjustment boundaries. Anisotropy of interfacial tension is expressed in the shapesof inclusions. However, even where strongly anisotropic mineralsare involved, inclusions with partly planar boundaries occuralongside inclusions with completely curved boundaries.  相似文献   

11.
Shape, size and orientation measurements of quartz grains sampled along two transects that cross zones of increasing metamorphic grade in the Otago Schist, New Zealand, reveal the role of quartz in the progressive development of metamorphic foliation. Sedimentary compaction and diagenesis contributed little to the formation of a shape‐preferred orientation (SPO) within the analysed samples. Metamorphic foliation was initiated at sub‐greenschist facies conditions as part of a composite S1‐bedding structure parallel to the axial planes of tight to isoclinal F1 folds. An important component of this foliation is a pronounced quartz SPO that formed dominantly by the effect of dissolution–precipitation creep on detrital grains in association with F1 strain. With increasing grade, the following trends are evident from the SPO data: (i) a progressive increase in the aspect ratio of grains in sections parallel to lineation, and the development of blade‐shaped grains; (ii) the early development of a strong shape preferred orientation so that blade lengths define the linear aspect of the foliation (lineation) and the intermediate axes of the blades define a partial girdle about the lineation; (iii) a slight thinning and reduction in volume of grains in the one transect; and (iv) an actual increase in thickness and volume in the survivor grains of the second transect. The highest‐grade samples, within the chlorite zone of the greenschist facies, record segregation into quartz‐ and mica‐rich layers. This segregation resulted largely from F2 crenulation and marks a key change in the distribution, deformation and SPO of the quartz grains. The contribution of quartz SPO to defining the foliation lessens as the previously discrete and aligned detrital quartz grains are replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and reduced alignment. Pressure solution now affects the margins of quartz‐rich layers rather than individual grains. In higher‐grade samples, therefore, the rock structure is characterized increasingly by segregation layering parallel to a foliation defined predominantly by mica SPO.  相似文献   

12.
Statistical size distribution and scanning electron microscopic studies of suspended sand grains in the Loire River at Montjean, France were carried out over a period of a year (hydrological cycle) to discern seasonal variations. The sand fraction in suspension is better sorted during winter (average mean = 0.69 mm, median = 0.65–0.95 mm, sorting value, σ= 1.1–1.35) and is dominated by quartzo-feldspathic minerals. During summer, on the contrary, this fraction is rich in mica minerals and is poorly sorted (average mean = 1.21 mm, median = 0.33–0.95 mm, sorting value, σ= 1.4–2.0). The grain size of the coarser fraction shows a tendency to increase with river discharge up to an optimum discharge of about 1000 m3 s-1 and thereafter decreases. Scanning electron photomicroscopy of quartz grains from the suspended sand population indicates that both mechanical and chemical features occur. The former (conchoidal fractures, mechanical fractures and breakage) dominate in the samples collected near the water surface during winter floods and the latter (solution pits, vermicular features, silica flowers, neogene silica and diatoms) in the samples from the bottom during low summer flows. During summer, chemical action takes place on sediments prior to or upon their deposition. Sediments that are resuspended during winter floods undergo mainly physical processes (attrition/abrasion). Mixed surface features are, therefore, observed during average and low river discharge. Inheritance of these surface features from the source area is, however, not completely excluded. Thus, the history of quartz grains in suspension can be reconstructed from SEM exoscopic studies: therefore, it can be proposed that the fluvial quartz grain surface textures result from a combination of alternating chemical and physical processes.  相似文献   

13.
Microfabrics of detrital quartz grains and quartz cement of four gold- and uranium-bearing conglomerates of various goldfields of the Witwatersrand Basin, South Africa, were investigated by optical and cathodoluminescence microscopy. The study revealed that the vast majority of quartz grains (<5 mm) originate from felsic magmatic source rocks. Cherts and polymineralic mineral grains, pointing to greenschist to amphibolite facies metasedimentary rocks, follow in abundance, whereas diagenetic to weakly metamorphosed quartzites are subordinate. Material from hydrothermal vein quartz is almost completely lacking, both in the sand and in the pebble fraction. No obvious relationships are discernible between the varying proportions of detrital siliciclastic components of the different reefs and their ore grades. Assuming a sedimentological control of gold distribution, this finding supports multiple sources for the detrital components, which were thoroughly mixed during transport. The post-depositional history of the sediments is characterized by a complex polyphase succession of deformation, cementation and hydrothermal alteration events. Both fragmentation and pressure solution features within detrital quartz, quartz cement and round grains of pyrite, zircon, chromite and uraninite demonstrate that these materials were present in the conglomerates during diagenesis, and, thus, are true detrital grains with abraded, rounded grain morphologies. By analogy, it is assumed that gold is also a detrital component, although most of the gold grains display characteristics of hydrothermal overprinting. During subsequent metamorphism, micro-shear zones are developed, and brittle-ductile crystal-plastic deformation and limited quartz recrystallization occur. Maximum temperatures of about 350 °C were reached on the prograde metamorphic path. Recrystallization and redistribution of detrital siliciclastic and ore minerals took place, and various hydrothermal/metamorphic minerals including chlorite, sericite, pyrophyllite and chloritoid were formed. These redistribution processes involved existing detrital minerals only and were generally isochemical because little evidence exists for the development of a secondary porosity and permeability that would allow major external inputs into the Witwatersrand conglomerates. Most of the gold grains have hydrothermal characteristics, as evidenced by their authigenic, crystalline shapes and their chemical compositions. However, these features are regarded to result from overprinting. Most likely, the gold grains experienced more drastic modifications relative to other ore components because of the ductile and mobile nature of gold. The retrograde metamorphic path is characterized by percolating radioactive fluids at T<300 °C, recorded by radiation damage indicated by cathodoluminescence alteration rims along quartz grain boundaries and microcracks. The degrees of radiation damage observed are proportional to the uranium contents of the conglomerate ores. The collective evidence of our study supports the modified placer model for the genesis of the Witwatersrand ores. This model explains most of the observations on the detrital mineral assemblage and its post-depositional modification elegantly and in a satisfactory manner.  相似文献   

14.
Petrological evidence is provided for anatexis of ultrahigh‐pressure (UHP) metamorphic quartzite in the Sulu orogen. Some feldspar grains exhibit elongated, highly cuspate shapes or occur as interstitial, cuspate phases constituting interconnected networks along grain boundaries. Elongated veinlets composed of plagioclase + quartz ± K‐feldspar also occur in grain boundaries. These features provide compelling evidence for anatexis of the UHP quartzite. Zircon grains from impure quartzite are all metamorphic growth with highly irregular shape. They contain inclusions of coesite, jadeite, rutile and lower pressure minerals, including multiphase solid inclusions that are composed of two or more phases of muscovite, quartz, K‐feldspar and plagioclase. All zircon grains exhibit steep REE patterns, similar U–Pb ages and Hf isotope compositions with a weighted mean of 218 ± 2 Ma. Most grains have similar δ18O values of ?0.6 to 0.1‰, but a few fall in the range ?5.2 to ?4.3‰. Thus, these grains would have grown from anatectic melts at various pressures. Zircon O isotope differences indicate that anatectic melts were derived from different sources with contrasting O isotopes, but similar Hf isotopes, that is, one from the quartzite itself and the other probably from the country‐rock granitic gneiss. Zircon grains from pure quartzite contain relict magmatic cores and significant metamorphic overgrowths. Domains that contain eclogite facies minerals exhibit flat HREE patterns, no Eu anomalies and concordant U–Pb ages of c. 220 Ma. Similar U–Pb ages are also obtained for domains that contain lower pressure minerals and exhibit steep REE patterns and marked negative Eu anomalies. These observations indicate that zircon records subsolidus overgrowth at eclogite facies conditions but suprasolidus growth at lower pressures. Zircon enclosed by garnet gave consistent U–Pb ages of c. 214 Ma. Such garnet is interpreted as a peritectic product of the anatectic reaction that involves felsic minerals and possibly amphibole and titanite. The REE patterns of epidote and titanite also record multistage growth and metasomatism by anatectic melts. Therefore, the anatexis of UHP metamorphic rocks is evident during continental collision in the Triassic.  相似文献   

15.
There are many observations in naturally deformed rocks on the effects of mineral reactions on deformation, but few experimental data. In order to study the effects of chemical disequilibrium on deformation we have investigated the hydration reaction plagioclase + H2OM more albitic plagioclase + zoisite + kyanite + quartz. We utilized fine-grained (2-6 µm) plagioclase aggregates of two compositions (An54 and An60), both dried and with 0.1-0.4 wt% H2O present, in shear deformation experiments at two sets of conditions: 900 °C, 1.0 GPa (in the plagioclase stability field) and 750 °C, 1.5 GPa (in the zoisite stability field). Dry samples and those deformed in the plagioclase stability field underwent homogeneous shearing by dislocation creep, but samples with 0.1 to 0.4 wt% water deformed in the zoisite stability field showed extreme strain localization into very narrow (~1-3 µm) shear bands after low shear strain. In these samples the microstructures of reaction products in the matrix differ from those in the shear bands. In the matrix, large (up to 400 µm) zoisite crystals grew in the direction of finite extension, and relict plagioclase grains are surrounded by rims of recrystallized grains that are more albitic. In the shear bands, the reaction products albitic plagioclase, zoisite, white mica, and traces of kyanite form polyphase aggregates of very fine-grained (<0.1 µm) dislocation-free grains. Most of the sample strain after % ~2 has occurred within the shear bands, within which the dominant deformation mechanism is inferred to be diffusion-accommodated grain boundary sliding (DAGBS). The switch from dislocation creep in dry samples deformed without reaction to DAGBS in reacted samples is associated with a decrease in flow stress from ~800 to <200 MPa. These experiments demonstrate that heterogeneous nucleation driven in part by chemical disequilibrium can produce an extremely fine-grained polyphase assemblage, leading to a switch in deformation mechanism and significant weakening. Thus, localization of deformation in polyphase rocks may occur on any pressure (P),temperature (T)-path where the equilibrium composition of the constituent minerals changes.  相似文献   

16.
This paper describes a microstructural sequence of quartz schists (metamorphosed chert) in the Asemi river region of the Sambagawa metamorphic terrain in central Shikoku, southwest Japan. The Asemi river region is divided into three areas on the basis of characteristics of microstructures of quartz schists observed under the optical microscope: areas I, II and III, in ascending order of metamorphic grade. Microstructures in area I consist of finer, equant, equidimensional and polygonal quartz grains free from internal deformation features. Microstructures in area II are characterized by oblate or elliptical grains with remarkable undulatory extinction surrounded by serrated grain boundaries. Microstructures in area III consist mainly of coarser and equant grains without distinct internal deformation features.The formation conditions of these microstructures are discussed in the light of recent experimental results.  相似文献   

17.
We present microstructural analyses demonstrating how the geometrical distribution and interconnectivity of mica influences quartz crystallographic preferred orientation (CPO) development in naturally deformed rocks. We use a polymineralic (Qtz + Pl + Kfs + Bt + Ms ± Grt ± Tur) mylonite from the Zanskar Shear Zone, a section of the South Tibetan Detachment (NW Himalaya), to demonstrate how quartz CPO intensity decreases from quartz-dominated domains to micaceous domains, independently of whether or not quartz grains are pinned by mica grains. We then use a bimineralic (Qtz + Ms) mylonite from the Main Central Thrust (NW Himalaya) to show how increasing mica grain connectivity is concomitant with a systematic weakening of quartz CPO. Our results draw distinctions between CPO weakening due to: (i) second phase drag, leading to ineffective recovery in quartz; and (ii) increased transmission and localisation of strain between interconnected mica grains. In the latter case, well-connected micaceous layers take up most of the strain, weakening the rock and preventing straining of the stronger quartz matrix. Our findings suggest that rock weakening in quartz-rich crustal rocks is influenced not only by the presence of mica-rich layers but also the degree of mica grain connectivity, which allows for more effective strain localization through the entire rock mass.  相似文献   

18.
付旭东 《沉积学报》2017,35(1):67-74
巴丹吉林沙漠是世界上沙丘最高大的沙漠,其沙源研究对认识沙漠形成、高大沙山发育和防沙治沙工程有重要意义。石英是沙漠沉积物中常见的矿物,其氧同位素值可示踪物源。采集沙漠西北部、东部、东南部高大沙山、丘间低地与湖泊以及雅布赖山前的表层沉积物,测定了样品不同粒级的石英δ18O值。结果表明:①石英δ18O值随粒级减小有增大趋势,同一样品不同粒级石英δ18O值存在较大差异,相同粒级石英δ18O值也有变化。②石英δ18O值介于9.4‰~19.3‰,均值为13.3‰（n=55）;其中沙丘沙的石英δ18O值介于9.5‰~16.6‰,均值为12.9 ‰（n=39）;湖相沉积物的石英δ18O值介于9.4‰~19.3‰,均值为14.2‰（n=16）。③区域内,<16 μm粒级的石英δ18O值与16~64 μm、125~154 μm、200~250 μm、> 300 μm粒级的石英δ18O值都存在显著差异,但200~250 μm与 > 300 μm粒级的石英δ18O值没有显著差异;经区域对比,巴丹吉林沙漠 < 16 μm粒级的石英δ18O值与柴达木盆地沙漠、蒙古戈壁风成沉积物 < 16 μm石英δ18O值无显著差异,但巴丹吉林沙漠16~64 μm粒级的石英δ18O值与蒙古戈壁风成沉积物16~64 μm石英δ18O值存在显著差异;这似乎暗示研究区的细颗粒物质可能是远源的。巴丹吉林沙漠沉积物的石英δ18O值位于火成岩石英、砂岩和变质岩石英δ18O值分布阈值内,受区域地质条件、物源混合、粒级效应等因素的影响,砂粒级的石英δ18O值所指示的母岩成份特征与祁连山区岩石的岩性有较好吻合。  相似文献   

19.
糜棱岩韧性变形发生的应变局部化过程,尤其是多相糜棱岩第二相对基质相变形的影响一直是显微构造研究难点.研究表明糜棱岩借助颗粒边界滑移实现多相混合,形成多矿物相集合体.在多相糜棱岩内,第二相在基质相颗粒边界施加齐纳阻力,牵制基质相颗粒边界的迁移速率,破坏基质相颗粒的动态平衡过程,使基质相颗粒位于古应力计对应的颗粒粒度以下,导致基质相整体的表面积增大,促进扩散交换过程,提高了扩散蠕变,降低了基质相位错蠕变和结晶学优选方位(CPO)形成的效率,使变形机制从颗粒粒径不敏感蠕变机制(GSI)过渡为颗粒粒径敏感蠕变机制(GSS).另外,多相糜棱岩内的第二相具有诱导应变局部化的效应,使塑性应变局部化更为强烈,引起物质强度的变化,进而引起岩石变形过程和岩石圈流变行为的改变.选取秦岭群花岗质糜棱岩进行多相矿物糜棱岩定量化研究,结果显示花岗质糜棱岩伴随着云母含量的增多以及各相混合程度的增大,石英的颗粒粒度明显减小,CPO强度显著降低,基质相显微变形受第二相控制逐渐增强.  相似文献   

20.
南京周家山下蜀黄土石英颗粒特征及其物源意义   总被引：1，自引：1，他引：0  
下蜀黄土的成因和物源一直是学术界研究的热点。对南京周家山下蜀黄土的石英粒度和石英表面微结构进行分析,结果显示:粉砂粒级石英颗粒（5~50 μm）占绝对优势;<20 μm组分平均含量为42.76%,<30 μm组分平均含量为62.98%;粒度分布曲线和累积曲线总体具有颗粒偏细,呈正偏态,分选较差,峰形尖锐,双峰曲线不对称的特征;粒级-标准偏差曲线呈“双峰”分布,两个明显的标准偏差峰值分别出现在7.962 1 μm和39.905 2 μm。石英颗粒表面形态主要以次棱角状为主;表面机械结构具有丰富的蝶形坑、曲脊、贝壳状断口,部分表面出现平行节理面、V形坑;不同粒级组分表面形态和机械结构特征存在差异。分析表明,南京周家山下蜀黄土属典型风成成因堆积物,是多源区物质高度混合搬运堆积的结果。  相似文献   

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