首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Two types of garnet porphyroblast occur in the Schneeberg Complex of the Italian Alps. Type 1 porphyroblasts form ellipsoidal pods with a centre consisting of unstrained quartz, decussate mica and small garnet grains, and a margin containing large garnet grains. Orientation contrast imaging using the scanning electron microscope shows that the larger marginal garnet grains comprise a number of orientation subdomains. Individual garnet grains without subdomains are small (< 50 µm), faceted and idioblastic, and have simple zoning profiles with Ca‐rich cores and Ca‐poor rims. Subdomains of larger garnet grains are similar in size to the individual, small garnet grains. Type 2 porphyroblasts comprise only ellipsoidal garnet, with small subdomains in the centre and larger subdomains at the margin. Each subdomain has its own Ca high, Ca dropping towards subdomain boundaries. Garnet grains, with or without subdomains, all have the same Ca‐poor composition at rims in contact with other minerals. The compositional zonation patterns are best explained by simultaneous, multiple nucleation, followed by growth and amalgamation of individual garnet grains. The range of individual garnet and garnet subdomain sizes can be explained by a faster growth rate at the porphyroblast margin than in the centre. The difference between Type 1 and Type 2 porphyroblasts is probably related to the growth rate differential across the porphyroblast. Electron backscatter diffraction shows that small, individual garnet grains are randomly oriented. Large marginal garnet grains and subdomain‐bearing garnet grains have a strong preferred orientation, clustering around a single garnet orientation. Misorientations across subdomain boundaries are small and misorientation axes are randomly oriented with respect to crystallographic orientations. The only explanation that fits the observational data is that individual garnet grains rotated towards coincident orientations once they came into contact with each other. This process was driven by the reduction of subdomain boundary energy associated with misorientation loss. Rotation of garnet grains was accommodated by diffusion in the subdomain boundary and diffusional creep and rigid body rotation of other minerals (quartz and mica) around the garnet. An analytical model, in which the kinetics of garnet rotation are controlled by the rheology of surrounding quartz, suggests that, at the conditions of metamorphism, the rotation required to give a strong preferred orientation can occur on a similar time‐scale to that of porphyroblast growth.  相似文献   

2.
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   

3.
Microstructures indicating incongruent dissolution precipitation creep of garnet in eclogite-facies graphitic micaschist (Tauern window, Eastern Alps) are investigated. Garnet dissolution is observed where garnet poikiloblasts grown at eclogite facies metamorphism approached each other as a consequence of progressive deformation during exhumation, with estimated P-T-conditions between 570 °C, 1.7 GPa and 470 °C, 0.9 GPa. The poikiloblasts are separated by a dissolution seam and flanked by strain shadows filled with quartz, white mica, and chlorite; there is no evidence for crystal plastic deformation of garnet. Two cases are investigated: (A) stylolitic contact zone, (B) smooth contact zone. In both cases, internal fabrics of the poikiloblasts and concentric chemical zoning are truncated. Material previously forming inclusions in the garnet poikiloblasts is now passively enriched in a dissolution seam, the original microstructure of fine-grained mica–graphite aggregates remaining preserved. Though microstructures suggest that garnet dissolution was driven by local stress concentration, the level of differential stress remained too low for plastic deformation of the fine-grained white mica-graphite aggregates set free from the stress supporting garnet. Incongruent dissolution precipitation creep appears to be a particularly effective deformation mechanism at low stress in a subduction channel.  相似文献   

4.
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   

5.
Porphyroblastic garnet schists from northern Samos contain in their matrix the assemblage Ca‐rich garnet + phengite + paragonite ± chloritoid equilibrated at ～530 °C and ～19 kbar during early Tertiary metamorphism. These high‐pressure/low‐temperature (HP‐LT) metapelitic rocks also exhibit mineralogical and microstructural evidence of an older, higher temperature metamorphism. Large, centimetre‐sized Fe‐rich garnet showing growth zoning developed discontinuous, <0.5 mm thick, Ca‐rich and Mn‐poor overgrowths, compositionally matching small (<1 mm) high‐P matrix garnet. Because the discontinuous garnet rims are in textural and chemical equilibrium with Alpine high‐P minerals, the central parts of the garnet porphyroblasts were found to have formed prior to the Tertiary metamorphism. This is supported by electron microprobe U‐Th‐Pb dating of monazite inclusions yielding partly reset Variscan ages between 360 and 160 Ma. Monazite‐xenotime and garnet‐muscovite thermometry applied to inclusions in the pre‐Alpine garnet yielded temperatures of 600–625 °C (at 3–8 kbar). Prismatic Al‐rich pseudomorphs, possibly after kyanite/sillimanite, and inclusions in garnet composed of white K‐Na mica + quartz ± albite ± K feldspar, interpreted as possible replacements of an intermediate K‐Na feldspar, further support Variscan amphibolite facies conditions. The Samos metapelites thus experienced higher temperatures during the Variscan than during Alpine metamorphism. Diffusional relaxation was very limited between pre‐Alpine garnet and Alpine garnet; both were filled with Alpine garnet along overgrowths and fractures. Fluid‐mediated intergranular element transport, enhanced by deformation, appears crucial in transforming the Variscan garnet into a grossular richer composition during Alpine subduction‐zone metamorphism. At such conditions, dissolution–reprecipitation appears to be a much more effective mechanism for modifying garnet compositions than diffusion. Amphibolite facies conditions are typical for Variscan basement relics exposed in central Cycladic and Dodecanese islands as well as in eastern Crete. The Samos metapelites studied comprise a north‐eastern extension of these basement occurrences.  相似文献   

6.
Within a mica schist from the coesite-bearing Brossasco-Isasca Unit (Western Alps), microstructural analysis shows that Alpine garnet grains are aligned with the crenulated foliation. Garnet crystallographic orientation was analysed with electron backscatter diffraction (EBSD): the obtained crystallographic dispersion patterns and distribution patterns of misorientation axes suggest a strong parallelism of {110} garnet planes with a 56°W-dipping foliation. The data are interpreted as evidence for an epitaxial growth of garnet upon (001) biotite planes, sometime during and/or after dispersion of the biotite/garnet crystals from their initially foliation-parallel orientation by rotation about the Alpine crenulation axis. This interpretation is based on the comparison of the measured EBSD data with: (i) theoretical dispersion trajectories of garnet crystallographic data, (ii) numerically modelled pole figures, and (iii) numerically modelled misorientation axis distribution patterns. Our data suggest that epitaxial growth of garnet upon biotite is allowed by distortion of the pseudohexagonal basal oxygen ring structure on (001) biotite surfaces, and that distortion is driven by introduction of missing ions. Our data further suggest that the spatial distribution of precursor phases influences the distribution patterns of garnet within mica schists.  相似文献   

7.
冀磊  刘福来  王舫  田忠华 《岩石学报》2021,37(2):513-529
石榴夕线片麻岩是中、下地壳主要组成岩石之一,岩石内石榴石和夕线石的结晶学优选方位会显著影响地壳深部流变性质,因此探讨特征变质矿物的变形机制和主要受控因素对构造带深部演化过程有深远意义。本文选取红河-哀牢山韧性剪切带内石榴夕线片麻岩为研究对象,通过定向切片内显微构造、电子探针、X-ray成分扫描、电子背散射衍射(EBSD)和相平衡模拟综合研究,揭示出石榴石在溶解沉淀反应过程中具有明显的粒度敏感性,不同粒径石榴石表现出截然不同的长宽比、成分环带、包裹体排列方式和压力影发育情况。石榴石表面流体活动明显截切早期生长环带。EBSD分析揭示石榴石破碎颗粒以绕〈112〉轴机械旋转为主,溶解过程主要集中于颗粒表面和裂隙内高曲率位置。夕线石的EBSD结果表明基质内夕线石以绕〈010〉轴旋转为主,而流体作用明显区域夕线石则以(100)[001]滑移为主。岩石相平衡模拟限定岩石变质峰期P-T条件达高压麻粒岩相,退变过程中同剪切变形导致大量流体渗入而形成降温降压退变轨迹,由~9.5kbar、760℃演化至~6.0kbar、500~600℃,并在粗粒石榴石内保存早期进变质环带,剪切抬升过程中石榴石内普遍发育垂直剪切方向的裂隙,并在流体作用下进一步改造其形态。此研究揭示红河-哀牢山剪切带内除前人报道的石榴石高温韧性变形外,还存在大量中-上地壳层次同变质反应下的溶解-沉淀蠕变作用。因此,石榴石变质-变形的综合研究有助于揭示变质杂岩带挤压-剪切-伸展多阶段构造演化过程。  相似文献   

8.
A fluid inclusion study on metamorphic minerals of successive growth stages was performed on highly deformed paragneisses from the Nestos Shear Zone at Xanthi (Central Rhodope), in which microdiamonds provide unequivocal evidence for ultrahigh-pressure (UHP) metamorphism. The correlation of fluid inclusion density isochores and fluid inclusion reequilibration textures with geothermobarometric data and the relative chronology of micro- and macro-scale deformation stages allow a better understanding of both the fluid and metamorphic evolution along the P–T–d path. Textural evidence for subduction towards the NE is recorded by the orientation of intragranular NE-oriented fluid inclusion planes and the presence of single, annular fluid inclusion decrepitation textures. These textures occur within quartz “foam” structures enclosed in an earlier generation of garnets with prolate geometries and rarely within recrystallized matrix quartz, and reequilibrated both in composition and density during later stages of exhumation. No fluid inclusions pertaining to the postulated ultrahigh-pressure stage for microdiamond-bearing garnet–kyanite–gneisses have yet been found. The prolate shape of garnets developed during the earliest stages of exhumation that is recorded structurally by (L  S) tectonites, which subsequently accommodated progressive ductile SW shearing and folding up to shallow crustal levels. The majority of matrix kyanite and a later generation of garnet were formed during SW-directed shear under plane-strain conditions. Fluid inclusions entrapped in quartz during this stage of deformation underwent density loss and transformed to almost pure CO2 inclusions by preferential loss of H2O. Those inclusions armoured within garnet retained their primary 3-phase H2O–CO2 compositions. Reequilibration of fluid inclusions in quartz aggregates is most likely the result of recrystallization along with stress-induced, preferential H2O leakage along dislocations and planar lattice defects which results in the predominance of CO2 inclusions with supercritical densities. Carbonic fluid inclusions from adjacent kyanite–corundum-bearing pegmatoids and, the presence of shear-plane-parallel fluid inclusion planes within late quartz boudin structures consisting of pure CO2-fluid inclusions with negative crystal shapes, bear witness of the latest stage of deformation by NE-directed extensional shear.This study shows that the textures of early fluid inclusions that formed already during the prograde metamorphic path can be preserved and used to derive information about the kinematics of subduction that is difficult to obtain from other sources. The textures of early inclusions, together with later generations of unaltered primary and secondary inclusions in metamorphic index minerals that can be linked to specific deformation stages and even P–T conditions, are a welcome supplement for the reconstruction of a rather detailed P–T–d path.  相似文献   

9.
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   

10.
High-density CO2 inclusions occur abundantly in granulite fadesrocks (age of metamorphism 2–5b.y.) of the Nilgiri massif,southern India. The chronology of carbonic inclusions in thewidespread enderbitic granulites studied in relation to thedevelopment of micro-textures and mineral assemblages indicatesthat randomly oriented, negative-crystal-shaped CO2 inclusions(4–20 µim) in garnet and quartz grains (qtz I) armouredby garnet entrap syn-peak-metamorphic pore fluids. The moreabundant trail-bound CO2 inclusions in the deformed, polygonized,and partially recrystallized matrix quartz grains (qtz II andIII) and plagioclase grains were formed in connection with astage of compressional deformation and subsequent annealingrelated to the development of the late-Proterozoic Bhavani shearzone. These inclusions resulted from local re-equilibrationof the former peak-granulitic carbonic inclusions and re-entrapmentof released fluids. The presence of pure CO2 in all the inclusionsis confirmed by microthermometric data and laser-excited Ramanmicrospectrome-try. Temperatures of homogenization (liquid phase)are in the range of 50 to +20C, and the corresponding CO2 densitiesare between 1.154 and 0–807 g/cm3. Mineralogical thermobarometry on the enderbitic granulites documentsa continuous gradient of near-peak metamorphic conditions from750C, 9–10 kb in the northern part to 73OC, 7 kb inthe southwestern part of the Nilgiri massif. Uniform P, Testimates(600–650 C, 6–7 kb) for late coronitic garnet +quartz assemblages in enderbites and metadolerites indicatethat differential uplift of the massif to mid-crustal levelswas accomplished before late compressional deformation. In conformity,carbonic inclusions in quartz II and III are characterized byuniformly high density (1.154–1.08 g/cm3). In contrast,early carbonic inclusions in garnet and quartz I preserve thedensity contrast reflecting the regional P, T gradient duringnear-peak metamorphic fluid entrapment. The fluid inclusionsys-tematics indicate ‘near-isochoric’ uplift ofthe northern high-P domain, but near-isobaric cooling of thesouthwestern low-P domain. The carbonic fluids are thought tohave been derived either from internal sources during dehydration-meltingprocesses or from freezing synmetamorphic intrusives into thelower crust.  相似文献   

11.
Field evidence and fluid inclusion studies on South Indian incipient charnockites suggest that charnockite formation occurred during a decompressional brittle regime following the ‘peak’ of metamorphism and regional deformation. The most abundant type of inclusions in quartz and garnet grains in these charnockites contain high-density carbonic fluids, although lower-density fluids occur in younger arrays of inclusions. Discrete fluid inclusion generations optically are observed to decrepitate over well-defined temperature intervals, and quantitative measurements of CO2 abundance released from these inclusions by stepped thermal decrepitation show up to a four-fold increase (by volume) in the incipient charnockites relative to the adjacent gneisses from which they are derived. Studies based on optical thermometry, visual decrepitation and stepped-heating inclusion release together indicate that entrapment of carbonic fluids coincided with charnockite formation. We confirm that an influx of carbon dioxide-rich fluids is associated with the amphibolite-granulite transition, as recorded by the incipient charnockites, the remnants of which are commonly preserved as the earliest generation of high-density fluid inclusions.  相似文献   

12.
Metamorphic garnet commonly contains needle‐like rutile inclusions as well as equant rutile inclusions that surround quartz inclusions and range in size from submicrometer to nanometer. Although the origin of these equant rutile inclusions, that is, exsolution or non‐exsolution, has important implications for petrological and tectonic processes, the crystallographic characteristics of these inclusions have rarely been studied because of the small sizes and analytical difficulties involved. Here, we report the crystallographic characteristics pertinent to the genetic origin of minute equant rutile inclusions in cloudy, nearly spherically shaped garnet domains with Ti‐depleted compositions surrounding quartz inclusions in ultrahigh‐pressure garnet from several diamondiferous Erzgebirge quartzofeldspathic gneissic rock samples. TEM analyses show that the equant rutile crystals in cloudy garnet domains are partially bounded by the low‐energy {100}rt ± {110}rt ± {101}rt facets and have rather random crystallographic orientation relationships (CORs) with the garnet host, with preferential alignment of low‐energy lattice planes, for example, {100}rt//{112}grt, for some rutile crystals. Although the rather random CORs are unlikely to be attributed to solid‐state exsolution subjected to the stringent topotactic garnet lattice constraints, the characteristic subhedral {100}rt ± {110}rt ± {101}rt crystal forms of rutile can be rationalized by a metasomatic dissolution‐reprecipitation mechanism via a fluid phase. In this scenario, the quartz+fluid inclusions in garnet were first subjected to decompression microcracking during rock exhumation, followed by dissolution of Ti‐bearing garnet matrix at the crack tips or along the crack surfaces and subsequent reprecipitation of rutile, apatite, gahnite, akdalaite, and Ti‐depleted garnet. The rapid coalescence between rutile and garnet crystals in fluid or direct attachment of rutile crystals onto the dissolving crack surfaces would then yield the rather random CORs as reported here. These results, along with previous work on rutile needles, indicate rather diverse genesis of rutile inclusions in various crystal forms, thus shedding light on the controversial exsolution origin for other inclusion suite/microstructure in minerals.  相似文献   

13.
The development of microstructures and textures (i.e. crystallographic preferred orientations) during recrystallisation of naturally deformed quartz polycrystals has been studied via electron diffraction techniques in the scanning electron microscope. In the investigated sample series of quartz-rich rocks originating from different deformation regimes, the microstructural and textural changes in quartz have been significantly influenced by dynamic recrystallisation. Based on microstructural observations paired with orientation and misorientation analyses down to the scale of grains and subgrains, criteria could be established which characterise the dominant recrystallisation process and its influence on texture development. It is shown that the texture development during dynamic recrystallisation is controlled by a differential activation of slip systems in grains of ‘soft’ and ‘hard’ orientations. The analyses provide further evidence that specific grain orientations are preferred during crystal plastic deformation, recrystallisation and grain growth. The influence of twinning after the Dauphiné law was also investigated. Observations of a progressive reduction in the population of Dauphiné-twin boundaries during recrystallisation and a penetrative deformation in both hosts and twins indicate a generation prior to deformation and recrystallisation. A mechanical origin for twinning and possible influence on texture development was therefore discarded.  相似文献   

14.
The (ultra‐) high pressure eclogites from Sumdo area, recorded the subduction and exhumation process of the Paleo‐Tethys oceanic crust. Previous studies showed that there are significant differences in temperature and pressure conditions of the eclogites in four regions, e.g. Sumdo, Xindaduo, Bailang and Jilang. The cause of this differences remains unclear. Studying the peak metamorphic conditions and P‐T path of Sumdo eclogite is of great significance to reveal the subduction and exhumation mechanism of Paleo‐Tethys ocean. In this paper, we choose the Jilang eclogite as an example, which has a mineral assemblage of garnet, omphacite, phengite, hornblende, rutile, epidote, quartz and symplectit (diopside + amphibole + plagioclase), and minor biotite. Garnet has a “dirty” core with abundant mineral inclusions and a “clear” rim with less mineral inclusions, showing typical growth zoning. From the core to the rim, Prp content in garnet increasing while Grs content decreasing. P‐T pseudosection calculated with Domino constrained peak P‐T conditions of Jilang eclogite as 563°C, 2.4 GPa. Combined with petrographical observation, four stages of metamorphism have been recognized: (1) early stage prograde metamorphism represent by the core of garnet and mineral inclusions therein; (2) peak metamorphism represent by the rim of garnet, omphacite, phengite, glaucophane, rutile and quartz; (3) first stage of retrograde metamorphism characterized by decomposition of lawsonite to zoisite; (4) second stage of retrograde metamorphism characterized by symplectites surrounding omphacite and cornona rimmed garnet. Jilang eclogite shows a clockwise P‐T path, and near isothermal decompression during exhumation. It differs from eclogites in other area, which are hosted by garnet‐bearing mica schists or serpentinites. Jilang eclogites are enclosed in metamorphic quartzites, with relatively low P‐T conditions. We infer that the Jilang eclogite was derived from the shallow part of the subduction zone, and was exhumated by low density materials in the subduction channel.  相似文献   

15.
Devolatilization reactions during prograde metamorphism are a key control on the fluid distribution within subduction zones. Garnets in Mn-rich quartz schist within the Sanbagawa metamorphic belt of Japan are characterized by skeletal structures containing abundant quartz inclusions. Each quartz inclusion was angular-shaped, and showed random crystallographic orientations, suggesting that these quartz inclusions were trapped via grain boundary cracking during garnet growth. Such skeletal garnet within the quartz schist formed related to decarbonation reactions with a positive total volume change (?V t > 0), whereas the euhedral garnet within the pelitic schists formed as a result of dehydration reaction with negative ?V t values. Coupled hydrological–chemical–mechanical processes during metamorphic devolatilization reactions were investigated by a distinct element method (DEM) numerical simulation on a foliated rock that contained reactive minerals and non-reactive matrix minerals. Negative ?V t reactions cause a decrease in fluid pressure and do not produce fractures within the matrix. In contrast, a fluid pressure increase by positive ?V t reactions results in hydrofracturing of the matrix. This fracturing preferentially occurs along grain boundaries and causes episodic fluid pulses associated with the development of the fracture network. The precipitation of garnet within grain boundary fractures could explain the formation of the skeletal garnet. Our DEM model also suggests a strong influence of reaction-induced fracturing on anisotropic fluid flow, meaning that dominant fluid flow directions could easily change in response to changes in stress configuration and the magnitude of differential stress during prograde metamorphism within a subduction zone.  相似文献   

16.
Permian volcanic sediments at Bluff have been strained and thermally metamorphosed by Permian intrusives to metasediments of hornblende—hornfels facies. Quartz, which crystallised as a secondary mineral during metamorphism, has an unusual preferred orientation with c-axes either forming paired maxima in the plane containing the lineation (=maximum principal strain axis = direction of extension) and the perpendicular to schistosity (=minimum principal strain axis = shortening direction) or a broad maximum parallel to the lineation; the paired maxima are approximately 30° either side of the lineation. Some quartz grains are markedly elongate parallel to the lineation, and according to hypotheses of preferred orientation involving crystal plasticity, there should be some correlation between the shape of such grains and their c-axis orientations. Grain-size and shape analysis of Bluff quartz demonstrate that no such correlation exists; the analyses show that the preferred orientation results from oriented nucleation in the residual stress field immediately following the bulk straining of the rocks, with the distribution of c-axes as predicted by Kamb's hypothesis (1959). The time relationships of rock deformation, thermal metamorphism, and nucleation and growth of quartz are discussed.  相似文献   

17.
High-pressure schists (2–2.5 GPa) from the Eclogite Zone in the Tauern Window contain honeycomb garnet in which fine webs of garnet surround strain-free quartz ± carbonate grains. High-resolution X-ray computed tomography shows that the garnet webs form a cellular structure that coats all surfaces of the inclusions. Electron backscatter diffraction analysis shows that the garnet cells are crystallographically continuous with more massive garnet regions, and that the quartz ± carbonate inclusions have random orientations; in contrast, matrix quartz exhibits a prominent crystallographic preferred orientation (CPO). High-resolution transmission electron microscopy shows few dislocations in either the garnet or the inclusion quartz. Most honeycomb garnet is chemically homogeneous, but some displays asymmetric core–rim zoning. Taken together, these observations are most consistent with formation of the garnet sheets via precipitation from a wetting fluid along quartz–quartz grain boundaries, or possibly via wholesale precipitation of garnet + quartz ± carbonate from a fluid. In either case, a silicate-rich aqueous fluid must have been present. The likelihood that a fully wetting fluid existed at high pressure has important implications for rheology during subduction of metasedimentary rocks: strain may be accommodated by grain rotation and sliding in an aqueous silicate slurry, rather than via dislocation creep mechanisms at high pressures. The absence of a CPO in early quartz may thus point to involvement of a pervasive grain-boundary fluid rather than requiring low differential stresses during subduction.  相似文献   

18.
Eclogite, orthogneiss and, by association, metapelite from an island at 78°N in North‐East Greenland experienced ultrahigh‐pressure (UHP) metamorphism at approximately 970 °C and 3.6 GPa, at the end of the Caledonian collision, 360–350 Ma. Hydrous metapelites contain abundant leucocratic layers and lenses composed of medium‐grained, anhedral, equigranular quartz, antiperthitic plagioclase and K‐feldspar with minor small garnet and kyanite crystals. Leucosomes are generally parallel to the matrix foliation, are interlayered with residual quartz bands, anastomose around residual garnet and commonly cross‐cut micaceous segregations. Textures suggest that the leucosomes crystallized from a syntectonic melt, but crystallized at the end of local high‐grade deformation. The metapelite outcrop is < 1.5 km from kyanite eclogites with confirmed coesite, but the metapelites lack coesite and palisade textures diagnostic of coesite pseudomorphs. They do contain highly fractured garnet megacrysts with polycrystalline quartz inclusions (some surrounded by radial fractures) and Ti‐rich phengite inclusions that suggest the former presence of coesite. Polyphase inclusions in garnet contain reactants and products of the inferred dehydration melting reaction: Phe + Qtz = Ky + Kfs + Rt + melt. The reactants are thought to have been early inclusions of hydrous phases within garnet that melted and then crystallized new phases. Garnet surrounding these inclusions has patchy zoning with elevated Ca, consistent with experiments that produced similar patchy microstructures in garnet around inclusions with an unequivocal melt origin. The peak UHP metamorphic assemblage in these rocks is inferred to have been phengite, coesite, garnet, kyanite, rutile, fluid ± omphacite ± epidote. Phase diagrams indicate that dehydration melting of phengite in this assemblage would have occurred after decompression from peak pressure, but still above the coesite to quartz transition. Unusual crown‐ and moat‐like textures in garnet around some polycrystalline quartz inclusions are also consistent with the inference that melting took place at UHP conditions.  相似文献   

19.
Elongate and deformed garnets from Glenelg, NW Scotland, occurwithin a thin shear zone transecting an eclogite body that hasundergone partial retrogression to amphibolite facies at circa700°C. Optical microscopy, back-scattered electron imaging,electron probe microanalysis and electron back-scatter diffractionreveal garnet sub-structures that are developed as a functionof strain. Subgrains with low-angle misorientation boundariesoccur at low strain and garnet orientations are dispersed, aroundrational crystallographic axes, across these boundaries. Towardshigh-strain areas, boundary misorientations increase and thereis a loss of crystallographic control on misorientations, whichtend towards random. In high-strain areas, a polygonal garnetmicrostructure is developed. The garnet orientations are randomlydispersed around the original single-crystal orientation. Somegarnet grains are elongate and Ca-rich garnet occurs on thefaces of elongate grains oriented normal to the foliation. Commonly,the garnet grains are admixed with matrix minerals, and, wherein contact with other phases, garnet is well faceted. We suggestthat individual garnet porphyroclasts record an evolution fromlow-strain conditions, where dislocation creep and recoveryaccommodated deformation, through increasing strain, where dynamicrecrystallization occurred by subgrain rotation, to higheststrains, where recrystallized grains were able to deform bydiffusion creep assisted grain boundary sliding with associatedrotations. KEY WORDS: diffusion creep; EBSD; garnet; plastic deformation; recrystallization  相似文献   

20.
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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号