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1.
Melt infiltration into quartzite took place due to generation and migration of partial melts within the high‐grade metamorphic rocks of the Big Cottonwood (BC) formation in the Little Cottonwood contact aureole (UT, USA). Melt was produced by muscovite and biotite dehydration melting reactions in the BC formation, which contains pelite and quartzite interlayered on a centimetre to decimetre scale. In the migmatite zone, melt extraction from the pelites resulted in restitic schollen surrounded by K‐feldspar‐enriched quartzite. Melt accumulation occurred in extensional or transpressional domains such as boudin necks, veins and ductile shear zones, during intrusion‐related deformation in the contact aureole. The transition between the quartzofeldspathic segregations and quartzite shows a gradual change in texture. Here, thin K‐feldspar rims surround single, round quartz grains. The textures are interpreted as melt infiltration texture. Pervasive melt infiltration into the quartzite induced widening of the quartz–quartz grain boundaries, and led to progressive isolation of quartz grains. First as clusters of grains, and with increasing infiltration as single quartz grains in the K‐feldspar‐rich matrix of the melt segregation. A 3D–μCT reconstruction showed that melt formed an interconnected network in the quartzites. Despite abundant macroscopic evidence for deformation in the migmatite zone, individual quartz grains found in quartzofeldspathic segregations have a rounded crystal shape and lack quartz crystallographic orientation, as documented with electron backscatter diffraction (EBSD). Water‐rich melts, similar to pegmatitic melts documented in this field study, were able to infiltrate the quartz network and disaggregate grain coherency of the quartzites. The proposed mechanism can serve as a model to explain abundant xenocrysts found in magmatic systems. 相似文献
2.
Anisotropy of magnetic susceptibility (AMS) in micaceous quartzites with mean susceptibility (K
m) >50 × 10−6 SI units is known to be on account of the orientation distribution of the para/ferromagnetic minerals (e.g. micas, magnetite),
which comprise the minor phase in the rocks. However, the strain in such deformed micaceous quartzites is dominantly accommodated
by the quartz grains, which are the major phase in them. The objective of this paper is to explore the extent to which AMS
data from micaceous quartzites provide information about the shape of the strain ellipsoid. AMS analysis of 3 quartzite blocks
is performed, and the shape of the AMS ellipsoid is recorded to be oblate. From AMS data, the three principal planes of the
AMS ellipsoid are identified in each block and thin sections are prepared along them. Quartz grain shape (aspect ratio, R
q), intensity of quartz and mica shape preferred orientation (κq and κmi, respectively) and 2D strain (E) recorded by quartz are measured in each section. R
q, κq, κmi and E are all noted to be minimum in the section parallel to the magnetic foliation plane as compared to the other two sections.
This indicates that the quartz grains have oblate shapes in 3D and accommodated flattening strain, which is similar to the
shape of the AMS ellipsoid. The role of mica in causing Zener drag and pinning of quartz grain boundaries is discussed. It
is concluded that during progressive deformation, migration of pinned grain boundaries is inhibited. This causes enhanced
recrystallization at the grain boundaries adjacent to the pinned ones, thus guiding the shape modification of quartz grains.
A strong correlation is demonstrated between κq and κmi as well as κmi and E. It is inferred that fabric evolution of quartz was controlled by mica. Hence, the shape of the AMS ellipsoid, which is on
account of mica, provides information about shape of the strain ellipsoid. 相似文献
3.
Kinetics of diffusion-controlled growth of fayalite 总被引：1，自引：0，他引：1
The rate of growth of fayalite (Fe2SiO4) has been measured at one atmosphere total pressure, temperatures from 1000° to 1120° C, and oxygen fugacities controlled by CO/CO2 gas-mixing from 10-9.9 to 10-13.0atm, chosen to span the fayalite stability field. The fine-grained polycrystalline fayalite layer was formed by reacting the oxides FeO or Fe3O4 with a thin slice of single-crystal quartz. The rate of growth of the fayalite increases with increasing temperature and decreasing oxygen fugacity, and is consistent with a parabolic rate law, indicating that the growth rate is controlled by diffusion through the fayalite. Microstructural observations and platinum marker experiments suggest that the reaction phase is formed at the quartz-fayalite interface, and is therefore controlled by the diffusion of iron and oxygen. The parabolic rate constant was analyzed in terms of the oxide activity gradient to yield mean chemical diffusivities for the rate-limiting ionic species, assuming bulk transport through the fayalite layer. Given that iron diffusion in olivine polycrystals occurs either by lattice diffusion, which shows a positive dependence on oxygen activity, or by grain boundary diffusion, which would result in growth rates significantly faster than we observe, we conclude that the diffusivities derived in this study represent oxygen diffusion. However, since oxygen lattice diffusion in fayalite has been established to be much slower than our measurements, it is likely that the transport path for oxygen is along the grain boundaries. Thus, the mean grain boundary diffusivity of oxygen in fayalite $\bar D$ O gb (m2 s-1), using the measured grain size of 0.25 μm, is then given by $$\bar D_O^{gb} {\mathbf{ }}\delta = 1.28 \times 10^{ - 3} f_{O_2 }^{ - 0.17} {\mathbf{ }}e^{ - 540/RT} $$ , where δ is the grain boundary width (in m), and the activation energy is in kJ/mol. Assuming δ=10-9 m (Ricoult and Kohlstedt 1983), the oxygen grain boundary diffusivities are about a factor of 30 × slower than those reported by Watson (1986) for Fo90 olivine. 相似文献
4.
We have investigated grain boundary diffusion rates in enstatite by heating single crystals of quartz packed in powdered San Carlos olivine (Mg0.90Fe0.10)2SiO4 at controlled oxygen fugacities in the range 10?5.7 to 10?8.7?atm and temperatures from 1350° to 1450?°C for times from 5 to 100?h at 1?atm total pressure. Following the experiments, the thickness of the coherent polycrystalline reaction rim of pyroxene that had formed between the quartz and olivine was measured using backscatter scanning imaging in the electron microprobe. Quantitative microprobe analysis indicated that the composition of this reaction phase is (Mg0.92Fe0.08)2Si2O6. The rate of growth of the pyroxene increases with increasing temperature, is independent of the oxygen fugacity, and is consistent with a parabolic rate law, indicating that the growth rate is controlled by ionic diffusion through the pyroxene rim. Microstructural observations and platinum marker experiments suggest that the reaction phase is formed at the olivine-pyroxene interface, and is therefore controlled by the diffusion of silicon and oxygen. The parabolic rate constants determined from the experiments were analyzed in terms of the oxide activity gradient across the rim to yield mean effective diffusivities for the rate-limiting ionic species, assuming bulk transport through the pyroxene layer. These effective diffusivities are faster than the lattice diffusivities for the slowest species (silicon) calculated from creep experiments, but slower than measured lattice diffusivities for oxygen in enstatite. Thus, silicon grain boundary diffusion is most likely to be the rate-limiting process in the growth of the pyroxene rims. Also, as oxygen transport through the pyroxene rims must be faster than silicon transport, diffusion of oxygen along the grain boundaries must be faster than through the lattice. The grain boundary diffusivity for silicon in orthopyroxenite is then given by D¯gbSiδ=(3.3±3.0)×10?9f0.0O2e?400±65/RT?m3s?1, where the activation energy for diffusion is in kJ/mol, and δ is the grain boundary width in m. Calculated growth rates for enstatite under these conditions are significantly slower than predicted by an extrapolation from similar experiments performed at 1000?°C under high pressure (hydrous) conditions by Yund and Tullis (1992), perhaps due to water-enhancement of diffusion in their experiments. 相似文献
5.
Microstructures and quartz c-axis fabrics were analyzed in five quartzite samples collected across the eastern aureole of the Eureka Valley–Joshua Flat–Beer Creek composite pluton. Temperatures of deformation are estimated to be 740±50 °C based on a modified c-axis opening angle thermometer of Kruhl (J. Metamorph. Geol. 16 (1998) 142). In quartzite layers located closest (140 m) to the pluton-wall rock contact, flattened detrital grains are plastically deformed and partially recrystallized. The dominant recrystallization process is subgrain rotation (dislocation creep regime 2 of Hirth and Tullis (J. Struct. Geol. 14 (1992) 145)), although grain boundary migration (dislocation creep regime 3) is also evident. Complete recrystallization occurs in quartzite layers located at a distance of 240 m from the contact, and coincides with recrystallization taking place dominantly through grain boundary migration (regime 3). Within the quartzites, strain is calculated to be lowest in the layers closest to the pluton margin based on the aspect ratios of flattened detrital grains.The c-axis fabrics indicate that a slip operated within the quartzites closest to the pluton-wall rock contact and that with distance from the contact the operative slip systems gradually switch to prism [c] slip. The spatial inversion in microstructures and slip systems (apparent “high temperature” deformation and recrystallization further from the pluton-contact and apparent “low temperature” deformation and recrystallization closer to the pluton-contact) coincides with a change in minor phase mineral content of quartzite samples and also in composition of the surrounding rock units. Marble and calc-silicate assemblages dominate close to the pluton-wall rock contact, whereas mixed quartzite and pelite assemblages are dominant further from the contact.We suggest that a thick marble unit located between the pluton and the quartzite layers acted as a barrier to fluids emanating from the pluton. Decarbonation reactions in marble layers interbedded with the inner aureole quartzites and calc-silicate assemblages in the inner aureole quartzites may have produced high XCO2 (water absent) fluids during deformation. The presence of high XCO2 fluid is inferred from the prograde assemblage of quartz+calcite (and not wollastonite)+diopside±K-feldspar in the inner aureole quartzites. We suggest that it was these “dry” conditions that suppressed prism [c] slip and regime 3 recrystallization in the inner aureole and resulted in a slip and regime 2 recrystallization, which would normally be associated with lower deformation temperatures. In contrast, the prograde assemblage in the pelite-dominated outer part of the aureole is biotite+K-feldspar. These “wet” pelitic assemblages indicate fluids dominated by water in the outer part of the aureole and promoted prism [c] slip and regime 3 recrystallization. Because other variables could also have caused the spatial inversion of c-axis fabrics and recrystallization mechanisms, we briefly review those variables known to cause a transition in slip systems and dislocation creep regimes in quartz. Our conclusions are based on a small number of samples, and therefore, the unusual development of crystal fabrics and microstructures in the aureole to the EJB pluton suggests that further study is needed on the effect of fluid composition on crystal slip system activity and recrystallization mechanisms in naturally deformed rocks. 相似文献
6.
In the Lesser Garhwal Himalaya, the North Almora Thrust separates the overlying medium-grade Dudatoli-Almora crystallines
of Precambrian age from the unmetamorphosed to partly metamorphosed rocks of the Garhwal Group of Late Precambrian age. The
crystalline nappe sheet consists of flaggy to schistose quartzites, granite gneisses and garnetiferous mica schist members
in an ascending order. In different localities. different members of the Dudatoli-Almora crystallines are exposed along the
thrust plane. Southwest of Adbadri fine-grained mylonitized schistose quartzites of Dudatoli-AImora crystallines are in contact
with the underlying metabasites of the Garhwal Group. The mylonitized schistose quartzites consist of alternating thick (1
to 2m) quartzite and thin (10 to 20cm) micaceous quartzite bands. The micaceous quartzites can be further differentiated into
alternating quartz-rich (0-5 to 2.0 cm thick) and mica-rich (0.2 to 1.0 cm thick) layers. In the quartzites the C-surfaces
are parallel to the S-surfaces defined by the alternating quartz-rich and mica-rich layers. Further, the S-surfaces exhibit
almost similar folds with multiple wavelengths where the axial planes are nearly parallel and enveloping surfaces are oblique
to the lithological layering. The evolution of these folds has been envisaged in three phases of deformation on the basis
of field evidence, fold geometry and microstructures.
During the first phase buckle folds (F
1) developed in thin micaceous quartzite layers. whereas thick quartzite bands underwent only layer parallel shortening. During
the second phase the stress orientation changed and the limbs ofF
1 folds were folded (F
2). During the third phase of deformation which coincided with thrusting, the rocks were sheared, mylonitized and developed
microstructures exhibiting dynamic recrystallization by the processes of subgrain rotation, and continual and discontinuai
grain boundary migration. This phase was also responsible for the development of C-surfaces parallel to the lithological layering.
Further, in the folded micaceous quartzite layers shearing resulted in the development of C-surfaces parallel to the axial
planes ofF
2 folds. 相似文献
7.
In the southern Korean Peninsula twelve quartzite strata occur in the Gyeonggi massif and Okcheon belt. Their geologic ages range from Precambrian to Upper Paleozoic. All quartzites in the Gyeonggi massif are of Precambrian in age and are characterized by high-grade metaquartzites; they are Seosan, Anyang, Yongmunsan and Uiam quartzites from west to east. Quartzite types occurring in the Okcheon belt are diverse from orthoquartzite to medium-grade metaquartzite. Orthoquartzites are all Paleozoic in age and are distributed mainly in the eastern Okcheon belt (Taebaeksan Basin) (Jangsan, Dongjeom and Jeongseon quartzites) with one in the central Okcheon belt (Mungyeong Quartzite). Low-grade metaquartzite is Hwasan quartzite in the western part and medium-grade metaquartzites are Daehyangsan and Geumsusan quatzites in the central part, and Yongamsan quartzite in the southwestern part of the Okcheon belt. Distribution of quartzite types in the southern Korean Peninsula is not related to the geologic age of quartzites. As a case study, quartzite characteristics were applied to a provenance study of quartzite clasts in the northwestern part of the Cretaceous Gyeongsang Basin. Quartzite clasts in the study area are interpreted to have been mostly derived from source quartzites in the Okcheon belt, which is consistent with the results of previous studies. 相似文献
8.
James M. Brenan 《Contributions to Mineralogy and Petrology》1993,115(2):215-224
To assess further the role of pore fluids in enhancing mass transport in deep-seated rocks, bulk diffusion coefficients (D-values) for chlorine have been measured at 1.0 GPa and 1000°C in texturally-equilibrated quartzites containing varying amounts (0.3–2.8 vol%) of H2O or a CO2–H2O mixture (X
CO2=0.4). Experiments were used to monitor transport predominantly through the fluid phase by employing a chemical tracer dissolved in the fluid (chlorine) that is virtually insoluble in the rock matrix (quartzite) but is somewhat soluble in a small fraction of dispersed indicator minerals (fluorapatite). For diffusion in H2O-bearing quartzite, experiments with 1 vol.% fluid exhibit a continuous decrease in D-values with decreasing porosity whereas an abrupt drop in the diffusion coefficient is indicated by the experiments with 0.3 vol.% fluid. At a given total porosity, diffusion coefficients obtained from quartzites containing the CO2–H2O fluid range from 80x to > 3000x lower than those from H2O-bearing experiments. Bulk transport measurements were correlated with textural observations and the observed reduction in bulk D-values for quartzites containing the CO2–H2O fluid reflects the overall isolated nature of porosity in such samples. The drop in the bulk D-value for quartzites with 0.3 vol.% H2O probably arises from the elimination of interconnected porosity owing to the presence of a sufficient number of dry grain edges. Textural observations, combined with transport measurements, are consistent with the pore structure predicted by dihedral angle measurements. However, due to anisotropy in the interfacial energy of quartz, long-range fluid connectivity is dramatically reduced in quartzites with low H2O contents, despite a median dihedral angle of less than (but near) 60°. Observed variations in chlorine D-values in samples with connected and non-connected porosity are found to be consistent with previous bulk property measurements on texturally-equilibrated, fluid-bearing rocks. Results of this study, combined with prior bulk diffusion measurements for oxygen, provides a general assessment of the effect of small amounts of fluid on the enhancement of mass transport in quartzose lithologies over a range of crustal P and T. At conditions of textural equilibrium, it is expected that the fluid phase provides little or no contribution to the long-range diffusive flux of solutes in quartz-rich rocks containing small amounts of H2O or CO2–H2O mixtures. 相似文献
9.
We ask the question whether petrofabric data from anisotropy of magnetic susceptibility (AMS) analysis of deformed quartzites gives information about shape preferred orientation (SPO) or crystallographic preferred orientation (CPO) of quartz. Since quartz is diamagnetic and has a negative magnetic susceptibility, 11 samples of nearly pure quartzites with a negative magnetic susceptibility were chosen for this study. After performing AMS analysis, electron backscatter diffraction (EBSD) analysis was done in thin sections prepared parallel to the K1K3 plane of the AMS ellipsoid. Results show that in all the samples quartz SPO is sub-parallel to the orientation of the magnetic foliation. However, in most samples no clear correspondance is observed between quartz CPO and K1 (magnetic lineation) direction. This is contrary to the parallelism observed between K1 direction and orientation of quartz c-axis in the case of undeformed single quartz crystal. Pole figures of quartz indicate that quartz c-axis tends to be parallel to K1 direction only in the case where intracrystalline deformation of quartz is accommodated by prism <c> slip. It is therefore established that AMS investigation of quartz from deformed rocks gives information of SPO. Thus, it is concluded that petrofabric information of quartzite obtained from AMS is a manifestation of its shape anisotropy and not crystallographic preferred orientation. 相似文献
10.
Helen C. Kerbey 《Proceedings of the Geologists' Association. Geologists' Association》2011,122(1):16-24
Flexible quartzitic rocks are found in various locations around the world and have been identified as both sandstones and quartzites. The term itacolumite was first used for a Brazilian quartzite thought to be a new type of rock in 1822. As analytical methods have improved flexibility has been shown to be due to irregular interlocking quartz grains surrounded by regular intergranular spaces. Platy minerals such as muscovite are sometimes present and probably add support to the structure, but are not necessary for flexibility. The underlying cause of the texture appears to be chemical dissolution of quartz at the grain boundary. 相似文献
11.
We studied ferrous paralava, a high-temperature rock, produced by complete fusion of the sedimentary protolith in the Ravat natural coal fire which has been on for over two thousand years. The paralava was sampled from the Fan-Yagnob coal deposit at the Kukhi-Malik site in the vicinity of former Ravat Village in central Tajikistan. This rock contains fayalite, sekaninaite, hercynite, Ti-magnetite, tridymite, and siliceous glass. Low-Ca pyroxene (clinoferrosilite), globules of sulfides (mainly pyrrhotite) and Fe-Ti oxides, secondary greenalite (after fayalite) and hematite are minor. Paralava includes xenoliths of partially molten clinkers (up to 20 vol.%) composed of mullite, cordierite, tridymite, and relict detrital quartz. We found relatively high Fe contents (100?Fe/(Fe+Mg) > 60) in mafic minerals, high K2O enrichment (up to 1.4 wt.%) in sekaninaite, and an unusually low CaO content (0.5 wt.%) in the rock. The Ravat paralava appears to be derived from a mixture of pelitic rocks (50–70%) and iron-rich rocks (30–50%), but without participation of calcareous material, which explains the low CaO and the absence of plagioclase and Ca-bearing pyroxene. The primary melt was as hot as >1210 °C, and the coal-fired fayalite-sekaninaite paralava crystallized at 1200–1100 °C, at a relatively low oxygen fugacity (near the QFM buffer), outside the zone of active aeration. Large-scale crystallization of ferrospinels and fayalite led to increasing Al2O3 and SiO2 in the melt whence sekaninaite and tridymite crystallized as later phases. The residual melt progressively acquired a more silicic-aluminous composition, rich in K2O, CaO, and P2O5, and became quenched to glass at >1080–1090 °C, when temperature dropped abruptly, possibly, by roof collapse or opening of large cracks, as it usually happens in underground coal fires. 相似文献
12.
Properties of fluids attending variable recrystallization of quartzite during contact metamorphism in the White‐Inyo Range,California 
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下载免费PDF全文 P. I. Nabelek S. K. Stephenson S. S. Morgan J. J. Student 《Journal of Metamorphic Geology》2017,35(3):357-369
Fluid inclusions in the metamorphic aureole of the Eureka Valley‐Joshua Flat‐Beer Creek (EJB) pluton in the White‐Inyo Range, California, reveal the compositions and origin of fluids that were present during variable recrystallization of quartzite with sedimentary grain shapes to metaquartzite with granoblastic texture. Metamorphosed sedimentary formations, including quartzites, marbles, calcsilicates and schists, became ductile and strongly attenuated in the aureole during growth of the magma chamber. The microstructures of quartzites have an unusual distribution in that within ~250 m from the pluton, where temperatures exceeded 650 °C, they exhibit relict sedimentary grain shapes, only small amount of grain boundary migration (GBM), and crystallographic preferred orientations (CPOs) dominated by <a> slip. At distances >250 m, quartzites were completely recrystallized by GBM and CPOs are indicative of prism [c] slip, characteristics that are typically associated with H2O‐assisted, high‐T recrystallization. The lack of extensive GBM in the inner aureole can be attributed to rapid replacement of H2O by CO2 produced by reaction of quartz grains with calcite cement that also produced interstitial wollastonite. Fluid inclusions in the inner aureole generally occur in margins of quartz grains and are either wholly aqueous (Type 1) or also contain H2S, CO2 and CH4 (Type 2). Type 2 inclusions occur only in some stratigraphic layers. In both inclusion types, NaCl and CaCl2, in variable proportions, dominate the solutes in the aqueous phase, whereas FeCl2 and KCl are less abundant solutes. The solutes indicate attainment of a degree of equilibrium with carbonates and schists that are interbedded with the quartzites. Some Types 1 and 2 inclusions in the inner aureole show evidence of decrepitation due to high amounts of strain and/or heating suffered by the host rocks, which suggests that they represent pore fluids that existed in the rocks prior to contact metamorphism. In addition to Type 1 inclusions, outer aureole quartzites also contain inclusions that contain CO2 vapour bubbles in addition to aqueous phase (Type 3). These inclusions only occur in interiors of granoblastic quartz that was produced by large amounts of GBM. The aqueous phase has identical ranges of first melting and final ice melting temperatures as Type 1 inclusions, suggesting that they have the same solute compositions. These inclusions are thought to represent the interstitial pore H2O that promoted recrystallization of quartz and reacted with graphite to produce CO2. Absence of significant amounts of CH4 in Type 3 inclusions is attributed to elevated fO2 that was buffered by mineral assemblages in interbedded schists. As opposed to the large amount of CO2 that was produced by the wollastonite‐forming reaction in the inner aureole to inhibit GBM, the amount of CO2 produced in the outer aureole by reaction between H2O and graphite was apparently insufficient to inhibit recrystallization of quartz. 相似文献
13.
Chemical etching of dislocations has been studied in natural and synthetic quartz single crystals, in deformed synthetic quartz and in naturally and experimentally deformed quartzites. The ability of different etchants to produce polished or preferentially etched surfaces on quartz is described. Dislocation etching was achieved on all crystal planes examined by using a saturated solution of ammonium bifluoride as the etchant. Appropriate etching times were determined for etching quartzites for grain size, subgrain boundaries, deformation lamellae, dislocations and twins. Growth and polished surfaces of synthetic single crystal quartz were similarly etched and dislocation etch pits, characteristic of various orientations were found. The use of ammonium bifluoride proved to be expecially advantageous for the basal plane, producing a polished surface with etch pits, suitable for dislocation etch pit counting. “Double” etch pits have been found on Dauphiné twin boundaries on the basal plane and the first order prism, using this etchant. Slip lines and deformation bands were suitably etched on deformed synthetic crystal surfaces for identification of the slip planes. Other acidic etchants have been explored and their application to the study of deformation structures in quartz crystals is discussed. 相似文献
14.
In this study several grain-scale microstructures are presented that are thought to demonstrate the migration direction of once-mobile grain boundaries in a naturally deformed quartzite. An analysis is presented of the sense of migration of the boundaries, and the characteristics of the patterns of relative grain growth and shrinkage. Grain-boundary migration seems to be correlated with the relative crystallographic orientations of neighbouring grains for the quartz—quartz grain boundaries, and the pattern of preferred grain growth is roughly symmetrical about the mica foliation plane. 相似文献
15.
Strain-rate estimation using fractal analysis of quartz grains in naturally deformed rocks 总被引：1，自引：0，他引：1
Manish A. Mamtani 《Journal of the Geological Society of India》2010,75(1):202-209
The area-perimeter fractal dimension (D) of quartz grains has earlier been proposed as a strain-rate gauge based on experimental
deformation of quartz aggregates. To test the application in naturally deformed rocks, D is calculated in (a) three quartzites
belonging to the Lunavada Group of rocks (Aravalli Mountain Belt, NW India) that developed textures between 420–600°C and
(b) one quartz reef sample from the Malanjkhand Granite (Central India), which underwent dynamic recrystallization between
250–400°C. Using the above T ranges and calculated D values, strain-rates are calculated for the two sets of samples. A 10−12.7 s−1 strain rate at 250°C is calculated for the quartz reef sample. However, at higher temperatures the calculated strain-rate
is >10−10 s−1 for the quartz reef and the quartzite samples. The quartzites show evidence of dynamic recrystallization by grain boundary
migration (GBM) and subgrain rotation (SGR), while the quartz reef is replete with evidence of bulging (BLG) recrystallization.
T and calculated strainrates are plotted on available recrystallization map of quartz. It is demonstrated that whilst the
T/strain-rate of the quartzites does not fall in the region of GBM and SGR, the T/strain-rate of the quartz reef falls in
the BLG region. The problems with strain-rate calculations using area perimeter fractal dimension are discussed. It is concluded
that the method of strain-rate calculation can be used only for lower T. 相似文献
16.
Silica mobility and fluid movement during metamorphism of the Connemara schists, Ireland 总被引：5，自引：1，他引：5
Quartz veins are developed in a wide range of metasediment types in the upper amphibolite facies rocks of Connemara, and attest to considerable migration of silica. Contrary to common assumptions, there is clear evidence that these veins do not primarily result from movement of fluid to regions of lower P–T down the regional geothermal gradient. Under amphibolite facies conditions, a dilute chloride fluid moving down temperature has the potential to alter 60g of plagioclase to muscovite for each gram of vein quartz precipitated, while cooling over the temperature interval from 650 to 500° C. The absence of significant metasomatic effects in the vein walls effectively precludes a simple origin from such through-flowing, externally derived fluids. The oxygen isotopic composition of matrix quartz shows considerable differences between different rock types (quartzite, pelite and marble), with a range of δ18OSMOW from c.+ 11.5% (quartzite) to + 18.5% (marble). In each rock type, vein quartz compositions closely match those of the matrix quartz. These results demonstrate the importance of local segregation processes in the formation of veins, and suggest that fluid convection cells were not developed during metamorphism on a scale larger than the individual sedimentary formations, if at all. Both oxygen isotope data and the absence of metasomatism indicate that veins form primarily by segregation of quartz from the host lithologies, with only a relatively minor component of through flow of externally derived fluid. Veins are clearly not the major pathways of metamorphic dewatering. It is proposed that abundant veins in the predominantly pelitic Ballynakill Formation formed during peak metamorphic D3 folding because the formation was embrittled by high fluid pressures but was capped by impermeable marble. Hence the pelitic formation fractured repeatedly and the pore fluid drained through the fractures to form veins, while irreversible loss through the rest of the succession was a much less important process. In the central mountains of Connemara, rather pure, unreactive quartzites are cut by widely spaced, laterally extensive quartz veins that are axial planar to D3 folds. These veins may mark pathways whereby metamorphic fluid made its way through the massive impermeable quartzite from lower parts of the nappe pile, but here too, oxygen isotope data indicate considerable segregation of locally derived quartz, reflecting the importance of pumping of fluid between wail rocks and fractures relative to the component of through flow. 相似文献
17.
In the Sambagawa schist, southwest Japan, while ductile deformation pervasively occurred at D1 phase during exhumation, low-angle normal faulting was locally intensive at D2 phase under the conditions of frictional–viscous transition of quartz (c. 300 °C) during further exhumation into the upper crustal level. Accordingly, the formation of D2 shear bands was overprinted on type I crossed girdle quartz c-axis fabrics and microstructures formed by intracrystalline plasticity at D1 phase in some quartz schists. The quartz c-axis fabrics became weak and finally random with increasing shear, accompanied by the decreasing degree of undulation of recrystallized quartz grain boundaries, which resulted from the increasing portion of straight grain boundaries coinciding with the interfaces between newly precipitated quartz and mica. We interpreted these facts as caused by increasing activity of pressure solution: the quartz grains were dissolved mostly at platy quartz–mica interface, and precipitated with random orientation and pinned by mica, thus having led to the obliteration of existing quartz c-axis fabrics. In the sheared quartz schist, the strength became reduced by the enhanced pressure solution creep not only due to the reduction of diffusion path length caused by increasing number of shear bands, but also to enhanced dissolution at the interphase boundaries. 相似文献
18.
硬玉石英岩是一种稀少且与流体作用相关的变质岩,同时出露于高压或超高压洋壳和陆壳俯冲带中.通过对中国东部大别造山带中出露达50 km2的含柯石英的超高压硬玉石英岩进行研究,综合全岩主微量元素、矿物Mg-O同位素和锆石学研究.结果表明,硬玉石英岩的原岩为古元古代TTG岩石,经历过弱化学风化和强物理风化作用,然后在三叠纪时期受到围岩富黑云母片麻岩分解脱水而产生的大量重Mg同位素流体交代,从而形成硬玉石英岩.考虑到这种受流体交代成因的硬玉石英岩在大别山广泛出露,表明其在三叠纪大陆深俯冲过程中存在着大规模的变质流体活动,这项研究首次报道了大陆俯冲带有大规模的流体活动存在,同时也挑战了传统观点认为的大陆俯冲带缺乏岛弧岩浆作用主要原因是缺乏足够量的流体活动. 相似文献
19.
Intergranular diffusion rates from the analysis of garnet surfaces: Implications for metamorphic equilibration 下载免费PDF全文
下载免费PDF全文 Novel approaches to garnet analysis have been used to assess rates of intergranular diffusion between different matrix phases and garnet porphyroblasts in a regionally metamorphosed staurolite‐mica‐schist from the Barrovian‐type area in Scotland. X‐ray maps and chemical traverses of planar porphyroblast surfaces reveal chemical heterogeneity of the garnet grain boundary linked to the nature of the adjacent matrix phase. The garnet preserves evidence of low temperature retrograde exchange with matrix minerals and diffusion profiles documenting cation movement along the garnet boundaries. Garnet–quartz and garnet–plagioclase boundaries preserve evidence of sluggish Mg, Mn and Fe diffusion at comparable rates to volume diffusion in garnet, whereas diffusion along garnet–biotite interfaces is much more effective. Evidence of particularly slow Al transport, probably coupled to Fe3+ exchange, is locally preserved on garnet surfaces adjacent to Fe‐oxide phases. The Ca distribution on the garnet surface shows the most complex behaviour, with long‐wavelength heterogeneities apparently unrelated to the matrix grain boundaries. This implies that the Ca content of garnet is controlled by local availability and is thought likely to reflect disequilibrium established during garnet growth. Geochemical anomalies on the garnet surfaces are also linked to the location of triple junctions between the porphyroblasts and the matrix phases, and imply enhanced transport along these channels. The slow rates of intergranular diffusion and the characteristics of different boundary types may explain many features associated with the prograde growth of garnet porphyroblasts. Thus, minerals such as quartz, Fe‐oxides and plagioclase whose boundaries with garnet are characterized by slow intergranular diffusion rates appear to be preferentially trapped as inclusions within porphyroblasts. As such grain boundary diffusion rates may be a significant kinetic impediment to metamorphic equilibrium and garnet may struggle to maintain chemical and textural equilibrium during growth in pelites. 相似文献
20.
Thomas Reinecke Heinz-Jürgen Bernhardt Richard Wirth 《Contributions to Mineralogy and Petrology》2000,139(5):584-606
Calcite in former aragonite–dolomite-bearing calc-schists from the ultrahigh-pressure metamorphic (UHPM) oceanic complex
at Lago di Cignana, Valtournanche, Italy, preserved different kinds of zoning patterns at calcite grain and phase boundaries.
These patterns are interpreted in terms of lattice diffusion and interfacial mass transport linked with a heterogeneous distribution
of fluid and its response to a changing state of stress. The succession of events that occurred during exhumation is as follows:
As the rocks entered the calcite stability field at T=530–550 °C, P ca. 1.2 GPa, aragonite occurring in the matrix and as inclusions in poikilitic garnet was completely transformed to calcite.
Combined evidence from microstructures and digital element distribution maps (Mn-, Mg-, Fe- and Ca–Kα radiation intensity
patterns) indicates that transformation rates have been much higher than rates of compositional equilibration of calcite (involving
resorption of dolomite and grain boundary transport of Mg, Fe and Ca). This rendered the phase transformation an isochemical
process. During subsequent cooling to T ca. 490 °C (where lattice diffusion effectively closed), grains of matrix calcite have developed diffusion-zoned rims, a
few hundred micrometres thick, with Mg and Fe increasing and Ca decreasing towards the phase boundary. Composition profiles
across concentrically zoned, large grains in geometrically simple surroundings can be successfully modelled with an error
function describing diffusion into a semi-infinite medium from a source of constant composition. The diffusion rims in matrix
calcite are continuous with quartz, phengite, paragonite and dolomite in the matrix. This points to an effective mass transport
on phase boundaries over a distance of several hundred micrometres, if matrix dolomite has supplied the Mg and Fe needed for
incorporation in calcite. In contrast, diffusion rims are lacking at calcite–calcite and most calcite–garnet boundaries, implying
that only very minor mass transport has occurred on these interfaces over the same T–t interval. From available grain boundary diffusion data and experimentally determined fluid–solid grain boundary structures,
inferred large differences in transport rates can be best explained by the discontinuous distribution of aqueous fluid along
grain/phase boundaries. Observed patterns of diffusion zoning indicate that fluid was distributed not only along grain-edge
channels, but spread out along most calcite–white mica and calcite–quartz two-grain junctions. On the other hand, the inferred
non-wetting of calcite grain boundaries in carbonate-rich domains is compatible with fluid–calcite–calcite dihedral angles
>60° determined by Holness and Graham (1995) for a wide range of fluid compositions under the P–T conditions of interest. Whereas differential stress has been very low at the stage of diffusion zoning (T > 490 °C), it increased as the rocks were cooling below 440 °C (at 0.3–0.5 GPa). Dislocation creep and the concomitant increase
of strain energy in matrix calcite induced migration recrystallisation of high-angle grain boundaries. For that stage, the
compositional microstructure of recrystallised calcite grain boundary domains indicates significant mass transport along calcite
two-grain junctions, which at the established low temperatures is likely to have been accomplished by ionic diffusion within
a hydrous grain boundary fluid film (“dynamic wetting” of migrating grain boundaries).
Received: 10 January 2000 / Accepted: 10 April 2000 相似文献