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1.
The bowing potential of granitic rocks: rock fabrics, thermal properties and residual strain 总被引：1，自引：0，他引：1
The bowing of natural stone panels is especially known for marble slabs. The bowing of granite is mainly known from tombstones
in subtropical humid climate. Field inspections in combination with laboratory investigations with respect to the thermal
expansion and the bowing potential was performed on two different granitoids (Cezlak granodiorite and Flossenbürg granite)
which differ in the composition and rock fabrics. In addition, to describe and explain the effect of bowing of granitoid facade
panels, neutron time-of-flight diffraction was applied to determine residual macro- and microstrain. The measurements were
combined with investigations of the crystallographic preferred orientation of quartz and biotite. Both samples show a significant
bowing as a function of panel thickness and destination temperature. In comparison to marbles the effect of bowing is more
pronounced in granitoids at temperatures of 120°C. The bowing as well as the thermal expansion of the Cezlak sample is also
anisotropic with respect to the rock fabrics. A quantitative estimate was performed based on the observed textures. The effect
of the locked-in stresses may also have a control on the bowing together with the thermal stresses related to the different
volume expansion of the rock-forming minerals. 相似文献
2.
Three calcitic marble types often used as cladding material and different in lattice preferred orientation (texture), grain shape preferred orientation, grain size distribution and grain interlocking are investigated to study the combined effect of thermal cycles (day-night) and moisture on the decay of marble, particularly on the bowing phenomenon. Repetitive heating-cooling under dry conditions leads to considerable inelastic residual strain only in the first thermal cycle. The residual strain continuously increases again if water is present, whereby the moisture content after a thermal cycle has a certain impact on the decay rate. The water-enhanced thermal dilatation strongly correlates with the deterioration rate obtained from the laboratory bow test. All applied approaches reveal that the texture in combination with the grain shape preferred orientation control the intensity and anisotropy of marble deterioration, thus, the cutting direction of facade panels has to be considered in terms of durability. On-site analysis from building facades confirm the laboratory results such as the bowing tendency of different marbles, directional dependence, relevance of moisture content during cyclical heating and the loss of strength due to environmental impact.Special Issue: Stone decay hazards 相似文献
3.
The veneer cladding of the Oeconomicum (OEC, Göttingen), the State Theatre of Darmstadt (STD, Darmstadt) and of the State and University Library (SUB, Göttingen) is characterised by pronounced bowing after a short time of exposure. Direct comparison of bowing data related to measurements from 2000 to 2003 at the SUB clearly show that the amplitude in bowing had significantly increased. The bowing is different in intensity and orientation (concave, convex). The cladding material (Peccia marble, Rosa Estremoz marble and Carrara marble) are different in lattice preferred orientation, grain size distribution and grain interlocking. Depending on the bowing, panels may show cracks mostly initiated at the dowels. The percentage of visible cracks and breakouts increases with the amplitude of bowing except for the STD. Repetitive heating–cooling under dry conditions leads to considerable inelastic residual strain only after the first or second thermal cycle. The residual strain continuously increases again if water is present, whereby the moisture content after a thermal cycle has a certain impact on the decay rate. The water-enhanced thermal dilatation strongly correlates with the deterioration rate obtained from the laboratory bow test. Detailed petrophysical investigations provide evidence that with increasing bowing a decrease of mechanical properties (flexural strength or breaking load at dowel hole) occur. Marble degradation is also connected with the increase in porosity and a general shift of the maximum pore radii to larger pore sizes. On-site damage analyses were combined with laboratory tests of the bowing potential to constrain factors that may influence the risk failure. The experimental bowing data clearly demonstrate that after 40 heating cycles combined with the effect of moisture a certain impact on the decay rate is observed. In the case of demounted panels the bowing tests show that already strongly deformed panels from the building exhibit a lower bowing potential than those with lower amplitudes of bowing. This is not the general case for all marble types. Finally, the artificial bowing causes a significant reduction of the flexural strength and the breaking load at the dowel hole. The strength loss of the experimentally aged claddings combined with on-site damage analyses led to conclusions concerning risk assessment and the predicted lifetime of the investigated marble claddings. 相似文献
4.
John M. Logan 《Environmental Geology》2004,46(3-4):456-467
Studies of marble panels from the exterior of two buildings document the processes leading to bowing of the material. Bowing of panels is most extensive in those areas that are exposed to direct or reflected thermal energy. The thermal anisotropic behavior of calcite results in grain-boundary separation, grain sliding and microfracturing. The resulting loss of strength is one factor leading to bowing. The development of bows further reduces the panel strength as the outer portions elongate by inelastic deformation mechanisms. Laboratory experiments cycling marble samples for over 200 cycles at three temperatures up to 107°C above room conditions show similar strength losses as the natural situation. The second factor contributing to the bowing process is the release of residual elastic strain. The strain is in part stored in the marble from its geologic history, but may also be accumulated during thermal cycling due to the properties of calcite. Marble panels have been found to bow when stored outside, but not attached to any framework, indicating that the release of residual strain is a critical factor in producing the bows. 相似文献
5.
Victoria Shushakova Edwin R. Fuller Jr. Florian Heidelbach David Mainprice Siegfried Siegesmund 《Environmental Earth Sciences》2013,69(4):1281-1297
Thermoelastic behavior of different marble types was analyzed using computational modeling and experimental measurements. Eight marble samples with different composition, grain size, grain boundary geometry, and texture were investigated. Calcitic and dolomitic marbles were considered. The average grain size varies from 75 μm to 1.75 mm; grain boundary geometry differs from nearly equigranular straight grain boundaries to inequigranular-interlobate grain boundaries. Four typical marble texture types were observed by EBSD measurements: weak texture; strong texture; girdle texture and high-temperature texture. These crystallographic orientations were used in conjunction with microstructure-based finite element analysis to compute the thermoelastic responses of marble upon heating. Microstructural response maps highlight regions and conditions in the marble fabric that are susceptible to degradation phenomena. This behavior was compared to the measured thermal expansion behavior, which shows increasing residual strains upon repetitive heating–cooling cycles. The thermal expansion behavior as a function of temperature changes can be classified into four categories: (a) isotropic thermal expansion with small or no residual strain; (b) anisotropic thermal expansion with small or no residual strain; (c) isotropic thermal expansion with a residual strain; and (d) anisotropic thermal expansion with residual strain. Thermal expansion coefficients were calculated for both simulated and experimental data and also modeled from the texture using the MTEX software. Fabric parameters control the amount and directional dependence of the thermal expansion. Marbles with strong texture show higher directional dependence of the thermal expansion coefficients and have smaller microstructural values of the maximum principal stress and strain energy density, the main precursors of microcracking throughout the marble fabric. In contrast, marbles with weak texture show isotropic thermal expansion behavior, have a higher propensity to microcracking, and exhibit higher values of maximum principal stress and strain energy density. Good agreement between the experimental and computational results is observed, demonstrating that microstructure-based finite-element simulations are an excellent tool for elucidating influences of rock fabric on thermoelastic behavior. 相似文献
6.
H.-R. Wenk M. Bortolotti N. Barton E. Oliver D. Brown 《Physics and Chemistry of Minerals》2007,34(9):599-607
Mechanical twinning in polycrystalline quartz was investigated in situ with time-of-flight neutron diffraction and a strain
diffractometer. Dauphiné twinning is highly temperature sensitive. It initiates at a macroscopic differential stress of 50–100 MPa
and, at 500°C, saturates at 400 MPa. From normalized diffraction intensities the patterns of preferred orientation (or texture)
can be inferred. They indicate a partial reversal of twinning during unloading. The remaining twins impose residual stresses
corresponding to elastic strains of 300–400 microstrain. Progressive twinning on loading and reversal during unloading, as
well as the temperature dependence, can be reproduced with finite element model simulations. 相似文献
7.
A. Frischbutter D. Neov Ch. Scheffzük M. Vrna K. Walther 《Journal of Structural Geology》2000,22(11-12)
Neutron diffraction methods (both time-of-flight- and angle-dispersive diffraction) are applied to intracrystalline strain measurements on geological samples undergoing uniaxial increasing compressional load. The experiments were carried out on Cretaceous sandstones from the Elbezone (East Germany), consisting of >95% quartz which are bedded but without crystallographic preferred orientation of quartz. From the stress–strain relation the Young’s modulus for our quartz sample was determined to be (72.2±2.9) GPa using results of the neutron time-of-flight method. The influence of different kinds of bedding in sandstones (laminated and convolute bedding) could be determined. We observed differences of factor 2 (convolute bedding) and 3 (laminated bedding) for the elastic stiffness, determined with angle dispersive neutron diffraction (crystallographic strain) and with strain gauges (mechanical strain). The data indicate which geological conditions may influence the stress–strain behaviour of geological materials. The influence of bedding on the stress–strain behaviour of a laminated bedded sandstone was indicated by direct residual stress measurements using neutron time-of-flight diffraction. The measurements were carried out six days after unloading the sample. Residual strain was measured for three positions from the centre to the periphery and within two radial directions of the cylinder. We observed that residual strain changes from extension to compression in a different manner for two perpendicular directions of the bedding plane. 相似文献
8.
Enrico Sassoni Serena Andreotti George W. Scherer Elisa Franzoni Siegfried Siegesmund 《Environmental Earth Sciences》2018,77(10):387
Bowing of thin marble slabs is a phenomenon affecting both historic monuments and modern buildings. In spite of the ubiquity and destructiveness of this phenomenon, no fully satisfactory treatment is currently available to arrest and/or prevent bowing. In this study, a treatment based on formation of hydroxyapatite (HAP) was investigated as a possible route to arrest and possibly prevent bowing of Carrara marble slabs. Four different formulations of the HAP treatment were tested and compared to ammonium oxalate and ethyl silicate (widely used in the practice of marble conservation). The treatments were applied onto pre-weathered and unweathered specimens to investigate their ability to arrest and prevent bowing, respectively. Marble behavior was studied in terms of residual strain and bowing after thermal cycles up to 90 °C in dry and wet conditions. Marble cohesion was assessed before and after the thermal cycles by ultrasound. The HAP treatments exhibited promising results, as the residual strain and the bowing after the cycles were always lower or equal to the untreated references, while marble cohesion was always higher. Surprisingly, ammonium oxalate caused marked worsening of marble thermal behavior. In the case of ethyl silicate, most of the initial benefit after consolidation was lost after the thermal cycles. In general, the results of the study point out the importance of evaluating marble thermal behavior to assess the suitability of any conservation treatment and suggest that treatments able to strengthen marble without causing excessive pore occlusion and stiffening are preferable to enhance durability to thermal cycles. 相似文献
9.
Folding experiments have been carried out on single-layers of Carrara marble and Solnhofen limestone at a confining pressure of 275 bars, temperature of 400°C, and strain rates of 5.5×10−7 to 8.2×10−7. The marble and limestone layers were embedded in a rock-salt matrix and in a matrix of a mixture of 60% fine-grained halite and 40% fine-grained calcite, respectively, and deformed to different percentages of bulk shortening. Aspect ratios of the layers varied between 11.25 and 15. The stress-strain relationship reveals that strain increased with a very small increment in compressive stresses, once folding was initiated.With progressive deformation the bulk strain is compensated by folding along one fold hinge. The resulting folds are concentric and a combination of class 1a, 1b and 3 type. The changes in the arc length, layer thickness, limb dip and wavelength with progressive folding in marble layers, are discussed.The microstructure and texture of the folded marble and limestone layers have been investigated optically and by means of an X-ray texture goniometer. The inner fold arc exhibits a strong preferred orientation, whereas in the outer fold are the preferred orientation is poorly developed. Differences in the fabric in medium-grained marble and fine-grained limestone layers have been attributed to the difference in mechanism of deformation. 相似文献
10.
Influence of shape fabric and crystal texture on marble degradation phenomena: simulations 总被引：2，自引：1，他引：1
Victoria Shushakova Edwin R. Fuller Jr Siegfried Siegesmund 《Environmental Earth Sciences》2011,63(7-8):1587-1601
Microstructure-based finite element simulations were used to study the influence of grain shape fabric and crystal texture on thermoelastic responses related to marble degradation phenomena. Calcite was used as an illustrative example for studying extremes of shape preferred orientation (SPO) in shape fabric and lattice preferred orientation (LPO) in crystal texture. Three SPOs were analyzed: equiaxed grains, elongated grains, and a mixture of equiaxed and elongated grains. Three LPOs were considered: a random orientation distribution function and two degrees of strong directional crystal texture. Finally, the correlation between the direction of the LPO with respect to that of the SPO was examined. Results show that certain combinations of SPO, LPO, and their directional relationship have significant influence on the thermomechanical behavior of marble. For instance, while there is no major dependence of the elastic strain energy density and the maximum principal stress on SPO for randomly textured microstructures, there is a strong synergy between LPO and its directional relationship with respect to the SPO direction. Microcracking precursors, elastic strain energy density, and maximum principal stress, decrease when the crystalline c-axes have fiber texture perpendicular to the SPO direction, but increase significantly when the c-axes have fiber texture parallel to the SPO direction. Moreover, the microstructural variability increases dramatically for these latter configurations. In general, the influence of LPO was as expected, namely, the strain energy density and the maximum principal stress decreased with more crystal texture, apart from for the exception noted above. Spatial variations of these precursors indicated regions in the microstructure with a propensity for microcracking. Unexpectedly, important variables were the microstructural standard deviations of the spatial distributions of the microcracking indicators. These microstructural standard deviations were as large as or larger than the variables themselves. The elastic misfit-strain contributions to the coefficients of thermal expansion were also calculated, but their dependence was as expected. 相似文献
11.
12.
Bowing of dimensional granitic stones 总被引：1，自引：1，他引：0
Bowing is a well-known phenomenon seen in marbles used as building veneers. This form of rock weathering occurs as a result of external factors such as temperature, humidity, the system for anchoring the marble slabs or the panel dimensions. Under the same external conditions, many factors will determine the degree of deformation including petrography, thermal properties and residual locked stresses. The usual way to solve the problem of bowed marble slabs is to replace them with other materials, such as granites, in which the deformation still exists but is less common. In this study, eight ornamental granites with different mineralogy, grain size, grain shape, porosity and fabric were tested in a laboratory to assess their susceptibility to bowing. Three slabs of granite, each cut with a different orientation, were studied under different conditions of temperature (90 and 120°C) and water saturation (dry and wet) to investigate the influence of these factors together with that of anisotropy. At 90°C, only the granite with the coarsest grain size and low porosity exhibited deformation under wet conditions. At 120°C and wet conditions, three of the granites showed evident signs of bowing. Again, the granite with the coarsest grain size was the most deformed. It was concluded that the wide grain size distribution influences microcracking more than other expected factors, such as the quartz content of the rock. Also, mineral shape-preferred orientation and porosity play an important role in the bowing of the studied granites. 相似文献
13.
Kai Chen Martin Kunz Nobumichi Tamura Hans-Rudolf Wenk 《Physics and Chemistry of Minerals》2011,38(6):491-500
Microstructures of deformed calcite in marble from the Bergell Alps are studied by using a microfocused polychromatic synchrotron
X-ray beam. The high spatial resolution, together with orientation and strain resolutions, reveals twin plane orientation,
multiple twin lamellae, and strain distributions associated with the twins. Single and multiple mechanical twins on e = { 01[`1] 8 } e = \left\{ {01\overline{1} 8} \right\} systems are confirmed. Residual stresses are derived from the strain tensor that is derived from Laue diffraction patterns.
Average lattice strains from several hundred to over one thousand microstrains are detected in a deformed marble from the
Bergell Alps. Such strains suggest 60–120 MPa residual stresses. A detailed study of strain components shows that shear stresses
on twin planes are completely released. 相似文献
14.
Dauphiné twinning and texture memory in polycrystalline quartz. Part 3: texture memory during phase transformation 总被引：1，自引：0，他引：1
Samples of quartz-bearing rocks were heated above the α (trigonal)–β (hexagonal) phase transformation of quartz (625–950°C)
to explore changes in preferred orientation patterns. Textures were measured both in situ and ex situ with neutron, synchrotron
X-ray and electron backscatter diffraction. The trigonal–hexagonal phase transformation does not change the orientation of
c- and a-axes, but positive and negative rhombs become equal in the hexagonal β-phase. In naturally deformed quartzites measured by
neutron diffraction a perfect texture memory was observed, i.e. crystals returned to the same trigonal orientation they started
from, with no evidence of twin boundaries. Samples measured by electron back-scattered diffraction on surfaces show considerable
twinning and memory loss after the phase transformation. In experimentally deformed quartz rocks, where twinning was induced
mechanically before heating, the orientation memory is lost. A mechanical model can explain the memory loss but so far it
does not account for the persistence of the memory in quartzites. Stresses imposed by neighboring grains remain a likely cause
of texture memory in this mineral with a very high elastic anisotropy. If stresses are imposed experimentally the internal
stresses are released during the phase transformation and the material returns to its original state prior to deformation.
Similarly, on surfaces there are no tractions and thus texture memory is partially lost. 相似文献
15.
The lattice preferred orientation (LPO) of an anorthosite (composed of andesine) sampled from a highly deformed anorthositic mylonite (Grenville Province, Quebec) was measured by TOF neutron diffraction and SEM-EBSD. The quantitative texture analysis of neutron data was accomplished by using the Rietveld texture analysis with the WIMV algorithm, implemented in the program package Materials Analysis Using Diffraction (MAUD). The texture calculations of the EBSD data were performed by using the program BEARTEX. Analyses from neutron and electron diffraction data gave similar results if EBSD data are smoothed to account for grain statistics. The principal pole figures show (010) roughly parallel to the rock foliation, (001) poles exhibiting a low angle (25°) to the pole to foliation, and (100) poles close to the Y-direction (perpendicular to the lineation and foliation pole). The [100] crystallographic direction shows a maximum in the lineation direction, [010] directions concentrate near the foliation pole. The geological deformation conditions and the constructed pole figure patterns indicate that the preferred orientation could be attributed to intracrystalline slip dominantly on (010) with [100] as slip direction. Elastic properties, calculated by averaging, document weak anisotropy that has implications for the seismic structure of the lower crust. 相似文献
16.
The thermal rock properties are particularly important for natural stones whenever a temperature change may occur, which becomes particularly important when different materials are combined on any architectural structure. The thermal expansion of a rock is dependent on the coefficients of the expansion of the individual rock-forming minerals and the rock fabric. A systematic study on 65 different stones, mostly granitoids and others magmatic rocks, most of them are often used as dimensional building stones, was performed. Temperature and moisture are very important parameters in the natural environment. Therefore, the thermal expansion, and in addition the thermohygric expansion on selected examples, was measured. The data were also discussed considering the effect of the mineralogy and the temperature. A modeling approach was introduced to show how the mineralogy and the related single crystal properties affect the thermal properties and how good a simple calculation can help to characterize the measured thermal expansion of a rock. The directional dependence of the thermal expansion was also discussed and explained based on detailed rock fabric measurements. In this study, the bowing of granitoid samples was tested and compared with bowing phenomena of granitoid facade panels. The slabs were cut in different directions and were studied under different conditions of temperatures and water saturation. It could be clearly documented that the temperature and the moisture have a control on the bowing behavior. The implication of our data is that thermal expansion depends greatly on wetting and drying, i.e., the thermal cracking is characterized by the residual strain observed after cooling to room temperature. The sensitivity to the thermal cracking has a significant control on the application in architectural constructions. 相似文献
17.
S. Siegesmund K. Ullemeyer T. Weiss E. K. Tschegg 《International Journal of Earth Sciences》2000,89(1):170-182
Marbles as building stones as well as in their natural environments show complex weathering phenomena. The most important
damage scenario is based on the highly anisotropic thermal expansion coefficient α of calcite, i.e. extreme expansion parallel and contraction normal to the crystallographic c-axis. Therefore, the rock fabric
and especially the lattice-preferred orientation (texture) of calcite and/or dolomite as the predominant mineral phases in
marbles have a significant influence on the mechanical weathering. The textures of marbles from five different locations vary
from a more or less perfect prolate to moderate oblate shape of the [006] pole figure tensor. Accordingly, the texture-derived
bulk thermal dilatation anisotropy covers a broad range from –0.048 to 0.680. The modelled thermal dilatations correlate with
those obtained from experimental measurements. The difference in magnitude is basically explained by the microcrack fabrics
which was not considered in the computations. All samples show a deterioration due to thermal treatment regardless of the
strength of texture. The directional dependence of (a) the total magnitude of the thermal dilatation coefficient and (b) of
the residual strain is highest in marbles with a strong texture, whereas the Carrara marble with a weak texture exhibits a
uniform crack formation. The progressive loss of cohesion along grain boundaries due to dilatancy may serve as an example
for the initial stage of physical weathering.
Received: 10 February 1999 / Accepted: 16 October 1999 相似文献
18.
Victoria Shushakova Edwin R. Fuller Jr. Siegfried Siegesmund 《Environmental Earth Sciences》2013,69(4):1263-1279
Microstructure‐based finite-element analysis with a microcracking algorithm was used to simulate an actual degradation phenomenon of marble structures, i.e., microcracking. Both microcrack initiation and crack propagation were characterized, as were their dependence on lattice preferred orientation (LPO), grain shape preferred orientation (SPO), grain size, marble composition (calcite and dolomite) and grain‐boundary fracture toughness. Two LPOs were analyzed: a random orientation distribution function and an orientation distribution function with strong directional crystalline texture generated from a March–Dollase distribution. Three SPOs were considered: equiaxed grains; elongated grains and a mixture of equiaxed and elongated grains. Three different grain sizes were considered: fine grains of order 200 μm (only calcitic marble); medium size grains of order 1 mm (calcitic and dolomitic marbles); and large grains of order 2 mm (only dolomitic marble). The fracture surface energy for the grain boundaries, γig, was chosen to be 20 and 40 % of the fracture surface energy of a grain, γxtal, so that both intergranular and transgranular fracture were possible. Studies were performed on these idealized marble microstructures to elucidate the range of microcracking responses. Simulations were performed for both heating and cooling by 50 °C in steps of 1 °C. Microcracking results were correlated with the thermoelastic responses, which are indicators related to degradation. The results indicate that certain combinations of LPO, SPO, grain size, grain‐boundary fracture toughness and marble composition have a significant influence on the thermal-elastic response of marble. Microstructure with the smallest grain size and the highest degree of SPO and LPO had less of a tendency to microcrack. Additionally, with increasing SPO and LPO microcracking becomes more spatially anisotropic. A significant observation for all microstructures was an asymmetry in microcracking upon heating and cooling: more microcracking was observed upon cooling than upon heating. Given an identical microstructure and crystallographic texture, calcite showed larger thermal stresses than dolomite, had an earlier onset of microcracking upon heating and cooling, and a greater microcracked area at a given temperature differential. Thermal expansion coefficients with and without microcracking were also determined. 相似文献
19.
While crystallography conventionally presumes that a single crystal carries a unique crystallographic orientation, modern
experimental techniques reveal that a single crystal may exhibit an orientation distribution. However, this distribution is
largely concentrated; it is extremely concentrated when compared with orientation distributions of polycrystalline specimen.
A case study of a deformation experiment with a single hematite crystal is presented, where the experimental deformation induced
twining, which in turn changed a largely concentrated unimodal “parent” orientation distribution into a multimodal orientation
distribution with a major mode resembling the parent mode and three minor modes corresponding to the progressive twining.
The free and open source software MTEX for texture analysis was used to compute and visualize orientations density functions
from both integral orientation measurements, i.e. neutron diffraction pole intensity data, and individual orientation measurements,
i.e. electron back scatter diffraction data. Thus it is exemplified that MTEX is capable of analysing orientation data from
largely concentrated orientation distributions. 相似文献
20.
Quartz and calcite microfabric transitions in a pressure and temperature gradient,Sivrihisar, Turkey
Nicholas C.A. Seaton Christian Teyssier Donna L. Whitney Matthew T. Heizler 《Geodinamica Acta》2013,26(3-4):191-206
Interlayered quartzite and marble in the southern Sivrihisar Massif, Turkey, record metamorphic conditions ranging from high-pressure/low-temperature through a Barrovian overprint from chlorite- to sillimanite-zone conditions. This sequence was exhumed under transtension, producing macroscopic constrictional fabrics (L-tectonites) during crustal thinning. Quartz microstructures consist of dynamically recrystallized aggregates in the dislocation creep regime dominated by grain boundary migration. Quartz microstructures are relatively constant across the high metamorphic gradient, and crystallographic fabric patterns transition from plane strain to constriction strain. Calcite fabrics are characterized by progressive overprinting of a columnar texture inherited from the high-pressure polymorph aragonite. In the low-temperature Barrovian domain (<400?°C), shearing of calcite rods produced a very strong c-axis point maximum. At moderate temperature, calcite rods were partially to totally recrystallized and the strong preferred orientation maintained. At temperature >500?°C and high constriction strain, marble has no crystallographic fabric, likely reflecting a transition from dislocation creep to diffusion creep. Phengite in high-pressure/low-temperature marble and quartzite yields relatively simple age spectra with Late Cretaceous (88–82 Ma) 40Ar/39Ar ages. Barrovian muscovite records significantly younger ages (63–55 Ma). The transtension system and associated metamorphism may have occurred above a subduction zone in Paleocene–Eocene time as a precursor to intrusion of Eocene (~53 Ma) arc plutons. 相似文献