Recrystallized Quartz Grains Research Articles

The effect of deformation on the TitaniQ geothermobarometer: an experimental study

We performed high strain (up to 47 %) axial compression experiments on natural quartz single crystals with added rutile powder (TiO2) and ~0.2 wt% H2O to investigate the effects of deformation on the titanium-in-quartz (TitaniQ) geothermobarometer. One of the objectives was to study the relationships between different deformation mechanisms and incorporation of Ti into recrystallized quartz grains. Experiments were performed in a Griggs-type solid-medium deformation apparatus at confining pressures of 1.0–1.5 GPa and temperatures of 800–1,000 °C, at constant strain rates of 1 × 10−6 or 1 × 10−7 s−1. Mobility of Ti in the fluid phase and saturation of rutile at grain boundaries during the deformation experiments are indicated by precipitation of secondary rutile in cracks and along the grain boundaries of newly recrystallized quartz grains. Microstructural analysis by light and scanning electron microscopy (the latter including electron backscatter diffraction mapping of grain misorientations) shows that the strongly deformed quartz single crystals contain a wide variety of deformation microstructures and shows evidence for subgrain rotation (SGR) and grain boundary migration recrystallization (GBMR). In addition, substantial grain growth occurred in annealing experiments after deformation. The GBMR and grain growth are evidence of moving grain boundaries, a microstructure favored by high temperatures. Electron microprobe analysis shows no significant increase in Ti content in recrystallized quartz grains formed by SGR or by GBMR, nor in grains grown by annealing. This result indicates that neither SGR nor moving grain boundaries during GBMR and grain growth are adequate processes to facilitate re-equilibration of the Ti content in experimentally deformed quartz crystals at the investigated conditions. More generally, our results suggest that exchange of Ti in quartz at low H2O contents (which may be realistic for natural deformation conditions) is still not fully understood. Thus, the application of the TitaniQ geothermobarometer to deformed metamorphic rocks at low fluid contents may not be as straightforward as previously thought and requires further research.

Fluid Inclusion Study of the Higher Himalayan Quartzitic Pelites, Garhwal Himalaya, India: Implications for Recrystallization History of Metasediments

Abstract Quartzitic pelites forms a part of Higher Himalayan Crystalline of higher geotectonic zone in Garhwal Himalaya. Quartzitic pelites (locally known as Pandukeshwar Quartzite) in Garhwal Himalaya is sandwiched between high grade metamorphic rocks of Central Crystallines and Badrinath Formation. Fluid inclusion studies are carried out on the detrital, and recrystallized quartz grains of quartzitic pelites to know about the fluid phases present during recrystallization processes at the time of maximum depth of burial. The quartzitic pelite (Pandukeshwar Quartzite) essentially consists of recrystallised quartz with accessory minerals like mica and feldspar. Fluid microthermometry study reveals the presence of three types of fluids: (i) high-salinity brine, (ii) CO2- H2O and (iii) H2O-NaCl. These fluids were trapped during the development of grain and recrystallization processes. The high saline brine inclusions and CO2- H2O fluid with the density of 0.90 to 0.97 gm/cm3 are remnants of provenance area. CO2 density in detrital quartz grains characterise the protolith of the sandstone as granite or metamorphic rock. The H2O–NaCl fluids involved in the recrystallization processes at temperature-pressure of 430-350°C; 4.8 to 0.5 Kbars as constrained by fluid isochores of CO2- H2O and H2O–NaCl inclusions and bulging and sub- grain development during recrystallization processes. The re-equilibration of the primary fluid due to elevated internal and confining pressure is evident from features like ‘C’ shaped cavities, stretching of the inclusions, their migration and decrepitation clusters. The observed inclusion morphology revealed that the rocks were exhumed along an isothermal decompression path.

Coupled micro-faulting and pressure solution creep overprinted on quartz schist deformed by intracrystalline plasticity during exhumation of the Sambagawa metamorphic rocks, southwest Japan

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.

Taking mylonites' temperatures

We measured titanium-in-quartz (TitaniQ) temperatures by ion microprobe for three deformed rocks having different quartz microstructure to assess the possibility of constrain- ing the temperature of mylonitization directly from dynamically recrystallized quartz grains. Calibration via ion microprobe indicates analytical precisions and accuracies of ~±2 °C and ±10 °C, respectively (2σ). High- versus low-temperature mylonites yield high versus low Ti concentrations and temperatures that are consistent with other fi eld studies of mylonites that have similar microstructures; these observations imply that TitaniQ accurately measures dynamic recrystallization temperatures. Variations in temperature in a single domain exceed analytical errors, implying that one microstructure in a rock can refl ect different tempera- tures and, presumably, strain rates. Combined with paleopiezometric and phase-equilibrium estimates of differential stress and water fugacity, strain rates may be estimated.

Continental rifting and metamorphic core complex formation ahead of the Woodlark spreading ridge, D'Entrecasteaux Islands, Papua New Guinea

We evaluate the role of a metamorphic core complex (MCC) on Normanby Island in the Woodlark rift. Located <30 km from an active mid‐ocean ridge (MOR), a >1 km thickness of blueschist‐derived mylonites formed in a midcrustal shear zone during the Pliocene at ∼400–500°C. This top‐to‐the‐north zone appears to have reactivated the gently dipping base of the Papuan ophiolite (Papuan Ultramafic Body, PUB), and its continued activity appears to control the north dipping asymmetry of active half grabens to the north of the MCC and rapid subsidence of the Woodlark Rise. Mylonites in the MCC's lower plate have been exhumed along a detachment as a result of >50 km of slip at rates of >12 mm/yr. The inactive, back‐tilted detachment preserves fault surface megamullions and mylonitic lineations parallel to the Plio‐Pleistocene plate motion. A second SE vergent detachment has been established on the opposite flank of this rolling‐hinge style MCC, probably since <0.5 Ma. Centimeters per year slip rates on these two faults can account for most of the Pleistocene plate motion in this eastern sector of the Woodlark rift, and confirm the important role of MCCs in exhuming very young HP rocks in this rift. Paleopiezometry of mylonites using recrystallized quartz grain size indicates flow stresses of ∼30 MPa before the rocks were overprinted by extension fractures. These results imply high pore fluid pressures (λ > 0.8) at depth, and provide a sufficient mechanism for activating low‐angle normal faults in the rift. MCC inception was not localized to the tip of the Woodlark MOR. Instead, extreme crustal thinning near the MCC preconditioned later continental breakup. The lower crust appears to be weak, thickening beneath unloaded footwalls to uplift MCCs above sea level, and flowing laterally to even out regional crustal thickness contrasts on a 1–6 m.y. timescale. Deep‐seated transforms separate rheologically distinct domains in which extension has been localized along the weak PUB to cause MCC formation, vs. those in which slip is distributed across an imbricate zone of more uniform strength normal faults. The Trobriand fault connects in the eastern Woodlark rift to the Owen Stanley fault in the Papuan Ranges, which is probably moving at nearly the full plate velocity.

Ultrafine-grained quartz mylonites from high-grade shear zones: Evidence for strong dry middle to lower crust

Creep strength of the crust depends upon the rheology of the most common mineral, usually quartz. Recrystallized quartz grains in many high-grade shear zones from the middle to lower crust are typically large (millimeter sized), implying active grain boundary migration, but equivalents from old polymetamorphic and water-deficient basement sheared at similar crustal depths can be very small. For the latter, strong crystallographic preferred orientation (CPO) of quartz, with c axes aligned close to Y, progressive misorientation of crystals, subgrain and dislocation development, and core-mantle structures with recrystallized grains of 2‐8 m size, all point to dislocation glide dominantly on the prism a system with recrystallization by subgrain rotation. Recrystallized grain size piezometry of such quartz indicates high flow stress in fine-grained shear zones, while the synkinematic metamorphic mineral assemblage and the CPO are typical of amphibolite facies conditions. This is evidence that middle to lower crust is not inevitably weak due to its high temperature: water content also has an important influence.

Microstructural and deformational studies on mylonite in the detachment faults of Yalashangbo dome, North Himalayan domes zone*

The Yalashangbo dome, located at the eastern end of North Himalayan domes zone, has a geometry and structure similar to those of a metamorphic core complex. Ductile shear zones formed the detachment system around the dome and these zones are composed of garnet-bearing phyllonite, mylonitic schist, mylonitic gneiss and mylonitic granite. Ductile shear fabrics developed well in my-lonitic rocks, and penetrative lineation and foliation were formed by stretched quartz and feldspar and preferred orientation of mica. Polar Mohr diagram method is used to calculate the kinematic vorticity numbers of the shear zones in the detachment system. Results indicate that the shear zone is a thinned shear zone (thinning of 23 % ) in an extensional setting which underwent a general shear dominated by simple shear. Comparison of the vorticity numbers between the northern and southern flanks of the Yalashangbo dome shows that the dome is an asymmetric system formed by a north-northwest-directed detaching unanimously. Statistical fractal analysis shows that the shapes of dynamically recrystallized quartz grains in the mylonites have characteristics of self-similarity, with fractal numbers ranging from 1.05 to 1.18. From these fractal numbers, the strain rate of the rock was deduced from 10-9.2S-1 to 10-7.3S-1, the differential paleao-stress was 13.7-25.6 MPa during the deformation happened at a temperature over 500 ℃. The ductile shear zones in the detachment system around Yalashangbo dome were formed under a high green-schist grade condition or happened simultaneously with the intrusion of granite.

Grain-size reduction of feldspars by fracturing and neocrystallization in a low-grade granitic mylonite and its rheological effect

Feldspar grain-size reduction occurred due to the fracturing of plagioclase and K-feldspar, myrmekite formation and neocrystallization of albitic plagioclase along shear fractures of K-feldspar porphyroclasts in the leucocratic granitic rocks from the Yecheon shear zone of South Korea that was deformed under a middle greenschist-facies condition. The neocrystallization of albitic plagioclase was induced by strain energy adjacent to the shear fractures and by chemical free energy due to the compositional disequilibrium between infiltrating Na-rich fluid and host K-feldspar. With increasing deformation from protomylonite to mylonite, alternating layers of feldspar, quartz and muscovite developed. The fine-grained feldspar-rich layers were deformed dominantly by granular flow, while quartz ribbons were deformed by dislocation creep. With layer development and a more distributed strain in the mylonite, lower stresses in the quartz-rich layers resulted in a larger size of dynamically recrystallized quartz grains than that of the protomylonite.

Introduction: linked processes of metamorphism and deformation

The interaction of deformation and metamorphism within the crust results in textures, structures and mineral growth that provide a record of tectonic activity as well as the progressive metamorphic and structural development of the crust. Analysis of these textures and structures is central to deciphering and evaluating the progressive nature of their development. Such rocks are commonly interpreted from the viewpoint of either structural or metamorphic processes; however, integration of these two perspectives has proved to be a powerful tool in enhancing our understanding of the structural development of metamorphic rocks at all scales. In recent decades, research into the linkages between metamorphism and deformation has increasingly involved the integration of textural, chemical and structural data. The present volume arose from a topical session titled Understanding Coupled Metamorphic and Deformational Processes: Advances in Integrated Textural, Chemical and Microstructural Analysis held at the 2003 annual meeting of the Geological Society of America, Seattle, USA. We were pleased that the session attracted 21 contributions from 48 geologists representing seven countries. The breadth of topics covered by the presentations, and the interest expressed by both participants and attendees encouraged us to assemble a set of thematic papers related to the topical session. The result is the present special issue of the Journal of Metamorphic Geology. Whitmeyer & Wintsch document the linkages between stress, metamorphic reactions, grain shapepreferred orientation (SPO) and reactivation. They describe the reactivation of an amphibolite facies ultramylonite along discrete greenschist facies mylonitic shear bands. A random SPO within the ultramylonite created a texturally hardened rock that displaced strain into adjacent weaker foliated gneiss. Later replacement reactions within the ultramylonite were concentrated at areas of high stress and resulted in a chemical and mechanical softening of the rock via the development of a SPO of muscovite, chlorite and quartz. Reactivation of the ultramylonite was localized along discrete mylonitic shear bands at the sites of these reactions and accompanying SPO development. The relationship between strain and SPO development is also explored by Stallard et al. who measured the 3D shape, size and orientation of quartz grains sampled along two transects of increasing metamorphic grade in the Otago Schist, New Zealand. Quartz SPO was initiated by dissolution–precipitation creep at sub-greenschist facies conditions, and was triggered by strain accompanying fold development. Detrital grains became blade-shaped, and were an important component of the metamorphic foliation. A second fold event, accompanied by crenulation cleavage development and resulting segregation of the rock mass, initiated a change in both the texture of the quartz SPO and deformation mechanisms. Previously discrete and aligned detrital quartz grains were replaced by aggregates and layers of dynamically recrystallized quartz grains of reduced aspect ratio and alignment. Pressure solution then affected the margins of quartz-rich layers rather than individual grains. The authors emphasize the contribution of quartz SPO to foliation development and the significance of fold events as a control on SPO formation and development. In a companion paper that analyses the 3D shape, size and orientation of white mica grains from the same samples, Stallard & Shelley describe the initiation and development of white mica SPO with increasing metamorphic grade. The character of white mica SPO is determined early in the deformation and metamorphic history. The onset of penetrative strain metamorphism is marked by the rapid elimination of poorly oriented large clastic micas in favour of numerous new smaller metamorphic grains of contrasting composition, higher aspect ratio and a strong SPO. The SPO results from the 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. Selverstone describes how changes in fluid composition correlate with changes in mechanical behaviour of graphitic schists during decompression. Interlayered graphitic and non-graphitic schists from the Tauern Window, Eastern Alps, record different mechanical behaviour during extensional exhumation. Graphitic schists contain brittle extensional structures oriented J. metamorphic Geol., 2005, 23, 409–410 doi:10.1111/j.1525-1314.2005.00593.x

The initiation and development of metamorphic foliation in the Otago Schist, Part 2: evidence from quartz grain‐shape data

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

Water distribution in dynamically recrystallized quartz grains: cathodoluminescence and micro-infrared spectroscopic mapping

Abstract The distribution of water in dynamically recrystallized quartz aggregates in granitic mylonites was investigated by cathodoluminescence (CL) observations and micro-infrared (IR) spectroscopic mapping. It is clear from CL observations and micro-IR spectroscopic mappings that the CL intensity and contrast depend on the density of defects (e.g. the content of molecular H 2 O and absorbed OH species). Dynamic recrystallization, as a recovery process of intracrystalline strain, decreases the densities of excess carriers originated from defects. The decrease in the densities of excess carriers decreases the CL intensity and contrast within each grain. To investigate the effect of water weakening in the rheological properties of the crust, CL spectroscopy and micro-IR spectroscopic mapping are essential in the determination of concentration and distribution of water in natural high-strain zones.

Deformation partitioning during folding and transposition of quartz layers

Banded iron formation (BIF) from the Quadrilátero Ferrı́fero (southeastern Brazil) shows a compositional layering with alternating iron-rich and quartz-rich layers. This layering was intensively folded and transposed at a centimeter/millimeter scale through a component of bedding-parallel shear related to flexural slip at middle to high greenschist facies conditions (400–450 °C). The microstructure and c-axis fabrics of normal limbs, inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy (SEM) and digital image analysis. In the normal limbs, recrystallized quartz grains show undulose extinction, relatively dry grain boundaries, c-axes at high angle to foliation and a pervasive grain shape fabric (GSF) indicating operation of crystal-plastic processes. In the inverted limb, quartz grains show more serrated and porous (“wet”) grain boundaries; the GSF is similar to that of the normal limb, but c-axes are oriented at 90° to those of the normal limb. We interpreted these characteristics as reflecting operation of solution-precipitation deformation in inverted limbs, as a consequence of grains having been rotated to an orientation that was hard to basal 〈 a〉 glide, but easy to dissolution-precipitation creep. This deformation partitioning between crystal-plasticity and solution-transfer during folding/transposition of quartz may explain the common occurrence of layered quartz rocks, where individual layers show alternating c-axis fabrics with opposite asymmetries but a consistent GSF orientation. Such characteristics may reflect an earlier event of pervasive folding/transposition of a preexisting layering.

Regional variation in exhumation and strain rate of the high‐pressure Sambagawa metamorphic rocks in central Shikoku, south‐west Japan

AbstractRegional variation in the P–T path of the Sambagawa metamorphic rocks, central Shikoku, Japan has been inferred from compositional zoning of metamorphic amphibole. Rocks constituting the northern part (Saruta River area) exhibit a hairpin type P–T path, where winchite/actinolite grew at the prograde stage, the peak metamorphism was recorded by the growth of barroisite to hornblende and sodic amphibole to winchite/actinolite grew at the retrograde stage. In the southern part (Asemi River area), rocks exhibit a clockwise type P–T path, where barroisite to hornblende core is rimmed by winchite to actinolite. The difference in P–T path could suggest a faster exhumation rate (i.e. more rapid decompression) in the southern than in the northern part. On the other hand, physical conditions of deformation during the exhumation stage have been independently inferred from microstructures in deformed quartz. Recrystallized quartz grains in rocks from the low‐grade (chlorite and garnet) zones are much more stretched in the southern part (aspect ratio ≥ 4.0) than in the northern part (aspect ratio< 4.0), indicating a higher strain rate in the former than in the latter. These facts may indicate that the exhumation and strain rates are correlated (i.e. the exhumation rate increases with increasing the strain rate). The difference in the exhumation rate inferred from amphibole zoning between the northern and southern parts could be explained by an extensional model involving normal faulting, where the lower plate can be exhumed faster than the upper plate due to the displacement along the fault. Furthermore, the model may explain the positive correlation between the exhumation and strain rates, because the lower plate tended to support more stress than the upper plate.

石英における微構造の発達

The statically recrystallized quartz grains, formed by the heating of the intrusion, show several characteristic microstructures associated with the grain boundary migration. Most of the new grains were formed by the strain-induced migration (bulging) of the original grain boundaries. The new grains contain much fewer fluid inclusions than the old ones, and the boundaries of the new grains contain fluid inclusion arrays. Thus the migrating boundaries result in a change in distribution of the inclusions, may make the picture even more complicated, such as “left over grains” and a cloud of fluid inclusions.

Cretaceous structural evolution of the Pelagonian crystalline in western Voras Mt (Macedonia, Northern Greece)

The kinematic of the Cretaceous deformation and the relationship between deformation and metamorphism of the Pelagonian crystalline was studied in Voras Mt (northern Greece). The Pelagonian crystalline in this area has been subdivided into a lower, core part, consisting mainly of gneissic rocks and schists and an upper, cover part, consistine of schists and quartzites with marble intercalations. Intensely deformed granitic rocks of Upper Paleozoic age intrude the Pelagonian crystalline basement. An S j foliation is the oldest fabric recognized in the Pelagonian crystalline. Sj is mainly defined by syn-Sjgarnet(Gr1), biotite(Btj), white mica (Wnij), chloritoid, kyanite and plagioclase in the metapelitic rocks and green amphibole, epidote, plagioclase, and biotiteiBtj) in the amphibolite. Garnet grows also in some cases post-kinematically. Ilmenite and tourmaline are often found in the pelitic rocks as well. S is overprinted by an S2 foliation that developed as a crenulation cleavage. In most places, however, S2 has destroyed all earlier fabrics and a single S2 fabric is present related to, isoclinal or sheath folds intrafolial in places. S2 in the metapelitic rocks is characterized by the syn-S2 development of chlorite, white mica(Wm2) and plagioclase. In the amphibolite S2 is mainly defined by the syn-kinematic development of actinolite, plagioclase, biotite(Bt2), white mica(Wm2) and chlorite. During D2 garnet(Gr1) and biotiteiBtj) are partially replaced by chlorite, while green amphibole is replaced by actinolite and chlorite. Chloritoid remains generally stable along the S2-planes but in some places transforms to chlorite and sericite. Furthermore, D2 was locally followed by a static post-kinematic annealing indicated by polygonal quartz microfabrics with equilibrated grain boundaries and triple points. The overall orientation of S2 is dome shaped with a gentle SW-ward and NE-ward dip in the southwestern and northeastern flanks of the dome respectively. Syn-S2 minerals defined a very well exposed NW-SE trending stretching lineation. Kinematic indicators show a main top to the SE sense of movement. An S3 crenulation cleavage associated with asymmetric NW-SE trending folds is also present in most parts of the core and cover rocks, possibly, related to a constrictional type of deformation. A well developed, S4 shear band cleavage is mainly present in the upper parts of the metamorphic dome and formed under cooler conditions. S4 shear bands are associated with a NW-SE developed stretching lineation defined by elongated and dynamically recrystallized quartz grains and a preferred orientation of white mica and chlorite. Along the S4 shear bands a transformation of garnet, biotite, chloritoid and amphibole into chlorite is always observed. S4 shear bands indicate a main top to the SE sense of movement. The P-T metamorphic conditions were derived from textural equilibria and mineral assemblages, as well as from the spatial distribution of the metamorphic minerals. Syn-Dj metamorphism reached the conditions of the boundaries between greenschist and amphibolite facies. Syn-D2 retrogression took place under greenschist facie conditions. K/Ar radiometric datings on coarse-grained syn-St and younger fine-grained syn-S2 micas define an Early Cretaceous cooling age ('135Ma) for the older event and a Mid- to Late Cretaceous age ('90-80Ma) for the second event. A white mica age of ca. 65Ma correlates with S4 shear band clevage. Furthermore, the intrusion age of a granitic body into the Pelagonian crystalline is dated using the Pb/Pb single zircon evaporation method. The estimated intrusion age of 300±3Ma suggests that the Pelagonian crystalline was affected by a pre-kinematic magmatic activity relative to its Cretaceous deformation.

FRACTAL GRAIN BOUNDARY MIGRATION

Fractal analysis on experimentally recrystallized quartz grain boundaries has been employed to relate the grain boundary complexities with deformation conditions, such as strain rate and temperature. The fractal dimensional increment of the grain boundaries, defined as (D-1), and the degree of irregularity in grain boundaries, is proportional to the logarithmic of the Zener–Hollomon parameter that is defined by strain rate and temperature (the Arrhenius term). The physical mean of the empirical relationship can be explained theoretically by a new grain boundary migration model (GBM or cell dynamics model) extended by the fractal concepts and the dimension analysis. This is a more general model than the migration growth model for the fractal grain boundaries.

C-Axis fabrics and microstructures in quartz schist from the Sambagawa metamorphic belt, central Shikoku, Japan

An integrated study on c-axis fabric and microstructure of plastically deformed and dynamically recrystallized quartz in quartz schist from the Sambagawa metamorphic belt in central Shikoku, Japan has revealed a correlation between c-axis fabrics and three-dimensional grain shape. Recrystallized quartz grains with prolate ( k > 1), oblate (0.2 < k < 1) and uniaxially oblate ( k < 0.2) shapes exhibit c-axis fabric patterns of cleft girdles around the principal finite elongation direction ( X), type I crossed girdles, and small circle girdles around the principal finite shortening direction ( Z), respectively. Fabric intensity calculated from the eigenvalues of orientation tensors of the c-axis fabrics increases with increasing magnitude of the apparent finite strain inferred from the shape of recrystallized quartz grains. These results indicate that the development of recrystallized grain shape as well as c-axis fabrics is primarily controlled by finite strain during progressive deformation. However, the finite strain estimated from three-dimensional grain shape is systematically deviated toward the flattening field compared to that estimated from the c-axis fabrics. We investigated several possible causes for the discrepancy, and propose that a scenario of coupled deformation and dynamic recrystallization in the quartz schist is responsible for the formation of these microstructures.

Influence of micaceous impurity on dynamically recrystallized quartz c-axis fabric in L- S tectonites from the Singhbhum Shear Zone and its footwall, Eastern India

When temperature and strain rate remain constant the quartz c-axis fabric in deformed pure quartz aggregates, is largely dependent on deformation kinematics. Asymmetry of the fabric, e.g. in type-I asymmetric crossed girdle pattern in natural quartz tectonites, simulated fabric or experimentally deformed quartz aggregates is generally related to sense of vorticity for a non-coaxial flow. Natural quartz tectonites, however, often contain micaceous impurities. Measurement on a sample of 59 quartz tectonites with mesoscopic L- S fabric and representing low T T m , deformation under non-coaxial flow, from the Singhbhum Shear Zone and Dhanjori quartzites. Eastern India provides the basic data to quantitatively assess the influence of mica on (i) asymmetry of quartx c-axis fabric and (ii) degree of crystallographic preferred orientation, i.e. fabric intensity, taking c-axes of dynamically recrystallized quartz grains as a fabric element. A fabric intensity parameter (κ) is defined as the ratio of the greatest eigenvalue to the least eigenvalue of the orientation tensor matrix corresponding to c-axis orientations in each measured specimen. The modal percent of mica (μ) in the sample varies from 2 to 35; that of recrystallized quartz grains (ν), as opposed to relict clasts. from 45 to 98. The asymmetry of the fabrics in the above sample, measured either as the Am statistic or as the angle between the central segment of the fabric skeleton and direction of mineral elongation lineation is independent of mica content. Correlation-regression analysis of the variables κ, μ, and ν. demonstrate a negative correlation between micaceous impurity and the fabric intensity parameter. The regression equation is of the form κ = 0.13 ν 1.11 μ −0.617.

Variscan deformation, microstructural zonation and extensional exhumation of the Moravian Cadomian basement

The Cadomian Dyje Batholith, in the foot–wall of the Variscan Moravian nappe pile, has been involved in Variscan ductile deformation. The Cadomian Brunovistulian rocks were obliquely underthrusted during Carboniferous dextral transpression.Strain intensity is inversely proportional to the distance from the contact of the Variscan thrust front. The microstructures of deformed granodiorites and quartz–diorites show a characteristic zonality marked by relatively high temperature flow in the west (550–580 °C) characterized by dynamic recrystallization of feldspars and grain boundary migration recrystallization of quartz. The size of quartz grains decreases with decreasing strain towards the east. At the easternmost part of the autochthonous Dyje massif, fracturing of feldspar and subgrain rotation recrystallization of quartz predominate. Flow stress estimates calculated from recrystallized quartz grain size show a regional increase of stress intensity from the highly strained margin towards the less deformed core of the Dyje massif. This microstructural zonation is oblique with respect to the major thrust boundary and corresponds roughly to metamorphic isogrades. The microstructural zonation reflects underthrusting of the Brunovistulian domain below the Moldanubian nappe.The main ductile tectonic event D1 is followed by a retrogressive brittle–ductile and brittle deformation D2. D2 results in the development of shear zones and faults superimposed on the D1 mylonite fabric. D2 is related to extension oblique to the D1 fabric, associated with detachment and the westward movement of the Moravian nappes. © Elsevier, Paris

Variscan deformation, microstructural zonation and extensional exhumation of the Moravian Cadomian basement

The Cadomian Dyje Batholith, in the foot-wall of the Variscan Moravian nappe pile, has been involved in Variscan ductile deformation. The Cadomian Brunovistulian rocks were obliquely underthrusted during Carboniferous dextral transpression. Strain intensity is inversely proportional to the distance from the contact of the Variscan thrust front. The microstructures of deformed granodiorites and quartz-diorites show a characteristic zonality marked by relatively high temperature flow in the west (550–580 °C) characterized by dynamic recrystallization of feldspars and grain boundary migration recrystallization of quartz. The size of quartz grains decreases with decreasing strain towards the east. At the easternmost part of the autochthonous Dyje massif, fracturing of feldspar and subgrain rotation recrystallization of quartz predominate. Flow stress estimates calculated from recrystallized quartz grain size show a regional increase of stress intensity from the highly strained margin towards the less deformed core of the Dyje massif. This microstructural zonation is oblique with respect to the major thrust boundary and corresponds roughly to metamorphic isogrades. The microstructural zonation reflects underthrusting of the Brunovistulian domain below the Moldanubian nappe. The main ductile tectonic event D 1 is followed by a retrogressive brittle-ductile and brittle deformation D 2. D 2 results in the development of shear zones and faults superimposed on the D 1 mylonite fabric. D 2 is related to extension oblique to the D 1 fabric, associated with detachment and the westward movement of the Moravian nappes.