Quartz Microstructures Research Articles

Interrogating an along-strike variation in the evolution and rheology of a large continental strike-slip fault zone

Along-strike variations in lithology, temperature, fluid activity, etc. can induce rheological changes in strike-slip faults that may be recorded by different fault rocks and fabrics. This article interrogates localized formation of granite-derived mylonite in a large strike-slip fault zone in which ultramylonite is the principle granite-derived fault rock to better understand this rock record of faulting. Microtextures show that rate-limiting deformation mechanisms in mylonite were dislocation creep in quartz and diffusion creep in very-fine-grained feldspar aggregates. Microtextures also show that mylonite formation is fully compatible with continuous viscous deformation whereas coeval ultramylonite along strike formed after whole-rock cataclasis at the brittle-viscous transition. Differential stresses determined from quartz aggregates in ultramylonites are 40–150% greater than stresses in mylonites. Hence, mylonites represent a local weak sector within this otherwise relatively strong fault zone. Mylonite formation is correlated with syndeformational chemical alteration as well as quartz microstructures and mineral assemblages indicating elevated deformation temperatures. Not all mylonite samples record significant chemical alteration. Therefore, mylonite formation likely records locally elevated temperatures. These results illustrate how a local shift in deformation conditions can affect the evolution and rheology of a large strike-slip fault zone, and how fault rocks record these processes.

Deformation conditions and kinematic vorticity within the footwall shear zone of the Wildhorse detachment system, Pioneer metamorphic core complex, Idaho

Detailed analysis of numerous sequential fabrics and microstructures preserved in the footwall shear zones of detachment fault systems can be used to clarify the spatial relationships and timing of deformation related to emplacement and exhumation of metamorphic core complexes. To quantify the kinematics during deformation, we conducted microstructural analyses along two ∼300 m transects across mylonitic sections of the footwall of the Eocene Wildhorse Detachment system within the Pioneer metamorphic core complex. These results enable comparison of deformation at different structural levels during exhumation and between monomineralic and polymineralic lithologies.In both transects, mylonitic rocks preserve normal-sense asymmetric structures such as recrystallized quartz oblique foliation, S/C–C′ structures, and asymmetric feldspar porphyroclasts. Quartz microstructures, CPO patterns, and kinematic vorticity analyses consistently indicate simple shear-dominated deformation, which is predicted for late stage, shallow-level deformation within a rolling hinge detachment system exhuming a metamorphic core complex. Variations in c-axis patterns and active slip systems are associated with temperature, influenced by structural level, and grain boundary processes related to lithology. To determine the degree of non-coaxiality and minimize limitations and uncertainties with kinematic vorticity methods, we used two methods to evaluate flow vorticity. Kinematic vorticity ranges from 0.55 to 0.95 for the oblique quartz foliation (δ/β) and C′-type shear bands (C′-c) methods. Quartz recrystallized grain size ranges from 10 to 37 μm, indicating maximum differential stresses of 80–110 MPa, which is consistent with the footwall shear zones of numerous metamorphic core complex detachment systems.

Zircon Microstructures in Large, Deeply Eroded Impact Structures and Terrestrial Seismites

Abstract Extraterrestrial cratering was a critical component in the evolution of the early Earth, but discovery of very ancient and deeply exhumed impact structures remains elusive, as identification tools are in short supply. The recognition of such structures is hindered by post-cratering geological processes, whereby impact-induced features common in younger, upper-crustal craters may be lost. In this study, we make a detailed analysis of planar microstructures in zircons from four large, confirmed impact structures (Manicouagan, Rochechouart, Sudbury, Vredefort) preserved at different crustal levels, from two previously described non-impact seismites in SW Norway and northern Italy, and from the 3.0 Ga Maniitsoq structure in West Greenland. A total of ~3400 zircon grains were studied using exterior and interior scanning and transmission electron microscopy. We show for the first time that shocked zircons contain two successive, principally different types of planar microstructures, only one of which is diagnostic of impact. Closely spaced, contiguous microplanes (CP) are formed first, presumably by the shock wave. In Manicouagan and Rochechouart zircons the exterior CPs have ultrathin interior counterparts of straight dislocation arrays, as identified in Manicouagan zircon using transmission electron microscopy. They have the same close spacing and orientations as the exterior CPs and are abundantly decorated with tiny pores down to less than 50 nm across. These interior CPs are identical to shock-induced decorated, partly annealed amorphous planar microstructures in quartz (planar deformation features, PDFs) and are interpreted as such. The second type is open planar fractures (PF). They are widely and irregularly spaced and texturally younger than the CPs. They re-use and displace the CP orientations, which they cut in stepwise fashion. We interpret these PFs as formed by impact-induced seismic shaking in the wake of the shock wave. We confirm two previous reports of isolated planar fractures in zircons from non-impact seismites, showing that PFs per se are not impact-diagnostic. There are no CPs in any of these zircons. Zircons from different parts of the Maniitsoq structure contain CPs in various states of preservation besides PFs, corroborating that this very large and very deeply exhumed structure resulted from an extraterrestrial impact.

High Temperature Flexural Strength, Microstructure, and Phase Evolution of Quartz Fiber/Boron Phenolic Resin Ceramizable Composite Modified with W and B4C

In order to investigate the effect of refractory metal on the high temperature properties of phenolic resin matrix composites, modified quartz fiber reinforced ceramizable composites were prepared by a molding process with a refractory component, tungsten, as the functional component and boron carbide as the ceramic forming agent. The effects of the tungsten and the boron carbide on the heat resistance of the composite were investigated. The results showed that the introduced refractory metal tungsten and the boron carbide can react to form tungsten borides and tungsten carbides at high temperature, and form a ceramic layer on the surface of the composite, which can fill the defects caused by pyrolysis of matrix and improve the high temperature performance of the composite. When the content of boron carbide was 10 wt% and the content of tungsten powder was 30 wt%, the flexural strength of the composite before and after heat treatment at 1200 °C were increased by 54.6% and 30.2% respectively compared with that without filler.

Early Paleozoic oblique convergence from subduction to collision: Insights from timing and structural style of the transpressional dextral shear zone in the Qilian orogen, northern Tibet of China

Transpressional shear zones commonly occur in ancient and modern convergent plate boundaries to accommodate oblique plate convergence. The early Paleozoic Qilian orogen in northeastern Tibet records the subduction of Proto-Tethyan Ocean lithosphere and the accretion-collision of various magmatic arcs and continental terranes. This study focused on the Datong ductile shear zone, which represents the central part of the WNW-ESE−striking ductile shear zone along the northern margin of the Qilian block in the Qilian orogen. This structure bears key information about the evolution of oblique convergence during the early Paleozoic orogeny. The kinematics and timing of the Datong ductile shear zone were investigated via field-based, microstructural, and mica 40Ar/39Ar dating analyses. Mesostructural and microstructural data showed predominantly dextral strike-slip shearing within the Datong ductile shear zone. Microstructural features and quartz c-axis crystallographic preferred orientation patterns indicated that dextral ductile shearing occurred under lower-amphibolite-facies conditions (∼500−550 °C and ∼5.6 kbar) within the shear zone. Microstructures of quartz showed subgrain rotation (SGR) and grain boundary migration (GBM), suggesting dislocation creep−dominated deformation. A strain rate of 10−12 s−1 and a differential stress of 25−39 MPa were estimated by the rheological flow law and quartz paleopiezometry. Finite strain measurements indicated that all deformed rocks of the Datong ductile shear zone exhibit a weakly oblate ellipsoid near the plane strain. Kinematic vorticity (ranging 0.47−0.83) analysis suggested the coexistence of simple shear and pure shear strains within the Datong ductile shear zone, indicating a transpressional setting. Biotite and muscovite 40Ar/39Ar data showed that transpressional shearing deformation started in the Ordovician (before 453 Ma) and lasted to the Silurian (ca. 430 Ma). Our new data combined with regional geological data show that the deformation type, kinematics, and dynamics of the Datong ductile shear zone were controlled by the southward oblique subduction of the Paleo-Qilian Ocean (Proto-Tethyan Ocean) and the following oblique collision between the Qilian block and the Alxa block. The intensive transpressional deformation along the northern Qilian block may reflect strong coupling between the subducting Paleo-Qilian oceanic slab and the overriding Qilian block as well as a high degree of convergence obliquity during the ongoing early Paleozoic convergence.

Mapping intragranular microstructures in quartz: the significance of Dauphiné twinning

Abstract Mapping on the microstructural scale can contribute significantly to conventional field and larger scale mapping and understanding of spatial, temporal and process-oriented relationships. Here, electron backscattered diffraction (EBSD)-based microstructural maps are presented of subgrain and Dauphiné twin boundaries in undeformed sedimentary quartzite. ‘Plane matching’ analysis permits determination of the complete orientation of boundaries. A workflow is presented to facilitate the necessary crystallographic calculations. The resulting maps indicate: (1) boundary plane rotation angle/axis pairs, including tilt–twist components; (2) boundary migration vectors; and (3) conventional EBSD misorientation angle/axis pairs. Subgrain boundaries are general with small misorientations and boundary plane normal directions sub-parallel to grain boundary stress concentrations; rotation axes are oriented sub-parallel to the bedding dip (017°/10°E). Most exhibit bi-direction boundary migration vectors parallel to bedding normal. The EBSD misorientation analysis results are different as they only recognize the parallelism of adjacent crystal lattices. The differences are especially apparent for Dauphiné twin boundaries. Maps are presented of twin boundaries using matched plane analysis, including explanation for lateral twin migration. Driving forces to move twin boundaries are also estimated by mapping variations in Young's modulus between parents and twins; differences are significant, indicating that the Young's modulus and driving forces do not need to be large, explaining the propensity for twinning in many quartzites.

Morphology and Composition Changes in Fluid Inclusions from Quartz under Progressive Deformation: Case Study of a Vein System in the Western Kelyan-Irokinda Fold Zone (Western Transbaikalia)

Abstract —The behavior of fluids during plastic deformation is studied from the morphology and distribution of fluid inclusions in quartz grains of different microstructure types from a vein system controlled by thrusting and strike-slip faulting in the eastern Sayan–Baikal fold area. The analytical work includes electron backscatter diffraction (EBSD) for quartz microstructure and crystallography, as well as Linkam heating-and-freezing analysis and Raman spectroscopy for the composition of fluid inclusions. The studied fluid inclusions are of seven types that differ in morphology and position in the deformed quartz structure. A model is suggested to describe successive structural changes of quartz aggregates during dislocation sliding and subsequent creep-related recrystallization associated with redistribution of fluid. Fluid inclusions undergo qualitative and quantitative changes due to water leakage at all stages of plastic deformation. The changes occur by two main mechanisms: (i) mass transfer during dislocation sliding at medium temperatures and strain rates and (ii) diffusion creep at low strain rates and high temperatures. The contribution of creep increases gradually with temperature, which maintains the interaction of inclusions with migrating grain boundaries.

Linked microstructural and geochemical evolution of mylonitic quartzite during exhumation of a core complex

A novel method for combining microstructural analysis and Ti-in-quartz measurements is used to characterize interactions between quartz deformation and Ti equilibration during progressive deformation and cooling in a mylonitic shear zone. Mylonitic quartzite from the Wildhorse detachment, the major bounding structure of the Pioneer metamorphic core complex (Idaho, USA), contains a continuum of microstructures: minimally deformed relict grains, quartz ribbons, and a matrix dominated by subgrain rotation recrystallization. EBSD and semi-quantitative Ti mapping via EPMA and cathodoluminescence imaging are used to correlate quartz microstructure and Ti concentration [Ti]. The Ti-in-quartz thermobarometer is used to estimate P-T conditions during and prior to mylonitization. Relict and ribbon quartz preserve high [Ti] (∼60 ppm) indicative of temperatures >650 °C, whereas recrystallized quartz contains low [Ti] (<5 ppm), reflecting equilibration to temperatures <450 °C. The high-T record may reflect either relict conditions prior to detachment deformation or conditions that prevailed in the early stages of strain localization in the detachment, whereas the low-T record likely reflects the final stages of ductile deformation. The preservation of multiple microstructures with distinct [Ti] facilitates construction of a multi-phase history of linked ductile deformation and Ti equilibration during cooling.

Development of the Median Tectonic Line-related shear zone, southwest Japan: An analysis of strain localization processes

The development of a mylonite zone along the Median Tectonic Line (MTL), southwest Japan, which developed in the Cretaceous granitic rocks, was examined to understand strain localization during cooling. The mylonites are E-W trending, north-dipping, and represent the hanging wall of the MTL. The mylonite zone includes both protomylonite and mylonite, which were derived from two distinct protoliths: tonalite (protomylonite) and granite (mylonite) and occur at distances of c. 0–300 m and 300–700 m from the MTL, respectively. We used quartz microstructures and crystallographic preferred orientations (CPOs) and two-feldspar thermometry to infer the spatiotemporal development of deformation conditions in the MTL mylonite zone. The mylonitic granite is sub-divided into southern and northern mylonite zones, which exhibit A- and B-type quartz microstructure formed by subgrain rotation and grain boundary migration recrystallization, respectively. In both the mylonitic granite and protomylonitic tonalite, quartz c-axis CPOs primarily display a moderate-temperature Y-maximum pattern, as well as a less common low-temperature type-I crossed-girdle pattern. In the northern zone, quartz deformation initially occurred at moderate temperatures of 435 °C–490 °C, forming mylonitic granite with B-type quartz microstructure. As temperature decreased to 355 °C–435 °C, the strain localized into the southern zone to form A-type quartz microstructure. The protomylonitic tonalite formed under similar temperature conditions. At temperatures below 355 °C, ductile deformation ceased in the majority of the granite and tonalite, with only a narrow zone of ultramylonite up to 50 m wide, deforming adjacent to the MTL at conditions approximating the brittle–ductile transition in quartz (c. 300 °C). Strain localization did not occur within a single shear zone, but in several deforming strands that formed in distinct protoliths, one of which developed into the narrow ultramylonite zone reworked as the brittle MTL, likely controlled by cooling.

Stress changes during exhumation of a high-pressure nappe (Phyllite-Quartzite unit, Greece): Insights from quartz microstructures and recrystallized grain size statistics

In this study, we carried out microstructural and grain size analysis of recrystallized quartz in quartz-rich samples collected from different structural levels of a 1.5–2.0 km thick high-pressure nappe (Phyllite-Quartzite unit, Greece). Exhumation-related deformation led to recrystallization of quartz by subgrain rotation (SGR) and the formation of different generations of recrystallized quartz with different grain sizes. Applying a new, fully automated histogram-based method, we reconstructed the “hidden” empirical probability density functions included in the histograms of the initial grain size distributions. Statistical analysis showed that most grain-size histograms display bimodal distributions with mean grain sizes in cluster A ranging from 46 to 48 μm, and a cluster B with mean grain sizes between 81 and 93 μm. Few grain size histograms display trimodal distributions and show an additional cluster C with mean grain sizes ranging from 100 to 115 μm. Microstructural observations indicate that the grains of cluster C are the oldest whereas those of cluster A are the youngest. Bimodal and trimodal distributions may reflect an episodic increase in the differential stress from 17 to 19 MPa and then to 31 MPa during exhumation, accompanied by a strain rate increase of at least one order of magnitude (from 10−13 s−1 to 10−12 s−1). This stress increase likely occurred at the early stages of the exhumation and may have lasted less than 0.5 Myr. Stress increase was followed by a stress drop expressed by partial foam structures of quartz in a few samples.

Microstructures in shocked quartz: linking nuclear airbursts and meteorite impacts

Many studies of hypervelocity impact craters have described the characteristics of quartz grains shock-metamorphosed at high pressures of &gt;10 GPa. In contrast, few studies have investigated shock metamorphism at lower shock pressures. In this study, we test the hypothesis that low-pressure shock metamorphism occurs in near-surface nuclear airbursts and that this process shares essential characteristics with crater-forming impact events. To investigate low-grade shock microstructures, we compared quartz grains from Meteor Crater, a 1.2-km-wide impact crater, to those from near-surface nuclear airbursts at the Alamogordo Bombing Range, New Mexico in 1945 and Kazakhstan in 1949/1953. This investigation utilized a comprehensive analytical suite of high-resolution techniques, including transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). Meteor Crater and the nuclear test sites all exhibit quartz grains with closely spaced, sub-micron-wide fractures that appear to have formed at low shock pressures. Significantly, these micro-fractures are closely associated with Dauphiné twins and are filled with amorphous silica (glass), widely considered a classic indicator of shock metamorphism. Thus, this study confirms that glass-filled shock fractures in quartz form during near-surface nuclear airbursts, as well as crater-forming impact events, and by extension, it suggests that they may form in any near-surface cosmic airbursts in which the shockwave is coupled to Earth’s surface, as has been proposed. The robust characterization of such events is crucial because of their potential catastrophic effects on the Earth’s environmental and biotic systems.

Fluid-enhanced grain-size reduction of K-feldspar from a natural middle crustal shear zone in northern Beijing, China

Grain-size reduction may lead to a transition from grain-size insensitive to grain-size sensitive deformation of rocks in the middle crustal level, thereby affecting the lithosphere's rheology significantly. However, the mechanisms of grain-size reduction of sheared rocks have long been debated. In this study, granitic mylonites from the Shuiyu shear zone of the Yunmengshan metamorphic core complex were sampled to investigate the mechanisms of grain-size reduction of K-feldspar during shearing in the middle crust. The mylonites are characterized by a typical microstructure of coarse-grained K-feldspar porphyroclasts surrounded by fine-grained matrix of oligoclase, K-feldspar, and quartz. Deformation temperatures are constrained from 400 °C to 550 °C, estimated using the feldspar and quartz microstructures, crystal-preferred orientation patterns of recrystallized quartz, and a two-feldspar geothermometer. Here, we show that the derivation of fine grains in the matrix from the K-feldspar porphyroclasts was accomplished via fluid-assisted subgrain rotation recrystallization. Walls of either well-organized or tangled dislocations constituted the boundaries of micron-scale subgrains along the margins of porphyroclasts. Hydrolytic weakening enhanced the dislocation motions to form subgrains. Furthermore, the subgrain rotation recrystallization promoted the replacement reaction of K-feldspar by sodium-rich fluid via increasing the areas of fluid/grain interfaces. The newly formed oligoclase and quartz were subsequently deformed by diffusion creep.

Tectonic setting, fluid inclusion and gold mineralization of the southwest Poli region (northern Cameroon Domain)

This paper documents original petrographic and structural data on metamorphic and magmatic rocks as well as the occurrence of gold mineralization in the Dourou Tchaga area (SW Poli) in the northwestern Cameroon domain. The study area is dominated by high-grade gneisses derived from a volcanic-sedimentary series alternating with orthogneiss and magmatic rocks. Four main deformation stages were distinguish comprising the early D1 and D2 deformations showing ductile behavior under amphibolite to granulite facies, followed by retrogression into greenschist facies. D3 and D4 are characterized by ductile-brittle and late brittle structures. Gold mineralization is hosted in laminated quartz veins in steeply dipping, ENE–WSW-trending, sinistral shear zone developed during D3. Host rocks are high-grade metamorphic paragneiss and orthogneiss. Ore minerals include native gold, galena, sphalerite, pyrite and chalcopyrite; wall rock alteration assemblage includes stable pyrite, magnetite and hematite. This assemblage together with specific quartz microstructures indicate gold mineralization at lower greenschist facies conditions. Gold is disseminated in quartz veins and wall rock alteration zones, where it occurs as free grains but also as inclusions in pyrite and sphalerite. Two types of fluid inclusions are distinguished: aqueous-carbonic CO2–H2O and aqueous (H2O), both with low to moderate (2.0–13.9 wt% NaCleq) salinity and variable CO2 content of 13–23 mole% for the aqueous-carbonic fluid inclusions. Fluid inclusions data and alteration assemblage provide evidence that gold precipitation in quartz veins occurred in response to an increase in fO2 caused by fluid pressure fluctuations in the shear zone. Gold mineralization in the northern Cameroon domain was formed during post-collisional stage and retrograde exhumation of the terrane in the greenschist facies toward the end of the Panafrican Orogeny.

Progressive Development of a Distributed Ductile Shear Zone beneath the Patagonian Retroarc Fold-Thrust Belt, Chile

Abstract The southern Patagonian Andes record Late Cretaceous–Paleogene compressional inversion of the Rocas Verdes backarc basin (RVB) and development of the Patagonian fold-thrust belt (FTB). A ductile décollement formed in the middle crust and accommodated underthrusting, thickening, and tectonic burial of the continental margin (Cordillera Darwin Metamorphic Complex (CDMC)) beneath the RVB. We present new geologic mapping, quartz microstructure, and crystallographic preferred orientation (CPO) fabric analyses to document the kinematic evolution and deformation conditions of the décollement. Within the CDMC, the décollement is defined by a quartz/chlorite composite schistose foliation (S1-2) that is progressively refolded by two generations of noncylindrical, tight, and isoclinal folds (F3–F4). Strain intensifies near the top of the CDMC, forming a &amp;gt;5 km thick shear zone that is defined by a penetrative L-S tectonite (S2/L2) and progressive noncylindrical folding (F3). Younger kink folds and steeply inclined tight folds (F4) with both north- and south-­dipping axial planes (S4) overprint D2 and D3 structures. Quartz textures from D2 fabrics show subgrain rotation and grain boundary migration recrystallization equivalent to regime 3, and quartz CPO patterns indicate mixed prism &amp;lt;a&amp;gt; and [c] slip systems with c-axis opening angles indicative of deformation temperatures between ~500° and &amp;gt;650°C. Approximately 40 km toward the foreland, the shear zone thins (~1 km thick) and is defined by the L-S tectonite (S2/L2) and tightening of recumbent isoclinal folds (F3). Quartz textures and CPO patterns indicate subgrain rotation recrystallization typical of regime 2 and dominantly basal &amp;lt;a&amp;gt; slip, and c-axis opening angles are consistent with deformation temperatures between ~375° and 575°C. Deformation occurred under greenschist and amphibolite facies conditions in the foreland and hinterland, respectively, indicating that the shear zone dipped shallowly toward the hinterland. The Magallanes décollement is an example of a regional ductile shear zone that accommodated distributed middle to lower crustal thickening below a retroarc FTB.

New constraints on the timing and character of the Laramide Orogeny and associated gold mineralization in SE California, USA

Abstract The timing of deformation and associated gold mineralization in SE California, USA, is contentious, partly due to the challenges involved with dating ductile deformation. We therefore combine modern geo- and thermochronology with field and microscopic observations to show that the Cargo Muchacho Mountains preserve evidence of northward thrusting in a kilometer-scale ductile shear zone during the Late Cretaceous Laramide Orogeny, accompanied by hydrothermal fluid flow, gold mineralization, and pegmatite emplacement. Penetrative strain was largely accommodated within the Jurassic metavolcaniclastic Tumco Formation, whereas intrusive Jurassic granitoids behaved as competent bodies. Quartz microstructures suggest deformation at ~500 °C, which is consistent with fabrics defined by amphibolite facies minerals. The timing of thrusting is constrained by dynamically recrystallized titanite with a U-Pb age of 68 ± 1 Ma and late syn-kinematic pegmatites that yield U-Pb zircon ages of 65.0 ± 4.2–63.2 ± 4.8 Ma. Syn-kinematic fluid flow was focused into a lateral thrust ramp where the shear zone foliation was deflected around a relatively rigid pluton, creating zones rich in magnetite-quartz veins and epidote, and precipitating gold associated with pyrite and chalcopyrite. Dating of these sulfides via Re-Os yields an age of 64.7 ± 0.8 Ma, which confirms a Laramide age for the gold mineralization. Together, apatite from the pegmatites and a nearby Jurassic granite yields a U-Pb age of 60.4 ± 3.5 Ma, reflecting cooling to below 530–450 °C. Comparison with published studies suggests that thick-skinned deformation in the Cargo Muchacho Mountains was driven by flat-slab subduction of the conjugate Hess Plateau, which occurred several million years after and to the south of flat-slab subduction of the conjugate Shatsky Rise. This suggests that the conjugate Hess Plateau may have been subducted up to several hundred kilometers farther north than previously thought. Metamorphic devolatilization of underplated Orocopia Schist likely generated the gold-bearing hydrothermal fluids, and anatexis of the schist formed the peraluminous pegmatites, which highlights the importance of schist underplating and devolatilization along much of the Californian and Mexican cordillera.

Formation of radiator structures in quartz veins - Phase-field modeling of multi-crack sealing

The present work showcases a comprehensive phase-field study on the formation of multi-crack-seal veins in quartz microstructures. The microstructure simulation framework Pace3D incorporates the modeling of both fracturing and sealing in polycrystalline rock systems, where crystallographic anisotropies, crack resistances and growth velocities of different facets are included in the energy density contributions. In a two grain system we exemplary show the formation of three different archetypes of radiators which are observed in natural quartz veins. We extend the phase-field studies to a polycrystalline host rock and investigate therein the effects of fracture aperture, presence of accessory minerals, and oblique opening trajectories with refracturing of fully and partially sealed veins on the occurring vein crystal morphology. Our results show that the shape of the forming grain boundaries depends on the crystal orientation of the participating grains, the location of a fracture through the crystals, and the sealing state. Evolution of more pronounced radiator structures are found in simulation studies with increasing aperture. A broad spectrum of crystal structures which are frequently observed in natural quartz vein microstructures is reproduced in simulations using the combined crack-seal phase-field model.

Steady-State Microstructures of Quartz Revisited: Evaluation of Stress States in Deformation Experiments Using a Solid-Medium Apparatus

Dynamically recrystallizing quartz is believed to approach a steady-state microstructure, which reflects flow stress in dislocation creep. In a classic experimental study performed by Masuda and Fujimura in 1981 using a solid-medium deformation apparatus, two types of steady-state microstructures of quartz, denoted as S and P, were found under varying temperature and strain rate conditions. However, the differential stresses did not systematically change with the deformation conditions, and unexpectedly high flow stresses (over 700 MPa) were recorded on some experimental runs compared with the applied confining pressure (400 MPa). Internal friction in the sample assembly is a possible cause of reported high differential stresses. Using a pyrophyllite assembly similar to that used in the previous work and setting up paired load cells above and below the sample assembly, we quantified the frictional stress acting on the sample and corrected the axial stress. The internal friction changed in a complicated manner during pressurization, heating, and axial deformation at a constant strain rate. Our results suggest that Masuda and Fujimura overestimated the differential stress by about 200 MPa in their 800 °C runs. Crystallographic fabrics in the previous experimental sample indicated that the development of elongated quartz grains, which are characteristics of Type-S microstructures, was associated with preferential growth of unfavorably oriented grains during dynamic recrystallization.

The rheology and deformation of the South Tibetan Detachment System as exposed at Zherger La, east-central Himalaya: Implications for exhumation of the Himalayan metamorphic core

Deformation processes and rheological changes within the South Tibetan Detachment System (STDS) played a key role in accommodating exhumation of the Greater Himalayan Sequence (GHS). We present a study for the STDS in Zherger La, Yadong region, east-central Himalaya, to better quantify the spatial and temporal variations of the shear zone's internal deformation and rheology properties. Quartz microstructures and crystallographic preferred orientations, together with quantitative estimates of deformation temperatures, differential stresses, strain rates and viscosities are deduced from the ductile segment of the detachment zone. The mylonites that comprise the exposed shear zone preserve a range of deformation conditions that are interpreted in terms of progressive exhumation. Strain localization within the STDS is indicated by the microstructural evolution from penetrative deformation of constitutive minerals to localized deformation into quartz, which exhibit interconnected networks and high strain zones. Deformation temperatures decrease from 510 ± 40 to 405 ± 50 °C and differential stresses increase from 24 to 100 MPa structurally upwards, as indicated by two-feldspar thermometry and quartz paleopiezometry, respectively. Strain rates derived from quartz dislocation creep flow law are on the order of 10−12-10−13 s−1 and are broadly consistent with macrostructurally-derived strain rates of 1.8–5.1 × 10−13 s−1, which are calculated from previously determined exhumation rates and shear zone width. The estimated differential stresses and strain rates are further utilized to determine the variation of viscosities across the STDS. The viscosities within the STDS range at 1019–1020 Pa s. In conjunction with the published numerical modelling and analog modeling studies, the estimated viscosity data are compatible with the channel flow model for exhumation.

A shape-change model for isolated K-feldspar inclusions within a shear zone developed in the Teshima granite, Ryoke metamorphic belt, Japan: Estimation of the duration of deformation in a natural shear zone

A shear zone developed in the Teshima granite in the Ryoke metamorphic belt of Japan contains isolated K-feldspar inclusions showing various shapes within deformed host quartz grains. We present a shape-change model for isolated K-feldspar inclusions, which is a mathematical equation that considers stress, inclusion size, temperature, and the duration of deformation. To calculate the shape-change pattern (i.e., the aspect ratio and size of inclusions) with respect to deformation duration using the model, we used values for deformation temperature (~500–600 °C) and paleo-stress (90 ± 10 MPa) estimated from deformed quartz microstructures within the shear zone. Shape-change patterns were obtained by calculating the model at the estimated range of temperatures with an interval of 25 °C and at the estimated paleo-stress. Shape-change patterns were subsequently corrected by compensating for the effect of post-deformation annealing during exhumation of the shear zone. Deformation duration was estimated by identifying the shape-change pattern calculated by the model that most closely corresponded to the observed shape-change pattern for the shear zone and considering the cooling rate of the Ryoke belt. The resultant deformation duration of the shear zone is estimated as 16,500–8400 yr under conditions of T = 550 °C and σ = 90 ± 10 MPa. Shear strain in the shear zone is calculated as 50–260 by multiplying the shear strain rate by the deformation duration.

Metamorphic tourmaline and its petrogenetic significance from the Maramureș Mountains (East Carpathians, Romania)

Abstract This study describes mineralogical and crystallochemical characteristics of metamorphic tourmalines from an Alpine shear zone in a Variscan metamorphic rock sequence from the Maramures region in the northern part of the East Carpathians. We use this mineral to unravel aspects of the evolution of the tourmaline bearing host rocks and compare the crystallo-chemical characteristics to other tourmalines from Alps. Petrographic and microstructural observations, as well as electron microprobe analyses on several zoned tourmalines and associated minerals (mica, feldspar) from mylonitic schist of the Rebra terrane (Maramureș Mountains), indicate that the pre-kinematic tourmalines belong to the alkali group (Na dominant), hydroxyl dominated on the crystallographic W-site and can be assigned to the species dravite and schorl. The tourmaline-bearing rocks have a metasedimentary protolith. The analysed porphyroblasts, rotated by simple shear, show corroded rim that are interpreted to have formed due to pressure release. Three main compositional zones were evidenced on a tourmaline porphyroblast: a core zone and two asymmetrically arranged inclusion-poor/free rims, all formed in pre-alpine prograde metamorphic conditions. Based on mineral microstructural relations and geothermobarometry (tourmaline–muscovite, tourmaline–plagioclase geothermometry and phengite geobarometry), the metamorphic peak conditions of the investigated Rebra terrane were evaluated to have been at a temperature of ca. 590 to 620 ± 22 °C and Pmin = 5.5 - 6.0 ± 0.5 kbar. By observing dynamically recrystallized microstructures in quartz and feldspar in the shear zone a temperature of 350 - 400 °C was estimated and the quartz paleopiezometry outlined a differential stress of about 1.5 kbar that implied only minor chemical change in tourmaline outer zone.