Kinematics of low-temperature intracrystalline deformation microstructures in quartz - Examples from quartz veins in the High-Ardenne slate belt (Belgium, Germany)

In order to use intracrystalline deformation structures in quartz as palaeopiezometer and as indicator of palaeostress/strain orientation, the microstructure formation processes and the controlling parameters should be fully constrained. Reviewing genetic interpretations of quartz deformation microstructures in the literature shows that many often genetically inspired terms are being intermixed. Moreover, the formation processes of these microstructures are still disputed. Therefore, a purely descriptive terminology is proposed, categorising the intracrystalline extinction bands: fine extinction bands (FEBs), wide extinction bands (WEBs) and localised extinction bands (LEBs) of which the LEBs are subdivided in blocky (bLEB), straight (sLEB) and granular (gLEB) morphological types. With polarised light microscopy, FEBs are narrow (<5µm wide), densely spaced bands with a difference in extinction angle <5°. WEBs are broader (<100µm wide) bands with a difference in extinction angle <20°, sub-parallel to the projection of the c-axis towards the thin section. LEBs occur in conjugate sets showing opposite crystal lattice rotations (<60°) with respect to the crystal lattice outside the LEBs. Vein-quartz samples from the High-Ardenne slate belt (Belgium, Germany), (de-)formed at different stages during the late Palaeozoic Variscan orogeny, are selected because of their well-known (de-)formation history reconstructed in previous studies. Using polarised light microscopy, universal stage microscopy, a range of scanning electron microscopy techniques (forescatter imaging, electron backscatter diffraction and cathodoluminescence) and transmission electron microscopy bright field imaging, we contribute to a better understanding of the extinction band formation processes and the interplay between brittle and crystal-plastic deformation. A close relationship between fluid inclusions and the deformation microstructures is shown to be more important than previously thought. Fluid inclusions are suggested to cause strain softening, to impede crystal-plastic deformation and in some cases to be redistributed themselves. FEBs are demonstrated to represent a range of different nanostructures arising from a variety of formation processes. Moreover, two new nanostructures are identified that are not related to FEBs before. FEBs observable with polarised light microscopy can result from the optical effect of bands with a variable crystal lattice orientation, or of bands with a high dislocation density. Multiple FEBs per grain are common. For WEBs parallel to the c-axis, we follow the interpretation as commonly presented in the literature, explaining WEBs as tilt walls formed by basal slip, though a bounding effect of fluid inclusion planes is additionally emphasized. Similar to WEBs, bLEBs are suggested to form by crystal-plastic deformation, bounded by existing fluid inclusion planes. bLEBs form parallel to the orientations of maximum shear stress by a range of slip systems. The blocking of dislocations against fluid inclusions causes fluid inclusion decrepitation, with the shape of the decrepitated fluid inclusions in turn affecting the crystal-plastic deformation. Two types of sLEBs are distinguished, fluid inclusion-rich continuous extinction bands and fluid inclusion-poor en échelon arranged extinction bands. The first type is related to crystal-plastic deformation taking place inside the width of a fluid inclusion plane. The fluid inclusions are commonly redistributed parallel to FEBs present, or parallel to the involved slip plane. The second type of sLEBs are interpreted to be elongate subgrains formed by difficultly activated slip systems, in strain-softened zones, or in WEBs that are strain hardened with regards to the easy slip systems. gLEBs form in zones of intense strain, by dislocation pile-up against particles (e.g. fluid inclusion, micas) and subsequent recovery. Dauphiné twin boundaries are never considered crucial in the microstructure formation, they merely influence each other. We do not recommend FEBs for palaeopiezometry. FEBs and LEBs are, however, not randomly oriented and can therefore be used as indicators for palaeostrain and possibly palaeostress, on the condition that a high amount of orientation data is collected, as not all FEBs and LEBs are formed parallel to the maximum shear stress orientation. For every studied deformation microstructure, a new or adapted formation process is put forward. Especially the interaction between crystal-plastic deformation and fluid inclusions, and the geometrical relationships between the microstructures are innovative. Since the proposed formation processes are diverse, the use of a purely descriptive terminology for the microstructures is highly recommended.status: publishe

Intracrystalline deformation microstructures in vein-quartz

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Low-temperature intracrystalline deformation microstructures in quartz

A review of numerous genetic interpretations of the individual low-temperature intracrystalline deformation microstructures in quartz shows that there is no consensus concerning their formation mechanisms. Therefore, we introduce a new, purely descriptive terminology for the three categories of intracrystalline deformation microstructures formed in the low-quartz stability field: fine extinction bands (FEB), wide extinction bands (WEB) and localised extinction bands (LEB). The localised extinction bands are further subdivided into blocky (bLEB), straight (sLEB) and granular (gLEB) morphological types. A detailed polarised light microscopy study of vein-quartz from the low-grade metamorphic High-Ardenne slate belt (Belgium) further reveals a series of particular geometric relationships between these newly defined intracrystalline deformation microstructures. These geometric relationships are largely unrecognised or underemphasised in the literature and need to be taken into account in any future genetic interpretation. Based on our observations and a critical assessment of the current genetic models, we argue that the interpretation of the pertinent microstructures in terms of ambient conditions and deformation history should be made with care, as long as the genesis of these microstructures is not better confined.status: publishe

Significance of geometrical relationships between low-temperature intracrystalline deformation microstructures in naturally deformed quartz

Although quartz is one of the most studied minerals in the Earth’s crust when it comes to its rheology, the interpretation of intracrystalline deformation microstructures with respect to deformation conditions and mechanisms, remains highly contentious. Moreover, inconsistent use of terminology for both deformation microstructures and mechanisms makes a correct assessment of observations and interpretations in published material very difficult. With respect to low-temperature intracrystalline deformation microstructures in quartz, different conflicting genetic models have been proposed. Most probably, the lack of consensus means that there is no unique interpretation for these microstructures, primarily because their initiation and development depend on many ambient conditions. We extensively studied these intracrystalline deformation microstructures by means of optical microscopy, Hot-Cathodoluminescence, SEM-Cathodoluminescence and Electron Backscatter Diffraction Orientation Imaging, in vein quartz of the High-Ardenne slate belt (Belgium, France, Luxemburg, Germany), (de)formed in a low-temperature regime. Firstly, we propose a new, purely descriptive terminology for the low-temperature intracrystalline deformation microstructures in naturally deformed quartz: fine extinction bands (FEB), wide extinction bands (WEB) and strings. The strings can be further subdivided into blocky (BS), straight (SS) and recrystallised (RS) morphological types. FEBs have consistently been called deformation lamellae in quartz and planar slip bands in metals. WEBs have been called deformation bands, prismatic kink bands or type II kink bands. Strings have formerly been called shear bands, deformation bands or type I kink bands. No distinction between blocky and straight morphological string types had ever been made. Secondly, a survey of the pre-recrystallisation stages in the history of the intracrystalline deformation microstructures reveals that the different types of low-temperature intracrystalline deformation microstructures in naturally deformed vein quartz show particular geometrical relationships, in our opinion a to date underexposed aspect of these microstructures. Several of these geometrical relationships will be presented and their potential implications with respect to deformation mechanisms and conditions will be discussed. The geometrical relationships observed may suggest a similar formation mechanism for the different microstructures, a weakening effect for successive microstructure formation and a strong dependency on the crystallographic orientation.status: publishe

Low-temperature intracrystalline deformation microstructures in naturally deformed quartz, haven't you noticed them in your samples?

With an estimated presence of 62%, quartz is the most common mineral in the continental crust. The understanding of the deformation properties of quartz is therefore crucial in understanding the rheological behaviour of the continental crust. Although quartz is considered to be one of the best-known minerals concerning its nature of deformation, it is still contentious to unequivocally interpret deformation microstructures with respect to deformation conditions and mechanisms. Inconsistent use of terminology and the use of genetic terminology makes it very difficult to correctly assess all observations and genetic interpretations in published material. A large variety of models for the formation of low-temperature intracrystalline deformation microstructures has been suggested. Contradictions between the models show that their validity is still ambiguous and that there is probably no unique interpretation, as the microstructures depend on many ambient conditions as temperature, strain, strain-rate, crystallographic orientation with respect to the stress state, stress level, pressure, presence of fluida, etc. Moreover, these intracrystalline deformation microstructures have been observed in experimentally and in naturally deformed quartz. There is, however, still a need for more detailed observations in quartz deformed in a larger range of natural conditions, in order to properly correlate experimental conditions with conditions in the Earth’s crust. The low-temperature deformation microstructures referred to, are commonly observed with optical microscopy: (1) zones with a misorientation of less than 10° to the host crystal, that often contain fluid inclusions along their boundaries; (2) narrow (<2μm thick), lenticular planar elements that have a misorientation around 2° to 5° with the host crystal, that can be undulatory and wavy and occur in closely spaced, parallel sets with a close spacing around 4-5μm; (3) elongate bands with undulose extinction, up to 100μm in width, that have a misorientation with the host crystal between 5° and 10° and are mostly elongate parallel to the optical c-axis; (4) conjugate strings of square to rectangular zones, around 20-30μm in width, in which the misorientation (up to 60°) with the host crystal is in an opposite direction with respect to the host crystal; (5) conjugate anastomosing narrow zones, around 5μm in width, in which the misorientation (up to 60°) with the host crystal is in an opposite direction with respect to the host crystal, containing a high amount of decrepitated fluid inclusions. We propose to name these features (1) subgrains, (2) fine extinction bands, (3) wide extinction bands, (4) blocky strings and (5) straight strings. We prefer this more descriptive terminology than the wide variety of names that is currently being used. For example, the wide extinction bands have been called deformation bands, prismatic kink bands and type II kink bands. Additionally, extensive use of microphotographs is imperative for accurate correlation between different studies. Because a picture is worth a thousand words!status: publishe

Low-temperature intracrystalline deformation microstructures in quartz

A review of numerous genetic interpretations of the individual low-temperature intracrystalline deformation microstructures in quartz shows that there is no consensus concerning their formation mechanisms. Therefore, we introduce a new, purely descriptive terminology for the three categories of intracrystalline deformation microstructures formed in the low-quartz stability field: fine extinction bands (FEB), wide extinction bands (WEB) and localised extinction bands (LEB). The localised extinction bands are further subdivided into blocky (bLEB), straight (sLEB) and granular (gLEB) morphological types. A detailed polarised light microscopy study of vein-quartz from the low-grade metamorphic High-Ardenne slate belt (Belgium) further reveals a series of particular geometric relationships between these newly defined intracrystalline deformation microstructures. These geometric relationships are largely unrecognised or underemphasised in the literature and need to be taken into account in any future genetic interpretation. Based on our observations and a critical assessment of the current genetic models, we argue that the interpretation of the pertinent microstructures in terms of ambient conditions and deformation history should be made with care, as long as the genesis of these microstructures is not better confined

Three sets of crystallographic sub-planar structures in quartz formed by tectonic deformation

In quartz, multiple sets of fine planar deformation microstructures that have specific crystallographic orientations parallel to planes with low Miller-Bravais indices are commonly considered as shock-induced planar deformation features (PDFs). 1 diagnostic of shock metamorphism. Using polarized light microscopy, we demonstrate that up to three sets of tectonically induced sub-planar fine extinction bands (FEBs), sub-parallel to the basal, γ, ω, and π crystallographic planes, are common in vein quartz in low-grade tectonometamorphic settings. We conclude that the observation of multiple (2-3) sets of fine scale, closely spaced, crystallographically controlled, sub-planar microstructures is not sufficient to unambiguously distinguish PDFs from tectonic FEBs

Brittle versus crystalplastic deformation in intracrystalline shear bands in quartz

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The relative importance of brittle versus crystalplastic deformation mechanisms in quartz in the semi-brittle regime

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Regional significance of non-cylindrical folding in the northwestern part of the High- Ardenne slate belt (Redu-Daverdisse, Belgium)

Regional mapping and a detailed geometric analysis of complex, mixed brittle-ductile, fold-related accommodation structures, along the well-exposed banks of the Lesse river between Redu and Daverdisse (Belgium), reveal that the finite strain in the predominantly incompetent Lower Devonian rock sequence in the northwestern part of the High-Ardenne slate belt deviates from pure flattening and approximates plane strain. This finite strain is materialised by different manifestations of non-cylindrical folding, i.e. regional en-echelon and periclinal fold geometries, mesoscale non-cylindrical folds with hinges showing variable plunges, oblique flexural slip and locally a non-axial planar, transecting cleavage. The observed non-cylindrical folding fits in the regional framework of the Meuse Valley Recess, a transpressional corridor in the Ardenne allochthon that developed on top of a buried, buttressing oblique ramp in the pre-structural basement. Differential propagation in the overriding Ardenne allochthonuous domain to the east and west of this buttressing oblique ramp led to a component of lateral shortening on top of the ramp, resulting in the rotation of the overall structural grain and the development of en-echelon, non-cylindrical folds. Our study suggests that the Meuse Valley Recess can be continued towards the southeast, at least affecting the northern parts of the High-Ardenne slate belt.status: publishe