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

Abstract

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