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_Journal of Structural Geology 33 (2011) 244-254_ _Natural annealing of dynamically recrystallised quartzite fabrics: Example from the Cévennes, SE French Massif Central_ _Clemens Augenstein*, Jean-Pierre Burg_ _Department Earth Sciences ETH and University Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland_ _Article info_ _Article history: Received 6 March 2010; Received in revised form 13 October 2010; Accepted 18 October 2010; Available online 5 November 2010_ _Keywords: Cévennes Massif Central Quartz CPO EBSD Annealing Dynamic recrystallisation_ _Abstract_ _Quartzite samples from the SW-Cévennes (French Massif Central) have been investigated in order to reveal the effects of annealing on a previously dynamically recrystallised quartz fabric. The studied quartzite is interlayered with turbiditic micaschist series, and after regional deformation the whole sequence was intruded by the St-Guiral granodiorite at 1-3 kbar and up to 650-700 x14C. Recorded crystallographic preferred orientation (CPO) patterns of the dynamically recrystallised quartzite are classified as type I with a monoclinic symmetry and type II with an orthorhombic symmetry. Type I is related to simple shear-dominated deformation and type II to apparent constriction. These two fabric types are consistent with structural observations that a constrictive and coaxial deformation regime controlled the formation of non-cylindrical and refolded folds. In the metamorphic aureole, which produced essentially static annealing, CPO patterns intensify slightly and grain size increases towards the granodiorite until the temperature at which inversion from low-to high quartz is triggered. This inversion caused a volume increase and subsequent intergranular stresses activated slip systems fitting the temperature and water content._ _This study shows that annealing of dynamically recrystallised quartz crystals can activate previously inactive slip systems, reactivate slip systems and amplify older CPO fabrics. This conclusion identifies limits for the use of annealed quartz fabrics in reconstructing structural histories when earlier dynamic recrystallisation has occurred._ _© 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _The effects of annealing on quartz CPO fabrics have been investigated in several experimental studies (e.g., Green, 1967; Green et al., 1970; Heilbronner and Tullis, 2002) and some natural examples (Park et al., 2001; Piazolo et al., 2005; Otani and Wallis, 2006; Trepmann et al., 2010). The major drawback of experimental studies is the difficulty in extrapolating the process to geological timescales. This is especially the case for the annealing of dynamically recrystallised fabrics. We consider natural dimensions in time and space are essential for understanding and applying quartz fabrics to geological problems, and this can only be achieved by studies on naturally recrystallised and annealed rocks. However, natural examples of recrystallised and subsequently annealed quartz fabrics are rare. Previous studies show very few or no comparisons of annealed versus un-annealed samples in the same_ _* Corresponding author. Present at: Research School of Earth Sciences, ANU, Canberra, ACT, 0200 Australia._ _E-mail address: clemens.augenstein@anu.edu.au (C. Augenstein)._ _0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.10.008_ _geological setting._ _Park et al. (2001) and Piazolo et al. (2005) describe only one annealed natural sample and do not compare its CPO with those from un-annealed samples. Otani and Wallis (2006) show examples where initial dynamic recrystallisation of quartz took place during a range of high-T low-P conditions, but only compare the CPOs of a small number of annealed to un-annealed samples. Trepmann et al. (2010) investigated two annealed samples without a direct comparison to un-annealed rocks from the same initial dynamic recrystallisation. In addition to the limited number of samples, previous studies on natural examples just apply the results from experiments thus assuming that annealing has no major effect on previously established quartz CPO patterns._ _Our study documents a natural example where dynamic recrystallisation occurred in a well-constrained setting and observed quartz fabrics represent dynamically recrystallised and annealed samples. Additionally, we have studied nearly twice the amount of samples analysed in previous studies._ _The dynamic recrystallisation recorded in our samples took place during medium-T and medium-P metamorphism and was overprinted by a high-T event related to a granodioritic intrusion._ _C. Augenstein, J.-P. Burg Journal of Structural Geology 33 (2011) 244-254_ _245_ _Fig. 1. Geological map of the study area with location of CPOs in Figs. 5 and 6. Inset: location of the Cévennes in the Massif Central and France._ _If static annealing changes pre-existing quartz CPOs, the stability of CPOs is questionable, and with it the robustness of the information derived from quartz CPOs in terms of regional kinematics._ _The studied quartzite layer extends from the contact to the granodiorite to beyond the contact aureole (Fig. 1). This allows comparison of CPO patterns of dynamically recrystallised samples from outside to inside the contact aureole and so documents the effects of a progressive increase in temperature on static annealing of dynamically recrystallised quartz fabrics. This rare geological setting was chosen to gain new insights into the behaviour of dynamically recrystallised quartz crystals during natural annealing._ _Detailed structural and petrological investigations have been carried out in order to set the kinematic context of the CPO patterns that were obtained by electron back scatter diffraction (EBSD) measurements in a scanning electron microscope (SEM) at the Electron Microscopy Center at the ETH Zurich (EMEZ)._ _2. Geological setting_ _The study area is located in the SE of the French Massif Central close to the village of St-Jean-du-Bruel and is part of the "Cévennes méridionales" (Fr.: southern Cévennes, Fig. 1). The Cévennes are part of the western European Variscan Belt that resulted from the Devonian-Carboniferous collision between Gondwana in the S with Laurussia in the N, and some intervening continental blocks (e.g., Matte, 2001). The Cévennes consist mainly of micaschists, gneisses and several granitic plutons. The micaschists are regarded as the southern para-autochthon on which high-grade thrust sheets were_ _emplaced southward with a displacement of about 200 km during the Variscan orogeny (Burg and Matte, 1978; Matte and Burg, 1981; Ledru et al., 1989; Matte, 1991; Arnaud et al., 2004). The Cévennes micaschists, in turn, have been thrust southward over the so-called_ _8 P (kbar)_ _6_ _KFMASH KMASH KASH_ _kyanite sillimanite_ _4_ _andalusite_ _2_ _0 500_ _phMlogg-ocpoirtdeiemruitescoKv-ifteeldspar alummuossciolicvaiteteqKu-afertlzdspar_ _550_ _600_ _biotite spinel K-mfeuldsscpoavritealumosilicate cordierite_ _T( ° C )_ _650_ _700_ _750_ _Fig. 2. P-T grid with relevant, calculated KFMASH system reactions plus H2O at the high-T side. Dot with error bars gives contact metamorphic conditions next to the St-Guiral granodiorite intrusion (GPS: 03 x142502.700E; 44 x142049,800N) from this study. Square_ _with error bars: conditions obtained by Najoui et al. (2000) about 3 km north of our sample location. Regional conditions determined by Arnaud (1999) lay out of this diagram, at T < 500 °C for P ca. 4 kbar._ _246_ _C. Augenstein, J.-P. Burg Journal of Structural Geology 33 (2011) 244-254_ _Fig. 3. Images from the study area. a) the studied quartzite layer in normal position between the light-coloured (below) and black (above) micaschist series; b) quartz rods defining the N-S trending mineral stretching lineation in a quartz-rich sequence; c) shear bands (C) in micaschists deforming the main foliation (S0 S1 S2) and providing evidence for top-to-SW shear; d) cut through a non-cylindrical F2 fold, "refolding" an F1 isoclinal fold; e) F2 recumbent fold with strongly fanning S2 sub-parallel to S0 and S1 in the lower, inverted limb, and markedly oblique to these earlier planar structural elements in the top, normal limb; note thinner bedding on inverted limb indicates stronger shearing f) F2 folds with transverse mineral (elongation) lineation on S2 in the lower, normal limb._ _C. Augenstein, J.-P. Burg Journal of Structural Geology 33 (2011) 244-254_ _247_ _Le Vigan and Monts-de-Lacaunes series (Demay, 1948; Burg and Matte, 1978; Arnaud et al., 2004). The greenschist to lower amphibolite facies metamorphism of the micaschists in the Cévennes méridionales was estimated at 400-500 °C and 4.3 ± 0.4 kbar (Arnaud, 1999) and dated at around 340 Ma (AreAr on biotite and muscovite; Caron, 1990; Faure, 1995) to the NE of the study area. Regional metamorphism was overprinted in most of the Cévennes méridionales by a high-T low-P metamorphism (>680 °C, 4-5 kbar) at about 325 Ma (AreAr on amphibole and biotite; Najoui et al., 2000). This high-T event was followed by the granodiorite intrusions of the St-Guiral-Aigoual-Liron pluton at 311-306 Ma (U-Pb on zircon and AreAr on muscovite and biotite; Brichau et al.,_ _2008). These granitic intrusions are related to extension at the end of the Variscan orogenesis (Burg et al., 1994; Faure, 1995). The synto post-orogenic extension lasted from 320 to 280 Ma (Arnaud et al., 2004). The working area (Fig. 1) includes the western margin of the St-Guiral pluton, which intruded at about 1-3 kbar and 650-700 °C (Najoui et al., 2000; and this study; Fig. 2). The two main metasedimentary formations of the Cévennes micaschists, a black (Cambrian?) and a lighter coloured (Ordovician?) metapelitic and sandy sequence, cover the area. The studied quartzite layer separates the two micaschists, is discontinuous and up to several tens of meters thick (>90% quartz, Fig. 3a) with primary thickness varying._ Ключевые слова: najoui, foliation, tectonophysics, micaschists, folding, experimental, mm, grain growth, dynamic, quartz fabric, parallel, shear, apparent constriction, data, non-cylindrical, grid, plastic deformation, water content, cpo pattern, high-t, m-plane pole, matte, contact aureole, constant orientation, static annealing, refolded fold, strain, science, demay, inversion, quartzite, measurement, matte arnaud, geological, caron, slip, quartz, faure, park, tectonics, lineation, fabric, foliation plane, geophysical, burg, study, size, event, hobbs schmid, ?nite, spear, type, kbar, massif central, rock, tullis, mica content, france, arnaud, meyer, structural, ebsd, static, sub-parallel, pole, high quartz, maximum, geology, cpo, massif, central, preferred, casey, regional scale, high-t event, study area, review, cvennes micaschists, main foliation, map area, c?vennes, structural geology, plot, experimental study, hobbs, deformation, annealed, iniesta, program, area, ledru, green, ramsay, fabric development, intrusion, -p, inverted limb, cvennes, augenstein, la, petrology, lister, fold, wa, content, c-axis, french, piazolo, rst step, a-axis, overprinted, dynamically, rhomb, direction, jeol superprobe, girdle, dynamically recrystallised, comparison, lloyd, thermocalc powell, recrystallisation, variscan belt, granodiorite intrusion, powell, quartz inversion, blacic, dehydration reaction, mica, sample, recrystallised, high, arear, granodiorite, contact metamorphism, journal structural, wallis, trepmann, previous, regional, axis, aureole, wenk, grain, coloured, ax, ma, temperature, mineral, metamorphic, inactive slip, wirth, st-guiral granodiorite, moni?, studied, increase, calculated, grain size, variscan, zurich, natural, isoclinal fold, perpendicular, pattern, ?nite strain, orientation, annealing, valle, metamorphism, observation, structural observation, generated, society, contact, history, curved hinge, nite strain, journal structural geology, mineralogy, plane, studied quartzite, journal, etchecopar, ii, holland, condition, shearing, thesis academie, burg journal, g?ze, dynamic recrystallisation, distribution