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_Journal of Structural Geology 32 (2010) 202–215_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg Static recrystallization of vein quartz pebbles in a high-pressure – low-temperature metamorphic conglomerate Claudia Trepmann*, Annette Lenze, Bernhard Stöckhert Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Sonderforschungsbereich 526, Germany Article info Article history: Received 6 April 2009 Received in revised form 11 November 2009 Accepted 15 November 2009 Available online 23 November 2009 Keywords: Quartz Static recrystallization Strain-induced grain boundary migration Recovery Fluid inclusions Abstract Microfabrics of quartz pebbles in HP-LT (c. 1 GPa, c. 350°C) metamorphic conglomerates are investigated. The conglomerate was deformed by dissolution-precipitation creep, while the interior of the pebbles remained undeformed. Different pebbles display a wide variety of quartz microstructures imported from source rocks. One type of pebble is derived from quartz veins; it shows old grains with numerous fluid inclusions, subgrains, and undulatory extinction, which are partly replaced by new grains devoid of inclusions and substructure. Free dislocation densities are on the order of 1012 m-2 in both grains. We conclude that: (1) the quartz vein underwent inhomogeneous crystal-plastic deformation in the source rock; (2) recrystallization took place by strain-induced grain boundary migration starting from small crystalline volumes poor in defects; (3) recrystallization was purely static and commenced during re-burial of the conglomerate; which (4) was simultaneously deformed by dissolution-precipitation creep at low differential stress, insufficient for crystal-plastic deformation of quartz; (5) fluid inclusions within old grains were eliminated and their fluid content was drained along migrating high-angle grain boundaries; and (6) strain-induced grain boundary migration ceased once the driving force became too low by static recovery (concurrent to recrystallization) within deformed old grains. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Recrystallization is the migration and/or formation of high-angle grain boundaries driven by the reduction of stored strain energy (White, 1977; Haessner and Hofmann, 1978; Vernon, 1981; Jessell, 1986; Urai et al., 1986; Drury and Urai, 1990). Recrystallization is a thermally activated process. Thus, it requires sufficient temperatures, usually above a homologous temperature Tm of about 0.4 (Tm denoting the absolute melting temperature of the material). Recrystallization can be dynamic or static. Dynamic recrystallization occurs contemporaneous with deformation, in the regime of dislocation creep. Static recrystallization occurs without deformation in the absence of differential stress. It reduces the stored strain energy in a material that has acquired a high dislocation density during a preceding stage of glide-controlled crystal-plastic deformation at lower temperatures and higher stress in the low-temperature plasticity regime, where recovery and recrystallization are not effective. Dynamic recrystallization is common in all earth materials deforming by creep at elevated temperatures (T > Tm above about 0.4) and at moderate to high-stress in the * Corresponding author. Tel.: +49 234 322 7765; fax: +49 234 321 4572. E-mail addresses: claudia.trepmann@rub.de (C. Trepmann), annette.lenze@rub.de (A. Lenze), bernhard.stoeckhert@rub.de (B. Stöckhert). 0191-8141 $ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2009.11.005 deeper crust and mantle, as demonstrated by the microstructure of exhumed metamorphic rocks. In contrast, static recrystallization is less commonly reported for natural earth materials. Here, the term static is taken to mean recrystallization without notable contemporaneous crystal-plastic deformation, hence at very low differential stress. Strictly speaking, the process should be termed quasistatic, as an environment where differential stress is approaching zero is probably a very rare exception in the interior of the Earth. Four different geological scenarios leading to static recrystallization can be envisaged. Each involves a preceding stage of crystal-plastic deformation at low-temperature and/or high strain rate. (A) A rock, where minerals had undergone glide-controlled plastic deformation at high differential stress in the upper crust, is progressively buried and finally enters a very low-stress environment with higher temperatures at greater depth. The relevance of scenario (A) in nature is difficult to assess: firstly, a tectonic situation where a rock is plastically deformed at moderate temperatures in the upper crust and then enters a nearly stress-free environment when buried into the deeper crust by active tectonics, is not very likely. Active tectonics can be expected to cause deformation by dislocation creep accompanied by dynamic recrystallization in a deeper crustal level. Secondly, microstructures developing during progressive burial would readily be overprinted during exhumation by a sequence of processes commonly including high-stress C. Trepmann et al. Journal of Structural Geology 32 (2010) 202–215 203 deformation at the crustal scale brittle-ductile transition horizon. This leaves little potential for identification of an early stage of static recrystallization during burial. (B) A rock, whose minerals had undergone some crystal-plastic deformation during a preceding stage of the tectonic history, is heated by intruding magma in the upper crust, which leads to stress relaxation on a regional scale and annealing processes on the microscale. This scenario may be common wherever large magma bodies intrude the upper crust in a tectonically calm environment, for instance in regions of intraplate hot spot magmatism. In this case, the intracrystalline strain energy has been acquired when the host rock passed upwards earlier through the crustal scale brittle-ductile transition horizon, for which the highest stress levels in the Earth’s interior are expected. Afterwards the temperatures were too low to drive thermally activated processes, until heating of the cool upper crust by intruding magma. The general validity of scenario (B) is supported by observations indicating static recrystallization and grain growth in contact aureoles surrounding plutons with a shallow level of emplacement (e.g., Buntebarth and Voll, 1991; Wirth, 1985; Piazolo et al., 2005; Otani and Wallis, 2006). (C) Instantaneous loading related to stress redistribution during seismogenic faulting causes glide-controlled crystal-plastic deformation, commonly localized along fractures and in small-scale plastic shear zones (Trepmann et al., 2007). This can take place at elevated temperatures in the middle or deeper crust (Trepmann and Stöckhert, 2003; Nüchter and Stöckhert, 2008; Birtel and Stöckhert, 2008). When the period of stress relaxation is short, quasi-static recrystallization restricted to the damaged crystal volume can take place (Trepmann et al., 2007). This scenario is probably common in the middle crust of tectonically active regions, at temperatures where the material undergoes viscous deformation by dislocation creep at a long term, but is strongly affected by stress redistribution during major seismogenic faulting in the overlying upper crust (Trepmann et al., 2007). (D) Erosion of rocks, whose minerals were plastically deformed at high-stresses and decreasing temperatures during the preceding exhumation, yields clastic grains with a high dislocation density into a sedimentary basin. When this sedimentary rock is buried and heated during basin subsidence (case D1), or incorporated into an accretionary complex deforming at very low-stress by dissolution-precipitation creep (case D2), static recrystallization is possible. Both scenarios are probably common, though – to our knowledge – not extensively reported so far. The first case (D1) may be widespread in deep sedimentary basins and at passive continental margins, where burial and diagenesis of sediments takes place without pervasive tectonic deformation and where the stress field is governed by overburden load. In principle, the situation could directly be observed in samples from the bottom part of the deepest gas wells, depending on the local geotherm. Observation of microstructures indicating static recrystallization in clastic quartz particles within the sedimentary rock may be valuable for the reconstruction of the thermal history. The record of the second case (D2) is potentially accessible in exhumed accretionary prisms (e.g., Platt, 1986, 1993; Ring et al., 1999) and shallow portions of subduction channels (e.g., Gerya et al., 2002; Gerya and Stöckhert, 2006; Bachmann et al., 2009). As in the case of pure subsidence and burial by younger sediments, pore fluid pressure is expected to reach near-lithostatic values at shallow depths of a few kilometres, tectonic deformation of clastic sediments being then probably governed by dissolution-precipitation creep at very low differential stress (e.g., Schwarz and Stöckhert, 1996; Stöckhert et al., 1999). Many rocks buried to and exhumed from depths of several tens of kilometres show no sign of crystal-plastic deformation during that part of their history (Stöckhert, 2002). In both cases outlined as scenario (D), the microfabrics of clastic minerals acquired by deformation during exhumation and cooling in their source area can be modified by static recrystallization at low differential stress. The driving force is the stored intracrystalline strain energy, bound to a high dislocation density, which is brought into the sediment. Heat input concomitant with burial in a low-stress environment, where bulk deformation or compaction of the rock is governed by dissolution-precipitation creep, then leads to Ключевые слова: quartz, deformed grain, crystal-plastic, tem figs, hp-lt metamorphism, ?uid inclusion, piazolo, plastically deformed, sample, nig, grain figs, isometric subgrains, tem sample, permian base, abundant lagbs, epting, microstructures, source, quartz pebble, tem micrographs, stereographic projection, represent hagbs, static recovery, special, wide variety, arrow figs, crystallographic, small, recrystallized grain, urai schenk, vein quartz, small circle, quartz vein, boundary grain, jessell, sto? ckhert, dislocation, tectonophysics, sedimentary rock, schenk urai, earth, point, complex, process, upper carboniferous, vein, passive enrichment, cad, society, grain, subduction channel, colour coded, high, case, pressure, pebble, conglomerate, voll, strain, type, grain edge, dislocation density, laboratory experiment, middle crust, differential, crystal-plastic deformation, mineral, ster, recrystallization, exhumation, source area, arrow, energy, cover boundary, seidel, geology, surrounding matrix, dislocation creep, number represent, sto, band contrast, ?uid, preceding stage, pervasive deformation, uid, sto ckhert, figs, structural geology, crossed polarizers, intruding magma, wirth, stress redistribution, low-stress environment, geological society, orientation, observed, theye, grain driven, foliation, tem micrograph, poirier, schenk, hagbs, creep, stress relaxation, misorientation angle, recovery, ebsd map, theye theye, geophysical, static recrystallization, deformation, differential stress, static, bali conglomerate, journal structural, dauphine twin, platt, driving force, moderate temperature, optical micrographs, sample location, dynamic, nappe pile, structure, upper crust, dynamic recrystallization, dashed, mineralogy, metamorphic, white, sigbm, optical, jolivet, dihedral angle, phyllite-quartzite unit, uid transport, lagbs figs, wa, grain boundary, crete, dissolutionprecipitation creep, internal foliation, density, inclusion, high-pressure, high-pressure rock, contrast, displayed, und, high pressure, type quartz, grain growth, journal structural geology, upper, london, der, sto?, free, crust, showing, c-axis orientation, prior, tectonics, source rock, metamorphism, trepmann journal, rock, microstructural evolution, indented contact, angle grain, strain shadow, type pebble, microstructure, growth, plattenkalk unit, observed diversity, unit, urai, temperature, northern coast, discussion, inserted symbol, misorientation, geological, uid inclusion, structural, stipp, ckhert, deformed, angle, notably modied, plattenkalk, crystal, boundary migration, orientation map, active tectonics, journal, external reservoir, represent lagbs, jahrbuch, elsevier, hall, ller, subgrains, migration, stress, tem, renner, early stage, black, dissolution-precipitation creep, grain devoid, crystallographic orientation, boundary, trepmann, inhomogeneous deformation, lagbs, black arrow, fassoulas, area