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_Journal of Structural Geology 32 (2010) 1404-1416_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _The initiation of strain localisation in plagioclase-rich rocks: Insights from detailed microstructural analyses_ _H. Svahnberg*, S. Piazolo_ _Department of Geological Sciences, Stockholm University, SE-106 91 Stockholm, Sweden_ _article info_ _Article history: Received 23 June 2009; Received in revised form 25 May 2010; Accepted 18 June 2010; Available online 25 June 2010_ _Keywords: Plagioclase Electron backscatter diffraction Crystallographic preferred orientation Subgrain rotation recrystallisation Grain boundary sliding Strain localisation_ _abstract_ _In order to shed light on the cause for onset of strain localisation in plagioclase-rich rocks we have performed detailed microstructural analyses on a sheared anorthosite-leucogabbro using optical microscopy, electron backscatter diffraction (EBSD) and chemical analyses. The analysed sample is from an Archaean unit, SW Greenland, deformed at lower to mid crustal conditions (T ~ 675-700 °C and moderate pressure). The initial deformation occurred dominantly by dislocation creep and the grain size was reduced primarily by subgrain rotation recrystallisation. Recrystallised plagioclase grains (average size 80 μm) are dominantly found in (i) clusters, (ii) lenses and (iii) continuous bands subparallel to shear zone boundaries. Recrystallised grains in clusters and lenses display inherited crystallographic orientations. Their bulk crystallographic preferred orientation (CPO) is random; however, crystallographic characteristics show that parent and daughter grains have the same misorientation axes and possibly the same active slip systems. Recrystallised grains in continuous bands show a CPO with a single dominant active slip system, (001)<110>, aligned with the structural (XYZ) framework. For these parent and daughter grains, misorientation axes are random and the dominant slip system is different. Grain rotations of recrystallised grains are traceable back to the orientation of the adjacent porphyroclast._ _We infer that the cause for strain localisation is recrystallisation and development of a CPO in continuous recrystallised bands. Microstructures in combination with misorientation and slip system analyses indicate a possible change from dislocation creep in clusters and lenses to dislocation-accommodated grain boundary sliding (DisGBS) in continuous bands. This inferred shift in dominant deformation mechanism would lower the strength of the shear zone._ _© 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Feldspars are the most abundant minerals in the crust and are therefore important minerals when deciphering crustal rheologies (Tullis, 2002). Plagioclase feldspars are common in the lower crust and several studies of plagioclase have been undertaken to evaluate its behaviour during deformation and role in strain partitioning (e.g. Tullis and Yund, 1985; Ji and Mainprize, 1988; Heidelbach et al., 2000; Kruse et al., 2001; Rybacki and Dresen, 2004; Baratoux et al., 2005; Mehl and Hirth, 2008; Kanagawa et al., 2008). Nevertheless, the control of slip system activation is still poorly constrained although several studies have identified important slip systems (e.g. Marshall and McLaren, 1977; Montardi and Mainprize, 1987; Stünitz et al., 2003)._ _Strain localisation predominantly occurs in faults and shear zones (e.g. White et al., 1980) and represents a mechanical weakening of the rock. One of the main factors contributing to strain localisation is grain size reduction (Rutter and Brodie, 1988; Montesi and Hirth, 2003). The importance of grain size reduction lies in the possible change in deformation mechanism, from grain size insensitive dislocation creep to grain size sensitive diffusion creep (with grain boundary sliding, GBS), that lowers the strength of a shear zone (e.g. De Bresser et al., 2001; Kenkmann and Dresen, 2002; Raimbourg et al., 2008)._ _Unequivocal microstructural criteria for determination of the dominant deformation mechanism do not exist since several mechanisms may operate simultaneously (e.g. Warren and Hirth, 2006; Dimanov et al., 2007). However, some microstructures are indicative of the involvement of certain mechanisms, e.g. lattice distortions and subgrain boundaries for dislocation creep and phase mixing for GBS (e.g. Kenkmann and Dresen, 2002; Dimanov et al., 2007; and references therein). Rocks subjected to recrystallisation during intracrystalline plastic deformation display a crystallographic preferred orientation (CPO) that can be used to infer the operative deformation mechanism and the active dominant slip systems (e.g. Passchier and Trouw, 2005). However, recent experimental studies of anorthite aggregates have shown that a CPO may also develop with a stress exponent of n ~ 1, resembling Newtonian creep (Gómez Barreiro et al., 2007). The electron backscatter diffraction system (EBSD) has proven successful to aid microstructural interpretations by supplying information on grain orientation relationships, since most processes have specific crystallographic relationships (e.g. Wheeler et al., 2001; Bestmann and Prior, 2003). In addition, analyses of crystal lattice misorientations across subgrain boundaries can help to determine the dominant slip system (Prior et al., 2002)._ _The aim of this contribution is to investigate the operative deformation mechanisms and slip systems in plagioclase porphyroclasts and recrystallised grains to understand the process of recrystallisation and the development of strain localisation in a plagioclase-rich rock. We use CPO and misorientation data collected automatically by the EBSD technique in combination with microstructural characterisation and chemical analyses._ _2. Geological setting and sample description_ _The sample for this study was collected from an amphibole-bearing anorthosite-leucogabbro unit at Qarliit Nunaat in southern West Greenland (Fig. 1aec). The area is part of the North Atlantic craton of Archaean rocks and a current terrane model proposed for the region (e.g. Friend et al., 1987; Friend and Nutman, 2005) locates the sample site in the Tasiusarsuaq terrane (age ~2920-2810 Ma; Nutman et al., 1989)._ _The area is structurally dominated by regional-scale tight to isoclinal upright folds and the sample site is located on a subvertical fold limb structurally below inferred supracrustal units and structurally above the tonalitic orthogneiss (Fig. 1c; Lee et al., 2006). At least two major phases of ductile deformation have affected the area (D1 ~ D2) and possibly two high-grade metamorphic events, an earlier upper amphibolite to granulite facies event (M1) and a peak granulite facies metamorphism at about 2810-2790 Ma (M2; e.g Crowley, 2002). The tonalite dominating the area has orthopyroxene in the matrix and in felsic veins, indicating crystallisation at high temperature metamorphic conditions. It is interpreted to be emplaced syn-tectonic during the peak metamorphic event (D2 ~ M2) and possibly concurrently with the regional folding. The area is at present mostly at upper amphibolite to granulite facies but with some local retrogression to amphibolite and greenschist facies metamorphism (D3 ~ M3). Combined field relationships and structures suggest a transpressional regime. PT estimates calculated from btegrtesil-bearing metapelite at the north-western tip of the anorthosite-leucogabbro yielded a temperature of 675-700 °C and pressure of 350-450 MPa (Jaconelli, 2009)._ _The sampled anorthositic unit is more than 300 m wide and dominated by an anastamosing shear zone network of centimeter to meter-sized higher strain areas surrounding less deformed lenses (Fig. 2a). These anastamosing shear zones are continuous along the deformed unit. Mylonitic foliation is sub-vertical, striking NW-SE and subparallel to shear zone boundaries, and defined by alternating compositional bands of plagioclase and aggregates of amphiboles and quartz ribbons in the central parts of the high strain zone. These quartz ribbons (up to 4 mm wide) are interpreted to be associated with the orthopyroxene-bearing tonalite intrusion and to have been emplaced as veins late during D2. Lineation defined by alignment of amphiboles is weak and trends to the SE. Our hand sample (hso468022) consists of a 10 cm thick fine-grained high strain zone between less deformed regions of leucogabbro-anorthosite. The less deformed parts contain centimeter-sized magmatic plagioclase crystals with recrystallised rims (Fig. 2b). Mafic minerals define a bending of an older foliation into the shear zone where there is a further grain size reduction of plagioclase and development of the mylonitic foliation (Fig. 2b)._ _We selected two representative thin sections, hso468022D and hso468022L, for a detailed study of the microstructural development using the EBSD technique (Fig. 2b). These sections show a progressive increase in the number of recrystallised grains that are present as small clusters, lenses and as mylonitic foliation-defining continuous bands. This sequence is interpreted to represent initial development of strain localisation. Because quartz veins were emplaced later during deformation, the strain localisation was shifted from the continuous bands to the weaker quartz ribbons. Later reactivation of the shear zones (D3 ~ M3) continued to localise the strain, at least partly, in the quartz ribbons, preserving the high temperature deformation microstructures in the anorthosite-leucogabbro. Thin sections were cut pe'_ Ключевые слова: e, r, o