(2006) 3–14www.elsevier.com/locate/tecto

Tectonophysics 427

Analysis of dynamic recrystallization and nucleationin a quartzite mylonite

Angela Halfpenny, David J. Prior ⁎, John Wheeler

Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, Merseyside, L69 3GP, U.K.

Received 29 July 2005; received in revised form 18 April 2006; accepted 11 May 2006Available online 30 August 2006

Abstract

Electron backscatter diffraction (EBSD) has been used to analyse a natural quartz mylonite from the Stack of Glencoul, NWScotland. This technique has been used to measure the misorientation between original protolith ‘parent’ grains and recrystallized‘daughter’ grains. The angle of misorientation is important because it has implications for the controlling recrystallizationmechanism. The sample exhibits approximately 65% a recrystallized microstructure. The average neighbor-daughter grain size is12 μm and the average subgrain size of the parent grain is 16 μm. The parent grains do not show a systematic increase inmisorientation from the centre of the grain to the edges. These data are inconsistent with the subgrain rotation recrystallizationmodel. We propose that recrystallization was facilitated by bulging during strain-induced grain boundary migration. However, themajority of misorientations between the parents and their neighbouring daughter grains are in the range 10–30° requiring thatanother process has operated to rotate grains to higher misorientations with respect to their parents. This process is likely to be grainboundary sliding.© 2006 Elsevier B.V. All rights reserved.

Keywords: Misorientation; EBSD; Dynamic Recrystallization; Nucleation

1. Introduction

Dynamically recrystallized rocks are of interest togeologists (Bell and Etheridge, 1976; Poirier andGuillopé, 1979; Urai et al., 1986; Lloyd and Freeman,1994) as these microstructures are used to interpret theprocesses of deformation, the conditions of deformationand the kinematic strain path. Dynamic recrystallization isdefined as being synchronous with deformation (Hobbs,1968; Hobbs et al., 1982; Urai et al., 1986) and is impor-tant because creep deformation of rocks to high strains is

⁎ Corresponding author. Tel.: +44 151 794 5174; fax: 44 151 794 5190.E-mail address: a.halfpenny@liverpool.ac.uk (A. Halfpenny).

0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tecto.2006.05.016

facilitated by the processes of recovery and recrystalliza-tion (Urai et al., 1986; Passchier and Trouw, 1996).Without recovery and/or recrystallization, steady-stateflow by dislocation movement would be impossible, asthe dislocations would interfere with each other causingwork hardening and embrittlement.

Recrystallization involves the formation/nucleationand growth of new grains. The individual mechanismsby which recrystallized grains nucleate and grow, andother processes that might operate in tandem, such asgrain boundary sliding, have specific, predictable effectson the crystallographic relationships between host andrecrystallized grains (Hobbs, 1968; Wheeler et al., 2001)or ‘parent’ and ‘daughter’ grains.

Fig. 1. Basic processes that can be involved in the development of recrystallized grains. (a) Progressive subgrain rotation leading to the developmentof high-angle grain boundaries and the formation of a new grain. (b) Bulge nucleation due to grain boundary migration. Bulge separated from parentgrain due to the formation of a bridging subgrain wall.

Fig. 2. Terminology diagram. (a) Original protolith grains are termedparents. (b) Recrystallized grains that are in contact with a parent grain aretermed neighbour–daughters. (c) All other recrystallized grains, whichhave come from the parent grains, are termed daughters. (d) When thedata from only 1 μm either side of the parent to neighbour–daughtersHAGB are studied, these are termed edges.

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Partially and completely recrystallized microstructurescan be distinguished. In partially recrystallized micro-structures, a bimodal grain size distribution is character-istic, with aggregates of small grains of nearly uniformsize between large grains exhibiting internal deformationfeatures such as undulose extinction and subgrain bound-aries. Fully recrystallizedmicrostructures are similar to anequilibrium non-recrystallized microstructure or foamtexture, exhibiting a uniform grain size and straight non-serrated grain boundaries.

There are several recrystallization mechanisms dis-cussed in the literature (e.g., Urai et al., 1986) Mechan-isms of specific interest are subgrain rotation (SGR)recrystallization and strain-induced grain boundary mi-gration (SIGBM) recrystallization, both of which arerecognised in quartz (White, 1982). Nucleation is theformation of a new strain-free volume of crystallinematerial surrounded by high-angle grain boundaries(HAGB). Low-angle grain boundaries (LAGB) oftencorrespond to subgrain boundaries/walls, defined by anarray of dislocations. In contrast, HAGBs do not generallyhave determinable structures. The minimum misorienta-tion angle by which a HAGB is defined is mineraldependent; in this paper, we use 10°, a value determinedfrom transmission electron microscopy research on thestructure of boundaries in quartz by White (1977). Asubgrain is a volume of crystalline material where at leasta portion of the boundary is an LAGBwhereas a grain is avolume of crystalline material completely surrounded byHAGBs. We now summarise separate mechanisms,which contribute to nucleation, recrystallization andconsequent modifications of microstructures.

1.1. Subgrain rotation (SGR) recrystallization

SGR is an extension of the recovery process: thedriving force for SGR is a reduction of the internal strainenergy of the deforming aggregate. This is achieved bythe movement and interaction of dislocations to form asubgrain wall (White, 1976; Urai et al., 1986). As moredislocations enter the subgrain wall, there is a progressiveincrease in misorientation between subgrains (Fig. 1a).This process is enhanced when dislocations are able toclimb. The misorientation angle will continue to increaseuntil the subgrain boundary becomes a HAGB and the

Fig. 3. Sketch map of northern Scotland modified from Law et al.(1986) showing the location of the Assynt region and the mylonites ofthe Moine Thrust zone. The sample came from the stack of Glencoul.

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subgrain can no longer be classified as part of the originalparent grain; this new daughter grain is often thought of asa nucleus and the process by which it is formed is knownas SGR recrystallization (Poirier and Guillopé, 1979). Asthe recrystallized daughter grains which are formed arerotated portions of the original parent grain, they mayexhibit an internal defect structure inherited from theparent grain (Drury and Urai, 1990). The primary line ofevidence for SGR recrystallization is core and mantlestructure, where the cores of original protolith grains passout transitionally into the mantle with increasing subgraindevelopment and misorientation. Around the core and

mantle structure are aggregates of recrystallized grainswith similar size and orientations to the nearby subgrains(Gifkins, 1976; White, 1976). Daughter grains developedby SGR recrystallization should be related to their parentsby an orientation relationship that will reflect the dislo-cations involved in deformation, recovery and SGR(Lloyd et al., 1997; Prior et al., 2002).

1.2. Strain-induced grain boundary migration (SIGBM)recrystallization

SIGBM recrystallization process is driven by the re-duction of the stored strain energy (the energy of dislo-cations, point defects, subgrain and grain boundaries)across an existing HAGB; it occurs principally when twoneighbouring grains having significantly different disloca-tion densities (Fig. 1b). SIGBMrarely occurs bymovementof all points along a boundary with equal velocity; hence,the microstructural signature of grain boundary migrationis highly serrated grain boundaries (White, 1982).

1.3. Bulge nucleation

Grain boundary bulges form along moving bound-aries during SIGBM, developing from HAGBs prefer-entially in areas of sharpest crystal bending (White,1977) or where neighbouring grain boundary segmentsare pinned (Jessell, 1987). Full isolation of the bulge canbe achieved by the development of a bridging subgrainboundary across the neck of the bulge and its conversionby progressive misorientation into a HAGB (Fig. 1b)(Urai et al., 1986). These bulges give rise to new strain-free grains (Bell and Etheridge, 1976). Bulge nucleatedgrains can be recognised by their polygonal structureand lack of internal deformation. The recrystallizedgrains will be related in orientation to the protolithparent grain, the angle depending on the magnitude andaxis of rotation of the SGR of the bridging boundary.

1.4. Grain boundary sliding

The driving force for GBS is strain incompatibility.GBS is the movement of grains relative to one another.GBS accommodated by frictional processes (cataclasis,particulate flow) is common and requires dilatancy. Non-dilatant GBS is limited to fine-grained rocks and/orrelatively high temperatures and is accompanied by a ratecontrolling accommodating process which can be diffu-sion, a solution or dislocation process or a combination ofthese (Evans and Langdon, 1976; Gifkins, 1976). Thepresence of voids or bubbles along a grain boundary willfacilitate GBS (White, 1977). Evidence indicating that

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GBS has become the dominant deformation mechanismincludes polygonal grains, smooth grain boundaries, voidsalong grain boundaries especially located at triplejunctions and alignment of groups of recrystallized grainsto formplanes of grain boundaries parallel to themyloniticfoliation (White, 1979).GBS is hypothesised toweaken orrandomise crystallographic preferred orientations (CPOs),even when accompanied by continued dislocation creep(Zhang et al., 1994; Bestmann and Prior, 2003; Storey andPrior, 2005). It may also randomise grain boundary mis-orientation axes (Jiang et al., 2000).

1.5. Aim of the research

Our aim is to understand the mechanisms ofnucleation and recrystallization. Key information is

Fig. 4. Microstructural maps. (a) Optical image of the area which was studiedmap produced from EBSD data. (c) A 2° boundary map. (d) A 5° boundary mof the chosen parent grains. Transects a, b and c mark the locations of the m

contained in the misorientation between parent anddaughter grains. The misorientation relationships can becompared with the predicted effects of commonrecrystallization and nucleation mechanisms to deter-mine the controlling mechanism(s). In this paper, wedefine a parent grain as an original protolith grain, whichmay have been deformed but is still recognisable. Adaughter grain is any recrystallized grain that is deducedto have formed from a parent grain. Neighbour–daughter grains (Fig. 2) are those daughters whichshare a grain boundary with a parent grain (although notnecessarily the parent grain from which they wereformed).

We use electron backscatter diffraction (EBSD) (Ven-ables and Harland, 1973; Dingley, 1984) to measure thefull crystallographic orientations of individual areas

via EBSD (field of view 700 μm). (b) Band contrast or pattern qualityap. (e) A 10° boundary map. (f) A 30° boundary map. (g) Location mapisorientation profiles presented in Fig. 7.

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b1 μm in diameter (Prior et al., 1999). Automated EBSDdata collection enables construction of quantitativemicrostructural maps (Adams et al., 1993; Prior et al.,2002) from which misorientations within grains and

Fig. 5. Equal area, lower hemisphere, looking NNE, stereonet of the (a) parenneighbour–daughter grains only (data within 1 μm of the HAGB between padiagram of the [c]-axis plot of (a). The diagram also shows the kinematic frafoliation), lineation (X) is horizontal within the foliation. The shear direction

between neighbouring grains can then be established.Using these data it is possible to analyse the evidence ofrecrystallization and to reconstruct the deformation andrecrystallization processes.

t grains only, (b) neighbour–daughter grains only, and (c) edges of therent and neighbour–daughters). Between (a) and (b) is a skeletonizedmework where foliation is E–W perpendicular to the page (Z=pole toof the Moine Thrust is top to the west.

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2. Sample

The sample used in this study is a natural myloniticquartzite from the Stack of Glencoul (Grid Ref. NC28882876), in the northern part of the Assynt region, NW

Fig. 6. (a) Band contrast or pattern quality map for parent grain 1 and surroundmarked on. 2°=yellow, 5°=lime green, 10°=blue, 20°=purple and N30°=bwithin 30° of the centre of the upper cluster in Fig. 5a (parent grain cluster owithin 30° of the centre of the lower cluster in Fig. 5a (parent grain orientatiothat they are dispersed more than 30° away from the centre of either clusterlocated around a single parent grain and that there has been some mixing of

Scotland (Fig. 3). Callaway (1884) was one of the first todescribe these rocks which are now classed as SNL andL–S tectonites. The protolith for these rocks was aCambrian quartzite, whichwas deformed along theMoineThrust at greenschist facies condition (Law et al., 1986).

ing daughter grains (location shown in Fig. 5) with the grain boundarieslack. (b) Texture component map where every grain whose c-axis wasrientation 1) was coloured red, whereas every grain whose c-axes wasn 2) was coloured blue. Some grains remain uncoloured due to the fact. (c–e) show that neighbour–daughters of both cluster orientations arethe daughters.

Fig. 7. Misorientation profiles plotting data relative to the first point onthe profile. The locations of the misorientation profiles are marked onFig. 4. (a) Transect inside parent grain 1 showing the internal defor-mation from the centre of the grain to the edge. Some of the LAGBshave been marked onto the profile. (b) Transect starting in thedaughters around parent grain 1 crossing the width of parent grain 1and ending in the daughter grains on the opposite side of the parent.The locations of some of the HAGBs and the neighbour–daughtershave been marked onto the profile. (c) Transect across neighbour–daughter grains only. The locations of some of the HAGBs between theneighbour–daughter grains have been marked on the profile.

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The Moine Thrust is a foliation parallel ductile contactbetween the mylonitic Cambrian quartzite and thesimilarly deformed overlying Moine metasediments(Christie, 1960; Christie, 1963; Weathers et al., 1979;Law et al., 1986; Law, 1987; Strine and Wojtal, 2004).The sample was collected 5 m below the Moine Thrustfrom the white Cambrian quartz mylonites and exhibits apartially recrystallized microstructure. The sample isequivalent to sample SG10 of a previous study (Law et al.,1986) and was chosen due to its clear dynamicallyrecrystallized microstructure which can be easily com-pared with other mylonitic samples. The sample containsapproximately 1% muscovite.

3. Methods

3.1. Sample preparation

Standard XZ (parallel to lineation and perpendicularto foliation), 30 μm thin sections were chemically–mechanically polished using SYTON fluid (Lloyd,1987) and carbon coated to prevent charging.

3.2. Data acquisition

Sample analysis areas were selected using optical andelectron microscopy utilising orientation contrast (OC)imaging (Prior et al., 1996). The OC images show wherecrystallographic orientations change. Full crystallo-graphic orientation data were obtained from automati-cally indexed EBSD patterns collected in a CamScanX500 Crystal Probe SEM fitted with a field emissiongun and a FASTRACK stage. The EBSD patterns werecollected using a 20 kVacceleration voltage and a beamcurrent of 30 nA. The working distance was 25 mm witha column tilt of 70°. Samples were mapped by the beammoving on a grid with a fixed step size of 2 μm, ensuringthat the recrystallized daughter grains and the subgrainscontained ample measurement points. The EBSDpatterns were imaged on a phosphor screen, viewed bya low-light CCD camera and indexed using the HKLTechnology manufacturer's software package Channel 5(Schmidt and Olesen, 1989). Average measuring timeper point was 0.2 s. The raw data have 62% indexed asquartz and 38% non-indexed (EBSD patterns with nosolution). The non-indexed points correspond to grainboundaries and secondary phases. Grain sizes and sub-grain sizes are calculated as the diameter of a circle ofequivalent area to the measured grain/subgrain area. Thedata shall be analysed using optical images (Fig. 4),stereonets (Fig. 5), EBSD maps (Fig. 6a and b) andmisorientation data (Figs. 7–9).

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4. Results

4.1. Microstructure characteristics

Parent grains were identified via their large size,internal deformation, elongation and highly serrated grainboundaries (Fig. 4). The parent grains defined foranalytical purposes do not include all the parent materialin the field of view. The daughter grains were identified bytheir small size, lack of internal deformation and polygonalappearance. The parent grains show an average aspectratio of 1:9, calculated from the EBSD maps and exhibitinternal distortion and subgrain boundaries. The averagesubgrain size within the parent is 16 μm whereas theaverage neighbour–daughter grain size is 12 μm (Fig. 6a).The internal substructure of the parent grains showsLAGBswithmisorientations greater than 2°. These bound-aries are not regularly arranged; the amount of misorien-tation on individual subgrain boundaries does not increasesystematically away from the core of the parent grain to theedge (Fig. 6a). Subgrain boundary traces are preferentiallyoriented subparallel to the grain long axes. The misorien-tation between the parent and daughter grains is between10° and 30° (Figs. 4 and 8a).

4.2. Orientation data

Parent grain [c]-axes form two fairly tight clusters, oneat the top of the stereonet and one at the bottom (Fig. 5a).Parent grains 2, 4, 5 and 6 cluster at the top of the stereonetandwill be referred to as parent grain cluster orientation 1,whereas parent grains 1, 3, 4, 7, 8, 9, 10 and 11 cluster atthe bottom of the pole figure and shall be referred to as

Fig. 8. Misorientation angles plotted as histograms of relative frequency (in dangles for parent to neighbour–daughter boundaries. (b) Misorientation anangles for neighbouring pixels within parent grains only.

parent grain cluster orientation 2 (grain numbers markedin Fig. 5). The daughter grain [c]-axes form the same twoclusters but they are more dispersed than the parent grainclusters (Fig. 5b and c). Neighbour–daughter grains (Fig.5c) show the same orientation distribution as all daughtergrains; there is no gradient in orientation dispersion withdistance away from the parent grains. The [c]-axis fabricskeleton exhibits an asymmetric single girdle (Fig. 5). Atexture component map (Fig. 6b) enables us to assess thedistribution of grains that fall into orientation clusters 1and 2. Most neighbour–daughter grains fall in the sameorientation cluster as their neighbouring parent. Similarly,most daughter grains fall in the same orientation cluster asneighbouring daughters. However, there are areas whereparents have neighbour–daughter grains from bothorientation clusters and there are areas of recrystallizedgrains where daughter grains from both orientationclusters are intermixed (Fig. 6c–e).

4.3. Misorientation data

Misorientation profiles show that there are significantdistortions (N10°) across and along the length of parentgrains (Fig. 7a), in addition to the LAGBs. Large steps inmisorientation occur at the boundaries between parentsand neighbour–daughters (Fig. 7b) and between daugh-ters (Fig. 7c). Statistically, themisorientations between theparents and their neighbour–daughter grains peak be-tween 10° and 30° (Figs. 6a and 8a). This is similar to therange ofmisorientations between daughter grains (Figs. 6aand 8b) except that the boundaries between daughtersinclude many more boundaries with misorientationsgreater than 30° and there is a stronger Dauphiné twin

egrees−1) for neighbour pairs and neighbour pixels. (a) Misorientationgles for boundaries between neighbour–daughters. (c) Misorientation

Fig. 9. Misorientation axes/angle pair data plotted up in 10° intervals of misorientation on inverse pole figures. (a–f) Plotting the raw data, whichclusters about the c-axis. (g–l) Data have been normalized to 8400±100 data points to create comparable contours. The data still clusters around [c]but is weakest in the plot of 10–20° misorientation angle. Number of data points used in each plot is shown above plot. Contours are between 1 and 5in multiples of mean uniform density.

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signature (misorientation peak at 60°). In contrast parentgrains contain only LAGBs and Dauphiné twins (Fig. 8c).Misorientation axes were plotted in a crystal referenceframe (i.e., an inverse pole figure). This reference frame issuitable for testing hypotheses about crystallographicallycontrolled deformation processes (i.e., slip systems)(Lloyd et al., 1997) and other processes, such as GBS,that are not dependent upon crystallography. Misorienta-tion axes of LAGBs cluster about the [c]-axis (Fig. 9a).LAGBs are located mostly within parent grains and datafrom only those parent grains shown in Fig. 4g show thesame pattern as all LAGBs. HAGB misorientation axes

Fig. 10. Summary diagram of the data collected on the microstructure showingand randomly arranged. The internal subgrain size is significantly larger thaparent grains and neighbour–daughter grains is between 10° and 30°. Neighboeach parent grain.

have been subdivided into 10° intervals (Fig. 9b–f).Although all show misorientation axes preferentiallylying parallel to [c], the degree of preference is variable; itis weakest in boundaries with misorientation between 10°and 20° (Fig. 9h).

5. Discussion

We suggest that the daughter grains whose clustercontains parent grain cluster orientation 1 have recrys-tallized from those parents and, similarly, daughtergrains whose cluster contains parent grain cluster

the misorientation of parental subgrain boundaries to be N2° but b10°n the neighbour–daughter grain size. The misorientation between theur–daughters of both parent grain cluster orientations 1 and 2 surround

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orientation 2 have recrystallized from those parents. Anobservation that needs explaining is the 10–30°misorientations of neighbour–daughter grains from theparent grain orientation; that is, there has been somerotation of the neighbour–daughters. Also, around asingle parent grain, representatives of both neighbour–daughter orientation clusters are seen. If the neighbour–daughters have recrystallized from both parent orienta-tion clusters, as suggested, then there has been somemixing of the neighbour–daughter grain populations.

5.1. Subgrain rotation recrystallization

The presence of subgrain boundaries within parentgrains, and the range of misorientations these show,suggest that recovery and SGR have accompanieddislocation creep. LAGB misorientation axes are prefer-entially orientated parallel to [c]: if we assume a tiltboundary geometry, this would be interpreted as thegeometrical consequence of an {m}⟨a⟩ slip (prism slip)system (Lloyd et al., 1997), (Lloyd, 2004). However, thisimplied slip system is incompatible with the overall CPO(Fig. 5) which would usually be interpreted as havingformed via dominant (c)⟨a⟩ slip (basal slip) (Law, 1987).We therefore propose that the subgrain walls have a twistcomponent. Lloyd (2004) discusses how combinations ofdifferent slips systems can give rise to twist boundaries.We do not concur with all aspects of his treatment, but doagree that themisorientation axis for a twist boundarywillbe perpendicular to all the dislocation line vectors in thatboundary. Therefore, two or more non-parallel popula-tions of screw dislocations formed by (c)⟨a⟩ slip can forma twist boundary with misorientation axis parallel to c.This is our provisional proposal to reconcile the CPO data(Fig. 5) with the misorientation axis data (Fig. 9).

Although we suggest that recovery processes includ-ing SGR were operative we still need to assess what rolethese processes played in recrystallization. When SGRrecrystallization is responsible for the nucleation, therecrystallized daughter grains should be initially of asimilar size to the internal subgrain size of the parentgrain (Urai et al., 1986). Our data show that the averageinternal subgrain size is 4/3 times that of the neighbour–daughters. Therefore, SGR alone cannot be the nucle-ation method unless there has been a phase of subgraingrowth within the parents after the recrystallization andnucleation of the neighbour–daughter grains. This isunlikely as subgrain boundaries have a much lowermobility than HAGBs (Humphreys and Hatherley,1996); thus, one would expect the daughter grains togrow faster under similar conditions. There is likely tobe an increased driving force for recrystallization

(higher strain energy) within parent grains relative todaughters. However, we would expect the drivingforce to be decreased by migration of the parent–daughter boundaries into the parent grain, not bymigration of the less mobile subgrain walls within theparent grain.

5.2. Strain-induced grain boundary migration recrys-tallization and bulge nucleation

The evidence for SIGBM is the highly serrated grainboundaries exhibited by the parent grains. In the bulgenucleation mechanism, the size of the bulge is not con-trolled by the subgrain size, so the size difference ofsubgrains and daughter grains is not a problem. A featureof bulge nucleation in dynamic recrystallization is that,while several boundaries of a new grain may be estab-lished by grain boundary migration, full isolation of thegrain may commonly be achieved by the developmentand SGR of a bridging subgrain boundary. The neigh-bour–daughter grains are misorientated between 10° and30° from the parent grain, with decreasing crystallo-graphic control, as this misorientation increases. It islikely therefore that another deformation mechanism hasincreased the misorientation. If no bridging subgrainboundary developed then bulge nuclei would have thesame orientation as parents. Any subsequent rotationwithout crystallographic control would eventually ran-domise misorientation axes (Jiang et al., 2000). Preferredalignment of misorientation axes parallel to [c]-axispersisting for all parent to neighbour–daughter bound-aries (Fig. 9) may be evidence that subgrain boundarybridging has occurred.

Initial bulgingwould have been of boundaries betweenoriginal parent grains. After the formation of neighbour–daughter grains between the two parents, subsequentbulging will be between these daughter grains and theparents. One might argue that the degree of dispersionobserved could be generated by the progressive misori-entation of each new ‘layer.’ However, there are nogradients in orientation dispersion and layers just a singlegrain wide between two parents are as dispersed as thickerdaughter grain bands.

5.3. Grain boundary sliding

The data show that there has been some mixing of theneighbour–daughter orientations so that both daughterorientations are now located around many individualparents. Moreover, the misorientation axes of 20–30°boundaries are less well aligned (parallel to [c]) than theaxes of lower angle boundaries or higher angle

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boundaries (Fig. 9). The higher angle boundary data isdistorted by the Dauphiné twin signature and is difficultto interpret. We interpret the weakening of the misorien-tation axis alignment as resulting from grain rotationsduring GBS. GBS can slide grains past each other andcause neighbour switching (White and Mawer, 1988),but GBS requires straight segments or boundaries forthe sliding to take place (White, 1977). The parentgrains show quite serrated grain boundaries, which arenot favourable for GBS, but the boundaries between therecrystallized grains are short, fairly straight and, there-fore, more favourable for GBS.

6. Summary and conclusions

The SEM/EBSD technique has been used to analysethe full crystallographic orientation of original quartzparent grains and recrystallized daughter grains in amylonitic quartzite which was deformed at greenschistfacies conditions. The parent grains have two orientationpopulations. Parent grains of orientation 1 have given riseto neighbour–daughter grains of orientation 1 and parentgrains of orientation 2 having produced neighbour–daughter grains of orientation 2. Neighbour–daughtergrains of both orientation 1 and 2 surround most of theparent grains.Mostmisorientation angles between parentsand neighbour–daughter grains are between 10° and 30°.The parent grains show internal deformation in the formof undulose extinction and subgrain boundaries, whichhave misorientation axes parallel to [c]. Such subgrainwalls can be formed by dislocations on (c)⟨a⟩ slip sys-tems, but if this is the case, they must have some twistcharacter. The subgrain boundary misorientations aregreater than 2°. There is no systematic increase inmisorientation from the centre of the parent grains to theedges. The average parent subgrain size is 16μm,whereasthe average daughter grain size is 12 μm. The subgrainsare significantly larger than the neighbour–daughtergrains. The neighbour–daughters show no internal defor-mation and are polygonal in shape. These data are sum-marised in a cartoon (Fig. 10). The conceptual modellisted below shows one way in which the microstructurecould have formed:

(1) Parent grains deform by dislocation creep accom-panied by recovery and subgrain rotation.

(2) Nucleation of the neighbour–daughter grains takesplace via bulging during strain-induced grainboundary migration. The sizes of daughter grainsthat are being formed via bulging are not controlledby the internal subgrain size of the parent and tendto be smaller than the subgrain size.

(3) Full isolation of the daughter grains is achieved bythe development of a bridging subgrain boundaryand its conversion by progressive misorientationinto a grain boundary.

(4) The neighbour–daughter grains are then rotatedfurther during grain boundary sliding to give thehigh observed misorientations between parentsand daughters and the weakening of crystallo-graphic control on misorientation axes. Recrys-tallized grains are able to slide past each othercausing neighbour switching and, hence, mixingof orientation populations.

Acknowledgements

Jan Tullis and Richard Law are thanked for their helpand stimulating discussions. Also Sandra Piazolo, PatTrimby and Geoff Lloyd are thanked for their usefulcomments and guidance. The detailed comments ofJosephWhite and John Fitz Gerald helped to improve theoriginal version of this manuscript. A NERC researchstudentship tied to Grant Number NER/A/S/2001/01181funded this project.

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