Quantitative structural analyses and numerical modelling of ...

100 L. BARATOUX ETAL.regime but under different thermal conditions(Stfpsk~i et al. 2001).Methods and techniquesMineral chemistryMinerals were analysed with a Cameca SX 100electron microprobe equipped with four WDSspectrometers at Blaize Pascal University,Clermont-Ferrand, France. Operating conditionswere 15kV, 10hA beam current, 2-5p~mbeam diameter, 20 s counting time, and naturalmineral standards. Some hornblende and plagioclasewere analysed using a CamScan $4scanning electron microscope and attachedLink EDX microanalytical system, at CharlesUniversity, Prague, Czech Republic. Operatingconditions were 20 kV, 1.8 nA beam current,1-3 p,m beam diameter, 120 s counting time,and mineral standards Structure Probe Instruments(SPI). Ca maps from plagioclase weremade at Claude Bernard University in Lyon,with operating conditions 15 kV, 15 nA beamcurrent and spatial resolution of 512 x 512pixels.Quantitative microstructural analysisQuantitative microstructural analysis of grainboundaries was carried out on traced and digitizedoutlines of grains in ESRI ArcView 3.2Desktop GIS environment. The map of grainboundaries was generated using ArcView extensionPoly (Lexa 2003). The resulting polygonshave been treated by MATLAB TM PolyLXToolbox (http://petrol.natur.cuni.cz/~ ondro;Lexa 2003), in which grain shape and grainboundary preferred orientations (SPO andGBPO, respectively) were analysed using themoments of inertia ellipse fitting and eigenanalysisof bulk orientation tensor techniques (Lexa,2003; modified SURFOR technique by Panozzo(1983) for GBPO). Their degree is expressed asthe eigenvalue ratio (r = E1/E2) of the weightedorientation tensor of grain shapes or boundaries.The orientation is defined by V~ and V2 eigenvectors.The grain sizes of the minerals werecalculated in terms of Ferret diameters of grainsection without stereological corrections.Crystallographic preferred orientationAmphibole and plagioclase crystallographic preferredorientations (CPO) were collected using ascanning electron microscope CamScan $4 inPrague and a JEOL JSM 5600 in Montpellierequipped with Channel5 electron backscatterdiffraction (EBSD) system from HKL Technology(Prior et al. 1999). Thin sections werepolished using 0.25 ~m diamond paste. Toremove all surface damage and minimize reliefbetween minerals, sections were chemicallypolished using a colloidal silica suspension. Allthin sections were carbon-coated. The coatingreduces the quality of the electron backscatterdiffraction patterns (EBSP) so that automaticindexing mode of the EBSP system could notbe used. Most data were therefore collectedmanually. Operating conditions were 20 kV inPrague and 15 kV in Montpellier, 5.6 nA beamcurrent, working distance 39 ram, and 2-5 ~mbeam diameter. For each measurement, threeEuler angles (vl, 05, v2) characterizing thelattice orientation as well as the nature of themineral were determined and stored. Practiceshows that plagioclase diffraction patterns donot change significantly from albite up to atleast An65 (Lapworth et al. 2002). Therefore,the Anorthite48 database was used for indexingplagioclase. Pole figures and inverse polefigures were projected using the software developedby D. Mainprice (ftp://ftp.dstu.univmontp2.fr/pub/TPHY/david/pc).Projectionsof crystallographic axes ([a], [b] and [c]) oroptical indicatrix (o~, /3, and 3/) are generallyused for plagioclase (e.g. Jensen & Starkey1985; Olsen & Kohlstedt 1985; Ji & Mainprice1988, 1990; Prior & Wheeler 1999).The degree of CPO has been quantified usingorientation tensor of crystallographic planesand directions (Mainprice, ftp://ftp.dstu.univmontp2.fr/pub.TPHY/david/pc).The intensityof CPO is given by the I parameter (Lisle 1985):3I = 15/2 x Z(Ei - 1/3) 2i=1where E~, E2, and E3 represent the eigenvalues ofthe preferred orientation of poles to planes anddirections of amphibole and plagioclase grainsplotted in pole figures. The values of I rangebetween 0 (no preferred orientation) and 5 (allfabric elements perfectly parallel one to another).MicrostructuresThe metagabbros from the eastern (lower) beltare affected by localized shear zones (Fig. lb)and all stages from non-deformed rock, protomylonite,mylonite, and up to highly strainedultramylonite are present (Figs 2a, b, c and d).In the western metagabbro (upper) belt, theprotolith stage is not preserved and only twodeformational stages can be distinguished252