Quantitative structural analyses and numerical modelling of ...

106 L. BARATOUX ET AL.Fig. 7. Drawings and micrographs of the western banded mylonite. (a) Lobate boundaries document grain boundarymigration (GBM) in coarse-grained plagioclases of banded mylonite (XPL). (b) Mechanical twins (MT) transect thegrowth-related zoning (GZ) in banded mylonites documenting a later deformation phase (XPL). (c) Digitized drawingof two plagioclase domains (W2-pll and W2-p12) used for quantitative textural analysis. (d) Hornblendes (W2) arecharacterized by straight grain boundaries meeting occasionally at triple points at variable angles. Many small grainsindicating high nucleation rate are present. SPO is rather low. Plagioclase is white, hornblende is light grey, pyroxeneis dark grey, and opaque minerals are black in all drawings.eastern counterpart. The increase in Na anddepletion in Ca in both metagabbro belts isrelated either to mylonitization or to primaryvariations due to postmagmatic processes.Trace elements show very similar trends forthe eastern and western metagabbros, suggestingthat they may originate from the same source.Metagabbros from both belts display a slightlynegative Nb anomaly normalized to MORB(Sun & McDonough 1989). Augen mylonitesfrom the western belt are depleted in Zr and Ticompared to eastern mylonites. Contents of Rb,Ba, and K are strongly variable in both belts,which is most probably related to postmagmaticalterations and/or mobility of these elementsduring metamorphism.Mineral chemistry and zoningSyndeformational chemical reactions arecommon in metabasic rocks (Brodie 1981;Brodie & Rutter 1985). Variations in plagioclasecomposition are shown in Figures 8 and 9.Amphibole compositions according to the classificationof Leake et al. (1997) are plotted inFigure 10. Representative analyses of mineralcompositions are listed in Tables 2 and 3.Mineral abbreviations are according to Kretz(1983).It is likely that the studied metagabbros experienceda metamorphic event prior to the maindeformation and metamorphism studied in thiswork. This is supported by existence of incompletelyamphibolized pyroxenes in undeformedrocks of low-strain domains. In addition, thecompositions of cores of large amphibole porphyroclasts(actinolites to actinolitic magnesiohornblendes)deviate from those of recrystallizedmagnesio-hornblendes in the eastern belt. Suchcompositions of primary amphiboles may indicateearly greenschist to amphibolite facies conditionspreceding the main higher gradeCarboniferous deformation.The peak metamorphic assemblages within theeastern metagabbro consist of P1 + Hbl + Qtz +Ttn _+ Ilm __%Mag. The composition of plagioclaseporphyroclasts in protomylonite (El)varies between Anso and An6o (Fig. 8a). Smallrecrystallized grains show a composition similarto the mother host grain corresponding toAnso-6o. More albitic compositions (An4o_45)occur along grain boundaries or triple junctionsof recrystallized grains (Fig. 9a). The geometryand sharp gradient in mineral zoning crosscuttingseveral grains is attributed to post-peak258