Quantitative structural analyses and numerical modelling of ...

108 L. BARATOUX ET AL.Fig. 9. BSE images of plagioclase and amphiboleand representative chemical maps of plagioclasecompositional variations. (a) Plagioclase inprotomylonite from the eastern belt (El) suffers fluidrelatedchemical variations modifying the compositionat triple points and grain boundaries. (b) Growth-relatedchemical zoning in mylonite from the eastern belt (E2).(c) Growth-related zoning in coarse-grained plagioclaseof banded mylonite from the western belt (W2).fluid-phase activity along grain boundaries(McCaig & Knipe 1990). Recrystallized matrixplagioclases in the mylonite (E2) have andesiticcompositions (An3o-5o) and show increases inanorthite content towards their rims (Fig. 9b),which is commonly ascribed to increasingmetamorphic conditions. Rare rounded porphyroclast,as well as matrix grains in ultramylonites(E3), underwent strong and irregularly developedlate albitization. Matrix grains correspond toAn3o_43 although oligoclase and albite compositions(An6_ 15) are also common along fractures,cleavages and grain boundaries. These irregularcompositional variations are attributed to late retrogressionunrelated to the main deformationprocess.Amphibole porphyroclasts in protomylonites(El) are magnesio-hornblendes with variablecontent in Si but constant XMg (Fig. 10a).Amphiboles with more actinolitic compositionreplacing former magmatic pyroxenes can befound in the cores of some grains. Small amphibolegrains show similar compositions comparedto the porphyroclasts, some of them being moretschermakitic. The amphiboles in mylonites(E2) are magnesio-hornblendes with slightlylower Si contents than those in the protomylonites.High Si/low A1 domains documentingincomplete transformation of pyroxenes intohornblendes are present in the cores of large porphyroclastsas well. Locked-up grains in theultramylonites (E3) vary between magnesiohornblendeand tschermakitic composition.Small new grains have slightly lower A1 contentscompared to the less deformed stages.In the western belt, the peak metamorphicassemblages correspond to P1 § Hbl _ Cpx _+Opx +_ Grt ___ Ttn + Ilm. A granulite faciesmineral assemblage, comprising also sapphirineand corundum, is not stable and re-equilibratedduring subsequent upper amphibolite faciesreworking. In the south, granulite facies assemblagesare absent and probably completelyre-equilibrated. In the north, a late metamorphicphase is documented by the presence of prismaticorthoamphibole (gedrite).Plagioclase porphyroclast compositions arevery similar to those of the eastern belt and varybetween An4o and An65. Recrystallized matrixgrains in the augen mylonite (W1) show locallyincreasing anorthite content up to An6o_9o comparedto the porphyroclasts, indicating thatchemical reactions took place during recrystallization.Sharp limited domains of An4o_45 compositionalong grain boundaries are also common,suggesting late fluid activity (Knipe & McCaig1994). Recrystallized plagioclase grains inbanded mylonites (W2) show stronger variabilityin compositions (An35-9o). Growth-relatedzoning due to increasing metamorphic grade isdocumented in Figure 9c. The cores have morealbitic composition (An34) compared to the rims(An6o_65) and are attributed to the early metamorphicstage. Modifications of plagioclasecomposition characteristic of fluid-related compositionalchanges are also developed in thebanded mylonite.Amphibole porphyroclast compositions (Fig.10b) were analysed in two different thin sectionsof augen mylonite (W1) and the difference intheir mineral chemistry may be explained bylocal bulk rock chemical variations. The amphiboleporphyroclasts correspond to magnesiohornblendeswith similar compositions to thosein the eastern belt. Porphyroclast rim compositionsshow a decrease in Si content withrespect to the cores. Neoblasts are marked260