Quantitative structural analyses and numerical modelling of ...

104 L. BARATOUX ETAL.only 10-20% of the total plagioclase volume.Plagioclases generally exhibit subequant shapeswith straight grain boundaries meeting at 120 ~triple joints (Fig 4b). However, in the vicinityof amphibole porphyroclasts, the plagioclasegrains are more elongate with higher aspectratios up to 2.5 (Fig. 4c).Hornblende grains are arranged in anastomosinginterconnected bands defining the myloniticfoliation. Most of the hornblende matrix grains,0.1-0.3 mm in length, are elongate in XZ sectionsattaining high aspect ratios (up to 6) whilethey are lozenge-shaped in the YZ sectionswith aspect ratios of 1.5-3. A total of 10-20%of the hornblende is present in a form oflocked-up porphyroclasts of 0.2-2 mm in sizeshowing undulatory or patchy extinction characteristicof strong internal strain. These porphyroclastsare elongate parallel to the foliation.Porphyroclasts are often transected by microshearzones (Fig. 4d) and small needle-shapedgrains consequently develop within these zonesby activation of the weak cleavage system{110}.Ultramylonite (E3). The ultramylonitic metagabbrosof the eastern sheet are marked by veryuniform mineral banding of fine-grained hornblendeand plagioclase layers ranging between 1 and5 mm in width (Fig. 5a). Asymmetric fabric ofhornblende and plagioclase is no longer present.Plagioclase forms continuous bands includingabundant interstitial grains of amphiboles(Fig. 5b). In contrast, amphibole-rich bands arealmost monomineral. Plagioclase is completelyrecrystallized, varying in size between 0.05 and0.3mm. The grains have smooth roundedshapes locally elongate parallel to the foliation.Rounded grains of 0.5 mm in size, interpretedas relicts of original porphyroclasts, locallyoccur in the fine-grained matrix. Plagioclasegrains are affected by strong retrograde sericitizationand albitization.Interstitial and matrix grains of hornblendereach 0.03-0.1 x 0.1-0.3 mm in size. Lockedupporphyroclasts of amphibole (0.5-1 mm insize) occur parallel to the foliation. However,they are less common than in the mylonite(E2). All amphibole grains are characterized byvery strong SPO (Fig. 5c).The mylonitic foliation is cross-cut by extensionalveins filled by a mixture of epidote andamphibole. These veins show that fluid activitywas high after the main mylonitic episode. Albitizationand sericitization of plagioclase in somesamples suggest that increased amount of waterwas present in these rocks also during late retrogression,which is most likely not related to themain process of mylonitization.Deformation of the western (upper)metagabbro sheetAugen mylonite (W1). In the western metagabbrobelt, the deformation was pervasive sothat initial protomylonite stages of deformationFig. 5. Drawings and micrograph of the eastern ultramylonite. (a) Plagioclase and amphibole banding (PPL).Plagioclase is darker than amphibole due to strong sericitization. (b) Plagioclase band in ultramylonite (E3)with abundant interstitial hornblende. Small grain size and subequant shapes are typical for these plagioclases.(c) Amphibole layer from ultramylonite (E3) is characterized by very fine grain size and strong SPO.256