Quantitative structural analyses and numerical modelling of ...

102 L. BARATOUX ETAL.is observed with increasing deformation. Modalproportions of amphibole and plagioclase in thewestern metagabbros are more variable thanthose in the eastern belt. Amphibole proportionmay vary between 20 and 80%, most likely dueto original magmatic compositional variations.For the purpose of our study, samples composedof c. 50% of amphibole and c. 50% of plagioclasewere chosen.Deformation of the eastern (lower)metagabbro sheetNon-deformed metagabbro (EO). At low strain,plagioclase and hornblende exhibit euhedral randomlyoriented crystals of 0.5-3 mm in size.Tapering mechanical twins and local undulatoryextinction occur in plagioclase. There is no evidencefor any kind of dynamic recrystallizationor crystallization of new grains. Undulatoryextinction locally attests to some bending ofamphibole grains. Amphibole porphyroclastsshow random spatial distribution and they aregenerally not interconnected.Protomylonite (El). At higher strains, about20-25% of the total volume of plagioclasegrains but only 8-10% of amphibole showevidence of strain, suggesting that the deformationwas accommodated mostly by plagioclaserecrystallization and associated grain-sizereduction. Plagioclase grains show polysynthetictwins according to albite and pericline laws(Tullis 1983) and patchy undulatory extinction.Large plagioclase porphyroclasts of 2-5 mmare cut by brittle fractures (Fig. 3a) reducingthe grain size to 0.5-1 mm. The fractures arefilled with small twin-free recrystallized grainsof 0.02-0.1 mm. Two recrystallization mechanismshave been identified: bulging and subgrainrotation recrystallization (Fig. 3b), as proposedby Poirier & Guillop6 (1979) or Fitz Gerald &StiJnitz (1993), leading to core-mantle structures.Core and mantle structures were observed withan intermediate zone of subgrains and newgrains of similar size developed along porphyroclastboundaries. Boundaries between the neoblastsbecome progressively straight, meeting attriple junctions of 120 ~ .Large porphyroclasts of hornblende revealstrong internal deformation such as kinking,bending leading to sweeping undulatory extinctionand (100) twinning. The twin planes arenot regular and they locally form finger-likestructures. Brittle fractures transecting largegrains are often present. Randomly orientedporphyroclasts of 2-5mm in size rotateFig. 3. Drawing and micrographs (XPL) of the eastern metagabbro protomylonite. (a) Initial stage of plagioclasedeformation characterized by brittle fractures cross-cutting large porphyroclasts. Digitized drawing was used for thequantitative textural analysis. Arrows mark three clasts derived by fracturing of one porphyroclast and showcorresponding grains in the digitized drawing and micrograph. Plagioclase is white, hornblende is light grey, andopaque minerals are black. (b) New grains develop preferentially along fractures by a mechanism of subgrain rotation(SR). Note that the neoblasts are twin-free. (P) refers to porphyroclast.254