Quantitative structural analyses and numerical modelling of ...

118 L. BARATOUX ETAL.Plagioclase - western belt(010) (001)N= 1509 MD = 3.64 9 MD = 3.93 9 MD = 3.169 MD = 5.23 9 MD = 4.39 9 MD = 5.41Lower hemisphere 9 MD = 6.00 9 MD = 4.38 9 MD = 4.77Fig. 15. Point and contour pole figures of plagioclase CPO in the western belt (lower hemisphere, equal areaprojections). Diagrams are contoured as multiples (0.5, 1.0, 1.5, 2.0, 2.5 .... • ) of uniform distribution. MD ismaximum density of data in the contour diagrams. Foliation is represented by the horizontal line, lineation is trendingE-W.rotation recrystallization mechanisms. The CPOis strong and indicates a possible activity ofthe (001)[110] slip system.The mylonitic stage is characterized by subequantshapes of recrystallized plagioclasegrains (low aspect ratios) with straight boundaries,weak shape preferred orientation andslightly higher degree of GBPO compared tothe protomylonite. Recrystallized grains showincreasing grain sizes compared to protomyloniticstage. Compositional zoning in plagioclaseindicates syndeformational growth of matrixgrains (Sodre Borges & White 1980). The plagioclaseCPO in the mylonite is fairly weak. Theseobservations suggest that grain boundary sliding(Boullier & Gu6guen 1975; Lapworth et al.2002), accompanied by some component ofcrystal plastic deformation in plagioclase, wasthe dominant deformation mechanism. Mixingof amphibole and ptagioclase grains could beanother argument for the presence of grainboundary sliding. Rosenberg & Sttinitz (2003)suggested for the syntectonically cooled anddeformed Bergell tonalite that mixing of plagioclaseand biotite in a fine-grained matrix togetherwith weakening of CPO implied a mechanism ofdiffusion-accommodated grain boundary sliding.A switch of deformation mechanism from dislocationcreep to grain size sensitive (GSS) flowdue to strain softening and grain size reduction iswell known from quartzo-feldspathic rocks (e.g.Walker et al. 1990; Tullis & Yund 1991). Verysmall grain size is required for activation ofGSS deformation: 'less than 10 ~m' byBoullier & Gu~guen (1975), 2-16 &m by Tullis& Yund (1991), and 3-30 p~m by Stfinitz &Fitz Gerald (1993). However, some studiessuggest grain boundary sliding deformation forgrain sizes of 100-150 I-~m for plagioclase(Jensen & Starkey 1985), 24-41 I~m for mixedplagioclase-hornblende layers (Kruse & Stfinitz1999), and 100-250 p,m for plagioclase(Lapworth et al. 2002). In our samples withweak or random CPO, the grain size variesbetween 30 and 150 txm. The occurrence ofgrain-boundary diffusion creep for such grainsize may be explained by very low strain rates(Fliervoet et al. 1999; Lapworth et al. 2002).Fine-grained plagioclases from high-straindomains adjacent to amphibole porphyroclastsand matrix plagioclase grains in ultramylonitesshow a decrease in average grain size coupledwith increased aspect ratios, strong SPO andGBPO. The CPO is strong for the ultramylon_itesample, suggesting activation of a (021) 1/2 [ 112]slip system. These features may indicate thatat very high strains the grain size sensitiveflow becomes less important in plagioclase.270