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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
TEXTURES OF NATURALLY DEFORMED METAGABBROS119[a](lOO)Hornblende[c][001]Lower [a]{110} hemisphere (100)[c][001] {110}a) Protomylonite - E 1 b) Ultramyionite - E3- a,~,~ -. .~- | 9 . .." 9N = 209N=989 MD = 7,69 9 MD = 9.57 9 MD = 4.619 MD = 15.98 9 MD = 12.08 9 MD = 5.55c) Augen mylonite - Wl d) Banded mylonite - W2N= 105 N= 107m MD = 7.93 9 MD = 9,94 =MD = 5.11 =MD = 5.15 = MD = 9,46 9 MD = 4.91Fig. 16. Point and contour pole figures of amphibole CPO in (a and b) the eastern belt and (c and d) the western belt(lower hemisphere, equal area projections). Contours are counted as multiples (1, 2, 3, 4 .... • ) of uniform distribution.MD is maximum density of data in the contour diagrams. Foliation is represented by the horizontal line, lineationis trending E-W. The CPO of porphyroclast is marked by a circle.A possible switch from diffusion-dominated flowin the mylonite to dislocation creep in the ultramylonitemay be due to a decrease in temperatureof deformation as described from felsic granulitesby Martelat et al. (1999). The switch fromthe GSS to the GSI regime for decreased grainsize is possible and implies an increase ofstrain rate and/or a decrease of temperature inthe shear zone (Handy 1990). A similar transitionfrom diffusion-dominated creep in plagioclaseaggregates in augen mylonites to dislocationcreep in banded ultramylonites was reportedby Schulmann et al. (1996) from deformedmetagranitoids.Dominant (010)[001] slip was identified inplagioclase from both CPO patterns and/orTEM observations (e.g. Olsen & Kohlstedt1984, 1985; Montardi & Mainprice 1987; Ji &Mainprice 1988, 1990). However, the observedCPO patterns for amphibolite facies metagabbrosare not compatible with activation of the classicslip system. Various other slip systems, whichmay be activated in plagioclase also, weredescribed by Marshall & McLaren (1977a, b),Olsen & Kohlstedt (1984, 1985), and Montardi& Mainprice (1987). In addition, St(initz et al.(2003) concluded that during experimentaldeformation of plagioclase several slip systems271
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“It strikes me that all our knowl
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Contentsviii4.7 Schulmann, Konopás
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Dedicated to my wife Markéta, daug
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Foreword 2First topic “Quantitati
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Foreword 4source and freely availab
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1. Quantitative analysis of deforma
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Chapter 2Mechanisms of lower crusta
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2. Mechanisms of lower crustal flow
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Chapter 3Quantitative analyses ofme
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3. Quantitative analyses of metamor
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Bibliography 42Culshaw, N., Beaumon
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Bibliography 44sources and trigger
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Bibliography 46Skrzypek, E., Štíp
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Journal of Structural Geology 23 20
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JOURNAL OF GEOPHYSICAL RESEARCH, VO
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DTD 5ARTICLE IN PRESSJournal of Str
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Table 1Statistical values of the qu
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Table 2Summary of parameters derive
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EXHUMATION IN LARGE HOT OROGEN 275m
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Probabi l irequencyProbabi l ireque
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EXHUMATION IN LARGE HOT OROGEN 279y
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EXHUMATION IN LARGE HOT OROGEN 281N
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EXHUMATION IN LARGE HOT OROGEN 283w
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EXHUMATION IN LARGE HOT OROGEN 285a
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EXHUMATION IN LARGE HOT OROGEN 287w
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EXHUMATION IN LARGE HOT OROGEN 289T
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EXHUMATION IN LARGE HOT OROGEN 291O
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EXHUMATION IN LARGE HOT OROGEN 293r
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EXHUMATION IN LARGE HOT OROGEN 295B
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EXHUMATION IN LARGE HOT OROGEN 297R
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ORIGIN OF FELSIC MIGMATITES 39Grain
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ORIGIN OF FELSIC MIGMATITES 45Fig.
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ORIGIN OF FELSIC MIGMATITES 49stron
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ORIGIN OF FELSIC MIGMATITES 51Cmı
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104 J. FRANĚK ET AL.in terms of th
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106 J. FRANĚK ET AL.Fig. 2. Struct
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108 J. FRANĚK ET AL.(a)perthite po
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