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660 O. LEXA ET AL.Fig. 7. Chemical profiles of garnet: (a) inlayered amphibolite showing flat patterns,(b) in sillimanite-bearing paragneiss withwell-developed flat profiles in core andmargin affected by late diffusion, (c) intonalitic migmatitic gneiss and (d) inGrt-amphibolite within metagabbro.Numbering of samples corresponds tonumbers in Tables 1, 2 and 7. See text fordiscussion.where N 0 and Gt are constants and may be related tonucleation density and growth rate of crystal (Cashman& Ferry, 1988; Marsh, 1988).CSD plots of all samples constructed according tothe method by Peterson (1996) exhibit linear correlationsbetween logarithm of population density (i.e.number of crystals per size per volume) and crystalsize (Fig. 9b). Therefore, applying the theory ofCSD, such distributions could be parameterized byzero size intercept N 0 (nucleation density) and slopeGt (growth rate multiplied by time). These twoparameters plotted in a N 0 –Gt space (Fig. 9) exhibitan inverse correlation, and the samples with acomparable metamorphic grade and deformationhistory form distinct clusters. The metagabbros(samples GLT1 & GLT2, Fig. 8b) sheared during theVariscan episode from the eastern mylonitic belt(c. 650 ± 50 °C) form a group with the highest N 0and lowest Gt values, while metagabbros (samplesGHT1 & GHT2) adjacent to the granodiorite sill(c. 750 ° ±50°C) exhibit a decrease in N 0 in conjunctionwith increasing Gt. The samples LAC1,LAC3 and TG2 (Fig. 8a,d) of banded amphibolitesand amphibole-bearing tonalitic gneisses thatoriginated during the Cambro-Ordovician metamorphism(c. 750–850 °C) plot in the centre of the diagram(lower N 0 , higher Gt). The undeformedVariscan granodiorite (sample T1, Fig. 8c) exhibitslowest values of N 0 and highest values of Gt.Grain shapes, shape preferred orientation and grainboundary preferred orientationPlagioclase is a low-symmetry mineral characterizedby weakly elongated to round shapes under mostmetamorphic conditions. Degree of elongation ofplagioclase grains is therefore an indication of intracrystallinedeformation. In contrast, amphibole ishighly elongate in low-metamorphic grade rocks, whileat high-grade ones it becomes spherical (Brodie &Rutter, 1987). Therefore, shape analysis combinedwith SPO provides information about the degree ofdeformation of both plagioclase and amphiboleaggregates.Both grain shapes and SPO were evaluated using thePolyLX toolbox (Lexa, 2003). The grain shapes arecharacterized by preferred orientation of the long axesand by mean axial ratio R of the best-fit ellipse on theindividual grains using the area-moments ellipse fittingmethod. To evaluate an overall single phase SPO,individual linear segments of grain boundaries ofindividual phases were treated as independent vectorscontributing to the bulk Scheidegger–Watson orientation-tensor.The eigenvalue analysis determines twoeigenvalues and assigns them to mutually perpendiculardirections. The ratio of eigenvalues R e is consideredas a rough estimate of the degree of SPO forthe given phase. Grain boundary preferred orientationwas assessed by a similar technique, with the bulkÓ 2005 Blackwell Publishing Ltd242
CONTRASTING TEXTURAL RECORD OF TWO METAMORPHIC EVENTS 661(a) (b)(c) (d)(d)Fig. 8. Representative digitized microstructures used in textural analyses. See text for detailed sample description.Scheidegger–Watson orientation-tensor formed fromthe individual linear segments of boundary tracesbetween the chosen phases. Resulting eigenvalues ratioR b is considered as a rough estimate of the degree ofGBPO.Results of the grain shape analysis and SPO analysisare shown in Fig. 10. Both amphibole (Fig. 10b) andplagioclase (Fig. 10a) show systematic changes. Whilesamples with preserved magmatic textures (T1) andLAC samples (LAC1, TG2) exhibit very weak SPOand lowest values of axial ratios (c. 1.5), samples ofmylonitic metagabbros show a significant increase inthe average axial ratio (2–3) and increase of the SPOdegree with decreasing temperature.Grain contact frequenciesGrain contact frequencies allow the statistical deviationfrom random spatial distribution of grainboundaries to be examined (Kretz, 1994). So far, thedegree of deviation of grain boundaries distributionfrom random distribution have been evaluated byplotting observed/expected ratio of like–like contactsof the two major minerals against each other. Here wepropose a diagram where v valuev ¼ Observed pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiExpected ð2ÞExpectedis plotted against the ratio of orientation-tensoreigenvalues, i.e. the degree of GBPO. The majoradvantage of this diagram is in simple visual evaluationof the degree of deviation from an expected randomdistribution of grain contacts (v ¼ 0).Results of this analysis are presented in Fig. 10. Theamphibolite facies mylonitic metagabbros exhibitalmost a random distribution at higher strains (GLT2)and tend to develop more of an aggregate-type ofmicrostructure (v values are positive for like–like andnegative for unlike contacts in Fig. 10c,d) at lowerstrains (GLT1) accompanied by a significant decreaseof preferred orientation of amphibole–amphibole andamphibole–plagioclase contacts. The high-grademylonitic metagabbros (GHT1 & GHT2) exhibit astrong aggregate distribution and a moderate preferredorientation of grain boundaries. Samples from thebanded amphibolite complex (TG2 & LAC1) show atendency towards a regular distribution (negative like–like v values and positive unlike v values) and a lowdegree of GBPO. The tonalitic gneiss weakly overprintedby the Variscan deformation (LAC3) showsÓ 2005 Blackwell Publishing Ltd243
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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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Table 1Statistical values of the qu
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Table 2Summary of parameters derive
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J. metamorphic Geol., 2008, 26, 273
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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 285a
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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 31Ó 20
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ORIGIN OF FELSIC MIGMATITES 33durin
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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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