Quantitative structural analyses and numerical modelling of ...

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