Quantitative structural analyses and numerical modelling of ...

B10406SCHULMANN ET AL.: RHEOLOGY OF PARTIALLY MOLTEN GNEISSESB10406Figure 8. (a) Detail from the backscattered scanning electron image of sample T1 (location of Figure 8ais shown in SEM image in Figure 6c). The image shows the distribution of albite seams along boundariesof K-feldspar grains, character of compositional zoning of the plagioclase in the top left, and intragranularfractures filled with albite (I.F.). (b) Rose diagrams show preferred orientation of 100 interstitial quartzand Pl2 seams in K-feldspar aggregate. The mean direction and standard circular deviation are shown inthe bottom of each diagram. The thick horizontal line represents the orientation of lineation (X), and thethin line shows the orientation of the mean direction.all the samples constructed according to the method byPeterson [1996] exhibit linear correlations between thelogarithm of the population density (i.e., the number ofcrystals per size per volume) and the crystal size (Figure 9b).Applying the theory of CSD, such distributions could beparameterized by the zero size intercept N 0 (nucleationdensity) and slope Gt (growth rate multiplied by time).These two parameters are plotted in a N 0 -Gt diagram[Lexa et al., 2005] where the samples form a distincttrend. Trends in the grain size distributions using CSDmethod are visualized in Figure 9b. The crystal sizedistribution plot of the plagioclase aggregates is the mostpronounced and shows a systematic decrease of N 0 and anincrease of the G t values with increasing degree ofdeformation i.e., from Type I to Type III rocks.5.2. Grain Shapes and Shape Preferred Orientation(SPO)[23] The aspect ratio median value of plagioclase andK-feldspar varies between 1.5 to 3.1 and no systematicpattern related to the type of rocks and the degree ofdeformation is obvious (Figure 10). However, the SPO ofplagioclase and K-feldspar in most of Type II and IIIorthogneiss samples is higher compared to the Type Isample with exceptionally high SPO for samples T1 andT2. There is a difference between the aggregate plagioclase(Pl1) and the interstitial albite (Pl2) marked by a systematicallyhigher SPO for the former compared to the latter.5.3. Grain Contact Frequencies (GCF) and GrainBoundary Preferred Orientation (GBPO)[24] The combination of the GCF analysis with thestudies of the preferred orientation of the like-like (like–like contacts = boundaries of minerals of the same species)and unlike grain boundaries (GBPO) yields importantinformation about the organization of the grain boundarieswith respect to the deformation processes [Lexa et al.,2005]. So far the degree of deviation of the grain boundarydistributions from the random distribution has been evaluatedby plotting the observed/expected ratio of the like–like8of20302