Quantitative structural analyses and numerical modelling of ...

B10406SCHULMANN ET AL.: RHEOLOGY OF PARTIALLY MOLTEN GNEISSESB10406Figure 9. (a) Grain size statistics for the studied samples presented in box-and-whiskers diagrams.Horizontal axis corresponds to the ferret diameter of grain size in micrometers, and the thick barrepresents the median of grain size distribution. Vertical axis shows samples arranged according to thedegree of deformation from least deformed S1 to most intensely deformed sample V1. B2 represents anexceptionally coarse-grained mylonitic banded orthogneiss. For each sample the plagioclase grain sizestatistics for interstitial phases (I = Pl2 grains), and for recrystallized grains forming aggregates (A = Pl1grains) are shown. The number of grains is indicated. Grain size distributions of isolated quartz in thematrix and quartz grains forming centimetric ribbons or augen are also differentiated. This diagram showsprogressive coarsening of both Pl1 (aggregate) and Pl2 (interstitial) grains. (b) Plot of crystal sizedistribution (CSD) for plagioclases (I = Pl2 interstitial grain, A = Pl1 recrystallized grains in aggregates).N 0 ln(mm 4 ) values on vertical axis represent density of grains per volume, and dimensionless Gt valuesreflect the grain size frequency distribution. This diagrams show decrease of N 0 values and increase of Gtvalues which in classical CSD plots represent decreasing values of the intercept of the CSD curve withvertical axis associated with decreasing slope [Higgins, 1998]. In the CSD theory this evolution meansdecreasing nucleation rate and increasing growth rate contribution to the shape of the grain size frequencyhistogram [Lexa et al., 2005].contacts of the two major minerals against each other [e.g.,McLellan, 1983]. Lexa et al. [2005] proposed a newdiagram where the c valuec ¼ Observed pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiExpectedExpectedis plotted against the ratio of orientation tensor eigenvaluesthat represent the degree of GBPO. If the solidified melt isidentified in a rock the GBPO may yield information aboutthe types of channel networks as defined by Sawyer [2001]and quantified by Hasalová etal.[2008] and Závada et al.[2007].[25] In this work we use a method proposed by Lexa et al.[2005] where the grain contact frequencies are used toassess the character of the spatial distributions of grainboundaries. Figure 11a shows that the plagioclase like-likecontacts for the Type I metagranite plots in an intermediatepart of the diagram and is characterized by a rather smallaggregate distribution. This is due to the presence of the twoplagioclase populations resulting from the recrystallizationof the large plagioclase crystals (high aggregate distribution)and from the disintegration of the large alkalinefeldspar crystals into a mixture of albite and K-feldspar(Figure 4). With increasing deformation a clear evolution ofthe grain contact frequency toward strongly aggregateddistribution for weakly deformed samples of the Type IImicrostructure can be observed. This is probably due to thecoalescence of feldspar and plagioclase layers. At very highstrain intensities the plagioclase grain contact frequenciesevolve toward the random or regular types of distribution inType III microstructure. This type of evolution indicates analmost perfect mixing of the plagioclase with other mineralphases. The degree of GBPO of the like-like boundaries forplagioclase does not evolve with the above described trendbut remains rather constant for any degree of the finitestrain. The K-feldspar grain contact frequency shows asimilar behavior to the plagioclase but for some samples arather strong GBPO coupled with a decreasing grain contactfrequency was observed. The evolution of the unlikeplagioclase-K-feldspar grain boundaries exactly mirrorsthe evolution of the plagioclase like-like contact frequencybehavior (Figure 11b). For the observed constant grain sizeit is a geometrical necessity that an increase of the like-likecontacts causes a corresponding decrease of the unlikecontacts of feldspars.6. Crystallographic Preferred Orientation[26] The lattice-preferred orientation (LPO) of quartz,plagioclase and K-feldspar was determined using the electronbackscatter diffraction (EBSD) technique [Bascou et9of20303