Quantitative structural analyses and numerical modelling of ...

B10406SCHULMANN ET AL.: RHEOLOGY OF PARTIALLY MOLTEN GNEISSESB10406examined for the interstitial and aggregate positions separately.The most striking feature is a progressive increase ofthe aggregate plagioclase grain size from 60 mm (medianvalue) in metagranite to 90 mm in augen orthogneiss to110 mm in banded orthogneiss, and to 170 mm forsample B2 (Figure 9a). We also observe a systematicallyhigher grain size of aggregate grains compared to theinterstitial plagioclase grains for all rock types. In addition,the median value of the aggregate plagioclase grainsis shifted toward the third quartile value, while in theinterstitial grains it is closer to the first quartile value.[22] In order to obtain information about the crystal nucleationand growth, the crystal size distribution (CSD) methodwas applied in the studied samples. The theory of CSD is awell-established technique in metallurgy, ceramics and chemicalengineering to reveal information about nucleation,growth rates and growth times of crystals [Randolph andLarson, 1971]. CSD in many metamorphic and igneous rocksshow a log linear relationship between the grain size L and thepopulation density N according to the equationN ¼ N 0 e L=GtFigure 6. Type II microstructure. (a) Micrograph ofsample M2 Type II microstructure characterized by largequartz ribbons and fine-grained feldspar matrix, (b) SEMimage of the M2 sample showing typical monomineralliclayering and plagioclase aggregate bridges, and (c) detailedSEM image of the T1 sample showing K-feldspar andplagioclase polycrystalline aggregate microstructure. Thecharacteristic feature is the high elongation of feldspargrains lined with Pl2 films, (d) corresponding ArcView GISdrawing of Type II microstructure of the T1 sample. TheSEM and ArcView GIS colors are the same as in Figure 4.where N 0 and Gt are constants and may be related to thenucleation density and growth rate of the crystals [Higgins,1998]. This technique was previously successfully appliedto the metamorphic rocks by Cashman and Ferry [1988],Hasalová etal.[2008], and Lexa et al. [2005]. CSD plots ofof the spatial distribution of the grain boundaries thatdevelops mostly because of solid state annealing [Flinn,1969], mechanical mixing [Kruse and Stünitz, 1999] orcrystallization of the interstitial melt [Dallain et al., 1999;Hasalová etal., 2008].5.1. Crystal Size Distribution[20] The grain size distributions of recrystallized quartz,K-feldspar and of plagioclase have been determined on thebasis of more than 500 measured grains per thin section foreach sample (except of about 200 measurements in coarsegrainedsample B2). Because of the ubiquitous lognormaldistribution of the grain populations, the median value wasconsidered to be the most reliable statistical value and thespread of the grain size distribution was evaluated as adifference between the third and first quartile instead of thestandard deviation.[21] The K-feldspar shows a similar grain size distributionfor all rock types and is characterized by the mediangrain size ranging from 60 to 180 mm (Figure 9a). Exceptsample B2 with exceptionally large grain size, the grain sizespread does not follow any systematic pattern. However, theinterstitial quartz shows an increasing median grain sizevalue from 20 to 50 mm in the Type II orthogneiss to 80–100 mm in Type III orthogneiss. The plagioclase wasFigure 7. Type III microstructure of banded orthogneiss.(a) Micrograph of sample D1 showing layers of quartzalternating with mixed layers of feldspars and quartz, (b)SEM image of the sample V1 showing quartz and feldsparmixing and poor definition of layer boundaries, (c) detailedSEM image of K-feldspar and plagioclase mixing in thematrix, and (d) corresponding ArcView GIS drawing. TheSEM and ArcView GIS colors are the same as in Figure 4.7of20301