Quantitative structural analyses and numerical modelling of ...

3. Quantitative analyses of metamorphic microstructures 35Figure 3.5: N 0 -Gt plots showing evolution of CSD from growth dominated to nucleationdominated microstructural stages within crustal channel. This evolution isinterpreted as result of syntectonic melt infiltration.originally high Gt/N 0 values towards low values in conjunction with increasing amountof mineral resorption and mineral overgrowth features. This particular evolution canbe only explained by increasing importance of nucleation rate which is incompatiblewith process of in-situ melting but reflect crystallization of melt in rock pores. TheseCSD plots were therefore used as a major argument for reactive porous flow in felsicrocks, a concept which was for the first time demonstrated in the continental crust. TheCSD results are supported by evolution in grain contact frequency plot which revealssystematically increasing importance of regular grain distributions which is driven tounexpected values. The regular grain distribution simply indicates progressive growthof new phases in the rock aggregate as the melt crystallizes in the rock. Finally, theprogressive melt infiltration is related with loss of grain shape preferred orientation coupledwith loss of aspect ratios, which in turn is connected with increasing developmentof preferred orientation of certain unlike boundaries. All that indicates that the porousflow was a dynamic process related to dynamic dilation of intragranular pores whichmaintained the porosity of rock at high level as long as the reactive porous flow wasactive.Our last studies are focused on quantification of microstructural evolution of granulitetextures (Franěk et al., 2011b). We discovered precursor rock of felsic granuliteswhich is interpreted as coarse grained orthogneiss, that was converted to granulite duringCarboniferous thickening and collision. These studies show that the hypersolvusalkaline feldspar is first decomposed into thick perthites that are rapidly replaced bypolycrystalline, equidimensional aggregate of pure K feldspar and plagioclase (Fig. 3.6).This decomposition is driven by heterogeneous nucleation process resulting from stored