Quantitative structural analyses and numerical modelling of ...

PRECURSOR AND RHEOLOGY OF VARISCAN GRANULITES 117equi-dimensional mosaic (Fig. 5e). Transposition of S1compositional layering into the S2 fabric via rotationand attenuation suggests intense deformation duringD2 (Fig. 9a). The quartz bands inherited from S1disintegrated to form the ribbons parallel to S2 andwere further dynamically recrystallized along ribbonboundaries, leading to significant reduction of quartzgrain size (Fig. 11a). Numerous new small quartzgrains nucleated in triple junctions of feldspar grains inthe matrix. The garnet grains became dispersed in thematrix and kyanite crystals deformed by kinking.Isolated biotite flakes lie parallel to S2 and, togetherwith quartz ribbons, define a strong L > S fabric.Areal analysis of a Type II matrix region covering100 feldspar grains yields an average feldspar compositionof 56.4% Or, 35.9% Ab and 7.7% An(Fig. 7a). The average composition of the K-feldspar is84.7% Or, 13.5% Ab and 1.8% An, and the plagioclaseis composed of 1.8% Or, 74.6% Ab and 23.6%An.Two compositional maps acquired from the feldspar-dominatedmatrix (one depicted in Fig. 10b)reveal typical mild zoning of K-feldspar, reflectingthe loss of Na around grain boundaries. Plagioclaseexhibits uniform An 23 composition with a weakincrease of 2–3% anorthite content towards rims.Locally, thin films of albite occur along plagioclase–K-feldspar and K-feldspar–K-feldspar boundaries;they are significantly thinner than rims in the Type Imicrostructure.Type III: microstructure of S3 amphibolite facies fabricThe S3 fabrics prevailing in the felsic granulites of theBLG reveal highly variable degrees of retrogressionof the previous mineral assemblage. Contemporaneoushydration of granulites heterogeneously increasestowards the boundaries of the granulite massif, whereit is accompanied by widespread partial melting andsegregation of the melt into mm–dm thick bandsparallel to S3 (Kodym, 1972; Franeˇk et al., 2006). Thequartz, K-feldspar and plagioclase show irregulargrain boundaries and develop into monomineralicaggregates (Fig. 12), whereas the quartz ribbons typicalfor the S2 entirely disappear. The aspect ratios ofquartz grains significantly decrease, compared with theS2 fabric, in conjunction with coarsening of feldsparmosaic (Figs 5f & 11b). The abundant biotite flakeslie parallel to the foliation planes and sillimanite insome instances defines the lineation. The resulting S3microstructure resembles a common Bt + Sil ± Grtorthogneiss more than a retrograde granulite.In order to eliminate the effect of strain localizationinto melt bands or biotite stripes that developed duringD3, only macroscopically homogeneous S3 sampleswith dispersed biotite were studied. The petrology andmicrochemistry of a steep fabric from a neighbouringKrˇisˇťanov Granulite Massif, analogous to the S3 in theBLG, are given in Verner et al. (2007).Quantitative microstructural analysisThe previous section defined three types of microstructuresdeveloped from the coarse-grained precursorof the felsic granulites: Type I corresponds togranular microstructure inside coarse perthite and atransitional type in the vicinity of coarse perthitegrains; Type II microstructure corresponds to thepervasively developed granulitic S2 fabric; and TypeIII microstructure corresponds to the S3 fabric developedunder amphibolite facies conditions. Accordingto the principle of fabric superposition (Wilson, 1961),the succession of microstructural types is seen as anevolutionary microstructural trend reflecting deformationstages along the exhumation path of thegranulites. The coarse-grained orthogneiss precursormicrostructure cannot be sufficiently describedusing quantitative methods. In order to comparethe three microstructural stages, we have quantifiedseveral parameters characterizing the microstructuresusing the PolyLX toolbox (Lexa et al., 2005) forthe MATLAB TMsoftware package and CSD-correctionprogram (e.g. Higgins, 1998). Four thin sections orientedparallel to the lineation and perpendicular to thefoliation (XZ sections) from representative samples ofthe S2 (956 & 1252 grains) and S3 fabrics (848 & 886grains) were digitalized in an ArcView GIS environment(Fig. 11a,b) and analysed by the PolyLX andCSD-correction software. In order to obtain statisticallysignificant results, we have merged four areas ofthe Type I microstructure yielding 801 grains. Beforemerging, the individual areas were rotated with respectto the trace of S2. In addition, the transitional matrixarea at the boundary between the perthite and D2matrix was digitalized, covering 868 grains.Different microstructural types have been quantifiedin plots of bulk grain-boundary preferred orientation(GBPO) against contact frequencies (Fig. 12) andslope of linear regression from crystal size distribution(CSD) analysis v. regression intercept (Fig. 13). Thegrain size as well as other quantitative characteristicsare statistically evaluated in Table 2, more detaileddescription of the quantitative data is given inAppendix S1.The contact frequency method (Kretz, 1969)compares the observed (O) count of contacts betweentwo minerals with the value expected (E) for a perfectlyrandom distribution. The calculation involves modalproportions of phases, only a scatter in grain size maycause a limited uncertainty of the results. The boundariesare designated as Ôlike–likeÕ for contacts betweengrains of one phase or ÔunlikeÕ for the case of boundarybetween two different phases. The resulting v 2 -value(O ) E) ⁄ sqrt(E) is a measure of the deviation ofmineral spatial distribution from random. It is positivefor like–like contacts and negative for unlike contactsif the corresponding microstructure tends to formmonomineralic aggregates (Lexa et al., 2005). On theother hand, the unlike contacts exhibit positive values,Ó 2010 Blackwell Publishing Ltd355