Quantitative structural analyses and numerical modelling of ...

PRECURSOR AND RHEOLOGY OF VARISCAN GRANULITES 105(a)(b)Fig. 1. (a) Position of the Bohemian Massif in the Variscan chain in Europe. (b) Simplified geological architecture of the BohemianMassif. Rectangle marks the study area depicted in Fig. 2. Modified after Franke (2000).Radiometric ages ascribed to protolith crystallizationof the South Bohemian felsic granulites give between469 ± 4 and 357 ± 2 Ma using the U–Pb method onzircon (Wendt et al., 1994; Kro¨ ner et al., 2000). Subsequentpeak HP metamorphism took place between351 ± 6 Ma (Wendt et al., 1994) and 341 ± 3Ma(Kro¨ ner et al., 2000). Retrogression under amphibolitefacies conditions proceeded immediately after exhumationto mid-crustal levels, between 340 ± 3 and338 ± 3 Ma (both U–Pb on zircon, Kro¨ ner et al.,2000). Cooling below 500 °C is constrained by the331 ± 1 Ma 40 Ar– 39 Ar age of hornblende from anamphibolite adjacent to the BLG (Kosˇler et al., 1999).Protolith to the felsic granulites is still debated, butthe majority of studies consider the protolith to havebeen a granitic igneous rock (e.g. Fiala et al., 1987;Jakesˇ, 1997; Kotkova´ & Harley, 1999; Finger et al.,2003; Janousˇek et al., 2004, 2006; Tropper et al., 2005;Janousˇek & Holub, 2007). Janousˇek et al. (2004) suggestedan Ordovician granitic protolith with a modelage of c. 450 Ma, which is supported by the radiometricU–Pb zircon ages of Kro¨ ner et al. (2000) andFriedl et al. (2003).Franeˇk et al. (2006) reported a succession of threeductile fabrics in the BLG (Fig. 2). The oldestsubhorizontal fabric S1 is preserved in an 8.5 km ·2.5 km elliptical area of the BLG and macroscopicallyis defined by an alternation of up to 1-cm thickwhite bands formed by recrystallized alkali feldspar,up to 1-cm thick quartz-rich bands and 1- to 3-mmthick darker plagioclase–garnet-dominated bands(Fig. 4a,b). Large perthite porphyroclasts, up to acentimetre in diameter, are preserved in the feldsparrichbands. The S1 foliation is folded by metre-scalepassive F2 folds with steeply inclined axial planesand pervasively developed penetrative cleavage S2,which is macroscopically characterized by strongsubhorizontal elongation of quartz ribbons andbiotite aggregates. Within the limbs, the S1 compositionallayering is stretched and progressively rotatedtowards the N–S striking steep S2 cleavage, whichcontains the subhorizontal L2 stretching and minerallineation. In strongly reworked areas, the only relictsof the S1 fabric are highly attenuated remnants of theS1 compositional layering parallel to S2. Macroscopically,the D2 structures are best seen in the formÓ 2010 Blackwell Publishing Ltd343