Quantitative structural analyses and numerical modelling of ...

58 J. FRANĚK ET AL.to c. 346 Ma (Janousˇek et al., 2004) and extension at c.340 Ma (Ža´k et al., 2005a), followed by the intrusionof undeformed granitoids along the boundary betweenthis complex and the Moldanubian domain at c.337 Ma (e.g. Janousˇek & Gerdes, 2003). This corroboratesages of 338–314 Ma for intrusion into theMonotonous Group of mostly post-tectonic granitoidsof the Moldanubian Pluton (e.g. Finger et al., 1997).In northern Bavaria, the Moldanubian rocks exhibityounger metamorphism associated with widespreadmigmatitization dated by U–Pb on monazite at c.326 Ma (Kalt et al., 1997) and c. 315 Ma (Schulzschmalschlager,1984). Syntectonic Bavarian granitoidsyield ages comparable to the MoldanubianPluton (335–322 Ma; e.g. Siebel et al., 2006).GEOLOGY AND STRUCTURAL GEOLOGY OFSOUTH BOHEMIAN MOLDANUBIAN DOMAINThis study focuses on a 130 · 30 km traverse throughthe whole South Bohemian Moldanubian domain fromthe Moldanubian Pluton in the SE to the CentralBohemian Plutonic Complex in the NW following the9HR seismic line (Fig. 4a). The SE part is characterizedby the occurrence of three large neighbouringbodies of felsic granulites (Blansky´ les, Krˇisˇtˇanov andPrachatice granulite massifs, see Fig. 5a). Discontinuousstripes of garnet or spinel peridotites and garnetpyroxenites are systematically found along the marginsof all the granulite massifs, and the Blansky´ les granulitemassif also contains numerous large bodies ofmantle-derived rocks in its interior.The adjacent Moldanubian domain consists of VariedGroup rocks to the SE and Monotonous Group rocks tothe NW. The individual massifs are separated by narrowzones of medium to high-grade paragneisses withaffinity to both Monotonous and Varied Groups. Thenorth-eastern margin of the Blansky´ les granulite massifis marked by a complex association of medium andhigh-grade rocks (Fig. 4a). Farther to the east all theunits become parallel to a major thrust zone defined bythe occurrence of eclogite bodies marking the ductilethrust of the Varied Group over the underlyingMonotonous Group (Rajlich et al., 1986; Vra´na &Sˇra´mek, 1999; Faryad et al., 2006). A zone of lowergrade muscovite-biotite paragneiss intruded by granitoidsof the Moldanubian Pluton occurs in the easternmostextremity of the studied area. The MonotonousGroup in the north-western part of the traverse isintercalated with several large NE–SW elongated bodiesof Varied Group and Gfo¨ hl orthogneiss (Fig. 4a).Earliest granulite facies fabrics S1 and S2The oldest identified fabrics (S1 and S2) are exceptionallywell preserved in an 8.4 · 2.5 km wide ellipticaldomain in the southern part of the Blansky´ lesgranulite massif (Figs 5a,b & 6a). Small relicts arefurther present throughout this massif and rarely alsoin the Prachatice granulite massif. In particular, in theBlansky´ les relict elliptical area the granulites exhibit apenetrative mylonitic foliation S2 (Fig. 5b) whichcontains relicts of S1 compositional layering, bothbearing a stable granulite facies mineral assemblageof Qtz + Kfs + Pl + Grt + Ky + Bt (Sˇtípska´ &Powell, 2005; Franěk et al., 2006). Mineral abbreviationsare after Kretz (1983).The onset of S2 fabric development is characterizedby pervasive recrystallization of the former coarsegrained S1 layered orthogneiss into a fine-grainedmosaic of plagioclase, K-feldspar and quartz, containinga small volume of former partial melt, nowindicated, for example, by cuspate shapes of theseminerals, distributed along feldspar boundaries(Franeˇk et al., 2010). The subsequent folding of S1 isaccompanied by progressive development of S2 axialplanecleavage and penetrative reworking of the S1fabric into the fine-grained, K-feldspar dominatedmatrix containing large quartz ribbons and biotiteflakes oriented into strong subhorizontal stretchinglineation (fig. 3e in Franěk et al., 2006). Plagioclasecoronas developed around kyanite and garnet in thematrix were interpreted as a product of reaction duringdecompression by Tajcˇmanova´ et al. (2007). The S2foliation from the Blansky´ les granulite massif strikesuniformly N–S showing variable dip of 50–90° to theW or less commonly to the E (Figs 4e, 5b & 7).Amphibolite facies D3 deformationIn all the Moldanubian rocks except the granulites, thefirst well-defined metamorphic fabric is a steepNE–SW trending amphibolite facies foliation(Fig. 6b,d). It is assigned as S3 due to a commonstructural concordance with a steep retrogressive S3fabric developed in the granulites.Because of strong later reworking, the S3 fabrics areusually well preserved only in competent lithologies.The only exceptions represent the southern part of theKaplice Zone muscovite paragneiss and the boundaryof the Moldanubian and the Tepla´-Barrandiandomains where steep NW-dipping S3 is present. Rarerelicts of S3 in metasedimentary rocks throughout thestudy area exhibit the same attitude. The S3 foliationoccasionally bears a weak mineral lineation or corrugations,both shallowly plunging NE and SW. In thevicinity of the regionally folded granulite massifs, thegenerally stable orientation of the S3 foliation ismodified by large wavelength late F3 folds.The S3 is defined by preferred orientation of micaand elongated quartz–feldspar aggregates in the Bt +Qtz + Pl + Kfs ± Sil ± Grt ± Ms paragneiss ofthe Monotonous and Varied Groups and by preferredorientation of leucosomes in migmatitic paragneissand the anatectic Gfo¨ hl gneiss. In the medium-gradeKaplice Zone paragneiss the S3 is defined by alignmentof muscovite and biotite and segregation of quartzlenses with cordierite or aluminosilicates (Vra´na et al.,Ó 2010 Blackwell Publishing Ltd210