Quantitative structural analyses and numerical modelling of ...

104 J. FRANĚK ET AL.in terms of thermal conditions and deformationalprocesses. Unravelling the complex deformationalbehaviour of the felsic granulites should assist broaderconsiderations of rheology and mechanical behaviourof orogenic lower crust during various stages ofdeformation and exhumation.The deformation mechanisms governing rheology oflower crustal rocks are often hard to determine due tochanges of the microstructures during exhumation,recrystallization in later deformations or retrogression(e.g. Rutter & Brodie, 1992). At granulite faciesconditions, the ductile strain is usually accommodatedby dislocation creep, or diffusion creep that may becomplemented by grain-boundary sliding (GBS)(Martelat et al., 1999; Garlick & Gromet, 2004),each mechanism having variable importance. Grainboundarydiffusion or the presence of silicate meltsmay favour the diffusion creep or GBS according tonew results of Schulmann et al. (2008) and Za´vadaet al. (2007). The GBS may then operate at the expenseof other mechanisms and promote granular flow,particularly in fine-grained rocks (Za´vada et al., 2007;Schulmann et al., 2008).Feldspar, the main constituent of the felsic granulites,is an essential component of the EarthÕs crust.Nevertheless, its deformational behaviour is not yetfully understood, mainly due to complex solid solutionmixing and variation in crystallographic structure withcooling (e.g. Ribbe, 1983; Putnis et al., 2003; Abartet al., 2009). A common process is exsolution withcooling, which modifies the rheological behaviour ofalkali feldspar and may lead to drastic weakening ofthe orogenic lower crust (Schulmann et al., 2008).Feldspar recrystallization, driven mainly by chemicaldisequilibrium of the Or–Ab–An solid solution,has been studied in natural examples (Stu¨ nitz, 1998;Putnis, 2002) or experimentally (Stu¨ nitz & Tullis,2001), but these studies have focused mainly onmedium-temperature water-assisted processes below500 °C. At these conditions the original chemicallyunstable feldspar undergoes dissolution and precipitatesas two separate feldspars of different composition.Such results cannot be easily extrapolated to thegranulite water under-saturated HT conditions of atleast 850 °C (OÕBrien & Ro¨ tzler, 2003; Sˇtı´pska´ &Powell, 2005), where the available fluid is representedby silicate melt, rather than water.In order to address the above-mentioned aspects, wepresent a microstructural analysis of exceptionallypreserved samples of lower-crustal felsic granulites froman 8.5 · 2.5 km domain (Franeˇk et al., 2006, 2011) withwell-preserved granulite facies fabrics. These rocks formpart of the Blansky´ les Granulite Massif (BLG) inthe southern Bohemian Moldanubian domain, whichbelongs to the Variscan collisional chain in centralEurope. The combined microstructural and petrologicalanalysis shows evidence of a complex evolution ofalkali feldspar rheology during granulite formation andexhumation. Changes in ductility are ascribed tochemically and deformationally driven recrystallization,variations in grain size as well as changingtemperature and the amount of interstitial melt.Geological settingThe Bohemian Massif (Fig. 1a,b) represents the easternexposure of the Variscan orogen in Europe. Duringthe Variscan orogenesis (380–300 Ma), involvingSaxothuringian oceanic subduction and subsequentcontinental underthrusting, a 300-km wide orogenicchain evolved. From the NW to the SE, the followingtectonic sequence is developed (Schulmann et al.,2009): the Saxothuringian domain represented byNeoproterozoic basement covered by Palaeozoicsedimentary rocks, the Tepla´ suture zone and thesupra-crustal Tepla´–Barrandian Unit. Further tothe SE, the arc-related granitoid plutons separate theTepla´–Barrandian folded sedimentary rocks from thehigh-grade Moldanubian Zone, which shows widespreadanatexis and contains slices of lower-crustal andmantle rocks. This pervasively deformed root domain isfurther to the east bounded by the Brunia microplate(e.g. Schulmann et al., 2005), which is only marginallyaffected by Variscan tectonometamorphic processes.The Moldanubian Zone consists of middle- andlower-crustal segments, offering an excellent opportunityto examine evidence of the exhumation processesoperating in a collisional setting. The exhumed lowercrust, designated as the Gfo¨ hl Unit (Fuchs, 1976), isrepresented by felsic granulites and anatectic gneissesthat enclose small bodies of mafic granulites, mantlerocks and eclogites. The mid-crustal paragneiss-dominatedlevel has been divided according to the prevailinglithology into the Monotonous Group, with only limitedcontent of intercalations, such as amphibolites orquartzites, and the Varied Group, bearing a largeproportion of intercalated amphibolites, quartzitesand marbles (Fuchs, 1976; Matte et al., 1990). Thestudied BLG is the largest granulite body in SouthernBohemia, and belongs to the Gfo¨ hl Unit, which islocated in a complex stack between the Monotonousand Varied groups, being accompanied by severalneighbouring granulite bodies (Fig. 2).Previous studies of South Bohemian granulitesP–T estimates (Fig. 3) of peak metamorphic conditionshave been calculated by various authors using eitherconventional thermobarometry yielding 1000 °C ⁄1.6 GPa (e.g. Vra´na, 1989; OÕBrien & Seifert, 1992;Carswell & OÕBrien, 1993; Cooke, 2000), thermodynamicmodelling in THERMOCALC software that yieldsa maximum of 850 °C ⁄ 1.6–1.8 GPa (Sˇtípska´ & Powell,2005) or TWEEQU yielding 970–1000 °C ⁄ 1.6–1.7 GPa(Kro¨ ner et al., 2000). The conditions for the amphibolitefacies overprint are estimated to 700–800 °C and0.5–0.8 GPa (Kro¨ ner et al., 2000; Sˇtı´pska´ & Powell,2005; Verner et al., 2007).Ó 2010 Blackwell Publishing Ltd342