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84 O. LEXA ET AL.mechanism lead to inversion of this crustal profile sothat the granulites presently form cores of crustalanticlines, surrounded by AGB, variably thinned duringextrusion, and by the Gfo¨ hl gneiss followed bymetamorphic sequences of the Monotonous and Variedgroups. In many places, the whole structure is eveninverted because of the lateral spreading of verticallyextruded lower crustal material (Fig. 2). This model iscorroborated by detailed structural and petrologicaldata, which show that the granulites and surroundingrocks exhibit common vertical fabrics which are associatedwith HP–high temperature (HT) mineralassemblages. The flat fabric, which locally may have abowl shape, is superimposed on vertical extrusionfabrics and developed during subsurface spreading ofthe extruded partially molten lower crust (7–4 kbar,700–650 °C; Racek et al., 2006; Tajcˇmanova´ et al.,2006; Hasalova´ et al., 2008b).CHRONOLOGY OF THE VARISCANCALC-ALKALINE TO POTASSIC MAGMATISM –CONSTRAINTS ON TIME-SCALES OF THEOCEANIC AND CONTINENTAL SUBDUCTIONThe contact of the Tepla´-Barrandian and Moldanubianunits is marked by voluminous granitic plutons ofVariscan age with (normal or potassic) calc-alkalinechemistry and large ion lithophile element (LILE) ⁄ -high field strength element (HFSE) enrichmentresembling magmatic associations of active continentalmargins. The oldest vestige of this igneous activityinitiated by a late Devonian–early CarboniferousAndean-type subduction is preserved in the Cˇista´ andSˇteˇnovice plutons (Venera et al., 2000; Zˇa´k et al.,2010) as well as orthogneisses in the roof of the largeCentral Bohemian Plutonic Complex, CBPC (protolithc. 370–360 Ma; Kosˇler et al., 1993). A newlyidentified member of the subduction-related associationis the mafic Lisˇov low-pressure granulite unit insouthern Bohemia, the protolith of which was emplacedat c. 360 Ma into middle crustal levels (15–20 km) of the same igneous arc (Janousˇek et al.,2006).After a significant time gap, subduction-relatedmembers of the CBPC were emplaced, including thenormal calc-alkaline gabbros, quartz diorites and tonalitesof the Sa´zava suite (354.1 ± 3.5 Ma; Janousˇeket al., 2004a) and the high-K calc-alkaline, mainlygranodioritic Blatna´ suite (347 + 4 ⁄ )3 Ma; Do¨ rr &Zulauf, 2010; 346.4 ± 1.1 Ma; Janousˇek et al., 2010)with associated monzonitic bodies. Finally, furthereast, commonly in association with HP granulite massifsand high-grade Gfo¨ hl orthogneisses, syn-deformationalor post-tectonic intrusions of earlyCarboniferous (343–336 Ma) (ultra-) potassic rockswere emplaced (Holub, 1997; Holub et al., 1997;Janousˇek et al., 2003; Verner et al., 2008; Kotkovaét al., 2010; Kusiak et al., 2010). The time lag betweenthe end of normal calc-alkaline magmatism (c. 354 Ma)and the onset of intrusion of K-rich magmas (c.346 Ma) can be related to the transition from oceanplate subduction to continental underthrusting.PETROLOGY AND GEOCHEMISTRY OFMOLDANUBIAN GRANULITES AND POTASSICMAGMATIC ROCKSA peculiar feature of the Moldanubian domain in theBohemian Massif is an intimate spatial and temporalassociation between felsic HP, kyanite–garnet granulitesand large ultrapotassic plutonic bodies (Janousˇek& Holub, 2007). Whereas the granulites tend to bedepleted in the radioactive elements U, Th and K, the(ultra-) potassic rocks are characterized by strongenrichment in these elements that shows clearly in theradiometric map (Fig. 4). Thus, understanding of theK, U and Th depletion ⁄ enrichment in individual rocktypes at various stages of the Viséan HP metamorphismand igneous activity seems to be a key to decipheringthe thermal history of the Moldanubianorogenic crust.Moldanubian HP granulitesHigh-pressure granulites represent a voluminous andubiquitous component of the Gfo¨ hl Assemblage inboth Austria and the Czech Republic. The most typicalare felsic types consisting essentially of garnet, quartzand hypersolvus feldspar, and commonly containingkyanite. Rutile, zircon, apatite, ilmenite ± monaziteare the common accessories (OÕBrien & Ro¨ tzler, 2003).The felsic Moldanubian granulites were considered asformer rhyolites ⁄ granites that acquired their highgrademetamorphic character during the Variscancollision and which suffered only limited HP melting(Fiala et al., 1987a; Vellmer, 1992; Janousˇek et al.,2004b). Indeed, some granulites in rare domains thatescaped later mylonitization ⁄ recrystallization aremigmatitic in appearance (Franeˇk et al., 2006). On theother hand, other authors (Vrańa & Jakesˇ, 1982; Jakesˇ,1997; Kotkova´ & Harley, 1999) suggested that thefelsic granulites represent HP granitic liquids thatformed and separated from their source during theVariscan metamorphic cycle. There seem to be severalarguments against such a model (Janousˇek et al.,2004b): (i) concentrations of Zr are far too lowcompared to calculated saturation levels at ‡900 °C,coupled with significant, mainly Ordovician zirconinheritance; (ii) consistently low (750 °C) zircon andmonazite saturation temperatures, whereas pre-Variscaninheritance is by no means rare, documenting thatsaturation was reached (Janousˇek, 2006); (iii) preservationof Ordovician–Silurian whole rock and thin slabRb–Sr ages corresponding to the principal inheritedcomponent in granulite zircon; and (iv) high heavy rareearth element + Y contents, ruling out the presence oflarge amounts of garnet in the residue and thus indicatinga rather low-P melting.Ó 2010 Blackwell Publishing Ltd186
HEAT SOURCES AND EXHUMATION MECHANISMS 85Fig. 4. Radiometric map of the south-eastern Bohemian Massif with isolines of natural air absorbed dose rate (nGy h )1 ). Themain bodies of ultrapotassic rocks (durbachite series and melasyenitoids sensu Holub, 1997) that show very high radioactivity areidentified by name. The Variscan granites (crosses) and HP granulites (dots) are outlined. Source: Czech Geological Survey MapServer, http://www.geology.cz.The most peculiar feature of the felsic Moldanubiangranulites is the lack of LILE depletion, except for Cs,U and Th (Fiala et al., 1987a,b; Janousˇek et al.,2004b). Thus, unlike in many other granulite terranesworldwide (Rudnick & Presper, 1990; Rudnick & Gao,2003), prograde metamorphism appears to have beenlargely isochemical. As shown by Janousˇek et al.(2004b), the composition of felsic Moldanubian granulitesmatches well with felsic Ordovician–Silurianmetaigneous rocks from the Saxothuringian domain,for instance orthogneisses and meta-rhyolites from theFichtelgebirge. The similarities include whole-rockgeochemistry (excluding the most mobile elements Cs,Rb, Th, U, Pb and Li – Fig. 5a), Sr–Nd isotopiccompositions and protolith ages of c. 480–455 Ma(Siebel et al., 1997; Wiegand, 1997), forming animportant maximum within the spectrum of inheritedages in the granulites.A good candidate for complementary small-volume,HP–HT (>900 °C) melt that managed to separatefrom the calc-alkaline granulites are the rare hyperpotassicgranulites from Plesˇovice, Blansky´ les Massif(Vrańa, 1989; Janousˇek et al., 2007). These rocks,which are composed essentially of K-feldspar, garnet,zircon and apatite, show rather extreme geochemicalcompositions (e.g. strong enrichments in Cs, Ba, Rb,U, Th, K, P and Zr; Fig. 5b). They yielded U–Pb zirconages of 338 ± 1 Ma (Aftalion et al., 1989) and337.13 ± 0.37 Ma (Sla´ma et al., 2008), which is closeto the established best estimate of the HP metamorphicclimax in the granulite massifs (c. 340 Ma – see above).Moreover, rare coeval glimmerite veins in peridotitefragments enclosed by granulite bodies show highconcentrations of LILE, U and Th, accompanied bylow Rb ⁄ Cs and K ⁄ Rb ratios as well as low HFSEcontents (Fig. 5c); the Sr and Nd isotopic compositionsoverlap with the granulites (Becker et al., 1999).The glimmerites were interpreted as having crystallizedfrom an ultrapotassic, F-rich aqueous-carbonic fluid,bearing a direct witness for the HP–HT devolatilizationof granulite massifs.Viséan (ultra-) potassic magmatism in the MoldanubiandomainIn the Moldanubian domain of the Bohemian Massif,relatively large volumes of (ultra-) potassic plutonicrocks constitute several plutons and stocks spatiallyassociated to granitoids of the Central Bohemian PlutonicComplex, the Moldanubian Plutonic Complex,Ó 2010 Blackwell Publishing Ltd187
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Contentsviii4.7 Schulmann, Konopás
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Dedicated to my wife Markéta, daug
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Foreword 2First topic “Quantitati
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Foreword 4source and freely availab
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1. Quantitative analysis of deforma
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Chapter 2Mechanisms of lower crusta
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2. Mechanisms of lower crustal flow
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Chapter 3Quantitative analyses ofme
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3. Quantitative analyses of metamor
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Bibliography 42Culshaw, N., Beaumon
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Bibliography 44sources and trigger
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Bibliography 46Skrzypek, E., Štíp
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Table 1Statistical values of the qu
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ORIGIN OF FELSIC MIGMATITES 31Ó 20
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ORIGIN OF FELSIC MIGMATITES 33durin
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ORIGIN OF FELSIC MIGMATITES 45Fig.
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ORIGIN OF FELSIC MIGMATITES 49stron
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