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92 O. LEXA ET AL.Fig. 10. P–T evolution of six tracked samples for models with different radioactive heat production values for the felsic lowercrust (FLC). Samples 1, 2 and 3 were located in felsic lower crust, sample 4 within the mafic layer (MLC) and samples 5 and 6 arelocated in the middle crust. (a) No radioactive elements removed, (b) 900 °C and (c) 1000 °C threshold for radioactive elementremoval from the lower crust. Circles mark 10 Myr time step.the partial melting connected with the granulite faciesmetamorphism.In conclusion, the thermal structure which fits bestthe petrological and geochronological data obtained sofar from the Moldanubian granulite massifs is thatcalculated with an initial radioactive heat productionof 4 lW m )3 located in the felsic layer. The P–T–tevolutions of the AGB (mafic lower crust) and of theoverlying mid-crustal rocks are also in good agreementwith the model results, despite the fact that the simulatedevolution represents only the initial part of acomplex polyphase exhumation.DISCUSSIONIn this article, we discuss a particular structural patternin the Variscan orogenic root in which orogenic lowercrust composed of felsic granulites of OrdovicianÓ 2010 Blackwell Publishing Ltd194
HEAT SOURCES AND EXHUMATION MECHANISMS 93protolith age forming cores of domes that are separatedfrom mid-crustal Neoproterozoic and Palaeozoicmetasedimentary rocks in synclines by a late Ordovician–Silurianmetabasic layer. We argue that the originof these structures was related to diapiric materialexchange within the orogenic lower crust. The keyelement in deciphering the Viséan development is theoccurrence of FLC underneath dense mafic crust asdepicted by geology and geophysics. To understandthe formation of this peculiar lithological sandwich thepossible emplacement models for the FLC at the bottomof the root need to be discussed. Subsequently, weshall address the particular role of this felsic layer forthe origin of the felsic granulite–(ultra-) potassicmagma association. Finally, a model of gravity inversionis discussed together with thermal consequencesfor P–T–t paths of rocks in different positions withrespect to the diapiric structure.Model of relamination of felsic crust to early Palaeozoicmafic lower crustThe relamination of FLC has been proposed as analternative to the model of Chemenda et al. (2000) toexplain the structure of the Tibetan Plateau. TheTibetan Plateau is characterized by an exceptionallylarge gravity low indicating dominantly a felsic rootunderplated by Indian felsic crust, the density of whichcorresponds to felsic granulite at a pressure of 20 kbar(Hetényi et al., 2007). Indeed, Le Pichon et al. (1997)argued that the high topography of the Tibetan Plateauis due to presence of low density granulites atdepth. A similar gravity low is typical of the AltiplanoPlateau in central Andes, having been interpreted as aresult of underthrusting of the Brazilian crust underneaththe Andean root (Oncken et al., 2006). Thegravity anomaly associated with the Moldanubiandomain resembles remarkably the Tibetan and Altiplanoplateaux density structure, which in this case isreduced by subsequent isostatic reequilibration (Burget al., 1994). The common denominator of all thesegeophysical observations is the occurrence of felsiccrust at lower crustal depths. This is explained either ascontinental crust underthrusting thin lithosphericmantle (Chemenda et al., 2000) or directly by influx offelsic crust into the orogenic root at Moho depth liftingthe original lower crust and depressing the mantlelithosphere (Behr, 1978; Plesch & Oncken, 1999;Avouac, 2008).In the western Bohemian Massif, the influx of ductilelower crust at granulite ⁄ eclogite facies conditions wasproposed by OÕBrien (2000). In Fig. 11a,b, the influx ofSaxothuringian crust into the root domain is visualizedin the form of a 10-km wide channel splitting the earlyPalaeozoic mafic lower crust from lithospheric mantle,whereas the other part of the continental crust iscontinuously subducted, contaminating the localmantle and sampling the mantle lithosphere. The termrelamination (Hacker et al., 2007) is accepted as beingsuitable for the addition of low density crust underneaththe dense root, in contrast to the term delaminationto describe loss of heavy root material and itsreplacement by the asthenosphere.As discussed before, oceanic subduction had to haveceased by 354–346 Ma to be replaced by continentalunderthrusting. Incidentally, a Sm–Nd age of354 ± 6 Ma was determined by dating of calcium-richcores of garnet from the South Bohemian granulitesindicating the onset of eclogitization of continentalcrust at this time (Prince et al., 2000). If true, such ascenario allows a 5–15 Myr period to c. 340 Ma, thegranulite facies metamorphic climax, for the subductedcontinental crust to thermally incubate and elevate theorogenic geotherm (England & Thompson, 1984).Based on P–T estimates, crustal thickening had toproduce a 70-km thick crust at this time (Fig. 11c).Melting of this continental crust started at c. 345 Ma,as indicated by high-K calc-alkaline magmatism of theBlatna´ suite. The maximum melt production at boththe base of this crust and in the underlying mantlelithosphere occurred during, or soon after, the HPmetamorphic climax at c. 340 Ma, as indicated byintrusions of ultrapotassic syenites at mid- to highcrustallevels.The origin of the felsic granulite–ultrapotassic plutonic rockassociation and the heat sourceRoberts & Finger (1997) proposed that heating ofrelatively refractory felsic metaigneous rocks, the likelysource of the felsic granulites, to temperatures as highas 1000 °C would result in production of 5–15 vol.%of partial melt. This notion was confirmed by thermodynamicmodelling of Janousˇek et al. (2004b), whosuggested that the initial melting, limited by micaavailability, would not exceed 10 vol.% for any of theprospective granulite decompression paths and rapidincrease of the melt fraction would occur only at1100 °C. However, it is important to correctly assessthe melt production for the most typical felsic granulites.The most likely protolith for the felsic granulitesis considered to be granite or orthogneiss of theÔFichtelgebirge chemistryÕ that must have lost somemelt during metamorphic evolution in order to preservethe high-pressure assemblage. Therefore, toestimate the degree of melt loss, pseudosections werecalculated using THERMOCALC for a granulite with theoldest known preserved fabric from the Blansky´ lesMassif (sample H296-S1 from Franeˇk et al., 2011b; fordetails, see Appendix).The sample modelled contains relict porphyroclastsof perthitic feldspar with inclusions of kyanite, garnetwith 24 mol.% of grossular, rutile and biotite, whichconstrains the peak P–T conditions to 16–18 kbarand 860 °C (Fig. 12a; Franeˇk et al., 2011b). It isnecessary to add 7 mol.% of melt into the rockcomposition to obtain assemblages containing biotiteand muscovite without garnet or alumosilicate atÓ 2010 Blackwell Publishing Ltd195
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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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Journal of Structural Geology 23 20
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DTD 5ARTICLE IN PRESSJournal of Str
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DTD 5ARTICLE IN PRESSL. Baratoux et
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Table 1Statistical values of the qu
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Table 2Summary of parameters derive
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J. metamorphic Geol., 2008, 26, 273
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EXHUMATION IN LARGE HOT OROGEN 275m
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Probabi l irequencyProbabi l ireque
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EXHUMATION IN LARGE HOT OROGEN 279y
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EXHUMATION IN LARGE HOT OROGEN 281N
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TEXTURES OF NATURALLY DEFORMED META
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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 35(a)Ty
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ORIGIN OF FELSIC MIGMATITES 39Grain
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ORIGIN OF FELSIC MIGMATITES 43(a)(b
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ORIGIN OF FELSIC MIGMATITES 45Fig.
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ORIGIN OF FELSIC MIGMATITES 49stron
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ORIGIN OF FELSIC MIGMATITES 51Cmı
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104 J. FRANĚK ET AL.in terms of th
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106 J. FRANĚK ET AL.Fig. 2. Struct
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108 J. FRANĚK ET AL.(a)perthite po
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116 J. FRANĚK ET AL.(a)(b)Fig. 10.
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130 J. FRANĚK ET AL.Southern Bohem
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