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86 O. LEXA ET AL.(a)(b)(c)and high-grade metamorphic rocks of the Gfo¨ hlAssemblage, most notably felsic HP granulites. Thisprominent group represents the late-syntectonic durbachiteseries (e.g. Cˇertovo brˇemeno, Trˇebı´cˇ and Knı´žecı´stolec intrusions – Zˇa´k et al., 2005; Verner et al., 2008)(Fig. 4) of quartz melasyenites to melagranites withhydrous ferromagnesian minerals, Mg-rich biotite andactinolitic amphibole. These are mainly coarsely porphyriticrocks that contain abundant K-feldspar phenocrystsand ultrapotassic mafic microgranularenclaves. The spatially associated, less deformed, oreven post-tectonic, biotite–two-pyroxene melasyenitesto melagranites (Ta´bor and Jihlava intrusions) arecharacterized by a ÔdrierÕ ferromagnesian mineralassemblage of orthopyroxene, clinopyroxene andMg-biotite and lack the porphyritic texture (Ža´k et al.,2005).The petrogenesis of the Moldanubian (ultra-)potassic igneous rocks has been a matter of debate aseven the basic, Mg and K-rich members or primitivelamprophyric dykes have a mixed geochemical character.While their high contents of Cr and Ni with highmg# point to derivation from an olivine-rich source(mantle peridotite), the elevated concentrations of U,Th, light rare earth element (LREE) and LILE,depletion in Ti, Nb and Ta (Fig. 5d) and highK 2 O ⁄ Na 2 O and Rb ⁄ Sr ratios apparently contradict amantle origin (Holub, 1997; Janousˇek & Holub, 2007).These features led several authors to invoke partialmelting of anomalous (LILE- and LREE-enriched)lithospheric mantle domains (e.g. Janousˇek et al.,1995; Holub, 1997; Wenzel et al., 1997, 2000; Janousˇek& Holub, 2007), followed by mixing with lower crustalleucogranitic melts (Holub, 1997; Gerdes et al., 2000).In any case, the crustal-like Sr–Nd isotopic signaturescannot be reconciled solely by crustal assimilation⁄ contamination during the ascent of any primitive,mantle-derived magmas and require contamination bythe subducted continental crust directly in the source(d)Fig. 5. Multi-element variation diagrams. (a) Box and percentileplots for felsic (SiO 2 >70 wt%) Moldanubian granulites,normalized to an average of felsic metaigneous rocks from theFichtelgebirge (Siebel et al., 1997; Wiegand, 1997). See Janousˇeket al. (2004b and references therein) for the data set. The distributionof each of the normalized trace-element contents isplotted as irregular polygons, the width of which at any givenheight is proportional to the empirical cumulative distribution.As in box plots, the median, 25th and 75th percentiles aremarked with horizontal lines across the box. Compositions ofthe hyperpotassic Plesˇovice granulites (data from Janousˇeket al., 2007) (b) and Lower Austrian glimmerite veins (Beckeret al., 1999) (c) normalized by an average of felsic (SiO 2>70 wt%) HP Moldanubian granulites, except Lisˇov (takenfrom Janousˇek et al., 2004b). (d) Multi-element variationdiagram for selected ultrapotassic rocks (durbachite series andsyenitoids) from the Moldanubian Zone of the BohemianMassif (Janousˇek & Holub, 2007) normalized by N-MORB andthen adjusted to Yb N = 1 to minimize the effects of fractionalcrystallization (Pearce & Stern, 2006).Ó 2010 Blackwell Publishing Ltd188
HEAT SOURCES AND EXHUMATION MECHANISMS 87(Janousˇek & Holub, 2007). As noted by the sameauthors, multi-element plots for the Moldanubian felsicgranulites (Fig. 5a) and ultrapotassic rocks(Fig. 5d) are largely mutually complementary. Themost striking are the cases of Cs, Rb, Th, U, Pb and Li,which are impoverished in the felsic granulites butstrongly enriched in the ultrapotassic magmatic rocks.NUMERICAL MODELLING CONSTRAINTS ONHEAT SOURCES AND EMPLACEMENTMECHANISMS OF THE OROGENIC LOWERCRUSTTo constrain the heat source driving the tectonic processesof vertical extrusion three contrasting, but notmutually exclusive, hypotheses will be tested. Heatcould be generated by: (i) in situ decay of radioactiveelements contained in the FLC (U, Th and K); (ii)radioactive elements present in the metasomatized orcrustally contaminated mantle; and (iii) a large-scalethermal anomaly generated in the mantle as a result ofslab break off or mantle delamination.Fig. 6. Model geometry, initial lithology distribution, boundaryconditions and location of tracked samples used for numericalsimulations.The annual thermal productions were calculatedfollowing the method of Kramers et al. (2001), usingdecay constants and specific heat production datasummarized by van Schmus (1995; table 8). The pastannual heat production (lW kg )1 ) can be obtainedfrom the elemental concentrations of K, U and Thusing the equation:HðlW kg 1 623:45 10 2:638 10Þ¼Ke 0:554t þ The0:0495t324:03777 10 9:396852 10þ Ue 0:985t þe 0:1551t ;ð1Þwhere t represents age in Ga and K, U, Th are concentrationsin ppm.Model setupThe numerical model studies outlined below describe thetransient thermal evolution of a thickened orogenicdomain (Fig. 6) characterized by the presence of tectonicallyaccreted felsic rocks, including granulites,within orogenic lower crust. This FLC directly underliesa mafic layer which was added to Neoproterozoic crustduring early Palaeozoic crustal stretching and magmaticunder-plating. The presence of low density FLC below adense mafic layer introduces significant gravitationalinstability within the lower crust (Gerya et al., 2001)which could trigger crustal diapirism (Ramberg, 1981;Perchuk, 1989) and perturbate the thermal field. We usethermal and dynamic numerical models to examine therole of high radioactive heat production located in theFLC as a main candidate triggering the gravitationalinstability as a result of pronounced progressive generationof heat and subsequent change in density becauseof thermal expansion.The model is set up to allow the definition of differentmaterial domains with different thermal and mechanicalproperties (Table 2) on high-resolution Lagrangianmarkers initially arranged in a rectangular grid (Gerya& Yuen, 2003). Properties are mapped to a Eulerianstaggered grid where governing equations are solved fortemperature change (DT) and velocity. In each time,step markers are advected according to the updatedvelocity field and all temperature-dependent variablesTable 1. Calculated radioactive heat production values for Fichtelgebirge metaigneous rocks, felsic Moldanubian granulites andMoldanubian peridotite (present and at 340 Ma).Rock type Age (Ma) Density (kg m )3 ) Concentrations (ppm) A (lW m )3 )K Th U40 K232 Th235 U238 U Present PastFichtelgebirge 340 2700 38642.550 13.00 9.00 7.3421 13.00 0.064 8.935 3.670 3.920Moldanubian granulites 340 2750 38601.045 2.10 1.00 7.3342 2.10 0.007 0.993 0.788 0.885Hornı´ Bory peridotite 340 3200 940.537 0.11 0.57 0.1787 0.11 0.004 0.568 0.176 0.214Data sources for averaged whole-rock compositions: Fichtebeirge metaigneous rocks: Siebel et al. (1997), Wiegand (1997).Felsic Moldanubian granulites (SiO 2 > 70 wt%): Janousˇek et al. (2004b and references therein).Hornı´ Bory peridotite: Ackerman et al. (2009).Ó 2010 Blackwell Publishing Ltd189
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“It strikes me that all our knowl
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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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Table 2Summary of parameters derive
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EXHUMATION IN LARGE HOT OROGEN 275m
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ORIGIN OF FELSIC MIGMATITES 31Ó 20
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ORIGIN OF FELSIC MIGMATITES 45Fig.
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104 J. FRANĚK ET AL.in terms of th
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