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B10406SCHULMANN ET AL.: RHEOLOGY OF PARTIALLY MOLTEN GNEISSESB10406Figure 1. Geological map of the central part of Bohemian Massif modified after Beneš [1964] andSynek and Oliverová [1993]. Black stars and capitalized letters refer to studied samples (B, D, H, M, R, S,T, and V). (a) Lower crustal rocks (granulites and eclogites), (b) the midcrustal monotonousmetasedimentary unit, (c) the midcrustal orthogneiss unit, (d) the kyanite micaschist unit, (e) theintrusives, (f) Cretaceous and Quaternary rocks, (g) the undifferenciated Lower Palaeozoic rocks, and(h) the Neo-Proterozoic metasediments.Schulmann et al. [1996] have shown that at amphibolitefacies the feldspars show evolution from dislocation creepto grain boundary sliding with increasing strain intensity inconjunction with variations in the activity of the quartz slipsystems. Therefore, the viscosity contrast between feldsparsand quartz was not constant but varied with increasingdegree of strain as the rheological role of individualminerals evolved as shown by a range of experimentaland natural studies [Handy, 1994b; Ji et al., 2000; Rybackiand Dresen, 2004; Stünitz and Fitz Gerald, 1993]. However,the aforementioned studies all neglect a possible roleof the melt on the deformation of the polyphase rocks.[4] The aim of this paper is to show, through detailedmicrostructural study and thermodynamical modeling, thecontribution of interstitial melt to the rheology of progressivelydeformed granites under midcrustal conditions. Weuse natural examples of a sequence of granite mylonites todocument the melt enhanced rheological inversion of disproportionatelystronger quartz compared to the weakfeldspars in midcrustal rocks. This work also shows thatwith increasing melt fraction the deformation mechanismsof feldspars varies from the grain boundary sliding accommodateddislocation creep to granular flow while the quartzis only deformed in the dislocation creep field.2. Geological Setting[5] The study area located in the central part of theBohemian Massif in the Czech Republic is known for theextreme deformation of porphyritic granites in a crustalscaleshear zone [Synek and Oliverová, 1993]. The porphyriticgranite mylonites studied are of Cambro-Ordovicianprotolith age and come from an orthogneiss-bearing, midcrustalunit that overlies kyanite micaschists in the west(Figure 1). In the east a similar rock assemblage occurs inan equivalent structural unit, although these orthogneissbodies exist within kyanite-sillimanite bearing micaschists.According to Schulmann et al. [2005] both units record aCarboniferous tectono-metamorphic history between 340and 325 Ma. The midcrustal unit is overlain by kyanitebearing migmatites and granulites of the orogenic lowercrustal unit that contain eclogite lenses with estimatedminimum pressures of 18–19 kbar and temperatures of800–900°C [Medaris et al., 1998]. The P-T conditions ofthe kyanite bearing micaschists in the footwall of themylonitic orthogneiss sheet have experienced temperaturesof 620–710°C at pressures of 6.5–9.5 kbar [Kachlík, 1999]similarly to 8–9 kbar and 610–660°C estimated in themicaschists in the eastern part of the studied area [Pitra andGuiraud, 1996].[6] The geological structure of the studied rocks wasdescribed by Synek and Oliverová [1993] who interpretedthe middle crustal orthogneiss-bearing unit and the overlyingorogenic lower crustal unit as a crustal nappe stackresulting from a Carboniferous deformation. On the basis ofthe structural position of the midcrustal orthogneiss and theregional metamorphic field gradient, the PT conditions ofmetamorphism and deformation of the studied rocks areestimated to be between 9 and 18 kbar and 650–850°C.More precise P-T estimations were not established becauseof the lack of pressure and temperature sensitive mineralassemblages that are necessary for standard thermobarometricmethods.3. Shape Analysis of Quartz and Feldspars[7] Undeformed porphyritic granitoids may serve as anexcellent example of multiphase mixtures of originallyspherical (ellipsoidal) clasts with constant phase fractions.When these rocks are subjected to deformation, mineralgrains reach different strain intensities, which can be easilyquantified using standard finite strain techniques [Ramsayand Huber, 1983]. Measurement of the shapes of naturallydeformed minerals in originally coarse-grained and porphyriticgranitoids may thus help to track the viscous behaviorof individual phases for different bulk strain intensities2of20296
B10406SCHULMANN ET AL.: RHEOLOGY OF PARTIALLY MOLTEN GNEISSESB10406Figure 2. Macroscopic samples of deformed metagranite divided in three types according to thedeformation intensity and the macroscopic appearance. X, Y, and Z refer to the axes of the finite strainellipsoid. (a) Sample S1 is a weakly deformed metagranite with large centimeter-sized grains of quartzand feldspar representing Type I rock. (b, c) Sample M1 is an augen orthogneiss corresponding to Type IIrocks and intermediate strain intensity (equivalent to samples T1, T2, M2, and R4), (d, e) Sample V1 is abanded mylonite corresponding to Type III rock and the highest intensity of deformation (equivalent toother highly strained samples R5, R3, V2, and H1).[Treagus, 2002]. This allows constraining the degree ofstrain partitioning in rocks and the viscosity ratios betweenthe individual mineral phases.[8] The shape analysis of the feldspars and the quartzpolycrystalline aggregates was carried out on 17 sectionscut both perpendicular to the foliation and parallel to thestretching lineation and perpendicular to both the foliationand the lineation, i.e., parallel to the XZ and YZ sections ofthe finite strain ellipsoid (Figure 2). The K-feldspar cannotbe distinguished from plagioclase in highly deformed macroscopicsamples and therefore both minerals were groupedtogether for finite strain measurements. All studied samplesare composed on average of 60–70% feldspars, 35–25%quartz and up to 10% of biotite and muscovite. The almostconstant mineral composition for highly variable strainintensities and constant bulk rock chemistry shows the lackof chemical variations with strain (Table 1).[9] In our study we classified three major types ofdeformed orthogneiss according to the deformation intensitiesat the macroscopic scale: Type I is represented byweakly deformed metagranite (Figure 2a); Type II correspondsto augen orthogneiss with quartz porphyroclast(Figures 2b and 2c); Type III is a banded mylonite orthogneiss(Figures 2d and 2e).[10] Mineral shape data are plotted into a Flinn diagram[Flinn, 1965] (Figure 3). The analyses of feldspar polycrystallineaggregates and quartz of the individual samples areconnected by tie lines with the vertical ellipse representingthe bulk strain value of the whole rock. An importantfeature of all the studied samples is that feldspars showhigher strain intensities than quartz for any bulk strain(Figures 3a and 3b). In several samples of the so-calledDoubravčany pencil gneiss, the strain intensities cannot beidentified because the stretching of feldspar and quartzlayers exceeds the length of the samples (Figure 2d). Thestrain symmetry, represented by the K values of Flinn[1965], varies from prolate to oblate shapes (K = 2.7 to0.3). For quartz with prolate shapes, the correspondingfeldspar strain symmetry is close to plane strain, while foroblate quartz, the feldspars show the same shape or slightlymore oblate shapes (Figure 3b). Strain intensities areexpressed using Ramsay’s D value, which is an alternativeexpression of viscosity ratio values of Gay [1968a, 1968b] alsoused by Schulmann et al. [1996]. The inspection of the diagramin Figure 3b shows that the highest strain ratios are achievedbetween D fel /D qtz for Type III orthogneiss and some Type IIorthogneiss samples marked by high bulk strain intensities. Therelatively small ratio between D fel /D qtz suggests similar yield-3of20297
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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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Journal of Structural Geology 23 20
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J. KonopaÂsek et al. / Journal of
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JOURNAL OF GEOPHYSICAL RESEARCH, VO
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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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EXHUMATION IN LARGE HOT OROGEN 283w
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EXHUMATION IN LARGE HOT OROGEN 285a
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EXHUMATION IN LARGE HOT OROGEN 287w
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EXHUMATION IN LARGE HOT OROGEN 289T
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EXHUMATION IN LARGE HOT OROGEN 291O
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EXHUMATION IN LARGE HOT OROGEN 293r
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EXHUMATION IN LARGE HOT OROGEN 295B
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EXHUMATION IN LARGE HOT OROGEN 297R
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Author's personal copyK. Schulmann
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J. metamorphic Geol., 2011, 29, 79-
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HEAT SOURCES AND EXHUMATION MECHANI
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