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B10210ZÁVADA ET AL.: EXTREME DUCTILITY OF FELDSPAR AGGREGATESB10210Figure 1. Simplified geological map of the studied area around the Eger river valley, NE of Stráž nadOhří town [Hradecký, 2002] (scale 1:25000) and the position of the studied area in the Bohemian Massif[Franke, 2000] and European Variscides. AM, Armorican massif; MC, Massif Central; BM, Bohemianmassif.1987; Daines and Kohlstedt, 1997; Gleason et al., 1999;Rosenberg and Handy, 2000]. This phenomenon can beexplained either by dilation of granular aggregates due tomelt overpressure [Renner et al., 2000], when hydrostaticmelt pressure overcomes the confining pressure, or bypassive melt migration into the incipient voids during grainboundary sliding (GBS) [Čadek, 1988].[4] However, studies of natural microstructures showingevidence of large finite strains and interstitial quenchedmelt, which would be indicative for pronounced meltenhancedweakening and grain-scale melt migration arepractically absent. This is due to two reasons: (1) Naturalmelt-bearing microstructures equilibrated at high-grade conditionsare likely to be overprinted by subsequent static ordynamic recrystallization [Rosenberg and Riller, 2000;Rosenberg, 2001]. (2) High finite strains, slow creep ratesand high homologous temperatures necessary to introducehigh-temperature melt-enhanced deformation mechanisms(e.g., diffusion creep or dislocation climb accommodatedGBS [Boullier and Gueguen, 1975]) cannot be attainedusing conventional experimental facilities. There are threeapproaches that may help to understand the rheologicalbehavior of minerals deformed at high-grade conditions inthe presence of melt and their influence on rheology ofpolyphase rocks: (1) Investigation of high-grade rocks,where microstructural record was preserved due to rapidcooling rates, (2) comparison of high-grade rock microstructureswith results of various experiments on creepproperties of metals, alloys and steels, and (3) applicationof more efficient rock deformation laboratory facilities (e.g.,Paterson rig apparatus).[5] In this work, we characterize deformation microstructuresin a strongly deformed high-grade banded orthogneiss,where melting was concurrent with deformation. Meltbearing microstructures have been preserved due to rapidcooling that effectively increased quenching of melt instudied rocks [Zulauf et al., 2002]. Deformation mechanismsof constituent mineral phases are investigated usingquantitative microstructural analysis. The mechanism responsiblefor preferred distribution of interstitial melt insample with maximum strain intensity is critically evaluatedusing modeling of melt volume produced by a metamorphicreaction and molar volume change of the system. The effectof mobile melt phase on deformation mechanisms and bulkrock rheology is discussed. Finally, we consider the apparentstrength relationship between quartz and feldspars.2. Geological Setting[6] The rocks studied belong to a lower crustal hightemperaturemetamorphic unit that was rapidly exhumedalong the major Variscan tectonic boundary in the BohemianMassif called the North Bohemian Shear Zone (NBSZ ofZulauf et al. [2002]). The trend and spatial limits of theNBSZ are not defined in the field, because it is mostlycovered by Neogene volcanics and sedimentary sequences(Figure 1). The sampled unit forms the uppermost thrustsheet within a crustal nappe stack that displays typicalmetamorphic inversion and is called the ‘‘Upper CrystallineNappe’’ (UCN) [Konopásek and Schulmann, 2005]. TheUCN in the studied area consists of banded orthogneisses,high-pressure granulites and migmatites (Figure 1). PTcalculations of metamorphic conditions for the partiallymolten orthogneisses in this study reveal pressures of900 MPa and temperatures 700 ± 20°C. In contrast,the granulites exhibit peak metamorphic conditions of1600 MPa and 800°C [Kotková etal., 1996] indicating thatthe granulites and orthogneisses have been juxtaposed atmiddle crustal levels during differential exhumation. Furthermore,very fast cooling rates of 50°C +25°C/ 17°CMa 1 were deduced [Zulauf et al., 2002] from the U-Pbdating of formation of zircons and monazites (342 ± 1 Ma)and 40 Ar- 39 Ar cooling ages of muscovites and biotites(341 ± 4 Ma) from the orthogneisses.2of15280
B10210ZÁVADA ET AL.: EXTREME DUCTILITY OF FELDSPAR AGGREGATESB10210Figure 2. (a) Traced and digitized mineral phase boundaries in XZ and YZ rock sections of themylonite type 3 sample (B109) and XZ section of a metagranite type 1 sample (B108) for comparison(mineral abbreviations after Kretz [1983], Mat means matrix and represents indiscernible mineral phases).(b) Flinn diagram of the phase aggregate’s shapes. R xy and R yz designate X/Y and Y/Z ratios of the strainellipsoid axes, respectively [Flinn, 1962].[7] Intensive north-south Variscan compression resultedin the development of vertical foliations in both the UCNand adjacent middle crustal rocks (LCN, Lower CrystallineNappe) [Konopásek et al., 2001] and also vertical stretchinglineations in the UCN. Zulauf et al. [2002] proposed thatthis vertical fabric is related to the rapid vertical motion oflower crustal rocks along the NBSZ and to intense deformationof gneisses and granulites. The UCN orthogneissshows significantly higher finite strains of feldspars comparedto relatively weakly deformed quartz suggestingextreme ductility of feldspars. In this work we focus onthe evaluation of the competency relationship betweenfeldspars and quartz, which can be of great importance forthe rheology of felsic lower crust.3. Finite Strain Analysis[8] In order to evaluate the influence of individual mineralson the rheology of UCN orthogneiss, the specimenshave been sampled according to the degree of finite strain.Plates cut along the XZ and YZ planes of the rock fabricellipsoid (XZ, parallel to lineation and perpendicular tofoliation; YZ, perpendicular to lineation and foliation) werecolored [Gabriel and Cox, 1929] in order to distinguish K-feldspar from plagioclase aggregates. The boundaries ofmineral phases in both XZ and YZ sections were thenmanually traced and digitized from photographs in ArcViewGIS environment (Figure 2a). The phase aggregate objectswere then statistically evaluated using the PolyLX Matlabtoolbox [Lexa et al., 2005] (http://petrol.natur.cuni.cz/ondro/polylx:home). Because both feldspars in bandedorthogneiss samples formed apparent infinite prolate-shapedaggregates, their strain was calculated using a method basedon normalization of their volume by the mean volume of thephase aggregates in sample with low finite strain intensity(where the phase objects are closed, sample B108). It isassumed that initially nonbanded feldspar aggregates coalescedtogether during the deformation. Quartz aggregatesin the XZ section appeared to retain similar cross-sectionalarea as in the sample with lowest strain intensity. Wetherefore suggest that quartz aggregates did not undergoextensive pinching by other phases during deformation,which would decrease their cross-sectional area and finitestrain estimate. Bulk deformation was calculated from allphases together as object area weighted geometrical mean.[9] Results of shape analysis are demonstrated in theFlinn diagram [Flinn, 1962] (Figure 2b). Sample B108shows the lowest degree of deformation and represents a‘‘metagranite’’ type 1. The ‘‘banded orthogneisses’’ type 2(samples B13, B109-2) show rod-shaped aggregates of bothfeldspars and plane strain to slightly prolate symmetry ofquartz. This type shows bulk intensities around D = 5 (D =(R 2 xy +R 2 yz ) 1/2 [Ramsay and Huber, 1983], D of K-feldsparclose to 10 and of plagioclase around 6. Maximum bulkintensity D = 16 was attained in sample B109 with K-feldspar and plagioclase reaching D = 26 and D = 50,respectively. This sample represents the final member ofmicrostructural evolution and will be designated the‘‘mylonite’’ type 3 sample. All stages of deformation showhigher finite strains in K-feldspar and plagioclase aggregatescompared to quartz.4. Deformation Microstructures[10] Microstructures and mineral textures of all rock‘‘types’’ were studied in detail in order to evaluate theevolution of deformation mechanisms with respect to increasingbulk rock strain. Quantitative microstructural analysiswas carried out using manually digitized grainboundaries [see Lexa et al., 2005] from several sequentialmicrophotographs of colored XZ thin sections. Texturalanalysis comprises quantification of the grain size and grainsize spread, grain shape (axial ratio) and shape preferredorientation (SPO), expressed as the ratio of eigenvalues of3of15281
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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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JOURNAL OF GEOPHYSICAL RESEARCH, VO
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SCHULMANN ET AL.: STRAIN DISTRIBUTI
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5 - 2 LEXA ET AL.: COLLISION IN WES
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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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J. metamorphic Geol., 2011, 29, 53-
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EXTRUSIONOFLOWERCRUSTINVARISCANOROG
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ORIGIN OF FELSIC MIGMATITES 45Fig.
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ORIGIN OF FELSIC MIGMATITES 47produ
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ORIGIN OF FELSIC MIGMATITES 49stron
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ORIGIN OF FELSIC MIGMATITES 51Cmı
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ORIGIN OF FELSIC MIGMATITES 53easte
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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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110 J. FRANĚK ET AL.(a)(b)(c) (d)
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112 J. FRANĚK ET AL.Table 1. Repre
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116 J. FRANĚK ET AL.(a)(b)Fig. 10.
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126 J. FRANĚK ET AL.development of
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130 J. FRANĚK ET AL.Southern Bohem
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