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B10210ZÁVADA ET AL.: EXTREME DUCTILITY OF FELDSPAR AGGREGATESB10210Figure 17. PT plot with contours of molar volume in Jbar 1 (equivalent to 10 1 cm 1 mol1 ) calculated for thesame system as shown in Figure 13. Mineral abbreviationsare after Kretz [1983].which soaks up the melt produced within plagioclase bands(the melt is passively redistributed, Figure 15). Consequently,plagioclase could not deform any further due to the loss ofintergranular melt that worked as a creep enhancing medium.[35] It has been shown that creation of melt-bearingporosity can be induced by fast melt production due tobreakdown of some hydrous phase in the rock (e.g., viadiscontinuous reaction) or due to high heating rate causingsubstantial overstepping of continuous melting reactionsand thus fast melt production as a result of the system’seffort to reach equilibrium state [e.g., Connolly et al., 1997;Rushmer, 2001]. In contrast, change in molar volume of therock during slow progress of continuous reactions will beprobably small.[36] In order to test this hypothesis, the P-T phasediagram section in Figure 13a was contoured for molarvolume of the system to see, whether the observed meltingreaction at estimated PT conditions may lead to rapidvolume increase. The result (Figure 17) shows that the onlyrapid increase in molar volume of the rock in given PTrange results from dehydration melting of muscovite at PTconditions higher than those reached by studied orthogneisssamples. In addition, the role of melt volume change andvelocity of the reaction is speculative for aggregates with‘‘incoherent’’ grain boundaries like both feldspars undergoingGBS in studied orthogneiss than rather ‘‘intact’’ solidphases around reaction sites in the experiments of Connollyet al. [1997].[37] The calculated volume of melt (2–4 vol %) suggeststhat the melt formed isolated melt films, pools or pockets. Inthe case of ‘‘overpressure’’ model, sufficient amount of meltneeds to be produced to induce considerable melt overpressureand melt redistribution. This critical melt volumecorresponds to creation of interconnected network of meltbetween the framework of grains, so that the melt can startincreasing its hydrostatic pressure toward the level ofmaximum compressive stress [Renner et al., 2000]. Thismelt volume is given by the melt connectivity transition(MCT) of F =0.07[Rosenberg and Handy, 2005] or similar‘‘liquid percolation threshold’’ (LPT) of F =0.08[Vigneresseet al., 1996], although this critical level is likely to depend onseveral variables (e.g., dihedral angles). It is possible that themelt migrates from locally overpressured isolated pockets athigh angle to the maximum compressive stress (s 1 )tointergranular boundaries subparallel with this direction(s 1 ). However, it is unlikely that the cohesion of grainboundaries and tensile strength of (001) intragranular cleavageof K-feldspar are significantly smaller than intergranularcohesion of plagioclase, which would be necessary conditionfor preferential hydraulic failure of K-feldspar, although themelt was produced in plagioclase bands.[38] In contrast, during cavitation, underpressure arisingfrom opening of incipient intergranular voids in K-feldsparis theoretically susceptible to ‘‘extract’’ melt from isolatedpools and films within plagioclase aggregates, in spite of itslittle amount. If distortion of plagioclase grains is easierthan for K-feldspar grains to accommodate GBS, then plagioclaseaggregates should also ‘‘extrude’’ excessive intergranularmelt into possible sink sites. This is in agreementwith analogue modeling results of Walte et al. [2005], whereaggregates composed of ‘‘weaker’’ grains (more susceptibleto plastic deformation) show higher ‘‘GBS locking transition’’than relatively stronger grains. Melt migration alongmutual boundaries of quartz was probably negligible incomparison to both feldspars. This is again controlled byrelative ‘‘weakness’’ of quartz grains with respect to ‘‘stronger’’grains of both feldspars and ‘‘welded’’ boundaries ofquartz. The viscosity ratio between melt and host aggregategrains thus increases from quartz to plagioclase and ishighest for K-feldspar [Walte et al., 2005].[39] Considering little amount of melt produced by themelting reaction and absence of intergranular melt pocketsperpendicular to the stretching lineation in plagioclase, weconsider the ‘‘cavitation’’ model to be the most likelymicrophysical mechanism explaining the dilation, microfracturationand preferential melt distribution in K-feldsparaggregates of type 3 microstructure.8.2. Impact of Impurities and MechanicalAnisotropy of Crystals on GBS[40] Experiments with aluminum in GBS regime withrelatively high-purity metals (e.g., aluminum of purity 4N(where N expresses the degree of purity of a material; 2N =99% purity, 4N = 99.99% purity, etc.), 650°K, strain rate10 9 s 1 ) or at high strain rates (e.g., 4N copper at 10 2 s 1 ,873°K [Čadek, 1988]) are marked by high relative elongationse, distinct contraction at the area of failure (necking)associated with intracrystalline fracturing. On theother hand, experiments with relatively low-purity metals(2N aluminum at 10 9 s 1 , 650°K) or at low strain rates(4N copper at 10 9 s 1 , 873 K) show small relativeelongations to failure, regular contraction along the deformedspecimen (homogeneous strain) and intergranularfracturing (Figure 18) [Sklenička et al., 1977; Čadek, 1988;Mohamed, 2002]. Therefore the higher purity material canwithstand higher relative elongations to creep failure (and atfaster strain rates) than its lower purity equivalent. Incontrast, stress accumulations around impurities and grainboundary ledges enhance diffusion-driven cavitation, cavitieseasily form at dislocation pileups around impuritieswithin the grains and creep failure will occur after smallerrelative elongations (and slower strain rates).12 of 15290
B10210ZÁVADA ET AL.: EXTREME DUCTILITY OF FELDSPAR AGGREGATESB10210Figure 18. Final creep failure microstructure and specimendistortion from tensile experiments with metals atconstant strain rate. [Sklenička et al., 1977; Čadek, 1988].Material of purity 2N contains more impurities than 4Nmaterial.[41] These results can be compared with different intensitiesof deformation and microstructures of both feldspars inthe studied orthogneiss (type 3 microstructure) sample.Feldspars can be also regarded as solid solutions like alloysor impure metals. We consider the three component K-feldspar with clearly defined solvus as a ‘‘less pure’’ equivalentto the two component oligoclase. This interpretation issupported by the presence of cryptoperthite exsolutionswithin some K-feldspar grains (lower left part ofFigure 10a), along which the tips of cavitation drivenfractures could nucleate and propagate. The perthite intergrowthsare commonly oriented along (001) plane [Spry,1969], which is fully compatible with orientation of most ofthe intragranular fractures reported in this work. In contrast,plagioclase shows higher finite strains in type 3 orthogneissthan K-feldspar and no intragranular fractures, which isconsistent with higher purity aluminum experiments orhigher purity metals in general [Čadek, 1988; Mohamed,2002]. Fractured grains in K-feldspar exhibit strong crystallographicpreferred orientation marked by (001) planesperpendicular to the axis of stretching direction (X direction)and their (010) plane coinciding with the Y direction of therock fabric’s coordinate system. Because propagation offractures (group 1 fractures, Figure 10a) driven by coalescenceof cavities is initiated from boundaries oriented at highangles to the maximum principal compressive stress direction,we can suggest that cavity coalescence was mosteffective along the K-feldspar (001) plane in [100] crystallographicdirection. Selection of suitably oriented K-feldspargrains for intragranular cavitation reflects the strong mechanicalcrystallographic anisotropy of K-feldspar [Smith,1974]. Group 2 fractures (Figure 10a) and curved or splittingfractures (Figure 10c) could develop due to cavitation andpropagation of fractures from triple-point junction boundaries.The propagation of fractures in K-feldspars in the caseof ‘‘melt overpressure’’ model would probably follow similarpathways as cavitation-driven fractures.8.3. Effect of Melt Phase on GBS[42] Several tensile experiments on creep properties ofmetals have shown dominantly GBS accommodated superplasticbehavior with peak of finite elongations at temperaturescoinciding with occurrence of melt phase along grain boundaries[Mabuchi et al., 1997]. This weakening was associatedwith effective contribution of the melt phase to annihilation ofgliding dislocations, therefore prevention of dislocation pileups,increased velocity of diffusive mass transfer and accommodationof stress concentrations that was also reflected bylower cavity densities in comparison with ‘‘melt-free’’ experiments[Koike et al., 1998]. Melt-enhanced weakening isfavored by low wettability of melt (high dihedral angles) andits relatively small amount. In contrast, high wettability (lowdihedral angle) melts thickly coating grain boundaries producedpremature necking and creep failure due to the loss ofgrain boundary cohesion [Mabuchi et al., 1997; Koike et al.,1998]. The same transition from superplastic flow to prematurefailure coinciding with almost complete wetting of grainboundaries (at length proportion of 70%) was demonstratedalso during compressive experiments [Pharr et al., 1989;Baudelet et al., 1992].[43] In summary, the material science experimentalresults show distinct rheological transition between melt(liquid)–enhanced GBS and collapse marked by increasedcavitation velocities at a melt volume threshold coincidingwith complete wetting of grain boundaries. This threshold issimilar to the concept of melt embrittlement at the onset ofmelt connectivity transition (MCT) proposed by Rosenbergand Handy [2005] from experimental data on quartzofeldspathicrocks. Collapse (rather than weakening) at theMCT was associated with localized intergranular and intragranularmicrocracking, frictional sliding and limited bodyrotation leading to the development of cataclastic zonesand produces heterogeneous and restricted deformation[Rutter and Neumann, 1995; Rosenberg and Handy,2005]. Rosenberg and Handy [2005] have regarded MCTas a rheological threshold even more important than thepreviously emphasized rheological critical melt percentage(RCMP) at F = 0.1–0.3 [Arzi, 1978; van der Molen andPaterson, 1979] at experimental conditions. However, theweakening mechanisms associated with MCT are incompatiblewith our observations.[44] Our strain measurements and material science experimentalresults show that weakening associated with meltenhancedGBS at low melt volumes (below the MCT) issubstantial and the aggregate is deformed relatively homogeneously.We therefore suggest that the degree of weakeninginduced by melt-enhanced GBS can be higher andmore important on large scales at natural conditions than theweakening mechanisms operative at experimental strainrates at the onset of melt connectivity transition as proposedby Rosenberg and Handy [2005].[45] The critical melt volume for transition from meltenhancedGBS to premature creep failure due to increasedmelt volume is likely to depend on several variables, such asdifferential stress [e.g., Bordeaux et al., 1994; Rosenberg,2001], strain rate, grain size [Bordeaux et al., 1994; Renner etal., 2000], melt volume and melt framework viscosity ratio[McKenzie, 1984; Pharr et al., 1989; Walte et al., 2005].[46] The presence of water without melt in the systemcould effectively enhance diffusion and dislocation creepaccommodated GBS of feldspars and produce similarmicrostructures as in this study [Tullis and Yund, 1991].However, microstructural observations and petrological datashow that melt was present in the system during deformation.The combined effect of water and melt on GBSenhancement is speculative (according to the weakeningmechanisms of melt discussed above), because the influenceof water on dihedral angles of melt in quartzo-feldspathicrocks is not yet well understood [Rushmer, 1996].13 of 15291
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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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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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CONTRASTING TEXTURAL RECORD OF TWO
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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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114 J. FRANĚK ET AL.at these P-T c
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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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