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48 P. HASALOVÁ ET AL.refractory. In the present case, Hasalova´ et al. (2008b)report continuous trends in whole-rock geochemistryand mineral compositions for the sequence of rocktypes, but different Nd isotopic composition for thetype I orthogneiss compared with the rest of the sequence,which precludes of in situ anatexis in a closedsystem.Melt infiltration modelThe discrepancies between the evolutionary trends wereport and generally accepted trends for anatecticterranes require an appropriate explanation that isconsistent with the structural, quantitative microstructuraland mineral compositional data. As a possibleexplanation, we introduce the concept of meltinfiltration from an external source, where melt passespervasively along grain boundaries through the wholerockvolume and changes macroscopic (Fig. 2) andmicroscopic (Fig. 3) appearance of the rock. Thisprocess is characterized by resorption of old phases,nucleation of new phases along high-energy like–likegrain boundaries and modification of mineral andwhole-rock compositions. These gradual changes areaccompanied by grain size reduction (Fig. 7) andprogressive disintegration of former aggregate (layered)distribution of original phases (Fig. 9). We suggestthat the individual migmatite types representdifferent degrees of equilibration between the host rockand migrating melt. It should be emphasized, that allthese processes occur along a retrograde path duringexhumation of the Gfo¨ hl Unit. We are aware that adecrease in P–T conditions during melt infiltration is afundamental and limiting factor for the model proposed.The amount of melt and its connectivity are criticalparameters controlling melt mobility and the rheologicalbehaviour of melt-present rocks. To constrainthese parameters both AMS and EBSD were used.Using AMS, it is possible to distinguish between solidstatedominated deformation mechanisms in themelanosome and free rigid body particle rotation in theleucosome (e.g. Ferré et al., 2003). On the other hand,using the EBSD technique enables us to distinguishdeformation mechanisms in the solid framework andto constrain the mechanical role of melt during thedeformation.AMS fabric origin: solid framework or melt controlleddeformationThe AMS study shows that the magnetic anisotropy isdominated by biotite. The oblate shape of magneticellipsoid and high degree of anisotropy of type I orthogneissand type II migmatite (Fig. 10a) are consistentwith strong preferred orientation of biotite and thefact that biotite has a intrinsically oblate shape of thesingle-grain magnetic ellipsoid (Zapletal, 1990; Martı´n-Hernandéz & Hirth, 2003). The type III and IV migmatitesreveal partly resorbed biotite flakes uniformlydispersed in the rock marked by slightly weaker degreeof magnetic anisotropy and less oblate fabric ellipsoidcompared with types I and II migmatite (Fig. 10a).This contrasts with common granites and diatexitesfrom other migmatitic terranes which show significantlylower values of degree of anisotropy and highlyvariable shapes of AMS ellipsoids (Fig. 10a; Bouchez,1997; Ferre´ et al., 2003).Numerous natural studies supported by numericalmodelling indicate that the magnetic susceptibility inviscously flowing magmas is characterized by a verylow degree of anisotropy, pulsatory fabrics and dominantlya plane strain AMS ellipsoid shape (Blumenfeld& Bouchez, 1988; Hrouda et al., 1994; Arbaretet al., 2000). A comparison of the AMS fabrics withthose of diatexites and results of numerical modelsindicate that the intensity of the AMS fabric of typesIII and IV migmatites does not originated from freelyrotated biotite in viscously flowing melt. On the contrary,we argue that the AMS fabric in all types ofmigmatites resembles fabrics usually acquired throughsolid-state deformation of a load-bearing framework,similar to melanosomes in migmatites (Ferre´ et al.,2003). To understand the mechanisms responsible fordevelopment of such fabrics the grain-scale deformationmechanisms and melt behaviour in individual rocktypes is discussed.Deformation mechanismsExperimental studies of low melt fraction rocks deformedunder high differential stress show that matrixminerals deform by grain boundary migrationaccommodated dislocation creep (DellÕ Angelo et al.,1987; Walte et al., 2005). Strong shape and GBPO offeldspar (Figs 8 & 9) as well as LPO of residual quartzgrains (Fig. 11a) in the type I orthogneiss may beinterpreted in terms of plastic deformation consistentwith a dislocation creep deformation mechanism(Rosenberg & Berger, 2001). However, the weak LPOof residual grains of both feldspars (Fig. 12a, e) in thetype I orthogneiss suggests a contribution of grainboundary sliding during the development of themicrostructure. In other words, the type I microstructurecorresponds to a transient microstructure interms of decreasing activity of dislocation creep andenhancement of diffusion controlled processes.Decrease in SPO and GBPO and constantly weakLPO in feldspar of type II migmatite (Figs 8 & 9) maybe interpreted as a result of melt-enhanced diffusioncreep (Garlick & Gromet, 2004). However, the largequartz grains reveal intense activity of basal Æaæ slipsuggesting important plastic yielding of this mineral(Fig. 11a). Elongate pockets inferred to represent formermelt oriented at a high angle to the stretchinglineation in the type I orthogneiss (Fig. 5a) and type IImigmatite indicate that the melt distribution wascontrolled by the deformation. This is supported by theÓ 2007 Blackwell Publishing Ltd334
ORIGIN OF FELSIC MIGMATITES 49strong LPO of interstitial plagioclase (Fig. 12f).Rosenberg & Riller (2000) reported that pockets withinquartz aggregates inferred to have been former meltare oriented at high angle to the foliation plane, possiblyclose to r 1 . Melt distribution in our samples issimilar to their results and also to experiments at highdifferential stresses (DellÕ Angelo & Tullis, 1988) andhigh confining pressures. In these experiments, meltaccumulated in pockets along faces of the grains subparallelto the main compressional stress direction r 1(DellÕ Angelo & Tullis, 1988; Daines & Kohlstedt,1997). Such a melt topology is also termed ÔdynamicwettingÕ (Jin et al., 1994).In types III and IV migmatites both residual andnew grains of K-feldspar and plagioclase exhibit lowSPO, GBPO and LPO (Figs 8, 9 & 12), indicatingabsence of dislocation creep, in contrast to quartz,which exhibits relatively strong crystallographic preferredorientation (Fig. 11a). The topology of formermelt is poorly constrained in both types III and IVmigmatite but, the SPO of the minor phases interpretedto have crystallized from melt shows a bimodaldistribution sub-perpendicular and sub-parallel to theS 2 foliation. These observations are neither compatiblewith high differential stress nor low differential stressexperiments, in which the melt occurs primarily intriple point junctions without any SPO (DellÕ Angeloet al., 1987; Gleason et al., 1999). However, in somenatural samples, former melt pockets are preferentiallylocated along grain boundaries parallel to the foliation(John & Stu¨ nitz, 1997; Sawyer, 1999; Rosenberg &Berger, 2001), indicating that the orientation of meltpocket in nature is not always in agreement withexperimental studies (Rosenberg, 2001). In this study,the melt pocket orientation sub-parallel to the foliationmay indicate low differential stress and high fluid/meltpressure as suggested by Cosgrove (1997).We conclude that during evolution from type Ibanded orthogneiss to type IV nebulitic migmatite meltwetted a majority of grain contacts. The AMS studyand quartz microfabrics in types II to IV migmatitessuggest that the melt fraction did not exceed the criticalamount to allow free relative movement of grainswithout interference, i.e. the melt fraction is below thecritical threshold (e.g. RCMP of Arzi, 1978; RPT ofVigneresse et al., 1996). Rosenberg & Handy (2005)argued that melt fractions of only / ¼ 0.07 (meltconnectivity threshold, MCT) will enable the formationof interconnected networks of melt under dynamicconditions which will lead to a substantial strengthdrop. These authors suggested that weakening at theMCT probably involves localized, inter- and intragranularmicrocracking, as well as limited rigid bodyrotation of grains, without an important contributionof dislocation creep and diffusion processes at grainboundaries. However, we do not observe any strainlocalization associated with brittle failure and thereforeit is suggested that the deformation has to beaccommodated by mechanisms operating homogeneouslyacross significant rocks volumes. Materialscience experiments (Mabuchi et al., 1997) show thatweakening due to melt-enhanced grain boundary slidingat low melt fraction is an efficient mechanismallowing homogeneous deformation. We suggest thatdeformation of both feldspars and quartz in the type IIto type IV migmatites occurred by melt-enhanced grainboundary sliding with a contribution to the overalldeformation by dislocation creep. These characteristicsare compatible with granular flow as described byPaterson (2001) accompanied by melt-enhanced diffusionand/or direct melt flow.CONCLUSIONSBased on a detail field and microstructural study, wedistinguish four types of gneiss/migmatite in the Gfo¨ hlgneiss complex: (i) banded orthogneiss (type I), withdistinct layers of recrystallized plagioclase, K-feldsparand quartz separated by layers of biotite; (ii) stromaticmigmatite (type II), composed of plagioclase and K-feldspar aggregates with subordinate quartz andirregular quartz aggregates – the boundaries betweenindividual aggregates are ill defined and rather diffuse;(iii) schlieren migmatite (type III), which consists ofplagioclase–quartz- and K-feldspar–quartz-enricheddomains with a foliation marked only by preferredorientation of biotite and sillimanite dispersed in therock; and, (iv) nebulitic migmatite (type IV), with norelicts of gneissosity. It is demonstrated that this is acontinuous sequence developed by melt-presentdeformation, in which the type I banded orthogneissesand type IV nebulitic migmatites are end-members.The progressive disintegration of the bandedmicrostructure and the development of nebuliticmigmatite is characterized by several systematic texturalchanges. The grain size of all felsic phasescontinuously decrease whereas the population densityof precipitated phases increases. The new phasespreferentially nucleate along high-energy like–likeboundaries, causing the development of a regular distributionof individual phases. Simultaneously, themodal proportions of felsic phases evolve towards aÔgranite minimumÕ composition. Further, this evolutionarytrend is accompanied by a decrease in grainSPO of all felsic phases. To explain these textural andcompositional changes we introduce a model of meltinfiltration from an external source in which melt isargued to pass pervasively along grain boundariesthrough the whole-rock volume. It is suggested that theindividual migmatite types represent different degreesof equilibration between the host rock and migratingmelt during the retrograde metamorphic evolution.The inferred melt topology in type I orthogneissexhibits elongated pockets of melt oriented at a highangle to the compositional banding, indicating that themelt distribution was controlled by deformation thesolid framework. Here, the microstructure exhibitsfeatures compatible with a combination of dislocationÓ 2007 Blackwell Publishing Ltd335
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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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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 287w
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EXHUMATION IN LARGE HOT OROGEN 295B
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EXHUMATION IN LARGE HOT OROGEN 297R
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