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664 O. LEXA ET AL.onset of dynamic recrystallization in weaklydeformed metagabbros show that the lower-temperaturemylonites exhibit a smaller initial recrystallizationgrain size than the new recrystallized grainsof the high-temperature mylonites (Baratoux et al.,2005). In addition, inspection of the N 0 –Gt plotshows that highly deformed samples (GLT2,GHT2a,b, T2) exhibit higher N 0 and lower Gt valuesrelative to weakly deformed samples (GLT1, GHT1).Based on these two observations, we suggest that thedifferences in CSD parameters are not the result oftextural coarsening but are merely controlled bytemperature- and strain rate-dependent mechanismsof dynamic recrystallization. Azpiroz & Ferna´ndez(2003) reported an increase in N 0 but a constant Gt(slope) with decreasing temperature and increasing Gtvalues for constant N 0 with increasing finite strain inthe Iberian Massif metabasites. However, it is proposedthat in our study the N 0 and Gt changesimultaneously due to temperature and strainintensity variations and that the temperature changeplays a key role in the resulting CSD shape.Samples from the banded amphibolite complex aremarked by lower N 0 and higher Gt values in comparisonwith the mylonitic metagabbros. The CSD isdeveloped in both amphibolites and tonalitic gneiss,which indicates that it is independent on a relativeproportion of amphibole and plagioclase in both rocktypes. These data in the N 0 –Gt diagram show a continuoustrend together with the above-described samplesfrom the mylonitic metagabbros. In addition,results of Hb–Pl thermometry reveal an increase inestimated temperatures from the eastern mylonitemetagabbros, through the western mylonite metagabbrosto the banded amphibolite complex. Such anevolutionary trend is likely to be interpreted as theresult of a textural coarsening comparable with theresults of Cashman & Ferry (1988) reinterpreted byHiggins (1998).Interpretation of spatial distribution minerals and grainboundariesSeng (1936) and later DeVore (1959) proposed that thespatial distribution of crystals in high-grade gneisses isdominantly determined by interfacial energy. Thisconcept was adopted by Flinn (1969) who explained aprevailing number of unlike boundaries in granulitesthrough the insertion of grains of one phase betweengrains of other phases. Flinn (1969) suggested that thisfeature is a consequence of a smaller interfacial energyof unlike boundaries in comparison with like–likeboundaries. However, Ramberg (1952) suggested thatdifferences in interfacial energies are too small to drivediffusional mass transfer in high-grade rocks.Modern material science experimental studies showthat during the wetting process the low-energy (lowmisorientationangle) boundaries in one phase arepreserved while another phase preferentially precipitateson high-energy (high-misorientation angle)boundaries (e.g. Kim & Rohrer, 2004). In other words,the highest energy boundaries are progressively eliminatedfrom an inherited population by ÔinfiltrationÕ ofthe other phase. This is in agreement with the knownfact that in granular-polygonal aggregates the minorphase precipitates on triple points to achieve lowertotal interfacial energy (Spry, 1969; Vernon, 1974).Such a tendency was documented by Dallain et al.(1999) who showed that the predominance of unlikecontacts in polycrystalline aggregates originatedthrough wetting of grain boundaries by fluids or melts,and subsequent precipitation of other phases on like–like contacts.In contrast, the solid-state differentiation resultingfrom dynamic recrystallization leads to the developmentof monomineralic aggregates or bands due tocoalescence of like phases at high strains (Schulmannet al., 1996; Kruse & Stu¨ nitz, 1999). Therefore, thelike–like contacts prevail and the so-called aggregatetypedistribution develops, which is a typical feature ofhigh-temperature deformation of polyphase rocks suchas gabbros and granites (Dallain et al., 1999; Baratouxet al., 2005).The high-temperature mylonitic metagabbros fromthe study area exhibit high-grain SPO and GPBOassociated with the development of a strong aggregatedistribution. The lower-temperature mylonitic metagabbrosare characterized by extreme values of SPO inconjunction with an almost random grain distribution.We suggest that in the case of high-temperaturemylonitic metagabbros the process controlling thedevelopment of a strong aggregate distribution is solidstatedifferentiation due to a different efficiency ofdislocation creep in hornblende and plagioclase. Thisprocess is likely to be accompanied by some diffusionalmass transfer responsible for preferential heterogeneousnucleation (Kruse & Stu¨ nitz, 1999) of interstitialplagioclase in coarse-grained amphibole aggregates(Baratoux et al., 2005). On the contrary, in the lowertemperaturemetagabbro mylonites, the random mineraldistribution and the lack of crystallographicpreferred orientation of plagioclase were interpreted tobe the result of mechanical mixing due to grainboundarysliding during granular flow.Samples from the banded amphibolite and tonaliticgneiss show a low SPO, a very weak elongation of bothplagioclase and amphibole, a weak GBPO connectedwith a weak dominance of unlike contacts indicating aregular to anticlustered grain distribution. Such a graindistribution and the large grain size of both phasesexclude mechanical mixing as a process explaining thistexture. We are of the opinion that the spatial distributionof plagioclase and amphibole (e.g. sampleLAC1) can result from heterogeneous nucleation ofplagioclase in an amphibole aggregates. However, anoriginal grain-size distribution characteristic of nucleationand growth processes is completely obliteratedby elimination of the small grains. Very low-grainÓ 2005 Blackwell Publishing Ltd246
CONTRASTING TEXTURAL RECORD OF TWO METAMORPHIC EVENTS 665elongation, SPO and GBPO, similar CSD of bothphases and regular grain distribution indicate that theaggregates tend to achieve a state with a minimuminterfacial energy. As mentioned above, the processesof heterogeneous nucleation of minor phase lead tooccupation of the high-energy grain boundaries atearly stages of the texture development. However, inthe rocks studied it is impossible to distinguish a minorphase from the host aggregate, which indicates that theprocess of interfacial energy reduction is more advanceddue to significant diffusional mass transfer. Asthe driving differences in bulk interfacial energies aretoo small (Ramberg, 1952), the only plausible explanationis the long time-scale of the process.Different time-scales of Cambro-Ordovician and Variscanmetamorphic eventsSˇtípska´ et al. (2001) proposed a tectonic model inwhich the Cambro-Ordovician metamorphism is relatedto large-scale rifting while the Variscan metamorphicevent was connected with a thermal effect inducedby the syntectonic granodiorite sill intrusion, and wasspatially restricted to the host rocks of the intrusion.The thermal time constant s is given by the relationship:s / l 2 j ;ð3Þwhere l is the characteristic length of thermal event andj is the thermal diffusivity (Carlslaw & Jaeger, 1959).This equation indicates that duration of thermalequilibration increases with the square of the size of thethermal anomaly. Based on the proposed tectonicmodel, we assume that a relaxation of the perturbedtemperature field generated by the continental riftdiffers in time-scale by at least one order of magnitudefrom that generated by an intrusion of several km insize. In other words, the metamorphic P–T conditionsattained at a similar depth level may yield similartemperatures but the time required for metamorphicequilibration and development of specific metamorphictextures may differ substantially.ACKNOWLEDGEMENTSThis research is a part of an ongoing collaborationbetween Charles University, Prague, Mainz University,Germany, and ETH Zu¨ rich, Switzerland. Financialsupport to P. Sˇtı´pska´ by the Charles University GrantAgency (grant no. 223/2002/B-GEO/PrF) and theCzech National Grant Agency (GA205/00/D043 andGA205/99/1195) is gratefully acknowledged. Themicroprobe work at ETH Zu¨ rich was financed by theSwiss National Foundation (ÔContinuous OrogenesisÕgrant to A.B. Thompson). Two visits of P. Sˇtı´pska´ toMainz University and a part of the microprobe workwere funded by the German Science Foundation(DFG, grant Kr 68-1, Kr 590/35). The grant no.24313005 of the Ministry of Education of the CzechRepublic provided funds for O. Lexa, P. Sˇtı´pska´ andK. Schulmann.REFERENCESAzpiroz, M. D. & Fernández, C., 2003. Characterization oftectono-metamorphic events using crystal size distribution(CSD) diagrams. A case study from the Acebuches metabasites(SW Spain). Journal of Structural Geology, 25, 935–947.Baratoux, L., Schulmann, K., Ulrich, S. & Lexa, O., 2005.Contrasting microstructures and deformation mechanisms inmetagabbro mylonites contemporaneously deformed underdifferent temperatures (c. 650 and c. 750 °C). In: DeformationMechanisms, Rheology and Tectonics: from Minerals to theLithosphere (eds Gapais, D., Brun, J. P. & Cobbold, P. R.),Geological Society Special Publications, pp. 97–125. GeologicalSociety of London, London, UK.Bohlen, S. R., Wall, V. J. & Boettcher, A. L., 1983. Experimentalinvestigations and geological applications of equilibria in thesystem FeO–TiO 2 –Al 2 O 3 –SiO 2 –H 2 O. American Mineralogist,68, 1049–1058.Brodie, K. H. & Rutter, E. H., 1987. The role of transiently finegrainedreaction-products in syntectonic metamorphism –natural and experimental examples. Canadian Journal of EarthSciences, 24, 556–564.Burg, J.-P., 1999. Ductile structures and instabilities: theirimplication for Variscan tectonics in the Ardennes. Tectonophysics,309, 1–25.Cahn, J. V., 1957. Nucleation on dislocations. Acta Metallurgica,5, 169–172.Carlslaw, H. S. & Jaeger, J. C., 1959. Conduction of Heat inSolids. Oxford University Press, Oxford.Cashman, K. V. & Ferry, J. M., 1988. Crystal size distribution(CSD) in rocks and the kinetics and dynamics of crystallization;3, Metamorphic crystallization. Contributions toMineralogy and Petrology, 99, 401–415.Dallain, C., Schulmann, K. & Ledru, P., 1999. Textural evolutionin the transition from subsolidus annealing to meltingprocess, Velay Dome, French Massif Central. Journal ofMetamorphic Geology, 17, 61–74.DeHoff, R. T., 1991. A geometrically general-theory of diffusioncontrolled coarsening. Acta Metallurgica Et Materialia, 39,2349–2360.DeVore, G. W., 1959. Role of minimum interfacial free energy indetermining the macroscopic features of minerals assemblages.Journal of Geology, 67, 211–227.Eberl, D. D., Drits, V. A. & Srodon J., 1998. Deducing growthmechanisms for minerals from the shapes of crystal size distributions.American Journal of Science, 298, 499–533.Ferry, J. M. & Spear, F. S., 1978. Experimental calibration of thepartitioning of Fe and Mg between biotite and garnet.Contributions to Mineralogy and Petrology, 66, 113–117.Flinn, D., 1969. Grain contacts in crystalline rocks. Lithos, 2,361–370.Gasparik, T. & Newton, R. C., 1984. The reversed aluminacontents of orthopyroxene in equilibrium with spinel andforsterite in the system MgO–Al 2 O 3 –SiO 2 . Contributions toMineralogy and Petrology, 85, 186–196.Hickey, K. A. & Bell, T. H., 1996. Syn-deformational graingrowth: matrix coarsening during foliation development andregional metamorphism rather than by static annealing.European Journal of Mineralogy, 8, 1351–1373.Higgins, M. D., 1998. Origin of anorthosite by textural coarsening:quantitative measurements of a natural sequence oftextural development. Journal of Petrology, 39, 1307–1323.Holland, T. J. B. & Blundy, J. D., 1994. Non-ideal interactionsin calcic amphiboles and their bearing on amphibole–plagio-Ó 2005 Blackwell Publishing Ltd247
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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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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 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 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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ORIGIN OF FELSIC MIGMATITES 31Ó 20
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ORIGIN OF FELSIC MIGMATITES 33durin
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ORIGIN OF FELSIC MIGMATITES 35(a)Ty
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ORIGIN OF FELSIC MIGMATITES 37(a)Pl
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ORIGIN OF FELSIC MIGMATITES 39Grain
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ORIGIN OF FELSIC MIGMATITES 43(a)(b
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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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