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38 P. HASALOVÁ ET AL.PlQtz70% 30%Banded orthogneiss (Type I)Stromatic migmatite (Type II)Schlieren migmatite (Type III)Nebulitic migmatite (Type IV)50%30%et al., 1974); and (iii) cuspate and lobate areas inferredto represent pools of crystallized melt (Jurewicz &Watson, 1984).The former presence of melt at grain scale was inferredfrom the following microstructures (Figs 4 & 5).(i) Plagioclase films between adjacent K-feldspargrains, inferred to represent a plagioclase componentcrystallized from melt (Fig. 5a, c). This plagioclase ischaracterized by more albitic composition and by differenttopology compared with original grains. (ii) Pl–Kfs–Kfs and Kfs–Kfs–Pl dihedral angles commonlylower than 30° (Fig. 5a, c), as observed in granitic meltcrystallized under experimental conditions (e.g.Laporte et al., 1997). (iii) Cuspate K-feldspar pools inplagioclase aggregates (Fig. 5b), inferred to represent aK-feldspar component crystallized from melt (Jurewicz& Watson, 1984; Sawyer, 1999, 2001). (iv) Normalzoning of plagioclase from An 10)30 to An 0)15 (Sawyer,1998; Marchildon & Brown, 2001) lining K-feldsparboundaries (Fig. 5c, d). An important feature is thepreferential orientation of plagioclase films coating K-feldspar boundaries in type I orthogneiss and type IImigmatite sub-perpendicular to the foliation (Fig. 5a),in contrast to the types III and IV migmatites, wherethese films are wider and do not show any opticallyvisible preferred orientation. Bulbous myrmekite(Fig. 4d) and new highly irregular lobate grains thatovergrow partially resorbed corroded feldspar grains(e.g. Fig. 4c) are similar to microstructures describedas typical of minerals reacting with melt (Mehnertet al., 1973; Bu¨ sch et al., 1974; McLellan, 1983).QUANTITATIVE TEXTURAL ANALYSISKfsFig. 6. Modal changes in both plagioclase (open symbols) andK-feldspar (closed symbols) aggregates in different rock typesplotted in a quartz–plagioclase–K-feldspar triangle. Arroweddashed lines indicate evolutionary trend from type I bandedorthogneiss to type IV nebulitic migmatite.The quantitative analysis of texture is based onstatistical evaluation of grain size distributions(Kretz, 1966, 1994; Ashworth, 1976; Ashworth &McLellan, 1985; Cashman & Ferry, 1988; Cashman& Marsh, 1988; Higgins, 1998; Berger & Roselle,2001), spatial distribution of minerals and GBPOs(Panozzo, 1983), and grain contact frequencies(Flinn, 1969; Kretz, 1969; McLellan, 1983; Kruse &Stu¨ nitz, 1999). In simple chemical systems, thesetextural parameters are more sensitive to changes ofphysical conditions than compositional characteristics.This is due to the high activation energies ofchemical reactions needed to produce new crystalgrowth compared with the small amount of latticestrain energy and grain boundary energy required todrive recrystallization processes (Spry, 1969; Stu¨ nitz,1998).In this study, the textures of three samples wereanalysed from each rock type, and in each samplemore than 1000 grains were evaluated in thin section.Due to significant textural variations, the individualK-feldspar-rich and plagioclase-rich domains wereanalysed separately. Maps of grains with full topologywere manually traced into the ESRI ArcViewDesktop GIS environment and grain boundaries weregenerated using the ArcView PolyLX extension (Lexa,2003). The ÔshapefilesÕ of individual digitalized thinsections are attached in Appendix S1 (Supplementarymaterial). Analysis of grain size, CSD, grain shapepreferred orientation (SPO), grain boundary preferredorientation (GBPO) and grain contact frequencieswere obtained using the MATLAB TM PolyLX toolbox(Lexa, 2003; http://petrol.natur.cuni.cz/ ondro/). Thegrain sizes of the minerals were evaluated in termsof Feret diameter (diameter of a circle having thesame area as the grain). Two methods were used todetermine the grain SPO: (1) mean directions usingcircular statistics; and (2) eigenvalue analysis ofScheidegger’s bulk orientation tensor calculated fromindividual long axes weighted by grain size (Lexaet al., 2005), where degree of SPO is expressed as theeigenvalues ratio Rg. GBPO was assessed by similartechniques, but the bulk orientation tensor is formedfrom the decomposed grain boundaries betweenchosen phases (Lexa et al., 2005) and the degree ofGBPO is expressed as the eigenvalues ratio Rb. Graincontact frequency, used to examine statistical deviationfrom a random spatial distribution of contactrelations between the individual minerals, was evaluatedin a manner similar to the method of Kretz(1969, 1994), except that contact frequencies wereobtained directly from grain map topologies insteadof using line intercepts.Results of the quantitative microstructural analysesshow an evolutionary trend from the banded orthogneiss,through the migmatite types II and III to thenebulitic migmatite. Therefore, in the following sectionsthe rock types are discussed as a sequence inwhich the type I orthogneiss and type IV nebuliticmigmatite are considered to be end-members of acontinuous microstructural evolution.Ó 2007 Blackwell Publishing Ltd324
ORIGIN OF FELSIC MIGMATITES 39Grain size analysisThe CSD is an important tool to estimate residencetime of magmas in magma chambers, cooling rates inrapidly quenched lavas, as well as to quantify texturesrelated to phenocrysts accumulation and fractionation(Cashman & Marsh, 1988; Marsh, 1988; Higgins,1998). In metamorphic petrology, CSD is used toobtain quantitative information concerning crystalnucleation and growth rates and nucleation densityand/or annealing (Randolph & Larson, 1971; Cashman& Ferry, 1988; Carlson, 1989; Waters & Lovegrove,2002). Hickey & Bell (1996) proposed thatduring dynamic recrystallization decreasing strain rateto temperature ratio (_e/T) leads to decrease in the ratioof nucleation and growth rate (N/G) and developmentof coarser grain size, whereas increasing _e/T leads toincreasing N/G and therefore to grain size decrease.This hypothesis is well documented in experimentalstudies with steel alloys (Sakai & Jonas, 1984) supportedby Azpiroz & Ferna´ndez (2003) and Lexa et al.(2005) in naturally deformed rocks. These authorsevaluate the role of recrystallization mechanisms on N/G ratio of the CSD. The CSD is commonly used informerly partially molten rocks to evaluate combinedprocess of resorption and grain size decrease in reactingphases in mesosome and nucleation and graingrowth and coarsening of minerals crystallizing inleucosomes (Dougan, 1983; McLellan, 1983; Ashworth& McLellan, 1985; Dallain et al., 1999). Because theprocesses controlling grain size distributions in thecrystallization of partially molten rocks are complexand interpretations uncertain, CSD has only rarelybeen used to describe textural evolution of migmatites(Berger & Roselle, 2001). In this work, the CSDmethods are used as a practical approach to parameterizegrain size frequency histograms and visualizetheir trends in a simple manner.Grain size statistics were evaluated for the four rocktypes for plagioclase, K-feldspar and quartz and theresults are presented in the form of average grain size,expressed as a median value of the Feret diameter, andgrain size range expressed as the difference between thethird and first quartiles instead of standard deviationbecause of the log-normal distribution of measureddata (Fig. 7a, Table 1). The results are also summarizedas CSD curves (plot of logarithms of populationdensity against crystal size) that were constructed usingthe method of Peterson (1996); values of the zero-sizeintercept (N 0 – population density interpreted as theratio of nucleation rate to growth rate) and negativeinverse of slope (Gt interpreted as a function of growthrate) of the linear parts of the CSD curves are plottedin Fig. 7b, c.Both plagioclase and K-feldspar in the type Iorthogneiss are characterized by log-normal grain sizedistribution exhibiting average grain size of 0.2 and0.2–0.5 mm respectively. Interstitial quartz yields significantlysmaller average grain size of 0.05–0.1 mm inboth domains. Quartz grains from polycrystalline ribbonswere not evaluated statistically but their grain sizeof 0.5–2.0 mm was estimated using an optical microscope.The grain size of new interstitial plagioclase inthe K-feldspar aggregates is close to 0.1 mm. The grainsize distributions from type I orthogneiss to type II,type III and type IV migmatites are characterized bythe following features. The average grain size of plagioclaseand K-feldspar decreases compared with typeI orthogneiss (Fig. 7a, Table 1). This is accompaniedby a continuous decrease in grain size range for bothfeldspars. The interstitial quartz grain size remainsfairly constant throughout all the stages of texturalevolution, ranging between 0.05 and 0.1 mm, beinglarger in the K-feldspar than in the plagioclasedomains (Fig. 7a). The grain size of minor plagioclasein the K-feldspar domains shows a bimodal distributionthat is attributed to the presence of small newlynucleated grains (0.06–0.1 mm) and to larger plagioclasegrains (0.2 mm) already present in the feldsparaggregates.The CSD of plagioclase indicate continuous increasein N 0 (nucleation density) values coupled with adecrease in Gt (growth rate) values from type Iorthogneiss towards type IV migmatite (Fig. 7b,Table 1). By contrast, K-feldspar shows a decrease inGt values from type I orthogneiss to type III migmatitewithout significant increase in N 0 values, which remainvery low. From type III to type IV migmatite a dramaticincrease in N 0 values is observed for K-feldsparat almost constant Gt values (Fig. 7c). This evolutionis clearly shown by steepening of the slopes of the CSDcurves accompanied by increase in their upper interceptwith the ordinate axis (insets in Fig. 7b, c).Grain shapes and grain shape preferred orientationGrain shape or grain aspect ratio together with grainSPO analyses provide important information aboutdeformation during or after leucosome formation(Mehnert, 1971; McLellan, 1983) or about degree ofinheritance of original anisotropy (Ashworth, 1979).Measurements of preferred orientations of inferredmelt-filled grain boundaries in rocks give insights intoprocesses of melt draining and melt transfer (Rosenberg& Handy, 2000, 2001; Sawyer, 2001).Grain shape and SPO statistics were evaluated in allthe textural types for plagioclase, K-feldspar, quartzand biotite. The results of SPO statistic are summarizedin a boxplot-type diagram, where the axial ratiosof the individual minerals are plotted against bulk SPO(Lexa et al., 2005) for the corresponding minerals(Fig. 8).Aspect ratios for both K-feldspar and plagioclaseshow small median values ranging from 1.5 to 1.7throughout the whole microstructural sequence(Fig. 8, Table 1). Quartz exhibits slightly smaller andstable aspect ratio close to 1.5. An important feature isthe continuous decrease in SPO of K-feldspar andÓ 2007 Blackwell Publishing Ltd325
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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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