Quantitative structural analyses and numerical modelling of ...

ORIGIN OF FELSIC MIGMATITES 47production of coarse-grained felsic mineral aggregatesresulting from increase in temperature (e.g. Dougan,1983; McLellan, 1983). This process is commonly followedby textural coarsening (Ashworth & McLellan,1985; Dallain et al., 1999; Berger & Roselle, 2001)explained by two competing approaches: the Lifshitz–Slyozov–Wagner (LSW) model (Lifshitz & Slyozov,1961), and the communicating neighbour theory (CNof DeHoff, 1991). Higgins (1998) showed that texturalcoarsening results in progressive decrease in N 0 valueand decrease in the slope of the CSD curve; he interpretedthis trend as a result of rapid undercoolingduring solidification of magma followed by reducedundercooling, suppression of nucleation and texturalcoarsening. However, our textural sequence exhibitsthe opposite trend in evolution of CSD curves, which isinterpreted as indicating that in situ partial melting andtextural coarsening are not responsible for the origin ofobserved CSDs.The observed CSD trend may be explained by one ofthree different mechanisms: (1) solid-state deformationunder decreasing temperature and or increasing strainrate (Hickey & Bell, 1996; Azpiroz & Ferna´ndez, 2003;Lexa et al., 2005); (2) a different degree of reactionoverstepping (Waters & Lovegrove, 2002; Moazzen &Modjarrad, 2005); and (3) a different degree of undercooling(Marsh, 1988).The grain size for dynamically recrystallized grainsin a power-law creep regime is a function of differentialstress (Twiss, 1977). Such grains are characterized bystrong shape and LPO and commonly solid-state differentiation(Baratoux et al., 2005; Lexa et al., 2005).However, this microstructural study does not revealany features in quartz, plagioclase and K-feldspar ofrock types II, III and IV which may indicate a dynamicrecrystallization processes operating under decreasingtemperature. Differences in the degree of reactionoverstepping have been documented in contact aureoles,but may be rejected in this case due to theregional nature of the metamorphism. However, therole of different degrees of undercooling relating to anoverall decrease in equilibration temperature cannot beexcluded.Our data indicate that the sequence of rock typesreflects the progressive resorption of residual grainsand crystallization of new grains from melt in intergranularspaces. Moreover, the trend of CSD curvessuggest a progressive increase in nucleation rate anddecrease in growth rate from type I orthogneiss totype IV nebulitic migmatite. This trend could beexplained by an increase in undercooling consistentwith the decreasing equilibration temperature wereport.The CSD trend is compatible with crystallization ofmelt in a progressively exhuming and rapidly coolingsystem. This is in accordance with exceptionally highcooling rates up to several hundred degrees celsius permillion years estimated for nearby granulites byTajcˇmanova´ et al. (2006).Interpretation of spatial distributions of phasesThe quantitative analysis of spatial distributions ofindividual phases shows that the intensification ofregular distribution (increasing amount of unlikecontacts; Fig. 9) correlates with an increasing degree ofhost rock–melt equilibration. The process of meltcrystallization leads to new mineral growth on thesurfaces of residual grains. This is responsible for theincrease in unlike grain boundaries, which commonlyretain melt–solid geometries. Our case study showsthat the development of a regular distribution of felsicphases is not related to solid-state annealing, as supposedby some authors (Flinn, 1969; McLellan, 1983;Lexa et al., 2005), but to the process of crystallizationof melt, consistent with precipitation of the minorphase on triple points in granular polygonal aggregatesto achieve lower total interfacial energy (Spry, 1969;Vernon, 1974). This process was documented by Dallainet al. (1999), who showed that the predominanceof unlike contacts in polycrystalline aggregates originatedthrough wetting of grain boundaries by fluids ormelt, and subsequent precipitation of other phases onlike–like contacts. However, we cannot exclude thepossibility that a regular distribution reported fromgranulites and high-grade gneisses (Flinn, 1969; Kretz,1994) results from solid-state annealing of rocks wheremelt crystallized. Therefore, the regular distributiondeveloped during melt crystallization may be inheritedand perhaps further accentuated during later thermaland textural re-equilibration.Origin of microstructural and compositional trendsThe sequence from type I orthogneiss to type IVmigmatite exhibit continuous trends in all quantitativeparameters (Table 1). The grain size decreases(Fig. 7a) and there is a progressive development of aregular distribution of all felsic phases (Fig. 9), whichis linked with mineral compositional trends indicatingtemperature decrease. These clear evolutionary trendsare incompatible with a process of partial melting ofdifferent protoliths. Partial melting of the same protolithmay develop continuous trends, but these shouldshow increase in grain size of individual felsic phases(Dallain et al., 1999) and different mineral compositionalevolution (e.g. Gardien et al., 1995; Greenfieldet al., 1998). Additionally, we show that the degree ofregular distribution for K-feldspar- and plagioclasedominatedaggregates evolves in the same mannerthroughout the microstructural sequence (Fig. 9).However, Dallain et al. (1999) reported significantlymore advanced regular distribution of plagioclasecomparedwith K-feldspar-rich aggregates in themicrostructural sequence originated by partial melting.These authors proposed that this microstructuralcontrast originated due to melting process preferentiallyoperating in mica–plagioclase rich aggregates,whereas the K-feldspar-rich aggregates were moreÓ 2007 Blackwell Publishing Ltd333