Quantitative structural analyses and numerical modelling of ...

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