Quantitative structural analyses and numerical modelling of ...

662 O. LEXA ET AL.Fig. 9. (a) Plot of crystal size distribution (CSD) parameters N 0 and Gt. Parameters are obtained from CSD plots, where straightline is fitted on plotted data. Gt represents the slope and N 0 the intercept of fitted line. Shaded ellipses from upper left to lowerright corner mark domains of LT metagabbros, HT metagabbros, banded amphibolites and magmatic rocks. See discussion.(b) Examples of CSD plots used for N 0 and Gt parameter estimates.transitional values of v and R b between those of thebanded amphibolite complex and the mylonitic metagabbrosamples. The Variscan undeformed granodioritesill shows nearly a random distribution of graincontacts and an absence of GBPO.DISCUSSIONThe study samples of mafic lithologies from theCambro-Ordovician and Variscan structures exhibitremarkably similar mineral assemblages. In addition,the presence of garnet in metamorphosed mafic rocksindicates the medium pressure conditions for bothCambro-Ordovician and Variscan structures. Localoccurrence of orthopyroxene bearing mineral assemblagesin the banded amphibolites as well as in thewestern metagabbro mylonite belt (Table 1) indicatesthat both Cambro-Ordovician and Variscan metamorphicevents reached a boundary between granuliteand amphibolite facies conditions.The metamorphic assemblages and P–T estimates ofthe Cambro-Ordovician metamorphic conditions ofbanded amphibolite complex calculated overlap withinthe likely uncertainties with those of the Variscanmylonitic metagabbros and granodiorite. Moreover, inthe southern part of the region studied, the Variscanand Cambro-Ordovician fabrics are concordant andtherefore slight differences in metamorphic conditionsare indistinguishable. We suggest that in this case thestandard methods of metamorphic petrology cannotdiscriminate the two distinct metamorphic events fromeach other.Interpretation of crystal size distributionsThe grain-size distribution in metamorphic rocks(Cashman & Ferry, 1988; Eberl et al., 1998) is primarilycontrolled by the interaction between nucleationrate which is sensitive to temperature overstepping(Cahn, 1957), and the growth rate which is anapproximately linear function of overstepping (Ridley& Thompson, 1986). A further process that influencesCSD is textural coarsening driven by a tendency todecrease the excess of interfacial energy to reach alower energy state (Voorhees, 1992). The transfer ofmaterial from smaller grains to larger ones occurs bydiffusion and is commonly expressed by the Lifshitz–Slyozov–Wagner (LSW) equation (Lifshitz & Slyozov,1961) for Ostwald ripening, or by the communicatingneighbours theory (CN) by DeHoff (1991) where adiffusion length scale is spatially dependent. This differencebetween the theories is expressed in the waythat the CSD is modified during coarsening (Higgins,1998). In the case of LSW the descriptive parametersN 0 and Gt remain constant, while the CN theoryimplies a decrease in N 0 and Gt during coarsening.Experimental studies have shown that the size ofdynamically recrystallized grains is strongly dependenton recrystallization mechanisms that are controlled bystress and temperature (Twiss, 1977). Low-temperatureÓ 2005 Blackwell Publishing Ltd244