Quantitative structural analyses and numerical modelling of ...

42 P. HASALOVÁ ET AL.Fig. 9. Grain boundary statistics plotted as the deviation from a random spatial distribution (grain contact frequency) v. degree ofgrain boundary preferred orientation (GBPO). For details see text. The degree of shading corresponds to the individual rock types.contact continuously increases (negative like–like v-values and positive unlike v-values) (Fig. 9). This isin a good accordance with the increasing amount ofinterstitial phases towards the type IV migmatite.Quartz exhibits the same strong regular distributionfrom the type I orthogneiss to the type IV migmatitein both feldspar domains.In K-feldspar-rich aggregates, the degree of GBPOof the K-feldspar like–like boundaries slightly decreasesfrom type I orthogneiss to type III migmatite,whereas type IV migmatite is characterized by an increasein the degree of K-feldspar like–like GBPO(Fig. 9, Table 1). The GBPO of plagioclase–plagioclaseboundaries in the plagioclase-rich aggregates issimilar to the evolution of K-feldspar like–likeboundaries. The GBPO of the K-feldspar–quartzboundaries as well as those of K-feldspar–plagioclaseboundaries are weak, and decrease throughout thetextural evolution (Fig. 9, Table 1).MINERAL FABRICIn rocks deformed in the presence of melt, the texturesof quartz and feldspar can be used to evaluate thedeformation mechanisms of the solid fraction as wellas the deformation of crystallizing intragranular melt(Za´vada et al., 2007). The mineral fabrics of ferromagnesianphases can be indirectly assessed usinganisotropy of magnetic susceptibility (AMS). TheAMS method has been recently used to determine thedegree of susceptibility, shape of the fabric ellipsoidand relative contribution of ferro- and para-magneticminerals to the bulk fabric in migmatites (Ferre´ et al.,2003, 2004).Anisotropy of magnetic susceptibilityTypes III and IV migmatites are macroscopically closeto isotropic, so that the mineral alignment defined bythe orientation of dispersed biotite is poorly defined(Fig. 2c, d). To better characterize the fabric, the AMSmethod was used to determine the internal fabric ofthese rocks. Oriented samples were collected using aportable drill at four sampling sites covering a sectionacross the well-defined structural sequence. The AMSdata were statistically evaluated using the Anisoftsoftware package (Jelínek, 1978; Hrouda et al., 1990).The low values of mean susceptibility (