Quantitative structural analyses and numerical modelling of ...

44 P. HASALOVÁ ET AL.(a)(b)Fig. 11. Characteristic c-axes preferred orientations of (a) old/relict quartz grains and (b) new quartz grains crystallized from areas ofinferred former melt for all rock types. The c-axis patterns of old/relict quartz grains in type I banded orthogneiss indicate prism Æcæslip system activity whereas in type II, III and IV migmatites basal Æaæ or rhomb Æa + cæ slip systems are dominant with minorprism Æaæ slip. New quartz grains inferred to have crystallized from melt in type I banded orthogneiss to type IV nebulitic migmatiteshow very weak LPO and nearly isotropic distribution of all quartz axes. Equal area projections, lower hemisphere, contoured atinterval of 0.5 times uniform distribution. Foliation is horizontal and lineation is in this plane in the E–W direction. N is the number ofmeasured grains. Maximum densities are marked on the bottom right of each pole figure. The dashed line represents the lowest contourlevel and the grey circle corresponds to the minimum density value.crystallized from melt are commonly weak with theexception of strong LPO of plagioclase in the type Iorthogneiss (Fig. 12f). Such slip-systems are supposedto be secondary and active if grains are in unsuitable(hard) orientation to the dominant slip-system[100](010) (Kruse et al., 2001).DISCUSSIONThis study presents a detailed microstructural andquantitative textural analysis of four types of migmatiticrocks identified in one of the largest (5000 km 2 )migmatitic complex of the eastern Variscan belt. Therock types are interpreted as representing a texturalsequence from banded orthogneiss via stromatic andschlieren migmatites to nebulitic migmatite. The possiblemechanisms that could account for the origin ofthis rock sequence involve: (i) genetically unrelatedmigmatites that have originated from distinct protoliths;(ii) variable degree of in situ partial melting of asingle protolith or different protoliths; and (iii) meltinfiltration from an external source through solid rockin which banded orthogneiss and nebulitic migmatiterepresent genetically linked end-members. Thesehypotheses are discussed further below.Spatial relationships of individual migmatite types withinthe shear zoneThe structural sequence described in this work indicatesan intimate relationship between types I to IIImigmatites and nebulitic type IV migmatite sheets thatcan be interpreted in terms of a shear zone, which wasexploited by rising magma (Brown et al., 1995; Collins& Sawyer, 1996; Brown & Solar, 1998b). We haveshown that the D 2 flat fabrics that cross-cut the steepfoliation S 1 developed at high-temperature solid-stateconditions (Fig. 1). Tajcˇmanova´ et al. (2006) andRacek et al. (2006) described a similar sequence ofsuperposed fabrics in lower crustal rocks several tensof kilometres to the north and south of the studied arearespectively. These authors proposed that the flat D 2deformation fabrics originated due to thrusting oforogenic lower crust over middle crustal units along alarge-scale retrograde shear zone. In agreement withthese authors, we suggest that the D 2 fabrics developedin a thrust related crustal-scale shear zone reportedalready by Urban (1992), Schulmann et al. (1994) andredefined later by Schulmann et al. (2005). The maindifference between other regions is in the degree of D 2reworking, which is so high in the studied area thatÓ 2007 Blackwell Publishing Ltd330