Quantitative structural analyses and numerical modelling of ...

ORIGIN OF FELSIC MIGMATITES 43(a)(b)Fig. 10. Plots to show the anisotropy of magnetic susceptibility (AMS). (a) P¢–T plot, where the P¢ parameter represents the degree ofmagnetic anisotropy and T is a shape parameter that describes the shape of the ellipsoid of magnetic susceptibility. T can take eitherpositive values (T > 0), characteristic for a planar fabric, or negative values (T < 0), typical for a linear fabric. Dashed ellipses showtwo distinct datasets. For comparison, data obtained by Schulmann K., Edel J.-B., Hasalova´ P., Lexa O., Jezˇek J. & Cosgrove J. W.(unpublished data) and Bouchez (1997) are shown. (b) Magnetic foliation (circles), plotted as the minimal susceptibility direction (K 3 ),perpendicular to the magnetic foliation, and magnetic lineation (squares), plotted as the maximal susceptibility direction (K 1 ).Lattice preferred orientationTo understand the deformation behaviour of individualphases, we measured and evaluated statistically thelattice preferred orientation (LPO) of aggregate grains(Pl, Kfs and Qtz) and grains apparently crystallizedfrom melt (Pl, Kfs and Qtz) separately (Fig. 12g, h).The LPO of quartz, plagioclase and K-feldspar weremeasured on a scanning electron microscope Cam-Scan3200 in the Czech Geological Survey using theelectron back-scattered diffraction technique (EBSD)and HKL technology (Adams et al., 1993; Bascouet al., 2001). Diffraction patterns were acquired at20 kV of accelerating voltage, 5 nA of probe currentand working distance of 33 mm from the thin sectionprepared from the structural XZ plane. The procedurewas carried out manually due to small differences indiffraction patterns. The chemistry and orientation ofindividual grains was controlled using a forescatterdetector with combination of orientation and chemicalcontrast. Thus, each individual grain is represented byonly one orientation measurement. The resulting polefigures are presented as lower hemisphere equal-areaprojections in which the trace of foliation is orientedalong the equator and the stretching lineation is in theE–W direction.Old quartz grains in ribbons of the type I orthogneissshow c-axes distributed in weak sub-maxima arrangedalong weakly developed small circles close tothe S 1 foliation trace. The most intense sub-maximaare developed close to the lineation direction. This typeof c-axis pattern may indicate preferential prism Æcæslip-system activity and dominantly coaxial deformation.The c-axes of large quartz grains in types II, IIIand IV migmatites reveal strong maxima either parallelto the S 2 foliation pole or close to the centre of thediagram. These c-axis patterns indicate mainly activityof basal Æaæ or rhomb Æa + cæ slip-systems and lessfrequently prism Æaæ slip (Fig. 11a). Towards types IIIand IV migmatites, the LPO of the matrix quartz becameless well developed, preserving activity of thesame slip-systems as in the previous microstructuraltypes (Fig. 11a). New quartz grains crystallized frommelt in type I orthogneiss, and type II migmatite showvery weak LPO and nearly random distribution of allquartz axes (Fig. 11b). Whereas old grains show progressiveweakening of the LPO from type II to type IVmigmatite, the new and randomly crystallized grainstends to develop weak crystal preferred orientationduring the same microstructural evolution from type IIto type IV migmatite (Fig. 11). It is difficult to distinguishold from new quartz grains in the type IV rockand therefore the LPO of quartz in this microstructurelinks LPO evolution between old and new grains in thefinal microstructural type.K-feldspar and plagioclase commonly show weakLPO in all rock types regardless the origin of grains.K-feldspar shows crystallographic patterns which arecompatible with dominant activity of the 1/2 [110](001)slip system (Willaime & Gandais, 1977; Willaime et al.,1979) (Fig. 12a, c). Contribution of other slip systemsas [100](010) (Fig. 12b) and [100](001) (Fig. 12d) hasalso been identified in both relict K-feldspar grains andin K-feldspar grains apparently crystallized from meltrespectively.Distribution of the main lattice directions of plagioclaserevealed slip parallel either to 1/2[1 10] on (001)and (11 1) planes (Fig. 12e) or to 1/2 [110] on (001) and(1 1 1) planes (Fig. 12f) for all types of rocks and bothaggregate grains and plagioclase inferred to havecrystallized from melt (Fig. 12g) (Olsen & Kohlstedt,1984). The textures of plagioclase inferred to haveÓ 2007 Blackwell Publishing Ltd329