Quantitative structural analyses and numerical modelling of ...

PRECURSOR AND RHEOLOGY OF VARISCAN GRANULITES 121Lattice-preferred orientationLattice-preferred orientation (LPO) measurements ofquartz, K-feldspar and plagioclase grains from all thedescribed microstructural types were additionallycarried out in order to evaluate operative deformationmechanisms. A strong LPO usually originates duringplastic deformation via dislocation creep, whereas thediffusion-accommodated GBS generally weakensthe LPO of deforming grains (e.g. Jiang et al., 2000).The slip systems accommodating dislocation creep arewell known for quartz (e.g. Schmid & Casey, 1986) andplagioclase (e.g. Tullis, 1983; Montardi & Mainprice,1987; Kruse et al., 2001; Stu¨ nitz et al., 2003), but lessknown for K-feldspar (Tullis, 1983). To ensure highquality of measurements, the crystal lattice orientationswere collected manually via EBSD in theLAREM laboratory of the Czech Geological Surveyand at the Institute of Petrology and StructuralGeology at Charles University in Prague. LPO data foreach sample (XZ thin section) were plotted separatelyas non-polar projections on a lower hemisphere andimportant slip planes and directions for quartz,K-feldspar and plagioclase were projected.Type I microstructureAt first, the EBSD study was carried out in domainsconsisting of perthite and the newly formed Type Imatrix enclosed within the perthite, therefore protectedfrom D2 and D3 deformation. The Type I K-feldsparand plagioclase grains reveal roughly the same latticeorientation as the K-feldspar host and plagioclaseexsolutions in parental perthite. When passing outsidethe protected matrix into the Transitional type IImicrostructure, a continuous increase of LPO scatteringwith respect to the parental perthite orientationoccurs in the matrix without any tendency to develop anew LPO (Fig. 14).Type II microstructureThe pole figures of matrix quartz show incomplete highangleType II cross-girdle (Lister & Price, 1978) of c-axes(Fig. 15a). High opening angle and central maximumare consistent with dominance of prism and slip systems, being variably accompanied by rhomb + slip (e.g. Lister & Dornsiepen, 1982).The large quartz ribbons have an irregular size anddistribution of subgrains, with inclined subgrainboundaries and a strong central maximum sometimesaccompanied with minor submaxima oriented closeto the lineation. This typical granulite pattern (Behr,1961) results from predominance of the prism slip, only rarely accompanied by subordinate prism slip (Schmid & Casey, 1986).In plagioclase, the LPO shows an incomplete girdleof (010) poles normal to the lineation direction andmaxima of [201], [101] and [001] subparallel to thelineation. It indicates the activity of primary [001] (010)Fig. 14. Electron back-scattered diffraction measurements of a perthite domain undergoing recrystallization to granular matrix andinitial D2 deformation. The crystal lattice orientation of both K-feldspar background and plagioclase exsolutions in parental perthiteare compared to both feldspars in the granulitic matrix located inside the perthite, at the edge of the perthite and in the deformedmatrix adjacent to the perthite grain. The elongation of quartz grains reflects D2 strain intensity and depicts orientation of S2 fabric.Lower hemisphere non-polar equal-area projections.Ó 2010 Blackwell Publishing Ltd359