Petrography and Succession of Granitoids from the Southern Part of the Strzelin Crystalline Massif (SW Poland) - Preliminary Data

parallel to the macroscopic foliation. In the upper sheet, most of the magmatic amphiboles and plagioclases are recrystallized forming monomineral bands of 0.1 – 1 cm in size. The plagioclase-plagioclase grain boundaries are strongly serrated, while the amphibole-amphibole boundaries are mostly straight and equilibrated. The quantitative microstructural analysis shows in the lower gabbro sheet an important increase in shape preferred orientation (SPO) of amphiboles and slight increase of SPO of plagioclase with increasing deformation. Both minerals achieve higher aspect ratio but they do not exhibit change in grain size distribution with increasing strain intensity. On the contrary, the SPO in the upper gabbro sheet as well as the aspect ratio of amphiboles slightly decrease with increasing deformation, whereas these parameters in plagioclases remain unchanged. Moreover, the grain size of amphibole decreases, while that of plagioclase increases with progressive deformation.The electron backscatter diffraction (EBSD) measurements of crystal preferred orientation (CPO) reveal similar trends for both metagabbro sheets. Amphibole is marked by a relatively strong CPO already at lower deformation intensities, whereas plagioclase displays very weak CPO. With progressive deformation, the CPO of amphibole further strengthens and becomes entirely random for plagioclase. The quantitative microstructural analysis and the EBSD study suggest that the deformation on a microscale changes depending on temperature and degree of deformation. In the lower sheet, the magmatic grains of amphibole firstly rotate to the easy slip direction, which is represented by the (100)[001] glide system oriented parallel to the foliation and lineation. When this orientation is achieved, the dislocation creep on (100)[001] takes place together with activation of (110)[001] weak cleavage planes inducing a strong rock anisotropy at high deformation intensities. Plagioclase recrystallizes mostly by fracturing and nucleation of new grains occurring in the highly strained zones and to limited extent by mechanism of subgrain rotation. At high strains, the deformation mechanism switches to grain boundary diffusion creep, which is a grainsize sensitive process resulting in a random CPO. In the upper sheet, most of the longest axes of magmatic amphiboles are already oriented parallel to the foliation, and the predominant (100)[001] glide is active. Moreover, the dislocation glide is accompanied by chemically induced grain boundary migration, which is manifested by different composition of the new and old grains. On the contrary, the plagioclase recrystallizes by subgrain rotation mechanism. At the later stages, the dominant recrystallization mechanism is grain boundary migration, which is either chemically or strain induced. It is indicated by strongly serrated plagioclase-plagioclase grain boundaries as well as by important differences in the plagioclase compositions. The processes described above result in strong anisotropy of the whole rock.