A study of deformed alkali feldspars from a Precambrian shear zone

Abstract

The specimens used in this study were quartzofelspathic gneisses collected from an upper amphibolite facies shear zone in the Ontario Gneiss Segment of the Grenville Province and were studied using optical, X-ray and EDS/SEM techniques. Results of these studies have shown that the majority of the deformation was due to ductile deformation which produced a significant amount of plastic strain in the feldspar porphyroclasts. Observations of these deformed porphyroclasts suggest that plagioclase and microcline have been dynamically recrystallized by rotation and progressive misorientation of subgrains within the host crystals. Increased deformation was found to decrease the amount of microcline present in the specimens. Other parameters such as Al-Si ordering and composition (as determined by XRD and EDS) were found to remain fairly constant with increasing deformation.