PROCESSES LEADING TO LOCALIZATION IN THE GRANITOID UNITS OF THE GRENVILLE FRONT TECTONIC ZONE, ONTARIO, CA.

The megacrystic granitoid units of the Grenville Front Tectonic Zone (GFTZ) in Ontario, Canada, from French River Outlet to the Orogenic Front, provide an excellent laboratory in which to examine the strength heterogeneities present during orogensis. Recent mapping in the area has constrained the boundaries of a series of kilometer scale shear zones in the GFTZ. Studies of four of the many NNE-SSW trending, steeply dipping shear zones in the GFTZ point to the critical role of strain weakening processes in their formation. Evidence of strain weakening is shown at the outcrop scale in the transformation of igneous features, such as a six-fold increase in the aspect ratios of magmatic enclaves and leucosomes. In all of these gradients, deformation resulted in significant grain size reduction and development of a mylonitic fabric in the amphibolite facies. Although the units are relatively modally consistent across the zones they do show a slight reduction in hydrous minerals from low to high strain. Petrologic, geochemical, and microstructural analyses of these shear zones look for evidence of (1) thermal perturbation; (2) stress heterogeneity; (3) mechanical change due to textural evolution; (4) incorporation or loss of fluids; and (5) mineralogical change due to metamorphic reactions as the causes of the observed strain weakening. Results of this study show that it is unlikely that any one strain weakening process could account for each of the km scale strength heterogeneities observed in this region. Instead, we propose that these shear zones form as the result of several interacting processes. Preliminary U-Pb Zircon ages show a series of events recorded by all the sheared rocks in the GFTZ at approx. 1.7 Ga and 1.4 Ga. However, some of the samples from rocks in the eastern shear zones record a younger age of 1 Ga that may be concurrent with the onset of Grenvillian deformation. We hypothesize that although the strain weakening of all the shear zones occurred during the Grenville Orogeny its timing was only recorded in some of the shear zones investigated, while others did not achieve conditions favorable for resetting the zircons.