HETEROGENEOUS DEFORMATION OF GABBROIC ROCKS IN THE CENTRAL METASEDIMENTARY BELT BOUNDARY THRUST ZONE, GRENVILLE PROVINCE CANADA

The Grenville province of southern Ontario, Canada is the product of many cycles of convergent tectonics between 1.4 to 1.0 Ga, resulting in terrane accretion and moderate to high grade metamorphic rocks. These rocks often exhibit high degrees of deformation and strain; shear zones are common. In southern Ontario these mid-crustal rocks are exhumed providing an opportunity to study the structural processes that acted during the Grenville orogeny. Of interest to this study is the Central Metasedimentary Belt boundary thrust zone (CMBbtz), a 200km by 10km area of anastomosing shear zones and thrust sheets separating two major crustal domains of the Grenville province. The CMBbtz is thought to have accommodated the majority of the thrusting of accreted terrane from the southeast onto the Laurentian continent to the northwest at about 1.0 Ga.

In this zone of structural and tectonic significance, we have investigated an outcrop of anorthositic gabbro that is near Gooderham to identify mechanisms of shear localization. This relatively small outcrop (25 meters) exhibits widely varying degrees of strain, from very low strain (apparent igneous texture) to very high strain. We have analyzed samples across the strain gradient to identify structural and microstructural differences, including in mode, grain size, shape preferred orientation, grain aspect ratio, mineral chemistry, and crystallographic preferred orientation. Preliminary analysis indicates that grain sizes of both hornblende and plagioclase are significantly reduced in the strained samples by fracturing of hornblende and subgrain boundary rotation-recrystallization of plagioclase. Additionally, biotite, a weaker mineral, is present in the majority of strained samples. Both of these factors could result in local weakening of this rock and strain localization.