STRAIN LOCALIZATION IN THE WHITESTONE ANORTHOSITE

Microstructural and geochemical study of an anorthositic mass deformed within an upper amphibolite facies shear zone in the Grenville Province of southern Ontario allows for the investigation of mechanisms that lead to strain localization in the deep orogenic crust. The Whitestone Anorthosite is an anorthositic to leucogabbroic sill that was metamorphosed to granulite facies during the Grenville orogeny at ca. 1160 Ma. Following peak metamorphism, the Whitestone Anorthosite was deformed along its margins during shear along the Parry Sound Shear Zone, a 1 to 3 km wide ductile shear zone that separates large amphibolite and granulite facies lithotectonic domains. The undeformed core of the Whitestone Anorthosite preserves igneous textures and the granulite facies assemblage. Deformed rocks are characterized by the development of a planar fabric dominated by centimeter- to meter-scale shear zones that anastomose around lozenges of less deformed rock similar in texture to the undeformed core. Strain gradients at shear zone margins are moderate to high. Within low-strain rocks, two morphologies of hornblende replaced clinopyroxene under amphibolite facies conditions: a monophase rim surrounds an intergrown hornblende-quartz core. The hornblende grains along the rims are enriched in Fe and Al relative to the hornblende in the core. In the moderately- to highly-strained rocks, all hornblende chemistries are identical and plagioclase shows a correlation between recrystallization and Ab content. In places, scapolite replaced the recrystallized plagioclase grains. These observations are consistent with a strain-related fluid presence.