THE METAMORPHIC AND STRUCTURAL HISTORY OF THE CENTRAL METASEDIMENTARY BELT BOUNDARY ZONE AT TWO OUTCROPS SOUTH OF KILLALOE, ONTARIO, CANADA

The Grenville Orogen in Ontario, Canada is composed of the Central Gneiss Belt and the Central Metasedimentary Belt. The Central Metasedimentary Belt boundary zone (CMBbz) is a 10 km thick, ~200 km long crustal-scale shear zone composed of stacked crystalline thrust sheets and ductile shear zones that juxtaposes the two. Our study area is in the northern CMBbz near the town of Killaloe, Ontario, Canada. The purpose of this study is to constrain the metamorphic and structural history of this segment of the CMBbz, which is made up of three nappes in this area. Specifically, we examined deformed gneisses at the base of the structurally lowest nappe, the Stafford sheet. We examined two outcrops that represent felsic and mafic orthogneiss lithologies of the Central Gneiss Belt’s Muskoka Domain. Differing outcrop assemblages and textures provide evidence for multiple deformation events in the shear zone. At one outcrop within the Stafford sheet, metamorphic foliations show variable orientations but generally dip to the north, and lineations are subhorizontal. Retrogressed pyroxenes within the mafic gneisses suggest that peak metamorphism occurred at the granulite facies. A second outcrop at the base of the Stafford sheet shows extremely well foliated straight gneisses. Strong foliation defined by straight gneiss dips shallowly to the east, and the lineation defined by quartz ribbons trends to the south. These quartz ribbons are strain-free and monomineralic, consistent with rapid recrystallization during high grade metamorphism. These observations are consistent with all of the deformation events occurring during a high-grade retrogression from granulite to upper amphibolite facies.