The Metamorphic and Structural History of the Central Metasedimentary Belt Boundary Zone South of Killaloe; Felsic and Mafic Gneisses of the Muskoka Domain, Retrogressed from Granulite Facies During Multiple Tectonic Events



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The Grenville Orogen in Ontario, Canada is composed of the Central Gneiss Belt (CGB) and the Central Metasedimentary Belt (CMB). 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 juxtapose the two. My study area is in the 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 locally made up of three nappes. Specifically, we examined deformed gneisses at the base of the structurally lowest nappe, the Stafford thrust sheet. I examined two outcrops that represent mixed felsic and mafic orthogneiss lithologies of the CGB. Differing outcrop assemblages and textures provide evidence for multiple deformation events in the shear zone. At one outcrop on Ruby Road within the Stafford thrust sheet, metamorphic foliation shows variable orientations but generally dips to the east, and lineation is subhorizontal. Retrogressed orthopyroxene within the mafic gneiss suggests that peak metamorphism occurred within the granulite facies. A second outcrop, on Brudenell Road at the base of the Stafford sheet shows extremely well foliated straight gneisses. Strong foliation defined by straight gneiss layering dips shallowly to the east, and the lineation defined by quartz ribbons trends to the south. Quartz ribbons formed from a flattening mechanism outlined by Mandal et al. (1997). Many quartz ribbons are strain-free and polycrystalline, 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.



Grenville, Central Metasedimentary Belt boundary zone, Killaloe, Quartz Ribbon