David Corrigan

M. Sc. Thesis

Geology and U-Pb Geochronology of the Key Harbour Area, Britt Domain, Southwest Grenville Province

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In a transect along Georgian Bay, rocks of the Key Harbour gneiss association form the lowermost structural level of the Central Gneiss Belt. Largely composed of leucocratic para- and orthogneisses, they have been strongly modified by a pervasive L-S fabric, in part associated with the formation of large-wavelength folds with hinges parallel to the transport direction (NW).

Southwest of Key Harbour, Grenvillian deformation (D3) is localized into shear zones, forming anastomosing arrays enclosing elongate zones of lower and more homogeneous strain. Crosscutting relationships preserved only in the inter-shear domains permit the establishment of a field chronology, and reveal an extensive series of pre-Grenvillian events. This chronology has been tested by U-Pb isotopic dating.

Leucogranite dated at 1684 Ma intruded previously deformed, partly migmatitic gneisses, including garnet amphibolites, in which granulite facies assemblages are locally preserved (M1-D1). After emplacement of mafic dykes, plutonism accompanied by high-level deformation and partial melting of country-rock (M2-D2) occurred between 1456-1422 Ma. This event was followed by a second episode of mafic dyking, including the 1238 Ma Sudbury swarm. Monazite in K-feldspar- kyanite-sillimanite paragneisses yields a U-Pb age of 1035 +/-1 Ma, interpreted as a minimum age for upper-amphibolite grade Grenvillian metamorphism (M3). Rocks of the Key Harbour area cooled through titanite blocking temperature between 1001 Ma and 1004 Ma. Zircons from syntectonic leucosomes developing in the neck of internal boudins indicate that ductile deformation continued at least until ca. 1015 Ma. E-W pegmatites dated by U-Pb zircon at 990 Ma intruded while the crust was still capable of ductile deformation.

It is proposed that the Grenville Province in the Key Harbour area consists of reworked mid- continental crust, as suggested by recent models.

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Supervisors: Nicholas Culshaw