Grenvillian metamorphism of monocyclic rocks, Georgian Bay, Ontario, Canada: Implications for convergence history

The Parry Sound and Shawanaga domains of the Central Gneiss Belt along Georgian Bay, Ontario, contain monocyclic rocks that originated at or near the southeastern margin of Laurentia between ca. 1450 and 1120 Ma. Their deformation and metamorphism are entirely attributable to Grenvillian orogenesis. Metamorphic assemblages, fabrics, and P-T-t paths from these rocks therefore provide important constraints on Grenvillian thermal and tectonic history. Rocks in the interior and basal Parry Sound assemblages of the northern Parry Sound domain were metamorphosed to granulite-facies conditions during an early phase of Grenvillian tectonism, i.e., at ca. 1161 and 1163 Ma, respectively. The most likely setting for high-P - high-T granulite-facies metamorphism in the interior Parry Sound assemblage was at or near the base of crust that wits underplated by voluminous mafic magma. In contrast, heat advected from anorthosite could account for intermediate-P granulite-facies metamorphism in the basal Parry Sound assemblage. In the upper part of the basal Parry Sound assemblage, retrogression from the granulite to upper-amphibolite facies likely occurred in response to thrust emplacement of the interior Parry Sound assemblage onto this part of the basal Parry Sound assemblage at ca. 1159 Ma. In the lower part of the basal Parry Sound assemblage, thrust deformation and reequilibration at lower-amphibolite-facies conditions took place at ca. 1120 Ma. In the southern Parry Sound domain, a highly attenuated: sequence of quartzites and mafic rocks, deposited some time after 1140-1120 Ma, was affected by upper-amphibolite-facies metamorphism before or at 1080 Ma Northwest of and structurally below the Parry Sound domain, rocks in the Shawanaga domain were affected by eclogite-facies metamorphism at ca. 1090-1085 Ma, suggesting deep burial or partial subduction of the Laurentian margin beneath the Central Metasedimentary Belt at this time. Widespread upper-amphibolite-facies metamorphism after ca. 1080 Ma was associated with a major phase of northwest-directed thrusting and crustal thickening. Sillimanite-grade conditions in the Shawanaga domain were maintained until at least ca. 1020 Ma, the time of major extensional deformation. Data on metamorphic grade and age are consistent with progressive northwest-directed juxtaposition of lithotectonic assemblages metamorphosed at progressively later times, and may thus record progressive or multistage convergence at the southeastern margin of Laurentia. Evidence for multiple phases of Grenvillian high-grade metamorphism in the Parry Sound and Shawanaga domains suggests that construction and interpretation of P-T-t paths from these rocks require careful assessment of timing and overprinting relationships. Derived P-T-t paths suggest that exhumation of interior Parry Sound granulites resulted from thrusting soon after peak metamorphism, whereas those for rocks of the Shawanaga domain suggest that exhumation was likely associated with both thrusting and extension.