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 Geor gian Bay, Ontario, contain monocyclic rocks that originated at or near the southeastern margin of Laurentia between ca. 1450 and 1120 Ma. Their deform ation and metamorphism are entirely attributable to Grenvillian orogenesis. Metamorphic assemblages, fabrics, and P-T-t paths from these rocks therefo re provide important constraints on Grenvillian thermal and tectonic histor y. 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, re spectively. The most likely setting for high-P - high-T granulite-facies me tamorphism in the interior Parry Sound assemblage was at or near the base o f 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-am phibolite facies likely occurred in response to thrust emplacement of the i nterior Parry Sound assemblage onto this part of the basal Parry Sound asse mblage at ca. 1159 Ma. In the lower part of the basal Parry Sound assemblag e, thrust deformation and reequilibration at lower-amphibolite-facies condi tions took place at ca. 1120 Ma. In the southern Parry Sound domain, a high ly attenuated: sequence of quartzites and mafic rocks, deposited some time after 1140-1120 Ma, was affected by upper-amphibolite-facies metamorphism b efore or at 1080 Ma Northwest of and structurally below the Parry Sound dom ain, rocks in the Shawanaga domain were affected by eclogite-facies metamor phism 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 as sociated with a major phase of northwest-directed thrusting and crustal thi ckening. Sillimanite-grade conditions in the Shawanaga domain were maintain ed until at least ca. 1020 Ma, the time of major extensional deformation. D ata on metamorphic grade and age are consistent with progressive northwest- directed juxtaposition of lithotectonic assemblages metamorphosed at progre ssively later times, and may thus record progressive or multistage converge nce at the southeastern margin of Laurentia. Evidence for multiple phases o f Grenvillian high-grade metamorphism in the Parry Sound and Shawanaga doma ins 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 granul ites resulted from thrusting soon after peak metamorphism, whereas those fo r rocks of the Shawanaga domain suggest that exhumation was likely associat ed with both thrusting and extension.