Duncan McLeish

The Central Gneiss Belt of the western Grenville orogen, Ontario, has been interpreted to represent the deep levels of ca. 1000 Ma. Himalayan scale mountain belt. Within this region, different lithotectonic domains, including the Kiosk and Algonquin domains, preserve different structures and metamorphic assemblages formed during ca. 100 million years of convergence. A field study of the poorly explored Kiosk and north-central Algonquin domains yields a better understanding of the tectonic evolution of the Grenville orogen.

Geological reconnaissance in northwestern Algonquin Provincial Park identifies a pervasive, straight, and strongly developed L=S thickening fabric in the Kiosk domain which is not developed at such a scale elsewhere within the Central Gneiss Belt. Field relations show this fabric to be one of the earliest Grenvillian fabrics preserved in the Central Gneiss Belt; neighbouring domains, in contrast, preserve highly contorted, S>L flow fabrics. Regional high-grade metamorphism resulting in granulite facies mineral assemblages is, for the most part, preserved in the Kiosk and Algonquin domains with notable retrogression to amphibolite facies occurring along the northern and southern boundaries of the Kiosk domain. Lithologically, the study area is characterized by small, highly deformed metaplutonic bodies set in a matrix of quartzofeldspathic and pelitic gneiss. Pressure and temperature estimates from metamorphic assemblages in metabasite and corundum bearing supracrustal rocks indicate that peak conditions of P = 14 kbar and T = 800^oC were achieved in the Kiosk domain.

In conclusion, the finding of an L=S thickening fabric that predates S>L flow fabrics is consistent with previously proposed tectonic models which show orogen development by continental thickening flowed by lower crustal extensional flow. The uncommon P-T signature of the Kiosk domain may be a relict record of the thermobarometric conditions which gave rise to the thickening fabric. Additionally, the finding of strong NW-SE oriented lineations in the Kiosk domain suggest a thrusting direction consistent with the NW directed emplacement of thrust sheets proposed by these models.

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Pages: 170
Supervisors: Nicholas Culshaw / Rebecca A. Jamieson