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The ∼2800-km-long Snowbird tectonic zone is one of the most controversial tectonic features of the Canadian shield. Metamorphic and in situ geochronologic data reported here reveal that a 1.9 Ga medium- to high-pressure belt extends along most of this tectonic zone. In contrast to recent interpretations, a collisional origin is indicated by the length of this metamorphic belt, tectonic thickening documented in parts of it, geological contrasts across it, and subduction-type microdiamonds within it. This collisional event marks a pre–1.865 Ga phase of the Hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of Laurentia.

The Anton terrane of the western Slave Province has been interpreted as a remnant of a meso-Archean (3500–2800 Ma) microcontinent. The largest coherent entity of the terrane is a previously unrecognized 50 by 250 km granulite complex, herein termed the Snare domain. The granulite facies mineral assemblages give the domain a characteristic magnetic signature that is distinct from areas of exposed meso-Archean basement in the Slave Province. The domain consists of interlayered 3100–2624 Ma orthogneisses and paragneisses and 2600–2585 Ma granitoid rocks; its structure was largely established during 2610–2590 Ma orogeny throughout the province that culminated in ca. 2590 Ma granulite facies metamorphism. Pressure-temperature ( P-T ) determinations yield peak conditions of 775–875 °C at 6–7 kbar. Shortly thereafter, 2–4 kbar of decompression occurred before the domain cooled below 450 °C. Decompression and retrogression textures are widespread but are most pervasive adjacent to late extensional detachment faults. U-Pb zircon age data suggest that the Snare domain was partially exhumed from mid-crustal levels ca. 2585–2580 Ma. We attribute exhumation to tectonic denudation of an overthickened crustal welt in the western Slave Province. Localization of the crustal welt in this part of the Slave Province may be related to the presence there of meso-Archean crust.