Structural, metamorphic, and U–Pb geochronological data bear on the distinction between Archean and Paleoproterozoic tectonism along the southeastern margin of the Rae Province on Baffin Island. Archean rocks include ca. 3.0–2.8 Ga gneiss, two greenstone belts of the Mary River Group, and various younger granitoid intrusions. In the greenstone belts, intermediate–felsic volcanism (2.74–2.725 Ga) was accompanied and outlasted by calc-alkaline plutonism (2.73–2.715 Ga). Deformation, low- to medium-pressure metamorphism, and peraluminous plutonism followed at ca. 2.7 Ga. Archean rocks and the locally overlying Piling Group (ca. 2.2–1.9 Ga) were deformed and metamorphosed together during development of the Paleoproterozoic Foxe fold belt. Tectonism is linked to the Isortoq fault zone, a major southeast-dipping structure marking an abrupt northwestward transition to granulite facies. Within a 5-km-wide zone, tight folds of the Archean Mary River Group give way down-section to moderately southeast-dipping, highly transposed, high-grade gneissic rocks. Several northeast- and north-striking ductile–brittle faults, some recording normal-sinistral oblique displacement, truncate early gneissosity and folds. This progression, along with U–Pb metamorphic ages, suggests early northwest-directed thrusting, starting at ca. 1.85 Ga, with peak metamorphism in the footwall at ca. 1.83–1.82 Ga. Later extensional displacement caused juxtaposition of lower grade on higher grade rocks. Archean ages of metamorphism (and deformation) are well preserved only in the hanging wall and the youngest metamorphic ages are restricted to the footwall. The data indicate that mountain building involved thrust-related thickening followed by gravitational collapse, a sequence characteristic of Phanerozoic orogens.

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