U–Pb zircon geochronology and depositional history of the Montresor group, Rae Province, Nunavut, Canada

Abstract

Paleoproterozoic metasedimentary successions of the northwestern Canadian Shield provide records of tectonic events, but the definition of depositional ages has proved elusive. Although previously poorly understood, the Montresor belt of western Nunavut yields new insight into the 2.2–1.8 Ga time window. On the basis of U–Pb analyses of detrital zircon in sedimentary rocks and igneous zircon in sills, we conclude that arenite of the lower Montresor group was deposited between 2.194 and 2.045 Ga, and arkose of the upper Montresor group after 1.924 Ga, adding constraints on the Rae cover sequence. The lower Montresor arenite yielded an older group (3.05–2.58 Ga) and a younger, more tightly constrained group (2.194 ± 0.014 Ga). Four of six zircon grains analyzed from a gabbro sill within the lower Montresor have discordant 207Pb/206Pb ages (2.71, 2.66, 2.53, and 2.39 Ga) and are considered to be inherited, whereas two grains provide an age of 2045 ± 13 Ma, interpreted to date crystallization and providing a minimum age for the lower Montresor package. Upper Montresor arkose contains detrital zircon with probability density peaks at 2.55–2.25 and 2.1–1.92 Ga, together with scattered older grains (3.8–2.65 Ga). The youngest grain yields an age of 1924 ± 6 Ma, establishing a maximum age for sandstone deposition. Provenance is inferred to have been from the west, where igneous sources of 2.5–2.3 Ga (Queen Maud block) and 2.03–1.89 Ga (Thelon orogen) are known. Collectively, the new ages suggest a minimum 120 million year gap between deposition of the pre-2045 ± 13 Ma lower and post-1924 ± 6 Ma upper parts of the Montresor group. Similar age constraints may apply to other parts of the Rae cover sequence.