New U-Pb geochronological and Sm-Nd isotopic results across the northwestern margin of the Superior Province in Manitoba, Canada, require a major reinterpretation of the nature, structure, and location of the Superior–Trans-Hudson boundary in the area. A significant finding of this study is that an area northwest of Assean Lake, previously interpreted to be dominantly Early Proterozoic, contains ancient crust. Detrital zircon U-Pb results from two metagraywacke samples in this ancient crustal segment reveal ≥ 3.75 Ga source material, consistent with crustal residence Nd model ages (TCR) of 3.85 and 3.76 Ga, and the U-Pb zircon age for one tonalite gneiss sample is ca. 3.54 Ga (TCR of 3.63 Ga). This newly discovered pre-3.5 Ga crust provides the first evidence for the possible preservation of some very old rocks (pre-3.8 Ga) in this region, older than any known Superior Province crust.