EARLY EVOLUTION OF MACKENZIE AND AMUNDSEN BASINS FROM DETRITAL ZIRCON GEOCHRONOLOGY

The middle to upper Mackenzie Mountains and Shaler supergroups (MMSG; SSG) were deposited in coeval, early to middle Neoproterozoic basins separated by a 500-km-wide paleopromontory in northwestern Laurentia (Northwest Territories and Yukon), as supported by litho- and chemostratigraphic correlation, biostratigraphy, and detrital zircon geochronology. The supergroups’ less well-known lower parts are addressed using detrital zircon geochronology (LA-ICP-MS; SHRIMP). Lowermost MMSG strata (Dolores Creek Fm. (YT) and Tabasco Fm. (NT)) are dominated by zircon derived from 1.7-1.5 Ga, North American magmatic gap (NAMG), and 1.9-1.8 Ga (Hudsonian) sources, with minor 1.2-1.0 Ga (Grenvillian) grains, and no material <1.0 Ga. The overlying Tsezotene Fm. (NT) contains a more significant, upward-increasing Grenvillian component; NAMG grains persist, but Archean, Paleoproterozoic, and Mesoproterozoic material diminishes. This spectrum persists into the upper MMSG. The Tarn Lake Fm. (YT) zircon spectrum is identical to that for the upper Tsezotene Formation (NT), supporting these units’ equivalence. The lowermost Shaler Supergroup (lower Escape Rapids Fm.) is dominated by Hudsonian and Archean zircon, resembles no other part of the SSG or MMSG, but is identical to the underlying Husky Creek Fm. (not part of SSG). The middle Escape Rapids Fm. is dominated by 1.7-1.4-Ga zircon with a significant Grenvillian component, closely resembling the Dolores Creek Fm., supporting these units’ proposed equivalence. The overlying Nelson Head Fm. (SSG) is dominated by Mesoproterozoic material, resembling the upper Tsezotene Fm. and Tarn Lake Fm. (MMSG), and younger MMSG units. Detrital zircon analysis supports previously proposed lithostratigraphic correlation of the lower MMSG and SSG, recording a history that includes erosion of an unknown, possibly exotic, early Mesoproterozoic source, and gradual inception of sediment delivery from the distant Grenville orogen. Detrital zircon spectra for the oldest material in the MMSG and SSG closely resemble those from uppermost Mesoproterozoic Bylot Supergroup (eastern Nunavut), suggesting that the depositional histories of otherwise unrelated epicratonic basins of Laurentia’s northern periphery overlapped during inception of Grenvillian orogenesis.