Archean and Proterozoic Age Distributions in Detrital Zircons Recovered from Pleistocene Esker Systems in the Acasta Gneiss Complex, NWTWednesday, November 21, 2018 - 11:20 to 11:39 Theatre 1
Bedrock terranes in excess of 3.7 Ga are exceedingly rare and contain information critical to further understanding the early Earth during its initial stages of differentiation and crust formation. The AGC (Acasta Gneiss Complex: 2.94 – 4.03 Ga) is one of only a handful of such Hadean/Eoarchean bedrock terranes worldwide. Despite its importance, the AGC is heavily under-mapped and under-explored. We exploit a Pleistocene esker system that traverses the AGC to efficiently sample fine-grained sediments derived from this complex gneiss terrane. Zircons extracted from these sediments are used to assess the relative proportion of different age bedrock units exposed at the surface.
The esker systems are composed of locally sourced AGC and regional bedrock that have been eroded, entrained, and deposited by the Laurentide Ice Sheet during the last glaciation. Detrital zircons, recovered from the sediment and dated by laser-ablation inductively-coupled-plasma mass-spectrometry, provide a mixed population of U-Pb ages representative of bedrock found up-ice-flow from the esker sampling sites. Zircons were recovered from two discrete grain size fractions, 1 – 50 mm and less than 250 µm, to assess the impact of grain size on sediment transport distance in esker systems.
At each sampling site along the esker transect, the detrital zircon U-Pb dates for the two grain size fractions yielded similar modes and abundances, with subtle differences related to grain size and source proximity. Prominent modes in age distributions from the samples coincide with known ages of currently mapped bedrock in the AGC, Wopmay Orogen, and Slave Craton. A mode of ~ 3.37 Ga is present in all samples, and is the dominant mode in two of the three westernmost samples. Recent high-resolution bedrock mapping of a small part of the AGC has identified km-sized plutonic bodies of this age. The prominence of this age population in the esker sediments strongly suggests that 3.37 Ga plutonism is widespread in the region. The easternmost sample from the esker transect was collected outside of the mapped AGC boundary, yet contains an abundance of zircons from 2.94 - 3.95 Ga. Given west-trending esker flow directions, the AGC likely extends further east than previously mapped, and undiscovered outcrops in excess of 3.7 Ga remain.