A NEW EOARCHEAN TERRANE IN ARCTIC CANADA - HOW EXTENSIVE IS ACASTA-LIKE CRUST?

The Slave Craton in the NWT, Canada serves as an excellent template for understanding continental evolution in the Archean. Most notable is the 4.02-2.95 Ga Acasta Gneiss Complex (Bowring et al., 1989; Stern and Bleeker, 1998), exposed on the western margin of the craton, which has been studied for nearly four decades. However, other extant basement gneiss outcrops, such as the northernmost Eokuk Uplift, are undercharacterized, with a high potential for containing older rocks.

The Eokuk Uplift is a known region of Mesoarchean crust in the Slave Craton – Emon et al. (1999) found basement gneiss protolith ages of 3.25 Ga to 2.59 Ga. Here we present new laser ablation split stream U-Pb and Hf isotope data obtained from zircon separates from 10 samples from the Eokuk Uplift region. These samples and two additional samples were also analyzed for whole rock and zircon geochemistry. The range of upper intercept ages in basement gneiss samples are 3.81 to 2.81 Ga. The weighted means of the initial ƐHf signatures in these samples range from –4.1 to –8.0, and the initial ƐHf signature for the 3.81 Ga sample is –4.1.

Samples from this region tend to be silica rich (approximately 72 wt.%), the range in Na2O content is 1.56 to 4.76 wt.%, and the range in Al2O3 content is 12.72 to 16.71 wt.%. Combined content of FeO, MnO, and MgO in zircon-yielding samples is low (generally <5 wt.%). Whole rock REE patterns reflect high LREE’s and lower HREE’s, similar to average continental crust.

The range of ages and the age vs Hf isotope systematics are very close to those of the Acasta Gneiss Complex (summary in Reimink et al., 2020). Time-integrated ƐHf evolution of the Eokuk Uplift samples show evidence for interaction with Hadean mafic crust. This similarity shows a connection between the Acasta Gneiss Complex and the Eokuk Uplift and may imply that the Acasta Gneiss Complex extends, in a fragmentary manner, for >300 km farther Northeast than currently known exposures. The Eokuk Uplift region represents an exciting new discovery of Eoarchean crust in the Slave Craton.