Beyond Acasta: A new Eoarchaean terrane identified in the Slave Craton

A principal limitation of our understanding of the early Earth is the paucity of samples preserved from this time. Only around a dozen locations worldwide retain crust from the Eoarchaean period (>3.6 billion years, Ga), with controversy often surrounding the true age of these ancient vestiges of our planet. One of the best records of formation and alteration events is the mineral zircon. However, distinguishing between multiple igneous and metamorphic events experienced by a zircon population within an Archaean gneiss defines a critical question: how old is this rock?

Each new tract of ancient crust discovered carries a disproportionate significance in helping to shed light on the magmatic and geodynamic conditions shaping our planet's earliest evolution. Therefore, the utmost care must be taken when evaluating whether a terrane does host such ancient lithologies.  We present a new U-Pb, Hf isotope and trace element dataset of zircons from 16 Archaean basement gneisses from the Eokuk Uplift, northwestern Slave Craton. Preliminary work from Eokuk showed one tonalitic gneiss with a U-Pb zircon crystallisation age of 3.813 Ga and an initial εHf value of -2.5, indicating the presence of Eoarchean crust derived from the partial melting of long-lived Hadean (~4.2 Ga) protocrust (Stoian 2023, Unpublished Thesis). Further investigation by depth profiling on 53 zircons to target the rim and outer mantle of this apparently Eoarchaean sample has revealed the presence of younger 3.150 Ga igneous crystallised zones (83% with Th/U> 0.3), with 60% of depth profiles yielding only this age. This compares with 13% of zircon depth profiles with only ~3.8 Ga ages and 6% drilling through both age domains. Whilst this complicates the argument that this sample represents an unambiguously Eoarchaean rock, younger igneous recrystallisation rims on Eoarchaean zircons are frequent in lithologies interpreted to be Eoarchaean from other terranes. Of the 114 spot analyses on grain interiors from this rock, 62% are ~3.8 Ga, with just 4% being ~3.15 Ga, and the rest being too discordant for age determination. We therefore conclude that this lithology records a dominant Eoarchean-aged zircon population, with depth profiling proving a robust tool to identify subsequent recrystallisation events.  

Our geochronology study also reveals an additional four lithologies with Eoarchaean zircon cores (~3.6 – 3.7 Ga) from two distinct outcrops ~1km north of the preliminary study site. These rocks have additional igneous crystallisation ages at ~3.14, ~3.33, and ~3.43 Ga identified by combined textural and geochemical analysis. ~2.90 Ga metamorphic rims (Th/U <0.1) are identified in two of the four depth-profiled samples. These results further demonstrate that Eoarchaean crust in the Eokuk Uplift was continually reworked throughout the Palaeo- Mesoarchaean.  

Previously, Eoarchaean-aged crust was only identified in the Slave Craton from the Acasta Gneiss Complex, some 275km south of the Eokuk Uplift. This new discovery provides the strongest case to date that larger packages of Eoarchaean crust exist beyond Acasta in the northwest Slave Craton. Further exploration and detailed mapping are required to determine the extent of Earth’s most recently discovered Eoarchaean terrane.