THE FORMATION OF THE SOUTH-EAST SUPERIOR CRATON BY TWO DISTINCT MECHANISMS

The formation of the continental crust, its timing, and the geodynamics that drove it, are of considerable importance in understanding the evolution of our planet and its progressive drive to become a habitable ecosystem. However, constraining the nature of geodynamics in the Archean, and the transition to modern-style tectonics, remains elusive. Here, we combine U-Pb geochronology, whole-rock geochemistry, Sm-Nd isotopes and in-situ zircon U-Pb-Hf-O-trace element data from across the Archean south-east Superior Craton, and apply this dataset spatially, and temporally, to investigate its time-space evolution. At >2695 Ma, the central and north-west Abitibi demonstrate more juvenile εHf, light-mantle-like δ¹⁸O, lower (Eu/Eu*)/Y*10000 (drier/shallower crust), reduced ΔFMQ, and less continental, more mantle-like, U_i/Yb and U_i/Nb, respectively, relative to surrounding crust, which contains older, ca. 2800-2750 Ma zircon ages. Furthermore, whole-rock Sr/Y and La/Sm demonstrate the presence of a High Sr/Y TTG component (mainly intrusive) surrounding zones of Low Sr/Y (mainly volcanic) component, the latter of which shows contamination trends with Mesoarchean crust. We interpret this to represent a continental-rift setting, driven by plume magmatism as represented by multiple komatiite suites. At ca. 2704-2695 Ma, there is a marked transition in multiple datasets; increases in δ¹⁸O, (Eu/Eu*)/Y*10000, ΔFMQ, and continental U_i/Yb and U_i/Nb data, together with more distinct arc-like trace element trends, suggest transition to north-dipping subduction. This process closed the rift system and initiated orogenesis. Crustal growth models indicate the need for a different geodynamic setting in the early Earth, with a transition at some time in the Meso-Neoarchean. We suggest that, not only does this study constrain the geodynamic setting which produced the majority of Earth’s crust, but also the timing at which it transitioned to subduction. This transition, via increased oxidation of melts and burial of carbon, would have had a significant effect on the Earth’s oxygen budget.