The formation of Neoarchean continental crust in the south-east Superior Craton by two distinct geodynamic processes

Abstract

The formation of the continental crust in the early Earth, and the geodynamics that drove it, are fundamental to understanding the evolution of our planet, but remain intensely debated. Here, we analysed 148 archived zircon separates of magmatic rocks for in-situ zircon U-Pb-Hf-O-trace element data, and compiled published geochronological, whole-rock geochemical and Sm-Nd isotopic data from across the south-east Superior Craton, Canada. We combine these data spatially and temporally to investigate the crustal evolution of this part of the craton in the Neoarchean, with a view to understanding its tectonic setting. In terms of zircon data, at >2704–2695 Ma, the central and north-west Abitibi demonstrate more juvenile εHf, light to mantle-like δ18O, lower (Eu/Eu*)/Y (drier/shallower crust), reduced ΔFMQ, less continental initial-U (Ui)/Yb, and more mantle-like Ui/Nb, relative to surrounding crust, which contains older, ca. 2800–2750 Ma inherited and magmatic 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, including; increases in δ18O, (Eu/Eu*)/Y, ΔFMQ, Ui/Yb and Ui/Nb data, together with more distinct arc-like trace element trends, suggesting a transition to north-dipping subduction. This process closed the rift system and initiated orogenesis. Subsequently, this study constrains the geodynamic setting which formed the majority of Neoarchean continental crust in the south-east Superior Craton, and the timing at which it transitioned to subduction. If these findings are replicated in other cratons, it suggests that plate tectonics was active by, or started at, ca. 2.7 Ga.