Sedimentary processes, provenance, and tectonic control on fluvial sandstone geochemistry during Superior craton stabilization

Abstract

ABSTRACT The Neoarchean marked an important turning point in the evolution of Earth when cratonization processes resulted in progressive amalgamation of relatively small crustal blocks into larger and thicker continental masses, which now comprise the ancient core of our continents. Although evidence of cratonization is preserved in the ancient continental cores, the conditions under which this geodynamic process operated and the nature of the involved crustal blocks are far from resolved. In the Superior craton, deep-crustal fault systems developed during the terminal stage of Neoarchean cratonization, as indicated by the cratonwide growth of relatively small, narrow, syn-to-late tectonic (ca. 2680–2670 Ma) sedimentary basins. The terrigenous debris eroded from the uplifted tectono-magmatic source regions was deposited as polymictic conglomerate and sand successions in fluvial-dominated basins. The composition of the sedimentary rocks in these unique basins, therefore, offers a unique record of crustal sources and depositional settings, with implications for the geodynamic processes that formed the world’s largest preserved craton. Here, we compare the geochemical compositions of sandstone samples from six sedimentary basins across the Abitibi greenstone belt and relate them to their mode of deposition, prevailing provenance, and geodynamic setting during crustal growth and craton stabilization. The sandstones represent first-cycle sediment that is poorly sorted and compositionally very immature, with variable Al2O3/TiO2 ratios and index of chemical variability values >1 (average of 1.36), reflecting a large proportion of framework silicate grains. The sandstones display chemical index of alteration values between 45 and 64 (average of 53), indicating that the detritus was eroded from source regions that experienced a very low degree of chemical weathering. This likely reflects a high-relief and active tectonic setting that could facilitate rapid erosion and uplift with a short transit time of the detritus from source to deposition. Multi-element variation diagrams and rare earth element patterns reveal that the lithological control on sandstone composition was dominated by older (>2695 Ma) pretectonic tonalite-trondhjemite-granodiorite and greenstone belt rocks. The sandstone units display large variations in the proportions of felsic, mafic, and ultramafic end-member contributions as a consequence of provenance variability. However, an average sandstone composition of ~65% felsic, ~30% mafic, and ~5% komatiite was observed across the basins. This observation is in agreement with recent models that predict the composition of the Neoarchean emerged continental crust for North America and supports the presence of a felsic-dominated Archean crust. The high proportion of felsic rocks in the upper crust requires continuous influx of H2O into the mantle and is best explained by subduction-related processes. In such a scenario, the detritus of the fluvial sandstones is best described as being controlled by uplifted and accreted continental arcs mainly composed of tonalite-trondhjemite-granodiorite and greenstone belt rocks.