Petrogenetic histories of pre-3750 Ma supracrustal rocks (Akilia association, West Greenland and Nuvvuagittuq supracrustal belt, Quebec, Canada)

The study of ancient crustal terranes enlightens our understanding of the evolution of Earth systems. From mantle processes, to oceanic and atmospheric chemistry, to the origins of life, the study of the earliest terranes allows us to follow the secular changes that have affected our planet. The expansion of the study of such ancient unrelated rock sequences also establishes a more global view of early Earth's history. In this study, detailed mapping and analysis of two unrelated Eoarchean supracrustal sequences reveal an earliest rock record consistent with previously investigated Archean and Proterozoic terranes.Minimum ages of ca. 3750 Ma for mapped areas, determined via ion microprobe U-Pb zircon isotope analyses of orthogneisses that preserve intrusive relationships establish the antiquity of these sequences, and pooled [Th/U]zircon, delta 18Ozircon values, and REE patterns of older zircon populations are consonant with igneous growth in the bulk composition of the host rocks. Whereas chemical and isotopic analyses, including whole-rock geochemistry, immobile trace elements, rare earth element patterns, and Delta33 S and delta18O isotopes, reveal that metamorphosed chemical (banded quartz- magnetite, -amphibole, and pyroxene rocks) and detrital (psammitic to conglomeratic) sediments are present throughout the Akilia association (AA) and the Nuvvuagittuq supracrustal belt (NSB) though subordinate to mafic and ultramafic rocks in volume. The remarkably consistent rock assembly in the AA, NSB and other Eoarchean terranes (e.g. Isua supracrustal belt, Greenland) underscores the widespread occurrence of rocks of this kind, and any exceptional circumstances that are used to explain the origin of any of these units must also elucidate the source of similar deposits in all known Eoarchean terranes.The presence of water borne sediments coupled with an absence of any glacial deposits (e.g. drop stones) suggest that the surface of Earth in the Eoarchean was relatively warm, despite a fainter sun, most likely due to high levels of greenhouse gasses. A possible biologically mediated deposition of banded-iron formations indicates that there may have been an active biosphere prior to the earliest supracrustal rock record. The young planet was a dynamic place, where lithospheric processes drove interconnected oceanic, atmospheric and biologic cycles, much as we see today.