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• Non-zero I/(Ca + Mg) values were detected in Archean and Paleoproterozoic shallow marine carbonates. • The samples are from several locations and range in age from 2.93 to 2.31 Ga. • These non-zero values indicate the original localized presence of dissolved marine oxygen. Marine oxygenation during the Archean and Paleoproterozoic is poorly constrained. Iodine in carbonate is a possible oxygen indicator. We obtained molar ratios of iodine to calcium-magnesium [I/(Ca + Mg)] for 2.93–2.31 Ga from shallow marine Ca-carbonate (limestone, dolostone) platform sediments at several locations in the southern Superior Province in Canada and the USA. Maximum I/(Ca + Mg) ratios obtained are 0.13, 0.26, 0.05 and 0.22 μmol/mol, for 2.93, 2.86, 2.80 and 2.31 Ga samples, respectively. These non-zero carbonate I/(Ca + Mg) ratios indicate localized oxygen in Archean and Paleoproterozoic shallow marine environments. These data support an early origin for biological oxygen-production and, specifically, the idea that mid-late Archean oxygenation was sufficient to facilitate Ca-carbonate precipitation in shallow marine 'oxygen oases' adjacent to anoxic iron-rich seas [ABSTRACT FROM AUTHOR]

[Display omitted] • Late Archean granitic plutons in northeastern Superior Province have diverse sources. • Four granitic suites record transition from plume-dominated regime to plate tectonics. • The ∼2730–2700 Ma subduction-related plutonism is the major crustal growth event. • The transition between tectonic regimes occurred over a short time span of 30 My. • The transition was markedly diachronous across the Superior Province. The Superior Province in Canada represents an excellent natural laboratory to test various models of crustal growth and evaluate tectonic regimes during the late Archean. Here, we present new geochemical and geochronological data for ca. 2.8–2.7 Ga plutons intruding different lithotectonic units of the north-eastern part of the Superior Province that demonstrate a complex nature and evolution of magma sources in relation to a changing tectonic setting. Four distinct plutonic suites were newly identified: (1) sodic tonalite–diorite (TD) with a composition resembling low-pressure TTG-like melts, (2) sodic tonalite–granodiorite–diorite (TGD) with medium/high-pressure TTG-like signatures, (3) Mg–K-rich monzogranite to monzodiorite (sanukitoids; MMD), and (4) K-rich granodiorite–granite–monzogranite (GGM). These suites are interpreted as recording a temporal evolution from plume-assisted melting of lower mafic continental crust through melting of a subducted oceanic slab at different depths to large-scale re-melting of the previously formed and amalgamated crustal units. In this evolutionary scheme, the ∼2730–2700 Ma subduction-related plutonism is the most voluminous addition of the juvenile arc material and thus represents the most significant crustal growth event. Altogether, the above inferences and published data suggest that the late Archean plutonism in the Superior Province formed during a transitional regime from plume-dominated to plate-tectonic over a short time span between ∼2730 and ∼2700 Ma and that this transition was markedly diachronous across the Province. [ABSTRACT FROM AUTHOR]