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• Ca. 3.75 Ga gneiss developed a gneissosity prior to ca. 3.3 Ga. • Ferroan, calc-alkaline ca. 3.3 Ga granitoids formed by remelting of Eoarchean crust. • Ca. 3.3 Ga granitoids were intruded by ca. 2.7 granite and by 2.5 Ga granitic dyke. • The magmatic history of the Saglek Block is similar to that of the SW Greenland. The Saglek Block in the Nain Province of northeast Canada is part of the North Atlantic Craton. It comprises Archean gneisses that record magmatic and metamorphic ages between ca. 3.9 and 2.5 Ga. In this study, a grey banded gneiss from Maidmonts Island records an age of 3.72 Ga, which is equated with Uivak I gneisses reported from across the Saglek Block, and contains 3.8 Ga xenocrysts. These rocks were deformed and metamorphosed prior to the intrusion of augen gneiss protoliths on both Maidmonts and Mentzel islands that record U-Pb zircon ages of 3.33 Ga. These rocks are composed of ferroan calc-alkaline granite and granodiorite and were likely generated by partial melting of pre-existing quartzo-feldspathic crust, as attested by the presence of ca. 3.8 Ga xenocrysts. Such augen gneiss was previously classified as ca. 3.6 Ga 'Uivak II gneiss', a term we argue should now be abandoned. The final magmatic events recorded on Maidmonts and Mentzel islands took place in the Neoproterozoic with the emplacement of granitic stocks at ca. 2.72 Ga, and sills and dykes at ca. 2.57 Ga. This sequence of magmatic events from the Eoarchean to Neoarchean is very similar to that recorded in the Itsaq Gneiss Complex of southwest Greenland, where, based on the Hf signature, it has been suggested that a change in tectonic environment resulted from the initiation of subduction at ca. 3.2 Ga. Although Lu-Hf and Sm-Nd isotopic signatures from the ca. 3.33 Ga gneisses have been interpreted in the literature as due to partial melting of Hadean mafic crust, alternatively, they can be generated by partial melting of Eoarchean continental crust in a continental arc setting. We argue this is more consistent with the non-TTG geochemistry of the augen gneisses. [ABSTRACT FROM AUTHOR]

• 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]