Pre-3750 Ma supracrustal rocks from the Nuvvuagittuq supracrustal belt, northern Québec

Geochemistry and U-Pb ion microprobe zircon geochronology guided by high-resolution mapping (1:50 scale) was used to define a minimum age of ca. 3750 Ma for supracrustal rocks from the Nuvvuagittuq supracrustal belt (NSB) in the northern Superior Province, Québec (Canada). Mineralogy and geochemistry of critical field relationships preserved at the Porpoise Cove locality describe a supracrustal succession of (mafic) amphibolites and ultramafic rocks, finely banded quartz-magnetite units with intermixed coarse-grained ferruginous quartz-pyroxene rocks and quartz-biotite schists that superficially resemble polymict meta-conglomerates with large (up to 10 cm) deformed polycrystalline quartz and mafic clasts. All units in the mapped outcrop have sharp lithological contacts. Narrow (dominantly trondhjemitic) orthogneiss sheets locally preserve intrusive contact relationships to the supracrustals. The total extent of the supracrustal enclave is ∼ 8 km2; it is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions. The quartz-biotite schists record complex zircon growth at ∼ 3500 and ∼ 2800 Ma, interpreted to reflect metamorphic events. Zircons separated from orthogneisses in the enclave – including one sheet that transects a banded quartz-pyroxene (± magnetite) unit – yield magmatic 207Pb/206Pb ages of ca. 3750 Ma. These ages are slightly younger than earlier provisional reports for an NSB orthogneiss from Porpoise Cove. The Nuvvuagittuq supracrustals are the oldest rocks thus far reported for the Minto Block, they overlap in age with the ca. 3.70–3.83 Ga Isua supracrustal belt and Akilia association rocks in West Greenland, and they represent an important new area for exploration of the early Earth.