Eoarchean to Neoarchean evolution of the Nuvvuagittuq Supracrustal belt: New insights from U-Pb zircon geochronology

Abstract

The Nuvvuagittuq Supracrustal belt in northern Québec is a rare ∼9 km² fragment of the Earth's early crust. The belt contains a metamorphosed volcano-sedimentary sequence that is at least Eoarchean in age, including amphibolites of the Ujaraaluk Unit that may have protoliths as ancient as 4400 million years old (Ma; O'Neil and others, 2012). Upper amphibolite facies metamorphism and high-strain deformation have obscured many primary field relationships and disturbed whole-rock isotope systematics, leading to debate over the interpretation of previous geochronological data. We report new SHRIMP U-Pb isotopic analyses of zircons from key meta-igneous and newly identified metasedimentary units of the belt. The analyzed samples fall into four categories: (a) felsic gneisses, including sheets interlayered with the supracrustal assemblage and a tonalitic orthogneiss at the margin of the belt; (b) a newly identified metasedimentary unit; (c) fuchsite-bearing quartz-rich layers; (d) mafic gneisses, including a garnet-biotite amphibolite from the Ujaraaluk Unit and a meta-gabbro. The felsic lithologies place constraints on the minimum age of the supracrustal assemblage; oscillatory-zoned zircons separated from a felsic orthogneiss sheet form a discordant array with an upper intercept age of 3774 ± 32 Ma, and the tonalitic orthogneiss yielded oscillatory zoned zircons with an upper intercept age of 3781 ± 11 Ma.

A metasedimentary unit, dominated by quartz, albite and clinozoisite, has been identified within a lower strain domain, interfolded with amphibolites of the Ujaraaluk Unit. A sample of this unit yielded zircons with highly variable CL structures and near concordant ages of between 3390 and 3780 Ma. Zircons from a fuchsite-bearing quartz layer have oscillatory zonation similar to the orthogneiss grains, and define an upper intercept age of 3794 ± 16 Ma. Petrologic observations and geochemical data suggest that this lithology is a metasomatically altered felsic orthogneiss band, rather than a meta-sedimentary rock belonging to the supracrustal package. Electron microscopy of polished thin sections was used to locate zircons within 6 samples of the Ujaraaluk Unit amphibolites and two samples of meta-gabbro. Large (≤500 μm) zircons separated from a garnet-biotite amphibolite, together with grains from a meta-gabbro, are sector zoned and yield ages of 2700 to 2640 Ma, recording the timing of pervasive high-temperature metamorphism in the region.

The new zircon U-Pb data add important new detail to the geological history of the Nuvvuagittuq belt. The volcano-sedimentary sequence and mafic-ultramafic intrusive rocks were formed prior to 3800 million years ago. Subsequent geological evolution included extensive and protracted early Archean TTG magmatism and crustal re-melting, Paleo- to Mesoarchean uplift and sedimentation and Neoarchean high-grade metamorphism and regional magmatism during consolidation of the Superior craton.