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

Abstract

The Nuvvuagittuq Supracrustal belt in northern Que ´bec is a rare ϳ9 km 2 fragment of the Earth's early crust. The belt contains a metamorphosed volcanosedimentary 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 m) 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 hightemperature 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 remelting, Paleo-to Mesoarchean uplift and sedimentation and Neoarchean high-grade metamorphism and regional magmatism during consolidation of the Superior craton.

Atmospheric record in the Hadean Eon from multiple sulfur isotope measurements in Nuvvuagittuq Greenstone Belt (Nunavik, Quebec)

“…S1) (17)(18)(19)(20)]. These age relationships are consistent with the inferred intrusive relationships among the throndhjemite bands, the gabbro sills, and the amphibolite (15).…”

Protocrustal evolution of the Nuvvuagittuq Supracrustal Belt as determined by high precision zircon Lu–Hf and U–Pb isotope data

“…Emplacement at 3772+5/−3 Ma of this felsic, presumably volcanic unit provides firm time constraints on the formation of (at least parts of) the NSB that was formed synchronously. This age is in disagreement with the minimum age proposed by Darling et al (2013) for the NSB of 3800 Ma. The latter authors claimbased on the observation of Cates and Mojzsis (2007) that one felsic band similar to that from which 1091F was sampled, had intrusive relationships to mafic rocks of the NSB -that also the felsic schist dated here and by David et al (2009) is of intrusive origin despite the observations reported in David et al (2009) of this felsic schist being concordant with surrounding mafic and metasedimentary rocks.…”

“…Several whole rock studies have been conducted in the NSB, but as the NSB was metamorphosed at ca. 3.66, 3.36 and 2.70 Ga (Cates and Mojzsis, 2009;David et al, 2009;O'Neil et al, 2012O'Neil et al, , 2013Darling et al, 2013), the isotope systems have been disturbed, as was recently shown by Roth et al (2013). Analysis of Lu-Hf in zircon provides a way around such problems because unrecrystallised zircon preserves the initial Hf-isotopic composition of its igneous host rock even through high temperature metamorphism that may lead to significant Pb-loss (Kinny et al, 1991;Kinny and Maas, 2003;Gerdes and Zeh, 2009;Lenting et al, 2010;Guitreau and Blichert-Toft, 2014).…”

“…Guitreau et al (2013) interpreted the positive correlation between 142 Nd/ 144 Nd and 147 Sm/ 144 Nd ratios, that was used to calculate the Hadean "scatterchron" (O'Neil et al, 2011), to represent an inherited isotopic signature from a mantle reservoir that had experienced fractionation and subsequent re-fertilisation in the early Hadean. U-Pb geochronology from different TTG rocks interpreted to be intrusive and/or volcanic have indicated formation of the NSB at or before 3750-3880 Ma (Cates and Mojzsis, 2007;David et al, 2009;Darling et al, 2013;O'Neil et al, 2013) and detrital zircons from what has been interpreted as metasedimentary units of 3780 ±22 Ma, show that the oldest zircon-bearing units of the NSB are most likely not older than 3800 Ma (Cates and Mojzsis, 2007;David et al, 2009;Cates et al, 2013). Later, laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) U-Pb and Lu-Hf isotope data from TTGs of the NSB reported by O'Neil et al (2013) were interpreted to indicate the presence of a Hadean crustal source in the host magma to the zircons, again taken as evidence for the presence of Hadean crust in the NSB.…”

Potassic, high-silica Hadean crust

“…2 and Dataset S1). These zircons are drawn from two NSB rocks: NSB11 is a green fuchsitic quartzite unit ∼20 cm wide, found within amphibolite units and similar to units Aqf of Cates and Mojzsis (19) and N09-28 of Darling et al (23). Unit NSB14 is a gray, fine-to medium-grained tonalitetrondhjemite-granodiorite (TTG) orthogneiss similar to units Ag of Cates and Mojzsis (19), ∼50 cm wide and intruding amphibolite units.…”