Formation age and metamorphic history of the Nuvvuagittuq Greenstone Belt

Abstract

The Nuvvuagittuq Greenstone Belt (NGB) in Northern Quebec, Canada, is dominated by mafic and ultramafic rocks metamorphosed to at least upper amphibolite facies. Rare felsic intrusive rocks provide zircon ages of up to ∼3.8Ga (David et al., 2009; Cates and Mojzsis, 2007) establishing the minimum formation age of the NGB as Eoarchean. Primary U-rich minerals that may provide reliable formation ages for the dominant mafic lithology, called the Ujaraaluk unit, have yet to be found. Metamorphic zircons, rutiles and monazites are present in the unit and give variably discordant results with 207Pb/206Pb ages ranging from 2.8Ga to 2.5Ga. The younger ages overlap 2686±4Ma zircon ages for intruding pegmatites (David et al., 2009) and Sm–Nd ages for garnet formation in the Ujaraaluk rocks suggesting this era as the time of peak metamorphism and metasomatism in the NGB, coeval with regional metamorphism of the Superior craton. Sm–Nd data for Ujaraaluk whole rocks scatter about a Sm/Nd vs. 143Nd/144Nd correlation (MSWD=134) whose slope would correspond to 3.6±0.2Ga if interpreted as an isochron. This “isochron” is seen to consist of a series of younger ∼3.2–2.5Ga slopes for the different geochemical groups within the Ujaraaluk, emanating from a baseline distribution older than 4Ga. The 146Sm–142Nd chronometer is less affected by metamorphism at 2.7Ga because of 146Sm extinction prior to ∼4Ga. Expansion of the 142Nd dataset for the Ujaraaluk and associated ultramafic rocks continues to show a good correlation between Sm/Nd ratio and 142Nd/144Nd that corresponds to an age of 4388+15−17Ma. The dataset now includes samples with superchondritic Sm/Nd ratios that extend the correlation to 142Nd excesses of up to 8ppm compared to the terrestrial standard with a total range in 142Nd/144Nd of 26ppm. The upper Sm/Nd ratio end of the Ujaraaluk correlation is defined by rocks that are interpreted to be cumulates to compositionally related extrusive rocks indicating that this crystal fractionation had to occur while 146Sm decay was active, i.e. well before 4Ga. Intruding gabbros give 143Nd and 142Nd isochron ages of respectively 4115±100Ma and 4313+41−69Ma, also supporting an Hadean age for the gabbros and providing a minimum age for the intruded Ujaraaluk unit. 3.6Ga tonalites surrounding the NGB, 3.8Ga trondhjemitic intrusive veins, and a 2.7Ga pegmatite show a deficit in 142Nd compared to the terrestrial standard. These felsic rocks plot to the low Sm/Nd ratio side of the Ujaraaluk isochron and do not show a correlation between their Sm/Nd and 142Nd/144Nd ratios, which can be explained if they are melts of ancient LREE-enriched mafic rocks, such as the Ujaraaluk, with the melting occurring after 146Sm was extinct. A subset of least disturbed Ujaraaluk samples has coherent isotopic compositions for both short-lived and long-lived Nd isotopic systems giving 143Nd and 142Nd isochron ages overlapping within error of 4321±160Ma (MSWD=6.3) and 4406+14−17Ma (MSWD=1.0), respectively. This age represents our best age estimate for the Ujaraaluk unit. The NGB thus preserves over 1.6 billion years of early Earth history including an expanse of mafic crust formed in the Hadean.