Lu-Hf Isotope Systematics of the Nuvvuagittuq Supracrustal Belt (Québec, Canada)

The Nuvvuagittuq supracrustal belt (NSB) in northern Québec (Canada) is a mafic terrane enclosed in the westernmost part of the Minto Block of the Superior Province. This locality became famous after the discovery by [1] of 142Nd deficits in the constituent amphibolites. The age of the NSB is a matter of debate because zircon U-Pb and whole-rock 147Sm-143Nd isotope systematics give Eoarchean ages of ~3.8 Ga, while the short-lived 146Sm-142Nd chronometer indicates a Hadean age of ~4.3 Ga. We present new Lu-Hf isotope data for mafic and felsic rocks from the NSB in an attempt to resolve this age issue and, hence, understand the origin of the negative 142Nd anomalies. The Lu-Hf data define a "scatterchron" yielding an Eoarchean age of 3864±70 Ma, which is consistent with 147Sm-143Nd and U-Pb zircon ages but in disagreement with the Hadean age inferred from 146Sm-142Nd systematics. We interpret the Lu-Hf age as the mean emplacement age of the different autochthonous units of the NSB. The observed alignment of the data along a Lu-Hf "scatterchron" precludes a Hadean age for the NSB because their isotopic characteristics appear to be controlled by long-term radiogenic ingrowth. A Hadean derivation should have caused age differences of hundreds of millions of years to manifest as strong deviations from the observed scatterchron. Furthermore, combined Lu-Hf and Sm-Nd data on the same NSB (Ca-poor cummingtonite- and hornblende-bearing) amphibolite samples define a mixing hyperbola at ca. 3800 Ma with end-member compositions representative of compositional groupings identified for these lithologies [2]. Low 142Nd/144Nd values relative to Bulk Silicate Earth are endemic to the "low-TiO2" amphibolite population, and can be attributed to a Hadean multi-stage history of its mantle source as indicated by rare-earth element patterns. The 142Nd deficits could have developed in response to a later re-fertilization episode within a mantle source depleted by primordial crust extraction at ca. 4500 Ma. A candidate location for preventing re-fertilized mantle domains from being stirred back into the "regular" mantle for ~500 Myr is continental lithosphere. We therefore surmise that the NSB formed at Eoarchean time in an evolving rift setting corresponding to a back-arc basin, which would account for the tapping and mixing of distinct mantle sources. [1] O'Neil, J., Carlson, R. W., Francis, D. & Stevenson, R. K., 2008. Science 321, 1828-1831. [2] O'Neil, J., Francis, D. & Carlson, R. W., 2011. J. Petrol. 52, 985-1009.