Reduced, reused and recycled: Detrital zircons define a maximum age for the Eoarchean (ca. 3750–3780 Ma) Nuvvuagittuq Supracrustal Belt, Québec (Canada)

A key discovery from the Hadean (pre-3850 Ma) detrital zircon record has been that the dichotomy of granitic and basaltic crust was established within about 160 Myr of Earth's formation (Harrison, 2009). Understanding the origin and fate of this primordial crust would greatly add to what we know about the geodynamics of the Hadean Earth. Insights emerge from 147,146Sm–143,142Nd isotope data reported from different Eoarchean terranes worldwide, including the Nuvvuagittuq Supracrustal Belt (NSB) in northern Québec. Some Ca-poor (cummingtonite-rich) amphibolites and granitoid gneisses of the NSB preserve lower 142Nd/144Nd than Bulk Silicate Earth (BSE); these also show positive correlations against 147Sm/144Nd that were used by O'Neil et al., 2008 and O'Neil et al., 2012 to assign a ca. 4400 Ma age. Alternatively, the compositions were inherited during the formation of the NSB at around 3800 Ma (Roth et al., 2013; Guitreau et al., 2013). To resolve this discrepancy, ion microprobe U–Pb ages are reported for detrital zircons from NSB meta-sediments from within the same supracrustal successions that preserve low 142Nd/144Nd. The youngest detrital zircon cores of igneous derivation define a maximum age for the NSB of ca. 3780 Ma. This age is about 600 Myr younger than that obtained from 142Nd/144Nd vs. 147Sm–143Nd regressions. Thus, just like the variable 142Nd/144Nd ratios reported for other Eoarchean terranes, non-BSE 142Nd/144Nd values of the NSB were inherited from an older component.

► Some rocks of the Nuvvuagittuq Supracrustal Belt preserve low 142Nd/144Nd.
► O'Neil et al. (2008) used this to assign a ca. 4300 Ma age to the rocks.
► Alternatively, these compositions were inherited from an ancient enriched source.
► Detrital zircon ages are ca. 3780 Ma for rocks of the Nuvvuagittuq belt.
► Anomalous Nd values of Nuvvuagittuq are unrelated to time of formation.