THE NUVVUAGITTUQ GREENSTONE BELT: A CASE OF HADEAN SUBDUCTION?

The long-lasting question of when modern-style subduction started on Earth is still far from resolved. Time estimates for establishment of plate tectonics similar to those operating today range from the Proterozoic to the Hadean. Most of the juvenile continental crust today is formed in convergent margin settings and it raises the question of whether the primitive crust was formed in similar environments. The Nuvvuagittuq Greenstone Belt (NGB) provides a glimpse of what may have been Earth’s primitive crust. Thin intrusions of trondhjemite put a minimum age of ~3.8 Ga on the NGB. The ¹⁴²Nd/¹⁴⁴Nd vs. Sm/Nd correlation of the dominant mafic lithology, called the Ujaraaluk unit, however suggests an igneous age between 4.3 and 4.4 Ga that would make the NGB the only remnant of Hadean crust preserved. The composition of the Ujaraaluk unit ranges from basalt to andesite and can be divided into three geochemically distinct groups following a chemostratigraphy. The base is characterized by high-Ti basalts of tholeiitic affinities, whereas the rocks at the top of the stratigraphy have low Ti contents and show compositional trends consistent with fractionation at elevated water pressures. The low-Ti Ujaraaluk have incompatible trace element compositions similar to modern-day boninites and calc-alkaline lavas. The chemical stratigraphy in the NGB closely matches that observed in the modern Izu-Bonin-Mariana forearc, suggesting similar petrological processes involved in its formation. Regardless of the igneous age of the NGB, the Ujaraaluk compositions suggest that some process similar to subduction, if not subduction itself, was active as early as the Hadean or Eoarchean. The NGB mafic rocks are surrounded and intruded by multiple generations of Eoarchean TTG. Deficits in ¹⁴²Nd compared to the terrestrial Nd standard have been measured in a number of Eoarchean and Neoarchean TTG surrounding the NGB area. Because ¹⁴²Nd anomalies can only be generated during the Hadean, this implies that crustal reworking of Hadean crust occurred for at least a billion years, from 3.8 Ga to 2.7 Ga within the Northeastern Superior Province. The isotopic data also suggests a long quiescence period of over half a billion years (from ~4.3 to 3.8 Ga) before the mafic primitive crust began to be extensively reworked to produce widespread TTG magmatism.