The Precambrian mafic magmatism of the Northeastern Superior Province, Canada

Major, trace and rare earth elements (REE), along with Nd isotopes and U-Pb ages, were determined for Archean greenstone belts (2.88-2.72 Ga) and Paleoproterozoic mafic dyke swarms (2.51-2.00 Ga) of the Northeastern Superior Province (NESP) of Canada in order to gain insights on the nature of mafic magmas across the Precambrian. Geographically separate, but coeval (2.78 Ga), greenstone assemblages with distinctive Fe-tholeiites occur across a large surface area of the NESP. This geographic separation, along with the numerous inheritance ages of greenstone lithologies that replicate those of older volcanic-plutonic assemblages, are consistent with these belts being the remnants of an extensive autochthonous mafic volcanic cover sequence. An increase of Th/Nb ratios in light REE-enriched basalts reflects a change in the nature of coeval crustal contaminants, from tonalite-trondhjemite prior to 2.75 Ga, to granite-granodiorite afterwards. The isotopically-enriched character of these magmas after 2.75 Ga implies more extensive contamination by a felsic crust affected by regional partial melting, succeeding the amalgamation of the Hudson Bay (TDM 4.3-3.1 Ga) and Riviere Arnaud (TDM < 3.1 Ga) terranes.;Paleoproterozoic mafic dyke swarms follow the stabilization of the Archean crust. The magmas of most dykes are sourced in fertile lherzolite at ∼1.5 GPa, but others are either sourced from deep (∼5 GPa) lherzolite or pyroxenite, Fe-rich mantle, or harzburgite. The presence of all dyke types within the older Hudson Bay Terrane reflects the heterogeneous nature of its lithospheric mantle. All contain a crustal component in the form of an increase in La/Yb with Zr/Nb ratios, and negative &egr;Nd values. In contrast, voluminous 2.2 Ga dykes of the Riviere Arnaud Terrane display an increase in La/Yb with a decrease in Zr/Nb ratios, and positive &egr;Nd values indicative of an alkaline mantle component similar to that of Paleoproterozoic alkaline rocks recognized across the NESP. Despite their 2.0--1.9 Ga ages, these alkaline rocks and coeval basaltic belts collectively display ca. 2.2 Ga Nd model ages that are identical to the numerous 2.2 Ga mafic dykes. This time correlation suggests that the composition of the mantle was modified by a pervasive metasomatism associated with the emplacement of the 2.2 Ga dykes. The secular variation of Archean and Paleoproterozoic mafic rocks of the NESP shows that their composition has been controlled by the nature of the crust and lithosphere they intruded.