Evidence for a widespread mafic cover sequence and its implications for continental growth in the Northeastern Superior Province

Abstract Archean greenstone belts of the Northeastern Superior Province (NESP) were emplaced over a 160 Ma period (2.88–2.72 Ga), spanning a major episode of crustal reworking in which early tonalite–trondhjemite plutonism evolved to dominant granite–granodiorite and pyroxene-bearing felsic plutonism. The numerous greenstone belts that crop up across the craton contain lavas belonging to three mafic volcanic suites: (1) Mg-tholeiites that have chemical compositions typical of many Archean basalts, with 4–10 wt.% MgO, 9–15 wt.% Fe 2 O 3 t, 0.4–1.2 wt.% TiO 2 and La/Sm ratios between 1 and 3. (2) Fe-tholeiites that have similar MgO contents and La/Sm ratios, but markedly higher Fe 2 O 3 t (11–20 wt.%), TiO 2 (1.0–2.6 wt.%), and Gd/Yb ratios that may reflect derivation from a distinct garnet-bearing mantle source. (3) Light rare earth element (LREE)-enriched rocks that have the chemical characteristics typical of ‘calc-alkaline’ mafic magmas, with higher SiO 2 , Al 2 O 3 and La/Sm ratios, but lower Fe 2 O 3 t and TiO 2 than the tholeiitic suites at a given MgO content. The presence of unconformities within single greenstone belts, numerous inheritance ages in volcanic rocks replicating those of older volcanic–plutonic assemblages, and the recognition of widespread, geographically separate, yet coeval (2.78 Ga) extrusive assemblages containing the distinctive Fe-tholeiites, do not support a plate tectonic model for a large portion of the NESP. The correlations in composition and ages of greenstone belts are more consistent with a model in which they represent the remnants of an extensive autochthonous mafic cover sequence. Nd isotopic evidence for the presence of evolved crust in some syn-volcanic felsic plutons and volcaniclastic rocks imply that this mafic cover sequence was erupted onto an older granitoid crust. The amalgamation of two isotopically distinct terranes at ca . 2.76–2.74 Ga created a proto-cratonic mass that consisted of a composite tonalite–trondhjemite–greenstone crust. Underplating by mafic mantle melts and widespread insulation after 2.75 Ga increased temperatures sufficiently to produce extensive melting of this crust to generate pyroxene-bearing felsic rocks and granodiorite to granite partial melts. The isotopically enriched character of mantle-derived magmas after 2.75 Ga may reflect more extensive contamination by a felsic crust affected by widespread partial melting. Furthermore, a decrease of Nb/Th ratios in the LREE-enriched mafic rocks younger than 2.75 Ga reflects a change in the nature of the crustal contaminant, from tonalite–trondhjemite prior to 2.75 Ga, to granite–granodiorite afterwards.