The Michael Gabbro (1426 Ma) and the Shabogamo Gabbro (1459 Ma) represent two large diabase intrusive episodes that affected the northwestern margin of the northeastern Grenville Province. Both have sustained variable effects of subsequent Grenvillian metamorphism. Other broadly contemporaneous magmatic activity in the region includes the formation of Elsonian anorthosite–granitoid complexes (1.46–1.29 Ga) that intruded Churchill Province and Nain Province rocks to the north, the Harp dykes (1.27 Ga), and Mealy dykes (~1.25 Ga). Petrologic and geochemical data show that the Michael Gabbro and Shabogamo Gabbro are similar, with the latter displaying more cumulate rock characteristics, and the former having compositions closer to those of rapidly cooled magma. Both have compositions comparable to those of other continental diabases and to some continental flood basalts. Sr and Nd initial isotopic compositions of Michael Gabbro (εNd(1426 Ma) = −4.7 to −6.0, ISr(1426 Ma) = 0.7032–0.7044) and Shabogamo Gabbro (εNd(1459 Ma) = −4.0 to −5.5, ISr(1459 Ma) = 0.7020–0.7060) are alike and overlap, suggesting similar sources and processes of development. Comparisons with other nearly contemporary mafic suites in central and southern Labrador show that only the Mealy dykes have a distinctly more radiogenic isotopic signature. Paradoxically, other mafic suites that intrude older basement rocks north of the Grenville Front have less enriched Nd signatures than do those that intrude younger basement rocks south of the front. The argument is made that the subcontinental lithospheric mantle, and not crustal contamination, played the most influential role in evolution of the magmas.