Geochemistry and petrogenesis of the early Archean mafic crust from the Saglek-Hebron Complex (Northern Labrador)

Abstract

The Saglek-Hebron Complex located in Northern Labrador, Canada, is one of the oldest granite-greenstone terrains on Earth. It is dominated by granitoids as old as 3.9 Ga and several enclaves of supracrustal rocks. We present here the largest whole-rock major and trace element geochemical dataset (over 100 samples) on the mantle-derived rocks from the Saglek-Hebron Complex to constrain their petrogenesis. The mafic amphibolites are metavolcanic rocks basaltic in composition, have tholeiitic affinities, and exhibit relatively homogeneous compositions with flat rare-earth element profiles. They include incompatible element enriched and depleted compositions, both which appear to be related by fractional crystallization of a gabbroic assemblage within volcanic flows. The enriched mafic rocks exhibit higher Ti contents compared to the more depleted mafic rocks, representing, respectively, evolved liquids and pyroxene-rich cumulate-liquid mixtures. Previous work has suggested the occurrence of two distinct supracrustal units in the Saglek-Hebron Complex, the Eoarchean Nulliak assemblage and the Mesoarchean Upernavik assemblage. Here we report that the petrological and geochemical composition of both assemblages is indistinguishable. This suggests either that the Nulliak and Upernavik metavolcanic rocks represent a single unit, or that they were formed by identical processes in a comparable context, despite an age difference of ∼400 Ma. Contrary to some other early metavolcanic belts, such as the Isua supracrustal belt and Nuvvuagittuq greenstone belt, none of the metavolcanic rocks from the Saglek-Hebron Complex studied here exhibit geochemical compositions that could be reminiscent of suprasubduction environments. The Saglek-Hebron Complex also includes two compositionally distinct groups of ultramafic rocks characterized by different Al/Ti ratios, Fe contents and controlled by olivine fractionation with different Fo contents. Ultramafic rocks from both groups appear to be cumulates derived from distinct parental magmas with a komatiitic basalt composition. One of these parental magmas may be genetically linked through fractional crystallization with the mafic metavolcanic rocks, whereas the other group is more difficult, at least geochemically, to relate to the Saglek-Hebron basaltic rocks.