Tectonic, stratigraphic, and geochemical comparisons between ca. 2500–2440 Ma mafic igneous events in the Canadian and Fennoscandian Shields

Abstract

Palaeoproterozoic mafic igneous rocks (2500–2440 Ma) are exposed in the form of layered intrusions, dykes, and volcanic rocks in the southern Canadian Shield and the Fennoscandian Shield. Both igneous events were associated with episodes of continental rifting. The tectonic and regional geological evolution of the two regions during the Palaeoproterozoic are strikingly similar. Further comparisons of the mineralogical and stratigraphic characteristics of the layered intrusions—the East Bull Lake suite in the Canadian Shield and the Fennoscandian suite in the Fennoscandian Shield—show additional similarities. Earlier investigations have identified possible parental magmas from mafic dykes for each of the layered intrusion suites. These include the so-called Streich dykes associated with the East Bull Lake suite and the Viianki dykes with the Fennoscandian suite. The Viianki dykes are more primitive in composition (Mg#=0.66–0.78) relative to the Streich dykes (Mg#=0.62–0.65), and probably represent primary mantle-derived magmas. Geochemical modelling suggest that these two parental magmas cannot be related through fractional crystallization. The differences in REE characteristics (La/Yb CN=3.6–9.1 for Viianki dykes; 3.6–3.7 for Streich dykes) and selected incompatible element ratios can, however, be accounted for by either differential amounts of upper crustal contamination, or different degrees of partial melting from a similar garnet-bearing mantle source. The data suggest that the Viianki dyke parental magmas may have been produced by a degree of partial melting that was more than a factor of two lower than that which gave rise to the more primitive parent of the Streich dyke magmas. We conclude that these early Palaeoproterozoic igneous events in the southern Canadian Shield and the Fennoscandian Shield could have been derived from a similar mantle source that was characterized by partial melt extraction of AMORB and enrichment by `subduction-like', LREE-enriched fluids during the Archaean.