The Red Lake greenstone belt, Superior Province: evidence of plume-related magmatism at 3 Ga and evidence of an older enriched source

The Red Lake greenstone belt of the Superior Province contains a number of volcano-sedimentary assemblages that range in age from similar to 2.99 Ga to similar to 2.74 Ga. The oldest of these is the 2.99-2.96 Ga Balmer assemblage which is part of the North Caribou terrane, one of the oldest portions of the Superior Province. The Balmer assemblage dominates the Red Lake greenstone belt and is significant in that it contains evidence for plume-generated magmatism, The assemblage is dominated by mafic volcanic flows with intercalated kormatiitic lavas, mafic and ultramafic sills and shallow water elastic and chemical sediments. The basalts have 5-12% MgO and mantle normalized signatures which range from slightly light rare earth element (REE)-depleted, through essentially flat, to light REE-enriched. Some lavas display slight negative Nb and Ti anomalies, indicating selective crustal contamination, The komatiites have 12-14% MgO, are the Al-undepleted type and are strongly depleted in light-REE. They have epsilon Nd values of ca -2. The geochemistry, combined with Sm-Nd isotope data, indicates that the komatiites came from a long term enriched source that was probably generated at ca 3.4 Ga. That source underwent a melting event that depleted it in light REE prior to komatiite generation. The presence of komatiites from a long term enriched source and the strong contrast in the REE profiles of the basalts and the komatiites are consistent with a plume model for the generation of the Balmer assemblage, the komatiites having originated in the plume core and the basalts from entrained upper mantle or mantle melted by the plume. (C) 1998 Elsevier Science B.V.