Archean Nb-enriched basalts in the northern Superior Province

Geochemical data from the ~ 2.9-Ga Northern Pickle assemblage of the Pickle Lake greenstone belt in the northern Superior Province distinguish three distinct volcanic suites. Suite I, from the north of the assemblage, comprises tholeiitic basalts with unfractionated rare earth elements (REE). Suite II basalts are light REE (LREE)-enriched with negative mantle-normalized Nb anomalies relative to Th and La, similar to the CA suite of Sajona et al. [J. Petrol. 37 (1996) 693]. Basalts from Suite III are also LREE-enriched but lack negative Nb anomalies. Normalized Nb abundances are either similar to or higher than Th and La, comparable to Nb-enriched basalts (NEB) from Phanerozoic arcs. Variable high field strength elements and heavy REE (HREE) systematics support the subdivision of Suite III basalts into two spatially distinct suites. Suite IIIa is interpreted to have melted at relatively shallow depths in the presence of Nb-Ti-bearing silicates whereas Suite IIIb melted in the presence of garnet. Variations in the geochemistry of the three suites can be accounted for by interaction among primitive mantle, adakite melts, and subduction-modified mantle. The association of the three suites is interpreted to be the result of rifting in a back-arc environment, characterized by Suite I tholeiites, permitting asthenospheric upwelling of subduction-modified NEB and associated arc-like volcanic rocks (Suites II and III). The association of arc-like basalts (Suite II) with NEB (Suite III) in the Northern Pickle assemblage extends the known occurence of Archean NEB beyond the ~ 2.7-Ga age of previously recognized examples. © 2002 Elsevier Science B.V. All rights reserved.