Komatiite–basalt–rhyolite volcanic associations in Northern Superior Province greenstone belts: significance of plume-arc interaction in the generation of the proto continental Superior Province

Abstract

Archean greenstone belts from the northern Superior Province, ranging in age from 3.0 to 2.9 Ga, comprise an association of komatiite–tholeiite sequences, some of which were erupted onto continental crust, intercalated with felsic volcanic rocks. Komatiites from the Uchi subprovince are characterised by variable LREE enrichment and negative Nb anomalies consistent with contamination of Munro-type Al-undepleted komatiites by a felsic melt. The majority of komatiites display variable HREE patterns and positive normalised V–Sc anomalies. Tholeiitic flows intercalated with the komatiites typically have flat rare earth element (REE) patterns with variable Nb anomalies and high Th/Ce ratios, consistent with minor degrees of contamination by sialic crust. Two suites of felsic volcanic rock, Type 1 and Type 2, have been recognised intercalated with the komatiite–tholeiite sequences. Both types display LREE enriched patterns and negative Nb and Ti anomalies, but Type 1 has strongly fractionated HREE patterns, whereas Type 2 HREE are generally flat and have elevated Ni and Cr contents. The predominant Type 1 rhyolite is directly comparable to southern Superior Province felsic volcanic rocks associated with oceanic arc sequences and inferred to be the products of oceanic slab melting. Type 2 rhyolite geochemical signatures may result from mixing of Type 1 rhyolites with tholeiitic magmas, or a contribution from mantle wedge sources located above the garnet stability field. The intercalation of komatiites and arc-type felsic volcanic rocks is best explained by interaction between an ascending mantle plume and adjacent subduction zones.