Trace element systematics of ultramafic and mafic volcanic rocks from the 3 Ga North Caribou greenstone belt, northwestern Superior Province

Abstract

High precision trace element data from ultramafic and mafic volcanic rocks from two units of the ∼3 Ga North Caribou greenstone belt in the northwestern Superior Province reveals new evidence for the contamination of early Archæan komatiites. Spinifex-textured komatiites (MgO=20−-27 wt.%) of the Opapimiskan–Markop unit are characterised by flat to moderately enriched light rare earth element (LREE) trace element patterns (La/Sm n=1–2) in conjunction with negative Nb and Ti anomalies (Nb/La pm=0.48–0.59; Ti/Ti*=0.6–0.8), consistent with contamination of a primary Munro-type Al-undepleted ultramafic melt by felsic material compositionally similar to Archæan tonalitic-trondhjemitic-granodioritic (TTG) suites. The trace element characteristics of these komatiites are distinct from both Proterozoic and Phanerozoic arc and rift picrites. Mg-tholeiites intimately associated with the komatiites display variable rare earth elements (REE) (La/Sm n=0.8–1.5) and Nb and Ti anomalies also consistent with variable degrees of contamination (Nb/La pm=0.58–0.98; Ti/Ti*=0.6–1.0). An intermediate suite of high-Mg andesites (54–60 wt.% SiO 2) associated with both the tholeiites and the komatiites can be derived by fractional crystallisation of a melt with the trace element characteristics of the most contaminated komatiites. Compositionally uniform Mg- to Fe-tholeiites of the South Rim unit are believed to overlie the Opapimiskan–Markop unit, possibly in stratigraphic contact. They are characterised by flat trace element patterns (La/Sm n=0.9–1.0), are uncontaminated, and comparable with modern day oceanic plateaux, such as Ontong Java. The two units likely represent the eruptive products of a single mantle plume that entrained tholeiitic melts from the upper mantle before impinging upon the Archæan crust at ∼3 Ga.