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

High precision trace element data from ultramafic and mafic volcanic rocks from two units of the similar to 3 Ga North Caribou greenstone belt in the northwestern Superior Province reveals new evidence for the contamination o f early Archaean komatiites. Spinifex-textured komatiites (MgO = 20--27wt.% ) of the Opapimiskan-Markop unit are characterised by flat to moderately en riched 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 A rchaean tonalitic-trondhjemitic-granodioritic (TTG) suites. The trace eleme nt characteristics of these komatiites are distinct from both Proterozoic a nd Phanerozoic are and rift picrites. Mg-tholeiites intimately associated w ith 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 con tamination (Nb/La-pm = 0.58-0.98; Ti/Ti* = 0.6-1.0). An intermediate suite of high-Mg andesites (54-60 wt.% SiO2) associated with both the tholeiites and the komatiites can be derived by fractional crystallisation of a melt w ith the trace element characteristics of the most contaminated komatiites. Compositionally uniform Mg-to Fe-tholeiites of the South Rim unit are belie ved to overlie the Opapimiskan-Markop unit, possibly in stratigraphic conta ct. They are characterised by flat trace element patterns (La/Sm-n = 0.9-1. 0), are uncontaminated, and comparable with modern day oceanic plateaux, su ch 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 Archaean crust at similar to 3 Ga. (C) 1999 Else vier Science B.V. All rights reserved.