Enriched komatiitic basalts from Newton Township, Ontario: their genesis by crustal contamination of depleted komatiite magma

Abstract

LREE-enriched komatiitic basalts are commonly found in association with LREE-depleted komatiites. This association is found in a sequence of late Archaean lavas from Newton Township, Ontario. The komatiitic lavas at Newton Township differ from most late Archaean examples in that both the komatiites and the komatiitic basalts are depleted in Al and in the HREE. The close association and distinctive Al and HREE depletions of the two lava types strongly suggest a common origin, despite their contrasting LREE patterns.A model is proposed whereby the LREE-depleted komatiites represent the parental magma to the LREE-enriched komatiitic basalts, the two being linked by a combination of crystal fractionation and crustal assimilation. The composition of the contaminant is estimated by comparing the LREE-enriched komatiitic basalts with the evolved part of a thick layered komatiite flow that has similar major element chemistry. The contaminant composition coincides closely with Taylor & McLennan's estimate of the composition of the Archaean upper crust. It is concluded that LREE-enriched komatiitic basalts can be produced from LREE-depleted komatiite parent magmas by combined assimilation and fractionation, and that such a process best explains the geochemistry and Nd isotopic features of most komatiitic basalts.