Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The petrology and geochemistry of komatiites at four localities in the Abitibi Greenstone Belt are presented. The effects of alteration, crystal fractionation and partial melting on their geochemistry are considered. It is concluded that Na, K, Ca, Sr and Rb have been disturbed during seafloor alteration, but the other major elements and the trace elements, Ni, Cr, Co, Sc, REE, Y, Zr and Hf have not been mobilized. Variations in the geochemistry within the komatiite flows are the result of olivine and sulphide fractionation. The komatiites consisting of branching olivines (skeletal textured komatiites) and coarse parallel plate olivines (A 3 spinifex textured zones) may represent crescumulates, rather than frozen liquids. Mass balance calculations indicate that the flows were open systems and could represent lava tubes. 26-34% partial melting of a primitive earth mantle (1.4 x C-1 chondrite) will produce melts similar to the komatiites, at each deposit, in terms of major elements, Ni, Cr, Co, Sc, HREE, and Y. However, on average, the concentrations of highly incompatible elements; LREE, Zr and Hf, in the komatiites only are 60-70% of the modelled concentrations, thus indicating that the komatiites were derived from a mantle depleted in highly incompatible elements.