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

Résumé

Cumulates of apatite and Fe-Ti oxides are form during the later stages of crystallization in many layered intrusions. In some cases they occur as layers or lenses of nelsonite composed of about one third of apatite and two thirds of Fe-Ti oxides. Associated with nelsonite are troctolite or diorite enriched in oxides. Two different models have been proposed for the origin of these rocks. Both models suggest that a Fe-Prich silicate liquid formed after considerable crystal fractionation of the mafic parental liquid. In the first model, Fe-Ti-oxide and apatite crystallize from this magma and accumulate on the crystal pile to form nelsonite. In the second model the silicate liquid splits into; a) an Fe-Ti-P liquid from which nelsonite forms and b) a SiO2-rich liquid.

We have investigated the whole rock and mineral compositions of nelsonites and associated rocks from the Sept- Îles Intrusive Suite, Quebec. In addition, we have determined the trace element concentrations of apatite in these rocks. Mass balance calculations show that apatite is the principle host for REE, Ca, U, Th and Sr in these rocks. To test the crystal accumulation model these trace elements were inverted to estimate the trace element composition of the magma at the time of apatite saturation. Mantle normalized trace element patterns of the estimated liquids are similar in shape to mantle normalized patterns from dykes believed to be feeders to the intrusion at this stratigraphic level. Apatite would not have been on the liquidus of magmas of the dyke compositions. A simulation (using PELE) was carried out in order to model crystallization of a magma derived from the dykes. The model mineral compositions and the trace element patterns including those of apatite match the observed compositions for Sept-Îles nelsonites and associated rocks. Thus we favour the crystal accumulation model for the formation of nelsonite and associated rocks.