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

Résumé

The Sept Iles Intrusive Suite, Quebec, Canada, comprises one of the world’s largest mafic layered intrusions. It is a natural laboratory to study differentiation processes of a ferrobasaltic melt whereby extreme fractional crystallisation of the mafic melt resulted in residual, felsic magma. Furthermore, liquid immiscibility played a role in forming Fe–Ti–P mineralisation (Fe-Ti oxides and apatite) in the apatite-bearing Critical Zone near the top of the mafic sequence. In this study, we explore and develop the use of trace elements in apatite as a petrogenetic indicator of these magmatic processes. A full suite of 31 trace elements was determined in apatite by LA-ICP-MS and electron microprobe from the mafic Layered Series and felsic Upper Series of the Sept-Iles Intrusive Suite. We show that the trace element content of cumulus apatite records the changing melt composition of the fractionating ferrobasalt from the mafic to the felsic rocks (increase in REE+Y, Th, U, K, Pb and Rb, and decrease in Sr, Ba, V and Mg of apatite), in accordance with the evolution of Sept-Iles liquid line of descent. In the mineralised Critical Zone, the fractionation trend overprints systematic chemical differences in apatite and plagioclase composition between alternating Fe–Ti–P-rich and -poor layers. This is attributed in part to partitioning behaviour of trace elements between immiscible Fe-rich and Si-rich silicate melts and the different cotectic proportions of apatite crystallising from these melts, which was calculated to be 5 to 8 % and 1 % for the Fe–Ti–P-rich and -poor, respectively. Crystal settling and accumulation is then responsible for the elevated proportion of apatite (7 to 36 vol.%) observed in Fe–Ti–P-rich layers. Furthermore, a series of new discrimination diagrams is proposed to help identify the provenance of igneous rocks using apatite chemistry. At Sept Iles, apatite from mafic rocks is distinguished from the felsic rocks using REE+Y, Sr and the Eu anomaly. Intercumulus apatite, which crystallised from the interstitial liquid, are readily distinguishable from cumulus apatite by enrichment in highly incompatible elements (higher Cl/F and Th/Lu ratios). This suggests that apatite could be used as an indicator mineral for the exploration of Fe–Ti–P mineralisation in mafic layered intrusions using detrital apatite.