The Sept Iles Intrusive Suite, Quebec, Canada

Abstract

The Sept Iles Intrusive Suite (Quebec, Canada) is made up of a large layered intrusion, late gabbro intrusions and a composite sill (Pointe du Criard Sill). The layered intrusion crystallized from a ferrobasaltic magma and is subdivided into a Layered Series of troctolite and gabbro, an anorthositic Upper Border Series and a granitic Upper Series. The formation of the Upper Border Series resulted from plagioclase flotation from the base to the roof of the magma chamber. Fractionation of troctolites and gabbros in the Layered Series resulted in SiO2-enrichment and FeOt-depletion of the residual melt, ultimately forming the granite of the Upper Series. The solidification history of the Layered Series was interrupted by two major and a series of minor influxes of ferrobasaltic melt, significantly enlarging the size of the initial magma chamber. As a consequence, the Layered Series can be subdivided into three megacyclic units (MCU I, II and III). Mixing between resident magma and undifferentiated melt during replenishments had an important influence on both mineral compositions and the liquid lines of descent during the crystallization of the various megacyclic units. It is shown that the liquid line of descent during crystallization of MCU II reached silicate liquid immiscibility. Immiscible melts crystallized two different types of apatite-bearing gabbros, one of which is a major P–Ti–Fe deposit. Cumulate rocks in the layered intrusion show a wide range of crystallized interstitial liquid content. Expelling of this liquid from the crystal mush during solidification is explained both by compaction and compositional convection, but the relative efficiency of these two processes is shown to change significantly with differentiation.