Abstract

The 564 million year-old Sept Iles layered intrusion is a ferrobasaltic mafic igneous intrusion located in Quebec, Canada. Ferrobasalts form the parent magmas of many important layered mafic intrusions such as the Skaergaard in Greenland and the Bushveld in South Africa, which contains the world’s richest reserves of platinum group elements, Fe, Cr, Ti and V. There are many contrasting ideas when it comes to the formation of layered intrusions: the liquid lines of descent, the processes that form the igneous layering, and the thickness and development of the crystal mush layer are just a few of the contentious issues amongst igneous petrologists. In order to help tease apart the late-stage magmatic history of the Sept Iles intrusion, microstructural evidence from the rocks must be incorporated into the existing natural and experimental evidence. In the Sept Iles intrusion, abundant microstructural evidence is present in the form of symplectites, “vermicular intergrowths” that can form on the edges of mineral grains from the breakdown of unstable phases. The spatial distribution of the symplectites as well as their geochemistry are being analyzed and related to large-scale magma chamber processes in order to gain a better understanding of the magmatic history of the Sept Iles intrusion.