540-Myr-old anorthosite complex in the Grenville Province of Québec, Canada

MASSIF-TYPE anorthosites and related rocks are an important component of the Grenville and Nain structural provinces of the Canadian Shield (Fig. 1), as well as in high grade metamorphic terrains in other parts of the world. There are many outstanding problems in the study of these rocks, not least of which is their age. Most geochronological data have been obtained from the associated granitic rocks of this suite as anorthosite itself contains very little rubidium and zirconium. Because of post-crystallisation deformation, however, the relationship between the anorthosite and the associated rocks is often tenuous and frequently disputed. Within the Grenville Province metamorphosed rocks believed to be genetically associated with anorthosites have yielded ages ranging from 1,020 to 1,130 Myr (refs 1–4). The high-grade of metamorphism surrounding the Morin and Lac St Jean anorthosites is believed by some researchers to be due to anorthosite emplacement. These ‘aureoles’ yield Rb–Sr ages5,6 of about 1,500 Myr. Anorthosite complexes in the Nain Province are undeformed, and the adamellite of the suite has yielded a U–Pb zircon age7 of about 1,300 Myr. Barton8 obtained a Rb–Sr isochron age of 1,418 Myr from a digested inclusion in the Nain anorthosite. Here we present geochronological data from the undeformed Sept Iles anorthosite, and show it to be Cambrian in age