LAUSR

The syenites in the western part of the Grenville Province in Quebec have been known since the 1990s, but few studies have been carried out on them. Over the last three years, a mapping project carried out by the Ministere de l’Energie et des Ressources naturelles has revealed the presence of several rare earth element (REE)–bearing syenitic intrusions in this area. In this paper, we present a geodynamic model for their formation based on geochemical, thermobarometric, and geochronological data. The intrusions were emplaced between 1038 +15/−13 Ma and 1009 ± 3 Ma. The syenites can be divided into two groups: (1) an older, REE-bearing group associated with a volcanic arc and (2) a younger group with lower REE contents associated with an anorogenic event. Formation temperatures were between 898 and 1005 °C, and pressures were between 2 and 10 kbar. This model involving two intrusive periods is in conflict with the model of a large, hot, long-duration collisional orogen proposed by Rivers et al. (2009) but is consistent with more recent studies.