Insights into a synorogenic origin for REE-bearing carbonatites in the Central Metasedimentary Belt Boundary Zone of the Grenville Province

Several carbonate-rich intrusions (i.e. carbonatites) have been documented along the northwestern limit of the Central Metasedimentary Belt (CMB) in the Grenville Province of Canada, i.e. the Central Metasedimentary Belt Boundary Zone (CMBBZ), but their origin and timing of emplacement remains contentious. Our study focuses on the tectono-magmatic setting of one REE-bearing carbonatite, the Baie-Mercier REE occurrence, and its associated carbonate and silicate rocks, located on islets within the Baskatong Reservoir in western Quebec, Canada. We present new LA-ICP-MS U-Pb zircon ages from four igneous rock samples within the exposed CMBBZ in the Baskatong Reservoir area in the Grenville Province in southwestern Quebec: 1) 1162 ± (5) [analytical error only: non propagated error] 16 [error including systematic uncertainties: propagated error] Ma for a syntectonic orthopyroxene leucosome located in the extensional quadrant of a sheared mafic boudin; 2) 1157 ± (4) 16 Ma for a syntectonic syenitic pegmatite composed of euhedral titanite phenocrysts preferentially aligned parallel to the host rock lineation, and characterized by syndeformation carbonatitic melt micro-textures; 3) 1149 ± (12) 18 Ma for a deformed late-tectonic pegmatite, and 4) 1022 ± (6) 15 Ma for a post-deformation dyke unit. These new age constraints, along with field observations, show that the REE-bearing carbonatite of Baie-Mercier was emplaced at ca. 1160 Ma during the Shawinigan Orogeny (1190–1140 Ma). Geochemical data point to a mantle-derived origin for the Baie-Mercier REE occurrence. Our findings further imply that this carbonatite could constitute one of the rare examples of synorogenic carbonatites worldwide.