SHAWINIGAN-AGED REE-BEARING CARBONATITES IN THE GRENVILLE PROVINCE: GEOCHRONOLOGICAL CONSTRAINTS WITHIN THE CENTRAL METASEDIMENTARY BELT BOUNDARY ZONE

Carbonatites and their associated rocks are increasingly tickling geologists’ interest due to their recognized potential as sources of rare earth elements (REEs), metals that are crucial to the development of renewable energy technologies. Tectonic environments for carbonatite emplacement are usually extensional, such as continental rifts and near-rift zones. Although orogenic or collision suture settings have been mentioned, they are uncommon.

We present field observations and geochronological constraints on REE-bearing carbonatites from the Baskatong reservoir in the western Grenville Province of Quebec that were proposed to be genetically associated with the syncollisional Kensington-Skootamatta (K-S) alkaline intrusive Suite (ca. 1090-1060 Ma). The Central Metasedimentary Belt Boundary Zone (CMBBZ) mapped in the reservoir represents a combination of marble, granitic gneiss, carbonatite and clinopyroxenite presenting mingling textures and metabasites associated with leucosome. Kinematic indicators suggest a sinistral-normal shearing movement in the zone. Significant deformation is observed in the carbonatite-clinopyroxenite mix forming L-tectonite structures within the CMBBZ. However, showings of carbonatites located outside of the CMBBZ are weakly deformed.

Our U-Pb geochronology results reveal a deformation age of ca. 1160 Ma for two samples located within the CMBBZ: (a) a syenitic pegmatite composed of euhedral titanite porphyroblasts forming a magmatic lineation (weighted mean ²⁰⁷Pb/²⁰⁶Pb age of ca. 1165 Ma), and (b) an orthopyroxene leucosome located in the extensive quadrant of a mafic sheared boudin (weighted mean ²⁰⁷Pb/²⁰⁶Pb age of ca. 1162 Ma). Additionally, magmatic micro-structures of carbonatitic melt are observed in sample (a). Pre-deformation timing is dated from a pegmatite associated with a clinopyroxenite sampled at Grand-Remous, yielding a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of ca. 1196 Ma.

These ages indicate that the formation of the carbonatites in the reservoir Baskatong is unrelated to the K-S Suite but aligns with the last phase of the Shawinigan orogeny (1200-1150 Ma). This implies that the emplacement of the studied carbonatites is synchronous with deformation.