François TURLIN's seminar, University of Quebec Montreal (UQAM)

The Lithospheric Metal Cycle: Example of Rare Earths in the Province of Grenville, Quebec

François TURLIN's seminar, 2019


Rare earths (REE) are metals of great economic interest and tracers of geological processes of first order. Yet their metallogenic cycle is still
unknown. The Grenville Province has numerous occurrences of magmatic REE, particularly associated with pegmatitic granite dykes ("PGDs"), and exposes its orogenic root, allowing access to processes occurring in a deep to intermediate continental crust at during a continental collision.

PGDs have a peraluminous signature that is contradictory with their enrichment in LREE up to ~ 7500 ppm carried by monazite or allanite.
The multi-method approach of petrogeochemistry, geochronology (U-Pb zircon, monazite), and isotopy (U-Pb-Hf-O zircon) of the PGDs makes it possible to characterize the source of these magmas and the LREEs they concentrate. The coupling of the petrochronology of their hosts (U-Pb monazite, apatite) brings constraints as to (i) the thermal evolution of the crustal host segment of the PGDs during the continental collision, and the (ii) geodynamic context of their petrogenesis. Finally, these elements serve as a framework for the discussion of petrogenetic hypotheses that led to such enrichment despite a peraluminous signature.