Hydrothermal activity during tectonic building of the Variscan orogen recorded by U-Pb systematics of xenotime in the GrSs Armoricain formation, Massif Armoricain, France

In the Saint-Aubin-des-ChA cent teaux deposit (Massif Armoricain, France), the Ordovician GrSs Armoricain sandstones have undergone several fluid-rock interaction events, including diagenetic cementation and orogenic base metal-As-Sb-Au mineralisation. Ironstone layers interbedded in the sandstones contain several generations of spectacular authigenic xenotime overgrowths that formed around detrital zircon grains in response to successive hydrothermal events. Textural and chemical characterisations allow to distinguish three generations of xenotime overgrowths, differing notably in their REE characteristics. In-situ U-Pb data obtained on these xenotime overgrowths show that their U-Pb systematics were largely disturbed by successive hydrothermal events over about 90 Ma between 330 and 420 Ma, a time interval encompassing most phases of the construction of the Variscan orogen in France. The younger dates cluster around 330-340 Ma and likely correspond to the age of the deposition of massive sulphides and base-metals in the Saint-Aubin-des-ChA cent teaux deposits, which is consistent with the structural contexts where they formed. Finally, this study shows that similarly to monazite, another phosphate widely used for U-Pb and Th-Pb dating studies, the U-Pb chronometric system in xenotime appears to be highly sensitive to fluid circulations.