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

International audienceIn the Saint-Aubin-des-Châteaux deposit (MassifArmoricain, France), the Ordovician Grès Armoricain sandstoneshave undergone several fluid-rock interaction events,including diagenetic cementation and orogenic base metal-As-Sb-Au mineralisation. Ironstone layers interbedded in thesandstones contain several generations of spectacularauthigenic xenotime overgrowths that formed around detritalzircon grains in response to successive hydrothermal events.Textural and chemical characterisations allow to distinguishthree generations of xenotime overgrowths, differing notablyin their REE characteristics. In-situ U-Pb data obtained onthese xenotime overgrowths show that their U-Pb systematicswere largely disturbed by successive hydrothermal eventsover about 90 Ma between~330 and~420 Ma, a time intervalencompassing most phases of the construction of the Variscanorogen in France. The younger dates cluster around~330–340 Ma and likely correspond to the age of the deposition ofmassive sulphides and base-metals in the Saint-Aubin-des-Châteaux deposits, which is consistent with the structural contextswhere they formed. Finally, this study shows that similarlyto monazite, another phosphate widely used for U-Pb andTh-Pb dating studies, the U-Pb chronometric system inxenotime appears to be highly sensitive to fluid circulations