Hydrothermal activity during tectonic building of the Variscan orogen recorded by U-Pb systematics of xenotime in the Grès Armoricain formation, Massif Armoricain, France
Résumé
In the Saint-Aubin-des-Châteaux deposit (Massif
Armoricain, France), the Ordovician Grès 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-
Châteaux 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.