Simultaneous resetting of the muscovite K-Ar and monazite U-Pb geochronometers: a story of fluids

Although water is ubiquitous in the continental crust, its effect on geochronometers through mineral reequilibration is rarely taken into account. Herein, we present 40Ar/39Ar analyses on muscovite and U-Pb isotopic data on zircon and monazite from a Variscan syn-tectonic granite from western France. Both the K-Ar in the muscovite and U-Pb in the monazite isotopic systems were hydrothermally reset, whereas the U-Pb radiogenic system in most of the zircons was unaffected and dates the granite emplacement age. Titanium chemical maps obtained on muscovites from various dated samples display a spectacular overprinting of their magmatic zoning resulting from increasing fluid-rock interaction. These results reiterate the need to combine geochronological data with petrological, mineralogical and geochemical studies to accurately interpret ages obtained in this type of geodynamical settings.