Petrological and geochronological discrimination of two white-mica generations in a granite cored from the Larderello-Travale geothermal field (Italy)

A magmatic muscovite in a granitic dyke cored at 4.1 km depth in the Larderello-Travale geothermal field is overgrown by hydrothermal muscovite. Electron-microprobe data on the white-mica separate confirms very small-scale intergrowths. The staircase-shaped Ar-39/Ar-40 age spectrum of the white-mica mixture anticorrelates with the Cl/K ratio and is best explained by the differential breakdown of the two-phase mixture during in-vacuo heating. This allows to infer an age for the hydrothermal mica, < 2.0 Ma, well resolved from that of the magmatic muscovite, greater than or equal to 2.5 Ma; the granite which initiated the thermal anomaly in Travale is thus at least 2.5 Ma old.