Petrological and geochronological discrimination of two white-mica generations in a granite cored from the Larderello-Travale geothermal field (Italy)
Villa, Igor M.; Ruggieri, Giovanni; Puxeddu, Mariano
European Journal of Mineralogy Volume 9 Number 3 (1997), p. 563 - 568
published: Jun 2, 1997
manuscript received: Jul 9, 1996
ArtNo. ESP147050903017, Price: 29.00 €
Abstract 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 39Ar/40Ar 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, ≥ 2.5 Ma; the granite which initiated the thermal anomaly in Travale is thus at least 2.5 Ma old.