Abstract

On the internal (Tyrrhenian) side of the northern Apennines, high heat flow and geothermal gradient in the Larderello geothermal field are evidence of a large-scale thermal anomaly, related to the emplacement of a large batholith of Pliocene age extending from the upper to middle crust. These intrusions relate to the Miocene to Recent northern Apennines magmatism, which developed as a result of crustal extension and upwelling of the asthenosphere at the base of the thinned crust. In the subsurface of the Larderello geothermal field, medium- and high-grade metamorphic rocks (Tuscan Metamorphic Complex) cored in several geothermal wells, have previously been considered to be Variscan basement. New structural and petrographic data on these metamorphic rocks indicate an Alpine tectonometamorphic history and provide evidence that medium- to high-grade mineral assemblages are related to Pliocene thermal metamorphism centred on intrusions at shallow depth. These data allow a new interpretation of the Tuscan Metamorphic Complex as a tectonic wedge of the Late Palaeozoic–Triassic units, and in particular highlight the considerable extent of Pliocene thermal metamorphism and hydrothermal circulation as a result of convective cooling of intrusions.

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