Deep Structure, Age and Evolution of the Larderello-Travale Geothermal Field

Abstract

A main seismic reflecting horizon (K), occurring in the depth range 3-6 km of the Larderello-Travale geothermal field, probably consists of fractured level, sometimes filled with hot fluids and/or authigenic minerals present at the contact between granites and the overlying basement, which is seen in drill cores to have undergone a hercynian metamorphism followed by a late Hercynian thermal event. After a polyphased Alpine metamorphism, a new thermal even then gave origin to the geothermal field and produced corundum and chiastolite in micaschist, leucogranitic dikelets and the resetting of metamorphic biotite ages to values into eh range of 1.6 to 3.5 Ma. After the climax of the thermal even (3.5 Ma ago), the geothermal field underwent a monotonic cooling with cooling rates in the range 16°-40°C/Ma, typical of a batholith. The different thermal regime in different wells easily explains the variations in the apparent ages of biotite, which is still partially open to 40Ar and 87Sr volume diffusion.