Geological model of a joung volcano-plutonic system: the geothermal region of Monte Amiata (Tuscany, Italy)

Geological, geophysical and petrologic data point to the presence of a granitic body below the geothermal region of Mt Amiata (central Italy). A broad area of about 900-1300 km2 centered on Amiata volcano shows a regional uplift of the Pliocene beds to 950 m a.s.l. The uplift began during the lower Pliocene, with a regression of the Pliocene sea from an uplifted area centered in the volcano zone. The temperature distribution below the Piancastagnaio field shows an updoming of the isotherms, forming a thermal high, probably present since the earliest stages of interaction between geothermal fluids and country rocks. A re-evaluation of the petrologic data from the xenoliths included in the lava flows allows an estimate of the P-T conditions of the magma body; a minimum temperature of 575°C and pressures of 1550-2200 bars can be estimated for the confining rocks around the magma body. Magmatologic data show a temperature of 800-900°C and a Pload - 1000 bar. Therefore the roof of the magma body should be present at about 6 km depth. Seismic reflection data reveal the continuous and widespread occurrence of a reflecting horizon (K) of all over the geothermal region. This horizon is present at a depth of 5-6 km. By analogy with Larderello, it is interpreted as a fracture interval filled with hot fluids, contact metamorphic minerals and hydrothermal minerals generated in the uppermost part of the granite and the basal levels of the wall rocks. By integrating geophysical and geological data, a two-dimensional gravimetric model of the volcano-plutonic system of Mt Amiata is proposed, with the following features: roof depth = 5-6 km, T = 820°C, d(magma) = 2.15 g cm-3, d(wall rock) = 2.8 g cm-3, shape of the intrusion is lens shaped or mushroom-like with possible thickening and roots just below Piancastagnaio. This model fits well with gravimetric data, which show a negative anomaly in correspondence with the uplifted area.