Metamorphic consequences of Neogene thermal anomaly in the northern Apennines (Radicondoli-Travale area, Larderello geothermal field Italy)

In the Radicondoli-Travale area of the Larderello geothermal field (Italy) new structural and petrologic data on some metamorphic units, cored from ge othermal wells, reveal the existence of a wide complex of hornfels rocks. T he development of the complex is related to the emplacement of Neogene magm atic rocks at a shallow depth within a Permo-Triassic terrigenous metasedim entary sequence of a low metamorphic grade. The sequence was subjected to a lpine tectonic-metamorphic events. This reconstruction gives new insights i n the tectonic setting of metamorphic units below the Tuscan Nappe, in sout hern Tuscany. Particularly, the medium and high grade metamorphic rocks see m strictly related to the development of Neogene thermal aureoles and do no t represent remnants of a Paleozoic basement. Furthermore, in this sector o f the Larderello geothermal system, the presence of deep geothermal reservo ir at a depth of 3 000 m is mainly linked to the enhanced permeability caus ed by fracturing in these hornfels rocks. This fracturing is a consequence of the dehydration reaction occurring in the metapelites due to Neogene the rmal metamorphism. This mechanism allowed the development of a long-lived h ydrothermal system, shown by the secondary mineralogical assemblages. These testify the presence of at least two hydrothermal stages which are well pr eserved in the less permeable units. (C) 2000 Editions scientifiques et med icales Elsevier SAS.