Alpine and Hercynian Orogenic phases in the Basement rocks of the Northern Apennines (Larderello Geothermal field, Southern Tuscany, Italy)

A microstructural - petrographic study was carried out on core samples from several deep wells of the Larderello geothermal field (southern Tuscany, Italy). This paper aims to outline the structural-metamorphic framework of the buried Palaeozoic - Middle Triassic sequences during Hercynian and/or Alpine times. A "cover' ("Tectonic wedges Complex', "Phyllitic-Quartzitic Group' and "Micaschist Group'), with a strong Alpine overprint, is detached from a Hercynian basement ("Gneiss Group'), weakly affected by Alpine events. The imprint of the Hercynian orogeny (a late Hercynian HT-LP event, the Sudetic phase, and, sometimes, relics probably belonging to the Breton phase) is clearly recognizable in most of the Palaeozoic sequences. Correlation between the Larderello buried Palaeozoic sequences and the ones outcropping in northeastern Sardinia is pointed out. -Authors