The Porto Azzurro unit (Mt. Calamita promontori, southeastern Elba island, Tuscany): stratigraphic, tectonic and metamorphic evolution

Elba Island has a key role in the reconstructions of the stratigraphic, tectonic, metamorphic and magmatic evolution of the Northern Tyrrhenian Sea and the inner part of the Northern Apennines chain. The Porto Azzurro Unit, cropping out in the south eastern part of Elba island, is the deepest tectonic unit of the central-eastern Elba structural pile of Tuscan, Ligurian and Ligurian-Piemontese Nappes, which were intruded by Late Tortonian-Lower Pliocene granitoids and mainly acidic dykes. Moreover, in this part of the island, the relationships between the emplacement of the plutonic bodies and the final deformations of the tectonic stack are easily detectable. To improve our geological knowledge of south eastern Elba, the authors carried out 1:10,000 geological mapping of the Calamita promontory (mostly composed of the Porto Azzurro Unit) and performed petrographical and structural studies on its rocks. The Porto Azzurro Unit consists of a Paleozoic, probably pre-Carboniferous basement (Mt. Calamita Metamorphic Complex), which is unconformably overlain by the ?Triassic Verrucano metasiliciclastics (Barabarca Quartzites) and ?Upper Triassic-?Hettangian metacarbonates. In the Mt. Calamita Metamorphic Complex, five main lithofacies were recognized and mapped. In particular, garnet-bearing-, albite mica-schists (lithofacies a) geometrically underlie a phyllitic-quartzitic unit (lithofacies b); porphyroids (lithofacies e), metabasite bodies (lithofacies d) and graphite-rich siliciclastics (lithofacies c) are also present. The rocks of lithofacies a are similar to those of the ?pre-Paleozoic-?Paleozoic Micaschist Complex of the Larderello geothermal field, whereas the other lithofacies can probably be correlated with the ?Ordovician formations of the Tuscan Metamorphic Succession (e.g. Apuan Alps). The complex deformation-metamorphic evolution of the Porto Azzurro Unit consists of the following events: a) a Variscan tectono-metamorphic event (D-x, recognized in the Mt. Calamita Metamorphic Complex, which is defined by a pre-Alpine foliation and mineral relicts (garnet); b) two Alpine tectono-metamorphic folding events (D-1 and D-2) in the greenschist facies, which also deformed the Mesozoic cover; c) a later folding event (D-3) which probably occurred during or immediately after the thermometa-morphic imprint (including the magnetite-rich skarn bodies), caused by Neogene magmatic intrusions; d) subsequently, the uplift of the magmatic bodies caused low-angle detachments within the Porto Azzurro Unit (between the Mt. Calamita Metamorphic Complex and the Mesozoic cover) and between the latter and the overlying tectonic units (e.g. Zuccale Fault between the Porto Azzurro Unit and the Cretaceous Flysch Unit); e) a final weak antiformal folding (D-4) of the whole promontory took place before the development of NW-SE and NE-SW trending high-angle normal fault systems, locally sealed by hydrothermal, sometimes Fe-rich mineralizations. The lithostratigraphical, tectonic, metamorphic and magmatic evolution of the Porto Azzurro Unit is similar to that detected for the Larderello geothermal region. Thus, the Mt. Calamita area can be considered as an older, but similar geological