STRUCTURAL EVOLUTION OF ANCHI-/EPIMETAMORPHIC UNITS OF CENTRAL AND EASTERN ELBA (ORTANO, ACQUADOLCE, MONTICIANO-ROCCASTRADA AND GRASSERA UNITS)

Abstract

The complex framework of the Elba tectonic pile has always been a matter of debate within the geology of the Northern Apennines and of the Tyrrhenian area. The structural data collected on the epi- and anchimetamorphic units of central and eastern Elba (Ortano Unit, Acquadolce Unit, Monticiano-Roccastrada Unit and Gràssera Unit) allow more detailed reconstructions of the complicate tectonic evolution of the Elba stack. Because of their similar structural framework, the Ortano Unit and the Monticiano-Roccastrada Unit acquired their main tectono-metamorphic imprints together as part of a single Paleozoic to Tertiary Tuscan-type succession. The probable Ligurian Piedmontese Acquadolce Unit, which occurs tectonically intercalated between the Tuscan epimetamorphic units, is characterized by a different tectono-metamorphic framework (e.g. shears show opposite sense relative to the Tuscan units). Moreover, the anchimetamorphic Gràssera Unit, that is another possible Ligurian Piedmontese unit, is located in between the non-metamorphic Ophiolitic Unit and the Tuscan Nappe. The present, complex piling up of Tuscan and Piedmontese units occurred within the time interval of 19 Ma (according to the radiometric age of the main foliation of the Acquadolce Unit) and 5 Ma (according to the radiometric age of the La Serra-Porto Azzurro granitoid which produced a well preserved thermometamorphic aureole throughout the stacked units). In more recent times (5-2 Ma), the uprising of the cooled La Serra-Porto Azzurro magmatic body remobilized the eastern Elba tectonic pile onto the Porto Azzurro Unit by low-angle detachment faults (Zuccale Fault). The whole Eastern Elba tectonic pile has been affected by a final, weak folding phase which occurred either during or immediately after the Zuccale Fault, and then by high angle normal faulting which is characterized be hematite-rich mineralizations.