GEOLOGY OF CENTRAL AND EASTERN ELBA ISLAND, ITALY

Abstract

The Elba Island is located in the Northern Tyrrhenian Sea at midway between Tuscany (Northern Apennines Chain) and Corsica (Alpine Corsica structural pile). The complex Elba I. stack of nappes, which is considered the innermost outcrop of the Northern Apennines Chain, is also well known for its Fe-ore bodies and the relationships between the emplacement of the Mio-Pliocene magmatic bodies and tectonics. The geological survey of Elba I. performed at a scale of 1:10,000 and 1:5,000 (geological map at 1:15,000) allowed a revision of the stratigraphic and structural setting of the central and eastern Elba I. This new scheme results more complex compared to Trevisan’s classical one, which was based only on five tectonic “Complexes” (Trevisan, 1950; Barberi et al., 1969). Nine tectonic units were defined, and they all pertain to the Tuscan and Ligurian (including the Ligurian-Piedmontese Units) paleogeographic domains. Before their final emplacement in the Elba’s tectonic pile during the 8.5 to 5.4 Ma time interval, some of these units were intruded by two acidic plutons (Mt. Capanne and La Serra-Porto Azzurro monzogranites), and by dikes of variable composition. A total of nine units were recognised, from bottom to top: 1- Porto Azzurro Unit (PU). It is made up of phyllites, quartzites and micaschists (Mt. Calamita Fm.), probably of Paleozoic age. It shows a strong static recrystallisation due to the La Serra-Porto Azzurro intrusion and the related aplitic dike network (6.0-5.4 Ma). On top of the Mt. Calamita Fm., crystalline dolostones and dolomitic marbles were recognised and were attributed to its Mesozoic cover. The aplitic dikes are cut along the tectonic contact (Zuccale Detachment Fault) with the overlying units described below. 2- Ortano Unit (UO). It includes metavolcanites (Porphyroids) and quartzitic-phyllitic metasediments (Capo d’Arco Schists) which can be correlated to the Ordovician formations of Central Sardinia and Tuscany (Apuan Alps). A few aplitic dikes were also recognised, and they occur along the coast between Capo d’Arco and Ortano Valley. 3- Acquadolce Unit (AU). It is composed of marbles, grading upwards into calcschists and, finally, into phyllites, metasiltstones and metasandstones with intercalations of calcschists which contain fossils of Early Cretaceous age. At its top a serpentinite slice crops out. This Unit has been attributed to the Ligurian Domain (Ligurian-Piedmontese Units) and can be correlated with the “Calcschists with ophiolites” of the Gorgona Island. Near Capo d’Arco Residence, some lamprophyric dikes (Casa Carpini Lamprophyries) also occur. Locally, the carbonate lithotypes are transformed into Fe-skarn bodies (e.g., Torre di Rio skarn). 4- Monticiano-Roccastrada Unit (MU). This Tuscan Unit largely consists of Upper Carboniferous-Triassic metasiliciclastic rocks (the Permian-Carboniferous Rio Marina Fm. and the Triassic “Verrucano” Group). It also includes a Jurassic to Oligocene epimetamorphic succession (from the Capo Castello Calcschists to the Pseudomacigno) which crops out along the coast between Capo Pero and Capo Castello, and in the Valle Giove mining area. 5- Tuscan Nappe (TN). South of the locality La Parata, this unit is composed only of calcareous-dolomitic, at times vacuolar, breccias (“Calcare Cavernoso”), while northwards these rocks are overlain by Upper Triassic to Hettangian shallow marine carbonates, and Sinemurian to Dogger carbonatic, siliceous and marly pelagic sediments. 6- Gràssera Unit (GU). It mostly consists of varicoloured slates with rare carbonate-siliceous and radiolarian cherts intercalations (Cavo Fm.). Between Cavo and La Parata, a basal decametric Calcschist Member also occurs. This anchimetamorphic unit, possibly of Cretaceous age, could have been originated in the Ligurian Domain: because of its peculiar lithologic association and metamorphic overprint it is considered a Ligurian-Piedmontese Unit.. 7- Ophiolitic Unit (OU). This Ligurian Unit is composed of seven tectonic subunits (Acquaviva “ASU”, Mt. Serra “SSU”, Capo Vita “CSU”, Sassi Turchini “TSU”, Volterraio “VSU”, Magazzini “MSU” and Bagnaia “BSU”), which are characterised by serpentinites, ophicalcites, Mg-gabbros, and by their Jurassic to Lower Cretaceous volcanic-sedimentary cover (Basalts, Mt. Alpe Cherts, Nisportino Fm., Calpionella Limestones and Palombini Shales). A shoshonitic dike (Mt. Castello Dike: 5.8 Ma) fills two ENE-WSW-trending normal faults cutting VSU in the Porto Azzurro area. Some calc-alkaline dikes (Mt. Capo Stella Dikes) were also identified in the Ligurian basalts along the western coast of Golfo Stella 8- Paleogene Flysch Unit (EU). It is constituted by shales with calcareous-marly, calcarenitic and arenaceous intercalations and, locally, by ophioliticcarbonate breccias (Colle Reciso Fm.). The fossiliferous content of the carbonate lithotypes points to a Middle Eocene age. This unit can be interpreted as a 98 syn-tectonic oceanic unit (Epiligurian Unit), which has the same paleogeographic origin of the Lanciaia Fm. in Southern Tuscany. Aplites (Capo Bianco Aplites: 7.9 Ma), locally sericitised (the so-called “Eurite”), and porphyries (Portoferraio Porphyries: 8.2 Ma and San Martino Porphyries: 7.4-7.2 Ma) intrude the sedimentary succession, but do not crosscut the basal contact with the underlying Ophiolitic Unit. 9- Cretaceous Flysch Unit (CU). It is a Ligurian, Helminthoid-type, oceanic succession. It consists of a basal tectonised complex, similar to OU (ophiolites, basalts and Jurassic-Cretaceous sedimentary cover slices), and of a sedimentary succession formed by Cretaceous Palombini Shales and Varicoloured Shales, which grade upwards into an arenaceous-conglomeratic (Ghiaieto Sandstones) and then to a calcareous-marly-arenaceous (Marina di Campo Fm.) flysch of Late Cretaceous Age. Similar to the EU, this unit is frequently intruded by locally thick acidic dikes and laccoliths. The structural setting of central and eastern Elba is characterised by a pile of eight structural units (Units 2-9), separated by low angle tectonic surfaces (thrusts and detachments), which lays onto the lowermost Porto Azzurro Unit 1, by a low-angle detachment fault marked by a decametric cataclastic horizon (Zuccale Fault and related cataclasite). The thrust surfaces (Late Eocene-Early Miocene) have been tentatively distinguished from the low-angle detachments, due to the extensional tectonics, which probably began during Burdigalian-Langhian, and continued during Messinian-Pliocene times, accompanied by magmatic intrusions. Other low angle tectonic surfaces are of complex interpretation because they derived from the superposition of tectonic events which occurred in different times and/or in different tectonic regimes. Among the high-angle faults, we recognised a NW-SE trending transfer fault system, which was preceded and followed by generations of normal faults, with WSW-NNE and N-S trends, respectively. The N-S-trending faults cut the whole tectonic pile, comprising all the detachment faults. The study of the tectonic relationships between the previous nine tectonic units and between these tectonic units and the Messinian-Pliocene magmatic events, suggests the following geological scenario for the evolution of the Elba Island: 1) Pre-magmatic stages (>8.5 Ma). They are recorded by: a- relics of the pre-Alpine schistosity within PU and UO, which can be attributed to the Sudetic phase of the Variscan orogeny; b- folding and thrusting of OU, EU and CU, with production of ophiolitic-carbonate breccias within PU, and the D1 tectono-metamorphic event (S1 relics) in AU, related to Eocene intra-oceanic deformation events; c- main deformation and metamorphic events of Tuscan (PU, UO, MU) and Ligurian-Piedmontese Units (and 19 Ma S2 in AU), overthrusting of the oceanic units (AU, OU, GU, EU+CU) onto the Tuscan ones, and a later refolding of the tectonic units, probably related to the Oligocene-Early Miocene collisional events; d- emplacement of AU between OU and MU, and of TN onto MU. The superposition of TN onto MU can be considered the older extensional event by low-angle detachments (Middle Miocene). 2) Syn-magmatic stages (8.5-5.4 Ma). This stage begins with the genesis and rise of anatectic melts due to the uplift of the asthenospheric mantle, within the stretched inner part of the Apenninic orogenic belt. During the uprise of the Mt. Capanne granitoid (6.8 Ma), the most of its cover, that was constituted by EU and CU (already injected by acidic dikes), was detached and shifted eastwards along a low-angle fault (Central Elba Fault, “CEF”). During this event the acidic dykes of the basal part of the flysch were sericitised (“eurite”: 6.7 Ma). Farther east, a shoshonitic dike intruded OU at 5.8 Ma and, possibly, lamprophyric dikes were emplaced within AU. A new uplift of the Mt. Capanne caused a further glide eastwards of EU+CU onto OU in the central Elba, and the development of transfer faults (as lateral ramps of detachments) within the Ligurian Units and, probably, the onset of the Zuccale Fault. At 6.0-5.4 Ma the emplacement of the La Serra-Porto Azzurro granitoid produced a wide thermometamorphic aureola and local skarn bodies within the host PU, UO, AU and MU. The uplift of this granitoid caused, or completed, the separation of the eastern and central Elba tectonic pile through the Zuccale detachment Fault. During this stage, the back-gliding of OU onto EU+CU in the Colle Reciso area, and the north- or north-eastwards gliding of CSU, completed the present tectonic pile of central and eastern Elba. 3) Post-magmatic events (<5.4 Ma). High-angle, N-S trending normal faults dismembered the orogenic pile and allowed the final circulation of idrothermal- mineralising fluids, with the formation of the hematite-rich ores of eastern Elba, dated ~5.3 Ma. Finally, our reconstructions of the tectonic evolution of the Elba I. is fitted in the geodynamic context of the orogenic system Corsica-Northern Apennines, as shown by a series of tectonic sketches starting from the Late Cretaceous.