Abstract

Magmatic activity in the western part of Elba Island at the north end of the Tyrrhenian Sea lasted approximately 1.5 Ma during the Late Miocene, building a complex of nested Christmas-tree laccoliths, a 10 km-diameter pluton (Monte Capanne) and, finally, the steeply-dipping Orano dyke swarm (ODS). This igneous activity occurred as an extensional regime and followed the wake of eastward-migrating compression of the Apennine front. The ODS consists of hybridized mantle-derived magmas, constituting about 200 dykes totalling a length of approximately 90 km. These dykes intruded the northwestern part of the pluton (NW of the Pomonte–Procchio geomorphic lineament) and its contact aureole, as well as several kilometres of sedimentary rock above. The ODS intruded near the close of pluton crystallization, above a source region marked by a positive magnetic anomaly located NW of the Pomonte–Procchio lineament. Dyke orientations are dominated by a major system trending N78E, with dykes concentrated in belts that locally produced up to 15% extension; between these belts, a minor system of Orano dykes dominates with N38W and N22E trends.

ODS emplacement patterns preserve the strain that resulted in exploitation of Riedel fractures in a NE–SW dextral shear zone; local internal zones of sinistral shear account for one set of the minor system. This shearing occurred between offset segments of the Elba Ridge in the western Elba transfer zone, where strain concentrated magma flow to build the western Elba magmatic complex. This zone developed as a result of different extension rates that produced north-trending Neogene–Quaternary sedimentary basins north and south of the zone. Such basins are connected regionally by NE-trending lineaments previously active during the formation and destruction of the Tethys Ocean. All the magmatic centres in the northern Tyrrhenian–Tuscan area are distributed along such lineaments and developed as a wave moving northeastward across the region, suggesting that magmatism was focused by transfer zone development as back-arc extension migrated in that direction and reactivated former faults.