INTRUSIVE SHEETS AND SHEETED INTRUSIONS AT ELBA ISLAND (ITALY)

Igneous activity at Elba Island (Tuscany) led to the emplacement of several magma bodies over a time span of about 1 Ma during the Late Miocene. A first intrusive cycle produced nine separated shallow-level granite porphyry sheets connected by feeder dikes, eventually constructing three nested Christmas-tree laccoliths. The laccolith layers were emplaced at depths between 1.9 and 3.7 km, as reconstructed by cross-section measurements, and space for magma was created by roof uplift. A second magmatic cycle led to the build-up of the granitic Monte Capanne pluton and a mafic dyke swarm. The pluton was constructed over a very short time span by three magma pulses stacked downward as subhorizontal intrusive sheets. The top of the intrusion reached a depth of about 5 km, and space for magma was created by roof uplift and tectonic-gravitational displacement of the overburden.

The laccolith intrusive layers are 50 to 700 m thick, with diameters of between 1.6 and 10 km. Length to thickness relationships for individual layers show a peculiar power-law correlation interpreted as the frozen evidence for the occurrence of a vertical inflation stage during laccolith growth. Also the reconstructed original dimensional parameters of the three intrusive sheets of the pluton display a power-law correlation indicative of a vertical inflation stage during pluton growth. Once the sheets constituting the multi-sheets laccoliths and those of the pluton are amalgamated in a single sheet (virtually for the laccoliths; actually observed for the pluton), their dimensional parameters fit those predicted for a single pluton.

We suggest the laccolith sheets failed to coalesce and form a larger plutons/laccoliths with typical dimensions owing to the availability of a large number of magma traps in the host crust that consists of a fault stack of bedded rocks. On the other hand, the magma batches building the Monte Capanne pluton exploited a major thrust fault separating rheologically distinctive tectonic units, with the three pulses of magma filling a single reservoir to build a “successful” pluton, as opposed to the “failed” plutons represented by the multi-sheet laccoliths. As a final consideration, we speculate that laccoliths and plutons represent different outcomes of the same geological process.