Two-stage growth of laccoliths at Elba Island (Italy)

At Elba Island, Italy, nine shallow-level late Miocene granite porphyry layers connected by feeder dikes built up three nested Christmas-tree laccoliths. Detailed mapping and reconstruction of tectonic history led to restoration of the original 5-km-thick sequence and determination of the dimensional parameters of each intrusive layer. The laccolith layers were emplaced at depths between 1.9 and 3.7 km, exploiting physical discontinuities that served as crustal magma traps inside a stack of nappes. The intrusive layers are 50-700 m thick, with diameters between 1.6 and 10 km. Length to thickness relationships for individual laccolith layers show a power-law correlation that does not fit the known dimensional distribution for laccoliths, but instead fits a line with a slope typical of the theoretical vertical-inflation stage of laccolith development. This is interpreted as the first reported natural example of the occurrence of a vertical-inflation stage during laccolith growth. The dimensional data for Elba intrusive layers also suggest that laccoliths and plutons commonly form by amalgamation of smaller sheet-like bodies, while multilayer laccoliths form when coalescence fails, possibly owing to the large availability of crustal magma traps