Two-stage growth of laccoliths at Elba Island, Italy.

At Elba Island (Italy), nine shallow-levei late Miocene granite porphyry layers connected by feeder dikes built up three nested Christmas-tree laccoliths. The intrusive units have been defined and correlated between exposures in western and centrai Elba on the basis of field,petrographic, and geochemical data, along with intrusive relationships. Detailed mapping and reconstuction of tectonic history led to restoration of the originai 5-km-thick sequence and determination of the dimensional parameters of each intrusive layer. Thè 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 individuai laccolith layers show a power-law correlation that does not fìt the known dimensional distribution for laccoliths, but instead fit 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 faiis, possibly owing to the large availability of crustal magma traps.