LACCOLITH GENERATION IN A BRITTLE CRUST RICH IN MAGMA TRAPS: THE ELBA EXAMPLE

A series of magmatic injections above what is now western Elba, Tuscany, constructed a nested set of multi-layer laccoliths during Late Miocene time. This activity occurred above a delaminating lower crust as rollback progressed eastward across the north end of the Tyrrhenian Sea. Decompression of rising asthenosphere generated mantle–derived magmas that transferred heat to the crust. The laccolith intrusions were derived by partial melting of lower crustal rocks, initially by muscovite dehydration melting, and subsequently with involvement of both muscovite and biotite along with increasing degrees of mixing with hybridized mafic melts preserved as microgranular mafic enclaves. Throughout this period of laccolith development, magma was transported over an extended period through a conduit system that defined a magmatic center the location of which was tectonically controlled at a transform offset.

Emplacement as discrete laccolith sheets was controlled by existing subhorizontal structures in a previously assembled sequence of oceanic ophiolitic and turbiditic units thrust onto the Apennine fold-and-thrust belt. Infilling of these magma traps occurred in the brittle upper crust in the Elba area under conditions of tensional stress, controlled by magma driving pressure that exceeded the vertical stress. These conditions allowed magma accommodation by lifting the roof of the system. Although the cumulative amount of 2.4 km of laccolith thickness was added within 3 km of host material, emplacement was sequentially above earlier injections such that no more than 3 km of overburden needed to be raised. Nevertheless, the cumulative effect was to generate a magmatic dome 10 km in diameter with slopes on the order of 25°. Ultimately the system failed structurally when an additional 3 km of magma was underplated to form the Monte Capanne pluton by amalgamation of three or more pulses of evolving magma. Failure occurred primarily along the most prominent trap structure, at the top of the Tethys ophiolite slice. The top half of the system slid eastward with back rotation such that the full upper section is now exposed in central Elba at sea level while the lower section is preserved in western Elba as an uplifted block at the same elevation.