Intrusive sheets and sheeted intrusions at Elba Island, Italy

The processes leading to successful versus failed coalescence of similar magma batches upon their emplacement are investigated at Elba Island (Tuscany), where several magma bodies were generated at a single magmatic center over a time span of ∼1 Ma during the Late Miocene. Three nested Christmas-tree laccoliths made up of separated, shallow-level felsic sheets were emplaced at 2-3 km depth with associated roof uplift. Then, at a deeper level, a granite pluton was constructed over a short time span by three magma pulses stacked downward as subhorizontal intrusive sheets, with space for magma generated mostly by roof uplift and tectonic-gravitational displacement of the overburden. Length-to-thickness relationships for individual laccolith layers, as well as for pluton sheets, show a power-law correlation interpreted as the frozen evidence for the occurrence of a vertical inflation stage during intrusion growth. We infer that laccolith sheets failed to coalesce and form a larger pluton because their magma driving pressure exceeded the lithostatic load in a crustal section rich in subhorizontal magma traps (a thrust stack of bedded rocks). However, the driving pressure of the first magma batch of the Monte Capanne pluton was presumably enhanced by an increased magma supply rate, so that the driving pressure exceeded the load at the level of a deeper magma trap represented by a major thrust fault. The following magma batches arrived in rapid succession and were not able to penetrate the still mushy tabular mass. Thus the laccolith sheets and the sheeted pluton represent different outcomes of similar processes occurring under slightly different conditions.