The Late Miocene Christmas-tree laccolith complex of the island of Elba.

In two separate areas of the west and centre of the Island of Elba (Italy), granite porphyries of late Miocene age are found as shallow-level intrusions inside a stack of nappes characterised by the occurrence of abundant physical discontinuities. Detailed mapping of intrusive rocks and their relations with country rocks, coupled with geochemical-isotopic-geochronologic data, show that outcrops in the west and centre of the Island of Elba expose the same rock types, with matching intrusive sequence, petrography and geochemical features. Structural and geological data indicate that these layers were originally part of a single sequence that was split by eastward-directed décollement and westward tilting. The two juxtaposed portions of the original sequence allow the restoration of a 5 km thick igneous-sedimentary sequence, made up of nine main intrusive layers, building three Christmas-tree laccoliths nested into each other to support a structural dome. During magma emplacement, the role of the neutral buoyancy level was of minor significance with respect to the role played by the relatively thin overburden and/or the large availability of magma traps inside the intruded crustal section. The emplacement of the Monte Capanne pluton into the base of the domal structure likely caused oversteepening and initiated decapitation of the complex with gravity sliding of the upper half off the top.