Rise and fall of a nested Christmas-tree laccolith complex, Elba Island, Italy

Abstract

In two separate areas of western and central Elba Island (Italy), Late Miocene granite porphyries are found as shallow-level intrusions inside a stack of nappes rich in physical discontinuities. Detailed mapping of intrusive rocks, along with their relations with country rocks, show that outcrops from western and central Elba Island 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 d6collement and tilting. The two juxtaposed portions of the original sequence allow the restoration of a 5-km thick sequence, made up of nine main intrusive layers, building three Christmas-tree laccoliths nested into each other to support a structural dome. During their construction, 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. 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.

Dynamics of magmatic intrusions in the upper crust: Theory and applications to laccoliths on Earth and the Moon

“…[5] Recent data on laccolith dimensions also show the same kind of power law relationships, but with an exponent larger than 1 and up to 1.5 which has been interpreted as reflecting the vertical inflation stage during laccolith growth [Rocchi et al, 2002;Mazzarini et al, 2004;Westerman et al, 2004;Corazzato and Gropelli, 2004].…”

Multi-stage emplacement of the Götemar Pluton, SE Sweden: new evidence inferred from field observations and microfabric analysis, including cathodoluminescence microscopy

“…The laccolithic complex was progressively built by stacking and upward inflation of sills (e.g. Westerman et al 2004;Morgan et al 2008). The emplacement locations were probably controlled by structural heterogeneities in the TIB batholith (Cruden 2008) and/or by a stress field in which horizontal extensional fracturing could occur.…”

When and where did hybridization occur? The case of the Monte Capanne Pluton, Italy

“…With emplacement of the Monte Capanne pluton and the Orano dyke swarm, the structural dome at this magmatic center was approaching instability with slopes on the order of 25° ( Fig. 4; Westerman et al 2004). …”

“…The relative chronology of the intrusive sequence has been fi rmly established on the basis of crosscutting relations supported by isotope chronology . The magmatic sequence led to the formation of a nested Christmas-tree laccolith complex with a total thickness of about 2400 m of porphyritic rocks emplaced at depths of 2 -3.5 km Westerman et al 2004). This complex was then intruded by the Monte Capanne pluton, portions of which were intruded by the Orano dyke swarm before complete consolidation.…”

“…At this stage, the nested Christmas-tree laccolith complex was fully developed with about 2400 m of new intrusive material added to a 2700 m section of host rock, forming a structural dome with a diameter of about 10 km and relief over 2 km (Fig. 4;Rocchi et al 2002;Westerman et al 2004).…”