Granite emplacement in an extensional setting: an AMS study of the magmatic structures of Monte Capanne (Elba, Italy)

Abstract

Monte Capanne, Elba is an excellent example of a granite pluton (42 km 2) emplaced 6 m.y. ago in a well-known extensional environment, that of the Tyrrhenian Sea. Its internal fabric has been studied using the magnitude of magnetic susceptibility and anisotropy measurements at 74 regularly distributed stations. Susceptibility magnitudes reveal a normal and asymmetrical petrographical zoning in the pluton. The magnetic lineations are remarkably well organized, mostly of NW-SE azimuth with slight plunges, and always normal to the walls of the late aplitic dykes. These lineations are also parallel to the stretching lineations of the immediate country rocks of the pluton. In the western and probably deepest part of the pluton a NE-SW trending magnetic/magmatic lineation is also imprinted. The following scenario is proposed for the emplacement of Monte Capanne during the opening of the Tyrrhenian Sea: An approximately E-W trending sinistral transform fault is taken as being responsible for the creation of a pull-apart void that permitted upwelling of the magma. Orientations of the magmatic structures at depth are parallel to the elongation of the feeding zone of this void, i.e. NE-SW. The very top part of the pluton was sheared parallel to the granite-country rock interface, i.e. NW-SE, parallel to the local stretching of the Tyrrhenian Sea at the time of emplacement.