THE FLAMANVILLE GRANITE (NORTHWEST FRANCE) - AN UNEQUIVOCAL EXAMPLE OF A SYNTECTONICALLY EXPANDING PLUTON

The Flamanville pluton was emplaced during the upper Carboniferous in Palaeozoic sedimentary cover forming one limb of the Siouville syncline, a Hercynian kilometre-scale fold in Normandy, Northwest France. We review the geological environment of the pluton with special emphasis on the Siouville syncline. The features described include gravity data, metamorphic environment, fold patterns in the contact metamorphic aureole, trajectories of principal strains in and around the pluton, finite strain and anisotropy of magnetic susceptibility within the pluton, and physical and kinematic aspects of pluton fabrics. The results emphasize that (1) the Flamanville pluton is syntectonic, (2) its emplacement involves lateral expansion of magma rather than spherical ballooning, and (3) granite fabrics do not reflect magmatic flow but are strain-controlled irrespective of grain-scale deformation processes and rheological state. The example of the Flamanville granite further suggests (1) that grain-scale deformation features are not critical in distinguishing tectonic from magmatic origin of granite fabrics and (2) that pluton formation within the soft sediments of the shallow upper crust most likely results from lateral expansion of magma injected through the brittle crust rather than from ballooning of a diapiric body.