Structural study of the Balmuccia Massif (Western Alps): A transition from mantle to lower crust

Abstract

Among the ultramafic bodies included in the Ivrea zone, the Balmuccia lherzolite massif offers a particularly well-preserved contact with the granulitic lower crust. The eastern margin of the massif presents a transition to the granulitic gabbros through a layered sequence including pyroxenites and dunites. A chronological evolution of textures in the peridotite is established, showing a continuous evolution from mantle-derived textures (porphyroclastic) located in the southwestern domain to recovered textures (equigranular) developed to the northeast along the preserved transition with the granulites. This recovery process is considered to be enhanced by fluid circulation, it would occur in granulite facies temperature conditions. The map of penetrative structures confirms this chronology: the foliation and lineation attitude follows a progressive evolution with a steep lineation associated with mantle textures and a flat one associated with crustal textures, this last orientation being concordant with that of the surrounding granulites. The structural analysis is consistent with a diapiric emplacement model of the Balmuccia lherzolite developed in asthenospheric mantle conditions and ending in the granulite facies conditions of the lower crust.