The Ivrea-Verbano Zne of northern Italy provides an opportunity to study directly the effects of intrusion of large volumes of mantle-derived melts into the lower continental crust. Alpine uplift has exposed a complex of mafic to intermediate plutonic rocks, which had intruded the lower continental crust during the Permian. Field mapping reveals a gross arcuate structure within the complex that has survived tilting and uplift. In terms of the orientation during intrusion, banding and foliation are subhorizontal near the base of the complex and steepen gradually up section. Outcrop relations indicate that the cumulates were profoundly affected by extensional deformation increases downward in the complex. The above characteristics are explained in terms of a model in which a huge volume of cumulates (8 km thick and >20 km long) crystallized from a small (≤1 km thick and ≤4 km wide), periodically refilled magma chamber which remained at a relatively fixed position through time. Large-scale ductile deformation of the complex and transport of cumulates downward and outward from the magma chamber occurred as a consequence of extensional tectonics. This process is capable of generating large sheets of gabbro in the lower crust. Gabbro complexes formed in this manner would be characterized by (1) widespread evidence of synmagmatic deformation dominated by stretching, (2) downward increasing intensity of deformation, (3) upward steepening layering and foliation that is concave toward the center of spreading, and (4) a scarcity of crosscutting intrusive relationships. ¿ American Geophysical Union 1994