The Sesia magmatic system constitutes and unprecedented exposure of a plumbing system of a caldera and, more in general, of a Silicic Large Igneous Province (SLIP), exposed with good continuity from the surface to a depth of about 25 km (Quick et al., 2009). In this framework, the Mafic Complex records processes that affected the deep crust beneath the caldera. The onset of volcanic activity correlates strictly with the climax of the growth of the upper Mafic Complex, when the crust was pervasively heated. Igneous activity may have continued for as much 10 million years. In the middle to upper crust, this activity was dominated by silicic melts produced by anatexis in the deep crust, but included a minor amount of mantle component. During the life of volcanic activity, the Mafic Complex was a huge, hot crystal mush body of slowly sinking cumulates, within which the magma chamber was limited to small bodies at the core of the arcuate structure, where injections of fresh, although still contaminated, mafic melt were mingled with dioritic cumulates. All available evidence indicates that large, predominantly molten magma chambers were not involved and that the magmatic plumbing system beneath the active volcanic field was composed of crystal mush bodies consistent with inferences from geophysical data (Guidarelli et al., 2006).
In Appendix A we propose a sequence of stops for a 2-days field excursion in the Sesia magmatic system.
In Appendix B we provide a JPEG copy of the Ivrea-Verbano geologic map from Quick et al., (2009)