ACKNOWLEDGMENTS: The SNF Ambizione Fellowship (grant PZ00P2_168166) and UGA Presidential Funds to M.P., the Alexander von Humboldt grant to L.Z., the project OROG3NY (SNF grants PP00P2_157627 and PP00P2_187199) to G.H., and SNF grant 200020_156421 to O.M. supported this research. We thank J. Connolly for precious suggestions about phase equilibria calculations. We acknowledge G. Dumond, W. Mooney, and E. Kissling for insightful comments on a previous version of the manuscript. We thank W.J. Shinevar for the constructive reviews and M. Edmonds for editorial handling. REFERENCES: Coltorti M, Siena F (1984) Mantle tectonite and fractionate peridotite at Finero (Italian Western Alps). Neues Jahrb Mineral Monatsh 149:225-244. Hartmann G, Wedepohl KH (1993) The composition of peridotite tectonites from the Ivrea Complex, northern Italy: residues from melt extraction. Geochem Cosmochim Acta 57:1761-1782. Lu M, Hofmann AW, Mazzucchelli M, Rivalenti G (1997a) The mafic-ultramafic complex near Finero Ivrea-Verbano zone: I. Chemistry of MORB-like magmas. Chem Geol 140:207-222. Lu M, Hofmann AW, Mazzucchelli M, Rivalenti G (1997b) The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone), II. Geochronology and isotope geochemistry. Chem Geol 140:223-235. Mazzucchelli M, Rivalenti G, Brunelli D, Zanetti A, Boari E (2009) Formation of Highly Refractory Dunite by Focused Percolation of Pyroxenite-Derived Melt in the Balmuccia Peridotite Massif (Italy). J Petrol 50:1205-1233. Mukasa SB, Shervais JW (1999) Growth of subcontinental lithosphere: evidence from repeated dike injections in the Balmuccia lherzolite Massif, Italian Alps. Dev Geotecton 24:287-316. Pin C, Sills JD (1986) Petrogenesis of layered gabbros and ultramafic rocks from Val Sesia, the Ivrea Zone, NW Italy. In The Nature of the Lower Continental Crust, edited by Dawson JB, Carswell DA, Hall J, Wedepohl, Geol Soc Spec Pub 24:231-249. Rivalenti G, Mazzucchelli M, Vannucci R, Hofmann AW, Ottolini L, Bottazzi P, Obermiller W (1995) The relationship between websterite and peridotite in the Balmuccia peridotite Massif, NW Italy, as revealed by trace element variations in clinopyroxene. Contrib Mineral Petrol 121:275-288. Shervais JW, Mukasa SB (1991) The Balmuccia orogenic lherzolite massif, Italy. J Petrol Spec Vol 2 (Lherzolite):155-174. Siena F, Coltorti M (1989) The petrogenesis of a hydrated mafic-ultramafic complex and the role of amphibole fractionation at Finero (Italian Western Alps). Neues Jahrb Mineral Monatsh 6:255-274. Sinigoi S, Antonini P, Demarchi G, Longinelli A, Mazzucchelli M, Negrini L, Rivalenti G (1991) Interactions of mantle and crustal magmas in the southern part of the Ivrea Zone (Italy). Contrib Mineral Petrol 108:385-395. Sinigoi S, Quick JE, Demarchi G, Klötzli U (2011) The role of crustal fertility in the generation of large silicic magmatic systems triggered by intrusion of mantle magma in the deep crust. Contrib Mineral Petrol 162:691-707. Stähle V, Frenzel G, Hess JC (2001) Permian metabasalt and Triassic alkaline dykes in the northern Ivrea zone: clues to the post-Variscan geodynamic evolution of the Southern Alps. Schweiz Mineral Petrogr Mitt 81:1-21. Voshage H, Hofmann AW, Mazzucchelli M, Rivalenti G, Sinigoi S, Raczek I, Demarchi G (1990) Isotopic evidence from the Ivrea Zone for a hybrid lower crust formed by magmatic underplating. Nature 437:731-736.

TABLE CAPTIONS Table 1: Average and specific bulk compositions of the representative exposed primitive rocks of the deep lower crust and upper mantle of the IVZ (Balmuccia and Lower Mafic Complex in Valsesia, Finero in Val Cannobina, and Sessera river in Val Sessera) used for the Perple_X calculations using the two thermodynamic models STX11 and HP02. Mn, Ni, P, and Cr were not used for the Perple_X calculations. Mg# is calculated as Mg / (Mg + Fe) (mole fractions). Symbols in the last row indicate the rock compositions that are reported (✓) and not reported (✗) in Figure 5. Average compositions are normalized to 100 wt.% on a volatile-free basis; specific compositions report original totals and H2O contents estimated based on the volumetric proportions hydrous minerals (amphibole and phlogopite) at the scale of the bulk rock. References: 1) Voshage et al. (1990); 2) Voshage et al. (1990), Shervais and Mukasa (1991), Hartmann and Wedepohl (1993), Rivalenti et al. (1995), and Mukasa and Shervais (1999); 3) Pin and Sills (1986); 4) Pin and Sills (1986), Shervais and Mukasa (1991), Hartmann and Wedepohl (1993), Rivalenti et al. (1995), and Mukasa and Shervais (1999); 5) Shervais and Mukasa (1991) and Mazzucchelli et al. (2009); 6) Sinigoi et al. (1991; 2011); 7) Hartmann and Wedepohl (1993); 8) Coltorti and Siena (1984); 9) Siena and Coltorti (1989), Lu et al. (1997a; b), and Stähle et al. (2001). Table 2: Summary of VP and r from the thermodynamic calculations based on representative rock compositions listed in Table 1. Mineral abbreviations: Pl = plagioclase, Spl = spinel, Grt = garnet, Ky = kyanite, Px = pyroxene, Cr-Di = Cr-diopisde, Am = amphibole, Phl = phlogopite.