Regional granulite facies metamorphism in the Ivrea zone: Is the Mafic Complex the smoking gun or a red herring?

Abstract

One widely accepted paradigm for the development of continental lower crust is that regional granulite facies metamorphism is caused by intrusion of mafic magma beneath or into the crust (magmatic accretion). The amphibolite to granulite facies supracrustal section exposed in the Ivrea zone (southern Alps, northern Italy) is commonly cited as a classic example establishing this postulated genetic relationship. Our interpretation of the pattern of metamorphic isograds, compositional trends in high-grade metasedimentary rocks, and textural evidence in metapelite, however, indicates that final emplacement of the mafic plutonic rocks (the Mafic Complex) occurred subsequent to the regional thermal maximum. Field and petrographic relations suggest that a spatially restricted contact-melting event in crustal rocks accompanied the emplacement of the Mafic Complex. This inference is consistent with leucosome compositions in migmatites and a low-pressure, high-temperature metamorphic overprint recorded by mineral assemblages in wall rocks proximal to the intrusion. Therefore, evidence of anatexis and metamorphism of crustal rocks associated unequivocally with emplacement of the Mafic Complex is found only within an ~2-km-wide contact aureole overlying the intrusion. The narrow aureole associated with emplacement of the Mafic Complex demonstrates that, in some cases, emplacement of large volumes of mafic magma within the crust does not inexorably lead to regional-scale granulite facies metamorphism and large ion lithophile element depletion by melt loss.