Metamorphic evolution of the south-western Ivrea-Verbano Zone (Ivrea Town area, NW Italy): metamorphic textures, mineral assemblages and P-T evolution.

In the Italian Alps, the Ivrea-Verbano Zone (IVZ) is known as one of the best preserved, i.e., not re-equilibrated during Alpine metamorphism, Variscan Units and, from SW to NE, it extends from Ivrea to Locarno. The sub-units constituting the IVZ are the Kinzigite Formation (supracrustal metapelites intercalated with marbles and metamafic rocks), the Peridotitic Massifs (Baldissero, Balmuccia and Finero) and the gabbroic Mafic Complex. The well-studied lithologies from Val Strona di Omegna and Val Sesia provide evidence of a nearly completed section from the lower crust (e.g., mafic granulites, metamafic migmatites, migmatitic metapelites) to the middle crust (e.g., amphibolites, marbles, calc-silicates and minor quartzites). In the present work, we focus on the less-studied area around Ivrea town to provide further insights into the P-T-X evolution of the IVZ.

Our field work shows that the main attribute of the Ivrea outcrops is the presence of metamafic rocks intercalated with enderbitic granulites and minor high-grade metapelites (stronalites). The lithologies and their field relationships are compatible with the metamafic septa intercalated with migmatitic meta-sediments of pelitic to psammitic composition and calc-silicates of the Kinzigite Formation described in the Val Strona di Omegna and Val Sesia areas.

The metabasites (orthopyroxene + clinopyroxene + plagioclase + amphibole + spinel + magnetite + ilmenite) are two-pyroxene granulites devoid of garnet. They are characterized by the widespread presence of brown amphibole, whose volume may locally exceed 20% of the rock (point-counting data). The enderbitic granulites consist of orthopyroxene + plagioclase + ilmenite + magnetite, relict calcic plagioclase + K-feldspar ± quartz, and minor retrograde amphibole and biotite. The stronalites are metapelites consisting of garnet + plagioclase + sillimanite + quartz + rutile + relict biotite. Despite the very simple mineralogy, more than one generations of the same mineral assemblage have been identified in the studied rocks by both textural relationships and mineral chemistry. These data suggest a complex metamorphic evolution of the studied area.

Preliminary P-T estimates (winTWQ software) have been obtained for each mineral assemblage of the two-pyroxene granulites. The results suggest a prograde-to-peak evolution under amphibolite- to granulite-facies conditions. Pressure is not higher than 5,5 – 6,5 kbar, in agreement with the absence of garnet. The temperature varies depending on the considered mineral assemblage.

Our data suggest that the Ivrea area belongs to the Kinzigite Formation and corresponds to a lower crust at the transition with a middle crust. The peculiar presence of the enderbitic granulites suggests a more complex evolution of this area with respect to the Val Strona di Omegna and Val Sesia areas.