Metamorphism and anatexis in the mafic complex contact aureole, Ivrea zone, northern Italy

Emplacement of mantle-derived magma (magmatic accretion) is often presumed or inferred to be an important cause of regional granulite facies metamorphism and crustal anatexis. The juxtaposition of mafic cumulates and regionally distributed granulite facies rocks has led some to consider the Ivrea zone (northern Italy, Southern Alps) as an important exposure that demonstrates this causal relationship. However, regional PTt paths indicated by metamorphic reaction textures and PT conditions inferred from geothermobarometry indicate that the emplacement of mafic plutonic rocks (Mafic Complex) at the Ivrea zone occurred during decompression from ambient pressures at the regional thermal maximum. Field and petrographic observations, supported by PT estimates, indicate that regional retrograde decompression and emplacement of the upper parts of the Mafic Complex probably accompanied extension during the Late Carboniferous-Early Permian. A spatially restricted decompression-melting event accompanied final emplacement, depleting supracrustal rocks enclosed by an similar to 2-3 km aureole overlying the upper Mafic Complex by 20-30% granite component. The upper Mafic Complex provided the thermal energy to reset mineral assemblages and locally overprint the regional prograde metamorphic zonation. The limited extent of the contact aureole suggests that magmatic accretion may not inexorably cause regional metamorphism and crustal anatexis.