Metamorphism and Anatexis in the Mafic Complex Contact Aureole, Ivrea Zone, Northern Italy

Abstract

Emplacement of mantle-derived magma (magmatic accretion) is the total thermal budget of the continental lower crust, often presumed or inferred to be an important cause of regional produce regional granulite facies metamorphism, and granulite facies metamorphism and crustal anatexis. The juxgenerate Yand heavy rare earth element (HREE)taposition of mafic cumulates and regionally distributed granulite depleted granitoids (Ellis, 1987). Accretion of mafic facies rocks has led some to consider the Ivrea zone (northern Italy, magma and migration of partial melt will internally Southern Alps) as an important exposure that demonstrates this stratify, chemically differentiate, and deplete the concausal relationship. However, regional PTt paths indicated by tinental lower crust in large ion lithophile elements metamorphic reaction textures and PT conditions inferred from (LILE). Magmatic accretion has been invoked to provide geothermobarometry indicate that the emplacement of mafic plutonic the heat and mass necessary for sustained magmatism in rocks (Mafic Complex) at the Ivrea zone occurred during detectonic settings such as Phanerozoic extensional terranes compression from ambient pressures at the regional thermal maximum. (Lister et al., 1986; Gans, 1987; Fountain, 1989; Mareschal Field and petrographic observations, supported by PT estimates, & Bergantz, 1990; Jarchow et al., 1993) and magmatic indicate that regional retrograde decompression and emplacement of arcs (Hamilton, 1981; Kay & Kay, 1981; Bohlen & the upper parts of the Mafic Complex probably accompanied Lindsley, 1987; Hildreth & Moorbath, 1988). extension during the Late Carboniferous–Early Permian. A spatially However, as few exposed sections of lower continental restricted decompression-melting event accompanied final emcrust show contiguous mafic intrusions and regionally placement, depleting supracrustal rocks enclosed by an >2–3 km distributed granulite facies rocks, estimates of the extent aureole overlying the upper Mafic Complex by 20–30% granite of anatexis and metamorphism accompanying magmatic component. The upper Mafic Complex provided the thermal energy accretion have relied on numerical and analog simto reset mineral assemblages and locally overprint the regional ulations. These models yield disparate results depending prograde metamorphic zonation. The limited extent of the contact on whether heat transfer within the mafic intrusion aureole suggests that magmatic accretion may not inexorably cause and surrounding country rocks is primarily convective regional metamorphism and crustal anatexis. (Campbell & Turner, 1987; Huppert & Sparks, 1988) or conductive (Marsh, 1989; Bergantz & Dawes, 1994; Barboza & Bergantz, 1996). A comparison with field evidence is required to discriminate between the model