A lead isotope study of the northeastern Ivrea zone and the adjoining Ceneri zone (N-Italy): evidence for a contaminated subcontinental mantle

Abstract

Lead isotope data on whole rocks, feldspars and sulfides from the Ivrea zone indicate that the magmas which formed the Hercynian (?) mafic complex derived from an isotopically anomalous mantle with a lead isotope composition that resembled average continental crustal leads. Contamination during magma emplacement played a minor role and probably accounts for second order variations of the μ values. Similar leads were observed in Caledonian orthogneisses, late-Hercynian granites and volcanics of the near-by Ceneri zone. Metasediments of the kinzigitic series of the Ivrea zone as well as metasediments of the Ceneri zone contain lead with high μ values which indicates of prolonged crustal history in agreement with the mean ages of 1900 to 2500 my of their detrital zircon populations. Amphibolites of the kinzigitic series still contain MORB-type leads, and contamination by metasediments played only a minor role in spite of the regional metamorphism in upper amphibolite facies. The trace leads of stratiform sulfide ores are variable mixtures of mantle-derived and crustal leads depending on the host rock lithologies which are either amphibolites, amphibolites plus metasediments or metasediments.