Age, Geochemistry and Petrogenesis of the Ultramafic Pipes in the Ivrea Zone, NW Italy

Pipe-like ultramafic bodies, hosting Ni–Cu–PGE sulphide deposits, INTRODUCTION intrude the Main Gabbro and the roof metasediments of the Ivrea The Ivrea Zone, in NW Italy, is believed to represent an Zone, NW Italy. These bodies were emplaced at 287 ± 3 Ma exposed section of the lower continental crust (Mehnert, and represent the last mantle-derived melts associated with an 1975; Garuti et al., 1980; Percival et al., 1992) whose underplating event that largely drove the crustal evolution of this area evolution has been largely driven by the underplating during the late Carboniferous (>300–290 Ma). The pipes are of mantle-derived basic magmas (the Basic Complex), composed of volatile-rich ultramafic rocks and gabbros with an associated with the uplift of large fragments of mantle alkaline signature simultaneously enriched in both incompatible and peridotites (the Mantle Tectonite massifs of Baldissero, the most compatible elements but depleted in elements of intermediate Balmuccia and Finero), beneath the metamorphic basecompatibility. The isotope composition of these pipe rocks is 290 Ma(Nd) ment of the Southern Alps (the Kinzigitic Formation) >3·7 to −1·9 and 290 Ma(Sr) >0·8–26. In a 290 Ma(Nd) vs (Fig. 1). Although this area has been the object of 290 Ma(Sr) diagram they define a linear array between the unnumerous studies that have significantly contributed to metasomatized and metasomatized peridotites of Finero, but distinctly the understanding of the evolution of the lower conoblique with respect to the trend defined by Balmuccia peridotites. tinental crust [see Percival et al. (1992) and references The S ranges from 0·0 to +0·9‰ and is indicative of a mantle therein], many aspects of the underplating process, espesource. We suggest that the pipes represent infiltration of melts derived cially the timing of emplacement of basic magmas and from a depleted mantle protolith flushed with alkaline metasomatic whether the environment was a mantle plume (e.g. fluids, probably of juvenile mantle origin, which underwent partial Shervais & Mukasa, 1991) or a subduction setting (e.g. melting as a consequence of the depression of the solidus owing to Hartmann & Wedepohl, 1993; Zanetti et al., 1999), are the increased activity of water and other volatiles. The similarity in still unresolved. age, trace-element, and isotopic signatures indicates that the pipes A particularly striking group of intrusions, whose study were probably produced in the course of the same metasomatic event may be relevant for understanding the geodynamic setting that affected the Finero ultramafic body. The overall geochemical and evolution of the Ivrea Zone, consists of pipe-like characteristics of the pipes are more consistent with magmatism related bodies formed of amphibole-rich peridotites and pyto a mantle plume than with a subduction setting. roxenites with minor hornblendites and gabbros, an-