Age, geochemistry and petrogenesis of the ultramafic pipes in the Ivrea Zone, NW Italy

Pipe-like ultramafic bodies, hosting Ni-Cu-PGE sulphide deposits, intrude the Main Gabble and the roof metasediments of the Ivrea Zone, NW Italy. These bodies were emplaced at 287 +/- 3 Ma and represent the last mantle-derived melts associated with an underplating event that largely drove the crustal evolution of this area during the late Carboniferous (similar to 300-290 Ma). nle Pipes are composed of volatile-rich ultramafic locks and gabbros with an alkaline signature simultaneously enriched in both incompatible and the most compatible elements but depleted in elements of intermediate compatibility. The isotope composition of these pipe rocks is E-290Ma(Nd) similar to3.7 to -1.9 and E-290Ma(Sr) similar to0.8-26. In a E-290Ma((Nd)) vs E290(Ma)(Sr) diagram they define a linear array between the unmetasomatized and metasomatized peridotites of Finero, but distinctlj oblique with respect to the trend defined by Balmuccia Peidotites. irhe delta S-34 ranges from 0.0 to + 0.9%(0) and is indicative of a mantle source. We suggest that the pipes represent infiltration of melts derived from a depleted mantle protolith flushed with alkaline metasomatic fluids, probably of juvenile mantle origin, which underwent partial melting as a consequence of the depression of the solidus owing to the increased activity of water and other volatiles. The similarity in age, trace-element, and isotopic signatures indicates that the Pipes were probably produced in the course of the same metasomatic event that affected the Finero ultramafic body. The overall geochemical characteristics of the pipes ale more consistent with magmatism related to a mantle plume than with a subduction setting.