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 t he Main Gabble and the roof metasediments of the Ivrea Zone, NW Italy. Thes e bodies were emplaced at 287 +/- 3 Ma and represent the last mantle-derive d melts associated with an underplating event that largely drove the crusta l evolution of this area during the late Carboniferous (similar to 300-290 Ma). nle Pipes are composed of volatile-rich ultramafic locks and gabbros w ith an alkaline signature simultaneously enriched in both incompatible and the most compatible elements but depleted in elements of intermediate compa tibility. The isotope composition of these pipe rocks is E-290Ma(Nd) simila r to3.7 to -1.9 and E-290Ma(Sr) similar to0.8-26. In a E-290Ma((Nd)) vs E29 0(Ma)(Sr) diagram they define a linear array between the unmetasomatized an d 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 p ipes represent infiltration of melts derived from a depleted mantle protoli th flushed with alkaline metasomatic fluids, probably of juvenile mantle or igin, 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 tha t the Pipes were probably produced in the course of the same metasomatic ev ent that affected the Finero ultramafic body. The overall geochemical chara cteristics of the pipes ale more consistent with magmatism related to a man tle plume than with a subduction setting.