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Journal of Petrology Volume 42 Number 2 Pages 433-457 2001
© Oxford University Press 2001
Age, Geochemistry and Petrogenesis of the Ultramafic Pipes in the Ivrea Zone, NW Italy
1DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF MODENA, ST EUFEMIA 19, MODENA 41100, ITALY
2DEPARTMENT OF MINERALOGY AND PETROLOGY, UNIVERSITY OF GRANADA, CAMPUS FUENTENUEVA, 18002 GRANADA, SPAIN
3DEPARTMENT OF EARTH SCIENCES AND ENVIRONMENTAL GEOLOGY, UNIVERSITY OF BOLOGNA, PIAZZA DI PORTA S. DONATA 1, I-40126 BOLOGNA, ITALY
Pipe-like ultramafic bodies, hosting Ni–Cu–PGE sulphide deposits, intrude the Main Gabbro 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 (
300–290 Ma). The pipes are composed of volatile-rich ultramafic rocks 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 
290 Ma(Nd) 3·7 to -1·9 and 290 Ma(Sr) 0·8–26. In a 290 Ma(Nd) vs 290 Ma(Sr) diagram they define a linear array between the unmetasomatized and metasomatized peridotites of Finero, but distinctly oblique with respect to the trend defined by Balmuccia peridotites. The 
34S ranges from 0·0 to +0·9
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 are more consistent with magmatism related to a mantle plume than with a subduction setting.
KEY WORDS: Basic Complex; Ivrea Zone; mantle plume; underplating; zircon age
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