New Sm-Nd ages for the Ivrea-Verbano Zone, Sesia and Sessera valleys (Northern-Italy)

The Ivrea-Verbano Zone (IVZ) is a slice of South-Alpine lower crust intrude d by mantle magma. The main episode of magmatic underplating occurred durin g the Permian time and was related to continental rifting. We have carried out mineral Sm-Nd isochron dating of samples from the IVZ, including a gabb ro dyke on which apparent Oligocene zircon ages have been reported, and a m etapelite. The gabbro dyke sampled yielded three isochrons (cpx, opx, plag, amph and whole rocks) with the same initial Nd-143/Nd-144 and an age of 27 4 +/- 11 Ma (95% c,l,, MSWD = 1.31) for the last major thermal event. Since this dyke is an embayment of the surrounding layered gabbro in the Balmucc ia Peridotite, this age also represents the minimum age for the cooling of mafic-ultramafic sequence and for the emplacement of the Balmuccia peridoti te in the IVZ. These conclusions are in contrast with those of Gebauer et a l, (1992), who reported Oligocene U-Pb ages for magmatic zircons extracted from the same rocks. In order to evaluate the effect of Alpine intrusions o n the IVZ, a sample of amphibole gabbro from the upper Sessera Valley was c ollected near Oligocene magmatic rocks. An internal Sm-Nd isochron (amph cpx + plag + wr) from this sample yielded an age of 267 +/- 21 Ma (95% c,l, , MSWD = 0.716). Thus, despite the vicinity of the Oligocene magmatic activ ity, no thermal overprint (at T> 650 degrees C) occurred in this part of th e IVZ more recently than 267 +/- 21 Ma. Furthermore, a paragneiss septum wi thin a shallower part of the intrusion in the lower Sessera Valley yielded a middle Triassic Sm-Nd garnet, biotite, plagioclase and whole rock age of 244 +/- 4 Ma (95% c.l), (C) 2000 Elsevier Science Ltd. All rights reserved.