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A Nd and Sr isotopic study of the Ivrea zone,Southern Alps,N-Italy
Authors:H Voshage  J C Hunziker  A W Hofmann  A Zingg
Institution:(1) Max-Planck-Institut für Chemie, Saarstrasse 23, D-6500 Mainz, Federal Republic of Germany;(2) Abteilung für Isotopengeologie, Universität Bern, Erlachstrasse 9a, CH-3012 Bern, Switzerland;(3) Geologisches Institut, Universität Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland
Abstract:The Ivrea zone represents a tilted cross section through deep continental crust. Sm-Nd isotopic data for peridotites from Baldissero and Balmuccia and for a suite of gabbros from the mafic formation adjacent to the Balmuccia peridotite provide evidence for an event of partial melting 607±19 Ma ago in an extended mantle source with epsiv 607 Nd =+0.4±0.3. The peridotites are interpreted as the corresponding melt residue, the lower part of the mafic formation as the complementary melts which underwent further differentiation immediately after extraction. The Finero body represents a complex with layers of phlogopite peridotite, hornblende peridotite, and amphibole-rich gabbro. The isotopic signatures fall into two groups: (1) highly radiogenic Nd and low-radiogenic Sr characterize the phlogopite-free, amphibole-rich rocks, whereas (2) low-radiogenic Nd and highly radiogenic Sr is found in ultramafics affected by ldquophlogopite metasomatismrdquo. Phlogopite metasomatism in the Ivrea zone is dated by a Rb-Sr whole rock isochron yielding 293±13 Ma. It was fed by K-rich fluids which were probably derived from metasediments. The high initial epsiv 293 Nd value of about +7.5 for phlogopite-free samples indicates a high time-integrated Sm/Nd ratio in the Finero protolith 293 Ma ago. Sm-Nd analyses of metapelites from the paragneiss series yield Proterozoic ldquocrustal residence agesrdquo of 1.2 to 1.8 Ga. Internal Sm-Nd isochrons for three garnetiferous rocks show that closure of garnet at temperatures around 600° C or even lower occurred about 250 Ma ago.
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