THE MAFIC-ULTRAMAFIC COMPLEX NEAR FINERO (IVREA-VERBANO ZONE) .2. GEOCHRONOLOGY AND ISOTOPE GEOCHEMISTRY

Whole-rock Nd and Sr isotopic compositions of the mafic-ultramafic com plex near Finero demonstrate that the magma was derived from a deplete d, perhaps MORE-type mantle reservoir. The Sm-Nd data for the Amphibol e Peridotite unit can be interpreted as an isochron with an apparent a ge of 533 +/- 20 Ma, which is consistent with a Pb-207/Pb-206 evaporat ion age of 549 +/- 12 Ma of a single zircon grain from the Internal Ga bbro unit. However, the interpretation of these apparent ages remains open to question. We therefore retain the alternative hypotheses that the intrusion occurred either about 533 or 270 Ma ago, the latter brin g the most likely age of emplacement of the much larger magma body nea r Balmuccia (Val Sesia). The implication of the older emplacement age (if correct) would be that the igneous complex may be related to the n umerous amphibolite units, which are intercalated with the metapelites of the overlying Kinzigite Formation, and together with them may cons titute an accretionary complex. In this ease, the mafic-ultramafic com plex itself might also be part of such an accretionary complex (as has been proposed for the Balmuccia peridotite). Internal Sm-Nd isochrons involving grt, cpx, plag and amph from the Internal Gabbro unit yield concordant ages of 231 +/- 23, 226 +/- 7,223 +/- 10, 214 +/- 17, and 203 +/- 13 Ma. These results confirm published evidence for a separate , regional heating event about 215 +/- 15 Ma ago. Initial epsilon(Nd)( 533) values average + 6.3 +/- 0.4 for six samples of the Amphibole Per idotite unit and + 6.0 +/- 1.2 for ten samples of the External Gabbro unit. Sr-87/Sr-86 ratios require little or no age correction and range from 0.7026 to 0.7047 (with two outliers at 0.7053 and 0.7071). Stron g correlations between Sr-87/Sr-86 and K2O and weaker correlations bet ween initial epsilon(Nd) and K2O imply a comparatively minor (less tha n or equal to 10%) contamination of the External Gabbro magma by crust al material and a later alteration by a crustal or seawater-derived fl uid. These results contrast sharply with the isotopic composition (neg ative epsilon(Nd) and high Sr-87/Sr-86 values) of the associated mantl e rocks, the Phlogopite Peridotite unit, which has been pervasively me tasomatized by crustal fluids. This type of metasomatism and its isoto pic signature are never seen in the magmatic complex. This evidence ru les out any direct genetic relationship between the igneous complex an d the mantle peridotite. The crust-mantle interaction is the opposite of that seen at Balmuccia, where the mantle peridotite is essentially 'pristine' and the magmatic body has been extensively contaminated by assimilation of crustal rocks. (C) 1997 Elsevier Science B.V.