Triassic emplacement of the External Gabbro unit of the Finero mafic complex: U-Pb SHRIMP zircon ages and their implications for the Ivrea-Verbano Zone, Western Italian Alps.

SRef-ID: 1607-7962/gra/EGU04-A-05072The Mafic Complex (MC) of the Ivrea Zone displays distinctive features in different sectors. Apart from petrology and geochemistry of the mantle peridotites (e.g. the strong metasomatism of Finero is absent in Balmuccia), the Finero region differs from the rest of the complex in stratigraphy, lithology and geochemistry of the mafic intrusives, and their relationships with the Kinzigite Formation, the metamorphic sequence into which the MC intruded. We give new evidence for a substantial difference also in the age of magmatism. The age of emplacement of the MC in the Val Sesia area has been recently proved to be homogeneous, between 283 and 289 Ma, after a thermal, upper-amphibolite event that affected the country rock at 320-to-310 Ma. At Finero, published Sm-Nd isochrones span 203-533 Ma, zircon ages span 208-549 Ma (Lu et al, 1997, Chem.Geol. 140, 223-235, Grieco et al, 2001, J. Pet. 52, 89-101). The more composite nature of the Finero sector is possibly hiding a more articulated geological evolution of the MC, as suggested by the presence of numerous high-T shear-zones (e.g., Kenkmann, 2000, J.Struc.Geol. 22, 471-487, Manckeltow etal., 2002, J.Struc.Geol. 24, 567-585, and ref. therein). We performed a detailed zircon study on samples from the External Gabbro unit of the complex, most of which have a homogeneous population of colorless zircons. The key for interpretation is one sample, bearing both magmatic, subeuhedral, pink crystals with oscillatory zoning in CL, and detrital zircons, rounded, small, colorless grains with blurred CL-patterns. The three events recorded by these zircons are clearly recognizable in CL, and were dated with SHRIMP U-Pb analyses. Magmatic growth of the zircons took place at 232+/-2 Ma and was overprinted at 214+/-5 Ma by a second event, dated by the rim-recrystallization ages. A 280-to-310 Ma age peak is clearly, though imprecisely constrained by the colorless zircons, proving that older ages are preserved, but must be considered detrital. Two problems arise: reason and significance of the older age peak. Together with the positive correlation between zircon yield and degree of crustal contamination (Nd-Sr isotopic compositions) of the studied samples, the U-Pb data allow to conclude that zircons and zirconium were inherited/digested from the country rock. The latter underwent a major event that reset the U-Pb system in zircon around 300 Ma, compatible with both magmatic and metamorphic ages in the MC of the Balmuccia sector. In the External Gabbro, though, no magmatic Carboniferous or Permian zircon was found: emplacement took place at 232 Ma, and a separate thermal event is dated at 214 Ma. The new data extend as far west as the Ivrea zone the evidence of the important Mesozoic magmatism of the central and eastern South Alpine. Mesozoic igneous activity in the Finero region is clearly distinct from the predominating Permo-Carboniferous magmatism in the Ivrea-Verbano Zone. Also, as for most gabbroic intrusions in the alpine-appenine system, ages in the Ivrea Zone cluster in two ranges, at either 285-310 Ma, or at 200-250 Ma. This distribution is thoroughly represented in the zircons from one sample only, the ages of which represent distinct episodes of heating, melting and metasomatism.