Triassic U-Pb SHRIMP Ages on magmatic Zircons from the External Gabbro unit of the Finero mafic complex, Ivrea Zone, Western Italian Alps.

abs. 236-38The northern part of the mafic-ultramafic Ivrea Verbano complex, the Finero region, differs from the rest of the complex in many features: petrology and geochemistry of mantle peridotites, stratigraphy, lithology and geochemistry of igneous bodies, relationships with the metamorphic sequence (Kinzigite Formation) into which the complex intruded. We provide evidence for a substantial difference also in the age of emplacement. The Mafic Complex (MC) in the Val Sesia area has been recently proved to have intruded between 283 and 289 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, and ref. therein).We performed a zircon study on 5 samples from the External Gabbro unit (EG) of the complex. Three events are revealed by the SHRIMP U-Pb results on the 2 most representative samples, one of which has a composite population of magmatic and detrital zircons, clearly distinguished for grain-morphology, color and CL-pattern. Primary crystallization of the pink magmatic zircons was dated at 232±2 Ma; these were overprinted at 214±5 Ma by a second event, dated by the rim-recrystallization ages. A 280-to-310 Ma age peak is clearly, but poorly constrained by the colorless zircons (U<20 ppm), proving that older ages are preserved, but must be considered detrital. This is further confirmed by the ages recorded in the second sample, which yielded no magmatic zircons, but only metamorphic grains. As for the significance of the older age peak, zircon yield and degree of crustal contamination (Nd-Sr isotopic compositions) of the studied samples suggest that zircons and zirconium were inherited/digested from the country rock. This latter underwent a major event that opened and reset the U-Pb system in zircon around 300 Ma, compatible with ages in the MC of the Balmuccia sector.In the EG, though, no magmatic Carboniferous or Permian zircon was found: the EG was emplaced in Anisian-Ladinian time at 232Ma, and a separate thermal event took place in Norian time at 214 Ma (Staehle et al., SMPM 70, 1990; von Quadt et al., SMPM 73, 1993). The clearly documented Mesozoic igneous activity, distinct from the predominant Permo-Carboniferous magmatism in the Ivrea-Verbano, and the possibly Mesozoic age of mantle metasomatism at Finero (Grieco et al, 2001, J. Pet.52, 89-101) suggest revisiting the current interpretations of Mesozoic magmatism in the Southern Alps of Lombardy and Trentino regions.