U-Pb SHRIMP and REEs - LA-ICPMS zircon data constrain the Anisian emplacement of the External Gabbro unit of the Finero Mafic Complex in the Ivrea-Verbano Zone (South-Alpine, NW-Italian Alps): new hints on the Triassic evolution of Adria.

abs. 10.1474, 01-0560.The northern part of the mafic-ultramafic Ivrea-Verbano (IV) complex, the Finero region, differs from the rest of the pluton in terms of petrology and geochemistry of mantle peridotites (e.g. the phlogoplte-bearing metasomatism shown by Flnero harzburgite is absent in the other mantle massifs of IV zone), as well as for the lithoiogy and geochemistry of mafic intrusions. Besides, the Finero Mafic Complex shows different relationship with the Kinzigite Formation, the metamorphic sequence into which it intruded. The emplacement age of the Mafic Complex in the Balmuccia area has been proved to be homogeneously between 282 and 189 Ma (Peressini & al, this congress).Conversely, in this contribution we provide evidence for a substantially different age of emplacement of the External Gabbro unit of the Finero Mafic Complex. We performed a detailed zircon study on 5 samples from the External Gabbro, most of which yielded a homogeneous population of colourless zircons. Only one sample showed a composite population, comprised of:a) maqmatic, subeuhedral, pink crystals with oscillatory zoning in cathodoluminescence (CL);b) detrital zircons, rounded, small, colourless grains with blurred, "granulitic" CL-patterns.Three events, clearly recognczable in CL, were dated with SHRIMP U-Pb analyses:1) primary magmatic crystallization of the pink zircons took place at 232 +/- 3 Ma;2) the pink zircon rims suffered a distinct recrystallization event at 215 +/- 4 Ma;3) a metamorphic age-peak at 280-310 Ma is clearly, although imprecisely (U below: 20 ppm), constrained by the colourless zircons.The U-Pb data of the colourless zircons are consistent with the age of the major event (ca. 300 Ma) that resets the U-Pb system of the zircons from the Kinzigite Formations, being compatible with both magmatic and metamorphic ages in othe sectors of the IV-zone (Vavra & al., Contr. Min. Pet., 134, 1999). This observation together with the degree of crustal contamination revealed by Nd and Sr isotopes on the same samples (Lu & al, Chem.Geol. 40, 1997), suggests that colourless zircons and zirconium were inherited/digested ffom the country rock. No magmatic Paleozoic zircon was found, as opposed to the predominant Permo-Carboniferous magmatism in the Baimuccia sector of the IV-Zone.Trace elements were measured on all zircon domains, and confirm the occurrence of a geochemical difference between the two age-groups. Triassic zircons have typical HREE-enriched strongly fractionated patterns from which those of the Permo-Carboniferous ones differr for the slightly more flattened HREE. Moreover, the Nb/Ta is ca. 1 for Mesozoic zircons, but ca. 10 for the Paleozoic ones. SHRIMP (U-Pb) and LA-ICP-MS (traces) zircon data presented in this study allow us to recognize an Anisian emplacement of the External Gabbro unit. A separate thermal event took place in Norian time at 215 Ma (see also Stähle & al, SMPM 70, 1990; von Quadt & al, SMPM 73, 1993, Vavra & Schaltegger, Contr. Min. Pet. 134, 1999). The Anisian emplacement of tholeiitic rocks in the western South-Alpine, and the possibly Triassic age of some events of mantle metasomatism in Finero (Grieco & al. 2001, J. Pet. 52, Zaccarini, in prep.), constitute importantinformationf or the interpretation of Mesozoic magmatism in the Southern Alps, and can have regional-scale implications for the geodynamic events preceding the opening of the Neo-Tethys.In the IV zone, magmatic ages cluster in two ranges:1. Permo-Carboniferous (280-315 MA, Peressini $ al., this congress);2. Early Mesozoic (Zircon U-Pb: Finero 210-235 Ma, this study, but also: Rimella 200-250 Ma Surface Controlled Alteration, and Val Fiorina, 215-245 Ma Vavra $ al., 1999; Sm/Nd ages Baldissero 160-200 Ma, Mazzucchelli $ al., 2005), an age distribution similar to that of most gabbroic intrusions in the Alpine-Appenine system (Rampone & al, J. Pet. 46, 2005).In Finero, the Paleozoic and the two Mesozoic age-peaks correspond to distinct episodes of heating, melting and metasomatism.