Mesozoic Magmatism in the Ivrea-Verbano Zone and its geodynamic implications.

abs. 10.1474, 01-0560.In the Southern Alps, Mesozoic magmatism is well documented in the Dolomite region and in the Lombard Pre-Alps. In the western portion of the Southern Alps, the Mesozoic magmatic activity has received much less attention.In the present study we report a review of geochronological evidences of the existence of Mesozoic magmatism in the portion of the Southern Alps west of the Lugano lake. The Mesozoic events are exclusively located: a) south of the Cremosina fault system; b) in the northeasternmost portion of the Ivrea-Verbano Zone.a) A suite of diorite-norite dykes occurring into the Baldissero mantle peridotite (Southern Ivrea-Verbano Zone) has been recently recognized. The age of dyke intrusion is currently constrained by: a) a two point mineral (plagioclase + clinopyroxene) best fit calculated from Sm-Nd isotopic data which yields a slope corresponding to an age of 18026 Ma, with a Ndi = 0.512804 and Ndi = 7.8 (Obermiller, 1984, PhD Thesis Un. Mainz, Germany); b) Re-Os model ages (Re depletion model age) between 140 and 190 Ma calculated on whole rock samples of the ambient peridotite (Mazzucchelli et al., 2004, EGU04 Geophys. Res. Abs., 6, 03966). Moreover a number of acid tuff layers from mm to several meters in thickness are present in the Mesozoic sedimentary cover of the Southern Alps from the Lugano to Biella area. They occur in the Crevacuore and Sostegno sedimentary succession, in the Villafortuna-Trecate oil field, and in the sedimentary cover of Monte San Giorgio (Ticino, Switzerland). In the Monte San Giorgio occurrence, high-resolution U-Pb zircon age gives 241±0.8 Ma (Mundil et al., 1996, Earth Planet. Sci. Letters, 141, 137-151). b) In the Basic Complex cropping out in the Finero and Val Grande area, most of the isotopic data invariably give Triassic or Early Jurassic ages. The Basic Complex in this area shows an antiformal structure, constituted by various cumulus rocks and gabbroic lithotypes (Internal Gabbro, Hornblende Peridotite and External Gabbro Units). At the core of the antiform, in tectonic contact with the rocks of the Complex, a mantle phlogopite-bearing peridotite occurs, whose metasomatic imprint was attributed to crustal components, deriving from a subducting slab. The age of metasomatism is Mesozoic [207 Ma - U/Pb on zircons from chromitites (Von Quadt et al., 1992, Ivrea-Verbano Zone Workshop, U.S. Geol. Survey Circular 1089, Abs., 20.); 226-177 Ma - Rb/Sr internal isochrons on amphibole and phlogopite pairs (Hartmann & Wedephol, 1993, Geochim. Cosmochim. Acta, 57, 1761-1782); 220 Ma - Ar/Ar on phlogopite (Hartmann & Wedephol, 1993)]. Magmatic, subeuhedral, pink crystals with oscillatory zoning in CathodoLuminescence (CL) from the External Gabbro Unit has been recently dated with SHRIMP (Peressini et al., 2005, this session). 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. These ages are well in accordance with the literature data reported for the Basic Complex. In spite of the similar ages, the Basic Complex does not record any evidence of the metasomatic agent which affected the mantle peridotite. The Finero Basic Complex is in tectonic contact, marked by a high-temperature ENE shear-zone, with the Permian (Peressini et al., 2005, this session) relatively anhydrous mafic-ultramafic sequences occurring in Val Sesia and on the right side of the Val d'Ossola.Presently, the evidences of Mesozoic magmatism in the westernmost sector of the Southern Alps are confined by tectonic lineaments to the southernmost and northernmost portion, respectively. This put new constraints for the comprehension of the geodynamic reconstruction of the whole Southern Alps.