U-PB ZIRCON DATING OF GRANITOIDS FROM THE DORA-MAIRA MASSIF (WESTERN ITALIAN ALPS)

Among the large number of granitic intrusions within the Dora-Maira ma ssif, several main types can be distinguished. In this study we report field, petrographic and geochemical investigations as well as zircon typology and conventional U-Pb zircon dating of plutons representing t hese types. The main results are as follows: the Punta Muret augengnei ss is a polymetamorphosed peraluminous granite of anatectic origin. It is 457 +/- 2 Ma old and represents one of the numerous Caledonian ort hogneisses of the Alpine basement. All other dated granites are of Lat e Variscan age. The Cavour leucogranite is an evolved granite of proba bly calc-alkaline affiliation, dated at 304 +/- 2 Ma. The dioritic and granodioritic facies of the Malanaggio diorite (auct.) are typical ca lc-alkaline rocks, whose respective age of 290 +/- 2 and 288 +/- 2 Ma overlap within errors. The Sangone and Freidour granite types have ver y similar alkali-calcic characteristics; their ages are poorly constra ined between 267-279 and 268-283 Ma, respectively. The new data for th e Dora-Maira granites are in keeping with models of the overall evolut ion of the Late- to Post-Variscan magmatism in the Alpine area in term s of age distribution and progressive geochemical evolution towards al kaline melts. In a first approximation, granitic rocks across the Vari scan belt seem to be increasingly younger towards the internal (southe rn) parts of the orogen. A Carboniferous, distensive Basin and Range s ituation is thought to be responsible for the magmatic activity. This tectonic context is comparable to the back-are opening of an active co ntinental margin. The observed southward migration of the magmatism co uld be linked to the roll-back of the subducting Paleotethyan oceanic plate along the Variscan cordillera.