SHRIMP and IDTIMS U-Pb zircon ages of the pre-Alpine basement in the Internal Western Alps (Savoy and Piemont)

Conventional and SHRIMP II U-Pb age determinations are reported for orthogn eiss zircons from four basement units of the Internal Western Alps, and for detrital zircons from the Belledonne External Crystalline Massif (ECM). Th e results lead to a reappraisal of the geotectonic evolution of the Pennini c basements (Brianconnais and Piemont) and their pre-Alpine origin. Except for the emplacement of the Cogne granodiorite (Valle d'Aosta) dated at 357 +/- 24 Ma (conventional IDTIMS analyses) and 356 +/- 3 Ma (SHRIMP analyses) , little evidence has been found for a Variscan imprint. However, results f rom the Peclet orthogneiss (482 +/- Ma SHRIMP) and Modane metagranite (452 +/- 5 Ma - SHRIMP) in the Sapey gneiss unit, and the Ambin metarhyolite (50 0 +/- 8 Ma SHRIMP) from the Ambin massif, show that a major plutonic and te ctonic event occurred at 450-500 Ma. Evidence has also been found for a maj or plutonic event of Permian age (269 +/- 6 Ma SHRIMP age from the Gran Par adise orthogneiss) which suggests important Paleotethyan activity at least in the Piemont basement. SHRIMP dating of detrital zircons from a metasedim ent of the Belledonne massif (ECM) and zircon cores and xenocrysts found in most of the analysed magmatic rocks show a large Pan-African age component (590-630 Ma) in both the External and Internal Alps. This suggests that th ere is little difference in the composition of the basement between (1) the ECM, which show a clear continuity with Variscan Europe, (2) the Penninic basements, which may represent allochthonous Alpine terranes, and (3) the S outhern Alpine and Austro-Alpine domains, classically attributed to an ''Af rican" indenter. They all belong to Gondwana but differ strongly in their V ariscan and Alpine history.