The Austroalpine Sesia±Lanzo inlier and upper Austroalpine Dent Blanche, Mt. Mary and Pillonet outliers occur on top of the western-Alpine orogenic wedge and, as a whole, override the structurally composite ophiolitic Piemonte zone. Instead, the Mt. Emilius, Glacier±Rafray, Etirol±Levaz and other lower Austroalpine eclogitic outliers are inserted within the Piemonte zone, between its upper (Combin) and lower (Zermatt±Saas) tectonic elements, or within the latter. Rb±Sr dating on phengitic micas show that the eclogitic imprint in the lower Austroalpine outliers, conventionally regarded as Late Cretaceous by comparison with the Sesia±Lanzo inlier, is of Eocene age (49±40 Ma), like the underlying Zermatt±Saas ophiolite (45±42 Ma) between the Aosta valley and Gran Paradiso massif. 40 Ar± 39 Ar plateau ages on the same mica concentrates of the ophiolitic Zermatt±Saas nappe (46±43 Ma) are consistent with Rb±Sr dating, whereas that on the Austroalpine Glacier±Rafray klippe (92 Ma) is influenced by argon excess. The lower Austroalpine outliers underwent the subduction metamorphism concurrently with the Zermatt±Saas nappe, 20±25 Ma later than the eclogitic Sesia±Lanzo inlier and blueschist Pillonet klippe. The temporal gap and present intra-ophiolitic position mean that the lower Austroalpine outliers were probably derived from an intraoceanic extensional allochthon (Mt. Emilius domain) stranded inside the Piemonte±Ligu-rian ocean far from the Dent Blanche±Sesia domain and Adriatic margin.Keywords 40Ar±39Ar´Eocene eclogitic metamorphism´Italian Western Alps´Rb±SrŹ ermatt±Saas ophiolite

À12, e Hf = À11) accounts for the enrichment of K and other large-ion-lithophile elements in the Italian volcanics. As shown by the relationship in e Hf -e Nd space and the lower-thanchondritic Hf/Sm ratio, this crustal component is dominated by pelagic sediments rather than terrigenous material. The overall scarcity of calc-alkaline compositions in the Italian volcanics and the presence of a HIMU component, which is the hallmark of hot spot basalts, raise the question of how plume mantle source contributes to volcanism in a subduction environment. At about 13 Ma, the Apennine collision terminated the westward subduction of the Adria plate under the European margin and rotated the direction of convergence to the northwest. The cumulative differential of subduction between the fossil plate under Tuscany and the active plate under Sicily since the opening of the Tyrrhenian Sea amounts to at least 300 km and is large enough to rift the dipping plate and open a plate window beneath the southern part of the peninsula. This model is consistent with recent high-resolution seismic tomography. We propose that the counterflow of mixed upper and lower mantle passing the trailing edge of the rifted plate is the source of Italian mafic volcanism. Alternatively, material from a so-far unidentified plume may be channeled through the plate window. The crustal signature is probably acquired by interaction of the mantle advected through the window with the upper part of the subducted plate.

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