Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt–Saas ophiolite, western Alps

Abstract

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. ⁴⁰Ar–³⁹Ar 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–Ligurian ocean far from the Dent Blanche–Sesia domain and Adriatic margin.