Giovanni Cortiana 1, Giorgio V. Dal Piaz* 1, Aldo Del Moro 2, Silvana Martin 1, Giorgio Pennacchioni 1, Paola Tartarotti 1
|Dipartimento di Geologia e Geofisica, Università degli Studi, Padova, Italy|
|Istituto di Geocronologia e Geochimica Isotopica, CNR, Pisa, Italy|
|Correspondence: via Giotto 1, I-35137 Padova, Italy (email@example.com)|
In the late few years the reliability of a generalised Eoalpine (Cretaceous-Paleocene) age of the high-P imprint wholly across the Western Alps, mainly inferred from K-Ar determinations, has been lowered by a growing body of Tertiary datings on retentive systems (Gebauer, 1999, for review) and recognition of Ar excess in phengitic micas (e.g., Arnaud and Kelley, 1995; Ruffet et al., 1997). Indeed, whereas the eclogitic climax in the Sesia-Lanzo zone was restricted to the Late Cretaceous, that in the ophiolitic Zermatt-Saas nappe (Bowtell et al., 1994; Cliff et al., 1998; Rubatto et al., 1998; Mayer et al., 1999) and its southern extension (Monviso) was rejuvenated to the Eocene. Therefore the hypothesis of two subduction events separated in space and time (Late Cretaceous vs Eocene in the Austroalpine and Penninic zone, respectively), recently favoured by some authors (e.g., Froitzheim et al., 1996; Michard et al., 1996; Rubatto et al., 1998), opposite to continuing contraction under persistent low thermal conditions.
It may be noted that in the north-western Alps the modern datings concern only the Sesia-Lanzo zone and the Zermatt-Saas nappe in the classic area, north of the Aosta-Ranzola fault, disregarding other poorly or never dated units of the Austroalpine-Piedmont nappe stack, like the Combin zone and the Austroalpine external slices. In this view, we understood a radiometric study on the relatively high-P Dent Blanche-Mont Mary-Pillonet thrust system (Cortiana et al. 1998), the high-P Austroalpine outliers and the underlying ophiolitic sequences.
This presentation focuses on the eclogitic metamorphism of previously undated Austroalpine outliers and underlying Zermatt-Saas ophiolites, mostly located between the Aosta valley and the Gran Paradiso massif. Rb-Sr datings on twelve phengitic and Mn-rich high-P white micas yield Eocene ages (45-42 Ma) in the Zermatt-Saas mafic eclogites and cover metasediments of the St. Marcel-Urtier area (Mt. Avic massif), similar to those previously obtained north of the Aosta-Ranzola fault. The same time interval (49-40 Ma) has been detected for the eclogitic imprint in the Mt. Emilius, Glacier-Rafray and Etirol-Levaz basement slices, showing that these external Austroalpine and Piedmont units were concurrently subducted during the Early-Middle Eocene, 25-20 Ma later than the relatively high-P Pillonet klippe (74 Ma), the eclogitic Sesia-Lanzo zone and possibly also the underlying Combin accretionary prism.
40Ar/39Ar determinations on the same mica concentrates are affected by Ar excess in the Austroalpine outliers (92.1 Ma), poorly or not in their high-P ophiolitic footwall (plateau ages: 46.1-43.4 Ma). In conclusion, the tectonic subdivision of the Austroalpine-Piedmont nappe stack into two contrasting groups of nappes (Ballèvre et al., 1986; Cortiana et al., 1998) is corroborated by diachronous subduction and exhumation histories: the upper and older group, non-eclogitic, is represented by the Dent Blanche-Mont Mary-Pillonet thrust system plus the Combin zone, the lower and younger group by the Austroalpine outliers plus the Zermatt-Saas nappe, all eclogitic.
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