The Pre-Alpine Basement of the Briançonnais (Wallis, Switzerland)

Abstract

The basement lithostratigraphy within the Middle Penninic domain in Wallis, Switzerland (formerly the Grand St. Bernard nappe), is described in order to try to produce reconstruction of the pre-Alpine history. The combined effect of pervasive Alpine deformation and metamorphism (mainly Tertiary greenschist facies overprint) as well as the lack of biostratigraphic and radiometric data make this attempt difficult. Polymetamorphic basements, affected successively by pre-Alpine metamorphism(s) and by Alpine metamorphism, are distinguished from monometamorphic basements, that apparently were only subject to this latter disturbance. The Pontis and Siviez-Mischabel nappes comprise both poly- and mono-metamorphic units while the rocks of the “Zone Houillère” and the Mont Fort nappe do not seem to have suffered Pre-Alpine metamorphism. Upper Proterozoic to lower Palaeozoic(?) siliciclastic meta-sediments of the polymetamorphic basement form a thick coarse-grained series overlaid by a dominantly pelitic series. Carbonate rocks (skarns, marbles) do not exist. A wide variety of magmatites (calc-alkaline granitoids, pyroxenites, felsic and mafic volcanites) are presumed to have been generated in different geodynamic environments from upper Proterozoic to lower Palaeozoic time. Two metamorphic preserved windows allow a partial reconstitution of the pre-Alpine metamorphic history: (1) outcrops of eclogites and retroeclogites (Siviez-Mischabel nappe), usually associated with banded amphibolites, show HP mineral assemblages (pyrope-rich almandine and omphacite) which are overprinted by an amphibolite-facies event (syn-Variscan?); (2) the most internal part of the Ruitor zone (Pontis nappe) is essentially composed of garnet-staurolite micaschists and aluminum silicate-bearing schists (andalusite-sillimanite). Textures and mineral chemistry support a possible evolution in staurolite-almandine subfacies and P-T estimates lead to a polyphase metamorphic trajectory, retrograde in pressure (10→2.5 kb) and thermally prograde (470→580 °C). Correlation with Variscan climax is likely, even if radiometric data is still lacking. Post-metamorphic late Carboniferous to Early Triassic fluviatil deposits form a thick sequence within the “Zone Houillère” and other parallel grabens(?). The most southern pull-apart(?) basin that is filled up with a thick volcano-sedimentary series (Carboniferous(?) Early Triassic) is largely incorporated into the Mont Fort nappe. Permian gabbros and subalkaline granitoids form sills and laccoliths intruding both poly- and mono-metamorphic basements. The major uncertainties still concern: (1) the precise age of all rocks and series, except those belonging to the “Zone Houillère”; (2) the initial nature of the polymetamorphic magmatites and the geodynamic context during their ascent; (3) the age and the geodynamic meaning of the different pre-Alpine metamorphic events (HP, HT), especially the age of Variscan(?) paroxysm (amphibolite-facies).