Radiolarites and radiolarians from the Chenaillet-Montgenèvre ophiolite (Piemonte zone, French-Italian Alps)

More than 100 years after initial biostratigraphic attempts (Parona, 1890; Squinabol, 1912), diagnostic radiolarians were finally extracted from the 'Rocher de la Perdrix' radiolarites, a sedimentary rock unit traditionally associated with the magmatic rocks of the Chenaillet-Montgenèvre ophiolite.

These radiolarians of Middle Jurassic age (tentatively middle Bathonian) also provide the first direct age determination from oceanic sediments on the Briançon map sheet. This age is older than those previously established on ophiolite sedimentary covers from this segment of the western Alps and shows that Lago Nero-Replatte thrust sheet basal radiolarites are older than the youngest intrusives from the overlying Chenaillet s.s. thrust sheet. It suggests that the two thrust sheets are distinct and distant pieces of lithosphere that were eventually stacked together. Regionally, the Lago Nero-Replatte sediments appear to be coeval to other Bathonian supraophiolitic radiolarites exposed in the Alps. These results strengthen the Bathonian correlation of widespread seafloor spreading in both western Tethys and the central Atlantic ocean.

Cordey F. & Bailly A. 2007. Alpine ocean seafloor spreading and onset of pelagic sedimentation: new radiolarian data from the Chenaillet-Montgenèvre ophiolite (French-Italian Alps), Geodinamica Acta 20/3, p. 131-138.

The Chenaillet-Montgenèvre ophiolite view from the "Rocher de la Perdrix" locality. Mt Chenaillet exposes mantle rocks (light coloured units on the base flanks) and oceanic crust (dark ruiniform pillow-basalts on top). Snowy peaks on the right-hand side are on the Italian side of the Piemonte zone.
The "Rocher de la Perdrix" locality view from the Chenaillet trail, with contact between Lago Nero and Chenaillet s.s. thrust sheets. So far, true radiolarites have not been found within the Chenaillet s.s. thrust sheet.
Close-up of oceanic sedimentary rocks at "Rocher de la Perdrix". Middle Jurassic radiolarians were extracted from the 'radiolarites' unit belonging to the Lago Nero-Replatte succession locally of blueschist metamorphic facies.
Contacts between calcschists (metamorphosed marls) and sheared radiolarian chert. Right: tectonic thinning and deformation of chert beds (see above).
Left: top of radiolarite succession comprising interbedded green and red chert. The massive beds on top are calcschists. Right: thicker chert beds yielding radiolarians of Middle Jurassic age.
Paleogeographic reconstruction of the Alpine Tethys and central Atlantic Ocean in late Middle Jurassic time (based on Bill et al. 2000) with location of middle Bathonian radiolarian-bearing series (~165 Ma) associated with oceanic crust (stars); Ba: Balagne; Ch: Chenaillet-Montgenèvre; Ge: Gets. Thick lines: seafloor spreading; grey pattern: oceanic crust.
Bathonian seafloor spreading from western Tethys and the Atlantic
A widespread group of radiolarite units associated with initial oceanic crust of Bathonian age is documented in the French-Swiss Alps (Gets nappe, Bill et al., 2000) and in Corsica (Balagne nappe, De Wever et al., 1987). Our results suggests that the Lago Nero-Replatte succession is part of this domain. It also means that radiolarites from the Piemonte zone could be more diachronous than previously established (middle Bathonian from this study, late Bathonian-early Callovian from Traversiera massif, middle-late Oxfordian from Queyras). It strenghtens the interpretation that the corresponding ocean was formed over an extended period of time. As stressed by previous authors (Bill et al. 2000), Bathonian was a time when siliceous sedimentation became common to the entire Alpine Tethys.

Other references

- Bill, M., O’Dogherty, L., Guex, J., Baumgartner, P.O., Masson, H., 2001. Radiolarite ages in Alpine-Mediterranean ophiolites: Constraints on the oceanic spreading and the Tethys–Atlantic connection, GSA Bulletin, 113, 129–43.
- De Wever, P., Danelian, T., Durand-Delga, M., Cordey, F., Kito, N., 1987. Datations des radiolarites post-ophiolitiques de Corse alpine à l’aide des Radiolaires, C. R. Acad. Sci. Paris 305, 893-900.
- Parona, C. F., 1890. Sugli schisti silicei a radiolarie di Cesana presso il Monginevra, Atti della reale Accademie delle Scienze di Torino, 27, 305-319.
- Squinabol, S., 1912. Radiolari della strada nazionale al Monginervo. Bollettino del reale Comitato geologico d’Italia, 43, 2-3, 281-289.

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