Alpine ocean seafloor spreading and onset of pelagic sedimentation: new radiolarian data from the Chenaillet-Montgenèvre ophiolite (French-Italian Alps)

Abstract

Radiolarians of Middle Jurassic age (tentatively middle Bathonian) provide the first direct age determination from oceanic sediments associated with the Chenaillet-Montgenèvre ophiolite (Piemonte zone, French-Italian Alps). This datum obtained from radiolarites of the Lago NeroReplatte thrust sheet is older than those previously established on ophiolite sedimentary covers from this segment of the western Alps. It also shows that Lago Nero-Replatte basal radiolarites are anterior to the youngest intrusives from the overlying Chenaillet s.s. thrust sheet. This chronological relationship implies either a late seafloor spreading-related magmatic activity in places younger than adjacent initial pelagic sedimentation, or more likely that the Lago Nero-Replatte and the Chenaillet s.s. thrust sheets are distinct and distant pieces of lithosphere that were eventually stacked together: the Lago Nero-Replatte unit was trapped within the accretionary wedge while the Chenaillet s.s., of a younger age and in a more distal position with regards to the European margin, was obducted. Regionally, the Lago Nero-Replatte sediments appear to be coeval to other Bathonian supraophiolitic radiolarites exposed in the western Alps. These results strengthen the Bathonian correlation of widespread seafloor spreading in both western Tethys and the central Atlantic ocean.

Mesozoic radiolarites – accumulation as a function of sea surface fertility on Tethyan margins and in ocean basins

“…Radiolarian cherts, once preserved as quartz, are quite resistant to metamorphism. Determinable radiolarians have been extracted from cherts that underwent blueschist‐grade high‐p metamorphism (De Wever & Caby, ; De Wever & Baumgartner, ; Cordey & Bailly, ; Baumgartner et al ., ). With increasing metamorphism and recrystallization, colours change from bright red to dusky red and finally to white quartzite.…”

Dating the Tethyan Ocean in the Western Alps with radiolarite pebbles from synorogenic Oligocene molasse basins (southeast France)

“…If we consider that in situ radiolarites of the Western Alps are associated with ophiolite basements, this age remains compatible with the classical scenario of synchronous opening for both the Central Atlantic and the Alpine Tethys in the Middle Jurassic (Lemoine et al 2000;Bill et al 2001;Cordey and Bailly 2007;de Graciansky et al 2010). However, we do not know if the sampled molasse units contain older oceanic remnants.…”

“…It was tentatively assigned to UAZ 6 of Middle Bathonian age. It could be the oldest radiolarite exposure of the Piemont zone (Cordey and Bailly 2007).…”

“…A significant improvement came with the successive dating of three radiolarite supraophiolitic localities: (1) the Cascavelier Peak near Saint-Véran in Southern Queyras (De Wever and Caby 1981;De Wever and Baumgartner 1995), (2) the Traversiera massif near the French-Italian border in the upper Ubaye valley (Schaaf et al 1985;De Wever et al 1987;De Wever and Baumgartner 1995), and (3) more recently the Rocher de la Perdrix in the Montgenèvre massif, located structurally below the Chenaillet ophiolite s.s. (Cordey and Bailly 2007). In these three sites ( Fig.…”

“…1), radiolarian faunas are preserved in low-temperature blueschist-bearing metacherts at the front of the Piemont ''Schistes lustrés'' complex (Gidon et al 1994;Tricart et al 2003). These three fossil localities directly argue for an ocean opening during Middle and Late Jurassic times (Cordey and Bailly 2007).…”