Datation Sm-Nd du métamorphisme éclogitique en Corse alpine : un argument pour l'existence au Crétacé supérieur d'une zone de subduction active localisée sous le bloc corso-sarde

Abstract

Alpine Cursica çorresponds to tectonically stacked units of basic, ultrabasic and sedimentary rocks (Lahondère and Lahondère, 1988). The basic and ultrabasic rocks are relics of the Ligurian-Piedmont ocean floor and the sedimentary rocks of the end-Jurassic to Early-Middle Cretaceous supra-ophiolitic cover (Caron et al., 1979; Lahondère and Caby, 1989). Intercalated gneiss bears witness to the involvement of crustal blocks in the orogenic process. Most units record a metamorphic evolution reaching lawsonite blueschist or glaucophane eclogite equilibrium (Ohnenstetter et al., 1976; Kienast, 1983; Péquignot et al., 1984; Lahondère, 1996). The eclogitic rocks belong to a single unit of serpentinized peridotite with eclogitic lenses derived from ancient granite, gabbro, basalt, radiolarite and sedimentary rocks (Lahondère, 1996; Lahondère and Lahondère, 1988). The unit corresponds to a paleodomain transitional between oceanic and continental conditions, probably represented today by the basement slices associated with the inner units. The eclogitic metasediments in the Golo valley overlie metabasalt and metaquartzite, locally manganese bearing (ancient radiolarites); they are calcareous schists with limestone intervals and two gneissic interbeds. The dated sample (VP427) is from the upper gneissic interbed where the rock is composed of quartz, jadeite, ferroglaucophane/crossite, paragonite, chloritoid, almandine garnet (chlorite, titanite, apatite and zircon). Major- and trace-element analysis has shown that this sample is derived from an ancient arkose composed of material eroded from calc-alkaline granite. The zircon typology is compatible with such an origin. The lower interbed is composed of metaconglomerate with an arkosic matrix. The analysed mineral phases were garnet, amphibole and clinopyroxene. A whole-rock powder was also analysed. The 147Sm/144Nd ratio is close to 0.65 for the garnet and between 0.14 and 0.20 for the whole-rock, glaucophane and clino-pyroxene. These data define an isochron at 83.8 ± 4.9 Ma (MSWD = 0.54).This age, the first obtained for the Alpine eclogites of Corsica, shows that subduction occured along the innermost part of the European margin during the Late Cretaceous. A recently proposed paleogeographic reconstruction (Lahondère, 1996) considers that the gneissic rocks of the inner units originally belonged to the same microblock, separated from the rest of the European margin by a small ophiolitic basin (present Balagne nappe). This configuration for the European paleomargin makes it possible to propose an original geodynamic scenario that takes into consideration the structure of both the inner and outer domains. Alpine compression began in the Late Cretaceous in response to the northward movement of the African and Adriatic plates; this led to the development of NW- dipping subduction beneath the southeast margin of the continental microblock which led to the progressive disappearance of the Ligurian-Piedmont ocean. The sinistral strike-slip nature of this subduction, suggested by the narrowness of the mountain belt and the orientation of eclogitic and post-eclogitic micro-structures, reflects the obliqueness between the subduction zone and the drift direction of the Adriatic plate. The localization of Eoalpine deformation within the microblock would explain the preservation of the Balagne domain and its ophiolitic substratum, where the accumulation of detrital sediments. rich in crystalline and sedimentary clasts derived from western Corsica, continued until the Eocene. The closure of this domain and deformation of the outermost parts of the margin did not occur until the Middle Eocene, in response to the collision between the Corsican and Apulian continental masses.