First report of an Ediacarian basement in the Western Alps: the Serre Chevalier crystalline unit (Briançonnais domain, France)

Abstract

We report new LA-ICP-MS U-Pb zircon ages of varied crystalline rocks occurring in an allochtonous unit of basement terrain located at the top of the Briançonnais nappes stack ~5 km west of Briançon city. Mapped as an undifferentiated metasedimentary basement on the geological map at 1/50,000 scale, this unit actually displays varied lithologies including alternating micaschist, paragneiss, quartzite, coarse-grained conglomerate and felsic (leptynite) to mafic (amphibolite) gneiss of magmatic origin. All rocks were metamorphosed in response to a dominant pre-Alpine event under garnet-amphibolite facies conditions. Retrogression under amphibolite to greenschits facies was associated with alpine events. Zircon ages were obtained on four types of rocks: (i) a micaceous quartzite from the core of the metasedimentary unit displays a dominant age population around 610 Ma and subordinated age populations at c. 940 Ma, 1825 Ma and 2100-2560 Ma ; (ii) a granite boulder from a coarse-grained conglomerate yields a dominant magmatic age of 582 ± 5 Ma and subordinated inherited ages between 1800 and 2200 Ma. Zircon metamorphic rims provide a concordant age of 492 ± 4 Ma ; (iii) a gneissic band in the vicinity of the conglomerate is dated at 597 ± 4 Ma ; (iv) a coarse-grained garnet amphibolite yields an age of 517 ± 3 Ma interpreted as the emplacement age of the protolith. The mainly Ediacarian record in the metasediments (quartzite and granite boulder in conglomerate) with no evidence of younger zircons indicates that this material resulted from the erosion of a nearby Ediacarian (Cadomian ?) basement. The age of 517 Ma obtained on the amphibolite provide a minimum age for the sedimentary succession which deposition is therefore bracketed between 582 Ma and 517 Ma. This interval is comparable to that of the metasedimentary units of the Brioverian group of the Armorican Massif. Based on their low Th/U ratio, the 492 Ma-old zircon rims in the granite boulder are attributed to an Upper Cambrian metamorphic event, but it is not yet clear whether this event corresponds to the garnet - amphibolite facies metamorphism recorded in the Serre Chevalier crystalline terrain.