Detrital zircons in blueschist-faciesmeta-conglomerates : Implications for theEarly Permian palaeo-topography of theWestern Alps

National audienceIn theWestern Alps, the Money Unit is a monocyclic unit, metamorphosedat blueschist facies during the Alpine cycle. It crops out as a tectonicwindow below the overthrusted polycyclic, eclogite-bearing unit of theGran Paradiso and its present position is the result of the Alpine tectonics.The Money Unit is considered to be Permo-Carboniferous in age,but no palaeontological or radiometric data constrain this interpretation.The Money unit exposes a sequence of clastic sediments and volcanics(Money Complex) that are intruded by a granitoid body (Erfaulet metagranite).In the Money Complex a polygenic (graphite-rich) and a monogenic(graphite-poor) meta-sedimentary formation have been recognized.The two sequences are separated by fine-grained biotite-amphibolegneisses, and by albite-bearing gneisses and amphibolites.Detrital zircon U-Pb geochronology in both meta-sedimentary formationsshows that (i) the main population is Cambrian and Ordovician inage, (ii) the youngest grains are Silurian and Early Devonian, and (iii)Carboniferous zircons are lacking. A study of the age distributions in theAlps suggests that potential source for the detrital material in the MoneyComplex is the Briançonnais basement. Late Carboniferous magmatismis widespread in the Helvetic Zone of the Alps. Permian magmatism isdominant in the Briançonnais, in the Austroalpine, and in the SouthernAlps. The lack of Carboniferous zircons in the Money Complex suggeststhat the detrital material did not derive from the erosion of theHelvetic Zone. The Helvetic Zone was separated from the Money basinby the Zone Houillère basin, where the main drainage pattern was developedfrom south to north and where the depocenters migrated northwardsfrom the Namurian to the Stephanian.We suggest that the MoneyComplex may had been located to the east of the main river drainageinside the Zone Houillère basin or alternatively may represent a smallbasin, located on the east of the Zone Houillère