Detrital zircon geochronology in blueschist‑facies meta‑conglomerates from the Western Alps: implications for the late Carboniferous to early Permian palaeogeography

International audienceIn the Western Alps, the Money Complex ofthe Gran Paradiso Massif, metamorphosed under blueschistfacies during the Alpine cycle, is considered to be Permo-Carboniferous in age, but no palaeontological or radiometricdata constrain this interpretation. A revision of thelithostratigraphy of the Money Complex allows recognizinga polygenic (graphite-rich) and a monogenic (graphitepoor)meta-sedimentary formation. Detrital zircon U–Pbgeochronology in both meta-sedimentary formations showsthat (i) the main population is Cambrian and Ordovician inage, (ii) the youngest grains are Silurian and Lower Devonian,and (iii) Carboniferous zircon grains are lacking. Acareful study of the age distributions in the Alps suggeststhat potential source for the detrital material in the MoneyComplex is the Briançonnais basement. Late Carboniferousmagmatism is widespread in the Helvetic Zone of the Alps.Permian magmatism is dominant in the Briançonnais, theAustroalpine and the Southalpine basements. The lack ofCarboniferous zircons in the Money Complex suggests thatthe detritus was not shed from the Helvetic zone, whichwas separated from the Money basin by the Zone Houillèrebasin, where the main drainage pattern was developed fromsouth to north and where the depocenters migrated northwardsfrom the Upper Missisippian to Upper Pennsylvanian.We suggest that the Money Complex may had beenlocated to the east of the main river drainage inside the Zone Houillère basin or alternatively may represent a smallbasin, located on the east of the Zone Houillère