Petrogenesis of graphitic cherts in the Armorican segment of the Cadomian orogenic belt (NW France)

Graphitic cherts are interbedded within terrigenous sediments in the Cadomi an orogenic belt of end-Proterozoic age. In the Armorican Massif (NW France ), the graphitic cherts are of two types: massive cherts essentially compos ed of quartz (SiO2 > 96%) and with rate sedimentary structures; laminated c herts containing up to 3.4% Al2O3 and 92-98% SiO2. Sedimentary structures o bserved in the laminated cherts are indicative of a restricted hypersaline tidal or supratidal environment. The origins of both types of chert are to be found in the diagenetic processes of silification of terrigenous and mix ed terrigenous-evaporitic facies. These processes, which could be mediated by the presence of organic matter, were controlled by the migration of the freshwater/saltwater mixing zone during periods of relative sea-level chang e. The proposed diagenetic origin for the cherts places a number of constra ints on their use in the establishment of stratigraphic correlations.