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

Authors
Citation
Mp. Dabard, Petrogenesis of graphitic cherts in the Armorican segment of the Cadomian orogenic belt (NW France), SEDIMENTOL, 47(4), 2000, pp. 787-800
Citations number
73
Categorie Soggetti
Earth Sciences
Journal title
SEDIMENTOLOGY
ISSN journal
00370746 → ACNP
Volume
47
Issue
4
Year of publication
2000
Pages
787 - 800
Database
ISI
SICI code
0037-0746(200008)47:4<787:POGCIT>2.0.ZU;2-8
Abstract
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.