Geochemistry and origin of Carboniferous (Mississippian; Viséan) bentonites in the Namur-Dinant basin, Belgium: evidence for a Variscan volcanic source

Several thin clay-rich horizons occur interbedded with Mississippian (Viséan) limestones in the Namur-Dinant Basin, southern Belgium. These have been interpreted as diagenetically altered volcanic ash layers (bentonites). Whole-rock geochemical analysis of several bentonites was undertaken to provide insight into their original magmatic composition. Ratios of several trace elements that are considered to be immobile during diagenetic alteration and indicators of petrogenetic processes suggest that most of the bentonites were originally trachyandesitic to trachytic ashes. Furthermore, the coarse grain size of euhedral zircon phenocrysts (up to 400 µm in length) suggests a proximal volcanic source. Zircon U-Pb isotopic ages and whole-rock geochemical data from these bentonites were compared with literature data from volcanic and intrusive rocks in adjacent areas, and suggest that the bentonites were sourced from volcanoes within the Variscan orogenic belt.