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

1. IntroductionSeveral clay-rich horizons interbedded with Mississippian (Visean) limestones in the Namur-Dinant Basin (NDB), Belgium, are interpreted to be diagenetically altered volcanic ash layers based on their lateral continuity, clay mineralogy and heavy mineral assemblages (e.g. Thorez & Pirlet, 1979; Delcambre, 1989, 1996). Whilst several aspects of these clay horizons have been studied in detail, their original (unaltered) volcanic composition and the location(s) of the source volcanoes are not well constrained. The whole-rock geochemistry of altered volcanic rocks such as bentonites and tonsteins has been employed in several other studies to gain information about the composition of the original (unaltered) ash layers and ultimately the magma from which they were derived (e.g. Spears & Kanaris-Sotiriou, 1979; Merriman & Roberts, 1990; Huff et al., 1993; Christidis et al., 1995). The major elements, which are routinely used to classify fresh or slightly altered volcanic rocks, are of limited use when classifying extensively altered volcanic rocks because several elements, including K and Na, are known to be mobile during weathering and diagenesis (Winchester & Floyd, 1977; Floyd & Winchester, 1978; Zielinski, 1982; Christidis, 1998). Instead, the classification of altered volcanic rocks relies on trace elements including Ti, the high-field-strength elements (HFSE) Hf, Nb, Ta, Zr and the rare-earth elements, which are generally considered to be immobile during most up