Typologie du zircon des bentonites du Frasnien (carrière du Lion, Belgique): utilité avant la datation U-Pb

Zircon morphology and size from bentonites near the Frasnian-Famennian boundary have been investigated. Belonging to the upper rhenana Conodont Zone, one bentonite from Steinbruch Schmidt type section (Germany) has been recently dated by U-Pb on zircon at 377.2 ± 1.7 Ma (Kaufmann et al., 2004). Seven bentonites from the Lion quarry near Frasnes (Belgium, fig. 1A) belong to the hassi (?), jamieae and lower rhenana Zones (fig. 1B). In Frasnes the argillaceous, cm-thick levels have a distinctive mixed-layer illite-smectite clay content, taken as derived from volcanic smectite (Deconinck et al., 2003; Gouwy et al., 2003). Zircon morphologies have been studied after Pupin (1976, 1980) together with the crystal size distributions. No zircon crystals were found over 125 µm in mesh size while most of the separated crystals are under 63 µm. The adjoining Frasnian calcschists yielded no zircon (greater than 32 µm, smallest mesh size). This favours an aeolian-controlled bentonite deposition during discrete events. Zircon morphologies (fig. 1C-D) characterize a volcanic origin for the Frasnes bentonites (4 with high T index, 1 from hyper-alkaline rocks), while Steinbruch Schmidt zircons are typical of plutonic sources (calc-alkaline granite and migmatite). Our provisional U-Pb on zircon age data are all identical, within error (2s), with the Steinbruch Schmidt age, especially the 376.4 ± 0.6 Ma of bentonite 3 belonging to the jamieae Zone (fig. 1B). The results may help bracketting the absolute age of the Frasnian-Famennian boundary. The cheap, naturalist techniques used for characterizing zircon morphology should become a standard operation prior to U-Pb dating of bentonites.