Thermoluminescence (TL) dating is a valuable tool for chronometric dating of heated minerals and has been shown to agree very well with independent age control. Comparison with argon dating of samples from identical events, however, revealed age underestimations of volcanic eruptions dated by orange-red TL (R-TL) of quartz extracts from some xenolith samples, while good agreement was obtained for others. The underestimation is attributed to an apparent signal loss ("anomalous fading") which was experimentally observed for some, but not all samples investigated. The presence of significant amounts of feldspar or tridymite, which could be related to the observations, is excluded by IRSL (Infrared Stimulated Luminescence) and XRD analysis. While the data is not entirely conclusive, it leads to the current working hypothesis that exposure to high temperatures might be responsible for an effect similar to the anomalous fading phenomena observed for some feldspar luminescence. It therefore appears to be prudent not to sample xenoliths from high temperature context, like basalt dykes in volcanic context.