The mineralogy and origin of some Silurian bentonites, Welsh Borderland, U.K.

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Abstract
Thirty clay‐rich horizons from the Wenlock and Ludlow Series of the Welsh Borderland were analysed to investigate their mineralogy and possible origins. Two mineralogical assemblages were determined: an assemblage of illite‐smectite and kaolinite of volcanic origin which was associated in many samples with an assemblage of illite and chlorite of detrital origin. Immobile chemical discriminants (Zr/TiO2 versus Nb/Y) infer an originally rhyolitic ash composition with a possible temporal progression, from the Wenlock to Ludlow, to more differentiated compositions. The thin nature of the bentonites and lack of juxtaposed subduction related calc‐alkaline magmatism suggests that the tuffs were derived from distant explosive eruptions from volcanic centres, possibly in NE Europe.