The mineralogy and origin of some Silurian bentonites, Welsh Borderland, U.K. |
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| Authors: | C TARQUIN TEALE D ALAN SPEARS |
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| Institution: | Department of Geology, Royal School of Mines, Imperial College of Science and Technology, Prince Consort Road, London SW7 2BP, U.K.;Department of Geology, University of Sheffield, Mappin Street, Sheffield SI 3JD, U.K. |
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| Abstract: | 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. |
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