Namurian bentonites in the Pennine Basin, UK - origin and magmatic affinities

Nine Namurian clay bands retrieved from boreholes in the northern part of t he Pennine Basin are, on the basis of their petrography, mineralogy and geo chemistry, shown to be volcanic in origin and are therefore bentonites. The bentonites, which have a fragmental texture, are normally graded and show rare preservation of shard textures, representing vitric tuff deposits that have been altered subsequently to clay-dominated horizons. Crystals are a minor component of the bentonites, but biotite, in particular, is concentra ted at the base of the beds. A clay mineral assemblage of mixed-layer illit e-smectite with subordinate kaolinite identifies most of the samples as K-b entonites, but kaolinite dominates two samples that can be classed as tonst eins. Temporal variation of salinity within the depositional basin is sugge sted to explain these different clay assemblages. The major element geochem istry of the bentonites reflects their clay mineralogy and the compositions of diagenetic minerals present, the latter including pyrite, carbonates an d hydroxyapatite. Enrichment of the bentonites in some trace elements (incl uding Ba, Sr, Pb, Cu and Ni) can be related to the presence of the diagenet ic minerals, but the extent to which the elements are added from external s ources as opposed to being redistributed within the ash is unclear. Immobil e trace element systematics suggest a rhyodacite/dacite composition for the original ash and derivation from the collision of plates, this being suppo rted by evidence provided by the rare earth elements (REE) in one group of samples. However, in another group of samples, variations in REE concentrat ions may be caused by mobility of these elements during alteration. The che mistry of the Namurian bentonites contrasts markedly with that of the local Carboniferous volcanics but is comparable, in some respects, with one grou p of Westphalian tonsteins, although the latter are more rhyolitic in chara cter. It is suggested that the Namurian bentonites and the Westphalian tons teins of acid affinities originated from volcanic activity associated with a destructive plate margin in the Variscan externides and that the observed compositional trend may reflect magma evolution possibly related to the pr ogressive east-west closure.