An Upper Carboniferous tonstein of volcanic origin

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Abstract
Previous work on the mineralogy of the volcanic supra‐Wyrley Yard tonstein is extended to include the opaque minerals. This has led to a better understanding of the major and trace element geochemistry of the tonstein. The complete mineral assemblage is kaolinite (dominant) with minor quartz, K‐mica, zircon, apatite, pyrite, sphalerite, galena, rutile, dolomite, barytes, goethite and organic matter.Information on element location in the tonstein has been obtained by using autoradiography and fission track radiography, the electron probe and by analysing separated fractions with an optical spectrograph.This has enabled the diagenetic history of individual elements to be determined, and the composition of the tonstein to be compared with igneous material. The high concentration of Th, U, Pb, Sn, Bi, Y, Be and B in the tonstein, and the low concentration of Ti, Cr, V, Ni, Cu and Co is indicative of an acidic ash. A source in the Armorican orogenic belt is postulated, based on the contrast with the basaltic Carboniferous volcanism and also the high concentrations of Sn, Pb and Zn.

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