EURAMERICAN TONSTEINS - OVERVIEW, MAGMATIC ORIGIN, AND DEPOSITIONAL-TECTONIC IMPLICATIONS

Carboniferous tonsteins (kaolinized volcanic-ash beds) of wide geograp hic distribution are known in both Europe and North America. Relict vo lcanic minerals common in these Euramerican tonsteins are volcanic qua rtz (including beta-quartz paramorphs), zircon and ilmenite; less comm on are magnetite, fayalite, rutile, monazite, xenotime, apatite and sa nidine. Data for two relatively thick (3-13 cm) and widespread (> 400 km) European tonsteins (Erda and Sub-Worsley Four-foot) indicate an in crease in detrital quartz near the top of the beds which indicates mix ing with normal clastic sediments, including the introduction of heavy detrital minerals (e.g., tourmaline and garnet). These thick tonstein s show multiple horizontal bedding, normal graded bedding, disturbed b edding, and centimeter-scale scour surfaces. The Fire Clay tonstein in North America represents from one to five separate volcanic air-fall ash deposits as determined by normal graded bedding and mineralogical analysis. These features indicate several episodes of volcanic-ash dep osition and very localized subsequent erosion and bioturbation. Electr on microprobe data from glass inclusions in volcanic quartz in Euramer ican tonsteins indicate a rhyolitic origin for these tonsteins and rev eal chemical ''fingerprints'' valuable for intra- and inter-basinal co rrelations. However, the tectonic framework for European and North Ame rican tonsteins was quite different. In Europe, volcanic-ash beds were associated with Variscan collisional tectonics, whereas in North Amer ica, volcanic ash was associated with Ouachita tectonic activity, expl osive volcanism from the Yucatan block, collision between the South Am erican and North American plates, and the formation of Pangea.