Apatite phenocryst compositions demonstrate a miscorrelation between the Millbrig and Kinnekulle K-bentonites of North America and Scandinavia

摘要

The Ordovician Millbrig K-bentonite of North America and the Kinnekulle "Big Bed" of Scandinavia have been considered correlative, thus representing one of the largest volcanic eruptions of the Phanerozoic. Whether these two K-bentonites correlate has implications for the paleogeographic reconstructions of Laurentia and Baltica, transoceanic biostratigraphic correlations of graptolites and conodonts, and the possibly global occurrence of the Guttenberg carbon isotope excursion. Trace element analyses of apatite phenocrysts have proven to be good discriminators for other Ordovician K-bentonites and it is shown here, on the basis of significantly different Mg, Cl, Mn, Fe, Ce, and Y concentrations in apatite phenocrysts from the Kinnekulle and Millbrig K-bentonites, that the beds must be viewed as representing two or more different eruptions. Vertical subsampling of several layers from each bed suggests that both beds are likely composed of multiple components, but none of the components coincide with one another. These results indicate that the Millbrig and Kinnekulle beds cannot both be derived from a single ultra-Plinian eruption, as previously proposed.