PIs will investigate the tectonic setting, magmatic origin and stratigraphic significance of altered volcanic ash beds (K- bentonites) in the Middle and Upper Ordovician strata of Baltoscandia and to compare these data with similar data from North America. Preliminary studies strongly suggest these K- bentonite beds on both sides of lapetus had a common source and a common tectonic setting. If this indication proves correct, the most widespread of these beds will have profound event- stratigraphic implications for the Ordovician, and our understanding of Laurentia and Baltica plate positions during the Taconian orogeny will be substantially improved. Similarly, the abundance of Silurian K-bentonites throughout Baltoscandia and the UK indicates ongoing plate-margin volcanism from Llandovery through Ludlow time. PIs will collect samples of the most extensive beds from cores and outcrops in Estonia, Sweden and Norway. They will add to existing information on the biostratigraphic position of these beds and use chemical fingerprinting of both bulk samples and selected phenocrysts to make accurate correlations with North American beds. These will be compared with trans-Atlantic stratigraphic relations of K- bentonites suggested by traditional biostratigraphy and by graphic correlation using conodonts. We will also obtain high- resolution Ar age spectra and U-Pb dates from biotite and zircon separates from biostratigraphically well-constrained K-bentonites that will add key ages to the Ordovician time scale. Trace element data will be plotted on discrimination diagrams to interpret the tectonic and magmatic settings of these beds.
