RESOLVING QUESTIONS OF CONSANGUINITY BETWEEN THE LATE ORDOVICIAN DEICKE, MILLBRIG AND KINNEKULLE K-BENTONITES IN NORTH AMERICA AND BALTOSCANDIA

Late Ordovician K-bentonites in eastern North America and northwestern Europe contain primary phenocrysts that provide useful information about their magmatic origin. The Deicke and Millbrig K-bentonites are two of the most widespread and persistent of the many beds of altered volcanic ash that occur in Middle to Upper Mohawkian/Champlainian (=Lower to Middle Caradoc) strata of eastern North America. The Kinnekulle K-bentonite is the thickest and most widespread among the many K-bentonites in the Ordovician of Baltoscandia and it is generally present in sections having an uninterrupted Johvian-Keilan succession. The possibility of a common source for these ash beds was suggested by Huff et al. (1992), and supportive chemo- and biostratigraphic data were recently presented by Saltzman et al. (2003). Major and trace element analysis of whole-rock Ordovician K-bentonites indicates that the parental magmas consisted of a calc-alkaline through peraluminous suite. The chemical compositions suggest these are subduction-related plate margin volcanics. Haynes et al. (1995) reported a compositional difference between Kinnekulle and Millbrig biotites with respect to their content of Fe, Mg, Al, Mn and Ti oxides. They further suggested that these variations represent separate eruptive events. However, Haynes et al. (1995) used data from only one Millbrig site in North America and one Kinnekulle site in Baltoscandia, so they failed to evaluate lateral variation as well as within bed variation in biotite compositions. Here we present a comprehensive study of K-bentonite biotite composition covering a more extensive geographic and stratigraphic range for these Ordovician beds. Over 700 Kinnekulle, Millbrig and Deicke biotite analyses representing 32 separate localities provide the most comprehensive view to date of the nature and extent of internal compositional variability within these widespread Laurentian and Baltoscandian ash beds. The data show clearly that the Kinnekulle and Millbrig are multiple event ash beds, some parts of which are chemically indistinguishable from one another. Published age dates are inconclusive as to the true ages of each bed. Tectonomagmatic and Mg number discrimination diagrams suggest that each bed represents a separate magmatic stage in the closing of the Iapetus Ocean.