Ordovician K-bentonites: Issues in interpreting and correlating ancient tephras

Prominent Upper Ordovician K-bentonites include the Deicke and Millbrig beds in the Late Ordovician of eastern North America and the Kinnekulle K-bentonite in the uppermost Sandbian of Baltoscandia. These beds are thought to represent some of the largest ash fall deposits known in the Phanerozoic record. This report presents a comprehensive study of biotite compositions from the three beds covering a far more extensive geographic range than has previously been described. These data show that the Kinnekulle and Millbrig are multiple event ash beds, some parts of which are indistinguishable from one another. Tectonomagmatic discrimination diagrams combined with Mg number data indicate that the Deicke–Millbrig–Kinnekulle sequence represents the progressive transformation from calc-alkaline to peraluminous magmatic sources, consistent with a model of progressively evolving magmatism during the closure of the Iapetus Ocean. It is concluded that the Millbrig and Kinnekulle beds are coeval and represent primarily simultaneous episodes of explosive volcanism although it cannot be excluded that some portions of both beds were produced by the same event.