Abstract

The large volume of Ordovician bentonites in the eastern and central United States indicates that they originated from a major volcanic system. The location and paleotectonic setting of this volcanic source is unknown. Nd and Sr isotopic data were collected from eight samples of primary apatite phenocrysts contained in four Ordovician bentonites, and U-Pb data were collected from one bentonite to determine the origin of this extensive volcanism. The range in initial isotopic ratios for the four bentonites is ϵNd = -3.5 to -5.2, 87Sr/86Sr = 0.71005 to 0.71200. The age of the Deicke bentonite is 457.1 ±1.0 (2σ), determined by a concordant U-Pb zircon fraction. Five other zircon fractions from the Deicke bentonite contain an inherited lead component. Lines drawn through the concordant point and the extremes of the discordant points have calculated upper intercepts with concordia of 1,090 Ma and 1,540 Ma. The Nd and Sr isotopic data and the U-Pb age data are consistent with the generation of the volcanic ash by anatexis of evolved continental crust. An evolved source region with volcanics of the same age and isotopic composition as the North American bentonites has not been found. This source region, however, may be the same as the source of Ordovician bentonites in Sweden because the ages of both groups of bentonites are identical, the 87Sr/86Sr ratios overlap, and zircon phenocrysts from both groups of bentonites contain an inherited Pb component. An appropriate tectonic setting for the generation of both groups of bentonites was a continental margin volcanic arc or an extensional regime associated with such an arc.

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