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Multimethod (K-Ar, Rb-Sr, Sm-Nd) dating of bentonite minerals from the eastern United States

Abstract:

Isotopic determinations (K-Ar, Rb-Sr and Sm-Nd), and trace and rare-earth elemental analyses were made on a few biotite and clay fractions of Palaeozoic bentonite units from the eastern US. The clay fractions were gently leached with dilute hydrochloric acid to study separately the acid-soluble minerals intimately associated with the extracted clay particles. The data highlight interesting potentials for this integrated approach to decipher complex tectonothermal evolutions of sedimentary basins. Biotite K-Ar ages are consistent with a Middle Ordovician stratigraphic age for the bentonite units with a mean age of 459 ± 10 Ma. The clay residues give a Sm-Nd isochron age of 397 ± 44 Ma, indicative of their crystallization during Acadian tectonothermal activity at about 200°C. The clay leachates, which are considered to represent mineral phases different from clay material, yield a distinct Sm-Nd isochron age of 285 ± 18 Ma which is indistinguishable from K-Ar ages obtained previously on the clays, suggesting a thermally induced diffusion of radiogenic 40Ar from clay particles during Alleghanian-Ouachita orogenic activity. The Rb-Sr system of the clay material seems to have been variably disturbed, except for the sample taken near the Allegheny Front for which an age of 179 ± 4 Ma suggests a further localized activity of the thrust system at about 130-150°C. Clearly the limited number of samples does not allow us to perfectly constrain an evolutionary model. However, analysis of the soluble minerals for their contents in metal and rare-earth elements suggests that metal-carrying fluids migrated during the Alleghenian-Ouachita orogenic activity in the eastern North American continent. Consequently, they could have contributed to the concentration of ore deposits in the region, but this possibility needs to be tested with a larger data base.