MULTIMETHOD (K-AR, RB-SR, SM-ND) DATING OF BENTONITE MINERALS FROM THE EASTERN UNITED-STATES

Isotopic determinations (K-Ar, Rb-Sr and Sm-Nd), and trace and rare-ea rth elemental analyses were made on a few biotite and clay fractions o f Palaeozoic bentonite units from the eastern United States. The clay fractions were gently leached with dilute hydrochloric acid to study s eparately the acid-soluble minerals intimately associated with the ext racted clay particles. The data highlight interesting potentials for t his 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 o f 459 +/- 10 Ma. The clay residues give a Sm-Nd isochron age of 397 +/ - 44 Ma, indicative of their crystallization during Acadian tectonothe rmal activity at about 200 degrees C. The clay leachates, which are co nsidered to represent mineral phases different from clay material, yie ld a distinct Sm-Nd isochron age of 285 +/- 18 Ma which is indistingui shable from K-Ar ages obtained previously on the clays, suggesting a t hermally induced diffusion of radiogenic Ar-40 from clay particles dur ing Alleghenian-Ouachita orogenic activity. The Rb-Sr system of the cl ay material seems to have been variably disturbed, except for the samp le taken near the Allegheny Front for which an age of 179 +/- 4 Ma sug gests a further localized activity of the thrust system at about 130-1 50 degrees 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-Ou achita orogenic activity in the eastern North American continent. Cons equently, they could have contributed to the concentration of ore depo sits in the region, but this possibility needs to be tested with a lar ger data base.