ABSTRACT: The
five-point Rb-Sr whole-rock
isochron age of 1.07 Ga for the
diabase sill at Bass Rapids, Grand Canyon,
has been regarded for 20 years
as an excellent example of the application
of conventional radioisotopic
dating. However, our new K-Ar, Rb-Sr, Sm-Nd and Pb-Pb radioisotope data from eleven whole-rock samples (eight diabase, three granophyre) and six mineral phases separated from one of the whole-rock diabase samples yield discordant whole-rock and mineral isochron “ages.”