Rb–Sr dating of the Bokan Mountain granite complex and its country rocks

Abstract

Seven out of 11 whole-rock samples of the Bokan granitic complex give a Rb–Sr isochron date of 151 ± 5 Ma (Late Jurassic) as a minimum age for the complex. This result is discordant with published Early to Middle Jurassic K–Ar and U–Pb dates for the same unique peralkaline rock suite. A Late Jurassic age is tenable if excess radiogenic Ar and a trace of xenocrystic zircon are postulated to explain the previously reported results and the U–Pb data are reinterpreted to give an age between 156 ± 6 and [Formula: see text]. Granitic country rock for the complex is dated by Rb–Sr at 432 ± 19 Ma (Early Silurian), in agreement with published K–Ar dates.