Rb-Sr GEOCHRONOLOGIC INVESTIGATION OF PRECAMBRIAN SILICIC ROCKS FROM THE ZUNI MOUNTAINS, NEW MEXICO

The Rb--Sr whole rock isochron age for metarhyolite from the Zuni Mountains, New Mexico, is 1.38 +- 0.03 b.y. with initial /sup 87/Sr//sup 86/Sr = 0.721 +- 0.009. The metarhyolite is intruded by a cogenetic suite of gneissic granites, biotite granites, and aplites which yield a Rb--Sr whole rock isochron age of 1.49 +- 0.09 b.y. with initial /sup 87/Sr//sup 86/Sr = 0.7062 +- 0.0008. Although the errors overlap, the metarhyolite has apparently been incipiently altered resulting in its apparent lower age. As the silicic plutonic rocks are all fresh (based on petrographic examination) and clearly intrusive into the metarhyolite, then 1.49 +- 0.09 b.y. is assigned as the minimum age for the Precambrian silicic rocks from the Zuni Mountains.