ABSTRACT

The abundances of rubidium and strontium have been determined in potassium feldspar separates and some whole rock samples from two Precambrian batholiths, the San Isabel batholith of the Wet Mountains, Colorado, U.S.A., and the Buchanan massif (Lone Grove pluton) of central Texas, U.S.A. Absolute abundances of the two elements in the feldspars vary widely, but the Rb/Sr ratio is relatively constant within mapped zones or facies. Selective precipitation of potassium over rubidium into early-crystallizing potassium feldspar leaves late-stage magmatic fluids enriched in rubidium and thus causes the absolute rubidium contents and the Rb/Sr ratios of feldspars and whole rocks to be highly dependent on the activity of volatiles. Escape of volatile-rich, end-stage fluids is apparently responsible for decrease in Rb/Sr ratios toward later-crystallizing rocks. The preservation of widely different Rb/Sr ratios in minerals in different facies of a comagmatic rock assemblage provides, through later partial melting, one method of generating non-zero initial apparent isochrons in whole-rock strontium isotope studies of igneous suites.