Abstract

Examples of discordant Rb-Sr mineral ages in granitic rocks from Australia illustrate a pattern of postcrystallization isotopic redistribution not previously recorded. Biotite retains the age of crystallization measured from total-rock samples, but K-feldspar gives a younger age and plagioclase an older age if the calculations are made using the total-rock initial Sr87/Sr86 ratio. Neither isotopic homogenization nor redistribution of Rb is considered probable under conditions which failed to affect the Rb-Sr and K-Ar ages of biotite in these rocks, and a mechanism of expulsion of radiogenic Sr87 from the K-feldspar is favored. The internal redistribution of Rb and Sr can be specified by four models of isotopic movement, and the pattern of response in any rock would depend on the intensity of the postcrystallization event.