A Rb-Sr isotopic study of weathering in the Mertondale granite,Western Australia

Fresh samples of the Mertondale granite give a Rb-Sr age of 2580 ± 16 m.y. (λRb⁸⁷ = 1.39 × 10^?11 yr^?1), representing the time of emplacement. Very little change in age is detected with progressive alteration of the granite, apparently associated with weathering, even when it is converted to a kaolinite-quartz assemblage. Preservation of the whole-rock age was unexpected, as the region has been subjected to Tertiary lateritisation to which the alteration of the granite would normally be attributed. We conclude that either the apparent weathering of the granite must be re-interpreted as a late-stage hydrothermal kaolinisation, or that the weathering occurred in the Archaean within about 15 m.y. of the granite's crystallisation.