SEQUENTIAL CLOSURE OF K-CA AND RB-SR ISOTOPIC SYSTEMS IN ARCHEAN MICAS

Comparative K-Ca and Rb-Sr data for extremely radiogenic muscovites (e psilon(Ca) = 2300 to 18,700; Sr-87/Sr-86 = 52 to 991) from two unmetam orphosed Archaean pegmatitic rocks from the Jack Hills in the northwes tern part of the Yilgarn Craton yield model ages ranging from 2580 Ma to 2100 Ma, with the K-Ca dates being younger than Rb-Sr. The muscovit es occur as two textural types: radial clusters of bladed crystals ('r osettes') in a massive 'pegmatitic' granite, and large euhedral 'books ' in cross-cutting pegmatites. There are marked differences between th e dates from the two mineral forms as well as between the two isotopic systems, and no individual date can be used to provide a point for [P -]T-t plots. The data can be interpreted collectively in terms of dela yed closure of the isotopic systems ina slowly cooled terrane. The clo sure temperature modelling demonstrates that the diffusion coefficient of Ca in muscovite is approximately an order of magnitude higher than that of Sr. A curved (assumed exponential) cooling trajectory, with e xtremely slow cooling (similar to 0.1 degrees C Ma(-1)) at similar to 2100 Ma, is required to reproduce the youngest K-Ca dates. Biotites (e psilon(Ca) similar to 150) from unmetamorphosed Archaean rocks from th e southwest of the craton show differences between K-Ca and Rb-Sr mode l ages (Rb-Sr data from Libby and de Laeter, 1979) that resemble the d ifferences in the Jack Hills muscovite dates. For biotites which have been affected by regional reheating at (or uplift until) similar to 50 0 Ma, the responses of the two isotopic systems are similar, but the a nalysed samples are not sufficiently radiogenic (epsilon(Ca) < 40) to permit identification of any closure sequence. Muscovites with a commo n cooling history but differing closure temperatures define empirical cooling curves in Sr-87/Rb-87 VS Ca-40*/K-40 plots. These can be used in conjunction with the K-Ca/Rb-Sr concordia to provide an improved m inimum age estimate for the samples. When only single samples (or very similar samples) are available, the difference between the K-Ca and R b-Sr dates gives a 'quality control' indication of the discrepancy bet ween the Rb-Sr date and crystallisation age.