A Rubidium—Strontium Geochronological Study of the Willyama Complex, Broken Hill, Australia

Rubidium-strontium isotopic measurements are reported for total-rock and minerals from igneous and high-grade metamorphic rocks from the Willyama Complex, Broken Hill, Australia. The results of measurements on total-rocks and some minerals from the high-grade gneisses indicate that nearly complete strontium isotopic redistribution occurred within individual rock units 1640±40 m. y. ago. This age is interpreted as that of the high-grade regional metamorphism which recrystallized the Willyama rocks to gneisses in the Broken Hill area. Analyses of total-rocks and some minerals from intrusive muscovite granites and pegmatites give consistent ages of 154O±5O m. y. indicating that these rocks were emplaced soon after the high-grade regional metamorphism. Rubidium-strontium isotopic analyses of all biotites, and of muscovites from pegmatites concordant in schists at Thackaringa reveal a 500 m. y. metamorphic episode of lesser intensity accompanied by pegmatite emplacement. This metamorphism was not of sufficient strength to open the total-rock systems significantly to rubidium and strontium isotopes. Biotites, however, appear to have lost their radiogenic strontium almost completely at this time and it is probable that this event accounts for the observed disturbance of the potash-feldspar rubidium-strontium systems in most gneiss and muscovite granite samples investigated. There is a close similarity between the rubidium-strontium age results and the Broken Hill model lead ages. This supports the hypothesis of two-stage lead development and, with the strontium isotope evidence, suggests that the region has evolved largely as a closed chemical system since at least the high-grade metamorphism.