The behaviour of lead isotopes during granulite facies metamorphism

Abstract

The U-Pb decay scheme can give the age of granulite facies metamorphism accompanied by the depletion of U, because of its sensitivity to changes in U/Pb ratio. Model Pb ages, which are generally reliable to within 100 Myr in the period considered, allow the dating of these U-deficient systems. Fractionation-corrected Pb isotopic compositions for central Australian granulites and their separated potassium feldspars were determined. Initial Pb for both rocks and feldspars has a model age which approximates one of two ages given by the Rb-Sr total rock method; the age is that of the granulite facies metamorphism. Isotopic data and bulk analyses demonstrate the depletion of U in all rocks, but in contrast show that Th is erratically depleted. The essentially single-stage nature of the Pb means that premetamorphic fractionation of U from Pb is not detected.