Electron-microscope and Ar isotope characterization of chemically heterogeneous amphiboles from the Palala Shear Zone, Limpopo Belt, South Africa

Amphiboles from two high-grade meatamorphic rocks in and near the Palala sh ear zone (a Proterozoic suture zone in the Limpopo Belt) were analysed by t he Ar-39-Ar-40 method. Their age spectra are discordant and several step ag es are older than the most probable age of peak metamorphism at 2.02 Ga. Cl /K vs. Ca/K correlations obtained from Ar isotopes suggest mixing of differ ent end members (at least two in one sample, four in the other). Neither op tical microscopy nor electron-microprobe profiling confirm the chemical het erogeneity. It is only by transmission electron microscopy that heterochemi cal amphiboles intergrown at a scale <1 mu m are resolved. Extrapolating to end-member amphiboles, ages can be modelled. In one sample, formed by pyro xene hydration, an excess radiogenic Ar-40/Ar-36 initial can be removed to give a 2.02 Ga age; in the other sample, the step ages pertaining to tscher makite and magnesiohornblende give approximate to 2.02 Ga, with both minor chlorite alteration and late actinolite overgrowths accounting for higher a nd lower outliers.