Abstract

Amphiboles from two high-grade meatamorphic rocks in and near the Palala shear zone (a Proterozoic suture zone in the Limpopo Belt) were analysed by the 39Ar-40Ar method. Their age spectra are discordant and several step ages are older than the most probable age of peak metamorphism at 2.02 Ga. C1/K vs. Ca/K correlations obtained from Ar isotopes suggest mixing of different end members (at least two in one sample, four in the other). Neither optical microscopy nor electron-microprobe profiling confirm the chemical heterogeneity. It is only by transmission electron microscopy that heterochemical amphiboles intergrown at a scale < 1 μm are resolved. Extrapolating to end-member amphiboles, ages can be modelled. In one sample, formed by pyroxene hydration, an excess radiogenic 40Ar/36Ar initial can be removed to give a 2.02 Ga age; in the other sample, the step ages pertaining to tschermakite and magnesiohornblende give ≈ 2.02 Ga, with both minor chlorite alteration and late actinolite overgrowths accounting for higher and lower outliers.

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