Electron-microscope and Ar isotope characterization of chemically heterogeneous amphiboles from the Palala Shear Zone, Limpopo Belt, South Africa
Belluso, Elena; Ruffini, Raffaella; Schaller, Mirjam; Villa, Igor M.
published: Feb 7, 2000
manuscript accepted: Sep 6, 1999
manuscript received: Sep 12, 1998
ArtNo. ESP147051201023, Price: 29.00 €
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. CI/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 < I 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.