Behaviour of biotites,amphiboles, plagioclases and K-feldspars in response to tectonic events with the 40Ar-39Ar radiometric method. Example of Corsican granite

Alpine deformation of Variscan granites in Corsica has resulted in fundamentally disturbing effects in minerals with regard to ³⁹Ar-⁴⁰Ar systematics. These deformational effects, as studied in quartz, biotite, feldspar, show an evolution in their development which is related to the intensity of deformation.Amphibole yields very disturbed age spectra, with loss of radiogenic Ar in low temperature steps, excess radiogenic Ar in intermediate temperature steps and undisturbed ages in high temperature steps. These effects similarly increase with the intensity of deformation.Biotite also shows disturbed spectra; we suggest that deformation induces equal loss of radiogenic Ar from all domains in this mineral and that this loss increases with the intensity of deformation. Only less-deformed biotite gives geologically significant age spectra.Spectra of deformed K-feldspar give results similar to those disturbed by thermal events. The most deformed K-feldspar gives high temperature spectra of Alpine age. High temperature steps however do not give the true age of the granite regardless of deformational intensity. Age spectra of plagioclase are quite disturbed and give geologically meaningless results. These results thus demonstrate the influence of deformation on the relative retentivities of terrestrial minerals.