Abstract
Eleven Rb–Sr measurements on total rock samples from granitic and aplitic orthogneisses of the Upper East-Alpine Silvretta nappe give two isochron ages of 428 ± 17 m.y. and 351 ± 14 m.y. The apparent ages are ascribed to intrusions of the Caledonian and Variscan orogeny. Mineral isochrons for individual gneiss samples yield a metamorphic cooling age of 295 ± 12 m.y. Sr evolution diagrams for other samples give mixed mica ages between Variscan and younger Alpine metamorphism. The influence of Alpine metamorphism on micas, the deficit of radiogenic Sr in feldspars, and their deviation from the isochron are discussed.
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