Abstract

Staurolite with a complex internal metamorphic and structural relationship occurs in metapelite from the Silvretta thrust sheet (eastern Alps). Stepwise leaching of a core and rim fraction of a single staurolite crystal resulted in a total Pb-Pb isochron of 308 ± 28 Ma. The Variscan metamorphism has been confirmed by a further single bulk staurolite Pb-Pb age of 298 ± 42 Ma, which is in accordance with U-Pb ages of 306 ± 8 Ma and 304 ± 6 Ma, respectively, obtained from combined U-Pb leaching of the same crystals. Direct dating of the staurolite-in reaction isograd was therefore successful. Our results date the prograde part of a Variscan counterclockwise pressure-temperature path in the Silvretta thrust sheet. Proximity with white-mica Rb-Sr and K-Ar cooling ages suggests that the whole Variscan episode was fast and took place within a few million years.

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