DATING A VARISCAN PRESSURE-TEMPERATURE LOOP WITH STAUROLITE

Staurolite with a complex internal metamorphic and structural relation ship occurs in metapelite from the Silvretta thrust sheet (eastern Alp s). Stepwise leaching of a core and rim fraction of a single staurolit e crystal resulted in a total Pb-Pb isochron of 308 +/- 128 Ma. The Va riscan metamorphism has been confirmed by a further single bulk stauro lite 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 combine d U-Pb leaching of the same crystals. Direct dating of the staurolite- in reaction isograd was therefore successful. Our results date the pro grade 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,vas fast and too k place within a few million years.