Evidence for high-temperature diffusional creep preserved by rapid cooling of lower crust (North Bohemian shear zone, Czech Republic)

Volume diffusion and dislocation creep at T similar to 800 degreesC led to high finite strain in granulite and orthogneiss of the Ohre crystalline complex (North Bohemian shear zone). Intragranular creep by volume diffusion is indicated by (i) lobate phase boundaries between feldspar and quartz, and (ii) removal of perthite lamellae and precipitation of tiny, aluminium-rich needles at the margins of K-feldspar. The striking diffusional-creep structures imply high interfacial free energy that has been preserved from equilibration as a result of rapid cooling. U-Pb dating of monazite (342 +/- 1 Ma) and 40Ar/39Ar dating of muscovite (341 +/- 4 Ma) of Kadafi orthogneiss result in a cooling rate of 50 + 25/-17 degreesC Myr(-1). This high value is attributed to collapse-related 'elevator-style' movements along the North Bohemian shear zone, resulting in the juxtaposition of upper crustal rocks of the Tepla-Barrandian unit against lower crustal rocks of the Erzgebirge crystalline complex.