Characterization and geodynamic implications of contrasting metamorphic evolution in juxtaposed high-pressure units of the western Erzgebirge (Saxony, Germany)

Three tectonometamorphic nappe units with contrasting P-T histories, the mica schist/eclogite unit, the garnet phyllite unit and the phyllite unit, became juxtaposed during and after exhumation; they form concentric zones around the core of an antiform of gneiss and gneiss/eclogite. EPMA data are presented for biotite, chlorite, chloritoid, staurolite, plagioclase, ilmenite, garnet and white mica, and used together with conventional thermobarometry. These results suggest that the lowermost mica schist/eclogite unit formed under high-P, intermediate-T conditions with minimum P of 12 kbar at 470-560 degrees C. A second stage of equilibration occurred during the production of the main foliation, under conditions of approximately 7-8 kbar and 500-630 degrees C, and a final stage is indicated under conditions of 2-3 kbar and 450-500 degrees C during the third deformational event. Well-defined P-T conditions of equilibration at approximately 9 kbar and 470 degrees C were obtained for the garnet phyllite unit. The P-T paths are clockwise for both units, with maximum P preceding the thermal climax. For the phyllite unit, P-T conditions are poorly constrained, but P of approximately 2 kbar are indicated with T >><$I> 300 degrees C. All these results reflect a complex metamorphic history on a framework of compression due to Variscan continent-continent collision followed by a multi-stage extensional event.