GEOTHERMOBAROMETRY AND GEOCHRONOLOGY ON MINERAL ASSEMBLAGES OF ORTHOGNEISSES AND RELATED METAPELITES OF THE SPESSART CRYSTALLINE COMPLEX, NW BAVARIA, GERMANY

Granitic orthogneisses of the Rotgneiss and the Haibach Gneiss type in the Spessart Crystalline Complex (SCC), an important constituent of t he Mid-German Crystalline Rise, contain the amphibolite-facies assembl age quartz + K-feldspar + oligoclase + muscovite + biotite +/- garnet. From the combined application of the garnet-biotite geothermometer, t he muscovite-biotite geothermobarometer and the phengite geobarometer to this assemblage, peak metamorphic temperatures of 600-650-degrees-C and pressures of 4-7 kbar are deduced, consistent with earlier estima tes based on mineral parageneses in staurolite-bearing metapelites and metabasites in the SCC. From an intercalated metapelitic schist of th e Schweinheim Formation temperatures of 590-620-degrees-C and pressure s of 6-8 kbar are estimated possibly indicating that the prograde meta morphic evolution passed through conditions with slightly higher P/T r atios. Some of the muscovites of the Rotgneiss and the Haibach gneiss, especially cores of zoned muscovite flakes, are characterized by high er Si contents, up to 6.6 per formula unit, indicating still lower tem peratures and/or higher pressures. Radiometric dating of mica concentr ates from the Haibach gneiss yielded conventional K-Ar dates of 323-32 5 +/- 2 Ma and 315-323 +/- 2 Ma for muscovite and biotite, respectivel y, and concordant Rb-Sr whole rock-muscovite dates of 323 and 326 +/- 7 Ma. A concordant Ar-Ar plateau date of 324 +/- 3 Ma was recorded on one muscovite from the Haibach gneiss. These model ages agree well wit h previously published K-Ar dates on hornblende, biotite and muscovite from different rock types in the SCC. They record a short period of r apid uplift following deformation and amphibolite-facies metamorphism near the Visean/Namurian boundary. Our results suggest that, during th e Variscan orogenesis, the SCC underwent a clockwise P-T path.