Zircon and monazite U-Pb ages of Variscan granitoid rocks and gneisses in the Moldanubian zone of eastern Bavaria, Germany

U-Pb ages were determined on monazites and zircons of high-grade metamorphic, anatectic and granitic rocks of the Moldanubian zone in Bavaria. Most of the monazite ages range between 316 and 324 Ma. Concordant zircon ages were obtained for the Kristallgranit 1 (315 +/- 4 Ma), indicating the same difference of about 30 Ma between the Rb-Sr whole rock isochronous age (349 +/- ii Ma) and the U-Pb mineral ages as they are known from the Weinsberg granite at the southern margin of the Bohemian Massif in the Lower Austrian Waldviertel. Concordant ages were also obtained for two fractions of long-prismatic zircons of the Oedwies granodiorite (318 +/- 2 Ma and 320 +/- 2 Ma, respectively). U-Pb data for zircon fractions > 100 mu m from two samples of the Oedwies granodiorite yield lower concordia intercepts at 326 +/- 5 Ma and 329 +/- 6 Ma. These ages are interpreted as crystallisation times of the monazites and zircons during late Variscan LP metamorphism and accompanying plutonism. Rb-Sr and K-Ar mineral ages cover the same period of time. Cooling from ea. 700 degrees C to ca. 350 degrees C apparently was too rapid to be resolved by these systems. The presence of Late Archean zircons in the detritus of the former elastic sediments is indicated by an upper concordia intercept near 2620 Ma. None of the fractions gave a concordant age or lower concordia intercept near 470 Ma, the age of the Early Paleozoic event. It is, however, indicated by the distribution of many discordant zircon data within the triangle between the concordia intercepts at 2300, 470, and 310 Ma.