U-PB DATA ON ZIRCONS FOR THE THERMAL PEAK OF METAMORPHISM IN THE VARISCAN ODENWALD, GERMANY

U-Pb data are presented for zircons from metamorphic rocks of the crys talline Odenwald that comprises part of the Mid-German Crystalline Ris e. The Odenwald can be divided into four principal domains with differ ent magmatic and metamorphic histories on the basis of published struc tural and petrological data and the presented ages. In the eastern (Bo llstein) Odenwald the metasedimentary rocks contain zircons derived fr om basement rocks that exceed 2.2 Ga in age. The intrusion of granitic magmas into these metasedimentary rocks took place before or close to the end of the thermal peak of metamorphism at 375 +/- 5 Ma ago. Rapi d uplift of the Bollstein Odenwald occurred at about 320 Ma bp, later than in the Bergstrasser Odenwald. The western (Bergstrasser) Odenwald can be subdivided, by geochronological, petrological and especially s tructural data, into a northern, a central and a southern part. In the northern part basic magmatism has an age of 362 +/- 7 Ma. The peak of thermal metamorphism in the southernmost Odenwald was accompanied by partial melting of metasedimentary rocks and the formation of new zirc ons with ages between 332 and 342 Ma. Probable peak metamorphic condit ions, revealed by two samples from the heterogeneous central part, occ urred at about 336 Ma, i.e. in the same time span as in the southernmo st part, but later than in the Bollstein area and the northern Odenwal d. Rapid uplift started earlier in the central and southern part and 1 0-20 Ma earlier than in the Bollstein Odenwald. Cooling from 500 to 30 0 degrees C occurred in the northern part in the time interval between the elevation of the other parts of the Bergstraesser Odenwald and th e Bollstein Odenwald. There is no indication for any thermal equilibri um between 2.25 Ga and 375 Ma recorded in the zircons of both areas.