RB-SR AND U-PB ISOTOPE STUDIES ON MIGMATITES FROM THE SCHWARZWALD (GERMANY) - CONSTRAINTS ON ISOTOPIC RESETTING DURING VARISCAN HIGH-TEMPERATURE METAMORPHISM

Mineral and isotope studies were undertaken on migmatites from the Sch warzwald, Moldanubian zone of the Variscan belt, The aims of the study were to date the migmatite formation and to determine the processes i nvolved in migmatization in order to evaluate their influence on isoto pic resetting. Textural evidence and the comparison of mineral composi tions from leucosomes and mesosomes of two centimetre-scale migmatite profiles, respectively, suggest that migmatitic textures and mineral a ssemblages were formed by metamorphic segregation (deformation-enhance d mass transport) rather than by partial melting (anatexis). The resul ts of Rb-Sr thin-slab dating on these profiles indicate that Sr isotop es were not completely reset during migmatization. No true isochron ag es, but ages of approximate isotopic homogenization were obtained on t he thin slabs by calculating Sr-87/Sr-86 ratios back to various stages in their evolution. The coincidence of these Rb-Sr data with U-Pb age s of monazites from migmatites and non-migmatitic gneisses shows that gneisses and migmatites were formed during the same high-temperature e vent in the Carboniferous (330-335 Ma). The observation that high-temp erature metamorphism failed to equilibrate Sr isotopes on the centimet re-scale imposes limitations on the use of conventional whole-rock iso chron techniques in dating migmatites.