PETROLOGY AND GEOCHRONOLOGY OF ECLOGITES FROM THE VARISCAN SCHWARZWALD (FRG)

The Moldanubian basement of the Schwarzwald contains basic to ultrabas ic rocks of both crustal and mantle origin which display high-pressure mineral assemblages or relics of such. In order to constrain the P-T- t evolution of the crustal high-pressure rocks, petrological and geoch ronological studies have been carried out on three eclogite samples. G eothermobarometric estimations indicate minimum metamorphic pressures of 1.6 GPa and equilibration temperatures of 670-750-degrees-C. Reacti on textures document various metamorphic stages during exhumation of t he high-pressure rocks. The age of high-pressure metamorphism is const rained by Sm-Nd isochrons of 332 +/- 13 Ma, 334 +/- 11 Ma, and 337 +/- 6 Ma defined by garnet, whole rock and clinopyroxene. For one sample, large garnets show prominent growth zoning in terms of major elements , Sm, Nd, and inclusions, dividing the grains into two growth stages. Sm-Nd isotope analyses on these garnets indicate that the time span be tween the two growth stages is too small to be resolved, reflecting a rather rapid metamorphic evolution. This result is further constrained by a Rb-Sr isochron age of 325 +/- 6 Ma on retrograde biotite and who le rock on the same sample. For one of the studied eclogites, formatio n of the magmatic precursor rocks is possibly approximated by the Ordo vician U-Pb upper intercept age of a discordia from zircons.