U-Pb and Sm-Nd dating of high-pressure granulite- and upper amphibolite facies rocks from SW Sweden

Geochronological data from different radiometric systems on high-grade meta morphic rocks in the Southwest Swedish Granulite Region, southwestern Swede n, are generally analytically precise but sometimes in conflict with each o ther. Age data from seven samples on which a variety of radiometric methods have been applied are presented. The age of peak metamorphism is determine d by metamorphic zircons from two garnet amphibolites at c. 975 Ma using th e conventional U-Pb isotope and Pb-Pb evaporation methods. Titanites from t he same type of rocks give ages of c. 923 +/- 3 Ma and 945 +/- 2 Ma. The Sm -Nd data on malic granulites and garnet amphibolites yield surprisingly you ng ages between 945 and 900 Ma, similar to previous Sm-Nd ages. These ages are comparable to, or even younger than, the Ar-40-Ar-39 hornblende ages th at range between 1030 and 930 Ma. Assuming that the Sm-Nd systematics are e ntirely controlled by volume diffusion, the closure temperature of the Sm-N d system in minerals from these rocks would be comparable to, or lower than , that of Ar diffusion in hornblende, i.e. around 500 degrees C. (C) 1998 E lsevier Science B.V. All rights reserved.