Radiometrie dating of the serorogenic Svecokarelian Enkullen and Fjällberg granites, south central Sweden

Abstract

Radiometric dating of the serorogenic Svecokarelian Enkullen and Fjällberg granites, south central Sweden, yield two parallel Rb-Sr whole-rock isochron ages of 1755 ± 17 Ma and 1758 ± 14 Ma, respectively. The initial ⁸⁷Sr/⁸⁶Sr ratios arc also similar; 0.7063 + 0.0056 and 0.7065 ± 0.0034, respectively. A U-Pb dating of zircons from the Fjällberg granite also gives a corrsponding age, i.e., an upper intercept with concordia of 1748⁺⁷⁴ _-45 Ma. A recalculation of the zircons with the U-Pb model of Wickman (1983) yields an age of 1765 Ma. The agreement between the Rb-Sr and U-Pb datings strongly indicates that they represent true primary crystallizing ages.