CHRONOLOGY OF ECLOGITE RETROGRESSION WITHIN THE SEVE NAPPE COMPLEX, RAVVEJAURE, SWEDEN - EVIDENCE FROM 40AR 39AR MINERAL AGES

Amphibole concentrates from four samples of variably retrogressed eclogite from the Tsakkok Lens of the Seve Nappe Complex display discordant 40Ar/39Ar age spectra suggestive of extraneous argon contamination. Ar-36/40Ar vs. 39Ar/40Ar isotope correlations are well-defined for two samples and yield ages of 464.7 +/- 1.3 Ma and 463.2 +/- 6.3 Ma. Inverse ordinate intercepts are very much larger than 295 and confirm significant extraneous argon components. Two white mica concentrates from metasedimentary units of the Tsakkok Lens display internally concordant 40Ar/39Ar age spectra which define plateau ages of 448.2 +/- 1.6 Ma and 468.4 +/- 0.9 Ma. An amphibole concentrate from a metamorphosed basic dike within the Sarek Lens of the Seve Nappe Complex yields an isotope correlation age of 463.8 +/- 12.3 Ma, suggesting a post-metamorphic thermal evolution similar to that of the Tsakkok Lens. The amphibolite and white mica ages are interpreted to date post-metamorphic cooling through appropriate temperatures for intracrystalline retention of argon following amphibolite facies retrogression of the eclogite facies assemblages. The Seve Nappe Complex appears to have originated within distal facies of the late Proterozoic-early Paleozoic miogeocline of Baltica. The present 40Ar/39Ar results provide a record of early Paleozoic (pre-Middle Ordovician) tectonothermal activity and therefore are compatible with previous suggestions of significant early Caledonian orogenesis within the Scandinavian Caledonides.