THE GROWTH OF EARLY PROTEROZOIC CRUST - NEW EVIDENCE FROM SVECOFENNIAN DETRITAL ZIRCONS

Ion microprobe U-Pb analyses of zircons from the 1.9 Gyr old Svecofennian metasediments demonstrate the presence of both Archaean and early Proterozoic grains, whereas all conventional multi-grain analyses give apparent U-Pb ages of about 2.3 Gyr. Most zircons are 1.9-2.0 Gyr old suggesting that a major, still unknown early Proterozoic terrain older than 1.9 Ga supplied detritus to the early orogenic Svecofennian turbidites. These results imply that the formation of early Proterozoic felsic crust started c. 100 Myr earlier than has previously been inferred from isotopic data. Models in which Nd model ages of 2.1-2.4 Gyr have been interpreted as real crustal formation ages are not supported by our results.