SVECOFENNIAN DETRITAL ZIRCON AGES - IMPLICATIONS FOR THE PRECAMBRIAN EVOLUTION OF THE BALTIC SHIELD

Metasediments intruded by 1.90-1.87 Ga old plutonic rocks form the old est major Proterozoic crustal component in the Svecofennian Domain of the Baltic (Fennoscandian) Shield. Their Nd,, model ages and conventio nal multigrain zircon U-Pb ages between 2.4 and 2.1 Ga have previously been interpreted either as mixing ages between similar to 1.9 Ga old juvenile materials and a minor Archaean component, or as actual rock a nd protolith ages. To resolve the ensuing controversy, 120 individual detrital zircons from Svecofennian metasediments in Sweden and Finland were analysed using the SHRIMP ion microprobe. The oldest materials i n this array are a 3.44 Ga old zircon from the Tampere Schist Belt in Finland and a 3.32 Ga old crystal from southeastern Sweden. About 30% of the analysed crystals are 2.97-2.60 Ga old, while similar to 65% ha ve ages between 2.12 and 1.88 Ga. Thus there is no evidence of 2.6-2.1 Ga old protoliths, but the age range of the Proterozoic zircons indic ates that a major area of 2.1-1.9 Ga old crust was in erosional positi on 1.9 Ga ago. This implies that the formation of Palaeoproterozoic cr ust in the Baltic Shield or its one-time close neighbourhood must have commenced 100-200 Ma earlier than hitherto assumed. In conjunction wi th previously obtained isotopic data, the youngest detritus ages of th e present study constrain the age of Svecofennian sedimentation. It ca n also be concluded that the Archaean zircons found in quartzites from southern Sweden may have been derived from source areas to the southw est of the central-Svecofennian marine depositional basin, the so-call ed Bothnian Basin, separating southern Sweden from the Archaean craton in the northeastern part of the Shield.