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 oldest major Proterozoic crustal component in the Svecofennian Domain of the Baltic (Fennoscandian) Shield. Their Nd,, model ages and conventional 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 and 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 in 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% have 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 indicates that a major area of 2.1-1.9 Ga old crust was in erosional position 1.9 Ga ago. This implies that the formation of Palaeoproterozoic crust in the Baltic Shield or its one-time close neighbourhood must have commenced 100-200 Ma earlier than hitherto assumed. In conjunction with previously obtained isotopic data, the youngest detritus ages of the present study constrain the age of Svecofennian sedimentation. It can also be concluded that the Archaean zircons found in quartzites from southern Sweden may have been derived from source areas to the southwest of the central-Svecofennian marine depositional basin, the so-called Bothnian Basin, separating southern Sweden from the Archaean craton in the northeastern part of the Shield.