THE DRIFT OF THE FENNOSCANDIAN AND UKRAINIAN SHIELDS DURING THE PRECAMBRIAN - A PALEOMAGNETIC ANALYSIS

A revised Precambrian (2.85-0.6 Ga) Apparent Polar Wander Path (APWP) for the Fennoscandian Shield, based on a new compilation and analysis of data, is presented. In fitting the APW path to successive Grand Mea n Palaeomagnetic poles (GMPs), we applied the spherical spline techniq ue originally developed by Jupp and Kent in 1987. The position and ori entation of the Fennoscandian Shield during 2.85-0.6 Ga was determined from the GMPs. Major palaeoclimatological findings are used to constr ain the palaeomagnetic interpretation of palaeolatitudes. The general drift of Fennoscandia, from relatively high latitudes in the late Arch aean-Early Proterozoic to nearly equatorial latitudes in the Middle Pr oterozoic, correlates with palaeoclimatological indications that a per iod of cold climate was followed by one of warm climate during this ti me interval. From the continuous APWP the APW velocities and latitudin al drift velocities of the shield were calculated. An accumulated APW curve was also calculated. The palaeomagnetic data are irregularly dis tributed and some periods are rather poorly represented. This means th at the calculated velocities can sometimes be artifacts of sampling. L ate Archaean and Early Proterozoic (2.85-1.90 Ga) data are too sparse to make these calculations meaningful and velocity calculations are th erefore restricted to data of 1.90 Ga and younger ages. The accumulate d APW curve shows a number of linear segments with varying slopes, ind icating sudden changes in drift rate. During the Middle Proterozoic (1 .90-1.35 Ga) there was a period when the rate of APW was constant and low and that of latitudinal drift also was low. This pattern changed a t ca. 1.35 Ga, and the following Middle-Late Proterozoic period can be described by rapid APW and strongly fluctuating drift velocities. Jot nian rifting and the intrusion of numerous dyke swarms (at ca. 1.25 Ga ) correlate with this shift in rate. These changes are attributed to c hanges in plate configuration. A new database for the Ukrainian Shield is also presented, and GMPs in the 2.32-1.20 Ga range are defined. Th e database is still inadequate and the comparison of the Ukrainian and Fennoscandian drift histories is therefore tentative. Similarities in position, latitudinal drift and rotation during the Early-Middle Prot erozoic are, nevertheless, evident. A close relationship between the s hields in this period is consistent with the low APW rate of Fennoscan dia, indicating that Fennoscandia may have been part of a larger conti nent, including the Ukraine, at that time. At ca. 1.2 Ga, the latitudi nal position of Ukraine differed significantly from that of Fennoscand ia, suggesting that the large shield split up between ca. 1.35 and 1.2 Ga. This would explain the change in APW rate at 1.35 Ga. The subsequ ent increase in rate was due to a reduction in the size of the shield. The discrepancy in palaeopositions of Fennoscandia and Ukraine at 1.2 Ga led Mikhailova and Kravchenko to suggest a late Precambrian time ( 1.07-0.57 Ga) for the accreation of Fennoscandia to the East European Platform (EEP). This may be correct as the rate of APW for Fennoscandi a decreased in the late Precambrian, reflecting such a consolidation.