THE SAAMIAN OF THE BELOMORIAN MOBILE BELT - NEW GEOCHRONOLOGICAL CONSTRAINTS

Most gneisses and amphibolites of the Belomorian Belt in the White Sea region have previously been considered to represent the oldest, ''Saamian'' crustal component in the Baltic Shield which has been referred to the Early Archaean. New U-Pb zircon ages of some of the most typical ''Saamian'' rocks from the northern segment of the Belomorian Belt are reported. These are: (1) similar to 2.65-2.60 Ga for the main (''Rebolian'') event of folding, high-P metamorphism and migmatization; (2) similar to 2.8 Ga for the protolith of an orthogneiss in a granulite-facies relic area within the region of Rebolian folding on the eastern shore of Lake Kovdozero; (3) similar to 2.74 Ga for tonalitic orthogneisses that dominate the Belomorian gneiss terrain around Kandalaksha Bay of the White Sea; (4) similar to 2.4 Ga for a gabbro-granite intrusion on the Tolstik Peninsula of the White Sea. This intrusion has previously been considered to be one of the type-rock units of the Early Archaean. These and other age determinations previously only reported in Russian constrain the age of the assumed ''Saamian'' in the Belomorian Belt. In conjunction with new field observations, they place most of the dated rock units within the same age group as the Late Archaean, Lopian rocks. As a result of the age determinations, the concept of a single array of Late Archaean geodynamic events in the Baltic Shield is advanced. The Belomorian Belt is interpreted as an edge of the Late Archaean granite-greenstone terrain in Karelia which had been involved in strong structural reworking during an event of tectonic stacking and thrusting similar to 2.65-2.60 Ga ago. Part of the Belomorian Belt was subsequently also affected by Palaeoproterozoic deformation. The new results throw doubt on the previously assumed Late Archaean, Lopian age of some metavolcanic rock units occurring within the Belomorian Belt but overlying the presently dated orthogneisses. These metavolcanics which comprise, for instance, the so-called Kolvitsa Belt and its continuation towards the northwest, are assigned tentatively to the Palaeoproterozoic. They may be age equivalents of Lapponian-Sumian, similar to 2.5-2.4 Ga old rocks underlying the Lapland Granulite Belt.