LATE ARCHEAN SM-ND MODEL AGES FROM THE VOCHE-LAMBINA AREA - IMPLICATIONS FOR THE AGE DISTRIBUTION OF ARCHEAN CRUST IN THE KOLA-PENINSULA, RUSSIA

Although an Early Archaean age for the ''Saamian'' gneisses has been g enerally accepted in the former Soviet Union, partly on the basis of t heir commonly granulite-facies metamorphic grade and partly on account of their structural complexity and complex intrusion history, these r ocks have yielded no precise isotopic ages in excess of similar to 2.9 Ga. Sm-Nd isotopic analyses are used to test the possible presence of Early Archaean (> similar to 3.5 Ga) ''Saamian'' gneisses at Voche-La mbina in the Kola Peninsula. Here tonalitic, trondhjemitic and granodi oritic gneisses, presumed to be Early Archaean (Saamian) in age, are o verlain by a sequence of Late Archaean (Lopian) supracrustals. A sampl e of the structurally oldest trondhjemitic gneiss, with a minimum age of 2.64 Ga, has a depleted mantle model age (t(DM)) of 2875 Ma. Tonali tic gneiss, also assumed on structural grounds to be Early Archaean, h as a t(DM) age of 2808 Ma identical to its U-Pb zircon age of 2807 +/- 9 Ma. This suggests that a depleted mantle model may be appropriate f or at least this part of the Kola region in the Late Archaean. Lopian rhyodacite, with a U-Pb zircon age of 2800 Ma and a Rb-Sr isochron age of 2540 +/- 83 Ma, has a t(DM) age of 2756 Ma. In each case the close agreement between the Sm-Nd model age and the time of intrusion or er uption leaves little scope for a significantly older crustal history f or these rocks. The data provide no evidence for a Saamian component i n this part of the Kola Peninsula.