PETROLOGY OF MAFIC LAVAS WITHIN THE ONEGA PLATEAU, CENTRAL KARELIA - EVIDENCE FOR 2.0 GA PLUME-RELATED CONTINENTAL CRUSTAL GROWTH IN THE BALTIC SHIELD

The Onega plateau constitutes part of a vast continental flood basalt province in the SE Baltic Shield, It consists of Jatulian-Ludikovian s ubmarine volcanic, volcaniclastic and sedimentary sequences attaining in places 4.5 km in thickness. The parental magmas of the lavas contai ned similar to 10% MgO and were derived from melts generated in the ga rnet stability field at depths 80-100 km. The Sm-Nd mineral and Pb-Pb whole-rock isochron ages of 1975 +/- 24 and 1980 +/- 57 Ma for the upp er part of the plateau and a SHRIMP U-Pb zircon age of 1976 +/- 9 Ma f or its lower part imply the formation of the entire sequence within a short time span. These ages coincide with those of picrites in the Pec henga-Imandra belt (the Kola Peninsula) and basalts in the Karasjok-Ki ttila belt (Norway and Finnmark). Together with lithostratigraphic, ch emical and isotope evidence, these ages suggest the derivation of the three provinces from a single large (similar to 2000 km in diameter) m antle plume. These plume-generated magmas covered similar to 600,000 k m(2) of the Baltic Shield and represent a major contribution of juveni le material to the existing continental crust at 2.0 Ga. The uppermost Onega plateau lavas have high (Nb/Th)(N) = 1.4-2.4, (Nb/La)(N) = 1.1- 1.3, positive epsilon Nd(T) of +3.2 and unradiogenic Pb-isotope compos ition (mu(1)= 8.57), comparable with those of modern oceanic plume-der ived magmas (oceanic flood basalt and ocean island basalt), These para meters are regarded as source characteristics. The lower sequences hav e (Nb/Th)(N) = 0.58-1.2, (Nb/La)(N) = 0.52-0.88 and epsilon Nd(T) = -2 .6. They have experienced mixing with 10-30% of continental crust and resemble contaminated lavas from other continental flood basalt provin ces. The estimated Nb/U ratios of 53 +/- 4 in the uncontaminated rocks are similar to those found in the modern mantle (similar to 47) sugge sting that by 2.0 Ga a volume of continental crust similar to the pres ent-day value already existed.