LEAD-ISOTOPE GEOCHEMISTRY OF PALEOPROTEROZOIC LAYERED INTRUSIONS IN THE EASTERN BALTIC SHIELD - INFERENCES ABOUT MAGMA SOURCES AND U-TH-PB FRACTIONATION IN THE CRUST-MANTLE SYSTEM

U-Pb data for plagioclase and sulfide are reported from the Burakovka and Olanga layered mafic-ultramafic complexes in Karelia. These 2.44-2 .45 Ga complexes differ in the age of their host rocks and in post-emp lacement metamorphic history. Acid leaching of optically discrete plag ioclase populations revealed three components of Pb present in the Ola nga plagioclases: initial, radiogenic, and alteration-related. Leachin g in HCl and HNO3 was found to remove most of U and radiogenic Pb from plagioclase and to fractionate U from Pb. The alteration component, a bundant in the Olanga plagioclase, is only partially removed by leachi ng. The Olanga sulfides also contain a significant radiogenic componen t. Plagioclase from the unmetamorphosed Burakovka Complex yields more reliable initial Pb ratios. The Pb isotopic compositions of plagioclas e from the three intrusions of the Olanga Complex form a single steep linear trend, which shows excess scatter due to residual radiogenic an d alteration Pb. Calculated single-stage mu(1) of about 7.9 in the Ki vakka intrusion is close to the Archean depleted mantle value, but com bined Pb, Sr, and Nd isotopic systematics suggest the presence of an e nriched component from either lithospheric or slightly contaminated pl ume source. internal variations in initial Pb and Sr isotopic ratios a nd Pb isotopic systematics of sulfides are explained by late-magmatic fluid exchange with 2.7 Ga country rocks. The Burakovka Complex, empla ced into the pre-3.1 Ga crust, shows large correlated variations in Pb -207/Pb-204, epsilon(Nd)(T) and Sr-87/Sr-86(T) in plagioclase. Initial Pb, Nd, and Sr isotopic variations in the Burakovka Complex are inter preted as a result of a multistage contamination process, including co ntamination en route and during emplacement and local assimilation of enclosing gneiss. In addition, the Burakovka mantle source was probabl y modified with addition of sediment-derived Pb by a subduction-zone f luid. Time-integrated Th/U approximate to 4.3 in mantle sources and cr ustal contaminants of the Olanga and Burakovka complexes are uniform a nd are similar to the Archean mantle value. This implies that U/Pb fra ctionation in the Archean-Paleoproterozoic crust-mantle system probabl y occurred by hydrothermal Pb transfer that fractionated Pb from U, bu t not Tn from U. Copyright (C) 1998 Elsevier Science Ltd.