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

Abstract

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-emplacement metamorphic history. Acid leaching of optically discrete plagioclase populations revealed three components of Pb present in the Olanga plagioclases: initial, radiogenic, and alteration-related. Leaching 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, abundant in the Olanga plagioclase, is only partially removed by leaching. The Olanga sulfides also contain a significant radiogenic component. Plagioclase from the unmetamorphosed Burakovka Complex yields more reliable initial Pb ratios.