The Late Archaean to Early Proterozoic lead isotopic evolution of the Northern Baltic Shield of Norway, Sweden and Finland

Abstract

The Late Archaean rocks of the Northern Baltic Shield comprise high-grade gneiss terrains and granite-greenstone terrains. The high-grade gneiss terrains consist mainly of older upper-crustal material which became metamorphosed during the Lopian orogeny (ca. 2.9–2.6 Ga). In contrast, the ∼2.7 Ga-old granite-greenstone terrains comprise mainly mantle-derived materials, At the southern and western edge, the Archaean craton became partially remobilized during the Svecofennian orogeny (2.0–1.75 Ga). This orogeny represents a major crust-forming event in the Baltic Shield, during which mantle material was added to the crust and older material was remobilized. Whole-rock, galena and sulfide trace lead are used to outline lead provinces on the Northern Baltic Shield. The Archaean lead from the Baltic Shield defines three secondary lead isochrons at ∼ 2.65 Ga with different initial lead compositions which reflect average pre-2.65 Ga μ 1 ratios of 8.0, 8.1 and 8.3, respectively. The variation of the initial lead composition of the high-grade gneisses and tonalites (Lofoten and Koitelainen area) probably indicates (1) variable crustal residence time and/or (2) different degrees of crustal contamination/assimilation before the 2.65 Ga metamorphic event, and the lead from the Eastern Finland greenstone belts indicates a heterogeneous mantle with respect to U/Pb before 2.65 Ga. Lead in Proterozoic rocks of the different lead provinces defines sets of subparallel mixing fields which can be modelled by a ≈2.65 Ga lead component and a ∼ 1.8 Ga lead component. The parallel offset of these fields reflects the pre-2.65 Ga μ 1 ratio of these terrains, i.e. it reveals the pre-2.65 Ga μ 1 of the involved Archaean rocks. Proterozoic terrains with involvement of Archaean lower-crustal lead show a poor correlation in the 206Pb/ 204Pb- 208Pb/ 204Pb diagram, whereas the other Proterozoic terrains show a good correlation in the 206Pb/ 204Pb- 208Pb/ 204Pb diagram.