GEOCHEMISTRY OF A 2.3 GA SUB-JATURIAN PALEOLATERITE IN THE HALLAVAARA AREA, EASTERN FINLAND

The ~2.3 Ga Sub-Jaturian paleosols of the Karelian Supergroup in eastern Finland developed on both glaciogenic deposits and Archean granitic rocks. The paleosols are recognized as ancient lateritic weathering profiles on the basis of mineralogical and geochemical data. A paleosol section from the Hallavaara area can be divided into a ferric iron-rich upper zone (hematite bearing sericite-quartz schist) and an iron-depleted pallid lower zone (sericite-quartz schist). Such features are similar to those of the ~2.2 Ga Hekpoort paleolaterites in southern Africa (Beukes et al., 2002) and also modern laterite profiles.

The Hallavaara paleosol shows weak negative Ce anomalies. The U/Zr ratios are constant in the ferric iron-rich upper zone, whereas the ratios in the iron-depleted lower zone are variable and relatively higher than the upper zone. The constant U/Zr ratios in the upper zone are probably controlled by zircon crystals that survived during the soil forming process. In contrast, the variable U/Zr ratios suggest the dissolution of U from silicates by an oxygenated soil water and redeposition of U in the lower reducing zone. The mineralogical and geochemical characteristics of the Hallavaara paleosols were, therefore, produced by chemical weathering under an oxygenated, rather than anoxic, atmosphere.