Abstract

We examined a complete late Paleoproterozoic paleoweathering profile beneath the Baraboo Quartzite from south-central Wisconsin, USA, in order to interpret biological and hydrological controls on ancient terrestrial weathering processes. Detailed thin-section investigations were used to identify characteristic pedogenic features associated with saprolitization and formation of an overlying regolith, and the isocon method (a geochemical mass-balance technique) was employed to characterize elemental gains and losses. The age of the sub-Baraboo profile is well constrained by the youngest detrital zircon ages previously reported from the basal Baraboo Quartzite (1712 Ma), and the 1749 Ma crystallization age of the underlying Baxter Hollow granite and rhyolite parent materials. Micromorphologic evidence of paleoweathering comprises clay illuviation features (indicative of clay movement), sepic-plasmic fabrics (formed by repeated wetting and drying cycles), redoximorphic features (redox-related depletions and concentrations of Fe indicative of variable drainage and redox conditions), and dissolution and alteration of feldspars and mafic minerals (now sericite and Fe-oxides). Geochemical evidence for weathering includes depth-dependent whole-rock losses of Na, Ca, Mg, Si, Ba, Sr, Fe, and Mn, and relative enrichment of several immobile elements; there is also evidence for post-weathering K metasomatism (previously dated as 1456 Ma) that includes 30% net additions of K and Rb. Geochemical patterns for the Baraboo profile resemble those of the previously studied and older Pronto (2.475–2.44 Ga) and Ville Marie (2.38–2.215 Ga) paleoweathering profiles, but with important differences related to Baraboo profile development beneath a regolith cover and under conditions of variable hydrology during weathering. In spite of its age and greenschist-facies metamorphism, the Baraboo paleoweathering profile is remarkably similar to some modern weathering profiles formed on granite.