NEOARCHEAN PALEOWEATHERING OF TONALITE AND METABASALT: IMPLICATIONS FOR RECONSTRUCTIONS OF 2.69 GA EARLY SOIL ECOSYSTEMS AND PALEOATMOSPHERIC CHEMISTRY

Field and laboratory investigations of a 2.69 Ga (²⁰⁷Pb/²⁰⁶Pb age of Saganaga tonalite) unconformity exposed in outcrop in northern Minnesota, USA, reveal evidence for development of a paleoweathering surface with geochemical biosignatures indicative of the presence of microbial communities and weakly oxygenated conditions. Weathering profiles are characterized by a 5-50 m thick regolith that consists of either saprolitized tonalite (plagioclase-rich rock) or basaltic metavolcanic rocks retaining relict rock structure (saprock), which is cross-cut by a major unconformity surface marking development of a terrestrially dominated successor basin. The regolith and unconformity are overlain by thick conglomerate deposits that contain both intrabasinal (saprock) as well as extrabasinal detritus. Thin-section microscopy and electron microprobe analyses reveal extensive hydrolysis and sericitization of feldspars, exfoliation and chloritization of biotite, and weathering of Fe-Ti oxides and Fe-Mg silicates. Geochemical analyses, assuming immobile Al during weathering, show depletion of Na₂O, CaO, and MgO, and enrichment of SiO₂, TiO₂, and K₂O, relative to least-weathered parent materials. K₂O enrichment is attributed to potassium metasomatism overprinting paleoweathering. Fe₂O₃ is lost in both the tonalite and metabasalt, consistent with previous estimates of low paleoatmospheric pO₂ and high pCO₂ during this time. Patterns of release of Zr, Sc, Y and Mn (basalt), and release of P₂O₅, V and Zn (tonalite), are consistent with recent laboratory experiments of basalt and granite weathering; these suggest the presence of organic ligands, and hence a primitive microbial community during weathering. Comparison with previous studies of weathered tonalite and basalt (Denison, 2.45 Ga) in Ontario, Canada reveal general similarities in paleoweathering for the two rock types, as well as important differences related to slightly increased paleoatmospheric pO₂ and increased terrestrial biosignature recorded in the Denison profile.