Geochemical and Textural Investigation of an Archean Paleosol, South Roberts Pit, Steep Rock Area, Ontario, Canada

Micromorphologic, geochemical and isotopic analysis of an apparent Archean paleosol exposed at South Roberts Pit, Steep Rock area, Superior Province, Canada are consistent with in situ, subaerial weathering followed by potassium metasomatism and greenschist metamorphism. The profile formed on tonalite of the 3.0 Ga Marmion Complex and is unconformably overlain by the basal conglomerates of the >2.7 Ga Steep Rock greenstone series. Only the lowermost part of the soil profile is preserved. Weathering of plagioclase, biotite, ilmenite, titanite, epidote, and apatite likely produced clays such as kaolinite and smectite (now altered to sericite and paragonite) and leucoxene. Micromorphologic evidence of soil-forming processes includes intertexic and agglomeroplasmic fabric, spalling quartz grains, and clay cutans. With increased weathering, quartz and plagioclase decrease and the sericite matrix increases. Rip-up clasts are present at and above the unconformity. Base cations (Ca, Mg, Na, P), Fe and Mn are depleted relative to parent material, while K is enriched. Chondrite-normalized rare earth element (REE) plots for the paleosol show light-REE enrichment, whereas heavy-REE patterns are mostly flat. Parent-normalized patterns indicate some REE mobilization and fractionation during pedogenesis. The low K-corrected chemical index of alteration and modest changes in the overall REE budget indicate that either the profile was not well developed or that the most heavily weathered sections were eroded away. Laser ablation-ICP-MS examination shows that the REE budget is largely controlled by titanite, which is generally LREE-depleted relative to its igneous parent. Eu anomalies (Eu/Eu*) in the paleosol are more negative than that of average parent, consistent with plagioclase weathering. Significant iron loss (>40%), Fe(III)/Fe(II) ratio of >1, and lack of strong Ce anomalies suggest that the paleosol could have formed under conditions sufficient to oxidize Fe, but not Ce, and that the profile was later stripped of Fe by reductive processes such as leaching by organic acids or hydrothermal alteration. Rb-Sr systematics for the paleosol were clearly disturbed; correlation of Rb and K indicate K-metasomatism most likely occurred during the ~2.7 Ga greenschist metamorphic event related to the Kenoran Orogeny. Sm-Nd isotope data for the paleosol form an apparent isochron with an age of 3018 ± 90 Ma. This age is indistinguishable from that of the Marmion Complex on which the profile formed, and suggests either that weathering took place very soon (less than ~ 60 my) after emplacement of the Marmion complex or that the Sm-Nd systematics were only slightly affected on a cm-m scale by weathering processes.