Metasaprolite in the McGrath Gneiss, Minnesota, USA: Viewing Paleoproterozoic weathering through a veil of metamorphism and metasomatism

•A Proterozoic metasaprolite has been identified by geochemical methods.•A moderate degree of weathering penetrated to a minimum depth of ∼420 cm.•The minimum total mass flux removed by weathering at 2.1 Ga is 2.4 moles/cm2.•The minimum mass of K2O added at 1.74 Ga is 0.33 moles/cm2.•Assuming 105 years of weathering, atmospheric pCO2 at 2.1 Ga was 3.7 × PAL.

A 2100 Ma, 420 cm-thick metasaprolite, which is devoid of plagioclase but contains abundant microcline, occurs in the Archean McGrath Gneiss beneath the overlying Paleoproterozoic Denham Formation. Despite being recrystallized under amphibolite-facies conditions and chemically modified by potassium metasomatism, mass fluxes related to weathering and metasomatism of the McGrath metasaprolite can be estimated by judicious application of several geochemical parameters and the A–C*N–K plot (Fedo et al., 1995). The metasaprolite yields values of 67–94 for the Plagioclase Index of Alteration, 62–72 for the Chemical Index of Alteration, and 47 for the Feldspar Index of Weathering, and the total mass flux removed by weathering is a minimum of 2.4 moles/cm2. Such values correspond to an intermediate degree, or intensity, of weathering compared to other, more intensely weathered paleosols in the region, such as those beneath the Baraboo and Sioux quartzites. Potassium metasomatism occurred at 1742 Ma during the geon 17 Yavapai tectonothermal event, when a minimum of 0.33 moles/cm2 K2O was added to the metasaprolite. Although the absence of pedogenic textures precludes interpretation of climatic conditions during weathering of the McGrath Gneiss, the level of atmospheric pCO2 can be estimated by application of Sheldon's (2006) method, which yields a minimum of 3.7 × PAL for a weathering duration of 100,000 years.