Extreme element mobility during transformation of Neoarchean (ca. 2.7 Ga) pillow basalts to a Paleoproterozoic (ca. 1.9 Ga) paleosol, Schreiber Beach, Ontario, Canada

Abstract

This study reports petrographic, major and trace element, and oxygen isotopic data for a subaerial weathering profile at Schreiber Beach, Ontario, Canada. The weathering profile developed on Neoarchean (ca. 2.7Ga) pillow basalts and is unconformably overlain by the Paleoproterozoic (ca. 1.88Ga) Gunflint Chert and basal conglomerates. This stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the Gunflint Formation. Rocks at Schreiber Beach have been classified as pillow cores, pillow rims, hyaloclastites, weathered red basalts, and weathered brown to green basalts. There are gradual textural, mineralogical, and geochemical transitions from unweathered basalts to intensely weathered hematite-bearing basalts with stratigraphic height.Rocks at Schreiber Beach underwent seafloor hydrothermal alteration and greenschist facies regional metamorphism in the Neoarchean. Following the subaerial weathering, they were buried and re-metamorphosed to greenschist facies in the Paleoproterozoic. Given the preservation of primary igneous textures and trace element patterns in pillow cores, they are used as a protolith to estimate element mobility during seafloor alteration, metamorphism, and subaerial weathering. Chloritized pillow rims and hyaloclastites are characterized by moderate losses or gains of major (Si, Ca, Na, Fe, Ti, K) and trace (LILE, REE, HFSE, and transition metals) elements during Neoarchean seafloor alteration. Alteration of these rocks at relatively low temperatures is indicated by enrichment in 18O (δ18O=+10.4 to +12.7‰) relative to fresh basalt (~+5.5‰).Subaerial weathering resulted in significant textural, mineralogical (e.g., hematite precipitation), and geochemical variations in metamorphosed basalts, causing additional redistribution of most major and trace elements and further enrichment in 18O. The highest δ18O values (+16.1 to +17.1‰) occur at the silica-rich base of the weathering profile. Many elements, including LILE, REE and HFSE, are strongly enriched (>100%) or depleted (up to 96%) relative to their original abundances. Alkaline metasomatism, which is a characteristic of Precambrian paleosols, is also evident at Schreiber Beach, where extreme enrichment in K (280 to 1000%), Rb (390 to 660%), and Cs (1160 to 2560%) occurs in the weathering profile.