WEATHERING OF GRANODIORITIC CRUST, LONG-TERM STORAGE OF ELEMENTS IN WEATHERING PROFILES, AND PETROGENESIS OF SILICICLASTIC SEDIMENTS

The bulk composition and mineralogy of the Toorongo Granodiorite, Aust ralia, are similar to average upper continental crust (AUCC). Weatheri ng characteristics of the Toorongo profile consequently provide insigh t into large-scale chemical weathering of the upper crust. In situ wea thered materials of the profile do not reflect parent granodiorite com position in quartz-plagioclase-K-feldspar (Q-P-K) or in quartz-feldspa r-rock fragment (Q-F-L) compositional space. Intensive in situ weather ing precludes sands, derived from mature weathering profiles through e rosion, from reflecting their provenance. Where intensive chemical wea thering has occurred, clay minerals and oxyhydroxides of the profile, and by inference muds derived therefrom, contain much more chemical in formation about provenance than do associated sands. Actinides, ran ea rth elements (REEs), many transition metals, and metalloids have accum ulated in deep parts of the weathering profile at concentrations much greater than observed in the fresh granodiorite. Mass balance consider ations require the bulk of these elements to have been derived from pr eviously weathered, and now eroded, granodiorite. These elements were, and are, continually cycled from the intensely weathered uppermost so il zone to deeper, less weathered, zones of the profile where they acc umulate. The profile therefore represents a large, continental element al storage reservoir, the storage capacity of which has increased over time. Wherever erosion is sufficiently slow and chemical weathering s ufficiently rapid, mature weathering profiles may become large, long-t erm storage reservoirs for actinides: REEs, and many other elements. T he total REE contents of extremely weathered soil material are somewha t less than in the parent granodiorite, but they are enriched twofold to threefold in the zone of intermediate weathering relative to parent . Similar variations in total REEs are observed in some muds when norm alized to their source (AUCC). These differences are attributed to a c ombination of chemical weathering and selective mass wasting of profil es. Homogenization of detritus in large sedimentary basins, however, p roduces muds with REE patterns and total REE contents similar to sourc e (AUCC). Nd/Sm ratios are not influenced by chemical weathering, alth ough both elements are mobilized by weathering and become enriched by over 200% relative to parent rock. Constancy of Nd/Sm in the profile i ndicates that Nd-Sm model ages derived from soils and sediments are no t affected by chemical weathering. The least mobile trace elements of the profile are Sc, Cu, Nb, and Ta, but others are more mobile. Thoriu m, for example, is mobilized during weathering of the Toorongo Granodi orite and displays a twofold increase in the profile, as does the Th/S c ratio. The ratio, however, varies by more than a hundredfold in majo r rock types so that Th/Sc (and other ratios) provides valuable inform ation about provenance, although sensitivity is diminished somewhat by the effects of chemical weathering. Copyright (C) 1997 Elsevier Scien ce Ltd.