Análisis de isótopos estables (δ18O-δD) en depósitos de arcillas de las Sierras de Tandil, provincia de Buenos Aires, Argentina

Abstract

Several clay deposits occur associated to the Precambrian igneous-metamorphic basement of the Sierras de Tandil, Buenos Aires Province and to the overlying Neoproterozoic-Palaeozoic sedimentary cover. The origin of these deposits is controversial, as they have variously been attributed to weathering of basement rocks, hydrothermal alteration, diagenesis, purely detrital deposition, or various combinations of the above, depending on locality and author(s). In order to substantiate, or else refute, previous work, we have determined δD and δ18O in several clay samples collected from deposits previously interpreted as formed by contrasting processes. Most values obtained range between δD =-85/-110‰ and δ18O =+10/+13‰. These detrital clays plot close and slightly to the left of the “kaolinite line” on a δD vs δ18O binary plot, evidencing equilibrium at temperatures slightly above ambient, and are compatible with mid-latitude meteoric waters of δ18O =-9/-12‰ and δD =-55/-85‰. Generally heavier values are, however, obtained for clays resulting from in situ weathering of the basement.