Weathering assessment in the Achala Batholith of the Sierra de Comechingones, Cordoba, Central Argentina.Ⅲ : appraising chemical weathering

摘要

La Trucha is a small (similar to 1.9 km(2)), mountainous (similar to 1300 m a.s.l.) granitic drainage basin, demonstrative of hundreds of second-order basins in the Achala Batholith of the Sierras Pampeanas of Cordoba, Argentina. Dominated by physical weathering and a weathering-limited denudation regime, the coarse-and fine-grained regolith has shown scant geochemical differences with the country rock. Erosion and sediment transport mainly occurs during torrential rain events (austral summer), whereas surface waters classify as dilute (750 T Sigma(+) 375 mu eq L-1) or very dilute (375 T Sigma(+) 185 mu eq L-1) water types. The dynamics of chemical weathering at La Trucha was approached through the use of major dissolved chemical components, comparing the performance of different methodologies in its assessment: mineral stability diagrams (i.e., chemical equilibria), the RE chemical weathering index, ternary diagrams, and PHREEQC inverse modeling. The latter approach allowed to probe into subtleties of the geochemical processes indicating that, besides the morphological dependency on mineral hydrolysis/dissolution or precipitation, there is a marked seasonal control that promotes or limits the occurrence of geochemical reactions.