Tectonic versus chemical-weathering controls on the composition of fluvial sands in tropical environments

The compositions of fluvial sands collected in mountainous regions along the volcanic front in Central America and along the Andean front in Venezuela and Colombia correlate with the tectonic setting of their source terrains. The sands derived from the Central American volcanic arc are compositionally distinct from those derived from the Andean terrains, and minor variations in tectonic style can also be discriminated. Subtle differences in tectonic setting and source rock composition are recognizable only when modification by chemical weathering is slight; they are strongest in sands from low-order streams draining mountainous areas with modest alluvial-plain development. Sands that have been exposed to weathering for extended periods during alluvial storage are progressively enriched in chemically stable phases. With increased weathering, sands from continental terrains converge toward a supermature quartz arenite end member, and the imprint left by source rock composition and tectonic setting is obscured. In intraoceanic terrains, a similar but much less pronounced enrichment in stable phases relative to less-stable phases is observed, similarly overprinting the tectonic signal. Although detailed tectonic information is easily lost, sands derived from intraoceanic terrains are distinguishable from those derived from continental terrains even after long exposure to intense chemical-weathering conditions.