Provenance of quaternary and modern alluvial deposits of the Amazonian floodplain (Brazil) inferred from major and trace elements and Pb–Nd–Sr isotopes

Abstract

Chemical and Pb, Nd and Sr isotope data from floodplain sedimentary deposits in the Solimões River, northwest Brazil, constrain the variability of the deposits through the Quaternary and help reconstruct the paleogeography of Amazonia since the uplift of the Andes. Compared to the modern alluvial deposits in the active channel, the Quaternary terrace deposits in the Solimões River have a higher chemical index of alteration (CIA), higher SiO2/Al2O3 ratios, higher concentrations of immobile elements, higher radiogenic Sr isotopic compositions, more negative εNd(t=0) values, and older Nd TDM ages. These geochemical characteristics indicate that the Quaternary sediment sources were composed by more felsic and/or recycled old continental crust derived rock materials than the younger source areas, which contributed major arc volcanic materials to the flood plain deposits. This shift in the composition of the sediment source areas indicates that a significant reorganization of the architecture of the Solimões River has occurred since the uplift of the Andes.