SIGNIFICANT CHANGES IN THE DETRITAL ZIRCON AGE SIGNATURE ALONG A TRANSECT OF THE MODERN AMAZON RIVER

Interpretations of U-Pb detrital zircon geochronology sometimes call for trans-continental transport of zircon populations. To test the attenuation rate of zircon populations during fluvial transport we studied the age distribution of zircons currently in transport along the Amazon River. The Amazon is ideal for this study: the only significant source of Phanerozoic zircons is in the river's Andean headwaters, and because much of the river's gradient lies in the Andes we can assume that there is a substantial contribution of those zircons to the system. The remainder of the basin has low gradient, and is underlain by a series of north-south trending basement belts that increase in age from about 1100 Ma just east of the Andes to Early Proterozoic and Archean near the river mouth. There are no Phanerozoic zircon sources in the mid and lower river basin, so any young zircons in our samples must come from the Andean headwaters.

We collected 13 fine- to medium-grained sand samples along 800 km of the middle Amazon between Alvarães and the Amazon/Madeira confluence, targeting sand bars emergent during low water. We obtained U-Pb ages by Laser Ablation-ICPMS analysis of ~180 grains per sample at the University of Texas at Austin. We found some young grains in all samples, and Cenozoic grains in most samples, indicating that Andean zircons are indeed transported 3000 km alongstream.

However, although young grains are present throughout, they show progressive downstream decrease in abundance. The farthest upstream samples have prominent populations of Andean Mesozoic zircons, but these dwindle in samples taken further down the river. The populations of more locally-derived 1100 Ma zircons are of lesser magnitude in the upstream samples and increase in relative size downstream. The furthest downstream sample has broader peaks around 150, 1100, and 1540 Ma and is composed of ~20% zircon older than 1500 Ma compared to less than 10% in the furthest upstream sample. The population distributions are explained by increasing dilution of the Andean signature by shield-derived Proterozoic (1.5-2.0 Ga) and some Archean zircons, in addition to progressive depletion of Andean zircons by deposition.