HOW FAR DO ZIRCONS TRAVEL? EVALUATING THE SIGNIFICANCE OF DETRITAL ZIRCON PROVENANCE USING THE MODERN AMAZON RIVER FLUVIAL SYSTEM

The utility of detrital zircon populations in provenance studies of ancient sediments is dependent on understanding zircon transport in fluvial and marine settings. We present new detrital zircon data for sediments from the Amazon River that demonstrate that zircon populations are dominated by local sources, suggesting that occurrence of far-traveled grains (1000’s of km) as distinct populations may be unlikely. We chose the Amazon River Basin to study zircon transport in a modern, continent-scale fluvial system because young Andean rocks in the headwaters provide a distinct and traceable source of detritus downstream. Two samples were collected near Coari, Brazil, 2000 km + from the headwaters (about halfway between the headwaters and the mouth of the river) to compare with published results for sediments collected at the mouth of the river (Rino et al., 2003). Dates (Pb-Pb for grains > 1 Ga and U-Pb for grains <1 Ga) were obtained by LA-MC-ICPMS at the University of Texas, Austin. Data for the two Coari samples are indistinguishable. Both samples yielded primarily Grenville-age grains (47% in the range 900-1300 Ma) with smaller populations of Paleozoic (18%), Mesozoic (13%) and Cenozoic-age (4%) grains. These results contrast sharply with results for zircons from sediment collected near the mouth of the river. Of the 396 zircons analyzed from that sample, only a single grain was Phanerozoic, ~9% were of Grenville age, and 76% were in the 2-3.5 Ga age range. It is unclear if Proterozoic and Archean zircons were derived from adjacent basement rocks or were reworked (over the preceding 2 Ga + years), but they require no long distance transport history. Most remarkable in the data is the attenuation of populations downstream: despite the abundance of Cenozoic sources in the headwaters, Cenozoic grains comprise only a small population ~2000 km downstream and are apparently not present as a population an additional ~1500 km downstream at the mouth of the river. It is unclear if population attenuation is the result of non-transport or dilution by local populations. However, the result is similar to those presented by Cawood et al. (2003) who also noted a link between zircon populations and local basement ages. Further sampling and analysis of sediments from along the entire length of the river will clarify these preliminary conclusions.