Andean fingerprint on placer sands from the southern Brazilian coast.

Abstract

On the passive margin of southern Brazil, where the availability of sediments and coastal system conditions were adequate for forming a Ti–Zr-bearing placer in the Holocene, could there be a source of detrital contribution other than the Precambrian South American cratons? To answer that, detrital zircon U–Pb–Hf analyses were performed using the LA-ICP-MS method. The U–Pb age pattern distribution of 866 zircon grains showed that 35.3% of the grains belong to the Neoproterozoic, which covers the Brasiliano orogenic belts. However, 9.1% of the total zircons analyzed (up to 12% in some samples) correspond to grains younger than 50 Ma, restricted to the Andean orogeny in southern South America. The Hf signature of the zircons stands out by pointing to five significant and distinct groups interpreted as coming from the Andean (0–50 Ma), Gondwanides (230–380 Ma), Famatinian (380–500 Ma), Brasiliano (850–541 Ma), and Grenville (900–1300 Ma) orogens, confirming an Andean fingerprint as source rocks in addition to the sediments originating from the craton. This Andean source is related to the distal sediment contribution from the La Plata River system and Argentina rivers that discharge in the Atlantic Coast. Based on a statistical and Hf isotope approach, we defined that around 50% of the zircons grains were transported from the Argentina coast and La Plata River by the littoral drift for more than 1000 km to their final sink in the southern Brazilian coast.