COMPARISON OF REE AND OTHER TRACE ELEMENTS IN ARGENTINE PAMPEAN AND CENTRAL US LOESS DEPOSITS

Classic Pleistocene to Holocene loess sections (glacial aeolian silt deposits) drape both the Argentine Pampas in South America and the central part of the US including Iowa, Nebraska and Kansas. Among various methods, geochemical tracers provide information on the source regions of the loess deposits, which include fluvial material from the Brazilian and Canadian shields to the north in both areas, sedimentary and igneous basement to the west and air fall ash from contemporary eruptions from the Andean arc region in South America and the Yellowstone volcanic center in North America. In the South American case, Patagonian sources to the south can be more mafic, although they also contain silicic Andean volcanic ashes and Andean arc sediments transported in rivers like the Rio Colorado. Here we present new geochemical analyses for the REEs, Ba, Ta and other diagnostic trace elements in some samples of Argentine loess from the Mar del Plata region on the Atlantic coast near 37°S and the Parana River near Buenos Aires at 35°S along with analyses of volcanic ashes from these same regions. The ashes are from the historic eruption of the Quizapu volcano in the south-central Andean volcanic arc, which covered the Argentine pampas and an ash from near Neocochea on the Atlantic coast south of Buenos Aires near 37°S. These analyses along with previous ones show the diversity of the sources of the Argentine and central US loesses. For example, the Pampean loess from near Mar del Plata has high Ba/Ta, La/Ta, and Ba/La ratios that reflect an Andean arc component as shown by comparison with the analyses of the Quizapu ash. In contrast, the Parana River loess has less volcanic arc-like trace element features as the Rio Parana does not drain the Andean arc terrane. In the loess from the Midwest of the US, relatively low Ba/Ta and La/Ta ratios reflect a provenance that includes the non-volcanic arc-like Yellowstone tephra and PreCambrian shield contributions. Loess from other parts of the world such as China and Germany show other trace element features like lower Ba/Ta ratios, whereas trace elements features in New Zealand loess can overlap those of North American and Argentine loess. These comparisons show that the sometime use of loess as a proxy for the composition of the upper crust needs to be viewed with caution.