U-PB AGES OF DETRITAL ZIRCON AND THEIR CORRELATION TO LOCAL TECTONICS OF THE ANDEAN PLATEAU, CHILE

The relative contributions of tectonism and volcanism to uplift of the Andean Plateau in Chile is debated. Ignimbrites ranging from 8.2 to 5.6 Ma are interbedded with gravels that were eroded from the edge of the Altiplano during tectonic uplift. We are investigating the age of the gravel horizons and their correspondence to the ignimbrites to obtain a clearer understanding on the relation between volcanism and uplift of the Altiplano. Three samples were taken from two gravel horizons in the Río Salado Valley in northeastern Chile. Approximately 100 detrital zircon grains extracted from each sample were analyzed using LA-ICP mass spectrometry to determine the ages and source areas of the gravel horizons. The lowest sample was taken directly above an evaporite unit, the Paciencia Group, from a pink gravel horizon. Detrital zircon grains extracted from this sample yielded U-Pb ages that are 86% Cenozoic but older than 10 Ma, 3% Cretaceous, and 11% Triassic and older. The remaining samples were taken 28m and 4m below the base of the Sifon ignimbrite, which is dated at 8.2 Ma, from a purple gravel horizon. U-Pb ages in the middle sample 28m under the Sifon ignimbrite have 7% grains less than 10 Ma, 33% Cenozoic but older than 10 Ma, and 60% Mesozoic and older. U-Pb ages in the uppermost sample 4m under the Sifon have 62% grains less than 10 Ma, 17% Cenozoic but older than 10 Ma, and 21% Mesozoic and older. Sources for these gravel layers include the underlying ignimbrites, Cenozoic sediments derived from the Altiplano, a Cretaceous arc 2.6km northwest, and Tertiary units from the Valle del Arcoiris 1.4km to the northeast. The youngest zircon grains derived from the gravel horizons are similar in age to the overlying Sifon ignimbrite, indicating that crustal erosion of the Altiplano and the aggradation of the gravel horizons occurred directly after the pulses of volcanism. Rising magma bodies formed during the subduction of the Nazca plate may have driven uplift and erosion of the Altiplano; therefore depositing the ignimbrite and gravel horizons. It can also be concluded that the source of the zircons was not limited to the sediments eroded from the young Altiplano, but also from the Cretaceous arc to the northwest and Tertiary units to the northeast.