143Nd/ 144Nd, 87Sr/ 86Sr, and incompatible element variations in calc-alkaline andesites and plateau lavas from South America

Trace element, Sr and Nd isotope data on Andean calc-alkaline andesites and plateau basalts from Patagonia are presented. Seven plateau lavas have ⁸⁷Sr/ ⁸⁶Sr ratios of 0.7033-0.7052 and ¹⁴³Nd/ ¹⁴⁴Nd of 0.51300-0.51262 and on a plot of ⁸⁷Sr/ ⁸⁶Sr versus ¹⁴³Nd/ ¹⁴⁴Nd they all lie within the region defined by Atlantic volcanic rocks. Six andesites from Ecuador have a narrow ¹⁴³Nd/ ¹⁴⁴Nd range of 0.51300-0.51285 while the Chilean calc-alkaline rocks have much lower ratios of 0.51257-0.51236. On the Nd versus Sr isotope diagram the Ecuador samples, and to a lesser extent the Chilean samples, are subparallel to the trend of the Patagonian and Atlantic lavas but displaced to more radiogenic compositions. Since both island-arc tholeiites and Andean andesites exhibit this shift to more radiogenic compositions, their Sr isotope geochemistry in particular may reflect similar processes along destructive plate margins in both continental and oceanic environments. The available data are briefly evaluated and a model is discussed whereby Rb, K, and Sr are preferentially released in H ₂O-rich fluids from the downgoing slab as a result of dehydration. Depending on the amount and composition of the Sr both in the overriding lithosphere and released from the slab this will variously affect the geochemistry of the resultant volcanics. In general, however, they are enriched in alkalis relative to say the rare earth elements and, providing their ⁸⁷Sr/ ⁸⁶Sr ratios are <0.706-0.707, they reflect displacement to more radiogenic Sr compositions due to the input of seawater-affected Sr from the subducted lithosphere.