Crustal recycling of metamorphic basement: Late palaeozoic granitoids of northern chile (similar to 22 degrees S). Implications for the composition of the Andean crust

Upper Palaeozoic silicic magmatism is widespread in the Central Andes, but its origin is poorly constrained. We investigated whole-rock chemical and i sotopic composition of Upper Palaeozoic granitoids and their Early Palaeozo ic high-grade country rocks in the Chilean Coastal Cordillera and Precordil lera at similar to 22 degrees S, in comparison with an Upper Cretaceous gra nitoid. The age of the Late Palaeozoic granitoids from a Rb-Si isochron of similar to 300 Ma ii consistent with K-Ar cooling ages of hornblende and bi otite. Similar major and trace element patterns as well as Nd and Pb isotop ic composition of Upper Palaeozoic granitoids and gneisses point to a sourc e of the granitoids that is similar to the gneisses at outcrop. Sr isotope ratios of the Upper Palaeozoic granitoids are less radiogenic than those in many of the gneisses. We propose a stratification of the Early Palaeozoic crust with a Rb-deficient granulitic mid-lower crust, resulting in less rad iogenic Sr compared with the upper crust, based on the interpretation of th e P-T-t history and isotopic composition of the Lower Palaeozoic meta-morph ic basement and of the isotopic composition of the Late Palaeozoic granitoi ds and younger magmatic rock. Nd isotopic composition is identical in lower and upper crust and in crustal melts from the Late Palaeozoic in Recent. T he Cretaceous granitoid evolved from partial melts of a mantle-derived sour ce with considerable contamination by the old crustal component. The crusta l that formed in the Early Palaeozoic is the major source of material for t he Cenozoic tectonic thickening of the Andean crust.