Crustal recycling of metamorphic basement: Late Palaeozoic granitoids of Northern Chile (~22°S). Implications for the composition of the Andean crust

Abstract

Upper Palaeozoic silicic magmatism is widespread in the Central Andes, but its origin is poorly constrained. We investigated whole-rock chemical and isotopic composition of Upper Palaeozoic granitoids and their Early Palaeozoic high-grade country rocks in the Chilean Coastal Cordillera and Precordillera at ~22°S, in comparison with an Upper Cretaceous granitoid. The age of the Late Palaeozoic granitoids from a Rb-Sr isochron of ~300 Ma is consistent with K-Ar cooling ages of hornblende and biotite. Similar major and trace element patterns as well as Nd and Pb isotopic composition of Upper Palaeozoic granitoids and gneisses point to a source 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 radiogenic Sr compared with the upper crust, based on the interpretation of the P-T-t history and isotopic composition of the Lower Palaeozoic metamorphic basement and of the isotopic composition of the Late Palaeozoic granitoids and younger magmatic rocks. Nd isotopic composition is identical in lower and upper crust and in crustal melts from the Late Palaeozoic to Recent. The Cretaceous granitoid evolved from partial melts of a mantle-derived source with considerable contamination by the old crustal component. The crust that formed in the Early Palaeozoic is the major source of material for the Cenozoic tectonic thickening of the Andean crust.