Composition and density model of the continental crust at an active continental margin—the Central Andes between 21° and 27°S

Abstract This paper derives an estimate of the average chemical composition of the Paleozoic Andean basement, based on the geological evolution and from compilations of geochemical and isotopic analyses of Early to Late Paleozoic age metamorphic and magmatic rocks between 21° and 27°S. Geochemical and isotopic data indicate that recycling of the Early Paleozoic metamorphic basement was the predominant process in the formation of felsic magmas in the Central Andes from the Odovician to late Paleozoic. The metamorphic basement, in turn, is derived in part from older intrusions and sedimentary rocks of Eocambrian age. Compositional characteristics of Ordovician and late Paleozoic sediments reflect the erosion of this crust. Mafic metamorphic and igneous rocks do not contribute significantly to the Paleozoic crust. The Paleozoic crust is mainly felsic in composition, which is supported by evidence from lower crustal xenoliths and limited exposures of lower crust. The crust consolidated in the Paleozoic remained largely intact until the formation of the overthickened Andean crust in the Cenozoic. The dominance of the Paleozoic crust in the crust of the present high plateaus of the central Andes is seen in the geochemical and isotopic signatures of the Cenozoic ignimbrite and andesite. Thermodynamic calculation of the stable mineral assemblages from the average bulk composition derived from our compilation indicate that the volumetrically most important minerals are quartz and feldspars under all reasonable P – T conditions expected for the thickened Andean crust. The resulting density–depth model is in accordance with geophysical observations of the velocity–density distribution at 21°–24°S. Radiogenic heat production is estimated from the average U–Th–K concentrations in the Paleozoic metamorphic and granitic rocks that are very similar to average values of the upper crust.