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

This paper derives an estimate of the average chemical composition of the P aleozoic Andean basement, based on the geological evolution and from compil ations of geochemical and isotopic analyses of Early to Late Paleozoic age metamorphic and magmatic rocks between 21 degrees and 27 degreesS. Geochemi cal and isotopic data indicate that recycling of the Early Paleozoic metamo rphic basement was the predominant process in the formation of felsic magma s in the Central Andes from the Odovician to late Paleozoic. The metamorphi c basement, in turn, is derived in part from older intrusions and sedimenta ry rocks of Eocambrian age, Compositional characteristics of Ordovician and late Paleozoic sediments reflect the erosion of this crust. Mafic metamorp hic and igneous rocks do not contribute significantly to the Paleozoic crus t. The Paleozoic crust is mainly felsic in composition, which is supported by evidence from lower crustal xenoliths and limited exposures of lower cru st. The crust consolidated in the Paleozoic remained largely intact until t he formation of the overthickened Andean crust in the Cenozoic. The dominan ce 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 Cen ozoic ignimbrite and andesite. Thermodynamic calculation of the stable mine ral assemblages from the average bulk composition derived from our compilat ion indicate that the volumetrically most important minerals are quartz and feldspars under all reasonable P-T conditions expected for the thickened A ndean crust. The resulting density-depth model is in accordance with geophy sical observations of the velocity-density distribution at 21 degrees -24 d egreesS. Radiogenic heat production is estimated from the average U-Th-K co ncentrations in the Paleozoic metamorphic and granitic rocks that are very similar to average values of the upper crust. (C) 2001 Elsevier Science B.V . All rights reserved.