Journal of Volcanology and Geothermal Research, 2 (1977)251--258 251 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands URANIUM IN VOLCANIC POCKS FROM THE CENTRAL ANDES M. ZENTILLI and J. DOSTAL Department of Geology, Dalhousie University, Halifax, N.S. (Canada) Department of Geology, Saint Mary's University, Halifax, N.S. (Canada) (Received December 6, 1976; revised version accepted March 18, 1977) ABSTRACT Zentilli, M. and Dostal, J., 1977. Uranium in volcanic rocks from the Central Andes. J. Volcanol. Geotherm. Res., 2:251--258. The distribution of U in volcanic rocks from two transects across the Central Andes (latitudes 27°--28°S and 16°--18° S, respectively) differs from that of K. For a given SiO2 content of the rocks, K systematically increase.s with the distance from the trench, while the highest U abundances are found in the rocks overlying the thickest segments of the continental crust, which are situated in the middle parts of the transects. It is suggested that this variation of U reflects crustal contamination. INTRODUCTION Variations in the chemical composition of volcanic rocks across orogenic belts associated with subduction zones have long been known. They are particularly well documented for K, which in calc-alkaline rocks tends to show an increase at any given SiO2 content with increasing distance of the eruptive centres from the adjacent oceanic trench (Dickinson, 1968, 1975; Hatherton and Dickinson, 1969). There are indications that other incompatible elements such as Sr, Ba, Ti and U show similar transversal variations (Jakes and White; 1972; Palacios and Oyarzfln, 1975), but owing to sparse data, these variations have not been convincingly documented. The purpose of this study is to evaluate the variation of U in Upper Cenozoic volcanic rocks from two transects of the Central Andes and to compare this variation with that of potassium. One transect is located at latitude 27°--28°S in northern Chile and Argentina, while the other one is at latitude 16°--18°S in southernmost Peru (Fig. 1). U was determined by a fission-track technique, the precision and accuracy of which are given by Dostal et al. (1975). The determinations of K were done by X-ray fluorescence. The abundances of U and K in rocks from the northern transect have been reported by Dupuy et al. (1976) and Dupuy and Lef~vre (1974).