Lead isotopic compositions of Andean igneous rocks, latitudes 26 degrees to 29 degrees S; petrologic and metallogenic implications

Abstract

Lead isotopic data are reported for 12 whole rock samples of Mesozoic to Quaternary plutonic and volcanic rocks from an intensely mineralized transect of the ensialic Central Andean orogen. In contrast to the initial Sr isotopic ratios and large ion lithophile element abundances of these rocks, the Pb isotope ratios show no trend either with decreasing age or with distance from the continental margin.The ratios 207 Pb/ 204 Pb and 208 Pb/ 204 Pb are significantly higher than those of oceanic tholeiite and alkali basalt from the Nazca plate, and we conclude that the lead cannot have been derived from subducted, metamorphosed basalt. However, there is extensive compositional overlap with the Pb isotopic ratios of ocean floor sediments and manganese nodules from the southeast Pacific.Considered in conjunction with the Sr isotope ratios and rare earth elements (and other large ion lithophile element) abundances of these rocks the new lead data are interpreted in terms of a two-stage magma generation model in which, though lead and strontium were derived from layers 1 and 2, respectively, of subducted oceanic lithosphere, the immediate source of the magmas was in the continental mantle peridotite which had become enriched in large ion lithophile elements through interaction with melts or waters rising from the subduction zone. It should be stressed, however, that the Pb isotopic ratios are not incompatible with a source in the sialic crust, although the wholesale anatexis of crust is here a very improbable mechanism for magma generation. Contributions from the continental crust may have occurred directly, through selective transfer to the invading magmas or, more remotely, following subduction and partial melting of material abraded from the leading edge of the continent.The relatively consistent nature of the rock lead compositions is paralleled by the broadly homogeneous metallogenic aspect of this transect, in which copper deposits almost everywhere predominate, and in which the longitudinal metal zonation, considered a characteristic of Andean-type orogens, is not developed. If rock and ore leads, and the economically important metals (Cu, Mo, Ag, Au), are cogenetic, arguments similar to those advanced to constrain the origin of the parent igneous rocks may be applied to delimit the source of the ore metals. Thus, metalliferous sediments such as those of the Nazca plate are improbable sources, whereas the metal-rich pelagic sediments and Mn nodules are promising candidates. However, the ore metals may have been derived from the deeper continental mantle or the sialic crust.