Quantitative models for crustal contamination in the Central and Northern Andes.

Abstract

Systematic variations in O, Sr, Nd and Pb isotope ratios and related trace-element abundances in modern andesitic lavas of the CVZ and NVZ are examined and compared in terms of crustal contamination models of assimilation and fractional crystallization (AFC). Isotopic and trace-element compositions of the 'primary' magmas are estimated and used to assess the degree of involvement of crustal material in the deep crust and/or subduction zone. Andesitic volcanics of S Peru (CVZ) are not markedly different in major-element chemistry from those of SW Colombia (NVZ), but they do exhibit large differences in isotopic and trace-element composition. Systematic geochemical variations in the CVZ lavas are shown to be the results of shallow to intermediate depth assimilation of Precambrian gneiss accompanied by dominantly plagioclase fractional crystallization. Correlated O-Sr-Nd isotope ratios in the NVZ andesites are not as easily described by simple AFC models, but do require significant involvement of crustal material at some stage of their magmatic evolution. Lead isotope ratios, which are strongly diagnostic of crustal contamination in the CVZ, exhibit minimal variation in the NVZ. -J.M.H.