Quantitative Models for Crustal Contamination in the Central and Northern Andes
Systematic variations in O, Sr, Nd and Pb isotope ratios and related trace element abundances in modern andesitic lavas of the central and northern Andes are examined and compared in terms of crustal contamination models of assimilation and fractional crystallization. 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 the subduction zone. Andesitic volcanics of south Peru are not markedly different in major element chemistry from those of southwestern Columbia, but they do exhibit large differences in isotopic and trace element composition. Systematic geochemical variations in the south Peruvian lavas are shown to be the result of shallow to intermediate depth assimilation of Precambrian gneiss accompanied by dominantly plagioclase fractional crystallization. Correlated O-Sr-Nd isotope ratios in southwestern Columbia andesites are not as easily described by simple assimilation-fractional crystallization models, but do require significant involvement of crustal material at some stage of their magmatic evolution. Pb isotope ratios, which are strongly diagnostic of crustal contamination in south Peruvian volcanics exhibit minimal variation in southwestern Columbia lavas. Geochemical variations due to fractional crystallization appear to be controlled more by fractionation of mafic minerals, including olivine, than by fractionation of plagioclase. These and other differences in the style of crustal contamination between the CVZ and NVZ are examined in light of dissimilar regional crustal structure and composition.
KeywordsFractional Crystallization Crustal Material Parental Magma Bouguer Anomaly Crustal Contamination
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