GEOCHEMICAL ZONATION OF VOLCANIC-ROCKS AND DEEP-STRUCTURE OF ECUADOR AND SOUTHERN COLOMBIA

The chemical composition of 104 samples of young volcanic rocks from t he Western and Eastern Cordilleras of Ecuador and southern Colombia wa s correlated with the deep structure of the Andean region bounded by l atitudes 1.5-degrees-N and 2-degrees-S and longitudes 77-degrees-W and 79-degrees-W. The geochemical zonation of the volcanic products appar ently depends on the depth of the primary source of calc-alkaline magm a in the partially melted part of the subduction zone. It was found th at the contents of K2O, U, Th, Rb, Sr, Ba, Zr, V, TiO2 and P2O5 in and esites significantly increase with the depth of the upper boundary of the intermediate depth aseismic gap in the Andean Wadati-Benioff zone. The differences in average elemental abundances for volcanic centers in southern Colombia suggest that recent calc-alkaline volcanism in Ec uador and southern Colombia are related to different subducted lithosp heric plates. Analysis of the geographical distribution of andesite an d dacite samples with apparent anomalous enrichment of both the compat ible and incompatible elements in relation to the subduction-dependent pattern of deep-seated fracture zones showed that the geochemistry of the volcanic products in convergent plate margins may be considerably influenced by the deep fault structure of the continental wedge and n eed not be identical with the geochemistry of the inferred primary mag ma.