Massimo Chiaradia

Aseismic ridge subduction is considered to exert major controls on volcanic activity, formation of porphyry Cu-Au deposits and the generation of juvenile Earth’s crust. Yet, there are almost no studies that have addressed in a systematic way the effects of this process on the geochemistry of arc magmas. Here we explore the role of the subducted aseismic Carnegie ridge on modulating frontal arc magma chemistry in the Ecuadorian-southernmost Colombian frontal arc. We show the occurrence of symmetric changes of several geochemical indices (Th, Nd, Sm, Nb, Th/La, Ba/Th) and evolutionary paths (Na2O-SiO2 correlations) in frontal volcanic arc rocks with respect to the equatorial latitude and thus to the projection of the crest of the subducting Carnegie ridge at the frontal arc. We explore the systematic along-arc changes of these geochemical indices through geochemical modelling using a Monte Carlo approach, and conclude that the trends are primarily controlled by intracrustal processes. In particular we argue that magmatic systems associated with the central part of the Ecuadorian volcanic arc evolve, through hot zone type processes such as fractionation, partial melting and mixing, at average greater depths than those in the northern and southern sides. We speculate that this is due to a systematic latitudinal change in the crustal stress regime within the overriding plate induced by the subduction of the Carnegie ridge.