Geochemistry, petrology and origin of the Cenozoic volcanic rocks of the Northern Andes in Ecuador

Abstract

The Cenozoic volcanic rocks of the northern Andes in Ecuador for which only little petrological and geochemical information was available until now, display a strong geochemical and petrological zoning from west to east. The magmas of the volcanoes of the Western Cordillera (WC) are unique to the Andes. They belong to andesite-plagidacite series which is characterized by low and fairly constant K 2O contents (1 ± 0.2%), low abundances of large cation elements (Rb, Sr, Ba), rare earth elements, highly charged cations (Zr, U, Th), and relatively high Ni and Cr abundances. The magmas of the volcanoes of the Eastern Cordillera (EC) belong to andesite andesitedacite-rhyolite series which is characterized by Rb, Sr, Ba, REE, Zr, U, and Th contents higher than in the rocks of the WC and varying with the K 2O content. They have Ni and Cr versus MgO correlations similar to the rocks of the Western Cordillera. Even higher contents of K 2O and the above-mentioned elements are encountered in the volcanic rocks of the Sub-Andean Area (SAA). All of the rocks have the same 37Sr/ 36Sr ratio of 0.704. The rocks are chemically more closely related to the Mesozoic to Tertiary rocks of the Basic Igneous Complex (BIC) of Ecuador and Colombia than to those of the Central Andes. It is suggested that the Cenozoic magmas of the volcanoes of the Western and Eastern Cordillera originated by hydrous partial melting of amphibolites and garnet amphibolites of the BIC-type in a subduction zone beneath the Cordilleras at pressures near 20 kbar which corresponds to a depth of about 70 km.