GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF VOLCANIC ROCKS FROM THE ANDEAN NORTHERN VOLCANIC ZONE, ECUADOR

Abstract

Whole-rock geochemical data on basaltic to rhyolitic samples from 12 volcanic centers are used to constrain the role of continental crust in the genesis of magmas formed beneath the anomalously wide subduction-related volcanic arc in Ecuador. Relatively homogeneous, mantle-like, isotopic compositions across the arc imply that the parental magmas in Ecuador were produced largely within the mantle wedge above the subduction zone and not by extensive melting of crustal rocks similar to those upon which the volcanoes were built. Cross-arc changes in 143Nd/144Nd and ?7/4Pb are interpreted to result from assimilation of geochemically mature continental crust, especially in the main arc area, 330–360?km from the trench. Mixing calculations limit the quantity of assimilated crust to less than ?10%. Most andesites and dacites in Ecuador have adakite-like trace element characteristics (e.g. Y <18?ppm, Yb <2?ppm, La/Yb >20, Sr/Y >40). Available whole-rock data do not provide a clear basis for distinguishing between slab-melting and deep crustal fractionation models for the genesis of Ecuador adakites; published data highlighting geochemical evolution within individual volcanoes, and in magmatic rocks produced throughout Ecuador since the Eocene, appear to support the deep fractionation model for the genesis of most evolved Ecuadoran lavas. A subset of andesites, which display a combination of high Sr (>900?ppm), ɛNd >4·1 and ?7/4Pb <6·0, appear to be the best candidates among Ecuador lavas for slab-melts associated with the subduction of the relatively young, over-thickened, oceanic crust of the Carnegie Ridge.