SUBDUCTION CONTROLS ON THE COMPOSITIONS OF LAVAS FROM THE ECUADORIAN ANDES

Three volcanoes of the Ecuadorian Andes, Atacazo, Antisana, and Sumaco , lie in a transect perpendicular to the trench and the main trend of the Andean are. Each of the volcanoes lies on crust of substantially d ifferent age. composition, and thickness. Few compositional or isotopi c features correspond in a straightforward way to the type of the crus t through which the magmas have passed. Isotopic data limit assimilati on to < 15% at each of the volcanoes. Instead, a systematic relationsh ip exists between the compositions of the lavas and the depth to the B enioff zone, suggesting that subduction imparts the principal control on the compositions of the magmas. Atacazo's lavas have low concentrat ions of the incompatible trace elements and very large LIL/HFS ratios. Sumaco's lavas are strongly enriched in the incompatible trace elemen ts and have small LIL/HFS ratios. Antisana's lavas are intermediate in almost every respect. These features are consistent with devolatiliza tion of the subducted slab controlling the extent of partial melting o f a depleted mantle source. A mixing and melting model suggests the vo lcanic front magmas are made by large extent of partial melting (simil ar to 15%) and include a large slab input (1.1% added to the depleted mantle), The magmas of the middle belt of volcanoes are made by smalle r extent of partial melting (3%), induced by moderate amounts of slab- derived fluid (0.06%). The back are magmas result from small degrees o f melting (2%) and small slab input. (C) 1998 Elsevier Science B.V.