Continental arc magmatism reflects a major process in plate tectonics, the convergence of oceanic and continental plates. The subduction of an oceanic plate beneath a continental plate results in creation of new continental crust and incorporation of crustal material into the convecting mantle. These are important processes that need to be understood in detail in order to understand the evolution of the earth. Andean volcanoes reflect the active subduction of the oceanic Nazca plate beneath the South American plate. In the Southern Volcanic Zone (SVZ) of the Andes, from 33o-42oS, there are major along-strike geologic changes; e.g., from north to south there are decreases in the age of the subducted crust and continental crust thickness. Concurrently, along and across strike of the Main Cordillera the lavas vary systematically in composition (major and trace element abundance, radiogenic isotope ratios). There are several alternative explanations of these geochemical trends. The P.I.'s previous and proposed studies have the objective of understanding the origin and evolution of volcanism in this region; specifically, to define the relative importance of components from the subducted crust, asthenosphere, lithosphere and continental crust to the arc magmas, and to determine the importance of variations in degree of melting, subsequent melt- solid segregation, magma mixing and crustal assimilation. To do this phenocryst phase compositions plus whole-rock major and trace element abundances and isotopic ratios (0, Sr, Nd, Pb) will be used. To date emphasis has been on stratovolcanoes on or near the volcanic front of the SVZ. This project will focus on recent basaltic lavas erupted 50 to 200 km east of the volcanic front. Because these back (retro) arc lavas are less evolved than volcanic front lavas, their geochemical characteristics more directly reflect source compositions (especially, important is the possibility of determining geochemical characteristics of continental mantle lithosphere) and mantle processes. In addition, along the volcanic front near a single stratovolcano (Villarrica), significant geochemical differences between lavas erupted from the stratovolcano and several surrounding (satellite), small basaltic cones have been found. These differences can be interpreted as reflecting much less crustal contamination in the satellite cone lavas, or the influence of source heterogeneities which are averaged out in long-lived magma chambers beneath the stratovolcano. These are important and very different alternative interpretations. To test the generality of results near the Villarrica stratovolcano and to distinguish between these interpretations, satellite cone lavas associated with other stratovolcanoes in the SVZ will be studied. ***//
