Petrogenetic Processes and Sources of Volcanic Rocks in the Central Andes: Constraints from Mafic Monogenetic Centers in Bolivia (Collaborative Research)

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Studies of volcanic products along the Andes show a variation in chemistry along strike and across strike with time. These variations can be explained by two contrasting models; (1) that the chemical variations simply reflect variations in the mantle source (primary magmas are not modified by interaction with the lithosphere); or (2) that these variations are due to modification of primary magmas due to interaction with the lithosphere through which they pass. In the Central Andes, the similarity of the compositions of the erupted andesites to typical continental crust, has led to the suggestion that these reflect the direct mantle to curst flux, and support a model for crustal growth through magmatic addition at the subduction zone Crustal thicknesses in the Central Andes reach~70km, and it is difficult to envisage that primary magmas would not be modified en route to the surface. The alternative interpretation is that Andean andesites are chemically similar to continental crust because they have been extensively contaminated by the crust. This project will address this key issue by studying mafic monogenetic centres in Bolivia. Available data suggests that these are the least differentiated lavas in the Central Andes and have 87SR/86SR similar to the source of island arc magmas. They therefore represent the closest examples to primary magmas in the Central Andes, and appear to be a vital window into the composition of the sub-Andean mantle.