Central Andean magmatism can be constrained by three ubiquitous end-members

摘要

The geochemical character of primary magmas and the nature of crustal melts remain poorly constrained at the active continental margin of the Central Andes, where the crustal thickness is >70km. We identify three end-members and constrain their full major and trace element compositions by polytopic vector analysis (PVA). The end-members include two mafic parent magmas: (1) a slightly evolved calcalkaline basaltic andesite derived from the fluid-fluxed mantle wedge that is typical of the Andes in general and (2) a strongly LREE-enriched basalt akin in its trace element pattern to back-arc shoshonites. (3) The third end-member is a variably HREE-depleted rhyodacite representing partial melts of lower to middle crustal lithologies. We propose that these end-members are ubiquitous and typical of such thick-crust settings, and can explain - in highly variable mixing proportions - the entire compositional range of Central Andean magmatism.