Mantle-drip magmatism beneath the Altiplano-Puna plateau, central Andes

Convective removal of continental lithospheric roots has been postulated to be the primary mechanism of recycling lithospheric mass into the asthenosphere under large plateaux such as the Altiplano-Puna in the central Andes. Convective instabilities are especially likely to develop where there is extensive intermediate arc-like magmatism in the upper plate, as the residual masses complementing these magmatic products are typically denser than the underlying mantle. Mafic volcanic rocks erupted on the central Andean Altiplano-Puna plateau during the past 25 m.y. contain evidence of this process. Here we use equilibration temperatures, age data, and geochemical constraints--primarily based on transition metals--to show that the most important source materials by mass for this mantle-derived magmatism are pyroxenites from the lower parts of the lithosphere, with only minor contributions from mantle peridotite. Pyroxenites are denser than typical upper mantle whether they are garnet bearing or not, and are therefore likely to contribute to destabilizing parts of the continental lithosphere. The pattern of melting is consistent with the process of foundering/dripping of small-scale (<50 km diameter) density anomalies in the lithosphere, where mafic volcanic fields on the plateau represent the manifestations of individual drips.