TRACKING AMPHIBOLE CRYSTAL POPULATIONS IN CONTINENTAL ARC MAGMAS THROUGH STUDY OF ANDESITES AND HORNBLENDITE CUMULATES IN THE CENTRAL ANDES, BOLIVIA

Partial melting of Earth’s mantle in various tectonic settings leads to a wide array of magmatic processes and products in both at-plate boundary and intraplate settings. Of these, the volcanic products at continental arcs are petrographically, mineralogically, and compositionally the most diverse. The magmatism associated with these systems is the result of a complex interplay between dehydration of the down going oceanic lithosphere, +/- sediment melting, partial melting of the arc mantle wedge, and +/- assimilation of the overriding continental lithosphere. In order to advance understanding of the magmatic processes occurring in these complex systems, study of the different components is necessary. This study focuses on the volcanic products (andesites) erupted from a monogenetic back arc volcano (Quillacas) on the Bolivian Altiplano associated with Central Andean arc volcanism. Particularly, these andesites are of interest because they contain crustal xenoliths from the underlying continental basement and rare hornblendite cumulates. Specifically, the crystal populations of the andesites are being investigated to evaluate the interaction between the different components of the magmatic system that exists within the arc crust. Major minerals include biotite, plagioclase, amphibole, oxides, and accessory apatite and zircon. Previous work by Velazquez Santana et. al (in review, Lithos) has characterized the chemistry of amphiboles in the hornblendites. Here, the textural and chemical characteristics of amphiboles in the andesites are compared to these. Amphiboles in the andesites are smaller in size than those in the hornblendites, and have LREE at LaN/SmN: 0.5-6.4 and HREE at DyN/LuN: 0-3.6, while the hornblendites exhibit LaN/SmN: 0.9-1.1 and DyN/LuN: 1.9-2.2. The differences between the two amphibole populations provide us with insights into the different crystal (in this case amphibole) populations within magmatic systems at continental arcs.