FAYALITE-BEARING RHYOLITE IN THE ALTIPLANO-PUNA VOLCANIC COMPLEX OF THE CENTRAL ANDES

The origin and evolution of the magmas during the Neogene ignimbrite flare-up of the Altiplano-Puna Volcanic Complex of the Central Andes is a model for continental magmatic systems. A pre-eruptive evolution dominated by Upper Crustal AFC of andesitic magmas derived from the 15 to 30 km deep Altiplano-Puna magma body has been well established. Among the rare extreme compositions erupted during the flare-up is contained in the zoned 4.59 to 4.09 Ma Caspana ignimbrite. Early erupted compositions are a fayalite-bearing high-Si rhyolite with later portions containing mixed rhyolite-andesite and andesitic pumices. A compositional gap of almost 16% SiO₂ is defined the two dominant compositions.

The rhyolite (SiO₂=75%) is crystal-poor with phenocrysts of fayalite, biotite, and plagioclase (An=15-20). The andesite (SiO₂=59%) contains ~20% phenocrysts of ~15% plagioclase (An=70-80), ~5% Mg-orthopyroxene, and ilmenite. Glomerocrysts containing the same assemblage can be found within the andesite with abundances of 55% orthopyroxene, 35% plagioclase, and 10% ilmenite. The iron rich rhyolite is thought to be derived by fractionation of the glomerocryst mineral assemblage in the andesite. This is supported by trace element and isotopic evidence.

The eruption stratigraphy indicates the coexistence of the two distinct compositions in a zoned magma reservoir with a large compositional gap. Intermediate composition magmas are bypassed if the fractionating assemblage defined a shallow cotectic over a short temperature decrease. The rhyolite that is produced is enriched in iron, stabilizing fayalite, Fe-rich biotite and Na-rich plagioclase. New mineral chemistry and thermodynamic data will be presented to further constrain the origin and evolution of this unusual fayalite bearing rhyolite in a continental arc magmatic complex.