PRODUCTION AND PRESERVATION OF SMALL VOLUME RHYOLITIC MELTS RECORDED IN THE MIDST OF A MONOTONOUS CONTINENTAL ARC FLARE-UP - THE HETEROGENOUS CASPANA IGNIMBRITE OF THE ALTIPLANO-PUNA VOLCANIC COMPLEX OF THE CENTRAL ANDES

The ~5 km³, 4.54 to 4.09 Ma Caspana Ignimbrite of the Altiplano-Puna Volcanic Complex (APVC) of the Central Andes records the eruption of an andesite and two distinct rhyolitic magmas. Ignimbrite stratigraphy and petrology connotes eruption from a vertical, heterogenous reservoir in a region where small volumes of magmas rarely survive homogenization prior to eruption. Phase 1 of the eruption, represented by a fallout deposit and thin flow unit tapped a crystal-poor, peraluminous rhyolite with petrological and geochemical characteristics best explained by partial melting of granodioritic roof rock. Phase 2 of the eruption records the emplacement of a more extensive flow unit containing crystal-poor, fayalite-bearing rhyolite and porphyritic to glomeroporphyritic andesite pumice containing noritic (plagioclase-orthopyroxene-Fe-Ti oxide) glomerocrysts. Magma Chamber Simulator (MCS) models indicate the Phase 2 rhyolite is derived from the andesite by nearly closed-system crystallization of the assemblage preserved in the glomerocrysts. The corresponding rhyolite-MELTS models show latent heat buffering promoted extraction of a rhyodacitic residual liquid and eventually rhyolite to create the pre-eruptive compositional gap of ~16 wt% SiO₂. Isotopic ratios support both closed system crystallization of the Phase 2 magmas and a separate petrogenetic origin for Phase 1. Modeled equilibrium pressures and temperatures of the andesite (400MPa, ~1060-930°C) and Phase 2 rhyolite (200MPa, 775°C) are consistent with a genetic relationship between the two, as are estimated water contents based on plagioclase-liquid hygrometry (4 vs. 5.1 wt%, respectively). The high Mg + Ca assemblage in the andesite reflects the oxidation conditions estimated from Fe-Ti oxide compositions (FMQ -1.03), and provides the high Fe²⁺/Mg and FeO/CaO leverage necessary to stabilize fayalite, a key phase in the Phase 2 rhyolite. The location of the Caspana system at the periphery of the regional thermal anomaly of the Altiplano-Puna Magma Body (APMB) permitted the small volume magma reservoir to preserve the disparate origins of rhyolitic liquids.