Early Miocene large-volume ignimbrites of the Oxaya Formation, Central Andes

During the early Miocene ignimbrite flare-up, significant parts of the Central Andes (17–20°S) were covered by large-volume ignimbrites. High-precision ²⁰⁶Pb/²³⁸U zircon dates constrain the flare-up in northern Chile at c. 18°S to a 3 myr period, starting with the deposition of the Poconchile ignimbrite at 22.736 ± 0.021 Ma. Of four main pulses, the two largest occurred at 21.924 ± 0.017 and 19.711 ± 0.036 Ma, when the >1000 km³ in volume Cardones and Oxaya ignimbrites erupted, respectively. The ignimbrites are high-SiO₂ rhyolites and show significant heterogeneities in crystal content, mineral proportions and trace-element compositions. The zoned Oxaya ignimbrite implies incremental extraction of a crystal-poor magma overlying a crystal-rich magma. In contrast, petrological and textural heterogeneities in pumice clasts are spread throughout the Cardones ignimbrite and we propose magma mixing caused by destabilization of multiple magma bodies within a magmatic mush system. Distal and medial deposits of the Cardones ignimbrite, with a maximum welded thickness of at least 1000 m, entirely covered the western flank of the Central Andes, which implies infill of a significant topographic relief. Both compaction and welding resulted in a maximum thickness reduction of around 30% for the Cardones ignimbrite.