Magma Evolution During a Single Caldera Cycle: Insights from Melt Inclusions of the 3.49 Ma Tara Supereruption, Guacha Caldera, SW Bolivia

Melt inclusions in quartz phenocrysts erupted during the 3.49 Ma Tara Ignimbrite supereruption (>800 km3 DRE) from the Guacha Caldera complex in SW Bolivia provide the rare opportunity to examine the magma evolution during a complete resurgent caldera cycle. Data from naturally glassy, crystal- and vapor-bubble free quartz-hosted melt inclusions from pumices in the climactic plinian and ignimbrite phase and lavas from the post-resurgent domes combined with data on matrix glass and whole rock compositions reveal that the three eruptive phases were genetically related and potentially derived from an andesitic magma represented by scoria in the ignimbrite. Variations in Cl, H2O, B and Li suggest the presence of a separate vapor phase in the pre-climactic Tara magma. H2O contents in the plinian pumice overlap with those in the ignimbrite pumice (2.2 to 6.0 and 2.1 to 5.4 wt.% H2O, respectively) but the ignimbrite-forming magma contains higher CO2 (<630 versus <300 ppm) suggesting a vertically arranged pre-climactic magma column from 7.5 km to 4 km. The initial plinian fall-producing eruption tapped a less-evolved, crystal-poor (~10%) roofward liquid that syneruptively mixed with the more crystalline (~45%) climactic, ignimbrite-forming magma. Melt inclusions from the highly-differentiated post-climactic magma contain no detectable CO2 and ~2% H2O in melt inclusions and connote a shallowing (<2 km) post-climactic magma system that crystallized a new generation of quartz crystals and accompanied the resurgent doming of the Guacha II caldera floor. The eruptive transition during the climactic eruption was not controlled by the volatile budget of the melt but more likely by external factors such as vent geometry and conduit evolution. Post climactic effusive volcanism reflects the degassed nature of the remnant magma.