Catastrophic Caldera-Forming (CCF) monotonous silicic magma reservoirs: Constraints from volatiles in melt inclusions from the 3·49 Ma Tara Supereruption, Guacha II Caldera, SW Bolivia

Abstract

Quartz-hosted melt inclusions from climactic and post-climactic deposits erupted from the 3·49±60·01 Ma Guacha II Caldera (G2C) in SW Bolivia provide insights into the volatile and melt evolution leading into and following a major caldera-forming eruption of monotonous silicic ignimbrite. The G2C, with its petrological and volcanological framework already in place, provides a unique opportunity to investigate the role of volatiles in controlling the plinian to ignimbrite transition as well as the evolution from explosive to post-climactic effusive behavior at a large silicic caldera-forming system. Melt inclusions from the fallout (<10 km3, dense rock equivalent, DRE) and ignimbrite (>800 km3 DRE) pumice have overlapping major and trace element compositions and pre-eruptive H2O and CO2 contents (2·1 to 6·0wt %, 36 to 630 ppm, respectively), indicating that they crystallized from the same pre-climactic monotonous silicic magma reservoir. Based on trace element and Cl systematics, the variations in H2O content likely reflect post-entrapment diffusive H loss from melt inclusions. We estimate that quartz crystallization took place at pressures of ~200MPa, in agreement with pressures derived from amphibole in the ignimbrite pumice (~200MPa). These results indicate that the preclimactic magma was vapor saturated at shallow crustal conditions (between 5 and 8km). Melt inclusions from the post-climactic Chajnantor Dome lava are the most evolved with respect to trace elements and REE. They have low H2O and Cl contents, and contain no detectable CO2, indicative of relatively shallow entrapment pressures (<100MPa). Melt inclusions from the post-climactic dome lava represent a pod of eruptible melt that was extracted from degassed, remnant climactic magma that shallowed (<4 km) and evolved following caldera collapse causing resurgent uplift. Melt inclusion trace element systematics can be explained by extensive degrees of crystal fractionation (≥70%) of plagioclase, quartz, sanidine, biotite, Fe-Ti oxides, apatite, and zircon in the pre-climactic melt that produced variable melt inclusion compositions in the fallout and ignimbrite pumice (between 100 and 500 ppmRb). In addition, quartz-hostedmelt inclusions fromthe fallout pumice are more evolved than their host pumice glass, supporting a model whereby some andesitic recharge magma fractionated within the pre-climactic reservoir and produced a relatively small volume of residual rhyolitic melt. Quartz from the monotonous silicic pre-climactic reservoir was inmixed into this less evolvedmelt prior to eruption and deposition as fallout pumice. The shift from fallout to ignimbrite is not accompanied by changes in pre-eruptive dissolved H2O content in the melt, suggesting that differences in other factors such as vent and conduit evolution were more important in controlling explosive eruptive style. In contrast, melt inclusions from the dome support the interpretation that lava domes represent degassed, remnant magma indicating a more primary role of volatile content on the explosive to effusive eruptive transition. Pre-eruptive volatile contents of the Tara magmas are broadly similar to those of other monotonous silicic magmas in the Central Andes and elsewhere, suggesting that similarities in the petrological and geochemical evolution of these supervolcanic systems extend to their volatile signatures.