Understanding large resurgent calderas and associated magma systems : the Pastos Grandes Caldera Complex, southwest Bolivia Public Deposited

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  • The Pastos Grandes Caldera Complex (PGCC) in southwest Bolivia has produced two large-volume (≥800 km³ DRE) dacite ignimbrites from a nested caldera source over a period of 5.5 Myr. In addition to the large-volume ignimbrites, a small-volume ignimbrite shield and post-climactic lavas define this composite system. Based on detailed field work and analysis of satellite imagery plus biotite ⁴⁰Ar/³⁹Ar dating, we summarize a revised stratigraphy, areal distribution and volume calculations for ignimbrites. From interpretations of stratigraphy and structures along the caldera margins, we propose an asymmetric collapse hinged along the north and western boundaries. Both the early Chuhuilla (5.45 ± 0.02 Ma), and the younger Pastos Grandes (2.89 ± 0.01 Ma) calderas share the hinge and much of the eastern collapse scarp, however the Chuhuilla caldera defines a much larger area (>1700 km²) compared to the Pastos Grandes (870 km²). It is proposed here that pre-existing regional tectonic weaknesses combined with influences of the magma body caused roof failure and caldera collapse. The vast majority of the ignimbrite volume lies within the Chuhuilla and Pastos Grandes calderas (92 and 75% respectively). Considerable intracaldera fill and the lack of preceding plinian deposits suggest that the caldera collapse was early and the eruptive column was not high and was short-lived. The physical properties of the ignimbrites, the limit of their areal distribution to regional topographic lows along with paleomagnetic characteristics support the idea of dense sluggish pyroclastic flows. New volume calculations using multiple methods update previous estimates, with the Chuhuilla now at approximately 1300 km³ and the Pastos Grandes at 800 km³. The ignimbrite volumes and spatial pattern of vents suggest that the caldera complex mirrors the construction of a long-lived composite batholith that focused spatially with time. Eruptions from the PGCC have produced compositionally restricted, high-K dacites with volumetrically minor rhyolites. Combined with granodiorite xenoliths, each caldera cycle contains a progression of textural maturity from ignimbrite, through post-climactic lavas, to remnant pluton. The chemical signatures in the PGCC mirror those of the host Altiplano Puna Volcanic Complex (APVC) and resemble typical arc characteristics (i.e. LIL enriched magmas - Ba/Nb of 4.7-4.8 and Ba/La of 1.6-2.0). However, overprinted on the arc signature are elevated radiogenic isotopes (⁸⁷Sr/⁸⁶Sr ~0.708 - 0.709), which suggest high degrees of crustal assimilation that is thought to be related to increased mantle input from melted asthenosphere. It is suggested here that a combination of assimilation and fractional crystallization from the regional mid-crustal parental source would create the magmas erupted in the PGCC. Subtle decreases in the ⁸⁷Sr/⁸⁶Sr from the Chuhuilla caldera cycle, to the younger Pastos Grandes cycle suggest higher amounts of crustal assimilation related to more heat flow during the peak of the flare up. Zircon chronochemistry reveals that the climactic, caldera-forming eruptions of the PGCC punctuate the protracted magma accumulation and storage periods. The combination of in-situ zircon U-Pb ages with indicators for geochemical evolution (e.g., Zr/Hf, Yb/Gd, Eu/Eu*, Th, U) magmatic temperatures (e.g., Ti) in zircon, reveals protracted magma presence before and after the climactic 2.89 ± 0.01 Ma Pastos Grandes Ignimbrite (PGI) supereruption (~800 km³ of magma) in southwest Bolivia. Zircons from PGI pumice and a lava dome define a pre-climactic magmatic stage of ~0.7 Myr duration prior to the climactic eruption and formation of the eponymous caldera. A further 0.4 Myr of post-climactic zircon crystallization is recorded in lava domes and cogenetic granodiorite clasts. Zircon crystallization is recorded for approximately 1.3 Myr for the Chuhuilla cycle; however the record is entirely pre-climactic. We propose a model for the Pastos Grandes cycle in which the climactic caldera-forming eruption vented the upper portions of the reservoir that were zircon saturated. Subsequently, deeper “remnant” dacite magma previously outside the zone of zircon saturation but crystallizing other major phases, rose to re-establish lithostatic equilibrium, commenced zircon crystallization anew, and drove resurgent volcanism and uplift. This ~1.1 Myr zircon crystallization history records the minimum duration of the lifetime of the PGI supereruption magma from its accumulation to post-climactic solidification. These data support >1 Myr magma lifetimes and a link between volcanic and plutonic realms in large sub-caldera magma reservoirs in the uppermost crust that feed some supereruptions.
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