Abstract

The Puyehue-Cord & oacute;n Caulle volcanic complex (PCC) is notable for explosive-effusive rhyodacitic-rhyolitic eruptions in 1921-1922, 1960, and 2011-2012 comprising >2.5 km(3) which have prompted many geophysical and petrologic investigations. The Puyehue composite volcano, which has erupted basaltic and rhyodacitic-rhyolitic lavas and tephras over the past 70 ka from vents 6 km southeast of the historic vents along Cord & oacute;n Caulle, has received much less attention. The PCC was enveloped by the Patagonian ice sheet (PIS) during the extended Last Glacial Maximum (eLGM) between 35 and 18 ka. Investigations that focused on the origin of historical Cord & oacute;n Caulle eruptions point toward shallow extraction of rhyodacitic-rhyolitic melt from basaltic crystal mush. Yet, there is limited understanding of how the magma plumbing system beneath the PCC evolved during the eLGM advance of the PIS and most of the post-glacial period that began following rapid ice retreat between 18 and 16 ka. We report new glass and mineral compositions from lava flows and tephra deposits erupted from Puyehue Volcano spanning the past 70 ka. Mineral compositions, thermobarometry, and rhyolite-MELTS modeling suggest that multiple discrete magma batches fed successive eruptions. Pre- to syn-eLGM (70 to 18 ka) basaltic andesitic to dacitic lavas were sourced from reservoirs between 9 and 24 km depth. During this period lavas and tephras reflect a large range in fO(2) buffering from triangle NNO-2 to +1 and preserve macro- to micro-scale evidence of mixing between basalt and rhyodacite. Immediately following the eLGM, between 18 and 2 ka, a reorganized plumbing system produced eruptions of more shallowly sourced dacites (similar to 12 km depth) and rhyodacites (similar to 5-7 km depth), as well as basaltic andesite and basalt that had been stored at depths of 10 to 32 km. As during the eLGM, magmas erupted between 18 and 2 ka were derived from multiple discrete reservoirs spanning a wide range of fO(2). This post-glacial phase of increased eruption rate also marks the emergence of exceptionally bimodal magmatism-including the first erupted rhyodacite from the PCC and coeval MgO-rich basalt. The most recent eruption from Puyehue Volcano at 940 yr BP, the 4.3 km(3) explosive crater-forming Mil Hojas dacite, shares glass and mineral compositions and a narrow range of T-fO(2) values (890-950 degrees C at triangle NNO between -0.8 to -0.2) with the historical rhyodacites-rhyolites erupted along Cord & oacute;n Caulle. These observations, coupled with petrologic modeling, plus modeling of the asymmetric growth and retreat of the PIS and associated crustal stress perturbations, imply that changes in surface loading propelled basaltic magma northwestward from beneath Puyehue Volcano in dikes that exploited basement faults. The resulting shallow basaltic magma reservoir underlying the entire PCC has sourced >12 km(3) of rhyodacitic-rhyolitic eruptions since 18 ka.