Magma Compositions and Eruptive Rates during Pre-, Syn- and Post-Last Glacial Maximum time at the Mocho-Choshuenco Volcanic Complex, Chile

Abstract:

The Mocho-Choshuenco Volcanic Complex (MCVC, 39.9°S) in the Central Southern Volcanic Zone (CSVZ) last erupted >150 years ago, yet >20 km³ of widespread Holocene tephra deposits attest to an elevated hazard. Moreover, the composition and explosivity of the Holocene eruptions have led to a model of magma reservoir response to rapid unloading of >1 km thick glaciers of the Patagonian ice sheet between 18 and 16 ka¹. The model suggests that unloading triggers dike ascent and eruption of long-stored silicic magma from 13 to 8 ka, followed by refilling of the reservoir to produce mafic eruptions from 7 to 3 ka, and uptick in andesitic activity in the last 2.4 ka¹. However, the Holocene tephras only provide a record of post-glacial explosive eruptions. To better understand pre-Holocene and effusive history and potential influences of the transition from >1 km thick ice, to ice-free conditions, we have collected samples from 40 Pleistocene to Holocene lava flows from mapped Mocho 2, 3, 4 & 5, Choshuenco 1 & 2, Alto Caunahue and Ranquil units². Imprecise ⁴⁰Ar/³⁹Ar ages suggest these range from 200 to 60 ka for the oldest (Mocho 2 & 3 and Choshuenco 1 & 2), whereas ¹⁴C ages and stratigraphy indicate that the youngest units (Mocho 4 & 5, Alto Caunahue and Ranquil) were emplaced after 18 ka². Multi-collector ⁴⁰Ar/³⁹Ar dating underway, coupled with major and trace element compositions from the Pleistocene lavas, will improve pre-Holocene age and compositional control for MCVC. The Pre-LGM lava compositions are basaltic andesite to andesite² and thus contrast markedly with the explosive post-LGM eruptions (Neltume, Pirehueico and Huilo). We aim to compare magma, melt inclusion, and mineral compositions and eruptive rates during pre-, syn- and post-Last Glacial Maximum time and thereby re-evaluate the model of magma reservoir response to rapid uploading of ice that began at 18 ka¹.

¹Rawson, H., et al. (2016) Geology.
²Moreno, H., Lara, L. (2007) SERNAGEOMIN, Carta Geológica de Chile.