Magmatic evolution and architecture of an arc-related, rhyolitic caldera complex: The late Pleistocene to Holocene Cerro Blanco volcanic complex, southern Puna, Argentina

Abstract

Through the lens of bulk-rock and matrix glass geochemistry, we investigated the magmatic evolution and pre-eruptive architecture of the siliceous magma complex beneath the Cerro Blanco volcanic complex, a Crater Lake–type caldera complex in the southern Puna Plateau of the Central Andes of Argentina. The Cerro Blanco volcanic complex has been the site of two caldera-forming eruptions with volcanic explosivity index (VEI) 6+ that emplaced the ca. 54 ka Campo Piedra Pomez ignimbrite and the ca. 4.2 ka Cerro Blanco ignimbrite. As such, it is the most productive recent explosive volcano in the Central Andes. The most recent eruptions (younger than 4.2 ka) are dominantly postcaldera effusions of crystal-rich domes and associated small explosive pulses. Previous work has demonstrated that andesitic recharge of and mixing with rhyolitic magma occurred at the base of the magma complex, at ~10 km depth. New isotopic data (Sr, Nd, Pb, and O) confirm that the Cerro Blanco volcanic complex rhyolite suite is part of a regional southern Puna, arc-related ignimbrite group. The suite defines a tight group of consanguineous siliceous magmas that serves as a model for the evolution of arc-related, caldera-forming silicic magma systems in the region and elsewhere. These data indicate that the rhyolites originated through limited assimilation of and mixing with upper-crustal lithologies by regional basaltic andesite parent materials, followed by extensive fractional crystallization. Least squares models of major elements in tandem with Rayleigh fractionation models for trace elements reveal that the internal variations among the rhyolites through time can be derived by extensive fractionation of a quartz–two feldspar (granitic minimum) assemblage with limited assimilation. The rare earth element character of local volumes of melt in some samples of the Campo Piedra Pomez ignimbrite basal fallout requires significant fractionation of amphibole. The distinctive major- and trace-element characteristics of bulk rock and matrix of the Campo Piedra Pomez and Cerro Blanco tephras provide useful geochemical fingerprints to facilitate regional tephrochronology. Available data indicate that rhyolites from other neighborhood centers, such as Cueros de Purulla, share bulk chemical characteristics with the Campo Piedra Pomez ignimbrite rhyolites, but they appear to be isotopically distinct. Pre-eruptive storage and final equilibration of the rhyolitic melts were estimated from matrix glass compositions projected onto the haplogranitic system (quartz-albite-orthoclase-H2O) and using rhyolite-MELTS models. These revealed equilibration pressures between 360 and 60 MPa (~10–2 km depth) with lowest pressures in the Holocene eruptions. Model temperatures for the suite ranged from 695 to 790 °C. Integrated together, our results reveal that the Cerro Blanco volcanic complex is a steady-state (low-magmatic-flux), arc-related complex, standing in contrast to the flare-up (high-magmatic-flux) supervolcanoes that dominate the Neogene volcanic stratigraphy. The silicic magmas of the Cerro Blanco volcanic complex were derived more directly from mafic and intermediate precursors through extensive fractional crystallization, albeit with some mixing and assimilation of local basement. Geochemical models and pressure-temperature estimates indicate that significant volumes of remnant cumulates of felsic and intermediate composition should dominate the polybaric magma complex beneath the Cerro Blanco volcanic complex, which gradually shallowed through time. Evolution to the most silicic compositions and final equilibration of some of the postcaldera domes occurred during ascent and decompression at depths less than 2 km. Our work connotes an incrementally accumulated (over at least 54 k.y.), upper-crustal pluton beneath the Cerro Blanco volcanic complex between 2 and 10 km depth. The composition of this pluton is predicted to be dominantly granitic, with deeper parts being granodioritic to tonalitic. The progressive solidification and eventual contraction of the magma complex may account for the decades of deflation that has characterized Cerro Blanco. The presently active geothermal anomaly and hydrothermal springs indicate the Cerro Blanco volcanic complex remains potentially active.

Probabilistic Volcanic Hazard Assessment of the 22.5–28°S Segment of the Central Volcanic Zone of the Andes

“…The Incapillo-Bonete magma body is deeper than the others (~30-50 km; Goss et al, 2011;Ward et al, 2017) and may need to grow or rise before large-volume volcanism starts. The Antofagasta volcanic cluster identified here is, apparently, not associated with a previously defined magma body nor mid-toupper crustal region of partial melt (Ward et al, 2017), although it is an actively deforming region, hosting the only known active caldera in the CVZ (Cerro Blanco) (Pritchard and Simons, 2002;Henderson and Pritchard, 2013;Vélez et al, 2021;de Silva et al, 2022).…”

“…The inclusion of a volumetric component into a well-tested spatio-temporal model may shed further light on how much each volcanic cluster identified here contributes in terms of erupted volume. This will, in our opinion, highlight the Antofagasta cluster, particularly due to the presence of the Cerro Blanco volcanic complex, which has long been recognized as the source of at least two eruptions on the scale of the largest eruptions to have occurred on Earth since the late Pleistocene (Báez et al, 2015;Báez et al, 2020a;Báez et al, 2020b;Fernández-Turiel et al, 2019;de Silva et al, 2022).…”

“…This region records ~35 Myr of uninterrupted volcanic activity, comprising a vast variety of activity types, volcanic landforms and erupted compositions. It includes the site of one of the largest (VEI 6+) Holocene explosive eruptions in the Andes, which occurred approximately 4.2 ka BP from the Cerro Blanco volcanic complex (26.8 °S/67.7 °W) (Báez et al, 2015;Fernández-Turiel et al, 2019;Báez et al, 2020a;de Silva et al, 2022), whereas the only confirmed historical (i.e., post ~1750 AD) eruptions in this arc segment occurred at Láscar (23.4 °S/67.7 °W), with the largest event (VEI of 3-4) in April 1993 (Gardeweg and Medina, 1994). However, much of the volcanic history of this area remains poorly constrained, even though many morphologically young volcanic features can be recognized throughout the region and several of these are considered potentially active (de Silva and Francis, 1991;González-Ferrán, 1995).…”