Crystallinity variability in rhyolites: Insights from the Tocomar volcanic system (Puna Plateau, NW Argentina)
The rhyolitic Tocomar volcanic center, located in the Northern Puna Plateau of the Central Andes, represents a Quaternary felsic magmatic system associated with an active geothermal field. Its eruptions produced pyroclastic deposits with variable crystallinity, ranging from extremely crystal-poor to...
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| Auteurs principaux: | , , , , , , , , , , , , |
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| Format: | Article (Journal) |
| Langue: | anglais |
| Publié: |
26 February 2026
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| In: |
Journal of volcanology and geothermal research
Year: 2026, Volume: 473, Pages: 1-21 |
| ISSN: | 1872-6097 |
| DOI: | 10.1016/j.jvolgeores.2026.108581 |
| Accès en ligne: | Verlag, lizenzpflichtig, Volltext: https://doi.org/10.1016/j.jvolgeores.2026.108581 Verlag, lizenzpflichtig, Volltext: https://www.sciencedirect.com/science/article/pii/S0377027326000545 
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| Notes sur l'auteur: | L. Bardelli, R. Filipovich, W. Báez, R. Becchio, M. Arnosio, A.K. Schmitt, M. Danišík, C. Zhang, L. Monaco, B. Giaccio, G. De Astis, J. Viramonte, G. Giordano |
| Résumé: | The rhyolitic Tocomar volcanic center, located in the Northern Puna Plateau of the Central Andes, represents a Quaternary felsic magmatic system associated with an active geothermal field. Its eruptions produced pyroclastic deposits with variable crystallinity, ranging from extremely crystal-poor to crystal-rich juvenile clasts, reflecting the pre-eruptive dynamics of magma generation and storage within an upper-crustal, felsic crystal-mush reservoir. Building on previous studies, we present an integrated geochemical (bulk-rock and matrix-glass major and trace elements, mineral chemistry) and petro-chronological (zircon UPb ages and trace-element compositions) characterization of the Tocomar volcanic rocks, and discuss the evolution of the magmatic system. Our results indicate that magmatism at Tocomar generated felsic magmas that erupted both as near-pure melts and as crystal-glass mixtures. Crystal-poor rhyolitic eruptions produced fallout deposits and ignimbrites whose geochemical signatures (e.g., extremely high Rb and low Ba, Sr, and Zr contents) are consistent with rhyolite extraction from a felsic crystal mush via crystal-melt separation processes such as hindered settling and compaction, rather than through melting of cumulates. In contrast, crystal-rich eruptions (rhyodacites), associated with phreatomagmatic pulses, display geochemical characteristics (e.g., high Ba, Sr, Zr/Hf, and Rb/Cs ratios) indicative of an “accumulated-like” origin involving both crystals and interstitial melt. These features are compatible with fluid-induced remobilization and subsequent eruption of portions of a crystal-mush reservoir. In such a scenario, the pre-eruptive magma-mush dynamics active in a local, extensional tectonic regime exerted a first-order control on the bulk geochemical composition of the erupted products and on the eruptive styles. The volcanic rocks also contain a complex zircon population, including Quaternary autocrysts and antecrysts as well as a variegate set of xenocrysts. The Quaternary zircons, common to both crystal-poor and crystal-rich rocks, indicate that magmatism at Tocomar was active between approximately between 1.1 and 0.9 Ma. These zircons exhibit variable Hf, Th, U, Ti, and Eu/Eu* compositions, reflecting prolonged, down-temperature crystallization paths compatible with a felsic mush environment and consistent with the geochemical characteristics of the host volcanic rocks. In contrast, four distinct types of zircon xenocrysts, distributed heterogeneously among the crystal-poor and crystal-rich magmas, record recycling processes operating in different portions of the plumbing system. Xenocrysts with UPb ages of ca. 490 Ma and ca. 1.48 Ga are systematically rimmed by Quaternary zircon rims, and are likely derived from the Altiplano-Puna Magmatic Body. In contrast, a Middle Miocene xenocrystic (c. 10-11 Ma) population occurs exclusively within the crystal-poor magmas, reflects a pre-eruptive to syn-eruptive recycling event of unerupted magmatic rocks from previous volcanic cycles. Recycling of such type of xenocrysts may represent a process also common to other crystal-poor rhyolites in the region. Currently, the geothermal system beneath Tocomar is sustained by a vertically extensive magmatic system, in which mafic to intermediate magmas replenishing the Altiplano-Puna Magmatic Body provide heat and volatiles to upper parts of the plumbing system. This interpretation is supported by the composition of the emitted volatile phases (CO2 and He), and by the presence near Tocomar of young mafic centers with eruptive ages of c. 150 and 50 ka. |
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| Description: | Gesehen am 26.05.2026 |
| Description matérielle: | Online Resource |
| ISSN: | 1872-6097 |
| DOI: | 10.1016/j.jvolgeores.2026.108581 |