11-2 Plio-Pleistocene Volcanism in the Southern Central Andes: Overo Volcano (34°30' S)

Session: Using Volcanic Deposits to Help Us Understand Volcanic and Magmatic Processes

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During Plio-Pleistocene times, the Southern Central Andes’ evolution was characterized by the steepening of the Nazca plate after a shallowing period in the Late Miocene. This geodynamic setting led to the westward retreat of arc-related magmatism and the development of N-S magmatic belts. Stratovolcanoes were emplaced immediately east of the arc, in a rear-arc position. Among them is the Overo volcano (34°30’S), whose main volcanic activity occurred between early Pleistocene and Holocene times, which exhibits remnant geothermal activity. Overo volcano is a stratovolcano with a central crater surrounded by monogenetic cones, whose volcanic activity alternated between explosive and effusive periods. Overo volcanism comprises subhorizontal levels of ignimbrites, porphyritic lavas, and volcanic autobreccias. The lavas are porphyritic basalts, with plagioclase, clinopyroxene, and olivine phenocrysts, within an intergranular groundmass. They are subalkaline rocks (SiO₂ = 54 -55.5 wt.%; Na₂O+K₂O = 4.6-5 wt.%), with trace element ratios indicating an arc-like signature (La/Ta = 51-64, Th/Hf = 2-2.4). Their Sm/Yb (2.5–2.8) and La/Sm (3.8–4.1) ratios suggest that these magmas equilibrated under low-pressure conditions. An increase in pyroclastic activity is represented by ignimbrite layers interbedded with ash fall tuffs of rhyolitic compositions (SiO₂ =74 wt.%,  alkalis =6.6 wt.%). Then, volcanism became predominantly effusive, characterized by porphyritic lavas with plagioclase phenocrysts in a glassy, fluidal-textured groundmass. Upwards, the lavas display columnar jointing and a lobate texture, composed of discrete, radially arranged subhorizontal tubes, interpreted as pillow lavas, indicating a subaqueous eruptive period. Finally, monogenetic cones and bomb fields are described surrounding the main crater. The cones are porphyritic lavas with plagioclase phenocrysts exhibiting disequilibrium textures and clinopyroxenes, in an intersertal groundmass. Overo volcanic activity culminates with pyroclastic activity and minor lava flows. The pyroclastic rocks are pyroclastic flow deposits interbedded with surge layers. Geochemically, they are subalkaline basaltic-andesites to andesites with a pronounced arc-like signature (La/Ta = 44-50). In summary, Overo volcano is characterized by alternating periods of explosive and effusive eruptions, represented by pyroclastic density currents, fallout deposits, and lava flows. The presence of pillow lavas suggests eruptions that occurred during syn-glacial stages. Overo volcanism took place during the steepening of the Nazca Plate in Pleistocene-Holocene times, and despite being away from the trench, it exhibits an arc-related signature and remnant geothermal activity.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4740

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