268-9 Decoding Magmatic Evolution in the Ecuadorian Rhyolite Province (ERP): Integrating Zircon Petrochronology and Glass Geochemistry

Session: Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder (Posters)

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Abstract:

Explosive rhyolitic eruptions pose serious hazards in volcanic regions worldwide. The Ecuadorian Rhyolite Province (ERP), situated within the eastern Cordillera of the Ecuadorian Andes, is a large volcanic field (~130 × 30 km) containing the Chacana rift caldera complex, Chalupas caldera, Cotopaxi, and the Pisayambo volcanics. Quito (pop. >2 million) lies along the ERP’s western margin, placing it near significant volcanic centers. Rhyolitic volcanism has occurred repeatedly from ~200 ka to <3 ka, beginning when Chalupas caldera produced a single 200 km³ rhyolitic pumiceous ignimbrite. Followed by Chacana eruptions producing rhyolitic lava flows and 100 km³ of pumice lapilli fallout. Recent activity (<100 ka) includes interleaved eruptions from all four centers. This repetitive eruptive history justifies the need to understand ERP magmatic evolution better and assess future eruption risk in Quito and northern Ecuador.

We are testing the hypothesis that ERP rhyolites were sourced from a long-lived magmatic system by integrating zircon petrochronology (U-Pb ages and trace elements) and glass geochemistry (major and trace elements). Zircons and glass from six ERP samples (three Chacana, two Chalupas, one unknown) were analyzed using LA-ICP-MS and EMPA. Glass major element geochemistry plotted on a TAS diagram reveals two compositional groups: high silica rhyolites (76–78 wt.% SiO₂) from Chacana and slightly less evolved rhyolites (72–74 wt.% SiO₂) from Chalupas with the unknown sample. This suggests the presence of two or more distinct magmatic groups.

Zircon U-Pb ages overlap with published ⁴⁰Ar/³⁹Ar eruption ages and display broad crystallization age ranges, consistent with protracted zircon growth and antecrysts incorporation. Trace element patterns among zircons show similar trends and, like glass major chemistry, also reveal two distinct chemical groups with some overlap, suggesting possible magma mixing or shared source components. Glass trace elements further support this division, reinforcing the presence of two distinct magmatic groups. Combined zircon and glass geochemistry suggests the unknown sample is genetically linked to the Chalupas system.

Ongoing work involves refining zircon age distributions and trace element patterns to reconstruct crystallization histories. By linking glass and zircon geochemistry, this study aims to test zircon-melt equilibrium, identify antecryst populations, and better constrain ERP magmatic evolution. These insights will improve our understanding of long-lived magma reservoirs and aid in forecasting future eruptions of highly siliceous centers in northern Ecuador.

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

doi: 10.1130/abs/2025AM-10815

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