127-3 Inter-Caldera Rocks of the San Luis Caldera Complex: Insights into Magmatic System and Evolution

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session

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Abstract:

Ignimbrite-producing, caldera-forming eruptions have represented some of the most massive and destructive geological events in history, making understanding their formation critical for effective hazard management. The central San Juan caldera cluster produced four of these eruptions in rapid succession during an “ignimbrite flare-up'' in the mid-Cenozoic Era, generating some of the most voluminous ignimbrites ever recorded. The tuffs themselves have been well-documented, but the intermediate lavas and small-volume intrusions that punctuated these eruptions are comparatively understudied. To better understand these units, we conducted SIMS U-Pb zircon geochronology, ICP-MS/ICP-OES bulk rock geochemical analysis, optical petrography, and plagioclase microtexture analysis of eleven intermediate inter-caldera samples. Our analyses fit these units into a model of a complex, dynamic, and tiered magmatic system injected episodically with mafic magma. Two samples originally mapped as a Stewart Peak intrusion and Fisher Dacite intrusion appear to be misclassified and instead represent an intracaldera tuff and lava flow, respectively. Additionally, the Captive Inca Dacite is compositionally similar to the immediately following Rat Creek Tuff and the Nelson Mountain Tuff一units potentially sourced from a shared reservoir. This temporal and compositional clustering suggests that the Captive Inca Dacite could represent an initial, small-volume eruption from the Rat Creek/Nelson Mountain reservoir, extending the timing and lateral extent of magma residence in the upper crust prior to the onset of this series of ignimbrite eruptions.

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

doi: 10.1130/abs/2025AM-5987

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