127-2 Precursor to the Ancient Rocky Mountain Super Volcanoes of Southern Colorado: Assessing the Petrogenesis and Crustal Evolution of the Pre-Caldera Conejos Lavas at the Platoro Caldera Complex

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

Presenting Author:

Authors:

Abstract:

The lava flows of the Conejos Formation serve as the initial phase of volcanism at Platoro Caldera and the wider Southern Rocky Mountain Volcanic Field - providing a basepoint for understanding the inception and early evolution of large-scale volcanic flare-ups and subsequent caldera-forming eruptions. Prior studies (mainly Colucci et al. 1991) have mapped and characterized three spatially, temporally, and chemically distinct sub-members of the Conejos at Platoro which erupted between 35 and 30 million years ago. Colucci primarily used bulk rock data for insight into parental magma composition and to create a model for pre-caldera magma petrogenesis, storage, and chemical evolution. However, limited mineral-centric data, outdated analytical methods, and lack of thermobarometry constraints leaves a restricted understanding of parental melt composition and limits resolution and interpretations of petrogenic models.

My research aims to investigate the magmatic, volcanic, and crustal evolution of the Conejos Formation at Platoro via in-situ EPMA analysis of pyroxene composition, zoning, and hosted melt inclusions. Critical questions that I seek to answer during my research are: What is the composition of the parental melts that formed each member of the Conejos at Platoro? And how do the magmatic storage conditions and crustal evolution vary by member and through time?

In-situ analysis of hundreds of pyroxenes from eight samples distributed across Platoro and representative of the three sub-units have revealed: Two-pyroxene thermobarometry places the magma storage conditions of my samples at a depth of 7 to 23 km and from 821 to 1027°C. Compositions of clinopyroxene zones of two of the sub-units follow the same trends in all major elements with the third differing in Ti and Mn concentrations with increasing SiO₂, potentially indicating that this third unit evolved separately or potentially resulted from a different magmatic parent. Upcoming analysis of melt inclusions and AFC modelling will further help to elucidate if these different lava flows are purely a result of magmatic evolution, or perhaps a difference in parental source.

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

doi: 10.1130/abs/2025AM-11068

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