73-6 The Compositional and Temporal Relationships Between Basaltic Lava Flows of the Beaver Ridge Subfield in Black Rock Desert, Utah

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

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Lava flows are the surface expression of magmatic compositions at the time of eruption. The geochemistry of individual flows can be used to interpret pre-eruptive processes along with magma chamber occurrence and longevity. The Quaternary Beaver Ridge subfield of central Utah’s Black Rock Desert is a small (~50 km²) area of mostly basaltic lava with ⁴⁰Ar/³⁹Ar eruption ages ranging from 0.3 Ma to ~1.2 Ma. The proximity of numerous flows in both space and time lead to questions about the longevity of magma chambers, and the timescales of processes that operate within them. To evaluate the petrogenetic relationships among the lavas, this work examines the petrography, geochemistry, and Pb isotopic ratios of the Beaver Ridge lavas. We use the Magma Chamber Simulator (MCS) to test if fractional crystallization can serve as a process to generate chemically distinct basalts. The MCS models suggest that at least three different magma sources are needed to produce the observed erupted compositions. Beaver Ridge 1 (BR1; 0.91 to 1.22 Ma) consists of at least seven lavas with compositions ranging from basalt to dacite. Beaver Ridge 2 (BR2; 0.52 to 0.88 Ma) consists of at least four lavas with compositions ranging from basalt to basaltic andesite. Beaver Ridge 3 (BR3; 0.30 to 0.39 Ma) consists of a basalt and a rhyolite lava flow that spatially correlate. BR1 and BR3 can be modeled as liquids resulting from a basaltic andesite parental composition, with a large range in the amount of crystallization to produce both the basalts and dacites sampled. However, BR1 and BR3 are considered as derivatives from different sources due to the significantly more radiogenic Pb isotopes of BR3. BR2 requires a more primitive parental composition, requiring >66% of the chamber mass to be crystalized to reach the composition of the observed samples and necessitating a hypothetical basaltic magma composition which has not been observed in the subfield. Combining these results with textural components observed in thin section, the conclusion of three different sources appears to be substantiated and allows us to think about the timescales that these magma chambers must have operated in order to produce these distinct groups. 

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

doi: 10.1130/abs/2025AM-8706

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