Whole-rock and mineral compositions of three eruptive deposits from the Twin Falls caldera, associated with the Yellowstone hotspot, provide a window into melt generation and evolution for hot, dry, A-type rhyolites. Three rhyolitic units were sampled via the Kimberly drill-core as a part of project HOTSPOT, a study focused on mantle plume and continental lithosphere interaction. Previous work has been done to collect high resolution U-Pb zircon ages, and Hf- and O-isotopic compositions. This study examined the geochemistry of apatite and zircon along with host rock compositions in the context of this previous work. The Kimberly core sampled the Shoshone Rhyolite (6.06 Ma, 120 m thick), Kimberly Member (7.70 Ma, 169 m thick), and Castleford Crossing Member (7.96 Ma, >1400 m thick). Apatite compositions more closely reflect the composition of their whole rock hosts than zircons. SiO2 content is higher in apatite of the Kimberly Member at (1.1 ± 0.75 wt.%), vs (0.72 ± 0.47 wt.%) for the Castleford Crossing and (0.84 ± 0.27 wt.%) for the Shoshone Rhyolite. REEs compensate for Si substitution in these apatites, with the Kimberly Member most enriched. Volatile contents in the apatites are typical of metaluminous A-type rhyolites, with very low Cl and high F concentrations. Average Ti-in-zircon crystallization temperatures were highest in the Castleford Crossing Member (847 ± 68°C), followed by the Shoshone Rhyolite (806 ± 78°C), and then the Kimberly Member (804 ± 70°C). Oxygen fugacity calculated from zircons has average ΔQFM values for the Shoshone (0.8), Kimberly (-0.2), and Castleford Crossing (0.2). Hf concentrations and Eu anomalies are comparable in zircons from all three units. REE patterns in zircons are also similar and concentrations of REEs in the Shoshone and Kimberly units are similar even though the whole rock compositions of all three units are distinct. Less than 15% of zircons in the Kimberly and Castleford Crossing rhyolites have CL-dark cores enriched in several REEs, U, and Th. These CL-dark cored zircons are likely xenocrysts entrained from chemically evolved granite and then overgrown with less enriched rims prior to eruption. There are several apatite grains with Si-LREE enriched rims in the Kimberly Member, which serves as further evidence of assimilation of silicic igneous rock by the Kimberly Member before eruption. Principal component analysis of the geochemical data distinguishes between the units using both whole-rock and apatite compositions. However, zircon compositions are not statistically distinguishable using PCA. A global comparison of Ti, U, Th, Yb, and Nb concentrations in zircons show that the zircons in the Central Snake River Plain are similar to zircons in Hawaiian basalts, while younger zircons from Yellowstone formed in cooler more differentiated magma. We propose that the zircon and apatite chemical patterns and trends confirm the A-type origin of Snake River Plain rhyolites and make it unlikely that they represent partial melts of felsic continental crust but are instead derived in large part from partial melts of young mafic crust--the midcrustal sill.
| Identifer | oai:union.ndltd.org:BGMYU2/oai:scholarsarchive.byu.edu:etd-11095 |
| Date | 14 August 2023 |
| Creators | Gale, Chesley Philip |
| Publisher | BYU ScholarsArchive |
| Source Sets | Brigham Young University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | https://lib.byu.edu/about/copyright/ |
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