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The Magmatic Evolution of the San Francisco Volcanic Field's Sugarloaf Mountain, Through U-Th Dating and Compositional Analyses

Huisa, Allison (2021) The Magmatic Evolution of the San Francisco Volcanic Field's Sugarloaf Mountain, Through U-Th Dating and Compositional Analyses. Masters thesis, Northern Arizona University.

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Rhyolites are the product of some of the largest eruptions in geologic history and come from shallow crustal reservoirs. There is considerable uncertainty as to how these magmas are stored and how they are remobilized for eruption. This can be linked to whether these magmas were stored in a less crystalline, readily eruptible state or in a more crystalline immobile state that becomes eruptible only after the reservoir is recharged by an influx of magma. In this thesis study, the storage conditions and magmatic evolution of a small-volume high-silica rhyolite is investigated using 230U-238Th dating of zircon and allanite surfaces, allanite geochemical analyses, and whole-rock major and trace element data. Sugarloaf Mountain is the youngest entirely silicic eruption within the San Francisco Volcanic Field in northern Arizona and is located at the mouth of San Francisco Mountain’s Inner Basin. This lava dome is composed of high silica rhyolite (>75 wt.% SiO2) that contains sanidine and accessory minerals such as zircon, allanite, and possibly chevkinite. Its sanidine 40Ar/39Ar date of 91±2 ka (Morgan et al. 2010) makes Sugarloaf Mountain an appropriate choice for 238U-230Th dating and compositional analyses. Precise age dating proved to be possible because of Sugarloaf’s zircon exceptionally high UO2 concentrations (0.7-2.3 wt.%). A minor population of zircons yields a weighted mean model age of ~95 ka (n=3; MSWD=~4). Most zircons yield a substantially younger weighted mean model age of ~75 ka (n =28; MSWD= 1.8 to 2.0) and a zircon isochron date of 76.5 ±7.3 ka. The allanites yield a range of slightly older weighted mean model ages from 72.6 ± 0.3 ka (n=61; MSWD=1.7) and 79.7 ± 0.2 ka (n=61; MSWD=1.4). The uniformity of the zircon and allanite surface ages indicate accessory phase co-crystallization and suggests that the younger dates are representative of final crystal rim growth. A zircon-allanite isochron date of 75.4 ± 1.0 ka, therefore, represents an eruption age for Sugarloaf Mountain, one that is ~15 kyr younger than the sanidine 40Ar/39Ar date provided by Morgan et al. (2010). Compositional analyses of the allanite rims and interiors provide evidence of disequilibrium and chemical fluctuations, indicated by, but not limited to, growth zoning and/or oscillatory zoning. Identification of sector zoning in some of the allanite interiors was an unexpected outcome of this work and is the first to be recorded for volcanic allanite. Cross-sectional analyses of the Sugarloaf allanite interiors to determine chemical melt evolution recorded by covariations between La/Nd and MnO/MgO values proved to be difficult because of the unusually low MgO values and the finding of sector-dependent elemental partitioning. Use of MnO/FeO instead of MnO/MgO as a proxy for differentiation was able to show that allanite interiors have a wider range in composition compared to the surfaces, indicating variable melt composition before eruption related to magma differentiation and recharge. The largely uniform zircon and allanite surface dates and the compositional uniformity of the allanite surfaces, together with zircon and allanite growth rates, indicate that the grains were immersed in the melt for at least 1.3 kyrs and grew in a single body of magma. The older zircon population could further indicate a previous period of warm storage before the younger-aged zircon and allanites grew after a ~20 ka period of cold storage. This study was able to provide a more precise and younger age for Sugarloaf Mountain, one that aligned with similarly aged eruptions within a ~250 km2 area, and insights into how Sugarloaf magma evolved before the eruption.

Item Type: Thesis (Masters)
Publisher’s Statement: © Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords: Allanite; Geochronology; Magma evolution; Rhyolite; Sugarloaf Mountain; Zircon; San Francisco Mountains (Ariz.)
Subjects: Q Science > QE Geology
NAU Depositing Author Academic Status: Student
Department/Unit: Graduate College > Theses and Dissertations
College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited: 02 Feb 2022 21:50
Last Modified: 02 Feb 2022 21:50
URI: https://openknowledge.nau.edu/id/eprint/5638

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