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High-SiO2 (>75 wt%) rhyolites are the most differentiated silicate magmas on Earth and are relatively scarce in the volcanic rock record, especially at subduction zones. This scarcity may be due to their narrow (<50°C) liquidus-solidus interval, which promotes extensive crystallization over small drops in temperature, preventing eruption. However, high-SiO2 rhyolites do erupt, sometimes in supervolcano quantities (≤1000 km 3 ), in regions of continental extension (e.g., Yellowstone, WY and Long Valley, CA). Therefore, understanding the magmatic architecture (i.e., temperature/pressure prior to eruption) and/or processes that enable successful eruption of high-SiO2 rhyolite are critical to investigate.

One of the greatest challenges in studying high-SiO2 rhyolites is the paucity of mineral- melt thermometers, barometers and hygrometers that can be accurately applied to them. This leads to conflicting results when thermometers and hygrometers calibrated on different magma compositions are applied to high-SiO2 rhyolites. Also, it is often not possible to use one of the only reliable thermometers, which is based on the equilibrium between two Fe-Ti oxides, because of post-eruptive alteration. In Chapter 2, a new biotite-melt thermometer is presented. New biotite-melt equilibrium experiments were conducted on high-SiO2 rhyolite between 675-800 °C and P H2O = 225-125 MPa. These experimental data were combined with biotite analyses in natural high-SiO2 rhyolites, for which high-quality Fe-Ti two-oxide temperatures are available (660-800 °C), to calibrate the new thermometer. It was successfully deployed on four Jurassic high-SiO2 rhyolite dikes, which contained pristine biotite, demonstrating its utility on samples for which Fe-Ti two-oxide thermometry is not possible.

In Chapter 3, the biotite-melt and Fe-Ti two-oxide thermometers were applied to a suite of high-SiO2 rhyolite domes and flows (from Glass Mountain, CA), which preceded a climactic, supervolcano eruption. A puzzling feature of these rhyolites is their highly variable phenocryst abundances (<1 to 20%), despite little change in their major-element compositions. One sample grew ~8% phenocrysts at a remarkably low temperature (660 ± 10 °C), which is below the water-saturated solidus at upper crustal conditions (≤300 MPa). Three hypotheses were tested to explain the variable phenocryst abundances: (1) they reflect equilibrium temperature, pressure, and melt H2O contents during crystal growth in magmatic reservoir(s), (2) the H2O-saturated granitic solidus is lower than previously documented, or (3) rapid phenocryst growth, following a kinetic delay to nucleation, occurred during magma ascent. Based on new experiments
conducted in this chapter, along with documented phenocryst compositions and textures, it shown that phenocrysts in the Glass Mountain high-SiO2 rhyolites grew during dike transport to the surface and not in a magma chamber.

In Chapter 4, U-Pb zircon crystallization ages (162-169 Ma) were obtained on a high-SiO2 rhyolite dike swarm in the Sierra Nevada, CA, which dates an episode of extension within the long-lived Mesozoic arc. These results are unexpected for two reasons: (1) this age range overlaps a magmatic flare-up Sierra Nevada arc (145-175 Ma), and (2) they differ from
previously published K-Ar ages for these dikes (168-209 Ma), which overlaps a lull in Sierran arc magmatism. However, the zircon ages for the dike swarm directly overlap the age (160-170 Ma) of the Coast Range Ophiolites, which formed in the forearc region due to trench-parallel spreading. This may have induced shearing and extension in the main Sierran arc.