Geology, 40Ar/39Ar geochronology, and lithium enrichment of the mid-miocene McDermitt volcanic field, Nevada and Oregon, United States [electronic resource]
Keywords: Geological and Environmental Sciences.

Catastrophic caldera-forming eruptions of intracontinental silicic magmas pose significant direct hazards to local communities and can severely impact global climate. Because these eruptions are infrequent on human time scales, studying ancient silicic systems is required to evaluate the frequency, styles, and causes of these cataclysmic events to inform hazard assessments at active centers. In addition, because many mineral resources are hosted within silicic volcanic centers, understanding the geology and geochemistry of these centers is essential for resource exploration and assessment efforts. In this dissertation, I use field relationships, 40Ar/39Ar geochronology, and trace-element geochemistry to establish a volcanic stratigraphy of metaluminous to weakly peralkaline ignimbrites and lavas from McDermitt Volcanic Field (Nevada and Oregon) and their relationships to regional ignimbrites from the nearby High Rock, Hawks Valley -- Lone Mountain, and Santa Rosa -- Calico centers. This data is used along with new geological mapping to identify two new calderas as sources for the oldest two ignimbrite eruptions from McDermitt Volcanic Field, the 16.47 Ma Tuff of Oregon Canyon and 16.42 Ma Tuff of Trout Creek Mountains. Topographic and structural margins of these calderas and the previously recognized Whitehorse Caldera, the source for the 15.56 Ma Tuff of Whitehorse Creek, are delineated in a new geological map. This reinterpretation of McDermitt Volcanic Field geology and synthesis with other centers make clear a southeastward propagation of silicic volcanism fueled by intrusion of Steens Basalt dikes during impingement of the Yellowstone plume head on the lithosphere. High-precision 40Ar/39Ar ages for McDermitt ignimbrites and associated rhyolitic lavas in conjunction with new ages obtained on regional trachytic to rhyolitic tuffs intercalated with lava flows of Steens Basalt are used to constrain the timing of the earliest Columbia River Basalt Group volcanism to ~16.64 -- 16.43 Ma. This volcanism postdates the onset of the Miocene Climatic Optimum, indicating that this global warming event was not caused by effusion of Steens and Columbia River flood basalt lavas. Lithium abundances of silicic magmas from McDermitt Volcanic Field and other rhyolitic centers from around the globe are determined by in situ measurements of homogenized melt inclusions in quartz phenocrysts using SHRIMP-RG. Highest concentrations occur in magmas that assimilate or incorporate high proportions of felsic continental crust. McDermitt Volcanic Field magmas formed in transitional continental crust and contain ~1,400 ppm lithium, an order of magnitude greater than magma at Pantelleria, Italy (~100 ppm), which erupted through thin continental crust, and significantly lower than magma at Hideaway Park, Colorado (~6,300 ppm), which formed on partial melting of thick continental lithosphere. Despite containing magmas with only moderate magmatic enrichment of lithium, the McDermitt Volcanic Field hosts the largest lithium deposit in the United States within caldera lake sediments along the ring fracture of the McDermitt Caldera. Using this deposit as a guide, a new model is presented in which intracontinental magmas only moderately enriched in lithium lead to economic deposits of lithium when erupted in large caldera settings. As demand for lithium rises due to its use in batteries for mobile electronics and hybrid and electric automobiles, lithium resource exploration efforts should focus on intracontinental calderas with preserved caldera lake sediments, including the newly identified calderas in the northern McDermitt Volcanic Field.