UC Riverside

California is geologically complex because of a long history of changing tectonic environments in the last ~300 my. Currently, the tectonic environments of California can be broadly split between subduction-driven in the north and transform motion-driven in the south, with the current transition between the two occurring ~325 kms northwest of San Francisco, at the Mendocino Triple Junction. Around 12 Ma, this transition location was in what is now Southern California and Northern Mexico. The full subduction of the Farallon plate under the North American plate and resulting attachment of the Pacific plate to the western edge of the North American plate led to the formation of the oblique-transform San Andreas fault system as well as the northern migration of the Mendocino Triple Junction. These different tectonic environments are evident through petrologic investigations of the rocks. Northern California has active subduction zone volcanism, the southern most active volcano being Lassen Peak. Southern California contains the western edge of the Basin and Range Province, where propagation of volcanism across the American Southwest has been attributed to lithospheric thinning and/or decompression melting. In the southernmost part of Southern California, where the San Andreas Fault system begins, ongoing continental rifting connected with the opening of the Gulf of California occurs beneath thick layers of Colorado River deposited sediments. This thesis reports microanalytical, geochemical analyses of various volcanic systems throughout California. Olivine hosted melt inclusions from the Basalts of Round Valley Butte, a basaltic cinder cone about 40 km east of Lassen Peak, were analyzed to evaluate the use of these investigative tools for pre-eruptive processes. The knowledge gained from Basalts of Round Valley Butte were then used on olivine hosted melt inclusions, melt embayments, and matrix glasses to characterize the melts beneath Pisgah Crater in the Mojave Desert to better constrain the mantle melting conditions that led to the formation of Basin and Range mafic volcanism. Finally, this study examines the effects of continental rifting heat sources on the mineralogy of the Salton Sea Geothermal Field and specifically how this heat source has affected the distribution of lithium between geothermal brines and minerals.