ABSTRACT Miocene–Pliocene volcanism around Lake Tahoe, California/Nevada, USA, part of the Southern Ancestral Cascades arc, ceased at around 3 Ma as the southern edge of the subducting Juan de Fuca plate migrated north of the region. Post–3 Ma, arc volcanism continued north of Lake Tahoe, but modern subduction and arc volcanism now occur only north of the Lassen volcanic center. Miocene–Pliocene Tahoe arc lavas appear to include an older mantle source component that is not common in Quaternary Lassen arc rocks. The goal of this work was to investigate how magma sources and/or volcanic processes transitioned in the northern Sierra Nevada between Lake Tahoe and Lassen. The Sierra Nevada between Lake Tahoe and Lassen, or the North Sierra segment of the Ancestral Cascades, includes eroded remnants of Ancestral Cascades volcanic rocks, including lava flow complexes, intrusions, and landslide/debris-flow deposits. Lava samples from the North Sierra segment include calc-alkaline basalts to dacites, with rare rhyolites. All North Sierra segment lavas exhibit normalized incompatible-element patterns with negative Nb, Ta, and Ti anomalies and positive Pb, Sr, and Ba anomalies. The North Sierra segment is geochemically split into two parts: a northern group including lavas from the Susanville area, and a southern group consisting of arc rocks from the Portola, Sierraville, Henness Pass, and Sagehen areas. With the exception of the Sagehen area, the North Sierra segment shows little variation in radiogenic isotope ratios with SiO 2 , indicating that assimilation of crustal rocks was outweighed by liquid-crystal crystallization during magma evolution. Trace-element and isotopic ratios in mafic rocks of the northern group are more typical of Lassen area Quaternary volcanic rocks, whereas those of southern group mafic rocks are more typical of Miocene–Pliocene arc lavas of the Lake Tahoe area. The isotopic distinction between Lassen-like and Tahoe-like arc lavas is likely controlled by basement age and lithology, where Lassen-like magmas were derived largely by mantle wedge melting and Tahoe-like magmas were primarily partial melts of metasomatized Sierran lithospheric mantle. The Susanville area represents the “transition zone” between these two geochemically distinct primary magma sources.

ABSTRACT Born in Chicago, Illinois, I am a beneficiary of doors being opened during the 1960s. I was able to attend the University of Chicago Laboratory High School, and after high school, I attended Michigan Technological University. Thinking I’d like to become an architect, I enrolled in the civil engineering program. During orientation week, I discovered the geology department, and as my first class as a civil engineering major was Principles of Geology, it wasn’t long before the hook was set! However, I never really considered a career as a geologist after college because I assumed the draft and the Vietnam War would determine my ultimate fate. But I wasn’t drafted, and, ultimately, I took a job with Shell Oil Company in New Orleans as an “engineer,” though I had a geology degree. Realizing this was not the job I wanted or the place I wanted to be, I sought an adventure by going to the University of Alaska to get a master’s degree. This led to many more adventures and challenges as I learned to adapt to a new environment far different from my past experience. As I finished the master’s degree, I was invited to join the U.S. Geological Survey (USGS) and began a long career with the Alaska group of the USGS. It, too, has been one continuing adventure, working with and learning from some amazing people through the years. I only hope that others can find their way to their own adventures in the geosciences. There have been barriers thrown up along the way at times, sometimes reflecting that I’m African American. Yet, in all cases, I’ve found ways to adapt or overcome. My style has been to get along, but sometimes I’ve needed to push through barriers to accomplish my goal at the time.