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.

The material covered over this six-day field trip explores the regional stratigraphy, tectonic setting, and mineral deposits of the central Sierra Madre Occidental and western part of the Mesa Central in Mexico. The course builds on similar field trips held in 1997, 2003, and 2014, adding updates of the geology based on new studies and opening of new roads in this rugged and poorly accessible region. A completely new section presents a detailed overview of the geology of the classic San Dimas mining district and the Sabinas and La Colorada mines. A road trip log with descriptions of each stop is included.

The material covered over this six-day field trip explores the regional stratigraphy, tectonic setting, and mineral deposits of the central Sierra Madre Occidental and western part of the Mesa Central in Mexico. The course builds on similar field trips held in 1997, 2003, and 2014, adding updates of the geology based on new studies and opening of new roads in this rugged and poorly accessible region. A completely new section presents a detailed overview of the geology of the classic San Dimas mining district and the Sabinas and La Colorada mines. A road trip log with descriptions of each stop is included.