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 The Franciscan Complex of California, the type example of an exhumed accretionary complex, records a protracted history of voluminous subduction accretion along the western margin of North America. Recent geochronological work has improved our knowledge of the timing of accretion, but the details of the accretionary history are disputed, in part, due to uncertainties in regional-scale correlations of different units. We present new detrital zircon U-Pb ages from two sites on opposite sides of San Francisco Bay in central California that confirm previously proposed correlations. Both sites are characterized by a structurally higher blueschist-facies unit (Angel Island unit) underlain by a prehnite-pumpellyite-facies unit (Alcatraz unit). The Angel Island unit yields maximum depositional ages (MDAs) ranging from 112 ± 1 Ma to 114 ± 1 Ma (±2σ), and the Alcatraz unit yields MDAs between 94 ± 2 Ma and 99 ± 1 Ma. Restoration of post-subduction dextral displacement suggests these sites were originally 44–78 km apart and much closer to other Franciscan units that are now exposed farther south in the Diablo Range. Comparison with detrital zircon dates from the Diablo Range supports correlations of the Bay Area units with certain units in the Diablo Range. In contrast, correlations with Franciscan units in the northern Coast Ranges of California are not robust: some units are clearly older than those in the Bay Area whereas others exhibit distinct differences in provenance. Integration of age data from throughout the Franciscan Complex indicates long-lived and episodic accretion from the Early Cretaceous to Paleogene. Although minor, sporadic accretion began earlier, significant accretion occurred during the interval 123–80 Ma and was followed by minor accretion at ca. 53–49 Ma. Periods of accretion and non-accretion were associated with arc magmatism in the Sierra Nevada–Klamath region, cessation of arc activity, and reorganization of paleodrainage systems, which implicates plate dynamics and sediment availability as major controls on the development of the Franciscan Complex.