Geochemical Transition from Miocene -- Pliocene to Quaternary Arc Volcanism in the Sierra Nevada, Northern California

The Northern Sierra region separates, by some 250km, the geochemically distinct lavas of the Southern Ancestral Cascades (Miocene-Pliocene) of the Lake Tahoe-Reno area, and lavas of the Southern Modern Cascades (Quaternary) of the Lassen Volcanic Centre (LVC). While minor geochemical differences in both major and trace elements exist, radiogenic isotope geochemistry provides the most information about the possible sources of the lavas. Isotopic values from the Lake Tahoe-Reno area are indicative of an older, subduction modified lithospheric mantle. Compared to Southern Modern Cascades lavas of the LVC and the Southern Ancestral Cascades lavas, the N.Sierra lavas have slightly more elevated 87Sr/86Sr and lower 143Nd/144Nd than do the LVC lavas, while exhibiting less enriched values compared to the Lake Tahoe area lavas. These geochemical signatures suggest that the older, more enriched lithospheric mantle may be thinning from south (Lake Tahoe area) to the north (LVC).