Petrogenetic connections between ash-flow tuffs and a granodioritic to granitic intrusive suite in the Sierra Nevada arc, California

Abstract

Trace-element analyses of zircons from ash-flow tuffs and granitic rocks in the east-central Sierran Nevada, combined with U-Pb zircon ages and trace-element abundances in whole rocks, allow us to interpret a shared magmatic heritage for widespread eruption of ash-flow tuffs and assembly of a typical Sierran granodioritic to granitic intrusive suite. The tuffs have 5%-15% phenocrysts of quartz, feldspar, biotite, and amphibole, silica contents ranging from 70% to 74%, and have variable alkali abundances but are consistently light rare-earth enriched with weak negative Eu anomalies, suggesting chemical classification as hydrothermally altered, arc-type, low-silica rhyolites. The tuffs yielded U-Pb zircon crystallization ages of 232, 224, and 219 Ma, indicating that explosive volcanism began before and continued throughout emplacement of granodioritic to granitic plutons in the underlying 226-218 Ma Scheelite Intrusive Suite, one of the oldest and largest intrusive suites in the Sierra Nevada batholith. Zircon crystals from ash-flow tuffs and contemporaneous felsic granodiorite and granite show distinct chemical zoning and extensive overlap in trace-element abundances, indicating crystallization of zircons from very similar melt compositions despite dissimilar late cooling histories. These data provide evidence for a unified volcano-plutonic model for Sierran arc magmatism that describes a long-lived, multi-stage magma system composed of an incrementally emplaced, granodioritic to granitic intrusive suite in the upper crust, overlain by widespread deposits of ignimbrite eruptions generated by thermal rejuvenation of felsic crystal mushes.