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Abstract

Trace-element analyses of zircons from ash-fl ow tuffs and granitic rocks in the eastcentral 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-fl ow 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 classifi cation as hydrothermally altered, arctype, 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-fl ow 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 unifi ed volcano-plutonic model for Sierran arc magmatism that describes a longlived, 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.

Granite provenance and intrusion in arcs: Evidence from diverse zircon types in Big Bear Lake Intrusive Suite, USA

“…We conclude that individual zoned intrusive suites in the California arc may have formed incrementally at a range of flux rates, from as low as <0.001 km 3 /yr to 0.005 km 3 /yr or more, with higher rates having the potential to generate and sustain comparatively large reservoirs of mobile magma. Such a range of flux rates is consistent with evidence that all three known pulses of voluminous intrusion in the Mesozoic California arc (Late Triassic, Middle to Late Jurassic, mid-to Late Cretaceous) were at least locally accompanied by eruption of widespread and locally thick, crystal-poor rhyolitic to crystal-rich dacitic ash-flow tuffs (Saleeby et al, 1990;Fiske and Tobisch, 1994;Schermer and Busby, 1994;Fohey-Breting et al, 2010;Barth et al, 2012). …”

Marine Volcaniclastic Record of Early Arc Evolution in the Eastern Ritter Range Pendant, Central Sierra Nevada, California

“…Grimes et al (2015) and Barth et al (2017) have established that zircon compositional ranges reflect specific tectonomagmatic environments. The zircon compositional data from the Ritter Range, combined with data from Saddlebag Lake pendant (Barth et al, 2012), not only provide information about melt evolution but also confirm the compositionally distinct nature of Jurassic melts, consistent with the differences in the whole rock major and trace element data presented above.…”

“…Studies of these incompletely preserved volcanic sections are critically important to understanding juvenile volcanic arc evolution and potential linkages between explosive volcanism and assembly of the underlying batholith. Geochemical and geochronological studies of the Sierra Nevada magmatic arc suggest a complex relationship between volcanism and batholith construction that remains controversial and poorly constrained (Barth et al, 2012(Barth et al, , 2013Bateman, 1992;Busby-Spera, 1984;Fiske & Tobisch, 1994;Glazner, 1991;Kistler & Swanson, 1981;Saleeby et al, 1990;Tobisch et al, 2000).…”

The Volcanic-Plutonic Connection

“…Less precise U/Pb methods (ion microprobe and laser-ablation ICPMS) are generally an order of magnitude less precise. For example, Barth et al (2012) found evidence for coeval volcanism and plutonism in the Mesozoic Sierran arc, but uncertainties on their ages of ±2 Ma (or more) are far too large to make a definite genetic link between systems that are known to crystallize zircon on timescales an order of magnitude shorter. Zimmerer and McIntosh (2013) obtained laser-ablation ICPMS geochronologic data for volcanic and plutonic rocks of the Oligocene Organ caldera of New Mexico and also concluded that extrusive and intrusive rocks were coeval within *600 ka.…”