Coexisting Discrete Bodies of Rhyolite and Punctuated Volcanism Characterize Yellowstone's Post‐Lava Creek Tuff Caldera Evolution

Abstract

Ion‐microprobe 206Pb/238U geochronology and trace element geochemistry of the unpolished rims and sectioned interiors of zircons from Yellowstone caldera's oldest post‐caldera lavas provide insight into the magmatic system during the prelude and aftermath of the caldera‐forming Lava Creek supereruption. The post‐caldera lavas compose the Upper Basin Member of the Plateau Rhyolite and fall into two groups based on zircon crystallization age: early lavas with zircon ages between ~750 and 550 ka and late lavas with zircon ages between ~350 and 250 ka. Zircons from the early‐erupted East Biscuit Basin flow yield U‐Pb dates and trace element compositions, which when considered with the Pb isotopic compositions of their coexisting feldspars and pyroxenes, point to an isotopically distinct parental melt present during crystallization of the Lava Creek magma but untapped by the supereruption. Distinct zircon crystallization ages and Pb‐isotope compositions of major minerals between the early and late Upper Basin Member groups suggest contrasting sources in the magma reservoir. As proxies for melt evolution, the zircons indicate that Yellowstone's post‐caldera rhyolites became more evolved between mid‐ to late‐Pleistocene time, during the same interval that melting of hydrothermally altered wall rock and recharge by new silicic magmas changed in their relative roles. The results from this study indicate that discrete and ephemeral bodies of silicic magma, at times within a mush dominated reservoir and including during the prelude to the Lava Creek eruption, have characterized Yellowstone's subvolcanic reservoir.

Volcano-pluton connections at the Lake City magmatic center (Colorado, USA)

“…On the other hand, residence times of eruptible magmas may be over estimated, and pseudo-geochemical trends may be resolved, if a zirconsaturated system spends a significant amount of time in a subsolidus state. This may be important for explaining some of the discrepancies that have been identified between magma residence times estimated from zircon geochronology and those from geospeedometry in other phases (e.g., Allan et al, 2013;Cooper and Kent, 2014;Till et al, 2019).…”

Postcaldera intrusive magmatism at the Platoro caldera complex, Southern Rocky Mountain volcanic field, Colorado, USA

“…13). Distinctive zircon trace element characteristics, such as those displayed by Chiquito Peak Tuff and the Alamosa River and Jasper monzonites, suggest crystallization from discrete, separate magma reservoirs (e.g., Till et al, 2019).…”

Extraction of high-silica granites from an upper crustal magma reservoir: Insights from the Narusongduo magmatic system, Gangdese arc

“…Fourth, the Narusongduo HSGs have low Zr/Hf ratios (average Zr/Hf = 28.4), which is a relatively robust proxy for identifying highly evolved magmas that were extracted from zircon-saturated mush zones (Linnen and Keppler, 2002;Claiborne et al, 2006;Deering and Bachmann, 2010;Deering et al, 2016;Wu et al, 2017). The samples of HSG have variable Zr/Hf ratios varying from 22 to 37, suggesting either that they were extracted from the andesitic magma reservoir at different cycles or that they were derived from disconnected melt lenses (e.g., Till et al 2019) with variable proportions of cumulate zircon, consistent with the diverse ages of zircon antecrysts (Figure 2). If the separation of melt-rich lenses occurred after zircon crystallization (Figure 7b), then the extracted melts would have significantly lower Zr/Hf ratios, such as the HSG samples with Zr/Hf = 22-24.…”