Cogenetic late Pleistocene rhyolite and cumulate diorites from Augustine Volcano revealed by SIMS²³⁸U‐²³⁰Th dating of zircon, and implications for silicic magma generation by extraction from mush

Abstract

Augustine Volcano, a frequently active andesitic island stratocone, erupted a late Pleistocene rhyolite pumice fall that is temporally linked through zircon geochronology to cumulate dioritic blocks brought to the surface in Augustine's 2006 eruption. Zircon from the rhyolite yield a 238 U-230 Th age of 25 ka for their unpolished rims, and their interiors yield a bimodal age populations at 26 ka and a minority at 41 ka. Zircon from dioritic blocks, ripped from Augustine's shallow magmatic plumbing system and ejected during the 2006 eruption, have interiors defining a 26 ka age population that is indistinguishable from that for the rhyolite; unpolished rims on the dioritic zircon are dominantly younger (12 ka) indicating subsequent crystallization. Zircon from rhyolite and diorite overlap in U, Hf, Ti, and REE concentrations although diorites also contain a second population of high-U, high temperature grains. Andesites that brought dioritic blocks to the surface in 2006 contain zircon with young (9 ka) rims and a scattering of older ages, but few zircon that crystallized during the 26 ka interval. Both the Pleistocene-age rhyolite and the 2006 dioritic inclusions plot along a whole-rock compositional trend distinct from mid-Holocenepresent andesites and dacites, and the diorites, rhyolite, and two early Holocene dacites define linear unmixing trends often oblique to the main andesite array and consistent with melt (rhyolite) extraction from a mush (dacites), leaving behind a cumulate amphibole-bearing residue (diorites). Rare zircon antecrysts up to 300 ka from all rock types indicate that a Quaternary center has been present longer than preserved surficial deposits.

A glimpse into Augustine Volcano's Pleistocene past: Insight from the petrology of a massive rhyolite deposit

“…Matrix glass compositions are consistent between the three samples at ~76.6 wt% SiO 2 . Both whole-rock and matrix compositions are similar to those reported by (Larsen et al, 2010) and Coombs and Vazquez (2014) for other Augustine rhyolite samples of 71-74 and 76-77 wt% SiO 2 , respectively.…”

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

“…The east Sierra Jurassic zircons have significantly higher Th/U, mostly 0.5-1.5, ranging up to 2.1 and extending to values higher than is Figure 13. (a and b) Uranium enrichment in arc zircons Claiborne, 2011;Coombs and Vazquez, 2014;Dilles et al, 2015). The well-known lithophile element enrichment of arc lavas from convergent-margin settings is reflected in relative U enrichment in zircons, where low to high U/Yb zircons formed in low to high K silicic arc melts.…”

A survey of Sierra Nevada magmatism using Great Valley detrital zircon trace-element geochemistry: View from the forearc

“…Arc zircon geochemistry is primarily controlled by the extent of melt fractionation during zircon growth and magma source variations that result in variable melt compositions at zircon saturation (Walker et al, 2010;Claiborne et al, 2010;Barth et al, 2013;Coombs and Vazquez, 2014). Arc zircons are characterized by enrichment in U and Th, reflecting the slab contribution to arc magmas, and depletion in Nb and heavy rare earth elements (HREEs), which are conserved in the slab (Barth et al, 2017;Schmitt et al, 2018).…”