14 Quaternary Volcanic Centers of the Cordillera

Friday, 14 May 2021
8:30 AM - 12:30 PM
Mackay School of Earth Sciences & Engineering Room

U-TH DATING OF ZIRCON AND ALLANITE RIMS FROM SUGARLOAF MOUNTAIN, A HIGH SILICA RHYOLITIC DOME WITHIN THE SAN FRANCISCO VOLCANIC FIELD, NORTHERN ARIZONA

Paper number: 14-1
Program: Theme Sessions
Time: 8:30 AM – 8:50 AM

Authors

Abstract

Sugarloaf Mountain is a high-silica (75.6 wt.%) rhyolite dome and has associated pumice deposits located within the San Francisco Peaks of northern Arizona. Sugarloaf rhyolite is significant because a sanidine 40Ar/39Ar plateau date of 91±2 ka and an inverse isochron date of 93±16 ka (MSWD:19) (Morgan et al., 2010) make it the youngest silicic eruption within the San Francisco Volcanic Field. The rhyolite is also noteworthy for its accessory mineral assemblage of zircon, allanite, and chevkinite. Results for allanites and/or chevkinites can be paired with those for zircons to obtain precise 238U-230Th isochron ages. The Stanford-USGS SHRIMP-RG ion microprobe was used for 238U-230Th dating of surfaces representing the final crystal growth on individual zircon and allanite crystals from five Sugarloaf samples. The zircon rims have unusually high uranium concentrations of up to 1.5 wt.%, and the allanites have thorium concentrations of 2-3 wt%. Additionally, 90% of the zircon rims yield a restricted range of model ages, assuming growth from a melt like the whole rock at 238U-230Th secular equilibrium, and a weighted mean age of 74.7+1.1/-1.0 ka (MSWD=1.6; n=31). Zircons and allanites together yield a combined 238U-230Th isochron date of 75.5± 0.8 ka, which is ~15 kyr younger than the reported 40Ar/39Ar dates. This result suggests that the Sugarloaf eruption occurred at a younger age than previously recognized.

Geological Society of America Abstracts with Programs. Vol. 53, No. 4

doi: 10.1130/abs/2021CD-363328

© Copyright 2021 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.